Experimental study on mechanical properties of polypropylene fiber foamed concrete after exposure to high temperatures

Foam concrete (FC) possesses beneficial properties such as low density and good thermal insulation compared to conventional concrete, but its high-temperature mechanical behavior remains to be fully understood. The present research investigates the post-exposure (20°C to 400°C) properties of polypropylene fiber foamed concrete (PPFFC). Specimens were characterized by a fixed density of 1000 kg/m³, a water-cement ratio of 0.5, and PPF dosages of 0%, 0.25%, 0.5%, 0.75%, and 1%. Tests included compressive strength, mass loss, and water absorption. Microstructure was analyzed by optical microscopy and SEM, and damage progression was monitored using acoustic emission (AE). This study demonstrates that incorporating polypropylene fiber (PPF) significantly enhances the residual strength and toughness of foam concrete after high-temperature exposure. Results indicate that at 400℃, the plain specimen (0% PPF) suffered total collapse with nearly 30% mass loss, whereas the 1% PPF specimen retained significant structural integrity with a mass loss of only about 15%. Notably, the residual compressive strength was substantially improved with PPF addition; for instance, at 400℃, the strength retention was most pronounced in specimens with 0.5% to 1% PPF compared to the control group. Microstructural analysis reveals a temperature-dependent mechanism: below 200°C, PPF bridges and restrains cracks; at 300°C, its melting forms channels that relieve steam pressure and mitigate spalling. Although Ca(OH)₂ decomposition at 400°C elevates internal pressure, PPF’s prior actions delay the failure process. Acoustic emission data confirm that PPF promotes energy dispersion, improving toughness. Based on a comprehensive assessment of mechanical strength, workability, and microstructural integrity, the optimal PPF dosage is identified as 0.5%-0.75%.