【作者】 刘汾涛；

【导师】 魏德敏；

【作者基本信息】 华南理工大学 ， 结构工程， 2010， 博士

【摘要】 碳纤维布(CFS)以其高强、轻质、耐久及施工便捷等优点,在混凝土结构加固领域得到了广泛应用。但碳纤维布及其与混凝土之间的环氧树脂类粘结材料在高温下会出现严重劣化,导致无防火保护的加固结构存在严重安全隐患。因此,积极开展碳纤维布加固混凝土结构的抗火性能研究,提出改善该类结构耐火性能的具体措施是十分必要的。本文从明火试验、数值模拟、参数分析和实用计算方法等方面,探讨了碳纤维布加固混凝土梁的火灾行为及耐火性能。本文主要工作如下:1.对4根碳纤维布抗弯加固混凝土梁和1根未加固对比梁进行了明火试验,前者碳纤维布表面涂抹有非膨胀型防火涂料。试验考察了梁跨中裂缝对受拉钢筋温度的影响,以及防火涂料厚度相对较薄时加固梁的破坏形态、高温变形和耐火极限。试验结果表明:(1)加固梁在达到耐火极限之前相当长一段时间内的挠度及裂缝宽度和深度较小,跨中裂缝对受拉钢筋温度影响有限;(2)混凝土的爆裂脱落可使加固梁的高温破坏位置发生显著改变;(3)在实际荷载比不大于0.5的情况下,即使防火涂料厚度较薄(10~20 mm),加固梁仍可达到2 h一级耐火要求。2.编制了设有防火保护层的碳纤维布加固混凝土梁的截面温度场计算程序,通过算例验证了程序的正确性。利用该程序,分析了防火材料类型及厚度、保护方式、截面尺寸对加固梁截面温度分布的影响。基于数值分析结果,提出了加固梁截面温度场的实用计算方法。研究结果表明:(1)除梁侧防火材料末端附近产生温度突变外,小U型防火保护方式下加固梁的截面温度分布总体上与底面防火保护方式下类似;(2)随着受火时间增加,梁底温度在底面防火材料的保护下虽然上升相对较缓,但也很快达到环氧类胶粘剂的失效温度;(3)与厚度相同的厚型钢结构防火涂料相比,水泥砂浆的防火保护效果明显偏弱。3.前期研究结果表明,常温下发生弯曲破坏的加固梁有可能在高温下出现破坏形态的转变,为此提出了高温下碳纤维布加固混凝土梁破坏形态转变临界状态(临界条件)的概念。利用编制的数值分析程序,针对常温下以弯曲破坏为主的加固梁,研究给出了相关参数(包括梁底防火保护层厚度、常温下构件抗剪承载力富裕程度、荷载比、加固量等)对加固梁破坏形态转变临界条件的影响规律,建立了该临界条件对应的参数表达式。在此基础上,提出了加固梁抗火设计的初步建议。研究结果表明,碳纤维布抗弯加固混凝土梁的底部防火涂料厚度应进行合理选择,以满足加固梁的耐火要求,同时避免出现高温剪切破坏。4.通过引入混凝土高温等效抗压强度,提出了碳纤维布加固混凝土梁高温抗弯承载力的一种简化计算方法。考察了防火涂料设置、碳纤维布加固量、受拉钢筋配筋率、混凝土保护层厚度等参数对加固梁高温抗弯承载力的影响规律,在此基础上建立了加固梁高温抗弯承载力随升温时间的定量衰减关系。研究结果表明:(1)利用该简化方法所得加固梁的耐火极限与试验结果吻合较好;(2)实际工程中梁侧防火涂料高度可以120 mm为限,在此范围内加固梁的高温抗弯承载力随着梁侧防火涂料高度的增加逐渐增大;(3)未设置防火涂料的加固梁要达到耐火极限2 h的要求较为困难;对于设置防火涂料的加固梁,即使涂料厚度只有10 mm,升温2 h后其抗弯承载力也比未设置时明显提高。5.开展了具有端部约束的碳纤维布加固混凝土梁的高温反应分析,初步揭示了轴向/转动约束刚度比、梁截面尺寸、跨度、荷载比、加固量、配筋率、保护层厚度和防火涂料厚度等参数对高温下约束加固梁的轴力及梁端弯矩的影响规律。通过大量数值分析,建立了高温下约束加固梁轴力和梁端弯矩的实用计算方法。研究结果表明:(1)随着升温时间增加,约束加固梁的轴力比总体呈现出先逐渐增大而后渐趋平缓甚至降低的趋势;(2)随着升温时间增加,约束加固梁的梁端弯矩总体呈现出先逐渐增大而后渐趋平缓的趋势;(3)防火保护(即使防火涂料厚度只有10 mm)对高温下约束加固梁梁端内力的影响非常明显。更多还原

【Abstract】 Carbon fiber sheet (CFS) has received widespread attention around the world as a relatively new material and technology for strengthening and repair of reinforced concrete structures in the past two decades. Compared with traditional strengthening technologies, CFS exhibits several advantages, including good resistance to corrosion, ease of application, and excellent mechanical strength. Research results have indicated that, deterioration of mechanical and bond properties of CFS with increasing temperature could result in the behavior of unprotected CFS-strengthened concrete members decreasing, which posed a significant risk for fire safety of these structures. Thus, a more complete understanding of the fire behaviors of CFS-strengthened concrete structures is required. In this paper, the fire resistance of reinforced concrete beams strengthened with CFS exposed to fire is discussed, through fire test, numerical simulation, parameter analysis and practical calculation method. The main research works are included as follows:1. Test results of five concrete beams in fire are presented in this paper, four of which were strengthened with carbon fiber sheet and protected by passive fire insulation, and the other unstrengthened one was tested as a comparison. The primary objectives of these tests are to investigate the influence of flexural cracks at mid span on the temperatures of tensile reinforcements, and to evaluate the failure mode, deformation and fire resistance of the strengthened beams with relatively thin fire insulation. Test results show that: (1) the deflection and crack width and depth of the strengthened beams increase very slowly for a long duration of the fire, leading the effect of flexural cracks on the temperatures of tensile rebars very limited; (2) spalling and debonding of concrete may cause a change in the failure location of the strengthened beams in fire; and (3) in the case that the actual load ratio is not larger than 0.5, the fire endurance of a strengthened beam with relatively thin fire insulation (e.g., 10~20 mm) can meet the requirement of 2 h in the design code.2. A computer program is developed to calculate the temperature fields of insulated concrete beams strengthened with carbon fiber sheet. This program is validated using experimental results from literatures. The influences of some parameters (e.g., type of insulation material, thickness of insulation, insulation scheme, and sectional size of beam) on temperature distributions of the strengthened beams in fire are analyzed using this program. Based on the numerical results, a simplified formula is proposed to predict the temperature fields of the CFS strengthened and insulated beams in fire. Simulation results show that: (1) the thermal fields of the CFS strengthened beams with minor U-shape insulation are similar to those with insulation at beam soffit, expect for locations close to the top end of minor U-shape insulation at beam sides; (2) the temperatures at beam soffit increase slowly with an increasing of heating time due to the protection of fire insulation, but they reach the failure temperature of epoxy quickly; and (3) the insulating effect of cement motor is much weaker than that of fireproof dope for steel structures.3. It is shown that elevated temperature may cause a change in the failure mode of concrete beams strengthened with CFS, as flexural failure at room temperature can be transformed into shear failure in fire. Hereby, the concept of the critical situation (i.e., flexural failure and shear failure occur simultaneously at high temperature) of the strengthened beams is proposed. In this paper, an analysis procedure for flexural capacity and shear capacity of RC beams strengthened in flexure using CFS at high temperature is discussed and validated by test results from literatures. A parametric study is conducted for the critical situation of the strengthened beams with fire insulation. The influences of some parameters, such as span-to-height ratio, confinement ratio, rich degree of shear capacity, thickness of concrete cover, thickness of fire insulation, and strengthening ratio, on the tensile reinforcement ratio related to the critical situation are examined. Based on the aforementioned analysis procedure and extensive numerical results, an empirical expression between the tension reinforcement ratio and the aforementioned parameters is suggested for the critical situation, which can be used to predict the failure mode of the strengthened beams in fire. Some recommendations for fire safety design of the flexurally strengthened and insulated beams are discussed preliminarily. It is important to recognize that increasing of the thickness of fire insulation is not always good for the fire performance of the strengthened beams. A balance between increasing of the flexural capacity of a strengthened beam and enhancing of the critical tensile reinforcement ratio should be made by appropriately determining the thickness of fire insulation through a trial-and-error process.4. Using the concept of equivalent compressive strength of concrete at high temperature, a simplified method is proposed for calculation of the flexural capacity of concrete beams strengthened with externally bonded carbon fiber sheet and subjected to fire. Then, the influence of some parameters (e.g., insulation condition, strengthening ratio, steel ratio, and thickness of concrete cover) on the flexural capacity of the strengthened beams in fire is discussed. Based on extensive parametric analysis, a regressive formula is suggested for the relationship between the flexural capacity of the strengthened beams and the heating time. Simulation results show that: (1) the fire resistance of the strengthened beams obtained using the aforementioned simplified method is in good agreement with the test result; (2) the insulation height at beam sides should be less than 120 mm, and the flexural capacity of the strengthened beams increases with an increasing of the insulation height within a range of 0～120 mm; and (3) it will likely be very difficult to achieve a 2 h fire endurance rating for an uninsulated CFS-strengthened beam. However, the flexural capacity of strengthened beams insulated with only 10 mm layer of fire insulation is obviously higher than that without fire insulation after 2 h of exposure to the fire.5. The CFS-strengthened beams with elastic axial and rotational restraints at beam ends are selected for numerical parametric study, and the effect of some parameters (i.e., axial/rotational restraint ratio, section size, length, load ratio, strengthening ratio, reinforcement ratio, thickness of concrete cover and thickness of fire insulation, etc.) on the axial force and bending moment at the end in restrained beams are analyzed. Based on the extensive simulation results, practical calculation methods for axial force and bending moment at the end of beams subjected to fire are proposed. Simulation results show that: (1) for axially-and-rotationally end restrained beams in fire, the axial force ratio increases gradually first, then varies gently, and finally decreases gradually; (2) the bending moment at the end of beam increases to the peak first, then becomes gentle in fire; and (3) the variation of internal force of restrained beams strengthened with CFS with time is different from that without fire insulation, even with 10 mm of fire insulation.更多还原