RESIDUAL FRACTURE-TOUGHNESS OF CONCRETE EXPOSED TO ELEVATED-TEMPERATURE

This paper uses an analytical-experimental model to evaluate the resid ual fracture toughness of concrete exposed to elevated temperatures. I n addition to the basic model of traction-Si ee cracks, this analytica l model also accounts for the closing pressure at the crack extension zone of notched concrete beam specimens. The stress beyond the tip of the crack extension zone is considered plastic with a magnitude equal to the modulus of rupture of the concrete. The crack opening displacem ents, failure load, initial crack length, and deflection at failure lo ads obtained from tests are incorporated in the analytical model to ev aluate the concrete roughness. A total of 45 6 x 6 x 30-in. (150 x 150 x 750-mm) three-point-bent beam specimens with a central notch were t ested. Twenty-one specimens had a 1-in. (25-mm) notch. Twenty-one had a 2.36-in. (60-mm) notch, and the remaining three had a 4.5-in (114-mm ) notch. Three crack sizes of 1, 2.36 and 4.5 in. were tested in three sets of specimens. Each set contained three beams, and each was teste d under ambient temperature conditions to validate the developed analy tical model. The remaining 36 specimens were tested after they were ex posed to one cycle of heating and air-cooling to room temperature. The temperatures considered were 50, 100, 150, 200, 250, and 300 C and ea ch specimen, was maintained at the desired temperature for a duration of 24 hours of heating under steady-state conditions. Results indicate that the residual concrete toughness decreases with an increase in te mperature. Effects of the cooling method of the specimens (air cooling , water cooling, foam cooling, etc.) are under further investigation b y the authors.