混凝土Ⅰ型裂缝扩展准则比较研究

针对混凝土I型裂缝扩展问题,分别采用以起裂韧度为参数的裂缝扩展准则、最大拉应力准则以及裂尖处应力强度因子为零的裂缝扩展准则,数值模拟了强度等级C20、C40、C60、C80和C100的混凝土三点弯曲梁裂缝扩展全过程,获取了试件的荷载-裂缝口张开位移(P-CMOD)曲线并与试验结果进行了比较。结果表明,三种准则中以起裂韧度为参数的裂缝扩展准则计算得到的峰值荷载及P-CMOD全曲线与试验结果差别最小。随着混凝土强度等级的提高,最大拉应力准则以及裂尖处应力强度因子为零的裂缝扩展准则计算出的P-CMOD曲线与试验结果相比均有较为明显的偏离,但以起裂韧度为参数的裂缝扩展准则计算结果与试验曲线更为吻合。试验与计算结果表明,以起裂韧度为参数的裂缝扩展准则更适用于不同强度混凝土材料的断裂分析。     

全级配混凝土I型裂缝扩展全过程数值模拟

该文将起裂断裂韧度作为裂缝扩展的判定依据,应用ANSYS软件,对全级配混凝土I型裂缝扩展过程进行数值模拟,分别计算了混凝土楔入劈拉试件的荷载-裂缝口张开位移曲线、临界裂缝长度和双K 断裂韧度,并与溪洛渡大坝的断裂试验结果进行比较,吻合良好。同时,结合试验数据,将该文计算结果与《水工混凝土断裂试验规程》规定的标准尺寸试件断裂参数计算结果进行对比。结果表明:规程规定的方法也适用于大尺寸非标准试件双K 断裂参数的计算,其误差率在5%以内。此外,对于全级配混凝土,只要通过试验测得其弹性模量、抗拉强度、抗压强度和起裂荷载,即可用该文提出的方法计算混凝土的双K 断裂韧度和裂缝扩展全过程。应用该方法还可以得到全级配混凝土的K_R阻力曲线。     

确定混凝土开裂与拉伸强度及双K断裂参数

该文考虑混凝土材料的非均质特性,发展了确定无尺寸效应的混凝土开裂强度与起裂韧度、拉伸强度与断裂韧度等材料参数的断裂理论与相应方法。基于三点弯曲、楔入劈拉、四点弯曲等不同类型混凝土试件的断裂试验,确定出对应的开裂强度与起裂韧度、拉伸强度与断裂韧度等材料参数,并与试验强度值及由双K断裂模型确定的双K断裂参数进行了比较,从而验证了所提模型与方法的合理性与适用性。基于确定的材料参数,分别建立了混凝土起裂与断裂破坏的全曲线,给出了确定无尺寸效应起裂韧度的混凝土试件最小理论尺寸。建立了起裂荷载与起裂韧度之间的解析公式,对三点弯曲、楔入劈拉、四点弯曲等不同类型混凝土试件的起裂荷载,以及不同混凝土的起裂韧度进行了成功预测。     

基于裂缝扩展准则的K_R阻力曲线研究

根据混凝土I型裂缝扩展准则,将起裂断裂韧度作为材料参数,提出了一种新的计算混凝土结构裂缝扩展过程KR阻力曲线模型。该模型认为,裂缝扩展阻力由混凝土材料固有的抗裂能力和粘聚力提供的阻力组成并等于裂缝扩展的驱动力;应用该模型并借助有限元法计算了混凝土三点弯曲梁、楔入劈拉试件的裂缝扩展量、断裂过程区（FPZ）长度,在此基础上...     

低热水泥全级配混凝土断裂试验及尺寸效应分析

该文分析了低热水泥全级配混凝土断裂性能的尺寸效应规律。通过在大坝工程施工现场浇筑成型试件有效高度为0.75 m、1.5 m和2.25 m的楔入劈拉试件并开展试验,获得低热水泥全级配混凝土稳定的双K断裂参数。基于试验结果,采用理论方法推求全级配混凝土无尺寸效应的失稳和起裂韧度,预测任意尺寸试件的最大和起裂水平荷载。研究发现,当韧带高度与骨料最大粒径比值大于等于6.0后,双K断裂参数趋于稳定。尺寸效应模型及其演化形式可用于确定全级配混凝土无尺寸效应的起裂和失稳断裂韧度,定量分析强度与韧度对裂缝扩展的影响,并可通过引入等效几何参数,分别建立任意尺寸试件最大和起裂水平荷载的预测方程。     

大初始缝高比混凝土试件双K断裂参数的试验研究

双K 断裂准则能够定量描述混凝土裂缝的起裂、稳定扩展和失稳断裂。该文采用混凝土三点弯曲梁试件,通过在试验中测得的起裂荷载P_ini、最大荷载P_max及临界裂缝口张开位移CMOD_C计算了初始缝高比为0.3~0.9共7组试件的起裂断裂韧度K_ICini 和失稳断裂韧度K_ICun 。结果表明,当初始缝高比为0.3~0.7时,混凝土裂缝扩展经历起裂、稳定扩展和失稳破坏3 个阶段,双K 参数均是与初始缝高比无关的材料参数;当初始缝高比大于或等于0.8 时,混凝土裂缝起裂后便进入失稳扩展阶段,起裂荷载即为最大荷载,且计算得到的K_ICini 仍与初始缝高比无关。因此,在确定K_ICini 时,仅需测得初始缝高比大于或等于0.8试件的P_max,将P_max作为P_ini直接计算得到K_ICini。同以往的试验方法相比,其结果更为准确且试验方法简单。     

采用峰值荷载法确定全级配水工混凝土断裂参数

该文采用峰值荷载法研究了全级配水工混凝土试件的断裂参数。通过有限元方法,得到了适用于全级配水工混凝土的非标准楔入劈拉试件的应力强度因子K、裂缝嘴张开口位移CMOD计算公式以及裂缝张开位移COD与CMOD比值COD/CMOD的相关曲线。基于相关断裂试验,通过该文所提公式得到了全级配水工混凝土的等效断裂韧度与临界裂缝尖端张开口位移CTOD_c等断裂参数。研究结果表明:峰值荷载法可应用于截面尺寸不同而缝高比相同的全级配水工混凝土试件,峰值荷载法只需实测各试件的峰值荷载,就可方便确定全级配水工混凝土的断裂参数。     

小尺寸混凝土试件双K断裂参数试验研究

采用最大尺寸为680mm×160mm×40mm的标准三点弯曲梁试件,利用在初始裂缝两侧粘贴电阻应变片并利用混凝土裂缝扩展到此处时其应变回缩的方法测得了起裂荷载Pini,在此基础上根据Pini及初始缝长a0得到了起裂断裂韧度KIiCni;根据在试验中测得的最大荷载Pmax及对应的裂缝口张开位移CMODC计算了混凝土等效裂缝长度aC,据此计算了失稳断裂韧度KIuCn。结果表明:采用电阻应变片法可准确测定混凝土的起裂荷载Pini,且方法简单。试验结果还表明:在本试验范围内,三点弯曲梁法测得的混凝土双K断裂参数KIiCni、KIuCn与试件高度无关,进一步说明了混凝土双K断裂参数可以作为描述混凝土裂缝扩展的断裂参数。     

电测法确定低强混凝土裂缝起裂和等效裂缝长度

该文设计了3种低强度混凝土三点弯曲切口梁,测试研究其双K断裂参数。试验中采用标距为5 mm和10 mm的应变片以半桥连接方式测量预制裂缝的起裂荷载,比较其工作性,发现应变片测量起裂荷载具有强度敏感性,短标距应变片更适宜测量本试验用低强度混凝土的裂缝起裂。试验中另沿韧带方向布置4组半桥应变片,根据各测点处拉应变回缩时的荷载与裂缝口张开位移,计算裂缝发展至测点高度时刻的等效裂缝长度,并与此时的实际缝长比较,结果表明双K断裂模型在预测裂缝长度方面具有较好的适用性。     

环氧沥青混凝土裂纹起裂与失稳扩展的临界特征

为了获取环氧沥青混凝土的裂纹起裂与失稳扩展临界点,重构了环氧沥青混凝土非均质(集料、砂浆和空隙)多层次(矿料级配)结构三维虚拟试件,采用离散元方法实施了单边切口小梁虚拟三点弯曲试验,借助数字摄像法捕捉了室内小梁试件表面裂纹发展情况,分析了裂缝发展过程中裂缝尖端张开位移(CTOD)和裂缝口张开位移(CMOD)之间的变化关系。研究结果表明:裂尖张开位移δ₂₅参数的使用为理解实际流入断裂带用于裂缝扩展的那部分能量开辟了一条新思路;裂缝尖端张开位移和裂缝口张开位移关系曲线的两个转折点描述了裂缝起裂和临界失稳扩展状态,环氧沥青混凝土断裂过程中裂缝的扩展经历了起裂、稳定扩展和失稳扩展3个阶段;δ₂₅-CMOD曲线分析法可以作为研究沥青混凝土材料断裂行为的辅助手段。     

Experimental and numerical investigations on fracture process zone of rock–concrete interface

A crack propagation criterion for a rock–concrete interface is employed to investigate the evolution of the fracture process zone (FPZ) in rock–concrete composite beams under three‐point bending (TPB). According to the criterion, cracking initiates along the interface when the difference between the mode I stress intensity factor at the crack tip caused by external loading and the one caused by the cohesive stress acting on the fictitious crack surfaces reaches the initial fracture toughness of a rock–concrete interface. From the experimental results of the composite beams with various initial crack lengths but equal depths under TPB, the interface fracture parameters are determined. In addition, the FPZ evolution in a TPB specimen is investigated by using a digital image correlation technique. Thus, the fracture processes of the rock–concrete composite beams can be simulated by introducing the initial fracture criterion to determine the crack propagation. By comparing the load versus crack mouth opening displacement curves and FPZ evolution, the numerical and experimental results show a reasonable agreement, which verifies the numerical method developed in this study for analysing the crack propagation along the rock–concrete interface. Finally, based on the numerical results, the effect of ligament length on the FPZ evolution and the variations of the fracture model during crack propagation are discussed for the rock–concrete interface fracture under TPB. The results indicate that ligament length significantly affects the FPZ evolution at the rock–concrete interface under TPB and the stress intensity factor ratio of modes II to I is influenced by the specimen size during the propagation of the interfacial crack.     

考虑骨料体积含量影响的混凝土准脆性断裂预测模型及应用

基于对准脆性断裂边界影响模型参数的分析,该文将平均骨料粒径d_ave引入模型中,得到了考虑骨料体积含量及尺寸影响的混凝土准脆性断裂预测模型。模型中的有效裂缝与特征裂纹的比值,明确表征了三分点加载单边切口梁(SENB)试件的尺寸及初始缝长度变化时服从的断裂失效准则;模型中d_ave及分散系数β_ave将影响最大荷载P_max作用下临界微裂纹扩展区的平均虚拟裂纹长度Δa_fic。通过SENB试件在P_max时的受力分析,得到了临界正应力σ_n、有效裂缝长度a_e、拉伸强度f_t及断裂韧度K_IC之间的关系式。通过Amparano的试验结果,当a_fic为0.8~1.4倍d_ave时,采用混凝土准脆性断裂模型能较好预测混凝土拉伸强度及断裂韧度。通过对Δa_fic=1.2d_ave时模型得到的预测曲线与试验结果的对比,证明了模型计算结果的可靠性。考虑骨料体积含量影响的混凝土准脆性断裂模型能基于RILEM规范中三分点加载SENB试验预测混凝土断裂韧度与拉伸强度。     

Effect of acid corrosion on crack propagation of concrete beams

The effect of acid corrosion on crack propagation of concrete beams was theoretically studied by the method of crack extension resistance curve. Based on this method, a calculation approach was proposed to determine fracture stress intensity factors in crack propagation of concrete beams. Loop iteration analysis was carried out to calculate maximum bearing capacity load, unstable crack toughness, resistance toughness curve, cohesive toughness curve and load–crack mouth opening displacement. Both bilinear and nonlinear softening traction–separation curves were adopted for each of these calculation parameters. The analysis results of each showed the effect of acid corrosion degrees. The influence of acid corrosion on fracture properties was discussed through the calculated results of cohesive toughness curves. These five kinds of simulated results were basically consistent, before the load attained the maximum value. However, with further crack propagation, cohesive toughness of nonlinear softening model was significantly larger than that of bilinear softening model, and the descending branch of P–CMOD curve by nonlinear law is higher than that by bilinear law. To validate the approach, tests of specimens under six different corrosion periods were experimentally studied, using three-point bending notched concrete beams soaked in sulphuric acid solution. The Double-K fracture parameters were investigated based on the test results, and load–crack mouth opening displacement curves for different acid conditions were obtained using synchronous sampling of a load sensor and clip-gauge. Numerical results by bilinear softening model showed a good correlation with the experimental ones.     

Interaction between tensile strength failure and mixed mode crack propagation in concrete

Crack size and structure size transitions are illustrated which connect the two limit-cases of ultimate tensile strength failure (small cracks and small structures) and mixed-mode crack propagation (large cracks and large structures). The problem of mixed-mode crack propagation in concrete is then faced. By increasing the size-scale of the element the influences of heterogeneity and cohesive crack tip forces disappear and crack branching is governed only by the linear elastic stress-singularity in the crack tip region. It is proved in this way that the fracture toughness of the material is measured by a unique parameter (G_IF, G_IC or K_IC) even for the mixed-mode condition. The ratio of the sliding or Mode II fracture toughness (G_IIF, G_IIC or K_IIC) to the opening or Mode I fracture toughness depends only on the crack branching criterion adopted and not on the material features. Eventually, very controversial experimental results recently obtained on the shear fracture of concrete are explained on the basis of the above-mentioned size-scale transition.     

基于虚拟裂缝模型的混凝土等效断裂韧度

本文采用虚拟裂缝模型,将临界裂缝尖端张开位移ＣＴＯＤｃ作为控制参数,利用三点弯曲梁试件通过迭代求得了混凝土裂缝亚临界扩展量的临界值△ａｃ,据此求得了混凝土起裂断裂韧度ＫｉｎｉＩｃ、等效断裂韧度ＫｕｎＩｃ值。计算结果表明,随着试件尺寸的增大,△ａｃ增大,但ＫｉｎｉＩｃ、ＫｕｎＩｃ值却是与试件尺寸无关的断裂参数。这表明线弹性断裂韧度准则可应用于混凝土结构的裂缝评定。     

Debonding of a thin rubberised and fibre-reinforced cement-based repairs: Analytical and experimental study

An analytical approach for the prediction of debonding initiation between a rubberised cement-based overlay and old concrete substrate under monotonous mechanical loading was applied. Based on the linear elastic fracture mechanics, a model has been developed taking into account the interlocking between two crack surfaces in the overlay. Assuming that the debonding initiation just occurs after the crack cutting the overlay layer reaches the overlay–substrate interface, the stress intensity factor of the debonding tip can be calculated, allowing prediction of stress fields near the interface debonding tip. Then with a criterion of debonding initiation and propagation depending on the interface tensile strength, the load associated could be determined and might be interesting for the design of thin bonded cement-based overlays. The adequateness of this analytical approach was verified by both experimental data and finite element calculations. It has been used to show the relevance of a cement-based material with low modulus of elasticity combined with a high residual post crack strength to achieve sustainable repairs.