“EXPERIMENTAL STUDY OF CORE DIAMETER VARING H/D RATIO ON CONCRETE CORE STRENGTH”

MID-TERM SUBMISSION REPORT

SUBMITTED BYPAWAN KUMAR AGGARWAL

UNIVERISTY ROLL NO.81402105007IN PARTIAL FULFILLMENT FOR THE AWARD OF DEGREE

OFMASTER OF TECHNOLOGY

INSTRUCTURAL ENGINEERING

Under the Guidance of SH.SANJEEV NAVAL DR.SANJAY SHARMA

ASSTT.PROF. & HEAD PROF. & HEADDEPTT.OF CIVIL ENGG. DEPTT.OF CIVIL ENGG.

DAVIET JALANDHAR NITTR CHANDIGAR HAT

PTU REGIONAL CENTRE

DAV INSTITUTE OF ENGINEERING & TECHNOLOGYJALANDHAR

2013

TABLE OF CONTENTS

S.NO. TITLE PAGE NO.

1. Introduction 5

2. Literature Review 7

3. Objectives 9

4. Material and Methods 10

5. Results and Discussion 13

6. Summary and Conclusion 17

7. References 18

LIST OF FIGURES4.1 Compression Testing Machine 11

4.2 Failure of cube at Maximum Load 11

4.3 Core drilling Machine 12

5.1 Correction factor of H/D ratio and core diameter 50mm 15

5.2 Correction factor of H/D ratio and core diameter 75mm 15

5.3 Correction factor of H/D ratio of core as per IS:516 16

LIST OF TABLES

4.1Compressive Strength of Standard Cubes and core 50mm

and 75 mm Diameter12

5.1 Correction factors for 75mm diameter cores 14

5.2 Correction factors for 50mm diameter cores 14

5.3

Comparison of actual compressive strength from C.T.M., and core strength as calculated as per curve of IS:516 and

fro equation no. 1 & 2 16

1. IntroductionFor the design of any concrete structure , the compressive strength of concrete is of prime

importance .If the strength of concrete is known, its capacity to withstand applied loads of any

nature can be ascertained. To check the quality and compressive strength of concrete, standard

test specimens are taken at the time of construction. These specimens are prepared and cured as

specified by relevant codes. The compressive strength of these specimens is generally tested at

the age of 7 days and 28 days. However 28 days strength is generally taken as compressive

strength of concrete.

But this 28 days strength of standard specimen is not always the actual strength of concrete

because:

i) It depends upon curing and compaction of concrete

ii) It is not possible to prepare and test standard specimens at a later stage, if the

current strength of structure is to be determined to check whether or not strength and

durability of concrete is adequate.

To overcome this problem one of the method is core testing of suspected structural elements. In

this method cores are drilled from the structure as per IS: 1199-1959[15] and tested for

compressive strength.

As per BS: 1881: part 4, 1970(2)[5], the need for compressive strength of cores to yield an

estimate of strength of suspected concrete is well established. This code also contains

recommendation for core testing of concrete. Also more detailed and comprehensive

recommendations for core testing is provided by Concrete society technical report no.11 of

1976[7].This report gives detailed procedure for testing and interpretation of results of core

testing. Both BS:1881 and the report of concrete society recommends the diameter of cores as

150mm and 100mm. However is not always possible to obtain cores of this diameter, with

required minimum H/D ratio of 1.0.This may be due to limitation of dimensions of the member

or critical reinforcement location. To overcome this problem CSTR 11,1976[7] has allowed the

use of 50mm and 75mm diameter cores.BS:1881, part (120) 1983[3], and ASTMC 42-

90,1994[2], allows a minimum core diameter of 102mm, provided that diameter of core is at

least 3times greater than maximum aggregate size of concrete mix. As per Bartlett F.M. et.al

1994[4], the diameter of core affects the compressive strength of cores. Arioz , O. et.al, 2006[1],

concluded through his research that though , smaller diameter cores should give higher average

strength than larger diameter cores for the same H/D ratio, due to size effect, but is not always

true , since the concrete is susceptible to micro cracking during drilling operation.

From the above discussion it is evident that core strength is dependent of H/D ratio and

diameter of the core specimen.IS:1199-1959[15] recommends the H/D ratio of core as 2, and IS:

516-1959[14], gives the correction factor to be applied , if the H/D ratio is less than 2.0.But this

relation does not give particular values of different core diameter.

2. LITRATURE REVIEWTo find the in situ strength of concrete, generally core strength test is used. In this method cores

are drilled from the structure, and are tested for compressive strength. Compressive strength of

the core is then converted into equivalent cube strength. Though it seems to be a simple

procedure, it is not easy to interpret the results. The following are some of the factors which

affect the accuracy of results:

1. Height to Diameter Ratio (H/D) ratio.

2. Moisture condition at the time of testing.

3. Age of concrete.

4. Strength level of concrete.

5. Disturbance during drilling operation of cores.

Also the condition of placing, compacting and curing may not be the same at site which were

used for preparing standard specimens.

J.H. Nevile et.al.(1956) [17], suggested that measured core strength increases with the decrease

of core size. The smaller size cores give more variable results. Mcintyre. et. al(1990) [20]

through their studies concluded that with the decrease in diameter, the homogeneity of the

material in test specimen diminishes, thereby affecting the internal failure mechanisms of the

specimen. Indelicato F.et.al(1993) [9] , showed that with the decrease in diameter, the

assessment of correct results become hard. The core strength obtained by converting these to

standard cube or cylinder may not give true value. Neville.et.al (1995) [21] , showed that as the

core diameter decreases, the volume the specimen also decreases significantly for a given H/D

ratio. Hence the strength of specimen decreases with the increase in its size. Bungey J.H. Et.al

(1996) [6] , showed that the properties of insitu concrete will vary with in a member , due to

difference in compaction and curing as well as non uniformity of material. The effects of

drilling are more pronounced for small diameter cores, as suggested by Barlet F.M. et al.

(1997) [4] They showed that is because damaged parts of the core have a constant thickness

independent of core diameter. They used 102mm diameter and 204mm height of the cores as

standard specimen and concluded that a factor of 0.98 and 1.06 has to be applied for cores of

152mm and 51mm respectively.

Indelicato F.et.al(1997) [10] through their tests on cores , showed that choice of smaller

diameter of cores is motivated by the need to reduce costs and minimize the damage to the

structure, and by possibility of drilling out samples more easily by means of smaller tools for

these reasons concrete society has recommended the ise of 50mm and 75 mm diameter cores.

Indelicato F.et.al (1998 and 1999) [11,12], concluded that potential strength of concrete is related

to quality of material used and is an estimate of standard test specimen. However in situ

strength of the concrete as it exists in construction is the end result of quality of concrete used

in the construction. Yusuhi Shmizu(2000) [23], studied the effect of design strength and date of

completion upon the compressive strength and showed that:

i) Compressive strength changes by the floor in the same building even though placing

concrete was same

ii) Average compressive strength increased and exhibited less chance to become low,

when date of completion become younger.

Erdogan T.Y. et. al(2003) [8] , indicate that variability of strength of small diameter cores is

greater than that of large diameter cores , even when strength are very close to each other . Jee

Namyong .et.al(2004) [18] in their study presented a regression equation for predicting

compressive strength of in-situ concrete based upon mix proportions and concluded that , water

cement ratio, cement contents, cement aggregate ratio are the main influential factors for

reliability prediction of compressive strength. Tuncan et.al(2006) [22] has shown that concrete

strength is a further factor which may influence the behavior of a core, and it is possible that

this also may affect the relative behavior of small and large cores. The complexity of these

problems contributed to the decision to confine the investigation to a single small core

size .M.Yaqub.et.al (2006) [19] in their research established the relation between core

compressive strength and cube compressive strength of hardened concrete in existing

structures, and concluded that core strength of 75mm cores is 69% of compressive strength of

150mmX150mmX150mm cubes. I. M. Nikbin et.al (2009) [16] revealed that the age of the

concrete was found to be an effective factor in the interpretation of the core strength results.

Test results also revealed that the l/d ratio of the specimen is more effective for small diameter

cores. The coefficient of variation of strength values was noticed to be somewhat higher for 50

mm diameter cores.

3. OBJECTIVESThe objectives are

1. To find the correction factors of 50mm dia core of M-25 mix with H/D Ratio i.e. 1.0, 1.25, 1.5, & 2.00

2. To find the correction factors of 75mm dia core of M-25 mix with H/D Ratio i.e. 1.0, 1.25, 1.5, & 2.00

3. To find the variation of cube & core strength by CTM

4. Materials and Methods

4.1 General:

This chapter deals with the materials used and methods adopted to conduct

the study of core diameter varying H/D ratio on concrete core strength”

4.2 Materials Used

4.2.1Cement:

The cement used should have a minimum compressive strength at different ages according

to the relevant IS specification–IS:269–1987 (33 grade OPC), IS: 8112–1982 (43 grade OPC),

IS:12269–1987 (53 grade OPC), IS:12230–1988 (Sulphate resisting cement), IS:1489-1976

(Portland Puzzolana cement), IS:455–1976 (Portland slag cement). All types of Portland

cements are interchangeable for mix design, and the most commonly used ones are OPC, PPC,

PSC, and SRC.

After water is added to the cement hydration occurs and continues as long as the relative

humidity in the pores is above 85 per cent and sufficient water is available for the chemical

reactions. On an average, 1 g of cement requires 0.253 g of water for complete hydration. As

hydration proceeds, the ingress of water by diffusion through the deposit of hydration products

around the original cement grain becomes more and more difficult, and the rate of hydration

continuously decreases. In mature paste, the particles of calcium-silicate hydrates form an

interlocking network which is a ‘gel’ having a specific surface of about 200 m2/g. This gel is

poorly crystalline, almost amorphous, and appears as randomly oriented layers of thin sheets or

buckled ribbon. The gel is the heart of the concrete and is a porous mass. The interstitial spaces

in the gel are called ‘gel pores’. The strength giving properties and phenomena, such as creep

and shrinkage are due to the porous structure of the gel, and the strength is due to the bond

afforded by the enormous surface area.

The ordinary Portland cement is the most important type of cement. It is classified into three

grades 33grade,43grade,53grade depending upon the compressive strength of cement at 28

Days.

Ambuja 43 grade OPC was used in this study. It was fresh and free from lumps. The properties

of the cement were determined and are given in Chapter IV.

4.2.2 Aggregate

The size of the aggregate, particle shape, colour, surface texture, density (heavyweight or

lightweight), impurities, all of which have an influence on the durability of concrete, should

conform to IS: 383–1970.

During the process of hydration the products of hydration completely surround and bind

together the aggregate particles in a solid hardened mass. Aggregates constitute nearly 70–75

per cent of the total volume of concrete. The strength of concrete is governed by the weakest

element, be it the cement paste, the aggregate or the interface of the aggregate-cement paste.

Strong aggregates are also more sound and durable in aggressive environments. The strength at

the aggregate mortar interface is perhaps more critical, hence the shape, size and texture of the

coarse aggregate is important. The aggregate should be clean, hard, strong, and durable, free

from chemicals or coatings of clay or other fine material that can affect the bond with the

cement paste.

Very sharp and rough aggregate particles or flat and elongated particles require more fine

material to produce a workable concrete. Accordingly the water requirement and there from the

cement content increases. Excellent concrete is made by using crushed stone but the particles

should be roughly cubical or spherical in shape. Natural rounded aggregates having a smooth

surface are better from the point of view of workability, but their bond with mortar may be

weaker and are likely to produce concrete of lower flexural strength.

The maximum size of aggregate governs the strength and workability of the concrete. For a

lean mix, a larger maximum size of aggregate gives better results, because for a given volume

of aggregate, the total surface area is less. Depending on the maximum size of aggregate, the

cement content for a specific strength is altered because at the same water-cement ratio

different ranges of strength are possible for different sizes of aggregate. However, for a mix of

high compressive strength a smaller maximum size of aggregate is preferable, and it is just not

economically possible to make concretes of 28-days compressive strengths exceeding

40 N/ mm2 using 40-mm aggregate.

A statement of fact valid for all mix design is: The smaller the maximum size of aggregate the

greater the proportion of fine aggregate needed for concretes of identical cement contents and

workability. Also, the lower the cement content of the mix, and/or the more angular the coarse

aggregate, the greater is the proportion of the fine aggregate required.

4.2.3 Water:

Fresh and clean tap water was used for casting the specimen in present study. The water

was relatively free from the organic matter, silt, oil, sugar, chloride and acidic material as per

Indian Standard.

4.3 METHODS OF CONCRETE MIX DESIGN

In the present study mix design was done by ISI mix design method.

ISI Mix Design method:

1. Determine the mean target strength fcm from the desired ‘characteristic strength’ fck :

fcm = fck+165.σ

Where the standard deviation σ depends on the quality control, which may be assumed

for design in the first instance, are listed in Table 8 of the Code. As test results of

samples are available, actual calculated value is to be used].

2. Determine the water-cement ratio, based on the 28-day strength of cement and the mean

target strength of concrete, using appropriate charts (such as Fig. 4.1); this ratio should

not exceed the limits specified in Table 5 of the Code (for durability considerations).

Fig. 4.1 Relation between water-cement ratio and compressive strength

3. Determine the water content Vw based on workability requirements, and select the ratio

of fine aggregate to coarse aggregate (by mass), based on the type and grading of the

aggregate; the former is generally in the range of 180–200 lit/m3 (unless admixtures are

employed), and the latter is generally 1:2 or in the range of 1:1½ to 1:2½.

4. Calculate the cement content Mc (in kg/m3) by dividing the water content by the water-

cement ratio, and ensure that the cement content is not less than that specified in the

Code [Tables 4 and 5] for durability considerations. [Note that the Code (Cl.8.2.4.2)

cautions against the use of cement content (not including fly ash and ground granulated

blast furnace slag) in excess of 450 kg/m3 in order to control shrinkage and thermal

cracks]. Also, calculate the masses of fine aggregate Mfa and coarse aggregate Mca

based on the ‘absolute volume principle’:

where ρc, ρf, ρac,, denote the mass densities of cement, fine aggregate and coarse

aggregate respectively, and Vv denotes the volume of voids (approx. 2 percent) per cubic

metre of concrete.

6. Determine the weight of ingredients per batch, based on the capacity of the concrete

mixer.

4.4 METHODS

The procedure of methods used for testing cement, coarse aggregates, fine aggregate and

concrete are given below:

4.4.1 Specific Gravity:

The specific gravity is a dimensionless unit defined as the ratio of density(mass of a unit

volume) of a substance to the density ( mass of same unit volume) of a reference substance.

The reference substance is water for liquids and air for gases. The specific gravity of the solid

is the ratio of its weight in air and weight immersed in water.

4.4.2 Standard Consistency of cement

300 g of cement is mixed with 25 per cent water. The paste is filled in the mould of Vicat’s

apparatus (Fig. 5.9) and the surface of the filled paste is smoothened and leveled. A square

needle 10 mm x 10 mm attached to the plunger is then lowered gently over the cement paste

surface and is released quickly. The plunger pierces the cement paste. The reading on the

attached scale is recorded. When the reading is 5-7 mm from the bottom of the mould, the

amount of water added is considered to be the correct percentage of water for normal consistency.

Fig.4.2 Vicat’s Apparatus

4.4.3 Determination of Initial and Final Setting Time

When water is added to cement, the resulting paste starts to stiffen and gain strength and

lose the consistency simultaneously. The term setting implies solidification of the plastic

cement paste. Initial and final setting times may be regarded as the two stiffening states of the

cement. The beginning of solidification, called the initial set, marks the point in time when the

paste has become unworkable. The time taken to solidify completely marks the final set, which

should not be too long in order to resume construction activity within a reasonable time after

the placement of concrete. Vicat’s apparatus used for the purpose is shown in Fig. 5.9. The

initial setting time may be defined as the time taken by the paste to stiffen to such an extent that

the Vicat’s needle is not permitted to move down through the paste to within 5 ± 0.5 mm

measured from the bottom of the mould. The final setting time is the time after which the paste

becomes so hard that the angular attachment to the needle, under standard weight, fails to leave

any mark on the hardened concrete. Initial and final setting times are the rheological properties

of cement

It is important to know the initial setting time, because of loss of useful properties of cement if

the cement mortar or concrete is placed in moulds after this time. The importance of final

setting time lies in the fact that the moulds can be removed after this time. The former defines

the limit of handling and the latter defines the beginning of development of mechanical

strength.

4.4.4 Compressive strength of cement:

Compressive strength is the basic data required for mix design. By this test, the quality and

the quantity of concrete can be controlled and the degree of adulteration can be checked.

The test specimens are 70.6 mm cubes having face area of about 5000 sq. mm. large size

specimen cubes cannot be made since cement shrinks and cracks may develop. The temperature

of water and test room should be 27°± 2°C. A mixture of cement and standard sand in the

proportion 1:3 by weight is mixed dry with a trowel for one minute and then with water until

the mixture is of uniform colour. Three specimen cubes are prepared. The material for each

cube is mixed separately. The quantities of cement, standard sand and water are 185 g, 555 g

and (P/4) + 3.5, respectively where P = percentage of water required to produce a paste of

standard consistency. The mould is filled completely with the cement paste and is placed on the

vibration table. Vibrations are imparted for about 2 minutes at a speed of 12000±400 per

minute. The cubes are then removed from the moulds and submerged in clean fresh water and

are taken out just prior to testing in a compression testing machine. Compressive strength is

taken to be the average of the results of the three cubes. The load is applied starting from zero

at a rate of 35 N/sq mm/ minute. The compressive strength is calculated from the crushing load

divided by the average area over which the load is applied. The result is expressed in N/mm2.

4.4.5 Sieve analysis for coarse and fine aggregates:

i) The sample was dried in an oven at a temperature of 110°C.

ii) The air dried sample was weighed and sieved successively on the appropriate sieves

starting with the large.

iii) Each sieve was shaken separately over a clean tray until not more than a trace passes ,

but in many cases for a period not less than two minutes. The shaking was done with a

varied motion, left to right ,backward and forward, circular clockwise , and with

frequent jarring, so that material is kept moving over the seve surface in frequently

changing directions.

iv) Lumps of the fine material if any was broken by gentle pressure with fingers against the

side of sieve. Light brushing with soft brush on the underside of the sieve was used to

clear the sieve openings.

v) On completion of sieving, the material retained on each sieve together with any material

cleaned from the mesh was weighed.

4.4.6 Compressive strength of concrete:

The required quantities of cement, coarse aggregates (20mm and 10mm) , and fine

aggregates were taken for 44 cubes, as per mix design. Water was added to the mix and

then mixed thoroughly for 3 to 4 minutes in mechanical mixer.

Cubes were cleaned and oil was applied. The concrete was filled into the cube moulds

and get vibrated to ensure proper compaction. The surface of the concrete was finished

level with the top of the mould using trowel. The finished specimen were left to harden

in air for 24 hours. The specimens were removed from the moulds after 24 hours of

casting and were placed in the mould tank, filled with potable water.

Specimens were taken out from the curing tank at the age of 28 days. Surface water was

wiped off.

In all 44 numbers of 150X150X150 mm (were casted and cured in the laboratory

condition) .50 and 75 mm diameter cores were cut from the cubes by using a diamond-

tipped core-cutter and trimmed to give over- all H/D ratios between 1.0 and 2.0. The

H/D ratios of capped core specimens were 2.0, 1.5, 1.75, 1.25 and 1.0. Fig.4.3 shows

core drilling operation. The compressive strength values of the standard specimens and

cores were determined at the age of 28 days.

The compressive strength of the standard specimens and the cores was determined by a

Compression testing machine. The compressive strength test results were taken as the

average of four specimens. A total of 4 standard and 40 core specimens were tested in

this investigation.

Mix Design

Properties of Materials

(i) Characteristic compressive strength at 28 days,

fck

25N/mm2

(ii) Maximum size of the available aggregate 20 mm

(iii) Shape of coarse aggregate angular

(iv) Degree of workability desired, compacting

factor

0.85

(v) Degree of quality control good

(vi) Type of exposure moderate

Test data for concrete making materials

Specific gravity of cement 3.15

Specific gravity of coarse aggregate 2.72

Specific gravity of fine aggregate 2.66

Water absorption (air dry to saturated surface dry)

in coarse aggregate, per cent 0.5

Surface moisture

Coarse aggregate nil

Fine aggregate, per cent 2

Compressive strength of cement 51 N/mm2

IS

Designatio

n

Cumulative % age passing Sand

Fraction I

10mm

Fraction II

20mm

40mm 100 100

25mm 100 100

20mm 100 88

12.5mm 100 24

10mm. 90 12 100

4.75mm 4 1 92

2.36mm 3 - 86

1.18mm - - 78

600µ - - 64

300µ - - 16

150µ - - 2

Passing

Through

150µ

- - -

Note :Sand conforming to zone II

Step 1: For the degree of quality control specified, namely, good the value of standard

deviation _ read from Table 11.7 = 5.3 N/mm2. Hence, the target mean strength for the desired

characteristic compressive strength

= 25 + 1.65 × 5.3

= 33.745 N/mm2

Step 2: Corresponding to this target mean strength the water cement ratio is read from the

appropriate curve corresponding to the 28 days strength of cement (Fig. 11.2). For a cement

strength of 51 N/mm2, curve D is selected and the water-cement ratio 0.46 obtained. This value

has now to be checked against the maximum limit of the water-cement ratio for the given

exposure condition. Table 11.1 for moderate exposure and reinforced concrete the maximum

water-cement ratio recommended is 0.50. Hence value of 0.46 obtained is acceptable.

Step 3: For maximum size of aggregate of 20 mm, the air content is taken as 2.0 per cent

(Table 11.11). Since the required grade of concrete is M 25 which is lower than M 35 grade,

water content per m3 of concrete = 186 litres and sand as percentage of total aggregate by

absolute volume = 35 (Table 11.9).

Since Table 11.9 is based on certain specific conditions, adjustments as per Table 11.10 have to

be made for any deviations in these values as given below.

Adjustment in Adjustment in

water, sand content

content, per

cent per cent of total

aggregates

by volume

(i) For sand conforming

to nil 1.50%

grading zone III

(ii) Increase in value of

the compacting factor nil 0.05/0.1x3=1.5 nil

by(0.85–0.80) = 0.05

(iii) Decrease in the value

of water-cement ratio by

nil Nil - 0.04/0.05x1=-0.8

(0.50–0.46) = 0.04

Over all adjustment 1.5 -2.8

Thus after incorporating the above adjustments the sand content = 35 – 2.8 = 32.2 and the water

content =

1.86 + 1.5 ×186/100= 188.8 lit.

Step 4: Determination of cement content:

Water-cement ratio = 0.46

Quantity of water after adjustment = 188.8 liters

Therefore, cement content =188.8/0.46= 410.43 kg

This cement content has now to be checked against the minimum cement content required for

mild exposure condition in reinforced concrete. The minimum cement content specified in

Table 11.1 is 300 kg/m3 from durability consideration. Therefore, the value of 410.43 kg/m3 is

acceptable.

Step 5: Now quantities of coarse and fine aggregates are worked out per m3 of concrete as

given below.

Volume of concrete = 1–0.02 (entrapped air)

= 0.98 m3

= 980 liters.

980 = 188.8 + 410.43/3.15+fs/0.322 / 2.66

fs= 566.08 kg

Similarly,

980 = 188.8 +410.43/3.15+Cs/(1 - 0.322) /2.72

Cs= 1218.81 kg

Therefore, the mix proportion becomes:

Cement Water Sand Coarse aggregate

410.43 188.8 566.08 1218.81

or 1 0.46 1.379 2.97

Adjustments required for water absorption:

Water absorbed by coarse aggregate

=1218.81 / 0.5X100= 6.09 lit

Free water in fine aggregate

=566.08X/100= 11.32 lit

Therefore, actual quantity of water required

= 188.8 + 6.09 – 11.32 = 183.57 lit

Actual quantity of coarse aggregate

1218.81 – 6.09 = 1212.72 and sand = 566.08 + 11.32 = 577.4

Therefore, the actual quantities of materials required are:

Cement Water Sand Coarse

410.43 kg 183.57 lit 577.4 kg 1212.72 kg.

Step 6: The fraction I and II of the coarse aggregate are to be now combined to give a

combined grading in accordance with IS: 383–1970 of 20 mm maximum size aggregate. To

obtain a combined grading, IS: 383–1970 recommends that the fraction passing the 10 mm

sieve shall be in the range of 25 to 55 per cent, or on an average of 40 per cent. Trials are then

made to combine the fraction I and II in the proportion of, say 40:60 to see whether the

combined grading is obtained. This is given below:

IS Sieve

Designation

Fraction I,10mm

MSA, 40 percent

Fraction

II ,20mm MSA,

60 percent

Combined

Grading

Desired Grading

20mm MSA , as

per IS:383

40mm 40 60 100 100

25mm 40 60 100 100

20mm 40 55.2 95.2 95 to 100

12.5mm 40 15 55 -

10mm 35.2 8.4 43.6 25 to 55

4.75mm 4.8 1.8 5.6 0 to 10

2.36mm 1.2 - 1.2 -

It can be observed that if the fractions I and II are combined in the ratio of 40:60 the desired

combined grading as recommended in IS: 383–1970 is obtained. The mix proportions for the

trial mix will now be

Cement Water Sand Coarse

fraction

10mm MSA

Aggregate

fraction II

20mm MSA

410.43 183.57 577.4 485.09 727.63

1 0.447 1.406 1.182 1.773

5.2 Measurement of compressive strength

For cubes and core drilled were tested with a compression machine with a

maximum capacity of 3000 kN, shown in Fig.4.1

Figure 5.1: Compression Testing Machine

Figure 5.2: Failure of cube under Maximum load

Figure 5.3: Core Drilling Machine

In the present study, the cores with two different diameters and four different H/D ratios were

tested and the effects of specimen as H/D ratios of core on concrete core strengths were

examined. The strengths of cores were compared to those of standard cube specimens. The

compressive strength developments of cube specimens are listed in Table 5.2.

Table 5.1 Compressive strength of standard cubes and cores50 mm dia and 75mm dia

H/D

RATIO

COMPRESSIVE STRENGTH in Mpa

Core Cube

50mm Dia 75mm Dia

1.00 22.28 21.45 31.54

1.25 22.9 22.2 31.54

1.50 23.53 22.94 31.54

1.75 24.15 24.19 31.54

2.00 24.9 24.94 31.54

5.3 Correction Factors as per IS: 516

Fig. 5.1: Correction factor as per IS:516

As shown in fig. IS: 516, give the correction factors to be applied if the H/D ratio is less than

2.0.But it does not give particular values for different diameters of core.

5.4 Correction factors as determined form cores:

In this research correction factors is established for core diameter 50mm and 75 mm

diameter with M25 concrete, for different H/D ratios

Correction Factor for a particular diameter with a particular H/D ratio =

Strength of core of particular diameter with particularHD

raio

Strengthof core of particular diameter withHD

ratio2.0

In this study for each mix the average values of measured core strength, were compared for each different value of H/D and expressed in terms of a core with H/D = 2.0, as correction factor for H/D=2.0 is 1.0. The numerical formula for 50mm core and 75 mm cores is given in equation no.1&2 respectively.

Table 5.2: Correction Factors for 75mm cores

No. Diameter(mm) H/D Ratio Core Compressive Strength(Mpa)

Correction Factor

1. 75 1 24.08 0.86

2. 75 1.25 24.94 0.89

3. 75 1.5 25.82 0.92

4. 75 1.75 27.13 0.96

5. 75 2 28.15 1.00

Table 5.3: Correction factors for 50mm cores

No. Diameter(mm) H/D Ratio Core Compressive Strength(Mpa)

Correction Factor

1. 50 1 24.16 0.85

2. 50 1.25 25.93 0.91

3. 50 1.5 26.66 0.93

4. 50 1.75 27.6 0.97

5. 50 2 28.54 1.00

The following graphs are obtained between the Correction factors and different Height/Diameter Ratio, for 50mm and 75mm Diameter cores.

0.8 1 1.2 1.4 1.6 1.8 2 2.20.75

0.80

0.85

0.90

0.95

1.00

1.05

f(x) = 0.212696078237582 ln(x) + 0.851222306424688R² = 0.989611318919525

50mm Diameter

50mm DiameterLogarithmic (50mm Diame-ter)

H/D

Corr

ectio

n Fa

ctor

Fig. 5.1: Correction factor for H/D ratio of core dia. 50 mm

0.8 1 1.2 1.4 1.6 1.8 2 2.20.75

0.80

0.85

0.90

0.95

1.00

1.05

f(x) = 0.209415938389719 ln(x) + 0.84567818033035R² = 0.970511784025761

75mm Diameter

75mm DiameterLogarithmic (75mm Diame-ter)

H/D

Corr

ectio

n Fa

ctor

Fig. 5.2: Correction factor for H/D ratio of core dia. 75 mm

k = 0.2127ln (λ) + 0.8512 (1) k= 0.2094ln (λ) + 0.8457 (2)

Where k is the correction factor of core strength for a core with H/D = λ

Table 5.3 Comparison of actual compressive strength from C.T.M., and core strength as calculated as per curve of IS: 516 and from equation no. 1 & 2

No.

Dia

met

er(m

m)

H/D

Rat

io

Cor

e C

ompr

essi

ve

Str

engt

h(M

pa)

Cor

rect

ion

Fac

tor

as

per

IS:5

16

Cor

rect

ion

Fac

tor

as

per

curv

e

Corrected Compressive Strength as per curve % age Variation

1.A

s pe

r IS

:516

2.A

s P

er

Cur

ve

3.A

s P

er

CT

M(A

ve

Bet

wee

n 1&

2

Bet

wee

n 2&

3

Bet

wee

n 1&

3

1 75 1 21.45 0.895 0.86 24.00 23.06 31.54 3.91 26.89 23.922 75 1.25 22.2 0.92 0.89 25.53 24.70 31.54 3.26 21.69 19.063 75 1.5 22.94 0.945 0.92 27.10 26.38 31.54 2.65 16.36 14.084 75 1.75 24.19 0.97 0.96 29.33 29.03 31.54 1.03 7.96 7.015 75 2 24.94 1 1 31.18 31.18 31.54 0.00 1.16 1.166 50 1 22.28 0.895 0.85 24.93 23.67 31.54 5.03 24.94 20.977 50 1.25 22.9 0.92 0.91 26.34 26.05 31.54 1.09 17.41 16.508 50 1.5 23.53 0.945 0.93 27.79 27.35 31.54 1.59 13.27 11.879 50 1.75 24.15 0.97 0.97 29.28 29.28 31.54 0.00 7.16 7.16

10 50 2 24.9 1 1 31.13 31.13 31.54 0.00 1.32 1.32

6. Summary and Conclusion RemarksFor small cores of 50 and 75 mm diameters, the following conclusion may be drawn:

It has been observed that there is variation of 12.30%age between the cube strength

tested by C.T.M., and that of core strength tested by C.T.M. as per IS:516 , and is

13.82% between the cube strength tested by C.T.M., and that of core strength tested by

C.T.M. as per curve obtained.

It is also observed that the correction factor as per IS:516 may be adopted for50mm and

75mm core diameter as well.

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Interpretation of Concrete Core Strength Results.” Magazine of Concrete Research, Vol. 58,

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concrete.

[3]BS 1881, Part 120 (1983). Method for determination of the compressive strength of concrete

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