土木外文翻译--高温下钢筋混凝土中钢筋的性能.doc

外文原文：Research Letters in Materials ScienceVolume 2008 (2008), Article ID 814137, 4 pagesdoi:10.1155/2008/814137Research Letter Properties of Reinforced Concrete Steel Rebars Exposed to High Temperaturesİlker Bekir Topu and Cenk Karakurt Department of Civil Engineering, Eskiehir Osmangazi University, 26480 Eskiehir, TurkeyReceived 12 February 2008; Accepted 31 March 2008Academic Editor: Rajiv S. MishraCopyright 2008 İlker Bekir Topu and Cenk Karakurt. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. AbstractThe deterioration of the mechanical properties of yield strength and modulus of elasticity is considered as the primary element affecting the performance of steel structures under fire. In this study, hot-rolled S220 and S420 reinforcement steel rebars were subjected to high temperatures to investigate the fire performance of these materials. It is aimed to determine the remaining mechanical properties of steel rebars after elevated temperatures. Steels were subjected to 20, 100, 200, 300, 500, 800, and 9 5 0 C temperatures for 3 hours and tensile tests were carried out. Effect of temperature on mechanical behavior of S220 and S420 were determined. All mechanical properties were reduced due to the temperature increase of the steel rebars. It is seen that mechanical properties of S420 steel was influenced more than S220 steel at elevated temperatures. 1. IntroductionFire remains one of the serious potential risks to most buildings and structures. Since concrete is widely used in construction, research on fire resistance of concrete becomes more and more important. Many researchers all over the world have done some researches on this subject. The mechanical properties of all common building materials decrease with elevation of temperature. The behavior of a reinforced concrete structure in fire conditions is governed by the properties of the constituent materials, concrete, and steel, at high temperature. Both concrete and steel undergo considerable change in their strength, physical properties, and stiffness by the effects of heating, and some of these changes are not recoverable after subsequent cooling 1.It is necessary to have safe, economical, and easily applicable design methods for steel members subjected to fire. However, without fire protection, steel structures may suffer serious damage or even collapse in a fire catastrophe. This is because the mechanical properties of steel deteriorate by heat during fires, and the yield strength of conventional steel at 600C is less than 1/3 of the specified yield strength at room temperature 2. Therefore, conventional steels normally require fire-resistant coating to be applied 3. The temperature increase in the steel member is governed by the principles of heat transfer. Consequently, it must be recognized that the temperature of the steel member(s) will not usually be the same as the fire temperature in a compartment or in the exterior flame plume.Protected steel will experience a much slower temperature rise during a fire exposure than unprotected steel. Also, fire effect on steel member is influenced with its distance from the center of the fire, and if more ventilation occurs near the steel in a fuel-controlled condition, wherein the ventilation helps to cool the steel by dissipating heat to the surrounding environment 4.Especially, temperature increase of steel and concrete in composite steel-concrete elements leads to a decrease of mechanical properties such as yield stress, Youngs modulus, and ultimate compressive strength of concrete 5. Thus, load bearing of steel decreases when steel or composite structure is subjected to a fire action. If the duration and the intensity of the fire are large enough, the load bearing resistance can fall to the level of the applied load resulting in the collapse of the structure. However, the failure of the World Trade Centre on 11th September 2001 and, in particular, of building WTC7 alerted the engineering profession to the possibility of connection failure under fire conditions 6. In this study, S220 and S420 ribbed concrete steel rebars were subjected to 7 different temperatures to determine the high temperature behavior of reinforcement steels.2. Experimental StudyExperimental studies were conducted with 10 and 16mm in diameter and 200mm in length S220 and S420 reinforcement steel rebars. Test specimens were subjected to 20, 100, 200, 300, 500, 800, and 950C temperatures in a high furnace for 3 hours, respectively. At the end of the curing process, steels were cooled naturally to the room temperature. Subsequently, tensile tests were applied to steel reinforcement rebars. According to EN 10002-1 tensile strength, yield strength and elongation of the steel rebars were determined for elevated temperatures 7. The steel specimens tensile strength tests were performed with 60 tons of loading capacity universal tensile strength test machine. The loading speed of the test machine is adjusted according to TS 708 code 8.3. Test Results and Evaluations3.1. Stress-Strain RelationsThe average values for stress-strain relationship for specimens that were exposed to various temperatures are given in Figures 1 and 2. The curves in Figures 1 and 2 were drawn with the average test results of 10 and 16mm in diameter steel specimens. The test conditions were meant to simulate a building that had a fire so the changes in the mechanical properties of reinforcing steels used in structures exposed to high temperature could be determined. As seen from Figure 1, temperatures below 500C have no significant effect on mechanical properties of preheated and cooled S220 steel rebars. The yield strength and splitting tensile strengths of S220 steels were similar up to this temperature. However, the yield strength and splitting tensile strength of the S220 steel rebars are reducing with the increase of temperatures over 800C. A similar behavior can be seen from the test results of S420-ribbed steel rebars (Figure 2). All high temperature subjected steel specimens became more ductile temperatures above 800C.Figure 1: Stress-strain curve of S220 steel rebar.Figure 2: Stress-strain curve of S420-ribbed steel rebar.3.2. Yield StrengthYield strength of both reinforcing steel rebars was affected with the elevated exposure temperatures. It can be concluded from Figure 3 that there is no variation in yield strength of reinforcing steels with cover up to 300C. Plain reinforcing steel rebars have experienced the strain hardening already for this temperature. According to Eurocode and TS EN 1993, before 400C there is no decrease in yield strength, but after this temperature a significant yield strength loss occurs 9. The yield strength losses of both S220 and S420 steel rebars were 46% and 84% for 800C exposure temperature, respectively. For further increase of temperature at 950C, yield strength decreases were 64% and 89%, respectively. According to these results, the remaining yield strength of S220 steel reinforcing rebar is higher than S420-ribbed steel rebar after high-temperature exposure.Figure 3: Yield strength of steel rebars against temperature.3.3. Tensile StrengthThe tensile strength variation of steel reinforcement rebars exposed to elevated temperatures is given in Figure 4. On the light of these results, there was no significance reducing of tensile strength for both types of steel rebars up to 500C temperature.The tensile strength losses of both S220 and S420 steel rebars were 51% and 85% for 800C exposure temperature, respectively. For the highest exposure temperature at 950C, tensile strength decreases were 60% and 90%, respectively. According to these results, the remaining tensile strength of S220 steel reinforcing rebar is higher than S420-ribbed steel rebar after high-temperature exposure. However, it should be considered that the possibility of complete strength loss of steel rebars at high temperatures when a structure is subjected to a huge fire. The remaining strengths of both reinforcing steel rebars after 500C are lower than the design strengths of these steels. Consequently, the remaining strength of the steel rebars in structures is influenced with the exposure time and type of fire depending on the heat transfer through concrete cover to steel parts 10.Figure 4: Tensile strength of steel rebars against temperature.3.4. ElongationThe relation between high temperature and splitting elongation ratio can be seen from Figure 5. The figure is demonstrated that both steel rebars show a same elongation behavior under elevated temperatures. The elongation ratios of S220 steel rebars are higher than S420 rebars depending on the ductile fracture behavior of this steel. After a fire inside the reinforced concrete building, the deflections of the structural members increase with the ductile behavior of the steel reinforcement at high temperatures.Figure 5: Elongation ratios of steel rebars against temperature.The elongation ratios were slightly increased up to 300C, however, above this temperature material becomes brittle with decrease of the elongation values. The elongation losses of both S220 and S420 steel rebars were 1.2% and 1.6% for 800C exposure temperature, respectively. For further increase of temperature at 950C, elongation ratios decreases were 1.6% and 3.3%, respectively. According to these results, the elongation capacity of S420 steel is lower than S220 steel under elevated temperatures. The S420 steel showed a brittle fracture behavior under elevated temperatures. This behavior is not sufficient for rebar steel in reinforced concrete structures.3.5. ToughnessThe energy absorbent capacity of materials used in construction should be higher against dynamic earthquake loads. The fracture energy of materials is defined with the toughness concept. The toughness values of the steel rebars used in experimental studies are given in Figure 6. According to test results, the toughness values of both types of steels were decreased after elevated temperature exposure. However, up to 300C, the toughness values were increased due to the ductile behavior of both steels. The toughness losses of both S220 and S420 steel rebars were 16% and 35% for 800C exposure temperature, respectively. For further increase of temperature at 950C, toughness decreases were 82% and 88%, respectively.Figure 6: Toughness of steel rebars against temperature.4. ConclusionsAs described in the previous studies, steel structural members loose strength under elevated temperatures. In this study, the mechanical properties of steel rebars were investigated which exposed to high temperatures and cooled to room temperature. According to test results, the most common reinforcing steel rebar S420 showed a brittle fracture mechanism under elevated temperatures. Splitting yield strength, tensile strength, elongation, and toughness values were low for S220 steel. These results demonstrate that S220 type of steel rebar is less affected than S420 steel under elevated temperatures. The authors suggest that the protective cover thickness should be higher for increasing the fire safety of reinforced concrete members.中文翻译：高温下钢筋混凝土中钢筋的性能摘要 处于高温环境的钢结构，屈服强度和弹性模量这两项机械指标的恶化被认为是影响其性能的主要因素。在这项研究中，热轧钢筋s220和s420将处于高温环境下以此探究这些材料的耐火性能。它的目的是确定经受高温的钢筋剩下的力学性能。钢受到20、100、200、300、500、800和9 5 0C温度并进行三小时的拉伸试验。得出了温度对s220和s420两种材料的影响结果。由于温度的升高，钢筋的所有机械性能都减弱了。而且能够看到s420钢材比s220钢材更容易受到温度的影响。1.简介火是大多数建筑和结构的隐患。由于混凝土在工程中的广泛应用，关于混凝土耐火性能的研究变的越来越重要了。世界上的许多研究人员已经对该项目做出了研究。所有一般建筑材料的机械性能随着温度的升高会下降。钢筋混凝土结构在高温条件下的反应主要受构成材料，混凝土，钢筋性能控制。混凝土和钢筋受高温的影响其力学，机械性能会受到很大的影响，并且这种影响在冷却后是无法修复的。找到一种使钢材耐高温的安全，经济，容易实施的方法是必要的。然而，钢结构在没有耐火措施的情况下在一场火灾中会遭受严重的损毁甚至瓦解。这是因为在火灾条件下受热导致钢材的机械性能恶化。在600C高温下，普通钢筋的区服强度还不及室温下钢筋屈服强度的三分之一。因此，普通钢筋一般需要涂膜防火。钢材中温度的升高主要受温度传递的控制。总之，必须意识到钢材的温度不会总和火焰的温度一样。耐火处理后的钢材其暴露在火焰环境下温度是上升速度会比不做耐火处理的钢材慢的多。同样的，高温对钢材的影响程度受其到火源距离的影响，在一个受控于燃料的环境下如果钢材附近有通风设备，那么通风设备会帮助冷却钢材，将热量带到其周围的环境中去。尤其是在复合钢筋混凝土原件中，钢筋和混凝土温度的升高会导致诸如屈服应力，杨氏模量，和混凝土极限抗压强度这些机械性能的下降。此外，当钢筋或复合结构承受高温时，钢筋的承载能力会下降。如果火的持续时间和强度足够大，其荷载抵抗能力会降到与外部荷载相当的水平，从而导致结构的破坏。然而，2001年911事件美国世贸大厦的倒塌尤其是WTC7楼的倒塌向工程专业发出了警告，揭示出建筑倒塌与高温条件的关联。在这个研究中，s220和s420两种钢材会经受七种不同的温度，以此来得出钢筋对于高温的反应。2.实验性研究实验性研究会分析直径为10mm和16mm，长200mm的s220和s420钢筋，测试试样会放在温度分别为20,100,200,300,500,800和950C的火炉中三个小时。在固化过程的结尾，钢筋会被自然的回复到室温。随后，将对这些钢筋进行抗拉强度试验。根据EN 10002-1，钢筋在温度升高条件下的抗拉强度，屈服强度和伸长率是已经确定的。钢筋试样的抗拉强度测试由60吨一般抗拉强度测试机器进行。测试机器的荷载施加速度按照TS 708 code。3.测试结果和评估3.1应力-应变 关系实验中不同温度下样本的应力应变关系均值已表示在图形1和2中。图形1和2中的曲线是通过直径为10和16的钢筋试样的平均试验值绘制的。该实验条件旨在模仿建筑遭受火灾时的状况，所以高温环境下结构中的钢筋的机械性能也就能够确定了。就像图形1中所表示的那样，低于500C的温度对于预热并冷却的s220钢筋的机械性能并没有太大的影响。S220钢筋的屈服强度和劈裂抗拉强度在这些温度下是相似的。然而，当温度上升至800C 以上时，s220钢筋的屈服强度和劈裂抗拉强度会出现下降。相似的反应同样发生在s420钢筋的实验结果中。暴露在高温下的钢筋在温度上升至800C以上时其延性会上升。Figure 1: Stress-strain curve of S220 steel rebar.Figure 2: Stress-strain curve of S420-ribbed steel rebar.图1：s220钢筋的应力应变曲线图2：s420钢筋的应力应变曲线3.2屈服强度两种钢筋的屈服强度都受到了外界温度升高的影响。从图形3中我们可以总结出钢筋的屈服强度在300C以内的条件下并没有太大的变化。光圆钢筋的机械加工中已经经历过这种温度。根据Eurocode 和 TS