Study on concrete strength under uniaxial and biaxial loadings from the view of fracture mechanics

Biaxial compression tests on cubic concrete specimens were performed on a newly designed triaxial machine. Specially designed steel brush platens were adopted in biaxial experiments to reduce the end friction in order to obtain real uniform compression state in the specimens. Concrete with different maximum aggregate sizes were tested for their biaxial compressive strength, with fracture energy and characteristic length examined at the same time. It was found that the biaxial compressive strength of concrete was intrinsically dependent on the fracture energy and characteristic length rather than the uniaxial strength. Therefore, by introducing the fracture parameters, a trend of biaxial failure envelope can be more accurately described than previous research in which the uniaxial strength was always taken as the basis for deriving the envelope. This finding may provide a more accurate design criterion based on fracture parameters in multiaxial problems. On the other hand, numerical simulation of concrete behavior under uniaxial tension, uniaxial compression and combined compression-tension was carried out based on a proposed local softening model by the nonlinear finite element software ATENA. Pure mode I crack was assumed under uniaxial tension and the equivalent wing crack system was employed for uniaxial compression and biaxial compression-tension. The local softening behavior was simulated by means of the interface material model in ATENA. The relationships among the uniaxial compressive strength, tensile strength and the fracture energy under tension were well reproduced compared with the empirical relations. This implies that the local softening law can be viewed as the linkage of these macroscopic properties. Numerical biaxial compressive-tensile strengths based on the local softening model also showed good agreement with available experimental results. Therefore, non-linear fracture mechanics with the local softening law at the sub-macro-scale seems to be suitable for analyzing concrete behaviors at the macro-scale