Studies on FRP-Concrete Interface
Dissertation Submitted to Tsinghua University in partial fulfillment of the requirement for the degree of Doctor of Engineering
by Xin-zheng Lu ( Civil Engineering )
Dissertation Supervisor		:	Professor Jian-jing Jiang
	October, 2004

摘  要

FRP-混凝土之间的界面力学行为及剥离破坏是FRP加固混凝土技术应用的关键基础问题，也是难点问题。本文对FRP-混凝土界面的三种主要剥离破坏形式：面内剪切剥离、受弯加固剥离和受剪加固剥离进行了系统的研究，提出了相应的数值计算模型，深入分析并揭示了FRP-混凝土的界面剥离破坏机理。基于对界面剥离破坏机理的充分认识，根据数值分析结果和试验结果，建立了界面粘结-滑移本构模型及受弯加固和受剪加固的剥离承载力的计算模型，并进一步提出了实用简化设计公式。与所收集的大量试验结果对比计算表明，本文的建议对FRP-混凝土界面行为模拟更为合理，所建议的设计公式优于现有文献公式，为安全、经济、合理地应用FRP加固混凝土结构技术提供了设计理论和方法。本文的主要研究工作和创新成果有：

(1)    首先针对面内剪切试验，采用所建议的混凝土非共轴转动裂缝模型，对FRP-混凝土界面行为进行了初步分析。为使数值分析更为稳定可靠，又进一步提出了精细单元有限元模型，深入研究了FRP-混凝土界面行为。并基于数值分析结果，提出了FRP-混凝土界面粘结-滑移本构模型及剥离承载力计算公式，与大量试验结果对比吻合良好；

(2)    基于精细单元有限元模型分析，得到了受弯加固剥离中的界面粘结-滑移关系，并提出了考虑裂缝宽度的双剥离破坏准则。利用无网格法，将传统混凝土有限元分析中的弥散裂缝模型和分离裂缝模型结合起来，提出了基于无网格法的组合裂缝模型，验证了双剥离破坏准则的正确性；

(3)    基于以上界面粘结-滑移关系和双剥离破坏准则，开发了相应的界面单元模型，使得受弯剥离可以通过普通钢筋混凝土有限元程序加以模拟。与大量试验对比表明，分析结果和试验结果吻合良好。在此基础上，根据有限元分析得到的界面粘结应力分布并加以简化处理，得到受弯剥离承载力的实用计算公式，与大量试验结果对比表明，计算结果与试验吻合良好，且过程简便；

(4)    根据受剪加固剥离问题的有限元分析和试验分析，讨论了FRP应力分布和斜裂缝宽度的关系，根据典型斜裂缝宽度分布规律和粘贴加固方式，提出了8种不同的受剪剥离简化计算模型，并讨论了受剪剥离的机理。基于最不利斜裂缝模型的分析结果，提出了受剪剥离承载力的计算公式。与大量试验结果对比吻合良好；

关键词：混凝土，加固，FRP，界面剥离，非线性分析

Abstract

External bonding of fiber reinforced polymer (FRP) plates or sheets has emerged as a popular method for the strengthening of reinforced concrete (RC) structures. In this strengthening method, the performance of the FRP-to-concrete interface in providing an effective stress transfer is of crucial importance. Indeed, a number of failure modes in FRP-strengthened RC members are directly caused by debonding of the FRP from the concrete. Therefore, for the safe and economic design of externally bonded FRP systems, a sound understanding of the behavior of the FRP-to-concrete interface needs to be developed.

In this thesis, the debonding failure behavior of three major FRP-concrete systems is investigated in detail: debonding in direct-pull tests, debonding in flexurally-strengthened beams and debonding in shear-strengthened beams. For each case, a systematic study is presented, which includes the numerical modeling of the FRP-strengthened structure and the detailed clarification of the debonding failure mechanism. Furthermore, the numerical results are verified and combined with available experimental results to establish interfacial bond-slip models and design models for debonding failures in flexurally- and shear-strengthened RC members. The proposed design models represent substantial improvements to existing models as evidenced through comparisons with existing test data.

The major contributions of the work presented in this thesis are listed as follows:

(1)   A finite element (FE) method for the simulation of direct-pull tests based on the non-coaxial rotating angel crack model (RACM) is first presented. FE results obtained using this method provides a preliminary clarification of the debonding process and mechanism. Next, a meso-scale FE model for debonding simulation is presented, which is capable of producing more reliable and detailed numerical results. The interfacial behavior is discussed in detail based on these meso-scale FE element results. In addition, new bond-slip models and bond strength models are proposed based on results from a numerical parametric study. The predictions of the proposed models are shown to be in close agreement with available test results.

(2)   An interfacial bond-slip model for use in the FE simulation of debonding failures in flexurally-strengthened RC beams (referred to as flexural debonding for brevity) developed from meso-scale FE results is described. A composite crack model based on the meshless method is also proposed in which the conventional smeared crack approach and the discrete crack approach for concrete are combined. Results from the composite crack model provides the basis of a dual debonding criterion for flexural debonding failures.

(3)   With the new bond-slip model and the novel dual debonding criterion, an interface element for flexural debonding failures is proposed so that flexural debonding can be accurately predicted with the conventional FE approach for concrete. The numerical predictions are compared with the results of 45 test beams and a good agreement is observed. Furthermore, based on the interfacial bond stress distribution from the FE model, and with some simplifications, a design model for flexural debonding is proposed. The predictions of the proposed design model agree well with the test results.

(4)   Based on both test results and numerical results, the relationship between the stress distribution in the FRP strips for shear strengthening of RC beams and the variation of the width of the critical shear crack is examined. Three typical width variations of the critical shear crack are then proposed, leading to eight simplified computational models for debonding failures in shear-strengthened concrete beams for different strengthening schemes. The debonding mechanism in shear-strengthened RC beams is discussed and a design model for shear strengthening is proposed based on the lower bound predictions of these eight computational models. This design model is compared and shown to agree well with the test results.

Key words: Concrete; Strengthening; FRP; Interfacial debonding; Nonlinear analysis

目  录

摘  要	I
ABSTRACT（英文摘要）	II
目  录	IV
第一章 绪论	1
1.1 FRP加固混凝土结构技术及研究概况	1
1.2 本课题研究目的、内容和路线	3

第二章 FRP-混凝土界面力学性能研究综述	5
2.1 引言	5
2.2 FRP-混凝土界面力学性能的试验研究	7
2.2.1 试验方案	7
2.2.2 试验结果	9
2.2.3 其他试验研究	11
2.3 FRP-混凝土界面数值模拟研究	12
2.4 FRP-混凝土界面的剥离承载力	13
2.4.1影响参数	13
2.4.2 界面剥离承载力模型	14
2.4.3不同剥离承载力模型对关键影响因素的考虑对比	17
2.5 FRP-混凝土界面粘结性能本构模型	18
2.5.1 概述	18
2.5.2 现有的本构模型	19
2.6 小结	23

第三章 基于宏观单元的界面力学性能的数值模拟	25
3.1 FRP-混凝土界面有限元分析的技术难点	25
3.2 非共轴转动裂缝混凝土本构模型	28
3.2.1 概述	28
3.2.2 强度准则	29
3.2.3 应力应变关系	30
3.2.4 裂缝模型和裂面剪力传递系数	30
3.3 有限元分析结果	32
3.3.1 面内剪切试验的有限元模型	32
3.3.2 参数讨论	33
3.3.4 模型验证	35
3.4 界面力学性能和剥离机理的讨论	38
3.5 小结	39

第四章 基于精细单元的界面力学性能的数值模拟	41
4.1 宏观单元模型的主要缺点	41
4.2 精细单元有限元模型	42
4.2.1 概述	42
4.2.2 FRP片材模型	43
4.2.3 混凝土模型	44
4.3 参数讨论	48
4.3.1 分析试件	48
4.3.2 裂面剪力模型	48
4.3.3 混凝土受拉软化模型	49
4.3.4 混凝土单元尺寸影响	50
4.3.5 对有限元模型的验证	51
4.4 界面剥离破坏过程	53
4.5 小结	58

第五章 界面粘结-滑移本构关系	59
5.1 引言	59
5.2 由精细单元模型得到的局部粘结-滑移关系	59
5.3 建议的粘结-滑移本构模型	61
5.3.1 精确模型	61
5.3.2 简化模型	65
5.3.3 双线性模型	65
5.4 建议的承载力模型	66
5.5 界面模型与试验结果对比	68
5.5.1 剥离承载力模型对比	68
5.5.2 本构模型对比	72
5.6 小结	79

第六章 抗弯加固剥离研究综述	80
6.1 引言	80
6.2 抗弯加固剥离试验研究	82
6.3 抗弯加固剥离数值分析	84
6.4 抗弯加固剥离承载力设计方法	85
6.4.1 概述	85
6.4.2 ACI 440建议的公式	86
6.4.3 黄-叶建议的公式	87
6.4.4 Teng等建议的公式	87
6.4.5 JSCE建议的公式	88
6.4.6 fib建议的公式	88
6.4.7 不同设计公式和计算结果的对比	89
6.5 小结	92

第七章 抗弯加固剥离的分析	94
7.1 引言	94
7.2 受弯剥离破坏试验研究	95
7.2.1 试验方案	95
7.2.2 试验结果	96
7.3 基于精细单元的有限元分析	97
7.3.1 精细单元有限元模型	97
7.3.2 界面粘结-滑移关系	99
7.3.3 双重剥离破坏准则	100
7.4 基于组合裂缝模型的数值分析	101
7.4.1 概述	101
7.4.2 基于无网格法的组合裂缝模型	102
7.4.3 计算模型和结果	105
7.4.4 小结	110
7.5 基于普通单元的有限元分析	110
7.5.1 概述	110
7.5.2 用户自定义界面单元	111
7.5.3 其他单元模型和本构模型	112
7.6 有限元分析结果与试验结果对比	114
7.6.1 典型试件对比	114
7.6.2 大量试验验证	120
7.6.3 参数讨论	124
7.7 受弯剥离承载力设计方法	124
7.7.1 简化的界面粘结应力分布	124
7.7.2 计算模型	126
7.8与试验结果对比	128
7.8.1 不同计算模型和试验结果对比	128
7.8.2 考虑保证率的设计公式	131
7.8.3 均布荷载情况	132
7.9 小结	132

第八章 抗剪加固剥离研究综述	134
8.1 引言	134
8.2 抗剪加固剥离的试验研究	135
8.2.1 U型粘贴	135
8.2.2 侧面粘贴	136
8.3 抗剪加固的剥离承载力公式	136
8.3.1 概述	136
8.3.2 ACI-440建议公式	137
8.3.3欧洲设计规范建议公式	138
8.3.4 英国设计规范建议公式	139
8.3.5 加拿大设计规范建议公式	140
8.3.6 日本设计规范建议公式	141
8.3.7 Chen & Teng公式	141
8.3.8 谭壮公式	143
8.3.9 张-叶-陆公式	143
8.3.10 U型加固计算值与试验值对比	143
8.3.11 侧面粘贴加固计算值与试验值对比	146
8.4 抗剪加固剥离的机理分析	148

第九章 抗剪加固剥离的分析	150
9.1 引言	150
9.2 试验研究	150
9.2.1 试验方案	150
9.2.2 试验结果	152
9.3 有限元分析	153
9.3.1 有限元模型	153
9.3.2 计算结果	154
9.4 FRP应力分布规律	158
9.4.1 斜裂缝形状的简化	158
9.4.2 滑移场模型	159
9.4.3 计算模型	160
9.4.4 计算结果	163
9.5 设计公式	169
9.6 小结	177

第十章 结论	178
参考文献	181
附录	192
致谢及声明	205
个人简历、在学期间的研究成果及发表的论文	206

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