Next: Introduction
FIBERS, PERCOLATION, AND SPALLING OF HIGH PERFORMANCE
CONCRETE
Dale P. Bentz
National Institute of Standards and Technology, Gaithersburg, MD USA
Abstract
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.