Residual mechanical properties of heated concrete incorporating different pozzolanic materials

Abstract

This paper aims at showing the effect of high temperatures on mechanical properties of concretes in which Portland cement (PC) has been partially replaced by pozzolanic materials. Three types of pozzolanic material, one natural pozzolana and two lignite fly ashes (one of low and one of high lime content) were used for the replacement. Concrete specimens were tested at four temperature levels of 200, 400, 600 and 800°C without any imposed load. The specimens for each of the chosen temperatures were heated under the same heating regime, so a comparison of their behaviour during heat exposure concerning their mechanical properties was possible. Tests of compressive strength, splitting tensile strength and modulus of elasticity were carried out on specimens cooled slowly to room temperature 1 day, 7 days, 1 month and 3 months after heating. Based on the results of this experimental work it can be said that concretes with pozzolanic materials added to the mixer in partial replacement of PC are more sensitive to exposure to fire than conventional concretes. A relatively greater drop in the strength of the concretes with pozzolanic materials was found especially in the temperature area of 200°C when they were compared with conventional concrete made from PC of Greek type (10% insoluble residue). This particular sensitivity was attributed to heat deformations of hardened pastes, as all the other testing parameters concerning consistency and rate of heating remained the same for all specimens. It is suggested that this is caused by the escape of the non-evaporable water that exists as combined water in hardened pastes of pozzolanic binding agents. These pastes are richer in compounds such as calcium aluminates, calcium aluminate sulfates and tobermorite gel (C-S-H) which are decomposed between 110 and 150°C.