concrete mix design - (part 1 & part 2)ppt7

Concrete Mix Design

CE2155 Structural Mechanics and Materials

byAssoc Professor T. H. Wee

Department of Civil EngineeringEmail: [email protected]

MIX PROPORTION

CE 2155 Structural Mechanics and Materials

Strength Workability Durability (with good resistance to exposure

environment eg. chloride attackcarbonationacid attacksulfate attackalkali-aggregate reaction, etc.)

Mix Proportion, Type of Cementitious Materials(including admixtures) used, etc.

Control of Non-Structural Cracks

(eg. plastic settlement crackplastic shrinkage crackthermal crack , Delayed Ettringite Formation due to high temperature rise etc.)

Good Concrete Practice (Handling)Quality Control of Concrete ProductionInvestigation (Destructive & Non-destructive Tests)

The successful use of concrete depends on: Proper design (taking the durability & deformation, etc. into consideration) Choice & Supply of sound constituent materials Proper handling & production process

W S

C G

(for 1 m3 of concrete)

An over-view of the requirements of good concrete in construction industry

MIX PROPORTION


Therefore, before starting to specify (or design), it is necessary to consider what properties/performance we want the concrete to have, and also what properties we do not want it to have.

Basic Considerations

Basic Considerations:

Workability - Minimum workability for adequate placement Compressive Strength -

Generally 28 days Other ages depending on the application

Durability - Chemical attacks Control of non-structural cracking, especially on early-

age thermal cracking and DEF Permeability/water absorption Repeated freeze/thaw action

Economy

MIX PROPORTION


Tests for Compressive Strength

The specimens are either: Cubes - 100 mm & 150 mm, or Cylinders - 150 mm dia. and 300 mm long

- 100 mm dia. and 200 mm long

After fully compacting the concrete into the moulds, the reference method is to cure the specimens in water at a controlled temperature.

The specimens are tested at a fixed age (usually at 7 and 28 days) in compression to failure, the load being applied at a controlled rate.

MIX PROPORTION


Compressive Strength Of Concrete When designing a structure, the designer will specify that the concrete

should have a given strength ( characteristic strength). When producing concrete, the mix is designed to have a specific mean strength.

Tests on samples show that the actual strength ( tested strength) deviates from the mean. The deviation or the spread of the test results depends on how closely the production process is controlled, and upon the properties of the individual materials in the mix.

The spread of test results approximates to the normal distribution curve, where is the mean strength and the spread of the results is measured by the standard deviation, .

For a set of n test results these are given by :-

However, when testing concrete it is usual to have only a small number of test results, so must be increased thus :-

x

StrengthPro

babi

lity

dens

ity

Poor control

Good control

X

MIX PROPORTION


General view of Quarry and Crushing plant installationsat KARIMUN, INDONESIA

MIX PROPORTION


Blasting of rock in action

Concrete Making Materials Aggregates

MIX PROPORTION


It can be seen from the normal distribution curve that the mean strength of a material is unsuitable for design purposes since a 50% of all test results might be expected to fall belowthe mean.

Based on BS EN 206, when concrete is classified with respect to its compressive strength of concrete, Table 7 (pls refer to next slide) apply.

Compressive Strength Of Concrete

Strength

Area = 0.05

Good control

Mean1.64 fcu

Prob

abilit

y de

nsity

From the diagram it is seen that :Mean strength = fcu + 1.64

Therefore, to obtain a concrete with characteristic strength 35 N/mm2 from a concrete batching plant for which a standard deviation of 5 N/mm2 is expected, the required mean strength will be :-

35 + (1.64 5) = 43.2 N/mm2

In SS and BS EN 206, the characteristic compressive strength at 28 days, fcu , is defined as value of strength below which 5% of the population of all possible strength determinations of the volume of concrete underconsideration, are expected to fall.

Mean strength = fcu + 1.64

MIX PROPORTION


BS EN 206 Part 1: 2000The strength grade of concrete should be selected from the table below:

MIX PROPORTION


Compressive Strength Of ConcreteTests are carried out on 150 mm cubes at an age of 28 days to give characteristic strength ( fcu).The concrete grade is expressed as Cn, where n is the cube strength fcu. (e.g. C40)

Poor quality control produces a wider spread of results, which leads to higher value of the standard deviation, .Very good quality control produces a very steep curve, and it can be seen that the better the control the lower the mean strength has to be produced for a specified value of fcu.

Strength

Area = 0.05

Good control

Mean1.64 fcu

Prob

abili

ty d

ensi

ty

Strength

Vgoo

dco

ntro

l Good control

Prob

abili

ty D

ensi

ty

1.64 - goodfcu1.64 - very good

Mean (vgood)

Poor control

Mean (good)

Mean (poor)

1.64 - poor

MIX PROPORTION


For the purpose of compliance based on estimated 28-day cube strength, the conditions are:

C20

C20

Acceptance of Concrete Strength - SS and BS EN

Note: The above requirement is more stringent when compared to ACI 301-05 (Specifications for Structural Concrete)

MIX PROPORTION


For a particular supply of a Grade 40 concrete, six batches were sampled each day for testing. The 28-day strength results for the first 12 batches are as follows:

Example:

OKFAIL40.841.012

OKFAIL40.142.511

OKFAIL40.143.010

FAILFAIL40.136.59

OKFAIL41.638.58

OKOK43.042.57

OKOK43.143.06

OKOK43.042.55

OKOK43.644.04

OKOK43.743.03

OKOK44.042.52

OKOK--45.51

Criteria BCriteria ARolling 4 AverageStrengthBatch No.

MIX PROPORTION


To provide structural safety, continuous control is necessary to ensure that the strength of the concrete as furnished is in satisfactory agreement with the value called for by the designer.

The ACI Code specifies that a pair of cylinders must be tested for each 150yd3 of concrete or for each 5000ft2 of surface area actually placed, but not less than once a day.

To ensure adequate concrete strength in spite of scatter, the ACI Code stipulates that concrete quality is satisfactory if

(1) no individual strength test results (the average of a pair of cylinder tests) falls below the required fc (specified strength) by more than 500 psi (= 3.5 MPa) when fc is 5000psi (=35 MPa) or less or by more than 0.10 fc when fc is more than 5000 psi (i.e. 35 MPa), and

(2) every arithmetic average of any three consecutive strength tests equals or exceeds fc

Acceptance of Concrete Strength ACI 301

MIX PROPORTION


Selection of Materials

Primary materials

Cement Water Aggregate

Coarse aggregate Fine aggregate (sand)

Optional materials (used upon requirement for specific application/job)

Mineral Admixture (Supplementary Cementing Materials) Chemical admixture Fiber Air entraining agent

W S

C G


MIX PROPORTION


Workability (flowability): For a given slump

water requirement when Max aggregate size Content of angular or rough-textured aggregate

particles Content of entrained air

CohesivenessCohesiveness Sand/coarse aggregate ratio Partial replacement of coarse sand by a fine sand At given w/c ratio, cement/aggregate ratio

W S

C G


Some basic rules for workability

MIX PROPORTION


Water-cement ratio (w/c)

Abrams Law: For given materials, the strength depends only on one factor the ratio of water to cement

c = compressive strengthA = empirical constant (96.5 MPa)B = Constant that depends mostly

on cement properties (~4)w/c = water to cement ratio by

weight

BA

cwc )/(5.1=

Fundamentals of Mix Design

MIX PROPORTION


Mix Design Process

Determine the job parameters Compressive strength Durability requirements? (w/c, cement content and

cement type, etc) Slump Aggregate properties, max. aggregate size Permissible allowable defects and standard deviation

Design the mix proportionsw/c ratio, cement ,water, aggregates and admixtures, etc.

Calculate batch weights Adjusting to the batch weights based on trial mix*

*There are various concrete mix design methods (eg. DOE & ACI Methods ). These methods are based on simplified classifications for type and quality of materials and it still remains to check whether or not the particular aggregates and cement selected for use in a given case will behave as anticipated. This is the object of making the trial mix, and the subsequent feedback of informaton from the trial mix is an essential part of the mix design process.

MIX PROPORTION


DOE Method of Mix Design British Method

The British method of concrete mix design, popularly referred to as the "DOE method", is used in the United Kingdom and other parts of the world and has a long established record.

The DOE method is based on various assumptions and requirements: Mixes are specified by the weights of the different materials

contained in a given volume of fully compacted concrete. It is assumed that the volume of freshly mixed concrete

equals the sum of the air content and of the absolute volumesof its constituent materials.

It is assumed that the strength of a concrete mix depends on: The Free water/Cement Ratio; The Coarse Aggregate Type; The Cement Properties.

MIX PROPORTION


DOE Method of mix deisgn British method

It is assumed that the workability of a concrete mix depends Primarily on The free water content; The fine aggregate type and, to a lesser degree, the coarse

aggregate type; The maximum size of coarse aggregate.

secondarily on: The percentage of the fine aggregate as a proportion of the

total aggregate content. The grading of the fine aggregate. The free water/Cement Ratio;

The DOE Method also provides guidance on the effects of air entrainment in a concrete mix.

DOE Method of Mix Design British Method

MIX PROPORTION


DOE Method of Mix Design Step 1

To Determine Free Water/Cement Ratio Required for Strength

Certain strengths are assumed at a w/c ratio of 0.5 for different cements and type of aggregate (refer to Table 2 given in the next slide). We mark a point corresponding to this strength at w/c ratio of 0.5 in Figure 4 (given in the next slide), through this point we now draw a curve parallel to the neighboring curves. Using this new curve, we read off the w/c ratio corresponding to the specified target mean strength.

A possible need for a lower w/c ratio for reasons of durability must not be forgotten.

The concept of target mean strength is introduced, this being equal to the specified characteristic strength plus a margin to allow for variability.

Mean strength = fcu + 1.64

MIX PROPORTION


DOE Method of Mix Design Stage I

Table 2 - Approximate compressive strength of concretes made with a free water/cement ratio of 0.5

Figure 4 Relation between compressive strength and free water/cement ratio

MIX PROPORTION


To Determine Free Water Content Required for Workability

This step is to determine the water content for the required workability, expressed either as slump or as Vebe time, recognizing the influence of the maximum size of aggregate and its type, namely crushed or uncrushed (refer to Table 3 given below)


Table 3 - Approximate water contents required to give various levels of workability

MIX PROPORTION


To Determine Required Cement Content Obtain the minimum cement content, which is required for

strength, by dividing the free water content obtained in Step 2 by the free water/cement ratio obtained in Step 1.

Check the minimum cement content, which is required for strength, against the maximum cement content, which is permitted, and give a warning if the former exceeds the latter.

Check the minimum cement content, which is required for strength, against the minimum cement content, which is allowable for durability, and adopt whichever is greater to be the cement content in the mix.

Divide the free water content by the cement content used in the mix to obtain a modified free water/cement ratio.


MIX PROPORTION


To Determine Total Aggregate Content Estimate the fresh/wet concrete density (using Figure 5 given

below) of fully compacted concrete based on the free water content estimated in Step 2 and specific gravity of aggregate inthe saturated surface-dry condition (SSD).

Total aggregate content (SSD)= Fresh density - (cement content + water content)


Figure 5 Estimated fresh/Wet density of fully compacted concrete (specific gravity is given for saturated surface-dry aggregate

MIX PROPORTION


Determine Fine and Coarse Aggregate Contents (Refer to Figure 6 given next slide)

Either use a specified value of the percentage of fine aggregate, or obtain the percentage of fine aggregate, which will provide the desired workability for concrete made with the given grading of fine aggregate (defined by its percentage passing a 600 m sieve), maximum size of coarse aggregate and the free w/c ratio obtained in Step 3.

Calculate the fine and coarse aggregate contents from the total aggregate content obtained in Step 4 and the percentage of fine aggregate.


MIX PROPORTION


Figure 6 Recommended proportion of fine aggregate (expressed as percentage of total aggregate) as function of free water/cement ratio for various workabilitiesand maximum sizes

*The numbers refer to percentage of fine aggregate passing 600 m sieve


**

MIX PROPORTION


Figure 6 (Cont) Recommended proportion of fine aggregate (expressed as percentage of total aggregate) as function of freewater/cement ratio for various workabilities and maximum sizes



*

MIX PROPORTION


Figure 6 (Cont) Recommended proportion of fine aggregate (expressed as percentage of total aggregate) as function of freewater/cement ratio for various workabilities and maximum sizes



*

MIX PROPORTION


Recommended Limiting Values for Composition & Properties of Concrete SS EN and BS EN 206

MIX PROPORTION


DOE Method of Mix Design Mix Design Form

MIX PROPORTION


Design concrete with a mean 28-day compressive strength (measured on cubes) of 44 MPa (which is equivalent to a cylinder compressive strength of 35 MPa); a slump of 50 mm; uncrushed fine and coarse aggregate with a maximum size of 20 mm; both with specific gravity of 2.64; 60 percent of fine aggregates passes the 600 m sieve; no air entrainment required and ordinary Portland cement to be used.

DOE Method of Mix Design Example

Solution:

Step 1: From Table 2, for OPC and

uncrushed aggregate, we find the 28-day strength to be 42 MPa.

Table 2 - Approximate compressive strength of concretes made with a free water/cement ratio of 0.5

MIX PROPORTION


DOE Method of Mix Design Example (Step 1)

We enter this value on the ordinate corresponding to w/cratio of 0.5 in Figure; this point marked A.

Through A, we draw a line parallel to the nearest curve until it intersects the ordinate corresponding to the specified strength of 44 Mpa; this is point B.

The ordinate through this point gives the w/c ratio of 0.48.

MIX PROPORTION



Step 2: From Table 3 given below, for 20 mm uncrushed aggregate and a slump of 50

mm, we find that the water requirement to be 180 kg/m3.

Step 3: The cement content is 180/0.48 = 375 kg/m3.

Table 3 - Approximate water contents required to give various levels of workability

MIX PROPORTION


Step 4: From the Figure 5 below, for a water content of 180 kg/m3 and 20 mm aggregate

with a specific gravity of 2.64, we read off the fresh density of concrete of 2400 kg/m3.

The total aggregate content is thus:

=(2400 375 -180) kg/m3

=1845 kg/m3


Figure 5 Estimated fresh density of fully compacted concrete (specific gravity is given for saturated surface-dry aggregate)

MIX PROPORTION


Step 5:

In Figure 6, we find the particular diagram for the 20 maximum size of aggregate and a slump encompassing the value of 50 mm.

On the line representing fine aggregate with 60 percent passing the 600 m sieve, at a w/c ratio of 0.48, the proportion of fine aggregate is 32 percent (by mass of total aggregate).

Hence, the fine aggregate content is 0.32 x 1845 = 590 kg/m3 and the coarse aggregate content is 1845 590 = 1255 kg/m3.


Figure 6 Recommended proportion of fine aggregate (expressed as percentage of total aggregate) as function of free water/cement ratio for various workabilities and maximum sizes

MIX PROPORTION


Fine Aggregate : Absorption Capacity = 1.5% Total Moisture Content = 4% Free Moisture Content = (4 1.5) %=2.5 %

Fine Aggregate Content = 590 kg/m3 of concrete (on SSD basis - from previous slide)= 590 + 590 (2.5 %) = 605 kg/m3 (if the above wet sand is used)

The extra water due to the use of wet sand has to be deducted accordingly.

Moisture Content (MC) of Aggregate - Example

MIX PROPORTION


Estimation of Required Average Compressive Strength from the Specified Compressive Strength

Because of the Statistical distribution of concrete strength

Concrete cannot be designed on the basis of specified strength

In order to compute the required average compressive strength of concrete mix, three information are needed:

Specified compressive strength fc Variability or standard deviations of concrete Allowable risk of having concrete with an unacceptable

strength

ACI Approach to Variability

MIX PROPORTION


Estimation of Required Average Strength (ACI 301)

The required average compressive strength fcr is determined as the larger value of the following two criteria:

(1) The probable frequency of the average of any 3 consecutivetests below specified strength fc should not exceed 1 in 100

fcr = fc + 2.33 s/ = fc + 1.34 swhere fcr = required average compressive strengthfc = specified compressive strengths = standard deviation

(2) (a) For fc 35 MPa, the probable frequency of tests more than 3.5 MPa below fc should not exceed 1 in 100

fcr = fc + 2.33 s - 3.5 (MPa)(b) For fc > 35 MPa, the probable frequency of tests below

0.90fc should not exceed 1 in 100fcr = 0.90 fc + 2.33 s

3

MIX PROPORTION


(When Data Are Available to Establish a Standard Deviation)

Specified compressive strength, f'c, MPa

Required average compressive strength, f'cr, MPa

35f'cr = f'c+ 1.34sf'cr = f'c + 2.33s 3.5

Use larger value

> 35f'cr = f'c+ 1.34sf'cr = 0.90f'c + 2.33s

Use larger value


MIX PROPORTION


Number of testsModification factor for

standard deviation

Less than 15 Use Table given in the next slide

15 1.16

20 1.08

25 1.03

30 or more 1.00

Modification Factor for Standard Deviation ( 30 Tests)

s is multiplied by the above factor


MIX PROPORTION


(When There Are Insufficient Data to Establish S)

Specified compressive strength, f'c, MPa

Required average compressive strength, f'cr, (MPa)

Less than 20 f'c + 7.0

20 to 35 f'c + 8.5

Over 35 1.1f'c + 5.0

These estimates are very conservative, and should not be used for large projects (over-design, non-economical)


MIX PROPORTION


0.866d0.866d*

Confinement at both ends

Cube specimen will have higher strength due to more confinement !!!

Cylinder vs Cube Specimens

Compressive Strength Test: Cubes - 100 mm & 150 mm, or Cylinders - 150 mm dia. and 300 mm long

- 100 mm dia. and 200 mm long

* Effective distance of end restraining effect

MIX PROPORTION


Effect of specimen parameters on strength

Height/diameter ratio of cylinder , the strength In general, larger specimen have lower strength

Correlation between cube and cylinder strength

Commonly assumed to befcu = 1.25 fcyl

Fcu/fcyl ranges from 1.3 for low-strength concrete to 1.04 for higher strength concrete

Cylinders are cast and tested in the same position, whereas for cubes, the loading direction is perpendicular to the casting direction

Cylinder vs Cube Specimens

MIX PROPORTION


ACI Method of Mix Design - Ten Steps

1. Required information

2. Choice of slump

3. Choice of maximum aggregate size

4. Estimation of mixing water and the air content

5. Selection of w/c or w/cm

6. Calculation of cement or cm content

7. Estimation of coarse aggregate content

8. Estimation of fine-aggregate content

9. Adjustments for moisture in the aggregates

10. Trial batch

MIX PROPORTION


Step 1 Required information Sieve analysis of fine and coarse aggregate, fineness modulus Dry-rodded unit weight of coarse aggregate Bulk specific gravity of materials Absorption capacity, or free moisture in the aggregate Information on structure including the type and dimensions of

structural members, minimum space between reinforcing bars

Required strength Exposure conditions Relationship between strength and w/c for available combinations of cement and aggregate Job specifications [e.g., max w/c, min. slump, strength at early

age (normally 28d)]

ACI Method of Mix Design

MIX PROPORTION


Step 2 Choice of slump

Recommended Slump Ranges

Concrete constructionSlump, mm

Maximum MinimumReinforced foundation walls and footings 75 25

Plain footings, caissons, and substructure walls 75 25

Beams and reinforced walls 100 25

Building columns 100 25

Pavements and slabs 75 25

Mass concrete 50 25

Recommended Slump Ranges (ACI 211)

MIX PROPORTION


Step 3 Choice of maximum size of aggregate

Using a large max size of a well-graded aggregate will produce less void space than using a smaller size

Large aggregates minimize the amount of water required, therefore reduce the amount of cement required.

The maximum allowable aggregate size is limited by

the dimensions of the structural elements and space between reinforcement

capabilities of construction equipment


MIX PROPORTION


Step 4 Estimation of mixing water and the air content

The quantity of water required to produce a given slump is dependent on the max size, shape and grading of

aggregate, amount of entrained air not greatly affected by cement content

Estimation of water from Table given in the next slide if no data are available for a given aggregate

The recommendations in the Table are reduced for other aggregate shapes by the amount stated below.

kg/m3


MIX PROPORTION


Approximate Water and Air Requirements for Different Slumps and Maximum Sizes of Aggregate

Water, kg/m3 of concrete, for indicated sizes of aggregate

Slump, mm9.5 mm

12.5 mm

19 mm

25 mm

37.5 mm

50 mm

75 mm

150 mm

25 to 50 210 200 185 180 160 155 130 113

75 to 100 225 215 200 195 175 170 145 124

150 to 175 240 230 210 205 185 180 160

Approximate amount of entrapped air in non-air-

entrained concrete, percent

3 2.5 2 1.5 1 0.5 0.3 0.2

Non-air-entrained concrete

Based on well-shaped, angular coarse aggregate


MIX PROPORTION


Air entrainment requirements

Air entrainment is required whenever concrete is exposed to freeze-thaw conditions

Air entrainment is also used for workability The amount of the air required varies with exposure conditions mild: indoor or outdoor service where concrete is not

exposed to freezing and de-icing salts. AEA may be used to improve workability

moderate: some freezing exposure occurs but concrete not exposed to moisture

severe size of the aggregates


MIX PROPORTION


Approximate Water and Air Requirements for Different Slumps and Maximum Sizes of Aggregate


Slump, mm 9.5 mm12.5 mm

19 mm

25 mm

37.5 mm

50 mm

75 mm

150 mm

25 to 50 180 175 165 160 145 140 120 107

75 to 100 200 190 180 175 160 155 135 119

150 to 175 215 205 190 185 170 165 155 -

Recommended average total air content, percent, for level of exposure

Mild exposure 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

Moderate exposure 6.0 5.5 5.0 4.5 4.5 4.0 3.5 3.0

Severe exposure 7.5 7.0 6.0 6.0 5.5 5.0 4.5 4.0

Air-entrained concrete


MIX PROPORTION


Compressive strength at 28

days, MPa

Water/Cement Ratio by mass



45 0.37 -

40 0.42 -

35 0.47 0.39

30 0.54 0.45

25 0.61 0.52

20 0.69 0.60

15 0.79 0.70

If no historical data are available make trial batches with different w/c, establish a relationship

between strength and w/c

estimation of w/c for the trial mixes from the table given below


Step 5 Selection of w/c or w/cm for strength

MIX PROPORTION


5. Selection of w/c or w/cm for durability

Durability Checking w/c against the max. allowable w/c for exposure

conditions Generally, more severe exposure conditions require

lower w/c The minimum of the w/c for strength and durability is

selected for proportioning of the concrete


MIX PROPORTION


5. Selection of w/c or w/cm for concrete in severe exposure


MIX PROPORTION


Requirement for Concrete Exposed to Sulfates

Sulfate exposure

Sulfate (SO4) in soil, % by mass

Sulfate (SO4) in water, ppm Cement type

Maximum w/c-ratio, by mass

Minimum strength, f'c, MPa

Negligible < 0.10 < 150 No special type required

Moderate 0.10 to 0.20150 to 1500

II, MS, IP(MS), IS(MS), P(MS), I(PM)(MS), I(SM)(MS)

0.50 28

Severe 0.20 to 2.001500 to 10,000 V 0.45 31

Very severe > 2.00 > 10,000

V +pozzolansor slag 0.45 31


MIX PROPORTION


Step 6 Calculation of cement or cementitious material content

= mixing water (step 4) divided by the w/c (step 5) if the concrete is used in flatwork, check minimum

cement content requirement

Nominal maximum size of aggregate, mm

Cementing materials,kg/m3

37.5 280

25 310

19 320

12.5 350

9.5 360


MIX PROPORTION


V, workability (pavement) V, workability (pumping, shotcrete)

Maximum size of aggregate,

mm

Fineness modulus of sand

2.40 2.60 2.80 3.00

9.5 0.50 0.48 0.46 0.4412.5 0.59 0.57 0.55 0.5319 0.66 0.64 0.62 0.6025 0.71 0.69 0.67 0.65

37.5 0.76 0.74 0.72 0.7050 0.78 0.76 0.74 0.7275 0.82 0.80 0.78 0.76

150 0.87 0.85 0.83 0.81

Volume of Coarse Aggregate per Unit volume of Concrete

Step 7 Estimation of coarse aggregate content


MIX PROPORTION


For example, assume: Max. size of coarse aggregate 19 mm and Fineness modulus of sand = 2.8

From the previous table, Volume of coarse aggregate required for 1 m3 of concrete with the above aggregates is = 0.62

Assume: Dry rodded unit weight of coarse aggregate = 1567 kg/m3 (oven dry condition)

Estimated coarse aggregate content (Oven dry condition) = 0.62 x 1567 kg/m3 = 971.5 kg/m3

Estimated coarse aggregate content (SSD condition)= 971.5 x (1+Absorption capacity)

Step 7 Estimation of coarse aggregate content per m3 of concrete


MIX PROPORTION


Step 8 Estimation of fine aggregate content

Mass (Weight) methodWfa = Wc - Weight of other ingredients

Wfa = weight of fine aggregateWc = unit weight of concrete

Estimate according to Table below


MIX PROPORTION


Volume method The components weight and specific gravity are used to

determine the volumes of the water, coarse aggregate, and cement. These volume + volume of air are subtracted from a unit volume of concrete to determine the V of fine aggregate

convert the V to weight (generally using bulk SSD specific gravity)

Step 8 Estimation of fine aggregate content


MIX PROPORTION


Step 9 Adjustments for aggregate moisture

The mix proportions determined by steps 1 to 7 are assumed to be on a saturated surface dry (SSD) basis. If aggregate contains free moisture, the mixing water

should be and aggregates correspondingly according to the amount of free moisture in the aggregates.

If aggregate is air dry, the mixing water should be and aggregates correspondingly

Total water in aggregate absorption = free moisture


MIX PROPORTION


For example, assume Coarse aggregate with absorption capacity =1%, effective

absorption = 0.5%, and Fine aggregate with absorption capacity = 1.3%, total moisture

content 4.5% Assume a concrete mix proportion based on SSD: Cement = 400 kg/m3, Water = 200 kg/m3, Coarse aggregate

= 1050 kg/m3, Fine aggregate = 710 kg/m3, and Estimated unit weight = 2360 kg/m3

Actual mix proportion with the given aggregates CA: 1050 1050x0.5% = 1045 kg/m3

FA: free moisture = 4.5% 1.3% = 3.2%710 + 710x3.2% = 733 kg/m3

Water: 200 + 1050x0.5% - 710x3.2% = 182 kg/m3

Estimated unit weight = 2360 kg/m3

Step 9 Adjustments for coarse and fine aggregate moistures


MIX PROPORTION


Step 10 Trial batch

Purpose: Verifies that a concrete mixture meets design requirements prior to use in construction.

To determine Fresh concrete: slump, cohesiveness, segregation

tendency, unit weight, air content, finishing Hardened concrete: strength 28 days or other ages

To adjust concrete mixture accordingly Strength does not meet requirement (workability ok) Reduce w/c

o Keep water content unalteredo cement, aggregate


MIX PROPORTION


Step 10 Trial batch Adjust concrete mixture accordingly (contd) Workability does not meet requirement (strength ok) Keep w/c unaltered

o Slump too low water and cement content

( 6 kg/m3 water will slump by ~25 mm) Use WRA or superplasticizero Slump too high water and cement content the dosage of WRA or SP

Segregation fine aggregate and coarse aggregate accordinglyReplace coarse sand with a finer sand

Air content: 1% entrained air, water by 3 kg/m3


MIX PROPORTION


Design a concrete required for an exterior column located above ground level in an area where it will be wet and subjected to substantial freezing and thawing. The concrete is required to have an average 28-day compressive strength of 30 MPa. For the condition of placement, the slump should be between 75 and 100 mm. The column is 635 mm square with a minimum clear space for aggregate of 50 mm. The properties of the materials are as follows:

Cement Type I, specific gravity = 3.15Fine aggregate Bulk specific gravity (SSD) = 2.63; absorption

capacity = 1.3%; surface moisture = 4.2% based on SSD state; fineness modulus = 2.70.

Coarse aggregate Maximum size = 19 mm; Bulk specific gravity (SSD) = 2.68; absorption capacity = 1.0%; surface moisture = 0.5% based on SSD state; dry rodded unit weight = 1600 kg/m3

The sieve analyses of the coarse and fine aggregates fall withinthe limits specified in ASTM C33.

ACI Method of Mix Design Example

MIX PROPORTION


Solution:

Step 1: Required information This is already given in the previous slide.

Step 2: Choice of slump The slump also given, consistent with Table given below

ACI Method of Mix Design Steps 1 & 2

Concrete constructionSlump, mm

Maximum Minimum

Reinforced foundation walls and footings75 25

Plain footings, caissons, and substructure walls 75 25

Beams and reinforced walls 100 25

Building columns 100 25

Pavements and slabs 75 25

Mass concrete 50 25

Recommended Slump Ranges

MIX PROPORTION


Step 3: Maximum aggregate size The maximum aggregate size, 19 mm meets the limitations of

1) 1/5th of the minimum dimension between forms and2) 3/4th clear space.

Step 4: Estimation of mixing water and air content Since the concrete will be exposed to freezing and thawing, it must be air entrained. From the below table, the air content recommended for severe exposure is 6% and the water requirement is 180 kg/m3



Slump, mm 9.5 mm12.5 mm

19 mm

25 mm

37.5 mm

50 mm

75 mm

150 mm

25 to 50 180 175 165 160 145 140 120 107

75 to 100 200 190 180 175 160 155 135 119

150 to 175 215 205 190 185 170 165 155 -

Recommended average total air content, percent, for level of exposure

Mild exposure 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

Moderate exposure 6.0 5.5 5.0 4.5 4.5 4.0 3.5 3.0

Severe exposure 7.5 7.0 6.0 6.0 5.5 5.0 4.5 4.0

Table - Approximate Water and Air Requirements for Different Slumps and Maximum Sizes of Aggregate (for air-entrained concrete)

MIX PROPORTION


Step 5: Water/cement ratio From table give below, the estimate of required w/c ratio to a given 28-day compressive strength of 30 MPa is 0.45. This does not exceed the limits based on durability (Tables are given in the next slides)

ACI Method of Mix Design Step 5

Compressive strength at 28

days, MPa

Water/Cement Ratio by mass



45 0.37 -

40 0.42 -

35 0.47 0.39

30 0.54 0.45

25 0.61 0.52

20 0.69 0.60

15 0.79 0.70

Table- Relationship between w/c ratio and compressive strength

MIX PROPORTION



MIX PROPORTION



MIX PROPORTION


Step 6: Calculation of cement content The required cement content, based on the results of Steps 4 and 5, is 180/0.45 = 400 kg/m3

Step 7: Estimation of coarse aggregate content Interpolating in Table given below, for the fineness modulus of the fine aggregate of2.70, the volume of dry-rodded coarse aggregate per unit volume of concrete is 0.63. Therefore, the coarse aggregate will occupy 0.63m3/m3. The OD weight of CA is 0.63 x 1600 = 1008 kg. The SSD weight is 1008 x 1.01 = 1018 kg.


Maximum size of aggregate, mm

Fineness modulus of sand

2.40 2.60 2.80 3.00

9.5 0.50 0.48 0.46 0.44

12.5 0.59 0.57 0.55 0.53

19 0.66 0.64 0.62 0.60

25 0.71 0.69 0.67 0.65

37.5 0.76 0.74 0.72 0.70

50 0.78 0.76 0.74 0.72

75 0.82 0.80 0.78 0.76

150 0.87 0.85 0.83 0.81

Table - Volume of Coarse Aggregate per Unit volume of Concrete

MIX PROPORTION


Step 8: Estimation of fine aggregate content The fine aggregate content can be established either by the a) mass (weight) methodor b) absolute volume method. a) Mass (weight) method: From table given below, the estimated

concrete weight is 2280 kg/m3. The more exact calculation on

the following equation is

Um = 10 (2.66) (100 - 6) + 400 (1-2.66/3.15) 180 (2.66 - 1) = 2264 kg/m3 .

Therefore, the weight of fine aggregate(SSD) = 2264-180-400-1018 = 666 kg


MIX PROPORTION


Step 8: Volume method Knowing the weights and specific gravities of water, cement, and coarse aggregate and knowing air volume, we can calculate the volume per m3 occupied by the different ingredients.


Water 180/1000 0.180 m3

Cement 400/(3.15x1000) 0.127 m3

Coarse aggregate 1018/(2.68x1000) 0.380 m3

Air content 6/100 0.06 m3

Total volume 0.747 m3

Therefore, the fine aggregate must occupy a volume of 1 0.747 = 0.253 m3 . The required SSD weight of fine aggregate

= 0.253 x 2.63 x 1000 = 665 kg. This is essentially same as the weight calculated according to the weight method

MIX PROPORTION


Step 9: Adjustment of moisture in the aggregate Since the aggregates will be neither SSD nor OD in the field, it is necessary to adjust the aggregate weights for the amount of water contained in the aggregate (note that very dry aggregates will absorb water from the mix, and this too must be allowed for.). Only surface water need be considered; absorbed water does not become part of mixing water. For the given moisture contents the adjusted aggregate weights become:


Coarse aggregate 1018 x 1.005 1023 kg/m3

Fine aggregate 665 x 1.042 693 kg/m3

Surface moisture contributed by the coarse aggregate is 0.5%; by the fine aggregate 4.2%. Therefore, the mixing water = 180 - 1018 x 0.005 - 665 x 0.042 = 147 kg/m3

MIX PROPORTION


Step 10: Trial batch A trial batch is now made using the proportions calculated. The properties of the concrete in the trial batch (including unit weight) must be compared with the desired properties, and the mix design must be corrected as described. To illustrate this process, consider the following trial batch results:

Small trial batched are prepared based on the example for 0.015 m3. The desired slump is 100 mm and the desired air content is 6%. During the course of trial match, we find that extra water is needed to achieve the desired slump. The final properties of the concrete are slump = 75 mm, air content = 5% and unit weight = 2286 kg/m3. The weights used in the trial batches are expressed in terms of SSD aggregate:


Cement 6.00 kg

Coarse aggregate (SSD) 15.27 kg

Fine aggregate (SSD) 9.98 kg

Water 2.84 kg

Total batch weight 34.09 kg

MIX PROPORTION


Based on the manufacturers recommendations, air entraining agentwas added at a rate of 0.23 L/m3.

The batch weights and unit weight can now be used to determine the actual quantities used on a m3 basis.

Batch factor = 2286/34.09 = 67.06 batched/m3


Cement 6.00 x 67.06 = 402 kg

Coarse aggregate (SSD) 15.27 x 67.06 = 1024 kg

Fine aggregate (SSD) 9.98 x 67.06 = 669 kg

Water 2.84 x 67.06 = 190 kg

Note that, due to extra water required and lower air content, the actual weights of the ingredients differ from the original values.

The mix design must now be modified to obtain the desired slump,air content and w/c ratio.

MIX PROPORTION


The water content for a 100 mm slump will be 190 kg/m3 + 6 kg/m3[to increase the slump from 88 mm] 3 kg/m3 [to take into account the extra slump that will be obtained as the 5% actual air content is increased to the desired 6%] = 193 kg/m3.We now proceed to Step 5 to recalculate the batch weights.Step 5: w/c = 0.45 is unchangedStep 6: Cement content = 193/0.45 = 429 kgStep 7: Coarse aggregate (SSD content ) = 1018 kg is unchangedStep 8: Fine aggregate (SSD) content : for this problem we will use the volume method.


Water 193/1000 0.193 m3

Cement 429/(3.15x1000) 0.136 m3

Coarse aggregate 1018/(2.68x1000) 0.380 m3

Air content 6/100 0.06 m3

Total volume 0.769m3

Absolute volume of fine aggregate = 1-0.769 = 0.231 m3.SSD weight of fine aggregate = 0.231x2.63x1000 = 608 kg.

MIX PROPORTION


Chemical admixturesDepend on applications

Mineral admixturesDepend on applicationsWhen used to replacement on mass basis, need to adjust

aggregate as specific gravity of cement & mineral admixtures are different

Admixtures

Use of Admixtures

concrete mix design - (part 1 & part 2)ppt7

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strength deviates

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