Size effect on compressive behaviours of normal-strength concrete cubes made from demolished concrete blocks and fresh concrete

Abstract

This work presented in this paper contributes to a comprehensive research into structural elements containing demolished concrete blocks (DCBs) with a distinctly larger size than conventional recycled aggregates. In this research, 51 cubic specimens made from DCBs and fresh concrete (FC) with a replacement ratio of 30% were fabricated and tested under uniaxial compressive loadings. Two parameters were considered – the cube dimension and the characteristic size of DCBs. The influence of the two factors on the compressive behaviours of normal-strength concrete cubes made from DCBs and FC was examined in terms of the combined cubic compressive strength, the modulus of elasticity and the strain at peak stress. Based on the test results, a formula is presented to describe the relationship between the characteristic ratio (i.e. ratio of the characteristic size of DCBs to the cube dimension) and the combined cubic compressive strength of the specimens made from DCBs and FC, and a model is proposed to predict the combined cubic compressive strength of the specimens with various cube dimensions and characteristic ratios. The following results were obtained. In the case that the characteristic ratio is constant, the combined cubic compressive strength of the specimens made from DCBs and FC decreases significantly with an increase in cube dimension, and the effect of cube dimension on the combined strength accords with both the size effect law and the modified size effect law for conventional concrete. The effects of both the cube dimension and the characteristic size of DCBs on the specimen's modulus of elasticity and strain at peak stress are limited. In the case that the cube dimension is kept constant, the combined cubic compressive strength of the specimens made from DCBs and FC decreases gradually with an increase in characteristic ratio; the influence of cube dimension on this trend is very limited. Both the established formulae and the proposed model agree quite well with the test results.