Effect of geometry of concrete specimen on compressive strength

In this thesis the effect of shape and size of concrete specimen with different strength levels on compressive strength of concrete were investigated. For this purpose one type of cement, one type aggregate ( fine and coarse ) and one type water reducing chemical admixture were used. Besides, a fly ash and a silica fume were added to the high strength concrete mixtures. 11 different concrete mixtures having water / binder ( W/B ) ratios ranging beween 0.23 to 0.89 were prepared. 150 mm cube, 100 * 200 mm as well as 150 * 300 mm cylinder specimens were cast. 2, 7 and 28-day compressive strength of the concrete specimens were determined. The cube specimens were loaded in two different directions i.e., parallel and perpendicular to the casting direction. It was concluded that at 2-day age, 100 * 200 mm cylinder specimens showed a higher strength than 150 * 300 mm specimens. However, by elapsing time, in medium- and high- strength concrete mixtures, the difference between 100mm and 150 mm diameter cylinder specimens was about ± 5%. Irrespective of the concrete age, the differences between compressive strength values of 150 mm cube and 150 * 300 mm cylinder specimens varied in the range of 10% to 15% in low strength concretes. The difference between compressive strength of 150 mm cube specimens loaded parallel and perpendicular to the direction of casting was high at early ages, however, at later ages this difference decreased. Specially, with an increase in the level of compressive strength of concrete, the ratio between the strength of cube specimens loaded parallel and perpendicular to the direction of casting decreased at early ages. Keywords: Compressive strength of concrete, shape of concrete specimen, size of concrete specimen.