Shape effect on compressive mechanical properties of compound concrete containing demolished concrete lumps

The viability of directly using coarsely crushed demolished concrete lumps (DCLs) to replace a portion of the new pouring concrete in structural members has been demonstrated in previous research and in practice. However, there is no information focused on the shape effect on the compressive mechanical properties of the compound concrete now. Meanwhile, the lateral deformation characteristic (e.g., volume dilatancy) of the compound concrete made of DCLs and fresh concrete still remains not very clear. To deepen the knowledge related with this concrete type, a total of 93 specimens containing DCLs including cubes, cylinders, and prisms were fabricated and axially loaded up to failure. The replacement ratio of DCLs considered in this study covered a relatively wide range (i.e., 0%, 20%, 30% and 40%). The test results indicate that the compressive strength of cubic compound concrete samples is about 1.30 that of similar cylindrical samples, and is 8.3% larger than that for conventional concrete. The compressive strength of the compound concrete measured with prismatic samples is about 1.08 times that measured using cylindrical samples, while this ratio is about 1.0 for conventional concrete. A formula has been established to predict the compressive strength of the cubic, cylindrical, and prismatic compound concrete specimens with a lateral dimension of 150-300 mm, and the predicted strength is usually within ±10% of the measured value. The ratio of the critical load (i.e., the load at which the volume of the concrete starts to increase) to the ultimate load decreases slightly with increasing DCL replacement percentages.