Effects of Aggregate and Specimen Sizes on Lightweight Concrete Fracture Energy

To evaluate the effects of beam specimen depth and aggregate size on the fracture energy of lightweight concrete (LWC), different beam specimens designated into 32 notations were tested under three-point bending. In each of the all-lightweight concrete and sand-lightweight concrete groups, the maximum aggregate size varied between 4 and 19 mm; the beam depth ranged from 150 to 600 mm in each ready-mixed concrete batch with the same mix proportions. Based on experimental observations and verification of prior empirical models, simple closed-form equations were proposed to generalize the influence of the concrete unit weight on the size effect for the fracture energy of concrete. Test results clearly showed that when the maximum aggregate size is larger than 8 mm, the aggregate size in LWC has an insignificant effect on fracture parameters such as the fracture energy, crack opening mouth displacement, and characteristic length due to crack propagation through the lightweight aggregate particles. The fracture energy of LWC was lower than that of normal-weight concrete, indicating that the size-dependence of the fracture energy increases with decreasing concrete unit weight. A comparison between the predicted and experimental fracture energies revealed that the reliability of existing models significantly depends on the concrete type and ligament depth of the beam specimen, whereas the proposed model generally gives better agreement with the test data; it consistently predicts the trend of the size effect, regardless of the concrete unit weight.