【作者】 高洪波；

【导师】 徐世烺；

【作者基本信息】 大连理工大学 ， 结构工程， 2009， 博士

【摘要】 由于混凝土材料内部缺陷和施工阶段以及使用阶段各种因素的作用，混凝土结构尤其是大体积混凝土结构不可避免出现裂缝。因此，正确评估裂缝的稳定性、安全性，进而对混凝土结构进行合理的维修和加固，以及提高混凝土材料和大体积混凝土结构设计水平，一直是工程界和学术界极为关注的问题。而在混凝土建筑物设计阶段对混凝土拌和物设计和结构选型的抗裂设计，以及对已建混凝土建筑物出现的裂缝进行评估时，首先均需测定混凝土断裂韧度参数。基于此，本文结合水电水利行业标准DL/T5332-2005《水工混凝土断裂试验规程》（以下简称为《规程》）的制定和国家自然科学基金项目“基于粘聚力的混凝土Ⅱ型与复合型双K断裂准则及其参数确定（50178015）”，对混凝土Ⅰ型、Ⅱ型断裂参数确定开展了如下的研究工作：1．混凝土起裂荷载的确定、断裂韧度的尺寸效应和形状效应是《规程》制定过程中的重要问题。为了对《规程》的制定提供更充足的基础数据，受编制组委托，分析了最大尺寸为S×D×B=2200×550×240 mm³的三点弯曲梁试件和最大尺寸为2H×D×B=1200×1200×250mm³的楔入劈拉试件共140个不同级配及湿筛法大坝混凝土试件的断裂试验。结果表明：电阻应变片法测定起裂荷载的方法可行；三点弯曲梁法所测得的混凝土双K断裂韧度值无明显的尺寸效应；采用《规程》规定的楔入劈拉加载方式所测得的混凝土断裂韧度值表现出一定程度的尺寸效应；在相同配合比、相同高度条件下，两种试件测定的双K断裂韧度值无明显的差异。2．分析了《规程》规定的楔入劈拉加载方式所测得的混凝土断裂韧度值表现出一定程度的尺寸效应的原因是竖向分荷载P_V引起的附加力矩M_V对试件裂缝端部应力场和断裂参数计算具有影响，提出了在试件宽度1／4处施加楔入荷载以消除此影响从而测得混凝土真实断裂参数的新的试验方法——楔入式紧凑拉伸断裂试验方法。3．使用双K断裂模型韧度三参数定律确定混凝土起裂韧度时，韧度增值K_Ic^c的计算无论是积分计算方法还是简化计算方法都较复杂。为了《规程》在工程界的推广使用，提出了韧度增值K_Ic^c的二元拉格朗日插值计算方法。通过与积分和简化计算方法比较，表明此插值计算方法具有很高的精度，计算过程得到进一步的简化，使用计算器即可实现，非常便于工程人员实际使用。4．采用国际上新近提出的两端切口Ⅱ型断裂试件开展了混凝土Ⅱ型断裂试验，分析研究了试件的破坏机理。试验观测到裂缝起裂角θ₀约为0（?），且沿着韧带方向扩展；裂缝尖端韧带出现具有典型剪切特征的混凝土碎片，表明发生了剪切破坏模式。由试验测定的曲线上的特征点确定临界荷载，进而由此试件几何相应的应力强度因子公式计算出混凝土Ⅱ型断裂韧度。试验表明，两端切口Ⅱ型断裂试件是进行混凝土Ⅱ型断裂试验及测试混凝土K_Ⅱc合适的试件形式。5．基于课题组近年研究的基础上，提出了一种进行混凝土Ⅱ型断裂试验的初始无切口半边对称加载新的试件形式及其应力强度因子解析公式。此试件由于剪切应力集中在名义韧带端部区域形成剪切微裂区；当微裂区尖端Ⅱ型应力强度因子达到混凝土Ⅱ型断裂韧度时，试件发生Ⅱ型断裂破坏。因此，初始无切口半边对称加载试件断裂破坏机理与两端切口半边对称加载试件破坏机理本质上是相同的。6．试验观察到混凝土Ⅱ型断裂过程可分为三个明显的阶段：裂缝起裂、稳定扩展、失稳破坏，本文引入起裂断裂韧度K_Ⅱcⁱⁿⁱ和失稳断裂韧度K_Ⅱc^un两个断裂参数来描述混凝土结构Ⅱ型断裂裂缝起裂、稳定扩展、失稳破坏全过程，建立了判定混凝土结构Ⅱ型断裂破坏的Ⅱ型断裂双K断裂准则，即：K_Ⅱ=K_Ⅱcⁱⁿⁱ时，裂缝起裂；K_Ⅱcⁱⁿⁱ<K_Ⅱ<K_Ⅱc^un时，裂缝稳定扩展；K_Ⅱ≥K_Ⅱc^un时，裂缝失稳扩展。更多还原

【Abstract】 Due to inherent defects of concrete materials and various influences from construction and operation periods of structures, concrete is inevitable to crack, especially for mass concrete structures. Thus the appropriate appraisal of crack stability, safety and consequent maintenance and reinforcement of concrete structures, and the improvement of concrete materials and mass concrete structure design have always been the issues of concern in the engineering and academic fields. For crack-resistance design during concrete mixing stages and structure pattern choices, together with the evaluation of cracks occurring in existing concrete buildings, the first thing is to determine the concrete fracture toughness. Considering these facts, the author carried out the researches on determination of modeⅠand modeⅡfracture parameters for concrete shown as follows. And this research is part of the professional standard of water conservancy and hydro power profession: Norm for Fracture Test of Hydraulic Concrete （DL/T5332-2005） and National Nature Science Foundation project "The modeⅡand mixed mode double-K fracture criterion and the determination of the corresponding double-K fracture parameters of concrete based on the cohesive force （50178015）" .1. The determination of crack initiation load, the size effect of fracture toughness as well as geometry influence are key issues in the norm. To provide sufficient test data for the norm, a total of 140 specimens were tested on three-point bending beams and wedge-splitting specimen fabricated with various grading aggregates and wet-screened components of dam concrete. The maximum size for three-point bending beams isS×D×B = 2200×550×240mm³ , and wedge-splitting specimens with 2H×D×S = 1200×1200×250mm³. The results show that its is feasible to use electric resistance strain gauges to determine the crack initiation load; no apparent size effect is observed in the double-K fracture toughness from three-point bending beams, while the double-K fracture toughness from wedge-splitting testing according to the norm shows a certain size effect; and the double-K fracture toughness from these geometries are almost of the same magnitude for the same aggregate grading and same specimen height.2. The size effect in the fracture toughness from wedge-splitting tests could be attributed to the influence of vertical component of load P_v and additional moment M_v on the stress field around the crack tip. Accordingly, a novel wedge-splitting test on compact tension specimen for true fracture parameters is proposed where the load is imposed at quarter-point of the specimen width to counteract the above-mentioned influences.3. In three-parameter law of fracture toughness for quasi-brittle materials, the cohesive force-induced fracture toughness K_Ic^c is difficult to be obtained whether by using integration or simplified method. For practical applications of the norm, an alternative determining method for K_Ic^c is put forward using a bivariate Lagrange interpolation function. By comparison withthe simplified method, the interpolation method is shown to have a higher accuracy. And still this much more simple calculating procedure can be achieved on a pocket calculator and it is very convenient for practical use.4. Failure mechanism for concrete is explored based on experiments on double-edge notched specimens. The measured initiation fracture angle is approximately O^-|o and the crack propagates along the ligament line. The presence of concrete fragments typical of shear fracture at the crack tip shows that the specimen is experiencing modeⅡfracture. Feature points on the experimental curve could be used to determine the critical load, and in turn the fracture toughness K_IIc is calculated by using the corresponding stress intensity factorformula for the given specimen. The proposed method is shown to be robust and applicable to predict modeⅡfracture.5. Based on the author’s recent research, a symmetrically non-notched specimen is recommended to conduct modeⅡfracture tests of concrete and its stress intensity factor analytical formula is presented. A mirco-shear zone is developed in the front of the nominal ligament as a result of high stress concentration. As the modeⅡstress intensity factor in the tip of mirco-shear zone reaches the modeⅡfracture toughness, the specimen will fail in modeⅡfracture. Consequently, failure mechanism for this specimen is essentially the same with double-edge notched specimens6. ModeⅡfracture process for concrete could be described by three observable stages: crack initiation, stable propagation and unstable failure. The initiation fracture toughness K_IIcⁱⁿⁱ and unstable fracture toughness K_IIc^un are introduced here to differentiate these three stages. Thus a modeⅡfracture double-K fracture criterion for concrete is established, i.e., when K_II = K_IIcⁱⁿⁱ the preformed crack begins to crack initially; when K_IIcⁱⁿⁱ< K_II < K_IIc^un thepropagating crack develops steadily; when K_II≥K_IIc^un the crack propagates unsteadily.更多还原