【文章內容簡介】 until the 26th day. At the end of 28 days of water curing, the specimens are dried and heated up to target temperatures and then cooled to room temperature either in air gradually or in water rapidly. Pullout tests are conducted 24 h after the specimens are cooled to room temperature and the residual bond strengths are determined. The pullout test is monly used to measure the steel–concrete bond strength [26], because it is easier according to other test methods and suitable for paring relative bond properties [27]. The pullout test machine, used in this study, is shown in Fig. 1. For investigating the effect of high temperatures over the bond strength between concrete–steel bars, the temperature values are chosen as 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600 and 700 176。C in this study. Also unheated specimens are produced and tested for paring with the heated specimens. For the heating process a laboratorytype furnace, capacity of 1000 176。C, is used. The rate of temperature increase of the furnace is 12–20 176。C/min. After heating specimens up to the reference temperature, the furnace temperature is kept constant for 3 h. All groups are cooled to room temperature either in air in laboratory conditions or in water, and the effects of cooling regimes are investigated. For each group 3 specimens are tested and the average of these specimens’ test results is recorded as the group39。s residual bond strength. 3. Results and discussion . Bond strength between concrete–steel bars exposed to elevated temperatures After the effect of the elevated temperatures in a range of 50–700 176。C, the bond strengths are shown in Fig. 2 and Fig. 3 for C20 and C35 concretes, respectively. The change ratios of the bond strengths by the effect of elevated temperatures are also presented in Table 6, Table 7 and Table 8, and are discussed below. Increases in residual bond strengths are observed up to 150 176。C for all embedment lengths. This is thought to be due to the increase in residual pressive strength for the same temperatures. The highest increase amounts of 6 cm embedment length are 14% and 12% for C20 and C35 concretes, respectively. These values are both taken from the specimens that are heated to 50 176。C and then cooled in water. Above 150 176。C, bond strength decreases with increase in temperature. For the temperature of 700 176。C, bond strengths are recorded as minimum values. C20 samples lost 89% and C35 samples lost 76% of their initial bond strengths for this temperature. Effect of cooling regimes is not distinguished for 6 cm embedment length. This is considered to be due to the insufficiency of embedment length. Steel bars slipped from the concrete at low pullout strengths for both air and watercooled specimens. For 10 cm embedment length, bond strengths of C20 specimens increased 10% and 9% for 50 and 100 176。C, but then there are decreases for all other temperatures. Bond strengths of specimens produced with C35 concrete showed losses for all temperatures, except for an increase with the amount of 5% for 100 176。C. Above 100 176。C, both aircooled and watercooled specimens’ bond strengths are found to be less than the unheated specimens for C20 and C35. The maximum strength loss is observed for 700 176。C. For this temperature, the loss amount is 74% for C20 concretes, while this value is 60% for C35. These bond strength losses are both obtained from the watercooled specimens. At this temperature strength losses for aircooled specimens ar