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COMPARISON OF CORE AND CUBE COMPRESSIVE STRENGTHOF HARDENED CONCRETE

M. Yaqub*, University of Engineering and Technology Taxila, PakistanM. Anjum Javed, Concordia University, Canada

31stConference on OUR WORLD IN CONCRETE & STRUCTURES: 16 - 17 August 2006,Singapore

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31st Conference on OUR WORLD IN CONCRETE & STRUCTURES: 16 17 August 2006, Singapore

COMPARISON OF CORE AND CUBE COMPRESSIVE STRENGTH

OF HARDENED CONCRETE

M. Yaqub*, Universityof Engineering and Technology Taxila, PakistanM. Anjum Javed, Concordia University, Canada

Abstract

This paper presents the result of core and cube compressive strength of hardenedconcrete. Non destructive test methods are used to investigate the properties andstrength of hardened concrete. In existing concrete structures there was no directrelation between the results of non-destructive tests. This paper describes therelation between core compressive strength and cube compressive strength ofhardened concrete in existing structures.

Sixteen cores (diameter 75mm) were extracted from the hardened concrete of 12-years old building. Twelve cores (48mm diameter) were also extracted from thesame hardened concrete building. Two sizes of cubes (150mm x 150 x 150mm) and(100mm x 100mm x 100mm) were made from the hardened concrete from stone

pieces which were taken from the structural members of building with help of stonecutting machine. The cores and cubes compressive strength was determined incompression testing machine. It was found that smaller size cubes (100mm x100mm x 100mm) and cores (48mm diameter) show greater strength as comparedto larger size cubes (150mm x 150 x 150mm) and cores (diameter 75mm). It wasalso found that the cubes (150mm x 150 x 150mm) compressive strength is 76% thecubes (100mm x 100mm x 100mm) compressive strength. The 75mm diametercores compressive strength is sixty nine percent the compressive strength of cubes(150mm x 150 x 150mm).

Keyword: Compressive strength, core strength, cube strength, Non-destructive tests.

1. IntroductionOnce concrete has hardened it can be subjected to wide range of tests to prove its ability to performas planned or to discover its characteristics if its history is unknown. For new concrete this usuallyinvolves casting specimens from fresh concrete and testing them for various properties as theconcrete matures. The concrete cube tests, concrete cylinder tests are the most familiar tests and areused as the standard method of measuring compressive strength for quality control purposes.Concrete beam specimens are cast to test for flexural strength and cast cylinder cone be used fortensile strength. Specimens for many other tests can be made at the same time to assess otherproperties.

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For existing concrete samples will need to be taken from the structure. Non destructive testingmethods are useful in some instances and can help identify areas from which samples should betaken. The normal method of concrete sampling is by coring although same chemical analysistechniques can be carried out on drilling dust samples.

In laboratory many techniques can be used to examine and test hardened concrete to assess a wideVarity of properties. Non destructive and semi-destructive methods play an important role in

evaluating the existing structure conditions [1, 2]. Non destructive test methods are used to indicateproperties other than strength [3, 4]. British Standard defined non destructive testing as a test thatdoes not impair the intended performance of the element or member under investigation [5]. The non-destructive evaluation techniques are used to assess the condition of concrete structures, to predictfuture performance and allow minor repair system. The best known non destructive evaluationtechniques use ultrasonic waves, core testing, Schmidt hammer test. The estimation of in placeconcrete strength requires that a known relationship between the results of in-situ testing and thestrength of concrete. For existing construction the relationship has to be assessed on site correlatingnon-destructive test results to strength of core. There was no any relation between core strength andcube strength of hardened concrete. In this research work a relationship between core strength andcube strength was developed.

This paper also describes the effect of diameter of core on compressive strength of concrete andeffect of size of cube on compressive strength of concrete.

2. Objectives

The objectives of this research were:

(1) To develop a relationship between core compressive strength and cubecompressive strength.

(2) To study the effect of core diameter on the compressive strength of concrete.(3) To study the effect of cube size on compressive strength of concrete.

3. Experimental Program

Sixteen and eleven cores of 75mm diameters and 48mm diameters were extracted from the 12-yearsold concrete buildings at different locations of the structural members. Some cores were taken fromcolumns, beams and some cores were taken from base slab, floor slabs and walls. In the samelocation concrete stone pieces were taken. The length of cores was variable as shown in the table 1& 2. The concrete stones were made into the regular cube shape of size 150mm x 150mm x 150mmand 100mm x100mm x 100mm by stone cutting machine. The cores were extracted by using rotarycutting machine with diamond bits.

The extracted cores were trimmed to make smooth edges parallel and then capped with sulphur tomake the ends smooth. After capping, cores were tested in compression testing machine in theconcrete laboratory. No rebars were found in the cores. The results of cores are shown in table 1 & 2.After making the regular shape of cubes of sizes 150mm x150mm x 150mm and 100mm x100mm x100mm they were tested in compression testing machine. The results of cubes are shown in table 3.

The average compressive strength of 150mm x 150mm x 150mm cubes was 24MPa [3508psi] andthe average compressive strength of 100mm x 100mm x 100mm was found to be 32MP [4608psi].The average corrected compressive strength of cores having diameter 75mm was 17MPa [2466psi].

The average compressive strength of cores having diameter of 48mm was 18MPa [2611psi]. All coreswere tested in dry condition. The strength of cores was calculated after applying all the factors givenin ASTM C42-90.

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Table 1 Results of Cores (75mm diameter)

Sr.No.

LocationWeight(gm)

Dia (mm)Length(mm)

Area(mm

2)

CorrectedCompressive

Strength of core(MPai)

1. RCC Column 1524 75 150 4418 12.6272. RCC Column 1516 75 150 4418 10.885

3. RCC Column 1608 75 150 4418 19.1584. RCC Column 1572 75 150 4418 19.3755. RCC Slab 1500 75 150 4418 14.15096. RCC Slab 1448 75 140.462 4418 26.9307. RCC Slab 1196 75 121.41 4418 23.21988. RCC Slab 1138 75 114.046 4418 25.7499. RCC Wall 1480 75 150 4418 7.61910. RCC Beam 1568 75 150 4418 21.77011. RCC Beam 1564 75 150 4418 21.53712. RCC Slab 1124 74.93 4.78 4418 14.14213. RCC Wall 1546 74.93 113.79 4418 13.59614. RCC Column 1562 74.93 113.79 4418 16.73315. RCC Beam 1518 74.93 113.79 4418 12.55016. RCC Beam 1486 74.93 150 4418 10.0145

Table 2 Results of Core (48mm diameter)

Sr.No.

LocationWeight(gm)

Dia (mm)Length(mm)

Area(mm

2)

CorrectedCompressive Strength

of core (MPa)1. CS-3 360.91 48 92.40 1809.557 15.8402. BS-2 392.92 48 96.64 1809.557 22.31173. BS-1 389.93 48 97.18 1809.557 23.37104. CS-1 400.53 48 100.12 1809.557 22.31175. GS-2 385.97 48 98.17 1809.557 13.8106. CS-2 398.36 48 100.15 1809.557 19.1207. R-7 300 48 77.26 1809.557 12.3528. R-3 258.52 48 64.77 1809.557 15.507

9. BS-4 250.31 48 64.70 1809.557 12.0010. R-3 (RAFT) 239.72 48 61.69 1809.557 15.87711. R-3 212.62 48 52.64 1809.557 21.347

Table 3 Results of Cubes

Sr.No.

Cube sizeLocation Corrected Compressive Strength of

core (MPa)

1. 150 x 150 x 150 Column 25.2. 150 x 150 x 150 Column 203. 150 x 150 x 150 Column 284. 100 x 100 x 100 Slab 335. 100 x 100 x 100 Slab 336. 100 x 100 x 100 Slab 28

7. 100 x 100 x 100 Slab 358. 100 x 100 x 100 Slab 329. 100 x 100 x 100 Wall 3210. 100 x 100 x 100 Wall 2611. 100 x 100 x 100 Wall 2712. 100 x 100 x 100 Wall 3513. 100 x 100 x 100 Slab 3114. 100 x 100 x 100 Slab 3515. 100 x 100 x 100 Slab 36

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4. Results and Discussion

The cores (diameter 48mm) show greater compressive strength of concrete as compared to the cores(diameter 75mm). The cubes (100mm x 100mm x 100mm) show greater compressive strength ascompared to cubes (150mm x 150mm x 150mm).

The smaller size cores have smaller cross-sectional area. The compressive strength is load per unit

area. Due to smaller size, the strength of smaller diameter cores was greater as compared to thelarger size diameter of cores. Similarly the smaller size cubes give greater strength as compared tolarger size cubes.

5. Relation between Core Compressive Strength and Cube Strength

It was found from the result; the compressive strength of cores (75mm diameter) is 0.693 times thecompressive strength of cubes (150mm x 150mm x150mm). The compressive strength of cores(75mm diameter) is 0.528 times the compressive strength of cubes (100mm x 100mm x 100mm). Thecore (48mm diameter) compressive strength is 0.724 times the compressive strength of cubes(150mm x 150mm x 150mm).

The compressive strength of cores (48mm diameter) is 0.552 times the compressive strength ofcubes (100mm x 100mm x 100mm).

6. Conclusion

From the results of the tested specimen the following conclusion were made.

1. The compare strength of cubes was greater than the compressive strength of cores.2. The smaller size cubes (100mm x 100mm x 100mm) give greater compressive strength of cubes

as compared to larger size (150mm x 150mm x 150mm).3. The smaller size cubes strength is 1.311 times larger size cube compressive strength.4. The strength of cores (48mm) is 5% greater than cores (75mm).

7. Reference

[1]. British standard institution, Guide to the use of non-destructive methods of test for hardenedconcrete, BS, 881, 1986, part 201.[2]. Fookes P.G concrete in the middle test past, present ad future , review Damage AssessmentRepair Techniques and strategies for reinforcement concrete Macmillan, G.L ed. Bahrain society ofEngineers 1991.[3]. Pocock ,D.C the selection o cost-effective repair strategies for corrosion damage concrete andetermination and respire of reinforced concrete In the Persian Gulf , oct,1997, Bahrain vol. ,PP147-161.[4]. Rasheed uzzafar , D and Gahtani, A.S corrosion of reinforcement in concrete structure in themiddle of east concrete international , American concrete institute , vol., 7. No 9. Sep.1985, PP , 48-55.[5]. Rasheed uzzafar Dakhil , F.H and Bader,A.M toward solving the concrete determination problemin the gulf region, the Arabian , Journal of science and Engineering , theme issue on concretedurability vol.11nov