Experimental characterization and physics-informed constitutive modeling of sustainable hybrid recycled and bio-based fiber-reinforced cementitious composites

Sustainable hybrid cementitious composites incorporating recycled and bio-based fibers offer a promising strategy for developing low-carbon construction materials with enhanced mechanical performance and fracture resistance. This study experimentally investigates the mechanical and post-cracking behavior of hybrid recycled steel- and bio-based fiber-reinforced cementitious composites at a fixed 50% recycled concrete aggregate (RCA) replacement level. Ten hybrid mixtures combining 0.3% recycled steel fibers recovered from end-of-life tires with bio-based coconut or kenaf fibers at volume fractions of 0.2–1.0% were produced and characterized, and compared against a reference mixture reinforced with 0.6% industrial steel fibers. All mixtures were characterized through compressive, splitting tensile, and three-point notched beam bending tests to evaluate compressive and tensile strength, load-CMOD response, work of fracture, flexural toughness, and residual post-cracking performance. Splitting tensile strength of the hybrid mixtures ranged from 2.92 to 4.05 MPa. The mixtures containing 0.3% recycled steel fibers with 0.6% kenaf or 0.4% coconut fibers ranked highest, reaching initial flexural tensile strengths of 6.14 and 5.63 MPa and flexural toughness values of 49.1 and 40.16 kN·mm. Because the mixing water was set at each mixture's optimum moisture content rather than held constant, the effective water-to-cement ratio varied between 0.50 and 0.62 and co-varies with fiber dosage: compressive and first-crack response are more strongly associated with the water-to-cement ratio, whereas the post-cracking response is consistent with a dependence on fiber composition. The experimentally characterized mechanical and fracture properties were subsequently integrated into a physics-informed constitutive modeling framework combining nonlinear finite element analysis, inverse analysis, and a tri-linear post-cracking constitutive law based on the RILEM TC 162-TDF framework. Every constitutive parameter is obtained in closed form from the measured compressive strength, secant modulus of elasticity and residual flexural strengths, so that the post-cracking law contains no freely fitted constant; the framework reproduced the measured load-CMOD responses of all ten hybrid mixtures within their experimental envelopes, and a unified regression relating the residual flexural force to crack mouth opening displacement (CMOD) and bio-based fiber volume fraction was derived from the calibrated responses. The proposed experimental- and physics-informed constitutive modeling framework provides a reproducible approach for constitutive characterization, performance prediction within the tested range, and comparative ranking of sustainable hybrid recycled steel- and bio-based fiber-reinforced cementitious composites for roller-compacted concrete pavements and other low-carbon structural applications.