Fiber Systems in Strain-Hardening Cementitious Composites: A Review of Multiscale Mechanisms and Engineering Applications

Strain-hardening cementitious composites (SHCCs) achieve tensile strain hardening and distributed multiple cracking through coordinated matrix cracking, fiber bridging, and interfacial stress transfer. This structured narrative review synthesizes evidence on single- and hybrid-fiber systems using a multiscale framework linking fiber properties and geometry, dispersion and orientation, fiber–matrix interfaces, crack-bridging response, strain-hardening criteria, and engineering performance. The evidence shows that no fiber is universally optimal: performance depends on matching fiber strength and geometry with matrix fracture properties, controlled debonding and frictional sliding, and reliable processing. Excessively weak interfaces provide insufficient bridging stress, whereas excessively strong interfaces promote fiber rupture and crack localization. Nominal fiber volume fraction does not equal the effective bridging population because dispersion and orientation vary with mixing, casting, pumping, spraying, and extrusion. Hybrid systems may broaden the crack-bridging range, but a positive hybrid effect requires comparison with single-fiber controls using the same matrix and total fiber volume fraction; poor rheology and competition among fibers can instead reduce performance. Engineering selection should therefore be application-specific and evidence-limited, with direct tensile behavior, crack control, durability, constructability, and life-cycle performance evaluated together.