Fracture evaluation of recycled steel fiber-reinforced geopolymers with varying compressive strength

The use of recycled steel fibers has gained importance in the manufacturing of geopolymer concrete due to environmental benefits. However, the previous works have neglected the relation between the compressive strength of geopolymer concrete and the efficiency of recycled steel fibers, though the efficiency of the industrial steel fibers in ordinary cement-based concrete remarkably relies on the concrete strength. The main objective of the present work is to shed light on this specific area of the geopolymer concretes. The main test parameters in the experimental study were the compressive strength (15, 40, and 70 MPa) and the recycled steel fiber content (0 %, 2 %, and 4 % by mass). A total of 27 prismatic notched beams from the 9 unique concrete mixtures were tested for three-point bending. The crack evolutions on the specimen surface were traced by using the digital image correlation method. The test results were evaluated in terms of fracture energy, critical crack mouth opening displacement (CMODc), critical crack length, and unstable fracture toughness. According to the test results, it can be noted that the fracture energy and the unstable fracture toughness of geopolymers were improved by the concrete strength grades though, they were hugely influenced by the raising recycled steel fiber ratios. Besides the laboratory study, the axial stress softening curves of geopolymer mixtures were derived from the load-deflection response of the notched beams by using the iterative inverse analysis method.