Computational modeling of fracture in capsule-based self-healing concrete: A 3D study

We present a three-dimensional (3D) numerical model to investigate complex fracture behavior using cohesive elements. An efficient packing algorithm is employed to create the mesoscale model of heterogeneous capsule-based self-healing concrete. Spherical aggregates are used and directly generated from specified size distributions with different volume fractions. Spherical capsules are also used and created based on a particular diameter, and wall thickness. Bilinear traction-separation laws of cohesive elements along the boundaries of the mortar matrix, aggregates, capsules, and their interfaces are pre-inserted to simulate crack initiation and propagation. These pre-inserted cohesive elements are also applied into the initial meshes of solid elements to account for fracture in the mortar matrix. Different realizations are carried out and statistically analyzed. The proposed model provides an effective tool for predicting the complex fracture response of capsule-based self-healing concrete at the meso-scale.     

The effects of interfacial strength on fractured microcapsule

The effects of interfacial strength on fractured microcapsule are investigated numerically. The interaction between crack and microcapsule embedded in mortar matrix is modeled based on cohesive approach. The microcapsules are modelled with variation of core-shell thickness ratio and potential cracks are represented by pre-inserted cohesive elements along the element boundaries of the mortar matrix, microcapsules core, microcapsule shell, and at the interfaces between these phases. Special attention is given to the effects of cohesive fracture on the microcapsule interface, namely fracture strength, on the load carrying capacity and fracture probability of the microcapsule. The effect of fracture properties on microcapsule is found to be significant factor on the load carrying capacity and crack propagation characteristics. Regardless of core-shell thickness ratio of microcapsule, the load carrying capacity of self-healing material under tension increases as interfacial strength of microcapsule shell increases. In addition, given the fixed fracture strength of the interface of microcapsule shell, the higher the ratio core-shell thickness, the higher the probability of microcapsules being fractured.     

Acoustic emission analysis for the quantification of autonomous crack healing in concrete

As half of the annual construction budget is spent on remediation of existing structures, self-healing of concrete, which is very sensitive to cracking, would be highly desirable. In this research, encapsulated healing agents were embedded in the concrete matrix in order to obtain self-healing properties. Upon cracking, the capsules break and the healing agent is released, resulting in crack repair. The efficiency of this crack healing technique was evaluated by means of mechanical tests and by using acoustic emission analysis. It was shown that due to autonomous crack repair, more than 80% of the original strength and stiffness can be regained. Events with an energy higher than the energy related to concrete cracking indicated breakage of the capsules. Upon reloading of beams with untreated cracks, the released energy was lower compared to beams with healed cracks. From this study it was shown that AE is a suitable technique to evaluate self-healing of cracks in concrete.     

混凝土I-型细观断裂模型及其在材料层次#br# 尺寸效应中的应用

混凝土材料的断裂破坏本质上是内部微细裂纹在荷载作用下不断萌生、扩展以及贯通的结果,断裂裂缝在细观层次上则是由砂浆裂缝、界面裂缝以及骨料裂缝3部分组合而成。文章基于细观力学和断裂力学基本理论,建立一类能够同时考虑细观裂缝在混凝土材料内部扩展过程中绕过骨料和穿透骨料发展的混凝土I 型细观断裂模型。与已有试验数据对比表明,文章模型能够有效预测混凝土断裂能等宏观力学参数随细观组分力学性能的变化规律。进而,基于建立的细观断裂模型,初步分析混凝土材料层次的强度尺寸效应,结果表明：当砂浆力学性能确定时,混凝土材料的名义强度与骨料强度和界面强度正相关;界面的力学性能能够显著影响混凝土材料名义强度等宏观力学参数随骨料尺寸的变化规律;高性能混凝土强度随骨料尺寸增大而增大,普通性能混凝土强度随骨料尺寸增大而减小。文章模型分析方法以期为基于性能设计的混凝土配合比研究奠定理论基础。     

骨料和砂浆等影响混凝土强度的细观层次机理分析

基于全级配三维细观混凝土随机模型分析方法,系统研究了混凝土中砂浆和骨料等各组分的受力特点,通过现场试验以及数值模拟分析,揭示了骨料降低砂浆强度的作用机理,提出混凝土宏观抗压强度与砂浆抗压强度间的修正关系,并讨论了骨料体积分数及混凝土强度分别对该修正系数的影响规律.研究表明：骨料的加入会降低水泥基材料的强度;混凝土材料的强度小于相同配比条件下的砂浆;用混凝土强度参数代替砂浆参数进行细观层次分析会造成较大误差,应进行修正;修正系数随骨料体积分数增加而线性增长,与混凝土强度关系不大.     

骨料缺陷对再生混凝土力学性能的影响

将2种普通混凝土破碎加工成再生粗骨料(RA),经620℃高温处理,剔除RA上的附着砂浆,得到再生粗骨料H-RA,然后配制再生骨料混凝土(RAC),测定其抗压强度、劈裂抗拉强度和断裂能.结果表明:RAC的力学性能显著下降,这归因于RA破碎加工导致的石子损伤及其表面的附着砂浆;在低水胶比条件下,RA中的石子损伤是导致RAC力学性能下降的主要因素,而在高水胶比条件下,导致RAC力学性能下降的主要因素则是石子表面的附着砂浆;吸水率与断裂能可敏锐反映RA的缺陷特征.     

混凝土四相复合模型的三维细观破坏模拟

考虑到混凝土内部细微裂隙的存在对混凝土强度和变形的重要影响,提出将混凝土视为砂浆、骨料、界面和随机缺陷组成的四相复合材料模型。混凝土细观单元服从应变空间内增量形式的弹塑性损伤本构,采用破坏单元网格消去法模拟混凝土裂纹扩展,数值模拟了三维混凝土单轴压缩和不同均质度试样的单轴拉伸弹塑性损伤破坏过程。结果表明：文中建立的混凝土四相复合材料模型中增加随机缺陷的模拟,一定程度上符合混凝土成形实际情况;随机缺陷的体积百分比是决定弱化混凝土宏观强度程度的主要因素;均质度低的单轴拉伸试样裂纹咬合明显,均质度高的试样断裂更     

混凝土梁破坏机制的声发射特性实验研究

记录了钢纤维混凝土,素混凝土和砂浆试件断裂过程中的声发射（AE）信号,并对其进行了分析和处理,通过对声发射信号持续时间设置滤值,以声发射信号的振幅分布特性为判据,并依据事件峰值对应的振幅大小,将它们的声发射信号持续时间分别划分为9个,7个和5个区段,并通过细胞细观层次上破坏机制的分析,分析得到了各破坏机制所对应的声发射参数特征。     

二维混凝土不规则骨料提取与重构技术

通过真实混凝土断层拍照成像及自行开发的图像处理技术,精确区分粗骨料与砂浆,进一步统计获取真实不规则骨料的数量与尺寸信息。开发了一种二维不规则颗粒重构技术,以颗粒尺寸为控制指标,通过一组特征值向量控制生成特定形状的不规则颗粒;进一步开发了相应的颗粒随机有序堆积算法,结合获取的实际颗粒数量与尺寸信息,重构一幅不规则颗粒图像。最后,通过数值模拟方法,研究粗骨料颗粒形状对氯离子传输过程的影响。结果表明,利用"中心生长法"重构的不规则颗粒体系能很好的模拟真实体系,稳态氯离子扩散系数偏差仅有1.35%。     

Micro-chemical/structural characterisation of thin layer masonry: A correlation with engineering performance

Masonry is one of the most ancient methods of construction. The types of blocks and mortars used over the centuries have developed considerably with the introduction of various types of materials. A multidisciplinary investigative approach, involving both engineering and material sciences expertise, will be necessary for a complete evaluation of the behaviour of masonry elements. From the engineering point of view, the transverse lateral load capacity of masonry built using solid dense concrete blocks with thin layer mortar, is up to four times that of similar blockwork constructed using conventional mortar. Both the mortar and block constituent material’s properties alter the joint strength with enhancements to tensile flexural bond strength. When thin layer masonry technology (TLMT) is employed, in conjunction with solid dense concrete blocks, the masonry behaves more as a concrete plate than conventional blockwork. The engineering properties of masonry elements are not only related to the engineering properties of the blocks and mortars used, but also to how the bond at their interface interacts. Testing on two different block types, and one mortar was undertaken to verify this hypothesis. Engineering testing was carried out to determine the flexural strength of the material. This approach was combined with petrographical examination of the two different types of block and the bond zone using micro-analytical techniques, (OTLM, SEM and EMPA). Observations using SEM and EMPA techniques revealed that these differences depend on the composition of both blocks and mortar, and in particular they are related to the petrographical composition of the parent material of the block. These aspects affect how the bond layer forms and influence the mechanical properties of the joint.     

裂隙岩体稳定/非稳定渗流数值模拟

 发展裂隙岩体稳定/非稳定的渗流数值模型,一方面,依照裂隙面的密度、产状、位置、大小和开度的统计分布规律,使用蒙特卡罗模拟技术将完整岩石切割为三维不规则块体集合。根据相邻块体单元间产生的裂隙单元,构建三维裂隙网络系统,并附加各裂隙单元的水力特性;另一方面,对裂隙单元进行三角形单元的有限元网格划分,运用变分原理导出裂隙单元的渗流有限元求解方程。采用离散元方法中的动态松驰技术,在无须组装整体渗透矩阵的情况下求解裂隙网格各结点的水头值。最后,通过典型算例验证程序的可靠性及适用性。     

Determination of mechanical parameters for elements in meso-mechanical models of concrete

The responses of cement mortar specimens of different dimensions under compression and tension were calculated based on the discrete element method with the modified-rigid-body-spring concrete model, in which the mechanical parameters derived from macro-scale material tests were applied directly to the mortar elements. By comparing the calculated results with those predicted by the Carpinteri and Weibull size effects laws, a series of formulas to convert the macro-scale mechanical parameters of mortar and interface to those at the meso-scale were proposed through a fitting analysis. Based on the proposed formulas, numerical simulation of axial compressive and tensile failure processes of concrete and cement mortar materials, respectively were conducted. The calculated results were a good match with the test results.     

极地低温环境下混凝土断裂性能试验研究

为研究寒区和极地低温环境下混凝土结构的抗裂性能,设计了36个试件,对其进行了三点弯曲梁断裂试验,分析了20~-80 ℃温度区间内混凝土双K断裂韧度、断裂能和特征长度等关键断裂参数。结果表明：低温环境下,混凝土粗骨料和砂浆界面强度有所提高,横截面上有更多粗骨料发生断裂破坏;随着温度的降低,混凝土梁开裂荷载及峰值荷载有所提高,荷载-位移曲线斜率变大;混凝土断裂性能在低温环境下明显增强,其起裂断裂韧度、失稳断裂韧度和断裂能均随温度的降低得到提高;低温环境下混凝土脆性变大,特征长度随温度的降低而减小。对低温作用下混凝土断裂能的增强规律进行定量分析,拓展CEB-FIP MC2010混凝土断裂能计算模型的适用范围。基于低温环境下立方体抗压和劈裂抗拉试验结果,分别提出了混凝土断裂能计算模型。     

Experimental investigation on concrete overlaid with textile reinforced mortar: Influences of mix,temperature, and chemical exposure

Textile reinforced mortar is widely used as an overlay for the repair, rehabilitation, and retrofitting of concrete structures. Recently, textile reinforced concrete has been identified as a suitable lining material for improving the durability of existing concrete structures. In this study, we developed a textile-reinforced mortar mix using river sand and evaluated the different characteristics of the textile-reinforced mortar under various exposure conditions. Studies were carried out in two phases. In the first phase, the pullout strength, temperature resistance, water absorption, and compressive and bending strength values of three different textile-reinforced mortar mixes with a single type of textile reinforcement were investigated. In the second phase, the chemical resistance of the mix that showed the best performance in the abovementioned tests was examined for use as an overlay for a concrete substrate. Investigations were performed on three different thicknesses of the textile reinforced mortar overlaid on concrete specimens that were subjected to acidic and alkaline environments. The flexural responses and degradations of the textile reinforced mortar overlaid specimens were examined by performing bending tests. The experimental findings indicated the feasibility of using textile reinforced mortar as an overlay for durable concrete construction practices.     

内置纤维胶液管钢筋混凝土梁裂缝自愈合行为试验和分析

混凝土及其结构能够自动适应环境, 在受到损伤后能够自行愈合, 是解决结构中的混凝土材料损伤的最佳途径。本文根据生物体损伤愈合的原理, 设计了一种具有裂缝自愈合行为、内置纤维胶液管的钢筋混凝土梁,混凝土受拉开裂时, 纤维胶液管破裂, 其中的修复胶粘剂流出, 愈合裂缝。首先, 分析了修复纤维对混凝土自愈合效果的影响因素; 然后, 从修复纤维微分单元的平衡状态, 得到了修复纤维能及时发挥修复作用的合理参数;最后, 通过三分点弯曲试验, 验证了梁的裂缝自愈合能力; 分别采用不同的修复胶粘剂进行了试验, 得到了一种适合钢筋混凝土梁裂缝自愈合的理想胶粘剂。     

混凝土裂缝自愈合的研究与进展

所谓自愈合是指当混凝土结构开裂其本身会自动修复裂缝。本文介绍了混凝土裂缝自愈合的相关研究,包括对混凝土天然愈合能力的研究,仿生自愈合的研究及在水泥基中掺入特殊材料实现自愈合的研究。然后进行了总结分析并对这一领域的研究进行了展望。     

离子传输型抗渗涂料对水泥基材料冻融自修复的影响

通过对水泥砂浆的基准样和离子传输型抗渗涂层试样冻融循环后,在不同养护时间和养护条件下抗渗性能和力学性能的测试,并利用SEM对试样微观结构的观察,研究了离子传输型抗渗涂料对水泥基材料冻融自修复作用的影响。结果表明:基准样在冻融循环后,通过养护能够实现一定程度的抗渗性能和力学性能的自修复,但是这种自修复的能力有限,而采用离子传输型抗渗涂料进行表面处理,可使水泥基材料具有很强的自修复能力,从而大大提高水泥基材料的抗渗性能和力学性能。SEM研究表明,离子传输型抗渗涂料主要是通过活性离子传输,在水泥基材料内部空隙和裂缝中反应生成大量针状结晶,堵塞内部孔隙,封闭毛细孔通道,从而提高水泥基材料的抗渗性能和力学性能。     

基于多场耦合的PCCP管道力学性能研究

为了探究预应力钢筒混凝土管(PCCP)在多场耦合下的力学性能,通过ABAQUS建立了管土相互作用三维模型,利用已有试验数据对PCCP结构模型进行了评估。之后,通过FLUENT建立了流场模型,并基于MpCCI实现了埋地PCCP管线的多场耦合计算。通过耦合计算,对PCCP管道力学性能进行了分析,研究了管体各层结构应力应变沿管道环向分布规律。最后,探究了断丝率对管道服役性能的影响。研究发现,砂浆、混凝土外芯和钢丝的应力应变最大位置发生在管侧位置,而混凝土内芯和钢筒在管侧位置的应力应变则小于管顶和管底位置;断丝率小于15%时,管道仍能承受工作内压0.8 MPa。     

Microwave-assisted beneficiation of recycled concrete aggregates

The presence of mortar has been reported as the main factor causing the lower quality of recycled concrete aggregates (RCA) when compared to natural aggregates (NA). A novel microwave-assisted technique to increase the quality of RCA by partially removing the mortar adhering to RCA particles and breaking up the lumps of mortar present in RCA is introduced in this paper. The technique takes advantage of the difference in the electromagnetic properties of the adhering mortar and natural aggregates to generate high thermal stresses within the mortar, especially at the interface with the embedded natural aggregates, to cause delamination. The stresses generated also result in the breaking up of the lumps of mortar into smaller pieces. The results of an experimental study conducted to investigate the capability of the microwave-assisted RCA beneficiation technique and to compare its efficiency with other beneficiation methods proposed in available literature are presented. Moreover, the effects of incorporating various amounts of un-treated and microwave-treated coarse RCA on the mechanical properties of concrete are investigated. The temperature distribution and stresses developed in RCA when subjected to microwave heating during the beneficiation process are numerically calculated for a better understanding of the processes involved.