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Dale P. Bentz
National Institute of Standards and Technology, Gaithersburg, MD USA
While the strength and durability of high performance concretes (HPCs) are often greatly superior to conventional concretes under ambient conditions, during exposure to a fire, their failure is sometimes rapid and dramatic, characterized by the explosive spalling of layers from the exposed concrete surface. This failure mode is rarely encountered in conventional concretes of higher w/c ratios. In these concretes, it is suggested that the interfacial transition zones (ITZ) surrounding each aggregate particle provide a convenient escape route for the vapor built up during the thermal exposure. In HPC, these ITZ regions are thinner and not percolated, but can be repercolated by the addition of just a few (0.2 % to 0.5 % by volume) fibers. Here, simulations are conducted to determine the relative efficiency of different length fibers in creating a percolated network, and investigate the effects of aggregate volume fraction and gradation on ITZ percolation. The percolation nature of the failure mechanism provides some insight into the inconsistent performance of HPC exposed to fire, as indicated by the widely variable performance of different HPCs observed in various studies.
Keywords: Fibers, high performance concrete, interfacial transition zone, microstructure, percolation, permeability, simulation, spalling.