Relationship between inter-aggregate spacing and the optimum fiber length for spalling protection of concrete in fire

Addition of polypropylene fibers in concrete for spalling protection in fire is well known, where the fibers melt in fire and percolate the concrete. However, the optimum fiber characteristics for spalling protection are not well understood. In this study, the optimum fiber length for spalling protection of concrete is found to be related to the inter-aggregate spacing of coarse aggregates, which is critical for the interconnectivity of adjacent aggregates surrounded by porous interfacial transition zones. The experimental test results showed that, for the concrete with maximum 20 mm of graded aggregates, with optimum 30 mm length fibers, almost no spalling was found; whereas other fiber concretes lost up to 92% of weight due to spalling. By evaluating the criteria of the optimum fiber length, this study also found the critical threshold of fiber numbers per unit volume.