Study on Mechanical Properties of Hybrid Fiber-Reinforced Phase Change Concrete

Phase change materials (PCMs) can mitigate early age thermal cracking in mass concrete but usually reduce mechanical strength due to their low stiffness and weak bonding to the cement matrix. This study investigates a hybrid fiber strategy using steel fibers (SF) and carbon fibers (CF) to compensate for this strength loss in phase change concrete (PCC). Single-factor tests and response surface methodology (RSM) were used to quantify the effects of phase change aggregate (PCA) replacement ratio and fiber contents and to optimize the hybrid system. Compared with plain PCC containing 50% PCA, mixtures with 1.0% SF and 0.3% CF increased the 28-day compressive/splitting tensile strengths by 11.15%/34.57% and 8.90%/27.78%, respectively, whereas the RSM-predicted optimal hybrid combination (0.895% SF + 0.291% CF) produced the greatest gains, with strength increases of 20.98% in compression and 42.28% in splitting tension. The quadratic RSM models (R2 > 0.97) predicted 28-day strengths of 50.57 MPa and 4.61 MPa, which agreed with experimental results within 5%. These findings confirm a clear synergistic effect between SF and CF in terms of 28-day mechanical strength performance within the investigated dosage ranges and demonstrate that hybrid fiber reinforcement is an effective way to enhance the mechanical performance of functional PCM-based concrete.