Specimen Size and Shape Effect on the Compressive Strength of Normal Strength Concrete

Present study deals with the effect of size and shape on the compressive strength of concrete. Mixes of five different strengths were designed, all between C20/25 and C50/60 strength class (normal strength concretes). Different size specimens were casted from all mixes in cube (edge length: 50 to 200 mm) and in cylinder (diameter: 60 to 150 mm) shapes as well. An equation was derived to estimate the compressive strength of the differently sized specimen. The parameters of the equation were optimized based on measurement data using nonlinear least-squares method with SSE (sum of squared errors) cost function. The parameter optimization was performed in different models (estimation based on the standard strength of cube or cylinder to approximate the different size specimens' cylinder or cube strength with the edge length, the surface area or the volume as dimension data). The test results showed good agreement between the laboratory measurements and the literature data (compressive strength is decreasing with the increase of the size of the specimen). The derived estimation models showed good correlation with the measurement data and with the literature estimation models, in some cases even with lower errors. The results indicated that size effect is stronger on concretes with lower strength class due to the higher level of inhomogeneity of the material. It was observed that size effect is more significant on cube specimens than on cylinder samples, which can be caused by the side ratios of the specimens and the size of the purely compressed zone. A limit value for the size of both cube and cylinder specimens was determined, above which the size effect on compressive strength can be neglected.