温度和荷载共同作用下FRP加固混凝土/钢结构的界面剥离机理及控制

Debonding of FRP-to-concrete/steel interfaces is a very frequently encountered failure mode in concrete/steel structures strengthened with externally bonded fiber reinforced polymer (FRP) composites. It is one of the most important fundamental research topics widely studied in the past two decades in the research field of FRP strengthening of existing structures. As a result, a lot of research achievements have been obtained and reflected in existing design guidelines. However, it is well noted that most of these research achievements were obtained based on the test results of FRP-strengthened concrete/steel members at a room temperature. In actual engineering practice, however, the service temperature may vary significantly from the construction temperature at which the externally bonded FRP composites are installed. Therefore, neglecting the temperature effects on the FRP strengthening design of existing concrete/steel structures may lead to many uncertainties and un-conservative design, which consequently jeopardize the structural safety. This research project aims to conduct an in-depth investigation into the debonding mechanisms of FRP-to-concrete/steel interfaces and FRP-strengthened concrete/steel flexural members, which are subjected to combined thermal and mechanical loadings. The effects of both temperature-induced thermal stresses at the interface and temperature-induced changes of the interfacial constitutive law will be carefully considered. The issue of how to incorporate carbon nano-tube (CNT) modified epoxies into the externally bonded FRP technique for the enhancement of debonding strength and lessening of the detrimental temperature effects will also be explored. The project will consist of both experimental and theoretical phases, with the deployment of advanced measurement techniques, theoretical derivations and numerical analyses. Upon the completion of the project, solid analytical solutions and design theories for predicting the plate-end debonding and intermediate crack-induced debonding of FRP-strengthened concrete/steel members that are subjected to combined thermal and mechanical loadings will be established. This will facilitate a more safe and cost-effective application of the externally bonded FRP strengthening technique in concrete/steel structures in Mainland China and the rest of the world.

FRP-混凝土/钢界面剥离破坏是FRP加固混凝土/钢结构中的最常见破坏模式，也是过去二十年中该研究领域的一个最重要的基础课题，大量相关研究成果亦已在现有规范中得以反映。但是，这些成果大多是基于常温试验室条件下的获得，而实际工程应用中结构的加固时的施工温度和服役温度往往相差较大，忽略温度效应的FRP加固设计蕴含着许多不确定性和安全隐患。本项目将考虑温度变化引起的界面应力及界面本构关系的变化，深入研究温度和荷载共同作用下的FRP及混凝土/钢界面及FRP加固混凝土/钢受弯构件的剥离强度及破坏机理，并建立基于碳纳米管改性环氧树脂的界面剥离控制技术。拟结合先进的测试手段，通过对界面、加固构件不同层次的试验、理论及数值分析，建立考虑服役温度的FRP加固混凝土/钢抗弯构件的端部剥离和中部剥离强度的分析方法和设计理论，为国内外外贴FRP在混凝土/钢结构加固中的安全、经济的应用提供可靠的理论和技术基础。

FRP-混凝土/钢界面剥离破坏是 FRP 加固混凝土/钢结构中的最常见破坏模式。实际工程中结构加固时的施工温度和服役温度往往相差较大，忽略温度效应的 FRP 加固设计蕴含着许多安全隐患。本项目从理论和实验两个层面深入研究了温度对于FRP加固钢/混凝土梁的力学性能影响。对FRP加固混凝土/钢的端部剥离、中部剥离破坏进行了严格的封闭理论解分析。对于端部剥离的情况，分别基于II型和混合型界面破坏的假设，对FRP加固混凝土/钢梁的端部剥离进行了理论分析，发现温度对端部剥离强度的影响和其对FRP-钢/混凝土节点剥离强度的影响相反，在界面本构不变的情况下，升温提高FRP-钢/混凝土节点的剥离强度，但降低FRP加固钢/混凝土结构的端部剥离强度。对于中部剥离的情况，基于II型界面破坏的假设，对带缺口的FRP加固混凝土/钢梁的剥离进行了理论分析，却发现温度对中部剥离强度的影响和其对FRP-钢/混凝土节点剥离强度的影响一致，在界面本构不变的情况下，升温会提高FRP加固钢/混凝土结构的中部剥离强度。反之亦然。上述理论模型利用有限元分析进行了验证，并基于上述理论模型，对FRP力学几何参数，钢/混凝土梁的力学几何参数以及界面的力学参数对FRP加固钢/混凝土梁的端部中部剥离荷载的影响进行了参数分析，为设计规范的制定提供参考。除了理论分析，对实际FRP和钢板接节点、FRP加固钢梁以及FRP加固混凝土梁在不同服役温度下的力学性能也进行了试验观察。试验结果表明：不同的服役温度对于构件的承载能力和失效模式都有不同程度的影响，升温情况下，在界面胶到达玻璃转移化温度前，温度对FRP加固钢/混凝土梁的剥离荷载的影响和理论分析一致，但继续升温会导致剥离荷载的下降。 降温情况下，FRP加固混凝土/钢梁的剥离荷载和理论预测一致，在低温至负20度的情况下，未发现剥离承载力的降低。本项目澄清了服役温度对FRP加固混凝土/钢结构梁的剥离荷载的影响，对FRP加固设计规范的完善，保证FRP加固钢/混凝土结构的安全性设计具有重要意义。