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2025-04-17 14:34 本頁面
【正文】 associated with a sudden expansion of % [11]. The structure of concrete material can be approximately classified into micro level (less than 1 μm), meso level (between 1 μm and 1 cm), and macro level (greater than 1 cm). For concrete subjected to high temperature, with the increase in temperature, strength and Young39。s modulus decrease at macro level, internal structures degenerate and micro defects develop at micro and meso levels [13]. The effect of fire and high temperature on the behaviour and properties of reinforced concrete studies includes pressive strength, modulus of elasticity, shear modulus, thermal conductivity, specific heat and creep of concrete along with modulus of elasticity and coefficient of thermal expansion and tensile strength of reinforcing steel. Information on the bond between concrete and steel is limited. In studying bond strength, pullout tests are applied and average loads are used to calculate average bond strength [14]. Some previous work was conducted on the bond between steel bars and concrete, the bond between fibres and concrete and the bond in other fibrereinforced posites [15], [16], [17], [18], [19], [20] and [21]. Most research data of residual properties of concrete after exposure to high temperature were obtained under conditions of natural cooling, which should differ obviously from cooling regimes in a real fire, where water spraying is usually used for fire extinguishing and consequently thermal shock is induced to concrete. It has been reported that water cooling caused more severe decrease in strength pared to natural cooling. Therefore, the effect of cooling regimes on the mechanical properties of concrete is of great concern [22], [23], [24] and [25]. As a result of these reasons summarised above, the effects of elevated temperatures and cooling regimes after the heating process on the residual bond strength between concrete and steel bars are examined in this experimental investigation. Normal strength concrete mixtures with the initial pressive strengths of 20 and 35 MPa, which are monly used in buildings in Turkey, are tested throughout the study. Reinforcement bars are embedded into concrete specimens with 3 different embedment lengths of 6, 10 and 16 cm. Specimens are cooled either in water or in air, after heating processes. 2. Materials and methods ASTM Type I, Portland Cement (PC), from A?kale Cement Factory in Erzurum, Turkey, was used in this investigation. Natural aggregate (NA) with a maximum size of 16 mm was obtained from Altunkent region in Erzincan, Turkey. Aggregate used in this study was river sand and gravel and it was siliceous aggregate. The chemical position of PC is summarised in Table 1, physical and mechanical properties of PC are given in Table 2 and the properties of aggregate are shown in Table 3. The diameter of the ribbed steel bars was 8 mm and the properties of the bars are listed in Table 4. Two different concrete mixes with initial pressive strengths of 20 and 35 MPa are produced in laboratorytype mixer with the capacity of 60 dm3. Ribbed steel bars 8 mm in diameter are embedded into concrete blocks with the embedment lengths of 6, 10 and 16 cm. For each group, six samples of 100 mm diameter and 200 mm height cylinders are prepared for all temperature values. Three of them are cooled in water and the other three specimens are cooled in laboratory conditions after heating. The specimens are placed in a water tank at 23177。2 176。C, 24 h after the casting and cured
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