Basically, the results of compression test carried out by using cubes are higher than that by cylinders. In compression test, the failure mode is in the form of tensile splitting induced by uniaxial compression. However, since the concrete samples tend to expand laterally under compression, the friction developed at the concrete-machine interface generates forces which apparently increase the compressive strength of concrete. However, when the ratio of height to width of sample increases, the effect of shear on compressive strength becomes smaller
So just a complement to what Dr Albayati wrote:
There are three standard compression tests - cubes, parallelepipeds and cylinders. The failure mode is tensile splitting due to Poisson effects (lateral expansion of the specimen) and brittle buckling of the splitted areas.
Most RC members are approximately 1-D or 2-D so if you have cubes you don't get represent adequately the actual members. You need parallelepipeds or cylinders. However if you use parallelepipeds you have increased friction and also stress concentration in the edges which may be inadequate for the real member.
So cylinders are the best option. The other good thing about them is they are easier to obtain with in-situ with a boring. Whatever specimens you test you need also to be alerted for scaling effect: you have standard dimensions of the samples for which you get the standard strength. If your samples are larger than standard you underestimate strength, if they are smaller you overestimate it so in both cases you need scaling factors.
Good luck!
It depends on the type of concrete you are about to prepare. Also take care of the maximum size of aggregates used, and match the minimum size of specimens that your Standard code of practice suggests.
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