Effect of Recycled Polypropylene Fibers on the Compressive and Tensile Properties of Concrete: Fiber Type, Aspect Ratio and Volume Fraction

This study aimed to evaluate whether short-cut recycled polypropylene (RPP) fibers can serve as an effective and sustainable alternative to virgin polypropylene (PP) fibers for improving the mechanical performance and crack-control behavior of concrete. For this purpose, PP and RPP fibers with two lengths, 3 mm and 6 mm, and three volume fractions, 0.1%, 0.2%, and 0.3%, were incorporated into C35 concrete. Compressive strength, splitting tensile strength, apparent stress–strain response, slump, failure morphology, and microstructural characteristics were examined at curing ages of 7 and 28 days. The results showed that both PP and RPP fibers improved crack distribution and failure integrity, while the mechanical enhancement strongly depended on fiber type, length, and dosage. Among all mixtures, RPPC3-0.1 exhibited the best overall performance; at 28 days, its compressive strength and splitting tensile strength were approximately 6% and 24% higher than those of plain concrete, respectively. These findings suggest that low-dosage short RPP fibers can effectively enhance the tensile-related performance of concrete while providing a feasible pathway for the value-added reuse of polypropylene waste.