Assessment of the fractured state of fire-damaged concrete
. This thesis reports on an experimental research programme into developing a new quantitative method, based on the Stiffness Damage Test (SDT), to be used in assessing fire-damaged concrete. The SDT subjects a concrete core to low-stress quasi-static load cycles. The energy dissipated during the load-cycle together with other characteristics of the stress-strain response are used as assessing tools of the fractured state of damaged concrete. In addition to the SDT, the extent of damage was also monitored by the following test methods: The Ultrasonic Pulse Velocity (UPV) technique The dynamic modulus measurements The crushing strength The full stress-strain response of fire-damaged concrete to failure The petrographic approach including: 0 Scanning Electron Microscopy (SEM) 0 X-Ray Diffraction (XRD) The investigated concrete was heated to various temperatures in the range (ambient - 5000C) to the point when the temperature became uniform and was then exposed to this temperature for various durations. Cooling of the heated concrete was either carried out in air or by means of spraying with tap water for a short duration. Both carbonate and siliceous aggregate concrete were investigated. Maximum aggregate sizes of 10mm and 20mm were used in the siliceous aggregate concrete. The SDT was shown to be a powerful method in quantifying the extent of fire-damage. The energy dissipated in a stress cycle was found to correlate extremely well with the % fired residual stiffness, R 2=0 . 95. Therefore, a new method of classification of fire-damage, based on such correlation is introduced. The currently accepted method of UPV was shown to be unsatisfactory in assessing fire-damaged concrete when moisture content is altered during the fire-fighting operations. The traditional method of assessing fire-damage using the phenomena of colour changes of heated concrete was also shown to underestimate the extent of internal disruption and damage. In this thesis a large body of evidence is presented to support the supposition that the extent of damage is not a sole function of the maximum temperature experienced. The extent of damage was found to be a complex function of: the maximum temperature, the method of cooling, the duration of exposure, and the type and size of aggregate used. Spraying hot concrete with water proved to be a major determinant in the post-firing residual stiffness of concrete.