Role of Superabsorbent Polymers in Balancing Shrinkage Mitigation and Mechanical Performance of Slurry-Infiltrated Fiber Concrete (SIFCON)

Ultra-High-Performance Concrete (UHPC) offers excellent strength and durability but suffers from autogenous shrinkage due to its low water-to-binder ratio. Internal curing using superabsorbent polymers (SAPs) is a promising method for mitigating this issue. This study investigates how different SAP particle sizes (D50: 215 µm and 725 µm) and internal curing water levels (36 kg/m3 and 60 kg/m3) affect the mechanical behavior of slurry-infiltrated fiber concrete (SIFCON) under sealed and drying conditions. Internal curing effectively reduced shrinkage but caused compressive strength losses (12.2–20.3%) and increased permeable pore volume (up to 31.1%). The shrinkage data utilized for this trade-off assessment were obtained from a parallel study conducted by the authors using identical material batches and environmental conditions. The finer SAP enhanced flexural strength by up to 9.3% and toughness, which is hypothesized to be associated with an improved fiber–matrix interfacial transition zone (ITZ), whereas the coarser SAP led to a flexural strength loss of up to 42.9% and reduced toughness, which may result from localized void formation. The multi-variable graphical trade-off assessment showed that the finer SAP achieved a more favorable balance between shrinkage mitigation and mechanical performance, highlighting the importance of SAP characteristics.