Compressive behaviors of cubes and cylinders made of normal-strength demolished concrete blocks and high-strength fresh concrete

• The mix of normal-strength DCBs and high-strength FC is feasible to applications.
• The weaker DCBs affect the specimens’ compressive behaviors more significantly.
• The effect of DCBs on cylindrical strength is greater than that on cubic strength.
• Formulas are proposed to describe the compressive behaviors of the specimens.

To reduce the cost of reuse of waste concrete, demolished concrete blocks (DCBs) having distinctly larger size than conventional recycled aggregates were adopted in researches and practical applications in the past several years for structural members, which were made of normal-strength DCBs and normal-strength fresh concrete (FC). To extend the use of DCBs to high-strength concrete structures, the mechanical behaviors of concrete mix made of normal-strength DCBs and high-strength FC should be investigated carefully. In this paper, thirty 300 mm cubic specimens and twenty-four cylindrical specimens (diameter 300 mm, height 600 mm) with different replacement ratios of DCBs (0%, 20%, and 33%) were fabricated using two kinds of high-strength FC (74.9 MPa, and 112.2 MPa) and two types of normal-strength DCBs (23.2 MPa, and 33.1 MPa), and the compressive behaviors of the cubes and cylinders under uniaxial loadings were experimentally studied. Based on the test results, formulas have been presented to determine the combined compressive strength, modulus of elasticity, and peak strain of the concrete mix made of normal-strength DCBs and high-strength FC. It is found that: (a) the specimens made of normal-strength DCBs and high-strength FC generally displayed a gradual failure, while the specimens made of high-strength FC alone exhibited a sudden failure, and the DCBs and FC bonded well, implying the mix of normal-strength DCBs and high-strength FC being feasible to practical applications; (b) the larger the difference between the demolished concrete’s strength and the fresh concrete’s strength is, the more significant the influence of the DCBs on the combined compressive strength, modulus of elasticity, and peak strain of the specimens is; (c) the influence of the DCBs on the combined compressive strength of the cylinders is more significant than that on the combined compressive strength of the cubes, leading the ratio of the combined cylindrical strength to the combined cubic strength being only about 0.65; and (d) the proposed formulas agree well with the test results on the whole.