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.

This paper presents the study of a two-hundred-year-old, demolded solid clay brick using the rebound hammer test for the estimation of the compressive strength. During the test, the location and face type influence on the rebound values are monitored and recorded. In addition, the calculation of the average rebound value has been modified to encounter the influence of location and face types. Furthermore, the estimated compressive strength is compared with the normalized mean compressive strength to check the accuracy of the rebound hammer test if it is within the confidential limit of ±25%. The result shows that the location and surface types have influence on the rebound value, which in turn affected the compressive strength.

Traditional supplementary cementitious materials (SCMs), like metakaolin or microsilica, highly improve various properties of concrete, making it more resistant against the effects of the environment. However, the cost of concrete is highly influenced by the applied amount of SCMs. Cellular concrete powder (CCP) is a by-product, produced by the cutting and curving process of cellular concrete blocks, which has the possibility to be used for similar purposes as SCMs. The present paper introduces the results of the laboratory experiments, conducted by the authors, and their analysis. The experiments were designed to investigate the effects of CCP on mortars, normal and high-strength concretes in terms of strength and durability. To get a deeper understanding of the processes taking place in a mortar or concrete containing CCP, thermogravimetric analysis was performed as well. CCP was applied in the mixes in different amount, and the performance of CCP was compared to SCMs. CCP was found to be a feasible alternative to SCMs in case of normal-strength concrete, while in case of high-strength concrete it did not improve the strength of the reference concrete. It could be traced back to the sulfate swelling of CCP particles. In optimal dosage, CCP was found to be preferable to metakaolin (MK) for durability characteristics. The combined usage of CCP and MK was investigated as well, but it did not result a concrete with better performance than the reference mix.