A series of fire tests was performed to develop empirical relationships
for the direct fire exposure time to small-scale specimens,
in accordance with American Society for Testing and
Materials [2] for a 2-h fire rating. The ASTM time-temperature
curve was followed during all pilot tests, which included the testing
of seven 6 in. 6 in. 21 in. (152.4 mm 152.4 mm
533.4 mm) concrete beams and mix sets of three 6 in. 12 in.
(152.4 mm 304.8 mm) concrete cylinders in direct fire exposure
for durations of 120, 60, and 30 min. It was observed that when the
small-scale specimens were exposed to the direct fire for the duration
of 2 h, they suffered extreme deterioration during testing and
after cooling. The prolonged exposure time initiated cracking, spalling,
and internal stresses with the integrity of the member declining
drastically throughout testing. In cooling, even more
deterioration occurred to the point where the small-scale specimens
were incapable of resisting any load. In various instances,
some specimens completely disintegrated after cooling.
Construction materials exposed to fire will experience changes in
material properties and, in some cases, may undergo creep, decomposition,
dehydration, and loss of material [20].When the duration
of the fire exposure was reduced to 60 min (1700F or 927 C) and
30 min (1550F or 843 C), the effects on the concrete specimens
were not so detrimental. The specimens exposed to direct fire for
30 min retained integrity better than those exposed for 60 min
(based on visual inspection); the 30 min exposure results are
shown in