Abstract:
Concrete loses strength when exposed to high temperature. The thermo-chemo-rheological
processes that take place within the concrete matrix at high temperature not only produce
gaseous matters and vapors but may also bring concrete to a new chemical composition.
Pressure built-up within concrete matrix due to formation of gases often leads to cracking
and eventual spalling of concrete. However, built-up pressure responsible for concrete
spalling should be directly related to the extent of permeable pores within the concrete
matrix. Thus, damage due to temperature episodes should be different in concretes having
different porosities. In general, concretes of higher strengths are known to be less porous.
Furthermore, brick aggregate concretes are generally more porous than stone aggregate
concretes. The porosities of recycled concrete fall in between. However, there is no
information based on a systematic study on strength losses for these fundamental cases.
The present work is motivated to characterize the effect of coarse aggregate porosity and
concrete porosity on the strength loss after different thermal episodes.
Concrete having 10 MPa, 20 MPa and 35 MPa made of four types of coarse aggregates
having different absorption capacities (0.8%-14.4%) were tested for compressive strengths
after heat treatments at a purpose built electrical furnace. Four different heating paths
having fast to gradually slower heating rates were used. Heating paths were so chosen that
to reach at 600°C temperature at one step, at two steps and at three steps. Variation in
heating paths was found to have significant effect on loss of compressive strength after
episodes of heat treatment. Absorption capacities of concrete (5.58% - 31.19%) were
measured. Losses of masses at different stages of heat treatment were also measured.
It is seen that compressive strength loss is generally proportional to the temperature and
heating rate for a given aggregate porosity. Strength loss was found to be proportional to
parent concrete strength. A conspicuous decrease of strength loss with increase in heat
input keeping maximum temperature of heating path at a constant value has been well
resolved. Statistical significance gathered from test data out of 384 specimens were
generally noted to be higher for slower heating rates and larger porosities. Concrete with
coarse aggregate porosity between 3% to 8% and concrete porosity between 10% to 20%
were found to be most susceptible to strength loss due to high temperature events.
