A NEW QUANTITATIVE METHOD OF ASSESSING FIRE DAMAGE TO CONCRETE STRUCTURES

This paper describes a laboratory investigation into the adoption of t he stiffness damage rest to assess fire-damaged concrete structures. L aboratory-prepared concrete cores (75 mm diameter, 175 mm long) were f ired under different heating regimes and their uniaxial compression st ress-strain response at low stress level was determined. The area of h ysteresis of the lend-unload loops and other characteristics of the re sponse such as the degree of concavity, the loading chord modulus, the unloading modulus and the residual plastic strain provide a quantitat ive measure of the extent of structural damage caused by thermal expos ure. Fire-damaged specimens were also monitored by measuring the ultra sonic pulse velocity. The microstructure of the damaged specimens was studied using scanning electron microscopy (SEM) and II-ray diffractio n. 320 degrees C marked the onset of significant modification in the c haracteristics of the stress-strain response loops, with a sudden incr ease in the damage index (area of hysteresis loops). The ultrasonic pu lse velocity in the fired concrete showed similar variation with tempe rature to that of the elastic properties. At temperatures higher than 320 degrees C, SEM photographs showed significant cracks in the cement paste, especially in the interfacial zone.