Consistency of water film thickness in UHPFRC matrix with various mixture proportions and constituents

The water film thickness (WFT) concept, critical for understanding workability and mechanical properties in cementitious materials, has been investigated for traditional materials but remains rarely explored in ultra high performance fiber reinforced cementitious composites (UHPFRC) system. This study aims to address this gap by systematically investigating the consistency of WFT across various UHPFRC mix designs, while ensuring high mechanical properties. A series of UHPFRC matrices were designed using modified Andreasen and Andersen (MAA) packing model. A modified wet packing method based on the mixer power measurement was utilized to accurately determine the maximum packing density (${\Phi }_{\max }$) and average WFT ($\overline{{\delta }_w}$) for each mix. Experimental evaluations included varying superplasticizer (SP) dosages, binder-to-total solid particle ratios (B/S), and supplementary cementitious materials (SCMs). Results revealed that increasing SP dosage reduced WFT and enhanced compressive and flexural strength up to an optimal SP/B of 0.50 %. Beyond this point, WFT stabilized (2.06 nm–3.61 nm). The stability of WFT was confirmed for mixes with varying B/S ratios (0.65–0.90) and SCM types, with values consistently ranging between 2.04 nm and 5.48 nm. The MAA packing model was validated, with a strong correlation between MAA index ($R^2$ ≥0.95) and enhanced mechanical properties. This study demonstrates the robustness of WFT as a parameter for governing UHPFRC workability and provides a framework for optimizing mix designs for superior performance (${\Phi }_{\max }\ge 0.78$, $f_{Uc}\ge 110MPa$ and $f_{Uft}\ge 12MPa$ at 7d). Recommendations for future research include investigating the influence of fibers (volume, type) on the wet packing density and the WFT concept in UHPFRC system.