Direct tension test of large concrete specimens - Part II: Insights into micromechanical fracture of concrete through acoustic emission and digital imaging

In the first part of the companion paper, experimental procedures for conducting direct tension tests on large concrete specimens were discussed, together with the fracture properties and size effects. In this paper, we delve into the details of micromechanisms of fracture and failure in concrete under direct tension using digital imaging and acoustic emission (AE) techniques. The surface cracking characteristics are obtained from imaging, while the evolution of microcracks in the fracture process zone (FPZ) are predicted through AE events and energy. The differences in microcrack initiation, and their coalescence to form macrocracks for different sizes of specimens are explained, and the size of the FPZ is estimated. It is concluded that in large-size specimens, the size of FPZ is relatively smaller with the formation of microcracks, their coalescence to form macrocracks, and the propagation of the final crack taking place almost simultaneously, leading to a brittle failure.