Determination of fracture process zone in hybrid fiber reinforced ultra-high performance concrete using acoustic emission entropy

This study investigates the evolution of fracture process zone (FPZ) in ultra-high performance fiber reinforced concrete (UHPFRC) using the acoustic emission (AE) technique.An important factor contributing to the fracture energy in concrete is the nucleation of its FPZ.The examination of FPZ helps in understanding the initiation and propagation of cracks in a material.The influence of different toughening mechanisms within the FPZ helps in predicting the structural integrity and performance of a material when subjected to an external loading.Thus, it is essential to investigate the evolution process of FPZ.In order to determine it, three-point bending tests with monotonic loading are performed on three notched beam specimen groups: ultra-high performance concrete (UHPC) without fibers, UHPFRC with 2% (by volume percentage of concrete, (V f )) straight micro-fiber, and a hybrid UHPFRC with 1% (V f ) straight micro-fibers and 1% (V f ) straight macro-fibers.Straight micro-steel fibers of 13 mm length and 0.2 mm diameter and straight macro-steel fiber of 26 mm length and 1 mm diameter have been used for the study.An AE parameter based on Shannon's entropy is used to examine the evolution of FPZ.The AE entropy performed more efficiently compared to the other conventional AE parameters on account of it being independent of threshold, and being more sensitive to changes in AE signal irregularities.It is observed that the entropy distribution is greatly influenced by the presence of fibers, indicating a more controlled and distributed cracking process in UHPFRC compared to the brittle behaviour of UHPC without fibers.It is also observed that the FPZ width decreases when straight micro-fibers is partially replaced with straight macro-fibers.