Influence of polypropylene fibers on thermal-induced permeability changes in concrete

During a fire, concrete structures experience rapid heating leading to thermal instability (spalling), compromising load-bearing capacity by reducing the concrete's cross-section or exposing steel reinforcement to flames.Polypropylene fibers are recognized for their effectiveness in mitigating instability risks.The study aims to enhance understanding of fiber's behaviour during heating to optimize their dosage and limit adverse effects on fresh concrete.Four concretes have been studied with calcareous gravels and mortar aggregates, with and without polypropylene fibers.Two samples of each concrete type were tested: C1-0 (calcareous aggregate), C1-18/32(0.5) (calcareous aggregate with fibers), C1MA (mortar aggregate), and C1MA-18/32(0.5)(mortar aggregate with fibers).The use of mortar aggregates allows us to investigate the influence of thermal mismatch and to better understand the interaction between several phenomena: cracking due to thermal mismatch and porosity and cracking due to fiber melting.Controlled heating from 80C to 450C at a rate of 2/min was conducted, followed by a 3-hour stabilization before radial permeability testing.Results indicate a temperature-dependent increase in intrinsic permeability across all concrete types, with mortar aggregates showing lower permeability values.Mortar aggregate induces minimal thermal mismatch compared to calcareous aggregate, owing to its similar expansion and shrinkage characteristics to cement paste.This reduced thermal mismatch results in fewer cracks when exposed to elevated temperatures, further contributing to the lower permeability of concretes containing mortar aggregate.In contrast, fiber-reinforced concretes exhibited higher permeability, primarily due to the melting and expansion of polypropylene fibers around 170C.Overall, these findings suggest that the mechanism that increases permeability and reduces spalling is primarily linked to the addition of fibers.While polypropylene fibers are effective in enhancing concrete's permeability and minimizing thermal instability, the thermal expansion of aggregates also has a high influence in this risk.Further research would explore the effects of mechanical loading on the intrinsic permeability of these concrete types and investigate polypropylene fibers with varying geometries and dosages to compare results with the current findings.