As part of its Partnership for High Performance Concrete Technology program [23], NIST is conducting research on a variety of topics critical to the performance of HPC. The fire performance of HPC is critical due to its use in high rise building and tunnel structures. Understanding the basic mechanisms behind the spalling behavior of HPC and the ability of polypropylene fibers to prevent spalling is critical to the design and construction of safe and durable structures. The simulations described in this paper provide further insights into these mechanisms and can provide an approach for examining the spalling susceptibility of a concrete based on any mixture proportions of interest.