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Zacoeb et al. Compressive Strength Assessment of Concrete Structures from Small Core by Point Load Test 263

INTRODUCTION

One of the reliable tests for assessing in-situstrength of concrete is coring. Coring mayprove expensive and the holes have to bebackfilled, but the resulting data are usuallyaccepted as the best evidence of the conditionof the concrete in place. It is established in JISA1107 (1993) that a core drilled specimendiameter of 100mm or three times of maximumcoarse aggregate size from a concretestructure member should be taken for

performing strength evaluation. Small cores areoften used as substitutes for large cores to testconcrete strength. They have the advantagesof being easily drilled and cut, minimumdamage to structures, and a lower capacitymachine is needed (Ruijie, 1996).

The main parameter for characterizing aconcrete in engineering practice is compressivestrength. Ibragimov (1989), the maximumaggregate size is considerable as played rolefor affecting the properties of concrete. Thestandard laboratory test ussualy requires astandard specimens, so indirect test areneeded. The PLT is intended as an index test

for the strength classification of rock materials,but it may also be widely used to predict othermaterial strength parameter. It is an attractivealternative method, because it can providesimilar data at a lower cost, a simplepreparation of specimen, and possibility on fieldapplication.

ISRM (1985), in order to estimate UCSindirectly, index-to-strength conversion factorsare developed. Richardson (1989) conducted apoint load tests of cast specimens with various

diameters. Zacoeb et al. (2007) showed astrong correlation between point load index ofcore drilled specimen (I

S) and compressive

strength of concrete core (fcc

). Ishibashi et al.

(2008) investigated the influence of h/d ratioand maximum aggregate size (Gmax) onconcrete core specimen by using PLT. Manyresearch works had been conducted toacknowledge with regard to PLT and resultedin widely used and other parameters. However,more experimental works helps to substantiate

the existing correlation.

COMPRESSIVE STRENGTH ASSESSMENT OF CONCRETE STRUCTURESFROM SMALL CORE BY POINT LOAD TEST

Achfas ZACOEB1)

and Koji ISHIBASHI2)

1)

Lecturer, Department of Civil Engineering, Brawijaya University, INDONESIAE-mail: [email protected])

Professor, Department of Civil Engineering, Saga University, JAPAN

E-mail: [email protected]

ABSTRACT

To assess a compressive strength from existing concrete structures by core drilling are usually gathered

with a diameter specimen of 100mm or three times of maximum coarse aggregate size and examined by

uniaxial compressive strength (UCS). It is relatively difficult to gather a large sized core, and a pit place

will be limited by main members. To get an alternative solution with smaller specimen, point load test

(PLT) has been selected which is a simple test and widely accepted in rock materials research, butrelatively new in concrete. The reliability of PLT is examined by extracting a lot of core drilled specimen

from ready mixed concrete blocks with maximum coarse aggregate size, Gmax of 20mm in representative

of architectural structures and 40mm in representative of civil structures on the range of concrete grade

from 16 to 50. Compressive strengths were classified into general categories, conversion factors were

determined, and scattering characteristics were also investigated. The relationship between point load

index (IS) and compressive strength of concrete core specimen (fcc) can be written as linear

approximation as fcc= k.IS C.

Keywords: Strength assessment, standard specimen, small core, point load index, linear approximation

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THEORITICAL BACKGROUND

Broch, et al (1972) started with a simpleformula taking an idealized failure plane ofdiametric core sample as shown in Figure 1.From this figure can be taken into account asconceptual model for derivation on point loadindex equation as:

2dPIS = (1)

where,IS : point load index (MPa)P : load (N)d : diameter of specimen (mm)

Figure 1 Specimen diametric of PLT on cores

By taking the circular area of the core intoaccount, an argument can be made that

Equation (1) should be written as:

2

4

d

PIS

= (2)

The user of this test soon noticed, that theresults of a diametric test were about 30%higher than those for an axial test using thesame specimen dimensions. Broch et al (1972)and ISRM (1985) suggested acknowledge thisdifference by applying a size correction andintroducing the equivalent core diameter of De.Hence, the Equation (2) can be re-written as:

2e

S D

P

I = (3)The variations of IS with specimen size and

shape lead to introduce a reference index IS(50)which corresponds to the IS of a diametricallyloaded rock core of 50mm diameter (Broch etal.1972). Accordingly, initial IS values arereduced to IS(50) by size correction factorsdetermined from empirical curves as a functionof d. It is indicated that the considerably largershape effect should be avoided by testingspecimens with specified geometries. ISRM

(1985) proposed a new correction functionwhich accounts for both size and shape effects

by utilizing the concept of equivalent corediameter (De). This function, known asgeometric correction factor Fis given by:

S)(S FII =50 (4)

where,F : the geometric correction factor

=

450

50

.

eD

(5)

The unique point load index can be obtainedby applying a size correction for the specimenas point load index of IS varies with corespecimen diameter of De. The size-correctedpoint load index of IS(50) for each specimen isdefined as the value of IS that would have beenmeasured on a standard specimen diameter ofDe = 50mm. In the case of specimen diameterof De other than 50mm, size correction must becalculated by using of Equation (5)

LABORATORY WORKS

The concrete block for core specimenextracting were sized of 300mm x 300mm x600mm made from ready-mixed normalconcrete with typical slump range value from 8to 12cm for most application as workabilitycontrol and divided into two groups as shown inTable 1. For curing, all concrete blockspecimens were covered with plastic sheetsand the humidity was set for about a week.

Commonly in Japan, for architectural structuressuch as building construction is using themaximum coarse aggregate size, Gmax of20mm. While for civil structures such as pier,abutment, bridge deck and check dam is usingthe maximum coarse aggregate size, Gmax of40mm.

Table 1 Group of concrete block

Group

Gmax(mm)

Grade Cement Type

16212436

I 20

50

OPC(OrdinaryPortlandCement)

162124

II 40

30

PBSC(Portland

Blast-FurnaceSlag Cement)

Core specimen diameter of 125, 100, 50 and35mm were extracted from the above

mentioned concrete block with the electric corepulling out machine. The wet type that used by

Idealized failure plane

Loading direction

Central axialdirection

d

h/2 h/2

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flowing some water during the core drilledprocess is applied, and the extraction speedwas assumed to be about 4cm/min. Thedirection of extraction is considered as thedirection of concrete placing as verticaldirection in assumption of practical work inconstruction. The situation of core specimenextraction is shown in Figure 2.

Figure 2 Core specimen extractions

The core specimen with h/dratio of 1.5 and2.0 were selected as core specimen of PLT inthis study. In order to establish a specific h/dratio, core specimens were cut both ends witha concrete cutting machine to become a fixedheight (h). The total number collection of eachspecimen is shown in Table 2.

Table 2 Total number of core specimens

Group Grade d(mm)

h/d Total

1.5 13535

2.0 1351.5 105

1650

2.0 991.5 90

352.0 901.5 60

2150

2.0 601.5 66

352.0 661.5 59

24

50 2.0 581.5 123

352.0 1261.5 111

3650

2.0 1081.5 67

352.0 661.5 73

I

5050

2.0 72

Table 2 (Cont.)

1.5 12635

2.0 1261.5 85

1650

2.0 871.5 154

352.0 1381.5 82

21

50 2.0 791.5 113

352.0 1131.5 87

2450

2.0 861.5 157

352.0 1721.5 113

II

3050

2.0 108

The specimen in PLT is taken and loadedbetween two hardened steel cones. The

system consists of a small hydraulic pump, ahydraulic jack, a pressure gauge andinterchangeable testing frame of very hightransverse stiffness. Spherically truncated,conical platens of the standard geometryshown in Figure 3 are to be used with thecylinder area of 14.52cm2. The platens shouldbe of hard material such as tungsten carbide orhardened steel so that they remain undamagedduring testing (ISRM, 1985).

Figure 3 Point load cone platen

The core specimen in this study is graduallyloaded by activating the hand pump until failureand determined this load as P. The point loadindex of ISwas calculated by using Equation (3)and for core specimen diameter of 35mm wascorrected to the standard core diameter aspoint load index of IS(50) for core specimendiameter of 50mm by using Equation (4). Theexamination is conducted by using PLTmachine with oil pressure cylinder type and

maximum load capacity of 98kN as shown inFigure 4.

Concrete placing

direction

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Figure 4 Setup of PLT

RESULTS AND DISCUSSION

Compressive strengthFrom a concrete block is extracted a core

specimen with diameter of 100 mm asminimum requirement and 125mm as threetimes of maximum coarse aggregate size (JISA1107, 1993), cut both ends of the core with aconcrete cutting machine, end face polished,processed it to become specific h/dof 2.0 andexamined by UCS test. The mean value of

compressive strength of concrete core, fcc isshown in Table 3, and assumed these valuesas reference on this study.

Table 3 Compressive strength

Group GradeAge

(Days)fcc

(MPa)

16 161 15.6

21 337 35.4

24 73 31.5

36 177 42.9

I

50 78 51.516 188 21.6

21 173 22.4

24 532 34.4II

30 118 32.2

Point load indexThe mean values of PLT were computed for

both diameter sizes as shown in Table 4.Scattering characteristics were alsoinvestigated by mentioning of CoV (coefficientof variation). The CoVis the degree to which aset of data points varies. When assessingprecision, the lower of CoV percentage, the

better of precision between replicates. ForGroup I, the level of CoVis almost same or lessthan that on actuality of ready-mixed concreteproduct (Saga, 2008) from 10 to 15%. It can bestated that the test results are satisfy enough.For group II, the CoV is bigger than therequirements (except for concrete grade of 30).

Table 4 Point load index

Group Graded

(mm)h/d

IS(MPa)

CoV(%)

1.5 2.27 11.435

2.0 2.32 13.3

1.5 1.87 10.716

502.0 1.93 10.8

1.5 3.24 11.435

2.0 3.31 12.4

1.5 2.57 9.721

502.0 2.61 11.1

1.5 3.21 12.435

2.0 3.28 13.1

1.5 2.71 8.124

502.0 2.77 9.3

1.5 3.61 11.935

2.0 3.69 13.5

1.5 3.06 10.436

502.0 3.02 10.9

1.5 3.95 8.135

2.0 4.05 9.6

1.5 3.27 8.2

I

50

50 2.0 3.34 8.9

1.5 2.20 26.135

2.0 2.30 31.9

1.5 1.85 18.716

502.0 1.95 18.1

1.5 2.52 23.035

2.0 2.55 24.3

1.5 2.12 21.021

502.0 2.00 18.0

1.5 2.90 26.335

2.0 2.92 27.0

1.5 2.31 18.22450

2.0 2.43 18.2

1.5 2.80 24.735

2.0 2.85 19.7

1.5 2.43 8.5

II

30

502.0 2.47 7.6

For Group I, the level of CoVfor h/dof 1.5 issmaller than h/dof 2.0. It can be stated that h/dratio of 1.5 is better than h/dof 2.0 for making a

PLT specimens from core drilled extraction.While for core specimen diameter, d is better

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using 50mm than 35mm, because the level ofCoV is also smaller. Beside this reason, it isalso fulfilled with the standard core diameterrequirements of 50mm. For all groups, it ispossible and acceptable for using a corediameter of 50mm and h/d ratio of 2.0 as PLTspecimen with results in the range of CoVfrom8 to 18%. Application of PLT for small diameter

of core specimen is not suggested for d/Gmaxratio below 1.25, considering the CoV resultsfor Gmax of 40mm and d of 35mm are largerthan 20%.

Correlation between point load index andcompressive strength

Point load index of core specimen diameterof 50mm, IS(50) is determined as standard value.Hence, the value of different core specimendiameter, IS(35) should be corrected in order toshow a relationship with I

S(50)by using Equation

(4) and (5). By correcting the point load index ofIS(35) and assuming as standard core specimendiameter of 50mm, will add the number of datafor analysis of point load index IS(50). The newresult for this combination is shown in Table 5corresponding with the compressive strength ofconcrete core (fcc) for each grade.

Table 5 Point load index and compressive strength

ISof h/dGroup Gradefcc

(MPa) 1.5 2.016 15.6 1.86 1.9321 35.4 2.57 2.6124 31.5 2.71 2.7736 42.9 3.07 3.03

I

50 51.5 3.27 3.3416 21.6 1.85 1.9521 22.4 2.12 2.0024 34.4 2.31 2.43

II

30 32.2 2.43 2.47

The correlation between point load index,IS(50) and compressive strength of concretecore, fcc for both of groups is shown graphicallyin Figure 5 and 6. It is clearly evident to showthe correlation by proposing a second order ofpolynomial and linear regression, respectively.It is proven by showing the square value ofcorrelation coefficient which judges theeffectiveness of a second order of polynomialapproximation curve for h/d= 1.5 is thought tobe similar for h/d= 2.0. Linier regression alsoshowed the same trend of effectiveness exceptfor group II.

y = 4.697x2 + 0.052x

R2 = 0.982

y = 4.860x2 - 0.757x

R2 = 0.981

y = 4.885x2

+ 1.951x

R2 = 0.957

y = 4.995x2 + 1.317x

R2 = 0.993

0

5

10

15

20

25

30

35

40

45

50

55

0 0.5 1 1.5 2 2.5 3 3.5 4

IS(50) (MPa)

f'cc(M

Pa)

Group I h/d=1.5

Group I h/d=2.0

Group II h/d=1.5

Group II h/d=2.0

Poly. (Group I h/d=1.5)

Poly. (Group I h/d=2.0)

Poly. (Group II h/d=1.5)

Poly. (Group II h/d=2.0)

Figure 5 Second order of polynomial regression

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y = 24.365x - 30.309

R2 = 0.953

y = 24.890x - 32.669

R2 = 0.953

y = 22.541x - 21.404

R2 = 0.749

y = 23.613x - 24.595

R2 = 0.959

0

5

10

15

20

25

30

35

40

45

50

55

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

IS(50) (MPa)

f'cc(M

Pa)

Group I h/d=1.5

Group I h/d=2.0

Group II h/d=1.5

Group II h/d=2.0

Linear (Group I h/d=1.5)

Linear (Group I h/d=2.0)Linear (Group II h/d=1.5)

Linear (Group II h/d=2.0)

Figure 6 Linear regression

JIS A5308 (2003) gives the compressivestrength range of ready-mixed concrete in fieldapplication from 18 to 45MPa. Hence, theapplication of PLT for estimating in-situstrength of concrete structure should beconfirmed in this range. So, the correlation waslimited to this range for core specimendiameter of 35 and 50mm as shown in Figure

7. When using the linear regression as shown

in Figure 6, the approximation line does notintercept in the origin point. However, Figure 7shown that the fitted curve will pass throughthe origin which aims to establish the relationof the whole area would be overestimated. It ispreferable to using linear approximation thanother modes in order to minimize the standardfor the assessment of risk.

0

5

10

15

20

25

30

35

40

45

50

55

1.30 1.50 1.70 1.90 2.10 2.30 2.50 2.70 2.90 3.10 3.30 3.50 3.70 3.90

IS(50) (MPa)

f'cc(M

Pa)

Group I h/d=1.5Group I h/d=2.0Group II h/d=1.5Group II h/d=2.0Linear (Group I h/d=1.5)

Linear (Group I h/d=2.0)Linear (Group II h/d=1.5)Linear (Group II h/d=2.0)Poly. (Group I h/d=1.5)Poly. (Group I h/d=2.0)Poly. (Group II h/d=1.5)Poly. (Group II h/d=2.0)

Therangeofestimation

Figure 7 Linear approximation for IS(50) to fcc

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The new geometric correction factorBy considering the Equation (4) and (5)

were proposed for rock specimen, so it is notsuitable for concrete regarding the issue ofhomogeneity. The previous section alreadymentioned that maximum coarse aggregatesize in concrete will affect the results of pointload index. A new correction factor of F is

proposed by following the format of previousEquation as:

S

X

)(S Id

I

=

5050 (6)

The value of Xcan be generated by usingdata from group I for core specimen diameterof 35mm. The selection of this data wasconsidered more reliable by showing a lowerCV. The solution is simple, because the natureof linear approximation as the origin. The

exponent value of X is calculated as 0.53 withcoefficient of correlation is 0.982. Finally, theexpression geometric correction factor forconcrete core specimen is given by:

530

50

.

eDF

= (7)

Table 6 shows the absolute relative errorbetween experimental and estimation valuesfor point load index of IS(35) to become standardpoint load index of IS(50) by using Equation (8).

The results are satisfied enough by showing avalue of absolute relative error less than 5% inthe case of dof 35mm and Gmaxof 20mm.

Table 6 Experimental and estimation of IS(50)

Point load index

(MPa)fcc

(MPa)

h/d

IS(35) IS(50)a

IS(50)b

Error

(%)

1.5 2.27 1.88 1.87 0.5315.6

2.0 2.32 1.92 1.93 0.52

1.5 3.24 2.68 2.57 4.2831.5

2.0 3.31 2.74 2.61 4.981.5 3.21 2.66 2.71 1.85

35.42.0 3.28 2.72 2.77 1.81

1.5 3.61 2.99 3.06 2.2942.9

2.0 3.69 3.06 3.02 1.32

1.5 3.95 3.27 3.27 0.0051.5

2.0 4.05 3.35 3.34 0.30*) a = estimation

b = experimental

Recalculation procedure is conducted by

using a new Equation (8) for correcting pointload index of core specimen diameter of 35mmand performing linear regression analysis to

propose a formula of compressive strengthestimation for equivalent core diameter of50mm as fcc= k.IS - Cas shown in Table 7.The coefficient of correlation, R2also shows animprovement in strong relationship betweenIS(50) and fcc.

Table 7 Formula of estimation

Grou

ph/d Formula of Estimation R

2

1.5 fcc= 24.4IS 30.3 0.953I

2.0 fcc= 24.9IS 32.7 0.953

1.5 fcc= 20.8IS 16.7 0.928II

2.0 fcc= 22.3IS 22.0 0.979

In consideration with the standard

compressive strength that used as linearapproximation for index-to-strength conversionfactor k is shown in Figure 8. The kvalue iscalculated by divided the compressive strength(fcc) with point load index (IS). While C isconstant depend on the linear regressionequation.

CONCLUSION

Based on this study, it can be concludedthatan approximation curve showed a strongcorrelation between point load index (IS) andcompressive strength (fcc) for core diameter of

35 and 50mm with height and diameter ratio ofh/d = 1.5 and 2.0. In addition for referenceindex IS(50), it can really deal with a linearapproximation. Considering the issue ofhomogeneity that concrete is compositematerial, a new correction factor is proposedfor core specimen diameters differ from 50mm

as

530

50

.d

F

= . To estimate a concrete

compressive strength can be conducted withproposed equation as fcc= k.IS- C.

There is a prospect that PLT can be appliedas indirect method to estimate a compressivestrength on concrete structure. Application ofPLT for in-situ concrete compressive strengthestimation should be confirmed in the range ofcompressive strength of ready-mixed concreteproduct from 18 to 45MPa. Considering themaximum coarse aggregate size of Gmax inconcrete, new criterion is proposed bydetermining the minimum value of d/Gmax ratioshould not less than 1.25.

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ACKNOWLEDGEMENTS

The research work for this study wasconducted by the first author during DoctorCourse in the Laboratory of StructuralEngineering and Mechanics, Department ofCivil Engineering, Saga University, Japanunder supervision of the second author.

Thanks to the Government of Japan forfinancial support trough the Monbukagakushoscholarship.

REFERENCES

Broch E. and Franklin J.A. (1972). The pointload strength test. International Journal ofRock Mechanics and Mineral Sciences. Vol.9(6): 669-676.

ISRM Commission on Testing Methods. (1985).Suggested method for determining point

load strength. International Journal of RockMechanics, Mineral Sciences and Geo-mechanics. Abstract Vol. 22: 51-60.

Ibragimov A.M. (1989). Effect of the maximumsize of coarse aggregate on the mainparameters of concrete. Journal of PowerTechnology and Engineering, Vol. 23: 141-144.

Ishibashi K., Zacoeb, A. and Ito Y. (2008).Influence of coarse aggregate size on theestimation of compressive strength of

concrete by point load testing. Journal ofStructures and Materials in CivilEngineering. Vol. 24: 108-115. (InJapanese).

Japanese Industrial Standard. (1993). JISA1107: Method of sampling and testing forcompressive strength of drilled cores ofconcrete. Referenced JIS Standard. (in

Japanese).Japanese Industrial Standard. (2003). JIS

A5308: Ready-mixed concrete. ReferencedJIS Standard. (In Japanese).

Richardson D.N. (1989). Point load test forestimating concrete compressive strength.ACI Materials Journal. Vol. 86(4): 409-416.

Ruijie, K.L. (1996). The diameter-compressiontest for small diameter cores. Journal ofMaterials and Structures, Vol. 29(1): 56-59.

Saga Fresh Concrete Industrial Union. (2008).Ready-mixed concrete products in SagaPrefectures, Fiscal Year of 2007. QualityAudit Report. (in Japanese).

Zacoeb A., Ishibashi, K and Ito Y. (2007).Estimating the compressive strength ofdrilled concrete cores by point load testing.Proceeding of the 29th JCI Annual Meeting,Sendai, Japan, July 11-13, 2007: 525-530.

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