Behavior of bond between reinforcement and concrete under fire

了解火灾下的粘结性能对于钢筋混凝土结构的设计和评估都是至关重要的。在恢复工作的情况下面临的挑战之一是评估现有成员的剩余能力。目前，对于暴露在火灾中的剩余债券能力的评估，没有明确的指导方针。规范给出的简化准则不足以现实地估计剩余的粘合能力。为了真实地模拟粘结行为，有必要区分可能的破坏模式(纯粘结破坏和/或混凝土劈裂破坏)，这是FIB模型代码2010对室温下的粘结所识别的。本规范建议了基于混凝土保护层、混凝土强度、钢筋直径等不同参数的拉拔破坏或劈裂破坏的粘结应力-滑移解析关系。尽管到目前为止已经进行了大量关于高温下粘结行为的研究工作，但仍然缺乏一个合适的模型来描述受热(真实火灾场景)和现实边界条件(无约束试件)下的粘结行为。现有的关于高温下粘结行为的大部分文献数据都是在稳态条件下通过防止混凝土开裂而获得的。此外，大多数实验都是在相对较低的升温速率下进行的。这些数据缺乏关于在真实火灾场景中产生的热致应力和随之而来的混凝土损伤对粘结行为的影响的信息。混凝土损伤的增加可能会在达到理论粘结强度之前导致混凝土开裂，从而可能高估理论上获得的剩余粘结能力。本研究项目的主要目标是研究粘结行为，并在考虑实际边界条件的情况下，开发一个实用的模型来估计暴露在标准火灾中的剩余粘结能力。拟议的研究工作包括一项通过数值模拟加强的实验活动。调查将使用暴露在标准燃烧曲线(ISO 834)下的梁端试件进行。将调查以下参数：加热时间、加热类型(单面或三面裸露)、混凝土保护层厚度、横向钢筋和钢筋直径。单面加热和三面加热这两种加热类型分别表示板和梁元素中的真实火灾暴露。实验结果将通过数值模拟得到进一步扩展。实验和数值计算的结果将被用来提出一个实用的模型来估算暴露在标准火灾中的剩余粘结能力。该模型将作为现有债券模型(FIB模型代码2010)的延伸而开发。

Understanding the bond behavior under fire is essential for both design and assessment of reinforced concrete structures. One of the challenges in case of reinstatement is the assessment of the residual capacity of the existing members. Currently, there are no clear guidelines on the evaluation of the residual bond capacity after exposure to fire. The simplified guidelines given by codes are insufficient for a realistic estimation of the residual bond capacity. In order to realistically model the bond behavior, it is necessary to differentiate between the possible failure modes (pure bond failure and/or concrete splitting failure), which is recognized by the fib Model Code 2010 for the bond at room temperature. This code recommends an analytical bond stress-slip relationship for pull-out failure or splitting failure based on different parameters such as concrete cover, concrete strength, rebar diameter etc. Even though a lot of research work on bond behavior under elevated temperatures has been performed so far, an appropriate model for the bond behavior under heating (real fire scenarios) and realistic boundary conditions (unconfined specimens) is still missing. The majority of the existing literature data on the bond behavior at high temperatures was obtained for steady state conditions and by preventing concrete splitting. Furthermore, the majority of the experiments were performed at relatively low heating rates. These data lack the information about the effect of thermally induced stresses and consequent concrete damage developed in real fire scenarios on bond behavior. Increased concrete damage can contribute to concrete splitting before the theoretical bond strength is achieved, resulting in a possible overestimation of the theoretically obtained residual bond capacity. The main objectives of the present research project are to investigate the bond behavior and to develop a practical model for the estimation of residual bond capacity after exposure to standard fire considering realistic boundary conditions. The proposed research work includes an experimental campaign augmented by numerical simulations. The investigations are to be performed using beam-end test specimens exposed to standard fire curve (ISO 834). The following parameters will be investigated: heating duration, heating type (one- or three-sided exposure), thickness of the concrete cover, transverse reinforcement and reinforcement diameter. The two heating types, one- and three- sided heating, represent the realistic fire exposure in slab and beam elements, respectively. Experimental results will be further extended by numerical simulations. The results of the experimental and numerical work will be used to propose a practical model for the estimation of residual bond capacity after exposure to standard fire. The model will be developed as an extension of the existing bond model (fib Model Code 2010).