A 3-D microstructural model for fiber-reinforced concrete has been presented and applied to examining the spalling phenomena of high performance concrete. The hypothesis that the percolation of the ITZ regions in the concrete is of paramount importance to spalling performance has been supported by numerous simulations and a review of documented experimental results. The efficiency of fibers to percolate a system of ITZs surrounding aggregates (or increase their percolated fraction) has been clearly demonstrated. The simulation results also suggest that 20 mm fibers will provide superior performance to 10 mm ones at equivalent volume fractions. The simulations, coupled with thermogravimetric analysis of the fibers and concretes, suggest that at the temperatures where most of the water vapor is generated in a high performance concrete, the fibers are softened and absorbed by the surrounding cement paste matrix. Measurements of the viscosity of the polymer melt indicate that while this flow within the fiber channels is possible, significant flow through the capillary pore network of the cement paste is unlikely. Thus, only those fibers in direct contact with the exterior surface are likely to be totally expelled by the pressure developing within the concrete during the fire exposure.
The developed computer program provides the opportunity to engineer a concrete with improved spalling resistance, by ensuring the percolation of the ITZ regions for the user-specified ITZ thickness. For accurate performance, the user must supply not only the aggregate volume fraction, but also the particle size distribution (based on a sieve analysis) and some insight into the specific geometrical properties (e.g., aspect ratios for 3-D ellipsoids) of the aggregate (and fiber) particles.