Effect of high elevated temperature on the strength of concrete

This work presents the results of an experimental program on the effect of elevatedtemperatures on the cornpressive strength and associated modes of failure of normalstrength concrete. high strength concrete and light weight concrete. The effect of five testmethods, represent five thermo-mechanicalloading combinations of temperature condition,on the relative compressive strength of concrete were investigated. This problem wastreated into two parts. The first part was concerned with the behavior of the differentconcrete mixes when exposed to high temperature under the residual unstressed test. In thistest. the strengths measured at room temperature were used as a reference. The specimenswere exposed to different target temperatures ranging from 100 °c to 700 °c for differentdurations (2. 4 and 6 hrs). The average heating rate was 8 °C/min. Five mixes wereconsidered through the present the experimental program. The first three mixes (I. 11 andIll) were for normal strength concrete made with three types of coarse aggregate (gravel.dolomite and basalt) and siliceous sand as fine aggregate. Mix IV was for high strengthconcrete made with basalt as coarse and fine aggregates. Mix V was for light weightconcrete made with leca as coarse aggregate and siliceous sand as fine aggregate. Thestrength and mass loss due to the exposure of the different concrete m ixes to the proposedelevated temperatures were measured and evaluated. Modes of failure and crack patterns ofthe different mixes were examined. Phase transformation and change in chemicalcomposition of the hydrated cement paste were mentioned based on the results of X-raydiffraction.The second part was concerned with the effect of different thermo-mechanical testmethods on the behavior of the five concrete mixes under high temperature. These testmethods include the stressed test, the unstressed test. residual stressed test. and failurestressed test. These test methods were applied on the five concrete mixtures mentionedabove. The specimens were subjected to target temperatures of 200 °c and 550 0c. Thespecimens were sustained under these temperatures for 1.5 hours. In the stressed andresidual stressed test. the specimens were subjected to load ratio of 40% and 60% of themaximum room temperature fracture load (RTFL). In the failure stressed test the timeelapsed between the start of heating until failure when the specimens were loaded by 80%or 60% from room temperature fracture load was recorded. The compressive strength due