Rehydration Model for Ultrahigh-Performance Concrete Matrix

Unhydrated cementitious materials in ultrahigh-performance concrete (UHPC) with a low water-to-binder ratio stop hydrating owing to water shortage but continue hydrating after re-exposure to water; this phenomenon is referred to as rehydration. Rehydration may either enhance or damage cement-based materials, and the mechanisms underlying it remain unclear. In this study, an accelerated rehydration test was conducted on an UHPC matrix to determine the chemically combined water contents and compressive strengths, and a rehydration model was developed based on the Krstulovi-Dabi hydration dynamics of cement and microstructure information of cement hydration; the model was used to analyze the mechanism behind the influence of rehydration on strength, combining the variations in micromorphology and pore structure. Results showed that the simulated values of rehydration model were in agreement with experimental values. During the early rehydration period, cement hydrated rapidly, and rehydration products filled and repaired the pores, improving the matrix properties. However, during the late rehydration period, cement hydrated slowly; and the volume expansion of rehydration products increased internal stress, resulting in microcracks and deterioration of the matrix properties.