Compressive behaviors of cylindrical concrete specimens made of demolished concrete blocks and fresh concrete

Abstract Adopting demolished concrete blocks (DCBs) rather than recycled aggregates in structural members may reduce the cost of reuse of waste concrete. Here DCBs have distinctly larger size than the conventional recycled aggregates. To investigate the effect of DCBs’ size on axial behaviors of the specimens made of DCBs and fresh concrete (FC), and to check whether the interface zones between DCBs and FC are weaker regions in the specimens, two series of tests were conducted in this paper. In the first series of tests, twenty-four cylindrical specimens made of DCBs and FC with a replacement ratio of 30% and twelve cylindrical specimens made of FC alone were fabricated and tested under uniaxial compressive loadings. The height-to-diameter ratio of all the specimens was the same as 2.0, and the diameters of the specimens ranged from 150 mm to 400 mm. The influences of both the cylinder dimensions and the characteristic size of DCBs on the specimens’ compressive strength, the modulus of elasticity, and the strain at peak stress have been experimentally investigated. Based on the test results, a formula is presented to describe the relationship between the characteristic ratio (i.e., a ratio of the characteristic size of DCBs to the cylinder diameter) and the compressive strength, and a model is established to determine the compressive strength of concrete cylinders with various diameters and different characteristic ratios. In the second series of tests, three 200 mm × 200 mm × 50 mm specimens cut from two 400 mm × 400 mm × 200 mm samples made of DCBs and FC were axially loaded, and the initiation and propagation of the specimens’ surface cracks were recorded using the two-dimensional digital image correlation technique. It is found that: (a) the effect of cylinder size on the strength of the specimens made of DCBs and FC is similar to the effect of cylinder size on the strength of the specimens made of FC alone, but the size effect on the relative strength for the former specimens is a little more distinct than the size effect on the relative strength for the latter specimens; (b) in the case that the cylinder dimensions keep constant, the strength of the specimens made of DCBs and FC decreases gradually with the increasing of the characteristic ratio; (c) for both the cylindrical specimens made of FC alone and those made of DCBs and FC, the cylinder dimensions and the characteristic size of DCBs have no clear effects on the modulus of elasticity and strain at peak stress; (d) both the presented formula and the established model mentioned above are in good agreement with the test results; and (e) microcracks do not concentrate in the interface zones between DCBs and FC, and these zones are not obviously weaker regions in structural members.