This report gives results of a laboratory study undertaken to evaluate the relative performance of limestone and dolostone aggregate concretes under sustained exposure to high temperatures. For each type of concrete, three water/cement (W/C) ratios ranging from 0.33 to 0.60 were investigated. The test specimens were exposed for up to four months to temperatures ranging from 76 to 600°C. The conditioning of the specimens prior to any temperature exposure consisted of moist curing for 28 days followed by storage under room conditions for 26 weeks.
The test results show the dolostone aggregate to be unstable under a sustained temperature of 150°C, confirming the results of a previous investigation. Such instability is again attributed to the slow oxidation of the pyrite contained in some of the aggregate particles. The resulting expansion causes disintegration of the aggregate and rupture of the concrete. Under similar exposure, concrete made with a limestone aggregate is found to be unaffected.
Except for these results, the loss of compressive strength of the specimens under exposure is generally independent of the type of aggregate and is proportional to the temperature. At temperatures of 150°C and higher, an increase in the length of exposure from 48 h to four months results in further decrease in strength. In all cases, however, any major strength loss is found to occur within the first month of exposure. In general, the leaner concretes are slightly less affected than the richer concretes in terms of relative strength loss after exposure.
Both pulse velocity and resonant frequency measurements appear to provide excellent means of monitoring the compressive strength loss during exposure.