The strength of concrete is considered as the most basic and important material property in the design of reinforced concrete structures. The two standard methods (ASTMC39-03 and BS1881-part 116) for determining the compressive strength of concrete are the testing to failure of cylinder and cube specimens respectively. It has become a problem to use this value as the control specimen sizes and shapes are different from country to another.
This study is made to comparison between propose hexagonal specimens with the standard cylindrical one in behaviour of specimens with different sizes under compression load. The new hexagonal specimen has a cross sectional area 4% greater than the cylindrical cross section. The volume of the hexagonal specimen is approximately equal to the cylindrical ones to give a greater uniformity of results. This proposed shape has height to average diameter ratio (H/D) = 1.95.
The hexagonal mould has the ease of making a control specimen while avoiding the capping process which is necessary in cylinders and reducing the cost and time it takes to prepare. When the hexagonal specimen cast in horizontal plane so no need for end preparation because the top and bottom surface of the specimen who will be in contact with platen of testing machine, are 100 % levelled and orthogonal to the height of the specimen. Results show that the mean ratio of compressive strength for hexagonal to cylindrical specimen is 0.95.The results of testing specimens show that the compressive strength of the specimen obviously increases with decreasing the ratio of H/D. This behaviour is proving that the hexagonal specimen acts like the standard cylindrical in stress distribution.
The using of hexagonal specimen to find the strength of concrete give better factor of safety in structural design that because strength determined from hexagonal specimens will be slightly smaller when compared with results determined from cylindrical and the behaviour of the hexagonal shape is similar to cylindrical one.