Influences of size and shape of demolished concrete lumps on compressive behavior of compound concrete

Abstract Previous research on the mechanical behavior of compound concrete (CC), which is made of demolished concrete lumps (DCLs) and fresh concrete (FC), has focused on the CC made with roughly spherical DCLs with specified sizes. Limited knowledge is available about the effects of size and shape of DCLs on the compressive behavior of CC, especially when high-strength FC is used. This study investigated the compressive behavior of CC containing DCLs with various characteristic sizes (50, 67, 100 and 133 mm) and shapes (oblate, spherical and slender). And the interfacial transition zones (ITZs) between FC and DCLs with different sizes and shapes were observed using scanning electron microscope (SEM). Laboratory compression tests show that the characteristic size of DCLs has little effect on the compressive strength or elastic modulus of CC made with high-strength FC. For a given lump size, the compressive strength is similar when slender or spherical DCLs are included, but oblate lumps give a compressive strength 10% lower. When the compressive stress exceeds 40% of the CC’s compressive strength, there is greater lateral deformation of specimens made with oblate DCLs compared with spherical or slender DCLs at a lump size of 133 mm. In addition, oblate DCLs bond to FC less strongly.