Effect of size and shape of test specimens on compressive strength of normal strength concrete

Abstract

Different codes (ASTM, BS etc) specify different size/shape test specimens for quality assurance testing. Cylinder specimens of 6×12 in. (150×300 mm) or 4×8 in. (100×200 mm) and cube specimens of 6×6 ×6 in. (150×150×150 mm) or 4×4×4 in. (100×100×100 mm) are widely used in different countries. In our country, design specifications refer to the compressive strength obtained from either testing 6×12 in. concrete cylinder or 6 in. concrete cube and tested as per relevant standards. However, 4×8 in. cylinder specimens are almost exclusively used in our country nowadays considering testing machine capacity, ease of handling, cost of materials etc. Therefore, a correlation between concrete compressive strengths of using cylinders and cube specimens are necessary. L'Hermite equation is widely used as a conversion factor between the standard cube and cylinder specimens. However, the applicability of this conversion factor may require to be evaluated in context of concrete as used in our country. Therefore, the main objective of this research is to study the effect of size and shape of test specimens on compressive strength of normal strength concrete. The variability of using L’Hermite equation as a conversion factor of compressive strength is also evaluated.

An experimental program is carried out to study the size and shape effect of concrete test specimens on normal strength concrete. A total of 324 nos. samples (90 nos. 6×12 in. cylinders; 90 nos. 4×8 in. cylinder; 72 nos. 6 in. cube and 72 nos. 4 in. cube) have been prepared and tested for unit weight, compressive strength, splitting tensile strength, static modulus of elasticity. In addition, a non-destructive test UPV is conducted to evaluate the concrete compressive strength. During the experimental study, 06 (six) strength levels (concrete mix design) i.e., 7 MPa (1000 psi) to 41 MPa (6000 psi) were obtained by mostly varying water-cement ratio, and other parameters such as maximum size of coarse aggregate, properties of fine and coarse aggregates and fine aggregate-to-total aggregate ratio were kept almost the same. 2% air content is considered in all concrete mixes. Parameters like curing condition, capping method, consolidation and testing method were controlled as per relevant standards (ASTM and BS) for the entire test program.

From the experimental results, it is observed that the smaller size cylinder specimen (4 × 8 in.) has higher compressive strength than that of larger sizes (6×12 in.) and the same trend is followed for the cube specimens. The ratio of compressive strength of 4×8 in. cylinder specimens to that of 6×12 in. specimens is 1.13 on an average with coefficient of variation of 6.2 percent. The average ratio of compressive strength of 4 in. cube specimens to that of 6 in. is approximately 1.06 with a coefficient of variation of 6.6 percent. The ratios of compressive strength between standard cylinder to standard cube are dependent on strength of concrete and the ratios increases with the increase of strength. Using L’Hermite equation to convert standard cube strength to cylinder strength is not conservative for strength level between 7 MPa and 41 MPa in case of concrete produced in our country. For the strength range between 41 MPa to 53 MPa, L’Hermite equation yields conservative values. When concrete strength using cylinder specimens is converted to cube strength according to LGED specification, it always overestimates the concrete strength for the range of 7 to 40 MPa concrete. For strength ranges between 10.5 to 62 MPa, REB specification provides better agreement with the test results.