A Refined Model of Viscous Stress for Free Water in Concrete Considering the Characteristics of Pore Structure

Abstract This study evaluates the mechanical effects of free water in wet-state concrete under dynamic loading, applying principles from the Stefan effect. A novel model of viscous stress is introduced, incorporating pore structure characteristics for the first time. Pore structure features of both mortar and aggregate are analyzed using mercury intrusion porosimetry. Equations are developed to integrate the differential pore volume and pore diameter of mortar and aggregate, facilitating the calculation of viscosity stress coefficients. A water content model, based on pore structure and water content assessments, is proposed to determine the distribution of unbound water. The results reveal that free water, within the lognormal distribution of pore sizes, fills nearly all available pore volume. However, when pore diameters follow a power law distribution, complete filling is not achievable. The viscous stress induced by pore water is primarily influenced by freely moving water in smaller pores. When free water adheres to apertures within a specific diameter range governed by the power law, substantial viscous stress occurs. The effect of viscous stress on larger pore diameters is negligible compared to the strength of the concrete matrix. The methodology for calculating pore water viscous stress in concrete, a composite material consisting of mortar and aggregate, is clearly outlined, and the improved model aligns well with experimental data.