COMPARISON OF CORE AND CUBE COMPRESSIVE STRENGTH OF HARDENED CONCRETE M. Yaqub*, University of Engineering and Technology Taxila, Pakistan M. Anjum Javed, Concordia University, Canada 31st Conference on OUR WORLD IN CONCRETE & STRUCTURES: 16-17 August 2006, Singapore Article Online Id: 100031053 The online version of this article can be found at: http://cipremier.com/100031053 This article is brought to you with the support of Singapore Concrete Institute www.scinst.org.sg All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information www.cipremier.com
31 st Conference on OUR WORLD IN CONCRETE & STRUCTURES: 16 17 August 2006, Singapore COMPARISON OF CORE AND CUBE COMPRESSIVE STRENGTH OF HARDENED CONCRETE M. Yaqub*, University of Engineering and Technology Taxila, Pakistan M. Anjum Javed, Concordia University, Canada Abstract This paper presents the result of core and cube compressive strength of hardened concrete. Non destructive test methods are used to investigate the properties and strength of hardened concrete. In existing concrete structures there was no direct relation between the results of non-destructive tests. This paper describes the relation between core compressive strength and cube compressive strength of hardened 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 the same 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 stone cutting machine. The cores and cubes compressive strength was determined in compression testing machine. It was found that smaller size cubes (100mm x 100mm x 100mm) and cores (48mm diameter) show greater strength as compared to larger size cubes (150mm x 150 x 150mm) and cores (diameter 75mm). It was also found that the cubes (150mm x 150 x 150mm) compressive strength is 76% the cubes (100mm x 100mm x 100mm) compressive strength. The 75mm diameter cores 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. Introduction Once concrete has hardened it can be subjected to wide range of tests to prove its ability to perform as planned or to discover its characteristics if its history is unknown. For new concrete this usually involves casting specimens from fresh concrete and testing them for various properties as the concrete matures. The concrete cube tests, concrete cylinder tests are the most familiar tests and are used 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 for tensile strength. Specimens for many other tests can be made at the same time to assess other properties.
For existing concrete samples will need to be taken from the structure. Non destructive testing methods are useful in some instances and can help identify areas from which samples should be taken. The normal method of concrete sampling is by coring although same chemical analysis techniques can be carried out on drilling dust samples. In laboratory many techniques can be used to examine and test hardened concrete to assess a wide Varity 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 indicate properties other than strength [3, 4]. British Standard defined non destructive testing as a test that does not impair the intended performance of the element or member under investigation [5]. The nondestructive evaluation techniques are used to assess the condition of concrete structures, to predict future performance and allow minor repair system. The best known non destructive evaluation techniques use ultrasonic waves, core testing, Schmidt hammer test. The estimation of in place concrete strength requires that a known relationship between the results of in-situ testing and the strength of concrete. For existing construction the relationship has to be assessed on site correlating non-destructive test results to strength of core. There was no any relation between core strength and cube strength of hardened concrete. In this research work a relationship between core strength and cube strength was developed. This paper also describes the effect of diameter of core on compressive strength of concrete and effect 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 cube compressive 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-years old concrete buildings at different locations of the structural members. Some cores were taken from columns, beams and some cores were taken from base slab, floor slabs and walls. In the same location 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 150mm and 100mm x100mm x 100mm by stone cutting machine. The cores were extracted by using rotary cutting machine with diamond bits. The extracted cores were trimmed to make smooth edges parallel and then capped with sulphur to make the ends smooth. After capping, cores were tested in compression testing machine in the concrete 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 x 100mm 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] and the 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 cores were tested in dry condition. The strength of cores was calculated after applying all the factors given in ASTM C42-90.
Sr. No. Table 1 Results of Cores (75mm diameter) Weight (gm) Dia (mm) Length (mm) Area (mm 2 ) Corrected Compressive Strength of core (MPai) 1. RCC Column 1524 75 150 4418 12.627 2. RCC Column 1516 75 150 4418 10.885 3. RCC Column 1608 75 150 4418 19.158 4. RCC Column 1572 75 150 4418 19.375 5. RCC Slab 1500 75 150 4418 14.1509 6. RCC Slab 1448 75 140.462 4418 26.930 7. RCC Slab 1196 75 121.41 4418 23.2198 8. RCC Slab 1138 75 114.046 4418 25.749 9. RCC Wall 1480 75 150 4418 7.619 10. RCC Beam 1568 75 150 4418 21.770 11. RCC Beam 1564 75 150 4418 21.537 12. RCC Slab 1124 74.93 4.78 4418 14.142 13. RCC Wall 1546 74.93 113.79 4418 13.596 14. RCC Column 1562 74.93 113.79 4418 16.733 15. RCC Beam 1518 74.93 113.79 4418 12.550 16. RCC Beam 1486 74.93 150 4418 10.0145 Sr. No. Weight (gm) Table 2 Results of Core (48mm diameter) Dia (mm) Length (mm) Area (mm 2 ) Corrected Compressive Strength of core (MPa) 1. CS-3 360.91 48 92.40 1809.557 15.840 2. BS-2 392.92 48 96.64 1809.557 22.3117 3. BS-1 389.93 48 97.18 1809.557 23.3710 4. CS-1 400.53 48 100.12 1809.557 22.3117 5. GS-2 385.97 48 98.17 1809.557 13.810 6. CS-2 398.36 48 100.15 1809.557 19.120 7. R-7 300 48 77.26 1809.557 12.352 8. R-3 258.52 48 64.77 1809.557 15.507 9. BS-4 250.31 48 64.70 1809.557 12.00 10. R-3 (RAFT) 239.72 48 61.69 1809.557 15.877 11. R-3 212.62 48 52.64 1809.557 21.347 Sr. No. Cube size Table 3 Results of Cubes Corrected Compressive Strength of core (MPa) 1. 150 x 150 x 150 Column 25. 2. 150 x 150 x 150 Column 20 3. 150 x 150 x 150 Column 28 4. 100 x 100 x 100 Slab 33 5. 100 x 100 x 100 Slab 33 6. 100 x 100 x 100 Slab 28 7. 100 x 100 x 100 Slab 35 8. 100 x 100 x 100 Slab 32 9. 100 x 100 x 100 Wall 32 10. 100 x 100 x 100 Wall 26 11. 100 x 100 x 100 Wall 27 12. 100 x 100 x 100 Wall 35 13. 100 x 100 x 100 Slab 31 14. 100 x 100 x 100 Slab 35 15. 100 x 100 x 100 Slab 36
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 as compared 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 the larger size diameter of cores. Similarly the smaller size cubes give greater strength as compared to larger 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 the compressive 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). The core (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 of cubes (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 hardened concrete, BS, 881, 1986, part 201. [2]. Fookes P.G concrete in the middle test past, present ad future, review Damage Assessment Repair Techniques and strategies for reinforcement concrete Macmillan, G.L ed. Bahrain society of Engineers 1991. [3]. Pocock,D.C the selection o cost-effective repair strategies for corrosion damage concrete an determination 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 the middle 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 problem in the gulf region, the Arabian, Journal of science and Engineering, theme issue on concrete durability vol.11nov