被动式约束混凝土机理研究及其在FRP加固数值模拟的应用

Under triaxial compression, concrete exhibits prominent enhancement in strength and ductility. It makes the confinement scheme as the efficient method to be applied in the field of concrete structural retrofit. Among various kinds of constitutive models for confined concrete, the plasticity-based model has been widely adopted by researchers and engineers due to its reasonability in its mechanics and feasibility in its engineering practice. Since the appearance of the application of fiber reinforced polymer (FRP) in concrete structural retrofit, the linear elasticity of FRP laminate results in concrete under continuously increasing confinement which is linearly related to the lateral deformation. It is a most successful application of passive confinement onto the concrete structures. However, the plasticity-based constitutive model has yet to be well established for the passively confined concrete. The research scope of the constitutive model in the sense of passive confinement mechanism will be under further investigation in this project. A specific triaxial compression test program, in which the lateral loading is restrained into passive confinement with arbitrary biaxial loading ratio, is proposed in this project. The experimental research will further assist the theoretical study on the yielding criterion, hardening criterion and flow rule. Through the calibration of the initial and subsequent yielding surfaces, the characteristics of friction angle and cohesion will be extensively investigated. In order to qualify the nonassociated flow rule for concrete, the plastic dilation angle will be calibrated with assistance of test data. The establishment of the comprehensive constitutive model for passively confined concrete will facilitate the finite element analysis for FRP confined concrete columns with arbitrary cross-section or under different loading scenario. It will fundamentally enhance the development of computer aid design for FRP retrofit in civil engineering.

混凝土在三向受压下，其强度及延性均有明显提高。基于此特性，约束混凝土成为常用的加固方法。现应用于约束混凝土的众多本构模型中，塑性模型以其理论上的契合性与工程设计上的易操作性而被广泛应用。随着纤维复合材料（FRP）约束混凝土技术的出现，被动式约束（围压变化受侧向变形控制）成为应用于混凝土结构加固重要的约束形式之一。然而该类模型在被动式约束应力路径下尚未架构完善。本项目将从被动式约束机理研究出发，拓展混凝土本构模型在FRP加固领域的应用：设计试验装置实现单向主动轴压与双向被动围压同步加载的试验，辅助理论研究；探讨混凝土塑性本构重要参数（摩擦角、粘结强度与塑性膨胀角）的特征，规范被动式约束下混凝土塑性本构的屈服准则、强化准则和流动准则；最终实现对有限元软件内置本构的二次开发，准确模拟FRP约束混凝土柱在不同截面形式，不同轴向力加载形式下的力学特性，为FRP加固的计算机辅助设计提供基础性研究依据。

纤维增强符合材料（简称，FRP）现已成为提升混凝土柱构件强度和延性的约束材料之一。由于FRP是线弹性材料，其在约束混凝土柱后的受力过程中产生被动式约束应力路径，即围压随侧向膨胀变形的增长而增长。混凝土在被动约束机制下具有不同与主动约束下的应力-应变关系。为能准确模拟FRP加固混凝土柱压弯性能，准确的混凝土本构模型至关重要。然而目前考虑被动约束的混凝土本构模型主要存在两大问题：1）本构试验局限于均匀约束，2）非均匀约束下的本构模型或为经验型应力-应变模型或依托主动约束工况下的三轴试验数据的三维本构。本项目着眼于被动式约束机理研究，为拓展混凝土本构模型在FRP加固领域的应用：1）三向加载试验装置研发的必要性论证；2）三向加载试验装置的设计；3）混凝土三向受压的力学性能的研究；4）数值模拟FRP 约束混凝土非圆截面柱的抗压性能。形成的结论如下：结合大量的试验数据与已有模型，阐明了混凝土的力学特性与围压的增长历史有关；建立了应力和应变与应力路径（即，围压增长率）的显式关系，便于应用于现有的应力-应变关系方程，同时论证了设计新型三轴试验装置的必要性；依托现有结构实验室的试验条件，建成了首个具被被动约束加载性能的真三轴加载系统，它由定制双向围压加载装置结合现有的单轴压缩试验机组成，大大降低研发新型试验加载装置的费用，通过一系列研发工作，实现了试验装置的有效性；基于此，进行了大批混凝土立方体试块三轴试验，建立了首批包含均匀被动约束双轴加载，不均匀被动约束双轴/单轴加载试验数据库；将混凝土在不均匀约束下的轴向应力-轴向应变全曲线及侧向应变-轴向应变特质曲线模型化，其中主要影响因素为最大约束刚度，不均匀约束比，最大侧向应变；基于现有的屈服函数/势函数方程模式，重新标定参数和增加参变量，从而实现不改变现有结构分析本构框架，提高复合结构分析准确度，降低结构分析的复杂度。最终可应用于考虑FRP约束效果的混凝土柱构件的数值分析。