Fineness of Densified Microsilica and Dispersion in Concrete Mixes

Volume 1 of 2

Anastasios M. Ioannides and Amar Deshini

for the Ohio Department of Transportation

Office of Research and Development

State Job Number 148000

June 2006

Fineness of Densified Microsilica and Dispersion in Concrete Mixes

State Job No.: 14800(0) FINAL REPORT

Prepared in cooperation with the Ohio Department of Transportation and the

U.S. Department of Transportation, Federal Highway Administration.

by

University of Cincinnati Cincinnati Infrastructure Institute

Department of Civil and Environmental Engineering Cincinnati, OH

August 2006

Research Team: Anastasios M. Ioannides and Richard A. Miller (co-PIs) Amarendranath Deshini, Jeff C. Mills, Kristina M. Walsh (Research Assistants)

ii

DISCLAIMER

The contents of this report reflect the views of the authors who are

responsible for the facts and the accuracy of the data presented

herein. The contents do not necessarily reflect the official views or

policies of the Ohio Department of Transportation or the Federal

Highway Administration. This report does not constitute a

standard, specification or regulation.

iii

FOREWORD

The investigation described in this Report was sponsored by the Ohio Department

of Transportation (ODOT) and by the Federal Highway Administration (FHWA) as Ohio

State Job No.: 14800(0); PID No.: 11340, under project AFineness of Densified

Microsilica and Dispersion in Concrete Mixes.@ The Principal Investigators were Drs

Anastasios M. Ioannides and Richard A. Miller, Department of Civil and Environmental

Engineering, University of Cincinnati. The ODOT Technical Liaison was Mr Bryan

Struble, the Research Manager was Mr Lloyd Welker, the Administrator for the Office of

Research and Development at ODOT was Ms Monique Evans, and the FHWA liaison in

Columbus, OH was Mr Herman Rodrigo. The assistance, cooperation and friendship of

these individuals was a major contributor to the success of the study, and their support is

gratefully acknowledged. Special thanks are also extended to Tim Jones, ODOT

laboratory technician, who conducted the tests on microsilica. The sand and both kinds

of coarse aggregates were supplied free of charge by Martin Marietta Materials, through

Mr Jim Martin. The cement was donated by CEMEX, through Mr Steve Reibold, and the

microsilica by ELKEM Materials, through Mr Tony N. Kojundic. The MB-AE 90 air

entrainer and the Rheobuild 1000 plasticizer were supplied at no cost by Master Builders,

Inc., through Mr Greg Wirthlin. The authors also acknowledge the contributions to the

project of graduate students Kristy M. Walsh and Jeff C. Mills. This Report will be

submitted by Amarendranath Deshini to the Division of Research and Advanced Studies

of the University of Cincinnati in partial fulfillment of the requirements for the degree of

iv

Master of Science in the Department of Civil and Environmental Engineering, in

December 2006.

v

ABSTRACT

This study explores the effect of densification of microsilica on the mechanical

and other engineering properties of concrete used on Ohio Department of Transportation

(ODOT) projects. American Society for Testing and Materials (ASTM) C 1240 requires

wet-sieved microsilica to pass a No. 325 sieve with no more than 10% retained.

Densified microsilica samples submitted to ODOT sometimes do not meet this

specification, since the sieving process may not be able to break the bonds formed due to

densification. During this study, No. 325 sieve tests on three microsilica types

(undensified, densified, and abused by prolonged exposure to moisture) were performed

at the ODOT laboratory, but none of the materials tested were found to conform to the

ASTM fineness specification. This calls into question the application of this procedure to

assessing the suitability of densified microsilica for use in concrete. In contrast, the

compressive and flexural strengths of concretes mixed with each of the three microsilica

types exceeded those envisaged by ODOT Item 499.03 Concrete-General:

Proportioning. As expected, undensified microsilica concrete yielded higher values than

its densified and abused microsilica counterparts at all ages, but this advantage was rather

limited. This was true for both natural and crushed coarse aggregate mixes. With very

few exceptions attributable to material and testing variability, trends observed with regard

to the effects of microsilica and coarse aggregate types, age and specimen size on the

development of strength were also as anticipated. Therefore, it is concluded that

vi

densified microsilica can be used on ODOT projects for the construction of pavements

and bridges.

vii

ACKNOWLEDGEMENTS

I would like to thank my graduate advisor Dr. Anastasios M. Ioannides and co-

advisor Dr. Richard A. Miller, without whom my thesis would not have happened. I

really appreciate the constant support I received from them. Their assistance not only

helped me to complete my Masters degree successfully, but also helped me shape my

professional career. I would also like to thank Dr. Sam M. Salem and Dr. Issam A.

Minkarah for readily accepting my request to be on my thesis committee.

I am ever grateful to my parents and to my sister for being with me when I am

low, and for the encouragement that I received from them from time to time. My special

thanks to Mr. Vamshidhar Thakkalapalli for his support, and for suggesting University of

Cincinnati for my graduate studies. The encouragement and valuable suggestions from

my roommates and friends (Sita, Ravi, Sharat, Karuna, Raju, Preethi, Pavan, etc., to name

but a few) can never be forgotten.

Lastly, I would like to express my gratitude to the Department of Civil and

Environmental Engineering for giving me the opportunity to pursue my Master of

Science in Construction Engineering and Management at University of Cincinnati.

During my studies at the University, I received financial assistance in the form of

Research and Teaching Assistantships (April 2002 to December 2003), and a University

Graduate Scholarship (September 2001 to April 2004).

viii

TABLE OF CONTENTS

Page

FOREWORD iii

ABSTRACT v

ACKNOWLEDGEMENTS vii

TABLE OF CONTENTS viii

LIST OF TABLES xiv

LIST OF FIGURES xvi

LIST OF SYMBOLS AND ABBREVIATIONS xviii

LIST OF SPECIFICATIONS CITED xxiii

SI* (MODERN METRIC) CONVERSION FACTORS xxv

1 INTRODUCTION 1

1.1 Problem Statement 1

1.2 Objectives 2

1.3 Technical Background 3

1.4 Report Organization 5

ix

2 LITERATURE REVIEW 7

2.1 Introduction 7

2.2 Production of Microsilica 8

2.3 Properties of Microsilica 9

2.4 Effect of Microsilica on Physical Properties of 11

Concrete

2.4.1 Water Demand 11

2.4.2 Cohesiveness 12

2.4.3 Slump and Workability 12

2.4.4 Air Content 13

2.4.5 Setting Time 13

2.4.6 Shrinkage and Cracking 14

2.4.7 Heat of Hydration 17

2.5 Effect of Microsilica on Mechanical Properties of 17

Concrete

2.5.1 Compressive Strength 17

2.5.2 Flexural Strength 18

2.5.3 Modulus of Elasticity 19

2.5.4 Bond Strength 19

2.5.5 Strength Development 20

2.6 Field Tests 22

x

2.7 Do Undispersed Agglomerates Matter? 24

3 MATERIALS AND PROCEDURES 27

3.1 Introduction 27

3.2 Materials Used 27

3.3 Tests on Microsilica 29

3.3.1 Preparation of Abused Microsilica 29

3.3.2 Fineness Tests 30

3.3.3 Gradation Tests 31

3.4 Tests on Aggregates 34

3.4.1 Specific Gravity and Absorption of Coarse Aggregate 34

3.4.2 Specific Gravity and Absorption of Fine Aggregate 35

3.4.3 Bulk Density of Coarse Aggregate 36

3.4.4 Sieve Analysis on Fine and Coarse Aggregates 37

3.4.5 Moisture Content of Fine and Coarse Aggregates 38

3.5 Mix Design 38

3.5.1 Constants and Variables 39

3.5.2 Ingredients 39

3.6 Mixing, Casting, and Curing Methods 41

3.6.1 Mixing Concrete 41

3.6.2 Casting Specimens 42

xi

3.6.3 Curing 43

3.7 Tests on Plastic Concrete 44

3.8 Strength Test Procedures on Hardened Concrete 45

3.8.1 Compressive Strength 45

3.8.2 Flexural Strength 46

4 TEST RESULTS 49

4.1 Introduction 49

4.2 Microsilica 49

4.2.1 Microsilica Fineness 49

4.2.2 Microsilica Gradation 50

4.3 Aggregates 50

4.4 Concrete Mixes 50

4.5 Mechanical Properties 51

5 DISCUSSION OF RESULTS 72

5.1 Introduction 72

5.2 Microsilica Fineness 72

5.3 Microsilica Gradation 74

5.4 Variability of Mechanical Tests 74

xii

5.4.1 Compressive Strength 75

5.4.2 Modulus of Rupture 76

5.5 Data Interpretation 76

5.5.1 Compressive Strength 77

5.5.2 Modulus of Rupture 79

5.6 Effect of Microsilica Type on Mechanical 79

Properties of Concrete

5.6.1 Compressive Strength 79

5.6.2 Modulus of Rupture 81

5.7 Effect of Coarse Aggregate Type on Mechanical 82

Properties of Concrete

5.7.1 Compressive Strength 82

5.7.2 Modulus of Rupture 83

5.8 Effect of Specimen Size on Mechanical Properties of 84

Concrete

6 CONCLUSIONS AND RECOMMENDATIONS 110

6.1 Summary 110

6.2 Conclusions 112

6.3 Recommendations 114

xiii

6.4 Implementation Plan 115

REFERENCES 117

xiv

LIST OF TABLES

Page

4.1 Results of Tests Conducted on Microsilica Types at ODOT 52

Laboratory

4.2 Hydrometer Test on Undensified Microsilica 53

4.3 Hydrometer Test on Densified Microsilica 54

4.4 Hydrometer Test on Abused Microsilica 55

4.5 Dry Sieve Analysis of Undensified Microsilica 56

4.6 Dry Sieve Analysis of Densified Microsilica 57

4.7 Dry Sieve Analysis of Abused Microsilica 58

4.8 (a) Aggregate Sieve Analysis 59

4.8 (b) Physical Aggregate Properties 59

4.9 Ingredients by Mix (per yd3 of concrete) 60

4.10 Specimens Cast by Batch 61

4.11 Physical Properties by Batch 62

4.12 Compressive Strength for Large Cylinders, f′c 63

4.13 Compressive Strength for Small Cylinders, f′c 65

4.14 Modulus of Rupture for Large Beams, MR 67

4.15 Modulus of Rupture for Small Beams, MR 69

5.1 Coefficients of Variation, COV (%), in Laboratory Test Results 86

5.2 Average Compressive Strength for Large Cylinders (psi) 87

5.3 Average Compressive Strength for Small Cylinders (psi) 87

xv

5.4 Relative Compressive StrengthValues for Large Cylinders (%) 88

5.5 Relative Compressive Strength Values for Small Cylinders (%) 88

5.6 Best-Fit Compressive Strength Values for Large Cylinders (psi) 89

5.7 Best-Fit Compressive Strength Values for Small Cylinders (psi) 89

5.8 Average Modulus of Rupture Values for Beams (psi) 90

5.9 Relative Modulus of Rupture Values for Beams (%) 90

5.10 Best-Fit Modulus of Rupture for Small Beams (psi) 90

5.11 Cylinder Size Factors (%) 91

xvi

LIST OF FIGURES

Page

4.1 Grain Size Distribution of Undensified, Densified, and Abused

Microsilica 71

5.1 Trend Line Curves for Large Cylinders 92

5.2 Trend Line Curves for Small Cylinders 93

5.3 Trend Line Curves for Small Beams 94

5.4 Effect of Microsilica Type on Large Cylinders with Natural 95

Aggregate

5.5 Effect of Microsilica Type on Small Cylinders with Natural 96

Aggregate

5.6 Effect of Microsilica Type on Large Cylinders with Crushed 97

Aggregate

5.7 Effect of Microsilica Type on Small Cylinders with Crushed 98

Aggregate

5.8 Effect of Microsilica Type on Small Beams with Natural 99

Aggregate

5.9 Effect of Microsilica Type on Small Beams with Crushed 100

Aggregate

5.10 Effect of Aggregate Type on Large Cylinders with Undensified 101

Microsilica

5.11 Effect of Aggregate Type on Small Cylinders with Undensified 102

xvii

Microsilica

5.12 Effect of Aggregate Type on Large Cylinders with Densified 103

Microsilica

5.13 Effect of Aggregate Type on Small Cylinders with Densified 104

Microsilica

5.14 Effect of Aggregate Type on Large Cylinders with Abused 105

Microsilica

5.15 Effect of Aggregate Type on Small Cylinders with Abused 106

Microsilica

5.16 Effect of Aggregate Type on Small Beams with Undensified 107

Microsilica

5.17 Effect of Aggregate Type on Small Beams with Densified 108

Microsilica

5.18 Effect of Aggregate Type on Small Beams with Abused 109

Microsilica

xviii

LIST OF SYMBOLS AND ABBREVIATIONS

°C: degree celcius

°F: degree Fahrenheit

µ: micro

γw: unit weight of water

%: percentage

A: % absorption

A: weight of oven-dry sample

AASHTO: American Association of State Highway and Transportation Officials

AC: abused microsilica concrete with crushed aggregate

ACI: American Concrete Institute

Al2O3: aluminum oxide

AN: abused microsilica concrete with crushed aggregate

ASG: apparent specific gravity

ASR: alkali silica reactivity

ASTM: American Society for Testing and Materials

b: average width

B: weight of pycnometer with water

B: weight of saturated surface dry sample in air

BSGSSD: bulk specific gravity at saturated surface dry condition

C: cement

C: weight of pycnometer with water

xix

C: weight of saturated sample in water

CaO: calcium oxide

C+P: cement + pozzolan

CA: coarse aggregate

COV: coefficient of variation

d: average depth

D: weight of oven-dry sample

D: diameter of particle

DC: densified microsilica with crushed aggregate

DN: densified microsilica with crushed aggregate

f ′: compressive strength

FA: fine aggregate

Fe2O3: ferric oxide

FM: fineness modulus

ft: feet

G: grams

G: mass of aggregate and measure

G1: specific gravity of liquid in which the sample was suspended

Gs: specific gravity of sample

Hrs: hours

in.: inch

K2O: potassium oxide

xx

kg: kilogram

km: kilometer

l: effective span

L: Distance from the surface of the suspension to the level at which the density of the

suspension is being measured

L: liter

Lb: pound

LOI: loss on ignition

m: meter

MgO: magnesium oxide

Min.: minutes

Mm: millimeter

MnO: manganese oxide

MR: modulus of rupture

MSSD: bulk density of aggregate at ssd condition

MW: mass of water

n: coefficient of viscosity of water

NA: not applicable

NA: not available

Na2O: sodium oxide

NCHRP: National Cooperative Highway Research Program

No.: number

xxi

NT: not tested

ODOT: Ohio Department of Transportation

oz: ounce

P: pozzolan

P: load at failure

PCC: Portland Cement Concrete

pcf: pounds per cubic feet

psi: pound per square inch

P2O2: phosphorus oxide

r: rate of application of load

Ractual: actual hydrometer reading

Rc: corrected hydrometer reading

S: weight of ssd sample

SEM: scanning electron microscopy

SiO: silica oxide

SiO2: silicon-di-oxide

SO3: sulfur trioxide

Sq: square

SSD: surface dry condition

T: Interval of time from beginning of sedimentation to the taking of the reading

T: mass of measure

TEM: transmission electron microscopy

xxii

TiO2: titanium dioxide

UC: undensified microsilica with crushed aggregate

UN: undensified microsilica with natural aggregate

US: United States of America

V: volume

Vair: volume of air

VSSD: volume at saturated surface condition

W: weight

W: density of water

w/c: water cement ratio

WCEM: weight of cement

WMS: weight of microsilica

WMS (SSD): weight of microsilica at ssd

WW (SSD): weight of water at ssd

yd: yard

µ: micro

γw: unit weight of water

No.: number

%: percentage

xxiii

LIST OF SPECIFICATIONS CITED

ASTM C 1240 – 01 Standard Specification for Use of Silica Fume as a Mineral Admixture in Hydraulic-Cement Concrete, Mortar and Grout ASTM C 430 – 96 Standard Test Method for Fineness of Hydraulic Cement by the 45-µm (No. 325) Sieve ASTM D 422 – 63 Standard Test Method for Particle-Size Analysis of Soils ASTM D 421 – 85 Standard Practice for Dry Preparation of Soil Samples for Particle-Size Analysis and Determination of Soil Constants ASTM D 854 – 06 Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer ASTM C 128 – 97 Standard Test Method for Specific Gravity and Absorption of Fine Aggregate ASTM C 29/C 29M – 97 Standard Test Method for Bulk Density (“Unit Weight”) and Voids in Aggregate ASTM C 136 – 96a Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates ODOT Supplemental Specification 848 Bridge Deck Repair and Overlay with Concrete Using Hydro-Demolition ODOT Item 499.03 Concrete-General: Proportioning ODOT Item 499.03 Concrete-General: Proportioning; Slump ASTM C 192/C 192M – 00 Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory ASTM C 143/C 143M – 00 Standard Test Method for Slump of Hydraulic-Cement Concrete ASTM C 231 – 97 Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method

xxiv

ASTM C 39/C 39M – 01 Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens ASTM C 78 – 02 Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading) ODOT Item 703.01 Aggregate-General: Size ODOT 703.02 Aggregate for Portland Cement Concrete: Fine Aggregate ASTM C 127 – 01 Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Coarse Aggregate AASHTO T 22-03 Compressive Strength of Cylindrical Concrete Specimens ASTM C 260 – 01 Standard Specification for Air-Entraining Admixtures for Concrete AASHTO M 154 – 05 Standard Specification for Air-Entraining Admixtures for Concrete U.S. Army Corps of Engineers CRD-C 13 Standard Specification for Air-Entraining Admixtures for Concrete ASTM C 566 – 89 Standard Test Method for Total Moisture Content of Aggregate by Drying ODOT 2002 Construction and Material Specifications

xxv

1

1 INTRODUCTION

1.1 Problem Statement

Microsilica has proven to be an excellent admixture for Portland cement concrete.

Addition of microsilica to a concrete mix usually results in significant improvements in

strength, durability and permeability. Some of the improvements to the concrete

properties occur because microsilica is a pozzolan. Pozzolans are finely divided silica

that combine with free lime (calcium hydroxide) to create more calcium silicate hydrate.

When conventional Portland cement hydrates, it produces both calcium silicate hydrate,

the chemical glue that makes the cement hard, and free lime, which is weak and highly

soluble. This strengthens significantly the cement matrix and decreases its permeability.

Microsilica is estimated to be about a hundred times finer than cement, giving it the

ability to plug voids between cement particles, and helping it increase the density of the

cement matrix (Malhotra, et al., 1987).

In its natural state, microsilica is extremely fine, having a particle size about the

same as that of cigarette smoke, and this makes it difficult to handle. Microsilica had

originally been used as a slurry, but this form was also found to be inconvenient for

ready-mix plants. The solution to this problem is densification. The microsilica is

bonded into larger particles that are far easier to handle (Malhotra, et al., 1987). There is

some concern, however, as to the effect of densification on the quality control process.

The American Society for Testing and Materials (ASTM) C 1240 – 01 Standard

Specification for Use of Silica Fume as a Mineral Admixture in Hydraulic-Cement

2

Concrete, Mortar and Grout requires in its Table 2 that wet-sieved microsilica pass the

No. 325 sieve (45 µm) with no more than 10% retained, and advises that care be

exercised “to avoid retaining agglomerations of extremely fine material.” In its

Appendix X2, however, ASTM 1240 – 01 also states: “The 45-µm (No. 325) sieve

specification is to be used to determine the amount of foreign material present…; good

judgment must be exercised to differentiate between easily dispersible agglomerates and

foreign materials.” The Ohio Department of Transportation (ODOT) has found that

densified microsilica samples submitted sometimes do not meet this specification,

apparently because wet-sieving is not capable of breaking the bonds formed during the

densification process. On the other hand, it may be argued that densification bonds are

temporary and are easily overcome during concrete mixing itself. It is also possible that

densified microsilica will still have adequate field performance even if a certain

percentage of its particles do not pass the No. 325 sieve.

1.2 Objectives

In this study, two important questions that arise out of the processes involved in

packaging and subsequent mixing microsilica into fresh concrete are explored. The first

pertains to increased particle size that results during densification. The second issue is

related to the possible repercussions of densification into larger particle sizes on the

mechanical and other engineering properties of microsilica concrete. Because this type

of concrete is produced and utilized around the world in increasing amounts (Helland, et

al., 1988), and because these two questions have not been answered conclusively yet in

3

the published literature, research reported herein assumes considerable significance, with

direct financial and engineering consequences. Whether densification increases the

amount retained on the No. 325 sieve to more than 10%, and whether such an increase is

detrimental to the engineering properties of microsilica concrete are issues that need to be

investigated using a well-designed factorial of experiments.

1.3 Technical Background

Microsilica is obtained as a by-product in the manufacture of silicon and

ferrosilicon, from a procedure that involves the reduction of high purity quartz with coal

at a temperature of 3300°F in an electric arc furnace. Consisting in excess of 85% of

amorphous non-crystalline silica (SiO2), microsilica is collected as the tiny particulate

matter present in the emissions from this combustion process, a material that would

otherwise have to be landfilled. Individual microsilica particles are spherical in shape

and measure about 0.1 µm in diameter, i.e., they are about 100 to 150 times smaller than

Portland cement particles. The bulk density of microsilica is in the range of 10 to 15 pcf

(150 to 250 kg/m3), and its specific surface area is on the order of 10,000 to 13,000 yd2/lb

(20,000 to 23,000 m2/kg) (Malhotra, et al., 1987).

Microsilica added to fresh concrete reacts with the calcium hydroxide produced

during the hydration of Portland cement to produce increased amounts of calcium silicate

hydrate. This results in a much stronger bond between the cement paste and the coarse

aggregate, thereby leading to increased compressive strength. Moreover, the additional

calcium silicate hydrates produced are much more resistant to chemical attack than the

4

weaker calcium hydroxide. Another beneficial mechanism operative when microsilica is

used derives from the fineness of its particles and is referred to as the micro-filler effect.

Filling of voids in the matrix leads to a much denser pore structure, and “reduces the

number and size of capillaries that would enable contaminants to infiltrate the concrete”

(www.norchem.com/appl-works.html; accessed: 07/21/05).

The combined action of microsilica as a pozzolan and as a filler, results in

concrete that can be of very high strength and durability. No more than thirty years ago,

6,000 psi concrete was considered to be high strength; using microsilica, compressive

strengths of up to 20,000 psi are reported in the literature. Similarly, the modulus of

elasticity and the flexural strength at 28 days are also higher than in ordinary Portland

cement concrete (Helland, et al., 1988). Improvements in durability and in scaling

resistance result from greatly reduced fluid permeability and ionic diffusivity and the

concomitant increased resistance to penetration by chloride ions, most notably present in

deicing or marine salts. Microsilica concrete can also exhibit very good freeze-thaw

durability provided the air entrained is controlled. Reduction in the alkalinity of the pore

solution and in the diffusion of alkali ions and water lead to a decrease in expansion and

in alkali-aggregate reactivity. High early strengths and resistance to abrasion are

additional benefits (Malhotra, et al., 1987).

Microsilica used in concrete is available in three forms: water slurry, dry

uncompacted powder, and dry densified (compacted) powder. The microsilica content of

the slurry form is about 50% by weight, the remainder being water. While used

commercially in this from, slurried microsilica can be difficult to handle in ready-mix

5

plants without special equipment. On the other hand, handling of the uncompacted

powder poses a potential health risk, since it may be breathed in by construction

personnel. Because of handling problems with both uncompacted and slurried

microsilica, compacted or densified microsilica is preferred. Dry compacted microsilica

is believed to have the same performance characteristics as the uncompacted material. At

typical densities of 40 pcf, its handling qualities approach those of Portland cement,

whose density is usually around 94 pcf. The bulk density of uncompacted microsilica is

typically 15 pcf. Compacted microsilica is virtually free of dust and lumps, flows readily

in pneumatic lines or along bucket elevators, and can be stored in ordinary cement silos

or transported in bulk cement tankers (Malhotra, et al., 1987).

1.4 Report Organization

This report is divided into six chapters. The first chapter provides a definition of

the problem explored, identifies the objectives of the study, and outlines the technical

background of the research. Chapter 2 presents a literature review into the use of

microsilica in concrete mixes, and the resulting effects on the physical, mechanical, and

durability properties of concrete. The third chapter offers a detailed description of the

mixing, casting, curing and testing procedures adopted during this project. Results from

the tests conducted are presented in Chapter 4, which is subdivided into three sections,

one for each of the microsilica types used, viz., densified, undensified and microsilica

abused in the laboratory by wetting and drying. The discussion of these results in the

fifth chapter involves a further subdivision, this time depending on the coarse aggregate

6

type employed, i.e., natural or crushed. In addition, the effects of specimen age and size

on the compressive and flexural strengths of microsilica concrete are examined. On the

basis of these tests, a number of recommendations to ODOT are formulated regarding the

future application of microsilica in concrete pavements and bridges. These

recommendations along with summary and conclusions are presented in Chapter 6.

A companion report, detailing the results of rapid chloride permeability testing on

specimens prepared during this project, has also been prepared under separate cover.

7

2 LITERATURE REVIEW

2.1 Introduction

This chapter summarizes the literature on the use of microsilica as a concrete

admixture, and describes its effect on the physical, mechanical and durability properties

of the resulting mix. In general, the properties of concrete depend on a number of

variables, making it hard to identify the exact cause of a particular behavior change. The

best available information from previous investigations and case histories is presented in

this chapter.

Microsilica was first introduced on an experimental basis as a concrete additive in

the Scandinavian countries in the 1950s (Helland, et al., 1988). The development of high

range water reducers in Europe and Japan in the early 1970s led to a reduction in the

water/cement ratio of microsilica concrete, while ensuring acceptable workability, and

contributed to the more widespread use of the product. Microsilica concrete highway

applications in the United States did not begin until the mid-1980s, with trial placements

of full-depth decks and overlays in the state of Ohio (Whiting and Detwiler, 1998).

By the early 1990s, microsilica was being used by nearly 30 state agencies to

varying degrees. Today, states like Ohio and New York place microsilica concrete

overlays every year, while others explore the use of microsilica on an experimental basis.

Microsilica contents have ranged from 5 to 12%, and typical water-to-cementitious

material ratios have been between 0.3 and 0.4. There is no consensus yet on the optimum

values of these percentages (Whiting and Detwiler, 1998).

8

2.2 Production of Microsilica

As already noted in Chapter 1, microsilica is a by-product of the ferro-silicon

alloy and silicon metal industries. Silicon, ferro-silicon, and other alloys of silicon are

produced in electric arc furnaces, where quartz is reduced by carbon at very high

temperatures. In the process, the silica oxide (SiO) vapors produced, oxidize and

condense to form very tiny spheres of noncrystalline silica (silicon dioxide or SiO2). The

latter is highly pozzolanic, and is recovered by passing the outgoing flue gas through a

baghouse filter (Malhotra, et al., 1987). It seems that microsilica containing more than

78% SiO2 in amorphous form can be used in cement and concrete. Malhotra and Mehta

(1996) maintain that “the current world production of microsilica appears to be about one

million tons per year” with Norway and the United States numbering among the major

producers. Nonetheless, they also note that “in spite of several technical advantages, only

a small percentage of the current supply of microsilica is being used as a mineral

admixture in the cement and concrete industries,” attributing this to the high cost of the

material, as well as handling difficulties.

Microsilica particles are extremely fine and have low bulk density, which makes

handling and transportation of the material difficult. Therefore, microsilica is generally

transported and used in the form of slurry or pellets, or as a densified powder

(www.silicafume.org/general-concrete.html; accessed: 07/22/05). Undensified

microsilica, harvested directly from the baghouse filters, is not commonly used. This

form would be very useful as an admixture in concrete, but it is very difficult to handle

because of its very low loose bulk density. Sometimes, undensified microsilica is

9

blended with cement to reduce handling and transportation costs. In order to make

microsilica easier to handle and transport, it is usually densified. Densification doubles

the loose bulk density of microsilica, and reduces the amount of dust created while

handling it. Densified microsilica is less costly than the undensified form, and is,

therefore, more economically attractive. The slurry form of microsilica contains 50%

water. Slurried microsilica has a number of advantages over undensified microsilica.

These advantages include lower transportation costs and the ability to be dispensed more

accurately. Its main disadvantage is that slurried microsilica must be protected from

freezing and from evaporation.

2.3 Properties of Microsilica

Microsilica particles are smooth, spherical, and have on average a particle

diameter between of 0.1 and 0.2 µm. This diameter is about a hundred times smaller than

that of Portland cement particles. The specific surface area of microsilica is measured by

nitrogen adsorption techniques. Most types of microsilica have an average specific

surface area of 1.07 × 105 ft2/lb (22 m2/g), although this parameter can vary between 6.34

× 104 (13) and 1.37 × 105 ft2/lb (28 m2/g). Undensified microsilica has an average bulk

density of 16 pcf (256 kg/m3), although this can vary between 5 (80) and 27 pcf (432

kg/m3). Densified forms of microsilica have bulk densities between 30 and 45 pcf. The

typical range for microsilica specific gravity is 2.2 to 2.3 (Luther and Smith, 1991).

When ordinary Portland cement hydrates, it produces calcium silicate hydrate, the

chemical glue that makes the cement hard, as well as lime (calcium hydroxide), which is

10

very weak and dissolves easily. When microsilica is added, the silica combines with the

free lime to create more calcium silicate hydrate. The primary active ingredient in

microsilica is silicon dioxide. Most types of microsilica have SiO2 contents above 85%.

Loss on Ignition (LOI) is used to infer the carbon content; most types of microsilica have

LOI values below 3% (Luther and Smith, 1991). Other chemical compounds like Al2O3,

SO3, Fe2O3, MnO, TiO2, CaO, K2O, P2O2, MgO, Na2O make up the other percentages in

microsilica.

Wolsiefer, et al. (1995) tested mixtures of concrete containing sixteen different

samples of microsilica. These samples differed with regard to the microsilica form

employed, as well as its silicon dioxide content. Their tests involved undensified,

densified, slurry, and a pelletized form of microsilica. The investigators found that using

different forms of microsilica had no significant effect on mechanical properties of

concrete, such as compressive and tensile strength. They also concluded that the shearing

forces applied during mixing were sufficient to break up agglomerations of microsilica

particles, even in pelletized form.

2.4 Effect of Microsilica on Physical Properties of Concrete

Due to its pozzolanic character, microsilica influences many of the physical

properties of concrete, including water demand, cohesiveness, bleeding, plastic

shrinkage, etc., as discussed below.

11

2.4.1 Water Demand

Microsilica particles tend to fill the void space between cement particles. This

void space is typically filled with so-called free water. Particle packing allows free water

to become available for hydration, thereby decreasing water demand in a microsilica

concrete mix. On the other hand, since microsilica particles have a relatively high

surface area, more water is adsorbed by the particles, and this tends to increase water

demand. Usually, such increases more than offset the water demand decreases stemming

from improved particle size distribution, so that the net effect is an increase in water

demand. Consequently, high range water reducers and superplasticizers are generally

added in microsilica concrete mixes to compensate the water demanding characteristic of

microsilica and maintain the required workability (Malhotra, et al., 1987).

The net effect of microsilica on the physical properties of concrete depends on a

number of factors, including the water-to-cementitious material ratio of the concrete,

microsilica content, and the presence of water reducers or superplasticizers. Typically,

when its concentrations are kept small, the overall effect of microsilica on water demand

is negligible. When the microsilica content is increased, however, the water demand will

also increase. Consequently, unless a superplasticizer is used, water must be added to

maintain workability (Malhotra, et al., 1987). Jacobsen and Sellevold (1997) tested the

frost resistance of microsilica concrete, and noted that one L/m3 of water should be used

for every kg/m3 of microsilica added in order to maintain a consistent level of

workability.

12

2.4.2 Cohesiveness

When microsilica is added to concrete of high cement content or low water-to-

cementitious material ratio, the concrete appears to be more cohesive (i.e., sticky and

gluey), making it more difficult to place and to consolidate. High cohesiveness often

results in a reduction in surface bleeding, i.e., in the development of a layer of water at

the top surface of freshly placed concrete,. Bleeding occurs due to excess water in the

concrete. Addition of microsilica reduces bleeding in concrete because of the high

affinity of condensed microsilica to water, thereby resulting in less water remaining

available for bleeding. Bleeding has mostly undesirable effects on concrete properties,

e.g., low strength, yet it helps protect against plastic shrinkage cracking, i.e., cracking of

the freshly placed concrete due to insufficient surface moisture. Consequently, it is

important for contractors to cure the concrete properly (Malhotra, et al., 1987).

Moreover, the addition of microsilica increases the concrete’s viscosity (i.e., its

resistance to flow). Consequently, a concrete containing microsilica is less prone to

segregation (i.e., the separation of its ingredients) than a mix without it. This effect

ensures a more uniform mixture and, therefore, results in higher strength, especially in

the presence of a superplasticizer (Malhotra, et al., 1987).

2.4.3 Slump and Workability

The addition of microsilica as a concrete admixture has been shown to decrease

both the slump and the workability of fresh concrete. In order to maintain a minimum

level of workability, a contractor must either add more water or use a water reducing

agent. In fact, it is generally considered that a somewhat higher slump must be achieved

13

with microsilica concrete in order to retain the same workability as ordinary Portland

cement concrete. Yet, at a low water/cement ratio, and if proper amounts of

superplasticizer are used, the workability of microsilica concrete may in fact be higher,

on account of the small particles displacing some of the water present in the matrix

(Malhotra, et al., 1987).

2.4.4 Air Content

It is generally possible to ensure the same amount of air in a mix containing

microsilica as in ordinary Portland cement concrete, even though the amount of an air

entraining admixture may need to be increased slightly. If a constant dosage of air

entrainer is used, the addition of microsilica can reduce the air volume in the mix. To

retain constant air content, the demand for an air entraining agent is higher in concrete

with microsilica than without it. This higher demand is caused by the high surface area

of the microsilica, and possibly by its carbon content (Malhotra, et al., 1987).

2.4.5 Setting Time

There is currently no consensus regarding how setting time of concrete is affected

by microsilica. Pinto and Hover (1997) conducted an extensive experiment to answer

this question. Their study consisted of two series of tests. In the first, the curing

temperature was held constant and nine separate high-strength mortar mixtures were

tested. In the second, the temperature was varied and six mixtures were examined. The

water-to-cementitious material ratio varied from 0.27 to 0.33. Three levels of microsilica

content were used: 0, 5, and 10%. Two levels of superplasticizer were also employed:

14

0.8 and 1.6%. The experiments showed that the presence of microsilica accelerated the

setting behavior.

On the other hand, during their experiments on the freeze-thaw durability of

concrete, Soeda, et al. (1999) noticed that the inclusion of microsilica increases setting

time by alleviating excessive viscosity stemming from a low water-binder ratio.

Investigations by Malhotra and Mehta (1996) have shown that ordinary concrete

mixtures incorporating small amounts of microsilica, i.e., up to 10% by weight of cement,

exhibit no significant difference in setting time compared to conventional mixes. Since

microsilica is typically used in combination with water reducers and superplasticizers, the

effects of microsilica on setting time of concrete tend to be masked.

For their part, Khayat and Aϊtcin (1992) reported that concretes with microsilica

take longer to set and achieve a given strength level than mixes without it. Moreover, the

addition of microsilica to concrete without the use of a water reducer or superplasticizer

delays setting time, especially when the microsilica content is high. Plasticizers are also

known to increase the setting time of concrete, so the combination of microsilica and

superplasticizers magnify the retarding effects of microsilica and make it difficult to

determine how much of the increased setting time is due to each of the microsilica and

the superplasticizer.

2.4.6 Shrinkage and Cracking

As concrete containing microsilica shows little to no surface bleeding, the risk of

plastic shrinkage is high. This can be a very serious problem under curing conditions of

elevated temperature, low humidity, and high wind, all of which contribute to rapid

15

evaporation of water from freshly placed concrete. Crack formation can begin soon after

casting, and can continue until the concrete starts to set (Malhotra, et al., 1987). To

overcome plastic shrinkage, the surface of concrete should be protected from evaporation

by covering it with plastic sheets or wet burlap, or by adding curing compounds and

evaporation retarders (Mehta and Monteiro, 1993).

Drying shrinkage depends heavily on the amount of microsilica used, the water-

to-cementitious material ratio, and the length, as well as method of curing. Generally, the

higher the amount of microsilica, the more prone a concrete becomes to drying shrinkage

and cracking. Microsilica concretes with a low water-to-cementitious material ratio

exhibit similar levels of shrinkage cracking as ordinary mixes. Microsilica concretes

moist cured for fewer than seven days exhibit higher shrinkage cracking levels than

mixes cured for at least seven days (Malhotra, et al., 1987).

Whiting and Detwiler (1998) conducted experiments to assess the effects of

microsilica on drying shrinkage and on cracking of bridge decks. Two classes of

concrete mixtures were included in their study: full-depth mixtures, containing 620 lb/yd3

of cementitious material with a maximum aggregate size of ¾ in.; and overlay mixtures

containing 700 lb/yd3 of cementitious material with a maximum aggregate size of 3/8 in.

For each mix design, separate test mixtures were prepared over a range of water-to-

cementitious material ratios and microsilica contents. The microsilica content ranged

from 0 to 12% for both mixture classes, whereas the water-to-cementitous material ratio

ranged from 0.35 to 0.45, and from 0.30 to 0.40 for full-depth and overlay mixtures,

16

respectively. Specimens from full-depth mixtures were moist cured for seven days

before testing, while overlay mixtures were moist cured for only three days.

Drying shrinkage was measured on beam specimens after 4, 7, 14, and 28 days,

and after 8, 16, 44, and 64 weeks. Cracking tendency was measured according to a

restrained-ring method developed under National Cooperative Highway Research

Program (NCHRP) Project 12-37 (Krauss and Rogalla, 1996). In the first phase of the

investigations, tests were carried out on both full-depth and overlay mixtures. In the

second, more comprehensive series of tests were performed on the full-depth mixtures

only, during which the more significant variables were examined using more replicates.

Chloride ion penetration was measured by exposing the concrete to a chloride solution

for 180 days, and measuring the chloride content close to the surface. This information

was used to develop chloride diffusivity coefficients for each mixture. Compressive

strength was measured on moist-cured cylinders after 7, 28, 56, and 90 days. The

researchers stressed the importance of proper moist curing techniques on drying

shrinkage. At early ages, inadequate curing, a high water-to-cementitious material ratio,

and/or a high cement factor can increase shrinkage cracking by up to 40% in microsilica

concretes used for bridge deck placement. Drying shrinkage increases with decreasing

water-to-cementitious material ratio, but this effect is more pronounced as microsilica

content increases. Specimens cured for only one day and containing between 6 and 9%

microsilica showed a high tendency for cracking. In specimens water-cured for seven

days, no significant correlation between microsilica content and cracking was observed.

17

2.4.7 Heat of Hydration

The addition of microsilica can cause a rise in the mix temperature in the first

couple of days, but will also lead to a significant overall decrease in temperature at later

ages. The initial temperature rise can be attributed to the acceleration of the cement

hydration reaction (Helland, et al., 1988).

2.5 Effect of Microsilica on Mechanical Properties of Concrete

2.5.1 Compressive Strength

Addition of microsilica increases the compressive strength of concrete. The main

reasons for the increase are the strong cement matrix formed due to the micro-filler effect

and the pozzolanic characteristics of microsilica. Various factors contribute to this

strength increase, including percentage of microsilica added to the mix, water-to-

cementitious material ratio, cementitious material content, dosage of superplasticizer,

temperature, humidity, and length as well as method of curing (Malhotra and Mehta,

1996).

Usually, the water demand of microsilica is very high. If slump is to be

maintained constant without using a superplasticizer, the water demand is directly

proportional to the percentage of microsilica replacement of cement. In this instance, the

increase in strength development due to the microsilica is offset by the loss of strength

that comes from a high water-to-cementitious material ratio (Malhotra, et al., 1987).

Malhotra and Mehta (1996) note that “in general, the use of superplasticizers is a

prerequisite to achieve the proper contribution of microsilica to concrete compressive

18

strength. In fact, many important applications of microsilica in concrete depend strictly

upon its utilization in conjunction with super plasticizing admixtures.” The effect of

microsilica on compressive strength is small when compared to that of a lower water-to-

cementitious material ratio. This effect on compressive strength is most pronounced

when 0 to 6% of cement is replaced by microsilica, but beyond 6% the strength increases

are minimal (Malhotra and Mehta, 1996).

The extent of compressive strength gain due to the microsilica addition depends

on the age of the concrete, the cement content, and the microsilica content. The age at

which microsilica starts to contribute to strength gain depends on the cement content and

the water-to-cementitious material ratio of the concrete. Generally, in concretes with

high water-to-cementitious material ratios, the microsilica takes a longer time to

contribute to compressive strength (Helland, et al., 1988). Prussack, et al. (2001)

observed that most of the contribution of microsilica to compressive strength

development occurs in the first 7 days under steam curing and in the first 28 days for

moist-cured concrete.

2.5.2 Flexural Strength

In general, the development of flexural strength in microsilica concrete is similar

to that in ordinary Portland cement concrete, but the contribution of drying is more

significant in the former. Moist cured concrete specimens show higher flexural strengths

than air-dried specimens. With or without microsilica, concretes cured with water will

have similar flexural-to-compressive strength ratios. When air cured, concrete containing

19

microsilica will generally have a lower flexural-to-compressive strength ratio (Malhotra,

et al., 1987).

2.5.3 Modulus of Elasticity

In general, the increase in compressive strength of concrete is accompanied by a

small increase in the Young’s modulus of elasticity. The addition of microsilica reduces

the porosity of the transition zone between the aggregate and the cement paste, allowing

the stiffness of the aggregate to contribute more to the overall stiffness of the concrete

(Helland, et al., 1988).

2.5.4 Bond Strength

Internal bleeding in concrete can cause free water to accumulate around coarse

aggregate and steel reinforcement. A high water content in a concrete mix can also

reduce the adhesion between the cement and the aggregates, or between the cement and

the reinforcing steel. As noted earlier, microsilica reduces the amount of bleeding,

thereby strengthening the bond between concrete and reinforcing steel. This effect is

attributable to a less porous transition zone between the cement and the aggregate. As a

result, pullout strength increases as the amount of microsilica increases (Malhotra and

Mehta, 1996).

Fitch and Abdulshafi (1998) tested a total of 62 bridge deck overlay specimens

for bond strength. After curing for 28 days, cylinders made with Portland cement

concrete were cut in half and one circular face was sandblasted to simulate the surface of

a bridge deck. The specimens were then placed at the bottom of a cylinder mold and

material from various batches made with microsilica concrete was placed over them. Six

20

different batches of microsilica concrete were prepared: three had natural gravel, whereas

the other three contained crushed limestone. Three different moisture conditions, viz.,

dry, saturated surface dry, and wet, were used for the Portland cement concrete base. The

specimens were then subjected to tensile loading to see whether failure occurred at the

overlay interface or within the Portland cement or the microsilica concrete matrix itself.

All of the specimens with a dry surface condition showed higher overlay bond

than concrete matrix strength. A majority of the specimens in the saturated surface dry

condition exhibited higher bond than concrete matrix strength. Specimens with a wet

surface condition generally had weaker bond than concrete matrix strength. These

findings show that microsilica concrete overlays achieve their highest bond strengths

when the Portland cement concrete base is dry.

2.5.5 Strength Development

Malhotra and Mehta (1996) state that “the main contribution of microsilica to

concrete strength development at normal room [curing] temperatures takes place between

the ages of about 3 and 28 days.” The overall strength development patterns in

microsilica concretes can vary according to concrete proportions and composition, and

are also affected by the curing conditions. High curing temperatures have a greater

strength accelerating effect on microsilica concretes than on comparable Portland cement

concrete mixes (Malhotra and Mehta, 1996).

As noted in previous paragraphs, curing conditions have a significant effect on a

number of properties of both fresh and hardened concrete. The inclusion of microsilica

as an admixture seems to magnify this effect, especially as far as strength development is

21

concerned. Microsilica concretes that are continuously moist cured show superior

mechanical properties than air cured ones (Malhotra, et al., 1987). In fact, Carette and

Malhotra (1992) found that air cured concrete containing microsilica may even show a

slight decrease in compressive strength after 100 days or more. Typically, seven days of

moist curing is sufficient to prevent this.

Sasatani, et al. (1995) performed tests for a period of five years, in which they

exposed concrete containing various admixtures to different weather conditions.

Concrete mixtures containing fly ash, blast-furnace slag, and microsilica were prepared.

Water-to-cementitious material ratios of 0.45, 0.55, and 0.65 were used for the control

concretes containing only ordinary Portland cement; for concretes containing admixtures

this ratio was fixed at 0.55. Replacement levels of ordinary Portland cement by fly ash,

blast-furnace slag, and microsilica were 30, 50 and 10%, respectively. Specimens were

cured in two different ways. In the first, specimens were cured in 20°C-water for 28

days, whereas in the second they were cured in 20°C-water for 7 days and then kept in a

dry environment for another 21 days. After curing, the specimens were exposed to four

separate environmental conditions: indoors, in 20°C-water; indoors, at 20°C and 60%

relative humidity; on the roof of a building at Kanazawa University located 15 km from

the sea; and at Matsuto Beach, facing the Sea of Japan. Chloride permeability and pore

size distribution were measured after 28 days of environmental exposure. Following

exposure times of 1, 3, and 5 years, compressive strength, pulse velocity, depth of

carbonation, and chloride ion penetration were also measured.

22

Air-dried 10% microsilica concrete showed a decrease in compressive strength

with exposure time over the first twelve months since casting. This may be ascribed to

internal stress built up due to uneven drying between the surface and concrete matrix.

Uneven drying probably results from the dense and discontinuous pore structure of

microsilica concrete. The effects of curing conditions were not as pronounced for

concrete exposed to outdoor conditions. This is because the concrete can absorb the

water needed for hydration from the environment.

2.6 Field Tests

Fitch and Abdulshafi (1998) conducted brief visual inspections on 145 microsilica

overlaid concrete decks. Of these, 27 decks showed noticeable cracks in the overlay

surface. The undersides of 84 of the decks were also visually inspected. Of these, 29

showed several transverse full-depth cracks, and evinced flow of water through the deck

resulting in deep chloride penetration. In-depth condition surveys were also performed

on 28 microsilica overlaid concrete decks. These surveys included manual sounding to

detect horizontal cracking, and core samples to be tested in the laboratory.

Approximately 39% of the concrete decks inspected exhibited cracking over more than

10% of their surface. Seven of the 28 decks showed maximum crack widths exceeding

0.20 mm.

Aϊtcin (1990) conducted three field experiments testing various properties of

concrete containing microsilica. In the first test, 50 m3 of microsilica concrete were used

for part of the three-lane Highway 25 in Montreal, Quebec, Canada. A reference mix of

23

concrete not containing microsilica was used nearby. The second field experiment

consisted of two sidewalks built in Sherbrooke, Quebec. Each sidewalk had sections of

microsilica, as well as of ordinary concrete. These sidewalks were chosen because of the

high amount of deicing salts they were routinely exposed to. In the third field

experiment, two experimental columns were cast during the construction of La

Laurentienne, a 26-story building in Montreal. Both columns contained microsilica, but

only one column was part of the building. The other was used as a mock column to study

the creep of the concrete.

Compressive strength and chloride ion permeability tests were performed on both

concrete cast during the experiment, and on cores drilled from the field for the first two

experiments. In the third, cores were drilled from the mock column two and four years

after casting. In the Highway 25 experiment, all the specimens had approximately the

same compressive strength after one year, but the reference concrete experienced a

greater strength increase between 28 days and one year. The chloride ion permeability

was much lower in the microsilica concrete than in the reference concrete. In the second

experiment, microsilica specimens cured in the laboratory had higher compressive

strengths than those from ordinary concrete, but in the field the situation was reversed.

After two years, no significant differences in compressive strength between the two sets

of specimens were found. Again, the microsilica concrete had much lower chloride

permeability. In the third experiment, the 28-day compressive strengths of the mock

column and the lab specimens were nearly identical, while the long-term strength gain in

the lab specimens was greater than that in the mock column. The chloride ion

24

permeability was extremely low. The combined results showed that microsilica concrete

exposed to outdoor conditions performed just as well as ordinary concrete. Yet, it

appeared that microsilica concrete suffered more from poor curing conditions. The

chloride-ion permeability was extremely low in microsilica concrete even after four to six

years.

2.7 Do Undispersed Agglomerates Matter?

The conventional wisdom on the repercussions of unbroken clusters in densified

microsilica is represented by the study conducted by Wolsiefer, et al. (1995), who tested

mixtures of concrete containing sixteen different samples of microsilica. These samples

differed with regard to the microsilica form employed as well as silicon dioxide content.

Thus, tests involved undensified, densified, slurry, and pelletized form of microsilica.

The investigators found that using different forms of microsilica had no significant effect

on the mechanical properties of concrete, such as compressive and tensile strengths, etc.

They also concluded that the shearing forces applied during mixing are sufficient to break

up agglomerations of microsilica particles, even in pelletized form.

On the other hand, in a “deliberately provocative” paper, Diamond and Sahu

(2003) set out to redress “the perceived failure of many respected sources of information

in the industry to properly convey a clear picture of the nature and particulate

characteristics of densified [microsilica].” They note with evident frustration, for

example, that “nowhere in the current [American Concrete Institute] ACI ‘Guide for the

Use of Silica Fume in Concrete’ is there mention of the fact that the actual size of the

25

densified silica fume, as supplied to the customer, is always in the range of hundreds of

µm...until and unless the fume is physically broken up by some process.” Although the

tone of this paper is unlike most in the professional literature, being rather aggressive and

almost personal, the issues it raises are worth considering seriously. Diamond and Sahu

(2003) insist that “it is extremely unlikely that complete dispersion ever takes place in

concrete mixing” analogous to that achieved using an “extremely powerful ultrasonic

system.” They cite a number of transmission and scanning electron microscopy (TEM

and SEM) studies, that suggest that clusters were noticed not only in densified microsilica

but also in undensified microsilica. This would be an indication that that these clusters

are not formed during the process of densification of commercial densified microsilica,

but would pose the same level of difficulty to break in the undensified material, as well.

The study by Diamond and Sahu (2003) is motivated by “two separate concerns

stemming from the failure of agglomerates of densified silica fume to be completely

dispersed in concrete mixing.” The first is that “to the degree that [undispersed]

agglomerates remain in concrete the expected fine particle packing benefit is lost,” since

such agglomerates are “often much coarser than cement.” The second concern is

potentially more important: “agglomerates can clearly act as extremely aggressive alkali-

silica reactive (ASR) aggregates,” despite the fact that microsilica “is ordinarily

considered as mitigating the effects of possible ASR when reactive sand or coarse

aggregate is components are present.”

The implications of the assertions by Diamond and Sahu (2003) are far reaching,

but only a few of their facets can be explored within the scope of this study. For

26

example, it is not feasible to conduct TEM or SEM measurements, but more conventional

means of establishing the gradation curves for the materials used herein can be employed.

The repercussions of agglomerates on fine particle packing can be examined using unit

weight determinations of the concrete prepared, as well as strength comparisons.

Regrettably, the potential for ASR deterioration cannot be reliably assessed in a study as

brief as the present one.

27

3 MATERIALS AND PROCEDURES

3.1 Introduction

The materials employed in this project are first enumerated in this chapter; they

include primarily the microsilica and the coarse aggregate types. Tests conducted on

these materials are outlined next, to illustrate conformity with the prescriptions by the

American Society for Testing and Materials (ASTM), by the Ohio Department of

Transportation (ODOT), or by the American Association of State Highway and

Transportation Officials (AASHTO). The derivation of the mix design for each of the

specimen lots cast is then described, emphasizing again adherence to the pertinent

governing specifications. The most arduous and time-consuming aspect of the project

was mixing, casting and curing the test specimens, the procedures for which are

discussed next. Finally, the tests conducted on the plastic and cured concrete samples are

described.

3.2 Materials Used

Sand, coarse aggregate, Type I-II Portland cement, water (www.cincinnati-

oh.gov/water; accessed: 07/22/05) from greater Cincinnati water works, microsilica,

superplasticizer, and air entrainer are the materials used in this project. The sand and

coarse aggregate were supplied free of charge by Martin Marietta Materials, a leading

supplier in the Cincinnati area. The sand was natural and came from their sand and

28

gravel facility in Ross, OH. Coarse aggregate was of two kinds, both in the No. 8

gradation with a nominal maximum aggregate size of 3/8 in.: natural river gravel, or

crushed limestone. The gravel was obtained from their gravel facility in Fairfield, OH,

whereas the stone was produced at their Phillipsburg quarry in Brookville, OH (Jim R.

Martin: personal communication, 10/14/02; www.martinmarietta.com; accessed:

12/02/02). The natural aggregate had a rounded shape and a smooth surface, whereas the

crushed aggregate had a more angular shape and a rougher surface. At given

water/cement ratio, concrete made with crushed aggregate is usually expected to have

higher compressive strength than concrete made with natural aggregate, as crushed

aggregate creates a stronger bond with cement mortar due to its rough texture (Mehta and

Monteiro, 1993).

The Portland cement Type I-II was donated by CEMEX from their operation in

Fairborn, OH (Steve Reibold: personal communication, 09/11/02; www.cemexusa.com;

accessed: 08/14/02), and the microsilica by ELKEM Materials from their location in

Alloy, WV (Tony N. Kojundic: personal communication, 08/07/02; www.materials.

elkem.com; accessed: 07/24/02). The other admixtures were supplied at no cost by

Master Builders, Inc. (Greg Wirthlin: personal communication, 08/07/02; www.

masterbuilders.com; accessed: 07/24/02). These were: MB-AE 90 air entrainer, meeting

the requirements of ASTM C 260, AASHTO M 154 and CRD-C 13, and recommended

for obtaining “adequate freeze-thaw durability in a properly proportioned concrete

mixture, if standard industry practices are followed;” and Rheobuild 1000 “high range,

water reducing admixture, formulated to produce rheoplastic concrete that flows easily,

29

maintaining high plasticity for time periods longer than conventional superplasticized

concrete,” and that “meets ASTM C 494 requirements for Type A, water reducing, and

Type F, high range water reducing, admixtures.” The research laboratory facilities on the

University of Cincinnati campus were used, except as noted.

3.3 Tests on Microsilica

Testing was performed on four types of microsilica, viz., undensified, densified,

and two types of abused microsilica. Undensified and densified forms are readily

available on the market, and were procured commercially. Abused microsilica was

prepared by the research team so as to represent a worst case scenario for the handling

and storage of commercially available material, particularly with regard to its exposure to

ambient moisture.

3.3.1 Preparation of Abused Microsilica

Two different abused microsilica samples were prepared. For the first sample of

abused microsilica (A), 9 kg of densified microsilica was weighed and soaked in a water

tub. The microsilica was stirred rigorously at the beginning, and the water level was

maintained to at least 3 in. above the surface of the settling solids. After a period of three

days, the microsilica had settled and the water above it was clean enough to be removed

using small plastic buckets. The wet sample was spread evenly onto plastic lined trays,

placed on a flat horizontal surface, and was left to dry at room temperature for nearly

three weeks. The second sample of abused microsilica (B) resulted from exposing

30

densified microsilica to high humidity in the curing room for two weeks before allowing

it to air-dry. In both cases, any clumps forming during drying were broken from time to

time using a small trowel. When completely dry, the microsilica was collected into

plastic bags, which were sealed and stored in a cool and dry place, until the time of

testing.

3.3.2 Fineness Tests

Samples of all types of microsilica used in this project were transported to the

ODOT laboratory in Columbus, OH on 05/14/03, for fineness testing by the agency’s

personnel, Mr. Tim Jones, who is responsible for checking conformance with ASTM C

430 – 96 Standard Test Method for Fineness of Hydraulic Cement by the 45-µm (No.

325) Sieve. The following equipment was used: small dessicator bowl with pouring lip,

2 in. dia., 3/4 in. tall; twelve No. 325 sieves, 1 ½ in. dia., 2 ¼ in. tall, each with 3 ½ in.

legs, each with its own calibration factor from the Cement and Concrete Reference

Laboratory (CCRL); electronic scale (Mettler Model PC 180), calibrated to 1/1000 g

precision; small paint brush; hot plate, 250-350°F temperature range; city water faucet;

pressure stabilizer; and temperature compensated pressure gage to 30 psi, 0.1 psi

precision. The ambient temperature in the laboratory was 78°F, and the humidity was

53%.

For each type of microsilica, three 1.000-g samples were weighed in the

dessicator bowl, and then pour into each sieve with the help of the brush and light tapping

of the bowl. First, each sieve was quickly rinsed with de-ionized water from a plastic

spray bottle with a narrow tip. Next, each sieve was subjected to a 10-psi stream of tap

31

water for a period of one minute. A constant stream of tap water was ensured by

connecting both the pressure stabilizer and the pressure gage to the end of the faucet,

which allowed easy control of the water pressure. The sieve was moved in a circular

motion to ensure that the entire sieve was irrigated evenly. The sieve was rotated at

approximately one rotation per second. For one sample from each type of microsilica, a

small paint brush was used in addition to the stream of water. In such cases, the sieve

remained stationary, but the brush was moved in a circular motion. Finally, each sieve

was rinsed again with de-ionized water, and the bottom of the sieve was blotted dry using

a towel. The sieves were then placed on a hot plate set to a temperature between 250 and

350°F. The samples were left here approximately for an hour and a half, or until all of

the moisture had evaporated. Once dry, the samples were allowed to cool off for another

hour, before the microsilica was removed from the sieves and was weighed in the

dessicator bowl again. The weight was recorded as the weight retained, and the sieve

correction factor was applied according to the specification.

3.3.2 Gradation Tests

The gradation of each of the three microsilica types was also determined using

ASTM D 422 – 63 Standard Test Method for Particle-Size Analysis of Soils (Hydrometer

Test), conducted in conjunction with ASTM D 421 – 85 Standard Practice for Dry

Preparation of Soil Samples for Particle-Size Analysis and Determination of Soil

Constants, and ASTM D 854 – 06 Standard Test Methods for Specific Gravity of Soil

Solids by Water Pycnometer. In each case, 15 g of oven dry microsilica material

previously washed through the No. 200 sieve, was mixed with 125 mL of 4% sodium

32

metaphosphate (NaPO3) solution, in a 250 mL beaker. The solution, whose trade name is

Calgon and whose function is that of a deflocculant or dispersant, had just been been

prepared by mixing 40 g of dry chemical with enough water to make 1000 mL. The

beaker was then covered with wet paper towels to minimize evaporation, and the contents

were left to stand for an hour. The mixture was subsequently transferred to a dispersion

cup, which was filled to about 2/3 of its volume with water, and was stirred for about 2

min. The suspension was then transferred into a sedimentation cylinder, more water was

added up to the 1000 mL mark, and the cylinder was capped and agitated for 1 min. Two

minutes after setting the cylinder down, the first reading was taken on the hydrometer,

which had been inserted into the cylinder about 20 s before the measurement.

Subsequent readings were taken at elapsed times of 4, 8, 16, 30, 60, 120, 240, 480, 960,

1920, 3840, and 5760 min. The temperature was also noted each time, and was used to

determine the viscosity of the water in the cylinder. The hydrometer readings were taken

to the top of the meniscus, and were later corrected as follows:

Tactualc CCRR +−= 0 (3.1)

where: Rc = corrected hydrometer reading; Ractual = actual hydrometer reading; C0 = “zero

correction” for impurities in tap water and for the use of a disperasal agent; CT =

temperature correction factor (Bowles, 1992).

The hydrometer percent finer, Fh, was then computed using:

%100×=s

ch M

RaF (3.2)

in which:

33

)1(65.265.1

−×

=s

s

GG

a (3.3)

Ms = mass of oven dry material previously washed through the No. 200 sieve used in the

test = 15 g (from above); and Gs = specific gravity of the solid particles in the sample.

This percentage is adjusted in proportion to the material that had been retained on the No.

200 sieve, F200, using the following expression:

200FFF hadjh ×= (3.4)

For each elapsed time, t, and depending on Rc, the effective sedimentation depth, L, is

read from Table 2 in the specification, whereupon the particle diameter, D, can be

obtained from Stokes’ formula, as follows:

tL

GGD

ws )(98030

−=

η (3.5)

with Gw = specific gravity of water in which the sample was suspended (=1.0); and η =

absolute viscosity of the suspension water, adjusted for temperature per the specification.

Prior to the hydrometer tests, a 200-g of oven dried sample of each microsilica

type was sieved mechanically through a stack of the following sieves: No. 10, 20, 40, 60,

140 and 200, following the procedures detailed by Bowles (1992). Moreover, the

specific gravity of the microsilica solids was determined for each type using the

procedure of ASTM D 854 – 06 Standard Test Methods for Specific Gravity of Soil

Solids by Water Pycnometer. ODOT Supplemental Specification 848 Bridge Deck

Repair and Overlay with Concrete Using Hydro-Demolition assumes that for microsilica

solids, Gs = 2.2.

34

3.4 Tests on Aggregates

3.4.1 Specific Gravity and Absorption of Coarse Aggregate

ASTM C 127 – 01 Standard Test Method for Density, Relative Density (Specific

Gravity), and Absorption of Coarse Aggregate was followed in conducting this test.

Coarse aggregate was taken in sufficient quantity and was immersed in water at room

temperature. After 24 ± 4 hours of soaking, it was removed from the water and was

rolled on a large absorbent towel, until all visible moisture was blotted from the surface

of the aggregate. The aggregate was then considered to be in the saturated surface dry

(SSD) condition. The test sample was weighed in air, before being placed in a container

and immersed in water at 73.4 ± 3°F, where its submerged weight was also determined.

The sample was then placed in an oven at 230 ± 9°F for a period of 24 ± 4 hours, after

which its oven-dried weight was measured. Three types of specific gravities were

determined, viz., bulk specific gravity for the oven-dried sample, bulk specific gravity for

the saturated surface dry sample, and the apparent specific gravity of the sample. In

addition, the absorption value for the aggregate was calculated. The following formulae

were used:

CBABSG−

= Dry),(Oven Gravity SpecificBulk (3.6)

CBBBSGSSD −

= Dry), Surface Saturated (Gravity SpecificBulk (3.7)

CAAASG−

= Gravity, SpecificApparent (3.8)

35

100)(% ,Absorption ×

−

=A

ABAbsorption (3.9)

where: A = weight of oven dry sample in air, g; B = weight of saturated surface dry

sample in air, g; and C = weight of saturated sample in water, g.

3.4.2 Specific Gravity and Absorption of Fine Aggregate

ASTM C 128 – 97 Standard Test Method for Specific Gravity and Absorption of

Fine Aggregate was followed in conducting this test. A 1000-g sample of sand was

immersed in water at room temperature for 24 ± 4 hours. The excess water was drained

and the sand was then spread over a flat nonabsorbent surface exposed to a gently

blowing current of air. The material was stirred frequently to ensure even drying. This

process was continued until the specimen no longer stood on its own when subjected to

the specified cone test, at which point the sand was considered to be in the saturated

surface dry condition. A pycnometer was filled partially with water and 500 g of this

saturated surface dry sample was added to it. The remaining volume of the pycnometer

was filled with water, up to the calibration mark. The mixture was agitated slightly in

order to remove the air bubbles. The total weight of the pycnometer with the sample and

water was then recorded. The pycnometer was emptied and the sample was dried in an

oven at 230 ± 9°F for 24 ± 4 hours. The sample was taken out of the oven and its weight

noted. The pycnometer was then filled only with water up to the calibration mark and its

weight was recorded again. The following calculations were performed to determine the

three specific gravities and the absorption value for the fine aggregate:

CSBABSG−+

= Dry),(Oven Gravity SpecificBulk (3.10)

36

CSBBBSGSSD −+

= Dry), Surface Saturated (Gravity SpecificBulk (3.11)

CABAASG−+

= Gravity, SpecificApparent (3.12)

100)(% ,Absorption ×

−

=A

ASAbsorption (3.13)

where: A = weight of oven dry sample in air, g; B = weight of pycnometer filled with

water, g; S = weight of saturated surface dry sample, g; and C = weight of pycnometer

with water, g.

3.4.3 Bulk Density of Coarse Aggregate

ASTM C 29/C 29M – 97 Standard Test Method for Bulk Density (“Unit Weight”)

and Voids in Aggregate was followed in performing this test. A sufficient amount of

aggregate was oven-dried for 24 ± 4 hours. The bucket measure was calibrated by filling

it with water and determining the weight of water in it. The unit weight of the water was

read off Table 3 (Density of Water) in the specification, as a function of its temperature.

The volume was determined using the formula:

w

wWV

γ= (3.14)

where: Ww = weight of water (lb); and γw = unit weight of water, pcf.

The effort to be expended in placing the aggregate in the bucket measure was

decided based on the size of aggregate. If the nominal maximum aggregate size was 1½

in. or less, a rodding procedure was used. This involved filling the measure in three

layers and rodding each one with a tamping rod 25 times. If the nominal maximum

37

aggregate size was greater than 1½ in., a jigging procedure was used. This involved

filling the measure in three layers, raising one side of the measure approximately 2 in.,

and allowing it to drop to the ground 25 times. After tamping the aggregate, the surface

of the measure was leveled so that no aggregates protruded over the top of the measure.

The mass of the measure plus contents, and mass of the measure alone were determined.

The following formulae were used:

VTGM )( Dry),(Oven Density Bulk −

= (3.15)

+=

1001 Dry), Surface (SaturatedDensity Bulk AMM SSD (3.16)

×−×

×=WS

MWSVoids )(100% Content, Void (3.17)

where: MSSD = bulk density of aggregate at SSD condition, pcf; G = mass of measure

plus aggregate, lb; T = mass of measure, lb; V = volume of measure, ft3; A = %

absorption; S = bulk specific gravity; and W = density of water (=62.4 pcf).

3.4.4 Sieve Analysis of Fine and Coarse Aggregate

This test was conducted in accordance with ASTM C 136 – 96a Standard Test

Method for Sieve Analysis of Fine and Coarse Aggregates. For the fine aggregates,

sieves No. 4, 8, 16, 30, 50, 100, and a pan were weighed and stacked in a column, and

between 300 and 1000 g of each sample were placed on the No. 4 sieve. The sieves were

locked on to a mechanical shaker and the shaker was run for 5 min. The sieves were then

released and they were weighed along with the material, and the readings noted. The

fineness modulus, FM, of the fine aggregate was calculated using the formula:

38

100RetainedsPercentageCumulative∑=FM (3.18)

The procedure for sieve analysis of coarse aggregates was the same as that for

fine, except that five additional sieves were used, viz., 3/8, 3/4, 1/2, 1, and 1½ in. sieves.

The fineness modulus was not calculated for coarse aggregates.

3.4.5 Moisture Content of Fine and Coarse Aggregates

ASTM C 566 – 89 Standard Test Method for Total Moisture Content of

Aggregate by Drying was followed in this case. The weight of a moisture tin was

recorded and an appropriate quantity of aggregate was placed in it. The weight of the

moisture tin plus moist sample was also recorded. The specimen was then kept in an

oven at a temperature of 230 ± 9°F for 24 ± 4 hours. The material was weighed again

after taking it out of the oven. The moisture content, w, was calculated using the

formula:

100×

−

=D

DWw (3.19)

where: W = weight of moist sample, g; and D = weight of oven-dried sample, g.

3.5 Mix Design

Concrete mix designs used in this project were prepared in accordance with

pertinent ODOT specifications, viz., ODOT Item 499.03 Concrete-General:

Proportioning of the ODOT 2002 Construction and Material Specifications; and ODOT

848.

39

3.5.1 Constants and Variables

Cement content of 700 lb/yd3, water/cement ratio of 0.36 and microsilica content

of 8% replacement of cement were maintained constant for all mixes. The amount of

water was variable, however, since it depends on the physical properties of the aggregate

used in any mix. The amount of MB-AE 90 air entrainer for each mix was set at 1.5

oz/100 lb (44.36 mL/100 kg) of cementitious materials, per manufacturer

recommendations (1/4 to 4 fl. oz/cwt, or 16 to 260 mL/100 kg), in order to maintain an

air content of 8 ± 2%, as specified by the ODOT specification cited. The Rheobuild 1000

plasticizer was taken as 650 mL/100 kg of cementitious materials for all mixes,

conforming to the manufacturer’s recommendations of 10 to 25 fl. oz/cwt, or 650 to 1000

mL/100 kg . Proper workability did not require the use of another water reducer.

Different properties of aggregate influence the water/cement ratio to a great

extent. Therefore, in order to maintain a constant water/cement ratio for all mixes, the

weights of the coarse aggregate (CA) and fine aggregate (FA) have to be adjusted for

each mix, based on their respective properties so as to maintain the specified proportion

of coarse to fine aggregate, CA:FA.

3.5.2 Ingredients

All determinations below are for the saturated surface dry condition, as indicated

by the subscript SSD in the equations presented. ODOT 499 assumes SSD bulk specific

gravity values of 2.62 for natural sand and gravel, and 2.65 for crushed limestone; the dry

values of 2.20 for microsilica and 3.15 are also used, per ODOT 848 and 499.03. The

mix design per cubic yard of concrete was adjusted after determining the actual BSGSSD

40

values for the aggregates. The saturated surface dry (SSD) volumes for CA and FA were

retained per the specification, and new SSD weights for CA and FA were backcalculated

using the lab BSGSSD values. The mix design was further modified in order to achieve

the desired percentage of cement-plus-pozzolan and air percentages % (c + p) and % air,

respectively. The weight of microsilica for determinations at SSD condition was first

calculated using the formula:

100)(%1

100)(%

)(

)( pc

pcWW

SSDCEM

SSDMS +−

+×

= (3.20)

where: WMS(SSD) = weight of microsilica for determinations at SSD condition; and

WCEM(SSD) = weight of cement for determinations at SSD condition = 700 lb/yd3, in

accordance with ODOT 848. As noted, both these weights are dry weights.

The weight of water and the volume of air were then calculated using the

formulae:

)()/()( MSCEMSSDW WWcwW +×= (3.21)

100%27 3 airftVAir

×= (3.22)

where: Ww(SSD) = weight of water for determinations at SSD condition; (w/c) =

water/cement ratio = 0.36, per the specification; and Vair = volume of air.

The volumes of cement, microsilica, and water were then calculated using for

each the formula:

41

wSSD

SSDSSD BSG

WV

γ×=

)( (3.23)

where: VSSD = volume at SSD condition; WSSD = weight at SSD condition; BSGSSD = bulk

specific gravity at SSD condition; and γw = unit weight of water = 62.4 pcf (assumed).

The sum of the volumes of cement, water, microsilica, and air were then

subtracted from a cubic yard (=27 ft3) to obtain the available volume for CA plus FA.

The proportions for CA and FA given by the specification were retained, and used to split

the available volume between CA and FA. Finally, the SSD weights for CA and FA were

backcalculated. The mix design charts are appended at the end of this report.

3.6 Mixing, Casting, and Curing Methods

ASTM C 192/C 192M – 00 Standard Practice for Making and Curing Concrete

Test Specimens in the Laboratory, and AASHTO T 126-86 Standard Method of Test for

Making and Curing Concrete Test Specimens in the Laboratory were adopted for mixing,

casting, and curing the concrete specimens, as described below.

3.6.1 Mixing Concrete

The ingredients used for the mixing process were weighed according to the mix

design. The concrete was mixed in a mechanical mixer of 5 ft3 capacity. The coarse

aggregate and about 20% of the mixing water into which the air entrainer and

superplasticizer had been dissolved were added in the mixer prior to turning it on. After

the mixer was started, the fine aggregate, the cement with the microsilica, and the

remaining water were added. After letting the mixer run for 3 min., it was rested for 3

42

min. It was then restarted, rotated for 2 more min., after which the concrete was poured

into a clean pan placed on a smooth rigid surface.

In order to cast the required number of specimens, nearly 4 ft3 of concrete needed

to be mixed. For densified microsilica concrete, 7 to 8 ft3 were used, as several large

specimens were cast for those mixes. The concrete mixer used by the research team had

a yield of just 2.5 ft3, and consequently the material had to be mixed in 4 batches. To

reduce the amount of concrete required, the investigators subsequently obtained ODOT’s

permission to cast smaller specimens for undensified and abused microsilica. This made

the mixing process more efficient and consistent. Thus, two batches were mixed for

concrete made from undensified and abused microsilica, while three to four batches were

made for mixes including densified microsilica.

3.6.2 Casting Specimens

Four different kinds of specimens were cast from the concrete mixed, viz., small

cylinders (4 × 8 in.), large cylinders (6 × 12 in.), large beams (6 × 6 × 21 in.), and small

beams (3½ × 4½ × 16 in.). The cylinders were tested to determine compressive strength,

and the beams were used for the calculation of flexural strength. The concrete was

placed in two layers in the molds for small cylinders, small beams and large beams, using

a scoop, and consolidated using a standard rod. For large cylinders, the concrete was

placed in three layers, and was rodded in the same way. Care was taken to avoid

segregation and to ensure that the concrete placed in each mold was representative of the

entire batch mixed. After the specimens were cast, they were transferred to rigid

43

horizontal surface, and were cured in air overnight. A plastic cover was placed over the

specimens to prevent undue moisture loss during this initial 24-hour period.

A total of six mixes of concrete were made with three kinds of microsilica

(densified, undensified, and abused) and two types of aggregate (natural and crushed).

The mixes were named Densified Natural (DN), Densified Crushed (DC), Undensified

Natural (UN), Undensified Crushed (UC), Abused Natural (AN), and Abused Crushed

(AC). The number of specimens cast in each mix will be reported in Chapter 4. Large

size specimens were more numerous than small ones in mixes DN and DC, whereas

smaller specimens outnumbered the larger ones in mixes UN, UC, AN, and AC. The

main motivation for switching to smaller specimens was the researchers’ desire to make

the mixing and casting process more efficient. This change was implemented following

ODOT staff consent. It is noted that ASTM C 192 / C 192 M specifies that “the diameter

of a cylindrical specimen or minimum cross-sectional dimension of a rectangular section

shall be at least three times the nominal maximum size of the coarse aggregate in the

concrete.” The maximum aggregate size used in this project is 3/8 in., so small cylinders

(4 × 8 in.) and small beams (3½ × 4½ × 16 in.) are also permissible.

3.6.3 Curing

Upon casting, the specimens were immediately covered using a plastic sheet in

order to avoid evaporation from the fresh concrete. After 24 hours, the specimens were

extruded from the molds. Air pressure was used to demold the cylinders. The specimens

were then cured submerged in water tubs in the University of Cincinnati moist room,

44

until the day and time of testing, 7 to 90 days later. Calcium hydroxide (lime) was added

to saturation in these tubs, so as to avoid leaching.

3.7 Tests on Plastic Concrete

Immediately after mixing each batch of concrete, slump, air content, and unit

weight determinations were conducted. Concrete slump is measured in inches, and is

used as a measure of workability and of consistency from batch to batch. The procedure

followed was as prescribed by ASTM C 143/C 143M – 00 Standard Test Method for

Slump of Hydraulic-Cement Concrete. The mold was first dampened and placed on a flat

surface. The concrete was then placed in three layers, each occupying approximately a

third of the total volume of the mold. The concrete placed in each layer was rodded 25

times with a standard rod. The excess concrete from the top layer was removed using a

rolling action of the rod, and the mold was immediately raised off the specimen. The

slump was measured by determining the vertical distance between the top surface of the

mold and the displaced original center of the top surface of the concrete specimen.

The air content of concrete was measured as stipulated in ASTM C 231 – 97

Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure

Method. The measuring bowl was placed on a level surface after the inner sides had been

dampened. The concrete was then placed in three equal layers, consolidating each layer

by 25 strokes of the standard rod. The extra concrete from the top layer was stricken off,

the rim of the bowl was thoroughly cleaned, and the bowl was sealed using the covering

lid. The air valve located between the air chamber and the measuring bowl was closed.

45

The petcocks were opened and water was injected into one of them, until water came out

from the other, which was then closed. Water was pumped into the other petcock, which

was closed when the water overflowed out of it. Air was pumped into the air chamber,

until the gage hand reached the initial pressure line. After a few seconds, the air valve

was opened, and the percentage of air was read on the dial.

The unit weight of concrete was calculated by dividing the weight into the

corresponding known volume of concrete. The concrete was first weighed in a container

of known volume after properly compacting it in three layers following the ASTM

standard procedure. The weight of the container was also noted after filling it completely

with water. The weight of the water alone was then calculated by subtracting the weight

of container from the weight of the container plus water. Dividing the weight of water by

its unit weight gives the volume of the container. The unit weight of concrete was then

determined by dividing the weight of concrete by the volume of the container.

3.8 Strength Test Procedures on Hardened Concrete

3.8.1 Compressive Strength

The procedure followed was as specified by ASTM C 39/C 39M – 01 Standard

Test Method for Compressive Strength of Cylindrical Concrete Specimens and by

AASHTO T 22-03 Compressive Strength of Cylindrical Concrete Specimens. The

diameter was measured twice at 90° to each other, and the average diameter was

calculated, from which the average cross sectional area was also determined. With the

hand finished surface at the top, the cylinder to be tested was placed on the loading table

46

of a 400-kip Tinius Olsen universal testing machine (http://www.tiniusolsen.com/

products/hydraulic/hydraulic-2000kn.html; accessed: 07/21/05). The cylinder was placed

such that its axis was aligned exactly with the center of thrust of the spherically seated

upper block of the loading head. After adjusting the initial reading to zero, the upper

block of the loading machine was brought down so that it just touched the top of the

cylinder. Load was applied at a uniform rate of 60,000 ± 25,000 lb/min. for large

cylinders, and 27,000 ± 12,000 lb/min. for small cylinders, until the specimen failed

completely. These rates conformed with ASTM C39/C 39 M, which specifies the rate of

load application as between 20 and 50 psi/s. Therefore, the lower limit is 20×60×π×d2/4

= 33,929 lb/min. for large cylinders (diameter, d = 6 in.), and 15,080 lb/min. for small

cylinders (d = 4 in.). Similarly, the upper limit is calculated as 84,823 and 37,699

lb/min., respectively. The compressive strength, f′c, of the concrete specimens was

calculated by dividing the maximum load at failure by the average cross sectional area of

the cylinder.

3.8.2 Flexural Strength

ASTM C 78 – 02 Standard Test Method for Flexural Strength of Concrete (Using

Simple Beam with Third-Point Loading) and AASHTO T 97 were adhered to. The beams

were simply supported on each end, so that the effective span for large beams was 18 in.

for large beams, and 13.5 in. for small beams. The specification requires that the

effective span should be three times the depth, which was 6 and 4.5 in., respectively.

After recording the dimensions of each beam on two opposite sides, the average

dimensions were calculated. Two load-applying blocks were placed on the top side of

47

the beam so that they were in contact with the surface of the beam at the third points

between the bottom supports. Load was applied at a rate of 1800 ± 300 lb/min. for large

beams and 775 ± 125 lb/min. for small beams, until the specimens failed completely,

whereupon the failure pattern was recorded. These rates conform to ASTM C 78 – 02,

which requires that the load be applied “at a rate that constantly increases the extreme

fiber stress between 125 and 175 psi/min. until rupture occurs.” To calculate such a rate,

the specification provides the following equation:

LdbSr

2

= (3.24)

in which: r = loading rate, lb/min.; S = rate of increase in extreme fiber stress, e.g., 125 or

175 psi/min.; b = average beam width, in.; d = average beam depth, in.; and L = effective

beam span, in.

Thus, from the lower limit of S = 125 psi/in., r is found to be 125×6×36/18 =

1500 lb/min. for large beams, and 125×3.5×20.25/13.5 = 656 lb/min. for small beams.

The values corresponding to the upper limit of S = 175 psi/min. are 2100 and 919 lb/min.,

respectively. The rates adopted are convenient rounded off values based on these

calculation results.

According to ASTM C 78 – 02, the modulus of rupture, MR, must be calculated

using one of the formulae below, depending on the location of the failure plane:

2dbLPM R = (3.25)

or:

48

2

3db

aPM R = (3.26)

where: P = ultimate load, lb; and a = average distance between the line of fracture and

the nearest support, in.

Equation (3.25) is to be used when failure occurs inside the middle third of the

span, whereas Equation (3.26) applies when failure occurs outside this region. None of

the specimens tested necessitated the latter equation.

49

4 TEST RESULTS

4.1 Introduction

The main purpose of this chapter is to present the data collected during this

project, including all information recorded, as well as any observations made while

performing the experiments. The raw data are tabulated along with derivative values

calculated from them. The information provided in this chapter will form the database

for the discussions to follow in Chapter 5, concerning the effect such parameters as age,

specimen size, microsilica type, and aggregate type.

4.2 Microsilica

4.2.1 Microsilica Fineness

The results of the fineness tests conducted on the four microsilica types

(undensified, densified, abused-A and abused-B) are presented in Table 4.1. These tests

were performed at the laboratory of the Ohio Department of Transportation (ODOT), and

some involved brushing the sample even though this is not prescribed by the American

Society for Testing and Materials (ASTM) in ASTM C 430 – 96 Standard Test Method

for Fineness of Hydraulic Cement by the 45-µm (No. 325) Sieve.

50

4.2.2 Microsilica Gradation

The calculations for the grain size distributions of undensified, densified and

abused microsilica are tabulated in Tables 4.2 through 4.7. The gradation curves plotted

in Fig. 4.1 represent the results of both the sieve and hydormeter tests.

4.3 Aggregates

The results of sieve analysis for the fine and coarse aggregate types are tabulated

in Table 4.8(a), which also includes the required percentages passing according to Ohio

Department of Transportation (ODOT) Items 703.01 Aggregate-General: Size and

703.02 Aggregate for Portland Cement Concrete: Fine Aggregate. Item 703.01 is

identical with the American Association of State Highway and Transportation Officials

(AASHTO) M 43. It can be seen that amounts passing are within the prescribed limits.

Each aggregate test was conducted three times by independent researchers, from

which average values and other statistics were determined. These results are tabulated in

Table 4.8(b), in which the coefficients of variation (COV) are also given. It is observed

that the COV did not exceed approximately 5% for most of the aggregate properties,

except for the absorption values. It can be said, therefore, that the values obtained by the

researchers are reliable from a statistical point of view.

4.4 Concrete Mixes

The amounts of each ingredient, used per cubic yard of concrete of each mix, are

given in Table 4.9. The mixes were named after the microsilica type and the nature of

51

the coarse aggregate used, as Densified Natural (DN), Densified Crushed (DC),

Undensified Natural (UN), Undensified Crushed (UC), Abused Natural (AN), and

Abused Crushed (AC).

A number of batches were prepared in order to obtain the required volume of each

mix. Table 4.10 enumerates the number of specimens cast from each batch, while Table

4.11 presents the physical properties of each batch, from which average values for each

mix were calculated.

4.5 Mechanical Properties

The results of the compressive strength tests are tabulated in Table 4.12 for large

cylinders, and in Table 4.13 for small cylinders. At each age, a minimum of three

cylinders were tested. All specimens gained a significant amount of strength by the age

of 28 days, but explosive failures (typical of high strength concrete) occurred at almost

all ages. Shearing of aggregate was observed in all specimens tested, and the ratio of the

frequency of this failure mode to that by aggregate pullout was 4:1 in most cases.

Modulus of rupture results can be found in Table 4.14 for large beams, and in

Table 4.15 for small beams. All beams failed with a vertical failure plane near the center

of the beam, between the two points of load application. In most of the beams, a crack

was observed to originate at the bottom of the beam and to propagate up. Examining the

specimens after failure, revealed that the failure mode was similar to that under

compressive strength testing, i.e., there was much more shearing in the aggregate itself

than aggregate pullout, typically in the ratio of 4:1.

52

Table 4.1 Results of Tests Conducted on Microsilica Types at ODOT Laboratory

Microsilica Type Sample No. Weight

Retained (g) Correction Factor (%)

Corrected % Retained

1 0.358 32.9 47.58 4 0.334 26.77 42.34 Undensified

Microsilica 9* 0.023 24.85 2.87 6 0.83 32.9 110.31 7 0.828 26.77 104.97 Densified

Microsilica 8* 0.767 44.56 110.88 3 0.596 84.14 109.75 5 0.606 52.59 92.47 Abused

Microsilica A 10* 0.506 35.03 68.33 2 0.836 30.79 109.34 11 0.832 24.85 103.88 Abused

Microsilica B 12* 0.766 44.56 110.73

Note: *Third replicate of each microsilica type pertains to a brushed specimen. specimen.

53

Table 4.2 Hydrometer Test on Undensified Microsilica

Calculated Gs = 2.060; a = 1.21; Percent finer than No. 200 sieve (wet sieving) = 87.80%

Elapsed Time (Min.)

Elapsed Time, t

(s)

Tempe-rature (°C)

Viscosity (Poise)

Act. Hyd. Rdg, R a

Corr. Hyd. Rdg, R c

Actual %

Finer

Adjusted % Finer

Hyd. Corr. Only for

Meniscus, R

L (cm) L/t D

(µm)

2 120 24 0.00914 20 18 145.2 127.5 21 12.9 0.1075 41.241

4 240 24 0.00914 18 16 129.1 113.3 19 13.2 0.055 29.499

8 480 24 0.00914 17 15 121 106.2 18 13.3 0.027708 20.938

16 960 24 0.00914 16 14 112.9 99.2 17 13.5 0.014063 14.916

30 1800 24 0.00914 14 12 96.8 85 15 13.8 0.007667 11.013

60 3600 24 0.00936 13 10.7 86.3 75.8 14 14 0.003889 7.938

120 7200 24 0.00914 12 10 80.7 70.8 13 13 0.001972 5.586

240 14400 23 0.00936 12 9.7 78.2 68.7 13 13 0.000986 3.997

480 28800 23 0.00936 11 8.7 70.2 61.6 12 12 0.000497 2.836

960 57600 23 0.00936 11 8.7 70.2 61.6 12 12 0.000248 2.006

1920 115200 23 0.00936 11 8.7 70.2 61.6 12 12 0.000124 1.418

3840 230400 23 0.00936 11 8.7 70.2 61.6 12 12 0.000062 1.003

5760 345600 23 0.00936 10 7.7 62.1 54.5 11 11 0.000042 0.824

54

Table 4.3 Hydrometer Test on Densified Microsilica

Calculated Gs = 1.970; a = 1.26; Percent finer than No. 200 sieve (wet sieving) = 24.93%

Elapsed Time (Min.)

Elapsed Time, t

(s)

Tempe-rature (°C)

Viscosity (Poise)

Act. Hyd. Rdg, R a

Corr. Hyd. Rdg, R c

Actual %

Finer

Adjusted % Finer

Hyd. Corr. Only for

Meniscus, R

L (cm) L/t D

(µm)

2 120 24 0.00914 16 14 118 29.4 17 13.5 0.1125 44.103

4 240 24 0.00914 13 11 92.7 23.1 14 14 0.05833 31.758

8 480 24 0.00914 9 7 59 14.7 10 14.7 0.03063 23.011

16 960 24 0.00914 9 7 59 14.7 10 14.7 0.01531 16.271

30 1800 24 0.00914 8 6 50.6 12.6 9 14.8 0.00822 11.923

60 3600 24 0.00936 7 5 42.2 10.5 8 15 0.00417 8.488

120 7200 24 0.00914 7 5 42.2 10.5 8 15 0.00208 6.002

240 14400 23 0.00936 7 4.7 39.6 9.9 8 15 0.00104 4.295

480 28800 23 0.00936 7 4.7 39.6 9.9 8 15 0.00052 3.037

960 57600 23 0.00936 7 4.7 31.2 9.9 8 15 0.00026 2.147

1920 115200 23 0.00936 6 3.7 31.2 7.8 7 15.2 0.00013 1.528

3840 230400 23 0.00936 6 3.7 31.2 7.8 7 15.2 0.00007 1.081

5760 345600 23 0.00936 6 3.7 31.2 7.8 7 15.2 0.00004 0.882

55

Table 4.4 Hydrometer Test on Abused Microsilica

Elapsed Time (Min.)

Elapsed Time, t

(s)

Tempe-rature (°C)

Viscosity (Poise)

Act. Hyd. Rdg, R a

Corr. Hyd. Rdg, R c

Actual %

Finer

Adjusted % Finer

Hyd. Corr. Only for

Meniscus, R

L (cm) L/t D

(µm)

2 120 24 0.00914 18 16 140.5 34.2 19 13.2 0.11 45.35

4 240 24 0.00914 15 13 114.1 27.8 16 13.7 0.057083 32.669

8 480 24 0.00914 10 8 70.2 17.1 11 14.5 0.030208 23.765

16 960 24 0.00914 9 7 61.4 15 10 14.7 0.015313 16.92

30 1800 24 0.00914 9 7 61.4 15 10 14.7 0.008167 12.357

60 3600 23 0.00936 9 7 61.4 15 10 14.7 0.004083 8.737

120 7200 24 0.00914 9 7 61.4 15 10 14.7 0.002042 6.178

240 14400 23 0.00936 9 6.7 58.8 14.3 10 14.7 0.001021 4.421

480 28800 23 0.00936 9 6.7 58.8 14.3 10 14.7 0.00051 3.126

960 57600 23 0.00936 9 6.7 58.8 14.3 10 14.7 0.000255 2.211

1920 115200 23 0.00936 8 5.7 50 12.2 9 14.8 0.000128 1.568

3840 230400 23 0.00936 9 6.7 58.8 14.3 10 14.7 0.000064 1.105

5760 345600 23 0.00936 9 6.7 58.8 14.3 10 14.7 0.000043 0.902

Calculated Gs = 1.897; a = 1.32; Percent finer than No. 200 sieve (wet sieving) = 24.34%

56

Table 4.5 Dry Sieve Analysis of Undensified Microsilica

Sieve Number

Sieve Size

(mm)

Tare (g)

Tare + Material

(g)

Weight of

Material Retained

(g)

% Retained

% Cumulative

Retained

% Passing

10 2 1250 1285 35 18.421 18.42 81.58 20 0.85 1155 1190 35 18.421 36.84 63.16 40 0.425 1120 1135 15 7.895 44.74 55.26 60 0.25 1125 1135 10 5.263 50.00 50.00 140 0.106 1110 1180 70 36.842 86.84 13.16 200 0.075 1070 1090 20 10.526 97.37 2.63 270 0.053 1130 1135 5 2.632 100.00 0.00 325 0.045 1035 1035 0 0 100.00 0.00 Pan 1140 1140 0 0 100.00 0.00

SUM 190 100

57

Table 4.6 Dry Sieve Analysis of Densified Microsilica

Sieve Number

Sieve Size

(mm)

Tare (g)

Tare + Material

(g)

Weight of

Material Retained

(g)

% Retained

% Cumulative

Retained

% Passing

10 2 1250 1250 0 0 0.00 100.00 20 0.85 1155 1190 35 11.111 11.11 88.89 40 0.425 1120 1225 105 33.333 44.44 55.56 60 0.25 1125 1170 45 14.286 58.73 41.27 140 0.106 1110 1195 85 26.984 85.71 14.29 200 0.075 1070 1105 35 11.111 96.83 3.18 270 0.053 1130 1140 10 3.175 100.00 0.00 325 0.045 1035 1035 0 0 100.00 0.00 Pan 1140 1140 0 0 100.00 0.00

SUM 315 100

58

Table 4.7 Dry Sieve Analysis of Abused Microsilica

Sieve Number

Sieve Size

(mm)

Tare (g)

Tare + Material

(g)

Weight of

Material Retained

(g)

% Retained

% Cumulative

Retained

% Passing

10 2 1250 1250 0 0 0.00 100.00 20 0.85 1155 1190 35 13.208 13.21 86.79 40 0.425 1120 1210 90 33.962 47.17 52.83 60 0.25 1125 1170 45 16.981 64.15 35.85 140 0.106 1110 1170 60 22.642 86.79 13.21 200 0.075 1070 1095 25 9.434 96.23 3.77 270 0.053 1130 1140 10 3.774 100.00 0.00 325 0.045 1035 1035 0 0 100.00 0.00 Pan 1140 1140 0 0 100.00 0.00

SUM 265 100.001

59

Table 4.8 (a) Aggregate Sieve Analysis

Sieve Size or Number

% Passing for No. 8 per Specification

% Passing for

Crushed No. 8

% Passing for Natural

No. 8

% Passing for Natural Sand Per

Specification

% Passing Average for

Natural Sand

1 ½ in. 100 100 100 100 100 1 in. 100 100 100 100 100 ¾ in. 100 100 100 100 100 ½ in. 100 100 100 100 100

3/8 in. 85-100 86 94 100 100 No. 4 10-30 11 20 95-100 100 No. 8 0-10 2 1 70-100 94 No. 16 0-5 1 0 38-80 72 No. 30 0 0 0 18-60 43 No. 50 0 0 0 5-30 14 No. 100 0 0 0 1-10 2

Pan 0 0 0 0-5 0

Table 4.8 (b) Physical Aggregate Properties

Natural Sand No. 8 Natural No. 8 Crushed Type of Test

Average COV (%) Average COV

(%) Average COV (%)

Apparent Specific Gravity 2.72 3.7 2.77 1.21 2.8 1.52

Bulk Specific Gravity

(Saturated Surface Dry)

2.62 6.1 2.61 3.04 2.59 1.45

Bulk Specific Gravity

(Oven Dry) 2.57 7.79 2.52 4.69 2.47 1.29

% Absorption 3.21 50.22 3.7 52.51 4.78 0.67 Fineness Modulus 3.11 17.94

Dry Unit Weight (pcf) 108.21 4.8 94 5.87

60

Table 4.9 Ingredients by Mix (per yd3 of concrete)

Mix

Ingredient UN UC DN DC AN AC

CEMEX Type 1/II cement (lb) 700 700 700 700 700 700

Martin Marietta Materials air-dry coarse aggregate (lb)

1339 1319 1339 1319 1339 1319

Martin Marietta Materials air-dry fine aggregate (lb)

1344 1355 1344 1355 1344 1355

ELKEM microsilica (lb) 61 61 61 61 61 61

City of Cincinnati water above air-dry (lb)

361 374 361 374 361 374

MB-AE 90 air entrainer (mL) 337.6 337.6 337.6 337.6 337.6 337.6

Rheobuild 1000 plasticizer (mL) 2070 2070 2070 2070 2070 2070

61

Table 4.10 Specimens Cast by Batch

Batch No. 1 2 3 4 5† Total Undensified Natural (UN)

Volume (ft3) 2.0 2.0 1.0 5.0 Small Cylinders 9 6 7 22 Large Cylinders 1 1 0 2

Small Beams 5 4 0 9 Large Beams 0 1 0 1

Undensified Crushed (UC) Volume (ft3) 2.0 2.0 1.0 5.0

Small Cylinders 10 7 8 25 Large Cylinders 1 1 0 2

Small Beams 5 4 0 9 Large Beams 0 1 0 1

Densified Natural (DN) Volume (ft3) 2.0 2.0 2.0 2.0 1.0 9.0

Small Cylinders 4 0 0 1 7 12 Large Cylinders 7 0 2 4 0 13

Small Beams 0 0 0 0 0 0 Large Beams 0 4 3 2 0 9

Densified Crushed (DC) Volume (ft3) 2.5 2.5 2.5 1.0 8.5

Small Cylinders 3 3 0 7 13 Large Cylinders 3 3 4 1 11

Small Beams 0 0 0 0 0 Large Beams 3 3 3 0 9

Abused Natural (AN) Volume (ft3) 1.8 1.8 1.0 4.6

Small Cylinders 8 9 8 25 Large Cylinders 1 2 1 4

Small Beams 5 4 0 9 Large Beams 0 0 0 0

Abused Crushed (AC) Volume (ft3) 1.8 1.8 1.0 4.6

Small Cylinders 8 9 7 24 Large Cylinders 1 2 1 4

Small Beams 5 4 0 9 Large Beams 0 0 0 0

Notes: †-Specimens cast in 2004; remainder cast in 2003.

62

Table 4.11 Physical Properties by Batch

Batch # 1 2 3 4 5† Average COV (%)

Undensified Natural (UN) Slump

(in.) 5.75 5.50 5.50 5.58 2.59

% Air 7.40 7.00 7.00 7.13 3.24 Unit

weight (pcf)

139.10 141.40 141.00 140.51 0.89

Undensified Crushed (UC) Slump

(in.) 5.75 5.25 4.00 5.00 18.03

% Air 8.70 7.10 7.80 7.87 10.20 Unit

weight (pcf)

139.40 141.60 138.60 139.86 1.11

Densified Natural (DN) Slump

(in.) 4.25 6.00 4.75 5.75 6.00 5.35 15.00

% Air 7.90 9.00 8.20 7.80 9.00 8.38 6.98 Unit

weight (pcf)

138.00 137.70 138.60 140.50 138.40 138.62 0.80

Densified Crushed (DC) Slump

(in.) 4.75 4.00 6.00 6.00 5.19 19.02

% Air 7.20 8.00 7.70 9.50 8.10 12.22 Unit

weight (pcf)

141.40 139.40 140.90 138.10 139.95 1.06

Abused Natural (AN) Slump

(in.) 7.50 6.50 5.50 6.5 15.38

% Air 9.00 9.50 8.00 8.83 8.65 Unit

weight (pcf)

137.30 135.50 138.30 137.04 1.02

Abused Crushed (AC) Slump

(in.) 6.50 7.00 6.50 6.67 4.33

% Air 10.50 10.00 10.00 10.17 2.84 Unit

weight (pcf)

134.80 134.60 135.80 135.08 0.46

Note: †-Specimens cast in 2004; remainder cast in 2002-2003.

63

Table 4.12 Compressive Strength of Large Cylinders, f′c

Mix/Age 7 days 28 days

UN C1 P (lb) 189958

Mean A (in.2) 27.69 f′c (psi) 6861

Mean f′c (psi, COV) 6861,

UC D1 P (lb) 186277

Mean A (in.2) 27.98 f′c (psi) 6657

Mean f′c (psi, COV) 6657,

DN A1 A3 A4 A1a A1b A4 P (lb) 122993 120727 127698 152968 152723 156605

Mean A (in.2) 28.57 28.57 28.57 28.27 27.98 27.98 f′c (psi) 4305 4226 4470 5410 5458 5597

Mean f′c (psi, COV) 4333, 2.87% 5488, 1.77%

DC B1 B2 B3 B1 B2 B3 P (lb) 137483 130945 124667 157817 162724 181443

Mean A (in.2) 28.87 28.27 28.27 28.27 28.28 28.28 f′c (psi) 4762 4631 4409 5582 5755 6417

Mean f′c (psi, COV) 4601, 3.88% 5918, 7.45%

AN G2 G1 P (lb) 102490 154065

Mean A (in.2) 27.98 27.98 f′c (psi) 3663 5506

Mean f′c (psi, COV) 3663, 5506,

AC I2 I2 P (lb) 108070 145031

Mean A (in.2) 28.28 28.87 f′c (psi) 3822 5024

Mean f′c (psi, COV) 3822, 5024,

64

Table 4.12 Compressive Strength of Large Cylinders, f′c (Cont’d)

Mix/Age 56 days 90 days

UN C2

P (lb) 205818 Mean A (in.2) 28.87

f′c (psi) 7129 Mean f′c

(psi, COV) 7129,

UC D2

P (lb) 210985 Mean A (in.2) 28.57

f′c (psi) 7385 Mean f′c

(psi, COV) 7385,

DN A1 A3 A4 A1 A1 A4 A1

P (lb) 175569 172942 171845 182309 175511 180865 183160 Mean A (in.2) 28.27 28.27 27.98 28.57 28.57 28.57 28.57

f′c (psi) 6209 6117 6141 6381 6143 6331 6411 Mean f′c

(psi, COV) 6156, 0.78% 6316, 1.90%

DC B1 B2 B3 B3 B6

P (lb) 205544 171138 169421 207146 190636 Mean A (in.2) 27.98 28.57 28.87 28.58 27.98

f′c (psi) 7346 5990 5869 7249 6813 Mean f′c

(psi, COV) 6401, 12.81% 7031, 4.38%

AN G2 G9

P (lb) 169969 181544 Mean A (in.2) 27.98 28.27

f′c (psi) 6074 6421 Mean f′c

(psi, COV) 6074, 6421,

AC I8 I8

P (lb) 175482 172221 Mean A (in.2) 28.57 28.28

f′c (psi) 6142 6090 Mean f′c

(psi, COV) 6142, 6090,

65

Table 4.13 Compressive Strength of Small Cylinders, f′c

Mix/Age 7 days 28 days

UN C1a C1b C2 C1a C1b C2 C6 C4

P (lb) 68791 62941 68063 85307 89279 90923 92999 86071

Mean A (in.2) 12.57 12.57 12.57 12.76 12.96 12.76 12.57 12.57 f′c (psi) 5473 5009 5416 6683 6888 7123 7401 6849

Mean f′c (psi, COV) 5299, 4.78% 6989, 3.99% UC D1a D1b D2 D1a D1b D2 D6 D4

P (lb) 70561 68308 71941 104065 95699 98017 89780 92015 Mean A (in.2) 12.57 12.57 12.57 12.18 12.37 12.57 12.76 12.37

f′c (psi) 5615 5436 5725 8546 7735 7800 7034 7438 Mean f′c (psi, COV) 5592, 2.61% 7711, 7.22%

DN A1 A1 A4 A5 P (lb) 57494 71430 61596 67457

Mean A (in.2) 12.76 12.37 12.76 12.76 f′c (psi) 4504 5774 4826 5285

Mean f′c (psi, COV) 4504, 5295, 8.95% DC B1 B2 B4 B5

P (lb) 65536 81796 74832 69245 Mean A (in.2) 12.76 12.18 12.76 12.57

f′c (psi) 5134 6717 5863 5510 Mean f′c (psi, COV) 5134, 6030, 10.29%

AN G1A G1B G2 G1 G2a G2b G4 G6 P (lb) 46014 51886 48793 65132 66524 63525 75059 75445

Mean A (in.2) 12.37 12.37 12.57 12.37 12.18 12.37 12.97 12.18 f′c (psi) 3719 4194 3883 5265 5463 5135 5789 6196

Mean f′c (psi, COV) 3932, 6.13% 5570, 7.69% AC I1 I2a I2b I1a I1b I2 I6 I4

P (lb) 51251 56157 52718 69133 65139 78617 65377 70121 Mean A (in.2) 12.57 12.37 12.37 12.57 12.57 12.57 12.57 12.57

f′c (psi) 4078 4539 4261 5501 5184 6256 5203 5580 Mean f′c (psi, COV) 4293, 5.40% 5545, 7.84%

66

Table 4.13 Compressive Strength of Small Cylinders, f′c (Cont’d)

Mix/Age 56 days 90 days

UN C1 C2a C2b C3 C5 C1 C2a C2b C7

P (lb) 99235 99923 92891 89557 88843 97717 96315 95778 91053

Mean A (in.2)

12.57 12.37 12.97 12.57 12.37 12.77 12.57 12.37 12.37

f′c (psi) 7897 8077 7165 7127 7181 7652 7665 7742 7360

Mean f′c (psi, COV) 7489, 6.16% 7604, 2.21%

UC D1a D1b D2 D3 D5 D1 D2a D2b D2c D2d D1 D7

P (lb) 94369 96110 109109 96845 94783 102908 89939 103200 107421 102850 91528 95652

Mean A (in.2)

12.57 12.57 12.57 12.57 12.57 12.37 12.76 12.37 12.37 12.37 12.37 12.76

f′c (psi) 7508 7646 8680 7707 7541 8318 7046 8342 8683 8313 7398 7494

Mean f′c (psi, COV) 7816, 6.26% 7942, 7.77%

DN A3 A6 A1h A1h A7

P (lb) 67126 72457 80545 84950 75330

Mean A (in.2)

12.57 12.56 12.37 12.76 12.37

f′c (psi) 5342 5766 6510 6655 6089

Mean f′c (psi, COV) 5554, 5.40% 6418, 4.58%

DC B2 B1h B1 B3 B1h B8

P (lb) 93327 96488 78415 74933 92455 80484

Mean A (in.2)

12.37 12.57 12.57 12.57 12.77 12.37

f′c (psi) 7544 7678 6240 5963 7241 6506

Mean f′c (psi, COV) 6856, 12.84% 6873, 7.55%

AN G1 G2a G2b G1 G5 G1a G1b G2 G2b G7 G8

P (lb) 75416 72367 76508 76771 73841 77445 72096 80855 69912 80913 82982

Mean A (in.2)

12.57 12.57 12.57 12.97 12.57 12.76 12.76 12.57 12.57 12.37 12.76

f′c (psi) 6001 5759 6088 5921 5876 6067 5648 6434 5563 6540 6501

Mean f′c (psi, COV) 5929, 2.11% 6126, 7.14%

AC I1 I2a I2b I1 I5 I1a I1b I2 I3

P (lb) 72096 79327 71501 67273 72201 77802 91983 83566 71992

Mean A (in.2)

12.37 12.57 12.57 12.57 12.57 12.57 12.76 12.76 12.37

f′c (psi) 5828 6313 5690 5353 5746 6191 7206 6547 5819

Mean f′c (psi, COV) 5786, 5.97% 6441, 9.17%

67

Table 4.14 Modulus of Rupture for Large Beams, MR

Mix 7 days 28 days UN

P (lb) bAVG (in.) dAVG (in.) MR (psi)

Mean MR (psi), COV

UC P (lb)

bAVG (in.) dAVG (in.) MR (psi)

Mean MR (psi), COV

DN A2 A3 A4 A2a A2b A3 P (lb) 8068.50 6446.30 7967.20 11086.00 8304.00 7567.30

bAVG (in.) 6.00 6.00 6.00 6.00 6.00 6.06 dAVG (in.) 6.00 6.00 6.00 6.00 6.00 6.00 MR (psi) 672 537 664 924 692 624

Mean MR (psi), COV 625, 12.13% 747, 21.05%

DC B1 B2 B3 B1 B2 B3 P (lb) 10874 7755.5 7158.4 7952.7 8623.1 9061

bAVG (in.) 6.00 6.13 6.00 6.06 6.06 6.06 dAVG (in.) 6.13 6.00 6.00 6.00 6.00 6.00 MR (psi) 870 633 597 656 704 747

Mean MR (psi) 700, 21.18% 702, 6.51% AN

P (lb) bAVG (in.) dAVG (in.) MR (psi)

Mean MR (psi), COV

AC P (lb)

bAVG (in.) dAVG (in.) MR (psi)

Mean MR (psi), COV

68

Table 4.14 Modulus of Rupture for Large Beams, MR (cont’d)

Mix 56 days 90 days UN

P (lb) bAVG (in.) dAVG (in.) MR (psi) Mean MR (psi, COV)

UC P (lb)

bAVG (in.) dAVG (in.) MR (psi) Mean MR (psi, COV)

DN A2 A3 A4 P (lb) 10246 8746.2 8436.8

bAVG (in.) 6.00 6.00 5.94 dAVG (in.) 6.00 6.09 6.03 MR (psi) 854 707 703 Mean MR (psi, COV) 755, 11.40%

DC B1 B2 B3 P (lb) 9221.2 7906.6 8226.9

bAVG (in.) 6.00 6.00 6.03 dAVG (in.) 6.06 6.16 6.00 MR (psi) 753 626 678 Mean MR (psi, COV) 686, 9.29%

AN P (lb)

bAVG (in.) dAVG (in.) MR (psi) Mean MR (psi, COV)

AC P (lb)

bAVG (in.) dAVG (in.) MR (psi) Mean MR (psi, COV)

69

Table 4.15 Modulus of Rupture of Small Beams, MR

Mix 7 days 28 days UN C1a C1b C2 C1 C2a C2b

P (lb) 4077.70 3724.80 3794.50 4333.70 4964.30 4427.80 bAVG (in.) 4.50 4.47 4.44 4.59 4.56 4.56 dAVG (in.) 3.53 3.53 3.5 3.66 3.53 3.75 MR (psi) 981 902 942 953 1178 932 Mean MR (psi, COV) 942, 4.17% 1021, 13.38%

UC D1a D1b D2 D1 D2a D2b P (lb) 3733.90 3295.10 3113.20 5127.20 4014.50 4551.80

bAVG (in.) 4.59 4.56 4.56 4.59 4.56 4.69 dAVG (in.) 3.88 3.63 3.56 3.75 3.66 3.56 MR (psi) 731 742 726 1071 889 1033 Mean MR (psi, COV) 733, 1.13% 998, 9.66%

DN

P (lb) bAVG (in.) dAVG (in.) MR (psi) Mean MR (psi, COV)

DC

P (lb) bAVG (in.) dAVG (in.) MR (psi) Mean MR (psi, COV)

AN G1a G1b G2 G1 G2a G2b P (lb) 3017.30 2935.00 3339.40 3945.60 4359.00 3988.10

bAVG (in.) 4.50 4.47 4.06 4.47 4.50 4.63 dAVG (in.) 3.53 3.50 3.63 3.56 3.56 3.50 MR (psi) 726 724 844 939 1030 950 Mean MR (psi, COV) 765, 9.03% 973, 5.11%

AC I1a I1b I2 I1 I2a I2b P (lb) 3154.80 3718.50 3296.10 3653.30 4093.10 4442.30

bAVG (in.) 4.50 4.50 4.53 4.56 4.56 4.56 dAVG (in.) 3.53 3.5 3.5 3.53 3.56 3.69 MR (psi) 759 911 802 867 954 1018 Mean MR (psi, COV) 824, 9.49% 946, 8.01%

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Table 4.15 Modulus of Rupture of Small Beams, MR (Cont’d)

Mix 56 days 90 days UN C1a C1b C2

P (lb) 3995.30 4341.90 3734.80 bAVG (in.) 4.56 4.72 4.53 dAVG (in.) 3.66 3.63 3.56 MR (psi) 884 945 877 Mean MR (psi, COV) 902, 4.17%

UC D1a D1b D2 P (lb) 3852.40 3753.80 3219.10

bAVG (in.) 4.56 4.50 4.63 dAVG (in.) 3.59 3.56 3.63 MR (psi) 883 887 715 Mean MR (psi, COV) 828, 11.85%

DN P (lb)

bAVG (in.) dAVG (in.) MR (psi) Mean MR (psi, COV)

DC P (lb)

bAVG (in.) dAVG (in.) MR (psi) Mean MR (psi, COV)

AN G1a G1b G2 P (lb) 4366.30 3742.00 3844.20

bAVG (in.) 4.63 4.53 4.59 dAVG (in.) 3.56 3.59 3.56 MR (psi) 1004 863 890 Mean MR (psi, COV) 919, 8.14%

AC I1a I1b I2 P (lb) 4314.70 4160.00 3951.00

bAVG (in.) 4.66 4.59 4.59 dAVG (in.) 3.56 3.63 3.59 MR (psi) 986 930 899 Mean MR (psi, COV) 938, 4.68%

71

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0001000.0010000.0100000.1000001.000000

Particle Diameter, mm

Perc

ent F

iner

Abused (Hydrometer)Densified (Hydrometer)Undensified (Hydrometer)Densified (Sieve) Abused (Sieve)Undensified (Sieve)

Fig 4.1 Grain Size Distribution of Undensified, Densified, and Abused Microsilica

72

5 DISCUSSION OF RESULTS

5.1 Introduction

The interpretation and discussion of the data collected in this project are presented in

this chapter. The main problems encountered in this task are identified first: the questionable

validity of results from the American Society for Testing and Materials (ASTM) C 430 – 96

Standard Test Method for Fineness of Hydraulic Cement by the 45-µm (No. 325) Sieve) as

performed by Ohio Department of Transportation (ODOT) personnel; and, the significant

variability of the results of the mechanical tests conducted at the University of Cincinnati. In

order to deal with the latter, an innovative approach to statistical data interpretation is

proposed, which can lead to a number of meaningful conclusions by imposing a set of

engineering boundary conditions, i.e., that strength increases with age at a decreasing rate.

5.2 Microsilica Fineness

It is anticipated that the smaller the particle size, the lower the percentage of material

retained, and the higher the fineness value. The undensified microsilica, as expected, had the

lowest amount of material retained. The abused samples were expected to have higher

amounts of material retained, but this was not the case. Abused microsilica A had the next

lowest amount, while abused microsilica B and densified microsilica had almost identical

amounts of material retained. It was also found that brushing the samples caused more

material to pass through the sieves.

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Looking at Table 4.1, it can be seen that many of the % Retained values exceed 100.

Obviously, such values are not reasonable. Thus, for the purposes of this project, any %

Retained amounts above 100 will be assumed to be 100%, indicating that none of the

material passed through the sieves. The corrected results show that only the undensified

microsilica and two of the abused microsilica A specimens were able to pass through the No.

325 sieve to any significant degree. The densified, as well as the abused microsilica B

specimens failed to pass through the sieves. According to ASTM C 1240 – 01 Standard

Specification for Use of Silica Fume as a Mineral Admixture in Hydraulic-Cement Concrete,

Mortar and Grout, microsilica used in concrete should have a fineness value greater than

90%, i.e., the % Retained should be below 90%. Therefore, it can be concluded that even the

undensified microsilica is not suitable for use in concrete, as 90% of the sample could only

pass through the sieve when, deviating from the specification, the sample was brushed

thoroughly. The results of testing by the water pressure method, as well as by using a brush,

do not seem to produce any useful information other than that all of the microsilica samples

failed this test.

Looking solely at the weight retained, it can be seen in addition that the two abused

samples of microsilica do not have the desired particle size. These samples were abused so

that they would have a larger particle size than both the undensified and densified samples.

The abused sample B had nearly identical results as the densified sample, while abused

sample A was actually finer than the densified sample. It is concluded that this test provides

no meaningful information for the purpose of assessing the engineering properties of

densified (or even of abused) microsilica, and should be dropped as a means of quality

assurance. decreasing rate.

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5.3 Microsilica Gradation

The hydrometer method in conjunction with dry sieve analysis is not the ideal manner

to explore microsilica gradation, but it was the only approach feasible during this limited

project. Nonetheless, the data obtained and presented in Fig. 4.1 corroborate some of the

concerns expressed by Diamond and Sahu (2003), who point out that “the actual size of the

densified silica fume, as supplied to the customer, is always in the range of hundreds of µm.”

In contrast, the data obtained in this study create no reasons for concern regarding “clusters”

even in the undensified microsilica itself, as Diamond and Sahu (2003) suggest. Although

dry sieving of undensified microsilica may lead to the impression that this type is hardly

different from the densified and abused varieties, the superiority of the former is clearly

visible in the hydrometer test results. For their part, the hydrometer gradations of the

densified and abused microsilica in Fig. 4.1 are practically indistinguishable, indicating that

abuse does not create agglomerations that survive the rigor of the hydrometer test

methodology. This observation reinforces the conclusion reached above concenring the

inadequacy of the microsilica test to distinguish among the microsilica types considered, yet

by itself sheds no light on the suitability of using densified microsilica in construction. The

latter question needs to be answered through additional testing, as discussed below.

5.4 Variability of Mechanical Tests

The coefficients of variation (COV) of the compressive strength and modulus of

rupture test data are discussed in this section. Even though the researchers had taken great

care during mixing, casting and curing procedures to ensure consistency, considerable

variation in the test results was observed ascribed mainly to the nature of concrete. From

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prior experience, the following COV values may be expected: 0-5%: uncommonly low; 5-

10%: excellent engineering work; 10-15%: good engineering work; > 15%: questionable

reliability. The uniformity of the mixing processes employed is reflected in the low COV

statistics obtained for the physical properties of each batch prepared. According to ODOT

Item 499.03 Concrete-General: Proportioning; Slump, the slump may vary between 4 and 8

in. for concrete in which chemical admixtures are used. It can be seen from Table 4.11 that

the average slump of all the mixes ranged between these values. Therefore, it can be said

that the concrete used in this project meets the ODOT slump specification, even though

slump was not a property that was controlled in itself. Similarly, the expected air content

was 8 ± 2%, according to ODOT Supplemental Specification 848 Bridge Deck Repair and

Overlay with Concrete Using Hydro-Demolition. Consequently, the average air content

values in Table 4.11 also met the ODOT specification, falling within the range specified,

except for a small deviation in a single batch.

5.4.1 Compressive Strength

The coefficients of variation for various mixes at ages of 7, 28, 56 and 90 days are

tabulated in Table 5.1. As noted in Chapter 4, a two letter designation is used to identify the

mixes, in terms of the microsilica and coarse aggregate types used, viz., Densified Natural

(DN), Densified Crushed (DC), Undensified Natural (UN), Undensified Crushed (UC),

Abused Natural (AN), and Abused Crushed (AC). Moreover, compressive test results are

designated as pertaining to large or to small cylinders, LC or SC, respectively. Low values

were obtained in the compressive strength, f′c, tests at almost all ages. From Table 5.1, it can

be seen that the average coefficients of variation for the compressive strength tests varied

mostly between 2 and 10% for all mixes, representing excellent engineering work. The

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actual values of f′c obtained can be found in Tables 4.12 and 4.13 for large and small

cylinders, respectively.

5.4.2 Modulus of Rupture

Coefficients of variation obtained for the modulus of rupture, MR, tests are also

tabulated in Table 5.1, in which they are designated as pertaining to large or to small beams,

LB or SB, respectively. It can be seen that COV values were higher for modulus of rupture

than for compressive strength testing. This is because beams are sensitive to even minor

changes in mixing, casting or testing methods. Similar variability has been reported in the

literature. The average coefficients of variation for MR varied between 7 and 15%, which

represent excellent to good engineering work. The actual values of MR obtained can be

found in Tables 4.14 and 4.15for large and small beams, respectively.

5.5 Data Interpretation

Despite the researchers’ efforts to ensure uniformity and consistency in the results

obtained, which are reflected in COV values representing excellent to good engineering

work, the data collected pose a difficult interpretation problem since the small number of

specimens tested does not permit an exclusively statistical analysis. Consequently, it has

been found useful to appeal in addition to a set of engineering boundary conditions to ensure

that strength increases with age at a decreasing rate. This approach has been found to

reinforce the statistical analyses performed, leading to a number of meaningful conclusions,

as described below.

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5.5.1 Compressive Strength

The average compressive strength of the various mixes at each age is tabulated in

Tables 5.2 and 5.3, pertaining to large and small cylinder specimens, respectively. From

these data, relative strength values at each age with respect to the corresponding 28-day

average strength were calculated, as found in Tables 5.4 and 5.5. For example, in Table 5.5,

the 7-day strength for the small cylinder mix with densified microsilica and natural aggregate

(DN) is 85% of its 28-day strength, while the 56-day strength for the same mix is 105% of

the corresponding 28-day strength. These values are calculated by dividing the strength at a

particular age by the strength at 28 days; e.g., from Table 5.3, the compressive strength for

small cylinders at 7 days for mix DN is 4504 psi; similarly, the strength at 28 days for the

same mix is 5295 psi. Therefore, the relative strength at 7 days with respect to the 28-day

strength is calculated as follows: (4504 / 5295) × 100 = 85%.

All relative strengths thus calculated were plotted against age, as shown in Figures

5.1 and 5.2, from the data for large and small cylinders, respectively. In each case, the

researchers then fitted a trend curve through the points, by adjusting slightly the line

predicted statistically, so as to conform to the boundary conditions that the slope should

decrease and the strength should increase with increasing age. It can be noted from Figures

5.1 and 5.2 that the trend curve is only slightly different from the unadjusted statistical lines.

The enforcement of boundary conditions results in a smooth curve that reproduces the

expected trends. From the trend line thus obtained, adjusted relative strength values at each

age were read. For example, in Fig. 5.1, the trend line indicates 75% strength at 7 days, and

98%, 112% and 117% at 28, 56 and 90 days of age, respectively. Thus, the strength gain

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between two ages can be determined; e.g., the strength gain between ages 28 and 90 from

Fig. 5.1 can be calculated as (117-98)/98 = 19%.

In this manner, a set of best-fit compressive strength values were obtained for each

mix, as presented in Tables 5.6 and 5.7, for large and small cylinder specimens, respectively,

and will be used below in the interpretation of the test results. The entire procedure leading

to the derivation of these best-fit strength values may be summarized as follows, for a

particular specimen size:

1. Obtain the laboratory data pertaining to each mix at every age.

2. Calculate the average strength value of each mix at every age.

3. Normalize these average values with respect to the corresponding 28-day strength, to

obtain the relative strengths for each mix at every age.

4. Plot the relative strengths for all mixes against age, and obtain a statistically predicted

line through these points.

5. Impose the engineering boundary conditions of strength increase at a decreasing rate

with time, to adjust the statistical line and thus obtain a smooth trend curve.

6. From the adjusted trend curve, read off the percentage strength gain with each age

increment.

7. Return to the laboratory data, and inspect them to identify the optimum age for each

mix, to be used in the best-fit strength derivation. One way to do this, is to select the age, for

which the coefficient of variation is the lowest.

8. Use the incremental strength gains from Step 6 in conjunction with the laboratory

strength at the optimum age for each mix from Step 7, in order to derive the best-fit strength

values for each mix at every age. For example, if laboratory 90-day compressive strength of

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mix DC is equal to 6873 psi (Table 5.3) and the lowest coefficient of variation occurs at this

point as shown in Table 5.1, then the best-fit 7-day strength is 6873 multiplied by strength

gain from 7 days to 90 days i.e., 6873 × (72/110) = 4499. Note that 72% and 110% are

obtained from the trend line from Fig. 5.2 at 7 days and 90 days, respectively.

5.5.2 Modulus of Rupture

The procedure followed in obtaining the best-fit modulus of rupture values is

analogous to that for compressive strength. Because of the small number of large and small

specimens tested, the analysis considered the two types of specimens collectively. The

average values of actual test data obtained for each mix at each age tested are presented in

Table 5.8, from which the corresponding values relative to the corresponding 28-day strength

are obtained, as shown in Table 5.9. The trend line for the modulus of rupture can be seen in

Fig. 5.3, from which the best-fit values in Table 5.10 are determined.

5.6 Effect of Microsilica Type on Mechanical Properties of Concrete

5.6.1 Compressive Strength

Natural Coarse Aggregate

Mixes DN, UN, and AN were made using natural aggregate, each with one of the

three types of microsilica, viz., densified, undensified and abused, respectively. The

compressive strength comparisons based on the type of microsilica are plotted in Figures 5.4

and 5.5 for large and small cylinders, respectively. It can be observed that the compressive

strength of undensified microsilica was greater than the other two microsilica concretes at all

ages, for both large and small cylinders. At 28 days, large cylinders made with undensified

microsilica concrete had nearly 576 psi (10%) and 923 (17%) psi higher compressive

80

strength than densified and abused microsilica concretes, respectively. In the compressive

strength results of small cylinders, the strength of undensified microsilica concrete was 1025

psi (18.5%) and 1137 psi (15%) psi greater than densified and abused microsilica concretes,

respectively.

The compressive strength of densified microsilica exceeded 5000 psi at 28 days for

both large and small specimens. In comparison, for mixes typically used in pavement

construction, ODOT 499.03 requires “an average compressive strength at 28 days of 4000 psi

for Class C, 3000 psi for Class F and 4500 psi for Class S.” Therefore, even though

densified microsilica had lower compressive strengths than the undensified material, it can

still be used on Ohio Department of Transportation (ODOT) projects, since it has adequate

strength to meet the requirements of this agency. In fact, even the abused microsilica results

appear adequate.

Crushed Coarse Aggregate

The effect of aggregate on concrete made with crushed aggregate and undensified

microsilica can be found in Figures 5.6 and 5.7, respectively, for large and small cylinders.

As was the case for concrete made with natural aggregate, the compressive strength of

concrete made with undensified microsilica and crushed aggregate, showed higher strengths

than densified and abused microsilica concretes. The abused microsilica exhibited in all

cases lower strengths compared to the other two types, as expected.

At 28 days, the compressive strength of large cylinders made with undensified

microsilica had 544 (9%) and 1088 (20%) psi higher strength than densified and abused

microsilica concretes, respectively. Similarly, the small cylinders made with undensified

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microsilica had 1442 (24%) and 1714 (30%) psi greater strength than densified and abused

microsilica concretes, respectively.

Once again, the densified microsilica with crushed aggregate showed higher strengths

than those prescribed by ODOT 499.03 at 28 days of curing in all cases. Therefore, densified

microsilica meets the ODOT specifications in all cases and can be used in the construction of

pavements and bridges.

5.6.2 Modulus of Rupture

Natural Coarse Aggregate

The effect of microsilica type on the modulus of rupture of concrete made with

natural aggregate can be found in Fig. 5.8. The beams made with undensified microsilica

showed somewhat higher strengths compared to densified and abused microsilica concretes.

Surprisingly, abused microsilica concrete had higher modulus of rupture than densified

microsilica concrete and was comparable to undensified microsilica concrete. At 28 days,

for example, the beams made with undensified microsilica had 22 psi (3%) and 73 psi (11%)

higher strengths than densified and abused microsilica concretes, respectively.

Crushed Coarse aggregate

Figure 5.9 shows the effect of microsilica type on concrete with crushed aggregate.

The results for modulus of rupture of concrete with crushed aggregate also showed some

surprising trends. Densified microsilica concrete exhibited lower values than undensified

and abused microsilica concretes at all ages. Comparing Figures 5.8 and 5.9, it is observed

that at 28 days concrete made with crushed aggregate showed slightly higher modulus of

rupture values than concrete with natural aggregate, as expected.

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5.7 Effect of Coarse Aggregate Type on Mechanical Properties of

Concrete

5.7.1 Compressive Strength

Undensified Microsilica

As expected, the compressive strength of large cylinders made with crushed

aggregate showed higher strengths than the concrete made with natural aggregate for large

and small cylinders at all ages and for all mixes. The effect of aggregate type on the

compressive strength of concrete made with undensified microsilica can be found in Figures

5.10 and 5.11, for large and small cylinders, respectively. Concrete made with undensified

microsilica and crushed aggregate showed higher strengths than the concrete made with same

microsilica type and natural aggregate. As expected, at 28 days of age, the compressive

strength of large cylinders from mix UC had 224 (4%) psi higher strength than large

cylinders from mix UN. When the compressive strengths of small cylinders made with

undensified microsilica are observed at 28 days, the concrete made with crushed aggregate

shows an increase of 810 (12%) psi over the concrete made with natural aggregate.

Densified Microsilica

Similarly, large and small cylinders made with densified microsilica and crushed

aggregate showed greater strengths at all ages than the concrete made from the same

microsilica and natural aggregate. The effect of aggregate type on compressive strength of

large and small cylinders made with densified microsilica is plotted in Fig. 5.12 and Fig.

5.13. Moreover, the difference in strengths due to aggregate type is higher for small than for

large cylinders. This trend was also observed for undensified microsilica concrete. At 28

days, large cylinders made with crushed aggregate had 256 (5%) psi higher strength than

83

natural aggregate concrete, whereas for small cylinders, the difference in the strengths of

crushed aggregate concrete and natural aggregate concrete was 393 (7%) psi.

Abused Microsilica

Abused microsilica concrete showed the same trends as undensified and densified

microsilica concretes. Figures 5.14 and 5.15 show the effect of aggregate type on the

compressive strength of abused microsilica concrete for large and small cylinders,

respectively. Concrete made with crushed aggregate had higher strengths than the concrete

made with natural aggregate at all ages, as expected. At 28 days, large cylinders made with

crushed aggregate had 59 (1%) psi higher strength than the ones made with natural

aggregate. In contrast, for small cylinders, the difference in strengths of crushed and natural

aggregate was 300 (6%) psi.

5.7.2 Modulus of Rupture

The modulus of rupture values showed similar trends as compressive strength results,

values being higher when crushed aggregate was used. The effects of aggregate type on the

three types of microsilica are explained in the following sections.

Undensified Microsilica

The effect of aggregate type on undensified microsilica concrete beams is plotted in

Fig. 5.16. At 28 days, the beams made with crushed aggregate had 13 psi (2 %) higher

flexural strength than beams made with natural aggregate.

Densified Microsilica

Rather surprisingly, beams made from densified microsilica concrete with natural

aggregate showed greater strength than those made with crushed aggregate at all ages.

Figure 5.17 shows the effect of aggregate type on densified microsilica concrete beams. It

84

can be seen that, at 28 days, beams made with natural aggregate had nearly 10 psi (1%)

higher flexural strength than beams made with crushed aggregate.

Abused Microsilica

The effect of aggregate type on abused microsilica concrete beams can be seen in Fig.

5.18. at 28 days, the modulus of rupture of beams with natural aggregate was surprisingly

somewhat higher when compared to beams with crushed aggregate. At other ages, however,

the beams with crushed aggregate exhibited greater values than the ones with natural

aggregate, as expected. In all cases, the variability in the values calculated far exceeds any

intrinsic differences due to aggregate type.

5.8 Effect of Specimen Size on Mechanical Properties of Concrete

Compressive Strength

Size factors pertaining to cylindrical specimens for mixes at various ages are

tabulated in Table 5.11. A size factor is calculated as the ratio of the compressive strength of

small cylinders to that of large cylinders, expressed as a percentage. Small cylinders, usually

have 5 to 8% higher strength than large cylinders (Mehta and Monteiro, 1993); therefore, size

factors are expected to be greater than 100%. From Table 5.11, it can be noticed that the size

factors obtained in this project do not follow any particular pattern. It can be seen that in for

mixes DN and AD, the compressive strength of the large cylinders was either higher than or

equal to that of the small cylinders at all ages.

85

Modulus of Rupture

Size factors for modulus of rupture results could not be calculated as either large

beams or small beams were tested at each age for all the mixes. Instead, a factor of 1.16 was

assumed based on past experience to convert the large beam values for mixes AN and DC to

the corresponding small beams values, so as to allow comparisons among aggregate and

microsilica types. Such comparisons suggest that the choice of the factor of 1.16 was

appropriate.

86

Table 5.1 Coefficients of Variation, COV (%), in Laboratory Test Results

Age (days) 7 28 56 90 Average Test

Mix: Undensified Natural (UN) f′c (LC) f′c (SC) 4.78 3.99 6.13 2.21 4.28

MR (LB) MR (SB) 4.17 13.38 4.17 7.24

Mix: Undensified Crushed (UC) f′c (LC) f′c (SC) 2.61 7.22 6.26 7.77 5.97

MR (LB) MR (SB) 1.13 9.66 11.85 7.55

Mix: Densified Natural (DN) f′c (LC) 2.87 1.77 0.78 1.90 1.83 f′c (SC) 8.95 5.40 4.58 6.31

MR (LB) 12.13 21.05 11.40 14.86 MR (SB)

Mix: Densified Crushed (DC) f′c (LC) 3.88 7.45 12.81 4.38 7.13 f′c (SC) 10.29 12.84 7.57 10.23

MR (LB) 21.18 6.51 9.29 12.33 MR (SB)

Mix: Abused Natural (AN) f′c (LC) f′c (SC) 6.13 7.69 2.11 7.14 5.77

MR (LB) MR (SB) 9.03 5.11 8.14 7.43

Mix: Abused Crushed (AC) f′c (LC) f′c (SC) 5.40 7.84 5.97 9.17 7.10

MR (LB) MR (SB) 9.49 8.01 4.68 7.39

87

Table 5.2 Average Compressive Strength for Large Cylinders (psi)

Age (days) UN UC DN DC AN AC

7 4333 4601 3663 3822 28 6861 6657 5488 5918 5506 5024 56 7129 7385 6156 6401 6074 6142 90 6316 7031 6421 6090

Note: The numbers in bold were used to calculate the best-fit values for each mix.

Table 5.3 Average Compressive Strength for Small Cylinders (psi)

Age (days) UN UC DN DC AN AC

7 5299 5592 4504 5134 3932 4293 28 6989 7711 5295 6030 5570 5545 56 7489 7816 5554 6856 5929 5786 90 7604 7942 6418 6873 6126 6441

Note: The numbers in bold were used to calculate the best-fit values for each mix.

88

Table 5.4 Relative Compressive Strength Values for Large Cylinders (%)

Mix Age (days) UN UC DN DC AN AC

7 79 78 67 76 28 100 100 100 100 100 100 56 104 111 112 108 110 122 90 115 119 117 121

Table 5.5 Relative Compressive Strength Values for Small Cylinders (%)

Mix Age (days) UN UC DN DC AN AC

7 76 73 85 85 71 77 28 100 100 100 100 100 100 56 107 101 105 114 106 104 90 109 103 121 114 110 116

89

Table 5.6 Best-Fit Compressive Strength Values for Large Cylinders (psi)

Age (days) UN UC DN DC AN AC

7 4774 4945 4333 4529 4068 4113 28 6238 6462 5662 5918 5315 5374 56 7129 7385 6471 6763 6074 6142 90 7448 7714 6760 7065 6346 6416

Note: The numbers in bold were used to calculate the best-fit values for each mix.

Table 5.7 Best-Fit Compressive Strength Values for Small Cylinders (psi)

Age (days) UN UC DN DC AN AC

7 4977 5592 4201 4499 4066 4293 28 6568 7378 5543 5936 5364 5664 56 7259 8155 6127 6561 5929 6261 90 7604 8543 6418 6873 6212 6559

Note: The numbers in bold were used to calculate the best-fit values for each mix.

90

Table 5.8 Average Modulus of Rupture Values for Beams (psi)

Age (days) UN UC DN DC AN AC

7 942 733 625 700 765 824 28 1021 988 747 702 973 946 56 902 828 755 686 919 938 90

Table 5.9 Relative Modulus of Rupture Values for Beams (%)

Age (days) UN UC DN DC AN AC

7 92 73 84 100 79 87 28 100 100 100 100 100 100 56 88 83 101 98 94 99 90

Table 5.10 Best-Fit Modulus of Rupture for Small Beams (psi)

Age (days) UN UC DN DC AN AC

7 722 733 700 691 735 751 28 851 864 825 815 867 885 56 902 916 875 864 919 938 90

Note: Large beams (6 × 6 × 21 in.) have been tested for Mixes DN and DC, whereas small beams (3½ × 4½ × 16 in.) have been tested for all other mixes. For mixes DN and DC, large beam lab data were converted to the small beam values in this Table through multiplication by a factor of 1.16, selected on the basis of past experience. The numbers in bold were used to calculate the best-fit values.

91

Table 5.11 Cylinder Size Factors (%)

Age (days) UN UC DN DC AN AC

7 104 113 97 99 100 104

28 105 114 98 100 101 105

56 102 110 95 97 98 102

90 102 111 95 97 98 102

Average by mix 104 112 96 98 99 103

Average by MS type

108 97 101

92

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Trend Curve

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Fig. 5.1 Trend Line Curves for Large Cylinders

93

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0 10 20 30 40 50 60 70 80 90 100

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Fig. 5.2 Trend Line Curves for Small Cylinders

94

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0 10 20 30 40 50 60 70 80 90 100Age, days

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Fig. 5.3 Trend Line Curves for Small Beams

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7 28 56 90Age, days

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Fig. 5.4 Effect of Microsilica Type on Large Cylinders with Natural Aggregate

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Fig. 5.5 Effect of Microsilica Type on Small Cylinders with Natural Aggregate

97

0

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7 28 56 90

Age, days

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Fig. 5.6 Effect of Microsilica Type on Large Cylinders with Crushed Aggregate

98

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Fig. 5.7 Effect of Microsilica Type on Small Cylinders with Crushed Aggregate

99

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7 28 56 90

Age, days

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ulus

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Fig. 5.8 Effect of Microsilica Type on Small Beams with Natural Aggregate

100

0

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7 28 56 90

Age, days

Mod

ulus

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Fig. 5.9 Effect of Microsilica Type on Small Beams with Crushed Aggregate

101

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Fig. 5.10 Effect of Aggregate Type on Large Cylinders with Undensified Microsilica

102

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Fig. 5.11 Effect of Aggregate Type on Small Cylinders with Undensified Microsilica

103

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Fig. 5.12 Effect of Aggregate Type on Large Cylinders with Densified Microsilica

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Fig. 5.13 Effect of Aggregate Type on Small Cylinders with Densified Microsilica

105

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Fig. 5.14 Effect of Aggregate Type on Large Cylinders with Abused Microsilica

106

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Fig. 5.15 Effect of Aggregate Type on Small Cylinders with Abused Microsilica

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0

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Fig. 5.16 Effect of Aggregate Type on Small Beams with Undensified Microsilica

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0

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Age, days

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Fig. 5.17 Effect of Aggregate Type on Small Beams with Densified Microsilica

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0

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Age, days

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Fig. 5.18 Effect of Aggregate Type on Small Beams with Abused Microsilica

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6 CONCLUSIONS AND RECOMMENDATIONS

6.1 Summary

This project explored the use of densified microsilica in concrete used in the

construction of pavements and structures by Ohio Department of Transportation

(ODOT). Of the various kinds of microsilica that are commercially available in the

market, only two were considered by the research team: undensified microsilica and

densified microsilica. A third type of microsilica, viz., abused microsilica, was also

explored. It is usually assumed that undensified microsilica will result in higher strengths

than the densified material, which is the form most commonly used in practice. Wishing

to compare densified microsilica’s performance to a worst case scenario, the investigators

prepared a quantity of abused microsilica by soaking the densified material in water and

drying it, thereby encouraging the formation of clumps. Microsilica thus prepared was

expected to in the weakest concrete, assuming that the consequence of densification is

lower strength. Comparing the engineering performance of densified microsilica to

undensified and abused materials brackets the range of situations that may be

encountered in the field.

Undensified microsilica is expected to pass the requirement that it should have a

fineness value greater than 90% found in American Standard for Testing and Materials

(ASTM) C 1240 – 01 Standard Specification for Use of Silica Fume as a Mineral

Admixture in Hydraulic-Cement Concrete, Mortar and Grout when it is wet-sieved in

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accordance with ASTM C 430 – 96 Standard Test Method for Fineness of Hydraulic

Cement by the 45-µm (No. 325) Sieve, whereas densified and abused microsilica may not

pass the sieve test. Abused microsilica was prepared by the research team in University

of Cincinnati concrete laboratory, while densified and undensified microsilica were

obtained free of charge from ELKEM chemicals. Cement, concrete admixtures, and

aggregate were provided free of charge by CEMEX, Master Builders, Inc., and Martin

Marietta Materials, respectively. The research team utilized the research facilities at

University of Cincinnati for performing the various tasks in this project.

After procuring all the materials needed for the project, the research team

conducted tests on the aggregates to determine their properties, and to formulate a mix

design for the concrete. Two kinds of coarse aggregate, viz., natural and crushed, with a

single gradation of No. 8 were used. ODOT Supplemental Specification 848 Bridge

Deck Repair and Overlay with Concrete Using Hydro-Demolition was used in preparing

the mix design, in conjunction with ODOT Item 499.03 Concrete-General:

Proportioning. Six concrete mixes were made with the three types of microsilica and the

two kinds of coarse aggregates.

Concrete specimens were tested to determine their compressive strength and

flexural strength. Cylinders and beams were cast for this purpose. Specimens were made

in two different sizes in each case, viz., small cylinders (4 × 8 in.), large cylinders (6 × 12

in.), small beams (3½ × 4½ × 16 in.), and large beams (6 × 6 × 21 in.). Cylinders were

tested to calculate compressive strength, whereas beams were used to determine flexural

strength. Three cylinders and three beams were tested at each age in most cases. Large-

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sized specimens were used for mixes made from densified microsilica, whereas small

specimens were used for the other mixes. The decision to change the specimen size from

large to small was taken by the investigators as it helped achieve higher efficiency and

consistency, without deviating from the ASTM specification that requires the minimum

specimen dimension to exceed three times the nominal maximum size of the coarse

aggregate (ASTM C 192/C 192M – 00 Standard Practice for Making and Curing

Concrete Test Specimens in the Laboratory). The same mixing, casting and testing

procedures were followed for all six mixes in order to maintain consistency.

Tests were also conducted on the microsilica material itself to check if it passes

the sieve test. The facilities at ODOT laboratory were utilized in conducting these tests.

The effect of densification of microsilica on its properties was evaluated from the results

of the above tests.

6.2 Conclusions

During tests conducted at the ODOT laboratory on the three microsilica types

used in this project, it was observed that none of the materials could actually meet the

ASTM C 1240 – 01 requirement when subjected to the sieve test of ASTM C 430 – 96.

Test results were not meaningful, since negative fineness values were obtained for some

of the samples. Even undensified microsilica could only meet the specifcation when a

non-prescribed brush was used. Therefore, it can be concluded that the sieve test is not

effective in predicting the performance of microsilica concrete that is to be used for

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ODOT projects, and that it is not appropriate for ODOT’s assessment of the suitability of

microsilica for use in concrete.

The tests conducted on the concrete specimens indicate that those made with

undensified microsilica show higher flexural and compressive strengths than concrete

made with densified microsilica and abused microsilica, for both natural and crushed

aggregate. The strengths of undensified microsilica concrete were followed by that of

densified and abused microsilica concretes in decreasing order. Trends observed in

almost all mixes with respect to increase in strength with age, microsilica type, aggregate

type and specimen size were as expected. The strength of concrete specimens from all

mixes increased with age; moreover, the strength increase was more rapid in the initial

ages than during later ages. Both large and small cylinders attained compressive

strengths in excess of those envisaged ODOT 499.03 at 28 days of curing. Even though

lower compressive strengths were obtained for densified and abused microsilica

concretes compared to undensified, they still had adequate strength as required for ODOT

projects. Therefore, densified microsilica can be used for construction of pavements and

bridges by ODOT even though it may fail the sieve test.

Just like compressive strength results, the modulus of rupture of undensified

microsilica concrete was greater than corresponding values of densified and abused

microsilica concretes, when natural aggregate was used. Yet, the modulus of rupture of

abused microsilica was nearly equal to that of undensified microsilica, and was greater

than that of densified microsilica concrete at all ages, for both types of aggregate.

114

Coefficients of variation were higher for the modulus of rupture than the compressive

strength results.

Concrete made with crushed aggregate showed higher compressive and flexural

strengths than concrete made with natural aggregate in most of the mixes. As noted

earlier, a combination of either three large cylinders with a small cylinder, or three small

cylinders with a single large cylinder, was used for any given mix to determine

compressive strength. It was observed that the small sized concrete specimens yielded

greater compressive strengths than large sized specimens.

As noted earlier, the compressive and flexural strengths of abused microsilica did

not differ much from that of densified microsilica. The abused microsilica was intended

to represent the worst possible situation that might arise in the field. The clumps formed

during the abusing process were broken using a trowel; therefore, it can be said that the

clusters of microsilica that are formed in the field due to moisture can easily broken

during the mixing process.

6.3 Recommendations

During the sieve tests conducted at ODOT, none of the three types of microsilica could

pass the ASTM C 1240 – 01 requirement. On the other hand, when results from

compressive and flexural strength tests are considered, all microsilica types achieved

strengths in excess of those envisaged by ODOT Item 499.03, irrespective of whether

they passed the No. 325 sieve test or not. Therefore, it is recommended that the No. 325

sieve test be abandoned as a consideration in assessing the suitability of microsilica for

115

use in concrete. This is not to imply that densification is no longer a concern for

microsilica users, nor that the No. 325 sieve test does not serve a useful purpose when

used by the manufacturers for quality assurance, as it was originally conceived to do The

recommendation simply asserts that as the No. 325 sieve is conducted by an agency such

as ODOT, it yields no meaningful information. It is also recommended that the

microsilica should be stored for limited time only, in areas of low humidity at room

temperatures, and that the mixing process should be careful and thorough, to limit the

amount of densification at mixing, and to permit any bonds to be broken.

6.4 Implementation Plan

IMPLEMENTATION STEPS & TIME FRAME: The recommendations above

can be implemented immediately by any ODOT District including microsilica in its

concrete mix design.

EXPECTED BENEFITS: The main benefits from this research will derive from

the use of densified microsilica from respected manufacturers in pavement and bridge

construction, if such use is justified based on the results from other, more specific and

expensive, studies. Another benefit will derive from the elimination of the No. 325 sieve

test at the ODOT laboratory for the purpose of assessing the suitability of microsilica for

use in concrete mixes.

EXPECTED RISKS, OBSTACLES, & STRATEGIES TO OVERCOME THEM:

It is anticipated that there may be a hesitation to abandon what may currently be the only

test conducted at the ODOT laboratory in order to assure the quality of densified

116

microsilica used in pavement and bridge construction. It is suggested that ODOT make

more stringent its microsilica procurement process, in order to ensure that material is

obtained from reliable manufacturers alone, whose declarations of suitability may be

accepted with confidence. The possibility of bonding the manufacturer to the

performance of the pavement or bridge concerned may also be considered.

OTHER ODOT OFFICES AFFECTED BY THE CHANGE: Any ODOT District

including microsilica in its concrete mix design.

PROGRESS REPORTING & TIME FRAME: To be determined by ODOT.

TECHNOLOGY TRANSFER METHODS TO BE USED: The Final Report from

this study will be made available to interested parties, either in hard copy, or in electronic

form, the latter to include either Word .doc format or pdf. At least one refereed journal

paper documenting this investigation will be prepared within a year from the completion

of this contract.

IMPLEMENTATION COST & SOURCE OF FUNDING: There are no costs

associated with implementing the findings of this study.

117

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