Polypropylene Fibers in Cementitious Composites: A Critical Review of Polymer Structure, Processing, Interfacial Behavior, and Performance

Abstract

Polypropylene (PP) fibers are widely used as secondary reinforcement in cementitious composites, yet their performance is governed as much by polymer structure and interfacial chemistry as by dosage. This review synthesizes 55 primary sources (2003–2025, concentrated post-2015) to examine how PP molecular structure, fiber-manufacturing history, morphology, surface properties and environmental ageing govern fiber–matrix interaction and multiscale composite performance. Isotacticity, molecular weight, crystallinity and drawing ratio are shown to set the fiber’s intrinsic tensile and modulus ceiling before it ever contacts cement paste. Because untreated PP presents a nonpolar surface (contact angle approximately 95–105 degrees, surface energy near 30 mN/m), wetting by the polar cement paste is poor, and the interfacial transition zone relies on friction and mechanical interlock rather than chemical bonding. Surface modification methods, including plasma and corona treatment, silane coupling, maleic-anhydride grafting and mineral coatings, are evaluated for their effect on contact angle, pull-out resistance and durability of the treatment under alkaline exposure. A consistent dosage confound is identified: reported optima cluster near 0.5–1.0% by fiber volume fraction, but many primary studies report weight-based dosage without specifying the denominator, so values are not directly comparable across the dataset. Fiber volume fraction and kg/m3 are adopted as the primary reporting units throughout. Thermal, oxidative, alkaline, freeze-thaw and UV ageing mechanisms are distinguished from ageing of the fiber–matrix interface and of the composite. Structural applications in pavements, precast elements, tunnel linings and impact-resistant elements are retained as evidence of structure–property translation rather than as the primary contribution. Sustainability is assessed quantitatively against steel, glass, basalt and natural fibers, without assuming PP is inherently sustainable. Priority research needs are standardized dosage reporting, quantitative surface-treatment durability data, and design guidance connecting polymer-level variables to composite performance.