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Concrete mix proportions were determined according to the rele-vant Turkish Standards TS EN 206-1 [23]. The mix designs wereintended to be representatives of concrete mixes typically used inTurkey. A constant water/cement (w/c) ratio of 0.50 was used for allthe test specimens.The experiments were carried out using a closed-loop servo-hydraulic dynamic testing machine, with a capacity of 2000 kN.Table 1Concrete mix proportions (kg/m3).Series w/c Cement Water River sand0–4 mmCrushedsand stone0–6 mmCrushedstone II6–12 mmCrushedstone III12–22 mmI 0.5 250 123 417 407 425 849II 0.5 350 172 373 365 381 761 The samples were tested under strain control procedures. A con-stant axial strain rate was used throughout the experiments.The concrete specimens were cured in lime saturated water at2072 1C for 27 days. The specimens were then removed from thecuring room and left to dry in air for 24 h. After being air-dried,the control specimens were then tested at laboratory temperatureof 20 1C to determine the properties of the concrete specimens atambient temperature.After being subjected to the same curing conditions as thecontrol specimens, the remaining test specimens, which weresupposed to be tested at elevated temperatures, were first putinto a ventilated oven and heated to a temperature of 100 1C for48 h and then allowed to air dry. A ventilated oven providesconvectional drying which allows homogenous drying of thespecimen. Inhomogeneous drying causes cracking and spallingof the concrete specimen.For the temperatures of 200 1C and above, an electrical furnaceused in this research as well as the heating regimes are shown inFig. 1a and b. The furnace was heated by means of exposedheating elements laid on the refractory walls of the insidechamber, which was approximately 700 by 1000 by 1000 mm indimension. The test specimens were stacked with sufficient spacebetween two adjacent specimens to obtain an uniform heating ineach specimen as in the case of Tolentino et al.’s experimentalstudy [24]. The samples were heated at a heating rate of 2 1C/minuntil the targeted test temperature was reached. The heating rateused in this study was chosen to be based on recommendationsobtained from previous conducted research [9,10]. However, it isworth mentioning that the rate of heating used is significantlylower than that specified by ASTM E 119 [25], which is about538 1C in the first 5 min. Once the targeted temperature wasreached, it was maintained for 45 min to achieve the thermalsteady state [26] and then allowed to cool down at a rate of1 1C/min. For illustration purposes, Fig. 2a and b shows thethermal-camera-photographs of cubes and beams concrete-specimens at temperature of 100 1C. It is worth noting that thespecimen surface temperature was assumed identical to thefurnace recorded temperature. In addition, the temperatures atthe different depth from the concrete surface were monitoredusing a control specimens equipped with a thermocouple. Inmeasuring temperature, several thermocouples were used, one onthe surface, one in the furnace and others at different depth (2, 3,4 and 5 cm) from the concrete surface. The variation of tempera-tures versus time both the temperature at the surface and insideof the specimen is given in Fig. 3.The technical literature specifies three methods to assess thestrength of concrete at elevated temperatures; namely: ‘‘stressedtests’’, ‘‘unstressed tests’’, and ‘‘unstressed residual strength test’’. Inthe ‘‘stressed tests’’, a preload is applied to the specimens prior toheating and is sustained during the heating period. Once the speci-men reaches its targeted heated temperature, load/strain is increaseduntil failure of the specimen. In the ‘‘unstressed tests’’, the specimen isheated, without preload, until the desired temperature is reached andmaintained while the specimen is being tested to failure. In the‘‘unstressed residual strength tests’’, the specimen is heated, withoutpreload, to the target temperature and then is allowed to cool,following a prescribed rate, to room temperature. Load/strain is thenapplied at room temperature till failure of the specimen. In this paper,the ‘‘unstressed residual strength test’’ was used, since this methodwas found to give the lowest strength results among the three testingmethods [27] and its results are most suitable for assessing thepost-fire (or residual) properties of concrete.After heating each specimen to its target temperature, thevelocity of ultrasonic transmission (VUT), compression and flexural tests were conducted on the specimens following currentASTM standards C597-09 [28], C78-09 [29], and British standardBS EN 12504-1:2000 [30], respectively. Fig. 4a and b showsplacement of typical beam-specimens in the furnace as well asa view of the compression testing machine, respectively.3. Results and discussionThe average compressive, flexural strength, and VUT testresults of the two test series at different temperatures are givenin Table 2. The average tabulated results will be used to assess theeffect of the two main variables (i.e., temperatures and cementdosages) on the mechanical properties of concrete. It is worthnoting that the word ‘‘average’’ will be omitted hereafter in thesubsequent sections, figures and tables.3.1. Residual compressive strengthThe residual compressive strength test results are shown inFig. 5. The figure shows the variation of the residual compressivestrength with temperature for the two test series (i.e., Series-I andSeries-II with 250 and 350 kg/m3cement dosages, respectively).The test results indicate that as the temperature increases theresidual compressive strength of concrete decreases.The results shown in Fig. 5 can be used to assess the effects ofthe tempretures and cement dosages on the residual compressivestrength of concrete. Fig. 6 plots the variation of the residual compressive strength of concrete to that of the control specimen(i.e., fıcðTÞ=fıc 20oC ðÞ) with temperature for the two test series. Fig. 6shows that both test series exhibit almost same strength loss withtemperature, implying that the effect of cement dosage on theresidual compressive strength of concrete is negligible within therange of temperatures being investigated in this research.The test results of Fig. 6 also show that when concrete isheated to about 100 1C, the average residual compressive strengthis reduced by about 9%. Near such a temperature, the free waterwithin the concrete starts to evaporate resulting in an increase inporosity and consequently a decrease in the residual compressivestrength of the concrete. At a temperature of 200 1C, a strengthreduction of about 17% is noticed. Such a reduction in strengthcan be attributed to the formation of internal cracks due to theevaporation of water and to the pore structure coarsening [31].At temperatures between 400 and 600 1C, the residual compressivestrength was found to reduce by about 33 to 48%; this reduction instrength is attributed to the dehydration of the CSH gel as well asto the volumetric expansion resulting from the transformationof the chemical compound Ca(OH)2 to CaO.