Effect of polypropylene fiber length on the mechanical properties, frost resistance, and microstructure of silica fume-modified self-compacting concrete

Abstract

This study investigates the influence of polypropylene fiber length (6–20 mm) on the mechanical properties, frost resistance, density, and microstructure of silica fume-modified self-compacting concrete (SCC). Microstructural analysis by scanning electron microscopy (SEM) was performed on the fiber-free reference mixture and the optimum fiber lengths of 12- and 15-mm. Polypropylene fibers affected the durability and strength performance of SCC: mixtures with 12 mm and 15 mm fibers performed best overall, reaching the highest 28-day compressive strengths (55.3 and 54.9 MPa), average density up to 2412 kg/m³, and frost resistance up to F350. Water absorption decreased from 4.8% in the fiber-free reference mixture to 4.1-4.2% in these mixtures. SEM observations indicated a relatively dense cement matrix with fewer visible pores and apparent fiber–matrix contact in the 12 mm and 15 mm mixtures. Because all mixtures contained the same silica fume dosage, its individual effect could not be experimentally isolated; based on the literature, its pozzolanic and microfiller effects may have contributed to the observed matrix morphology, while differences among mixtures were primarily associated with fiber presence and length. Overall, 12 mm and 15 mm fibers provided the best combination of mechanical and durability-related properties among the lengths tested.