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A severe loss in strengthwas noted when the specimens were heated to 800 1C. At such atemperature, the reduction in the residual compressive strength ofconcrete was found to be about 70%. This severe strength deteriora-tion is attributed to the decomposition of the CSH gel [32].For comparison purposes, the reduction in residual compres-sive strength of concrete with temperature as given by currentBritish standards [21] will be calculated and compared to the testresults of this paper. Eq. (1), shown below and obtained from BS EN1992-1-2:2004 standards [21], gives the reduction in compressivestrength as a function of the type of aggregate used (Siliceous versusCalcareous) and the temperature to which the concrete has beenexposed to. The reduction in compressive strength is defined by theratio of the residual compressive strength at temperature, T,tothatof concrete at normal ambient temperature. This ratio is denoted bykc ¼ fıcðTÞ=fıc 20oC ðÞin Eq. (1).Siliceous aggregatekc ¼ 1kc ¼ð1:1 0:0008 TÞkc ¼ 1:254 0:0013 T ðÞkc ¼ 0Calcareous aggregatekc ¼ 1kc ¼ 1:06 0:0005 T ðÞkc ¼ 1:471 0:0015 T ðÞkc ¼ 0Tr100 1C100 1CoTr400 1C500 1CrTr900 1C900 1CoTð1ÞFig. 7 plots the variation of the relative compressive strengthwith temperature as obtained from Eq. (1) for both siliceous andcalcareous aggregates. The figure also shows the experimentalresults for comparison purposes. Also the predicted unstressedresidual compressive strength results of ACI 216-1 for siliceousand carbonate aggregates is shown in Fig. 7. Fig. 7 shows that fortemperatures ranging between 100 and 400 1C, the predictedunstressed residual compressive strength results of ACI 216-1agree better with the experimental counterparts. On the otherhand, for temperatures exceeding 500 1C, the test results arefound to be in better agreement with the predicted results ofEq. (1). The discrepency between the experimental and predictedresults can be attributed to the fact that the proposed BS EN 1992-1-2:2004 equation and the predicted unstressed residualcompressive strength results of ACI 216-1 were mainly calibratedto assess the effect of aggregate type, and not cement dosage, onthe fire response of concrete.Regression analyses were carried out to determine a mathe-matical equation that best fit the variation of the experimentalresidual compressive strength results with temperatures. Theresults of these regression analyses are summarized by thefollowing newly proposed equation:fıcðTÞ¼ 1:0109 0:09 T100 fıc 20oC ðÞð2Þ where T is the temperature to which the concrete was exposed to(in 1C) and fıc 20oC ðÞis the concrete 28-days-compressive strength at20 1C.As noted, the newly proposed Eq. (2) does not depend on thecement dosage since in this research the effect of cement dosageon the variation of the residual compressive strength withtemperatures was found to be negligible. For comparison pur-poses, Fig. 8 shows the experimental relative residual strengthresults and the predicted results using the newly proposed Eq. (2).3.2. Residual flexural strengthThe residual flexural strength of the beam specimens wasobtained by subjecting the beam-specimens to three point bend-ing tests. All flexural tests were carried out after cooling thespecimens following the same procedures previously outlined forthe residual compressive testing. Fig. 8 shows the variation of theresidual flexural strength with temperature for the two test series(Series-I and II). Fig. 9 shows that when exposed to heat, theresidual flexural strength of concrete reduces almost linearly withincrease in temperature.The results presented in Fig. 9 can be used to assess the effectsof the cement dosages and temperature on the residual flexuralstrength of heated concrete. Fig. 10 shows the variation of therelative residual flexural strength with temperature for the twotest series (i.e., Series-I with cement dosage of 250 kg/m3andSeries-II with cement dosage of 350 kg/m3). The figure shows thatthe effect of cement dosage on the residual flexural strength ofconcrete can be ignored since both tests series exhibited almostthe same strength reduction-rate with increase in temperature.Here also, a mathematical equation, relating the residualflexural strength at a given temperature to that of concrete atambient temperature, is proposed and is given below:f cTðTÞ¼ 1:0101 0:115 T100 f cT 20 oC ðÞð3Þwhere T is the temperature to which the concrete was exposed to(in 1C) and fcT(20 1C) is the flexural strength of concrete at 20 1C.The newly proposed equation does not depend on the cementdosage since in this research the effect of cement dosage wasfound not to significantly affect the variation of the residualflexural strength with temperature.3.3. The velocity of ultrasonic transmission (VUT)The VUT is another type of tests that can be used to assess thedegree of damage in a concrete specimen when exposed to fire [33].In this investigation, and before conducting any compressivetesting on the cube specimens, the VUT (km/s) was measuredthrough the 100 mm cube-samples before and after exposing theconcrete cubes to high temperature. The results of the VUT-testsare given in Table 2 and shown in Fig. 11. The figure shows thatthe measured velocities at room temperature were almost similarfor all cube-samples of test Series I and II. However, decrease inthe velocities was observed for the samples that were exposed tohigh temperature (T4200 1C). The results also show thatthe effect of cement dosage on the variation of the VUT results with temperature can be ignored as both tests series exhibitedalmost similar trend. The decrease in VUT with increase intemperature is an indication of crack and cavity formation withhigh temperature. Hence, such a simple test can be used to assessthe degree of degradation of the mechanical properties of concreteonce exposed to high temperature. In fact, Durmus - and Arslan [34]and Chan et al. [35] proposed to use the porosity of concrete as atool to assess the post-fire mechanical properties of concrete.The test results carried out in this research also showed thatwhen concrete is exposed to high temperatures, some changes incolor occur. For examples, for temperatures less than 400 1C, theconcrete color did not change, while when heated to about400 to 600 1C the color of concrete changes from its original light-gray color to light-yellow–grayish color; and when heated to about800 1C, the color becomes yellow–grayish. Accordingly, the changein the concrete color can also be used as a tool to indicatedeterioration in the mechanical properties of concrete.4. ConclusionsBased on the unstressed residual strength tests conducted inthis research, the following conclusions can be made:(1) Increase in the cement dosage result in densification of themicrostructure in the transition zone which leads to animproved bond between cement paste and aggregate. Thisdensification effect increases the strength of the concrete atambient temperature [24]. However, the dehydration of CSHphase in cement paste of concrete exposed to high tempera-ture causes the deterioration in transition zone.