MULTI-SCALE MICROSTRUCTURAL MODELING TO PREDICT CHLORIDE ION DIFFUSIVITY FOR HIGH PERFORMANCE CONCRETE

摘要

As durability issues become increasingly prominent in concrete design, the need to predict the transport properties of a given concrete mixture becomes critical. For many degradation scenarios (corrosion, sulfate attack, etc.), the diffusion coefficient of the concrete is a critical parameter that helps determine its service life. The goal of current research at the National Institute of Standards and Technology is to develop predictive equations for the chloride ion diffusivity of concrete based on mixture proportions and expected degree of hydration. The basis for these predictions is a set of multi-scale computer-based microstructural models for cement-based materials. Modeling at the scale of micrometers (cement paste) provides the necessary information on the effects of water-to-cement (w/c) ratio, silica fume addition, and degree of hydration on the volume of capillary porosity, and its connectivity. Modeling at the scale of centimeters (concrete) provides information on the influence of volume fraction and gradation of aggregates, and interfacial transition zone (ITZ) microstructure on the effective diffusivity of the concrete composite. Connecting the models at these two scales allows for the quantitative estimation of the chloride ion diffusivity of a specific concrete. Based on the results of these models, the addition of silica fume is seen to be a very efficient means for reducing concrete diffusivity, particularly for low w/c ratio (< 0.4) systems.