Effect of coarse aggregate porosity on the compressive behavior of concrete after high temperature episodes

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.