【作者】 朱榆；

【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】 混凝土是一种由硬化水泥浆基体、骨料及基体与骨料间的过渡区组成的多相复合材料,其承载力及性质取决于各项的性质和它们之间的相互作用。水泥净浆和水泥砂浆是混凝土的基体成分,了解它们的基本断裂特性对于研究和提高混凝土材料的断裂性能具有十分重要的意义。由于混凝土的非均匀多相性,其内部不可避免存在空隙、微裂缝等缺陷,而且混凝土的抗拉强度远小于抗压强度,自身存在收缩和徐变,这些都是混凝土裂缝产生的主要原因。由于各种因素造成的混凝土开裂,使得实际工程中很多结构都是带裂缝工作的。近现代大量的试验研究和工程实践都表明:混凝土结构开裂几乎是不可避免的,在现有的经济和技术水平下科学的要求是将混凝土的裂缝控制在有害程度允许的范围内。基于此,本文首先研究了混凝土基体材料水泥净浆和水泥砂浆的断裂特性,随后通过在混凝土内部不同位置布置钢筋和在外部浇注UHTCC止裂带两种方式分别研究了它们对混凝土的阻裂作用。具体工作如下:（1）通过对156根不同强度、不同尺寸及不同初始缝高比的水泥净浆、水泥砂浆和混凝土三点弯曲梁试件的断裂试验,得到了荷载一裂缝口张开位移（P-CMOD）全曲线,采用全桥式应变片法测得了试件的起裂荷载,计算得到了起裂韧度的实测值,并分析了各试验参数对断裂韧度的影响。结果表明,水泥净浆的断裂过程并不是人们传统所认为的脆性断裂——一经起裂便失稳破坏,而是像混凝土一样在断裂过程中也要经过一个稳定的裂缝扩展过程才会达到失稳破坏。双K断裂模型也适用于水泥净浆和水泥砂浆等混凝土基体材料,且试件尺寸和初始缝高比对它们的双K断裂参数几乎没有影响,因此可以作为判定水泥净浆和水泥砂浆材料裂缝起裂、稳定扩展及失稳扩展的断裂模型。（2）采用幂函数和指数函数拟合法分别对试验测得的水泥净浆和水泥砂浆的P-δ曲线反弯点后的部分作了修正,得到了水泥净浆和水泥砂浆完整的断裂能,且所得到的断裂能没有尺寸效应。（3）进行了无任何阻裂措施的素混凝土三点弯曲切口梁的断裂试验,利用布置在试件上的全桥应变片和半桥应变片监测裂缝的扩展长度,测得了各测点处的开裂应变。根据所用混凝土的实际特性重新确定了非线性软化本构关系中的参数,并分析了该参数对起裂韧度理论值的影响,为下一步的阻裂研究奠定了基础。（4）通过在跨中预制缝的混凝土三点弯曲梁内的三种不同位置分别布置钢筋,研究了钢筋在混凝土裂缝扩展过程中的限裂作用。通过对裂缝扩展过程的分析发现,钢筋混凝土的断裂过程与素混凝土一样也可分为三个阶段:裂缝的起裂、稳定扩展和失稳扩展。因此,在引入适用于钢筋混凝土的起裂韧度K_Icⁱⁿⁱ和失稳韧度K_Ic^un两个断裂参数后,利用双K断裂准则对钢筋混凝土三点弯曲梁的裂缝扩展过程进行了描述,并通过试验对双K判定准则的适用性进行了验证。根据无量纲形式的BE CMOD/P与a/D之间的函数关系重新确定了钢筋混凝土三点弯曲梁裂缝长度的计算公式,并验证了计算公式的准确性,分析了不同裂缝长度计算公式对临界裂缝尖端张开位移的影响。（5）在混凝土三点弯曲切口梁的受拉面浇注UHTCC作为止裂带,研究其在混凝土裂缝扩展过程中的限裂作用,分析了UHTCC应变硬化特性对混凝土裂缝扩展过程的影响,根据界面变形一致的假定,忽略了UHTCC未开裂区域的弹性变形,提出了UHTCC总裂缝宽度w_u的确定方法,并由此得到了σ-w_u的简化模型,根据该简化模型便可以确定任意时刻UHTCC内的应力值。通过对UHTCC止裂带加固的混凝土三点弯曲切口梁裂缝扩展全过程的分析,引入适用于UHTCC加固混凝土三点弯曲梁的起裂韧度K_Icⁱⁿⁱ和失稳韧度K_Ic^un两个断裂参数,利用双K断裂准则对其裂缝扩展过程进行了描述,并通过试验对双K判定准则的适用性进行了验证。根据每根试件的实际裂缝扩展情况重新确定了适用于各试件的裂缝长度计算公式,并通过试验验证了所得公式的准确性。更多还原

【Abstract】 Concrete is a composite material,which can be properly represented by three phases in microstrueture:cement paste,aggregate as well as interfacial transition zone between them, the bearing capacity and property of which depends on the property of each component and interaction between them.As the matrix compositions of concrete,fracture properties of cement paste and mortar have great influence on researching and improving the fracture performance of concrete.Due to heterogeneous and multiphase of concrete,there inevitably exits gaps and micro-cracks within it,and there are some inherent characteristics of concrete, such as low tensile strength,shrinkage and creep,etc.These are the main reasons for concrete cracks.Concrete cracking caused by various factors makes many structures in actual project work with cracks.A large number of recent studies and engineering practices show that cracking of concrete structures is almost inevitable.In the existing level of.economic and technical,requirement of science is to control concrete cracks within the harmful degree. Therefore,the fracture properties of cement paste and mortar are studied firstly,and then resistance to crack propagation of concrete are researched by placing reinforcing bar at different positions in concrete and pouring Ultra High Toughness Cementitious Composite （UHTCC） on tension face of concrete.Details of the present study are introduced as follows:（1） Fracture tests are performed by 156 three-point bending beams of cement paste and mortar with different sizes,different strengths and different ratios of the notch length to beam depth.Complete load versus crack mouth opening displacement（P-CMOD） curves are directly obtained;the initial cracking loads are determined by using resistant strain gauges which make up a full-bridge circuit;the measured values of initial fracture toughness are calculated,and the influences of test parameters on fracture toughness are analysed.The results show that cement paste is not an ideal brittle material,and there also exist a nonlinear fracture process and a steady crack propagation stage before unstable fracture.Double-K fracture model is also suitable for cement paste and mortar,and there are no size effects on the double-K fracture paramenters.Therefore,the Double-K fracture model can be used to predict crack initiation,steady crack propagation and unstable fracture of cement paste and mortar.（2） A fit is made for the tail of the P-δcurves using power and exponential function, respectively,so complete fracture energy of cement paste and mortar are obtained,and the fracture energy are size-independent.（3） Fracture tests of concrete three-point bending notched beams without any crack control measures are carried out.The full-bridge strain gauges and the half-bridge strain gauges that arranged on specimence are used to monitor the length of crack propagation and measure the cracking strain of each measuring point.According to the actual characteristics of concrete used in tests,the paramenters in the nonlinear softening stress deformation relation are redetermined,and the influence of the parameters on theoretical values of initial fracture toughness is analysed.（4） Concrete three point bending notched beams with reinforcing bar placed at three different positions are used to study the effect of reinforcing bar on crack propagation of concrete.Through the analysis of crack propagation process,it is found that fracture process of reinforced concrete can be divided into three stages:crack initiation,stable extension,and unstable failure.Therefore,the initial fracture toughness K_Icⁱⁿⁱ and the unstable toughness K_Ic^un of reinforced concrete are introduced.Double-K fracture criteria are used for describing the whole process of crack propagation of reinforced concrete three-point bending notched beams, then applicability of the criteria is verified by tests.Formulas for calculating crack length of reinforced concrete three-point bending notched beams are reestablished based on the functional relation between nondimensional parameters BE CMOD/P and a/D and the accuracy of the calculation formulas is proved.The effect of different formulas for calculating crack length on the critical crack tip opening displacement is discussed.（5） UHTCC is poured on tension face of concrete three point bending notched beams as crack control strip to study the control action of UHTCC on crack propagation of concrete. The influence of strain hardening properties of UHTCC on crack propagation process of concrete is analyzed.Basing on the assumption that interface deformation is consistent and neglecting the elastic deformation of non-cracking regions of UHTCC,a method for determining total crack width of UHTCC is proposed and a simplified model ofσ-w_u is obtained subsequently.According to the simplified model,stress value of UHTCC at arbitrary hour can be determined.Through the analysis of whole process of crack propagation of concrete three-point bending notched beams retrofired with UHTCC,the initial fracture toughness K_Icⁱⁿⁱ and the unstable toughness K_Ic^un of concrete three-point bending beams retrofitted with UHTCC were introduced,and the double-K fracture criteria are used for describing the whole process of crack propagation of it,then applicability of the criteria is verified by tests.Formula for calculating crack length of each specimen is established based on practical situation of crack expansion,and the accuracy of the formulas is verified.更多还原