低温环境下不同地聚物砂浆力学性能及水化反应机理研究

为进一步提升低温环境下矿渣基地聚物砂浆的力学性能，以矿渣、硅灰和偏高岭土为主要原料制备了4种地聚物砂浆，探究不同原料组合对地聚物砂浆力学性能的提升效果，并结合扫描电镜和傅里叶红外光谱试验分析了不同组合地聚物砂浆的微观结构和硅灰、偏高岭土的改性机理。结果表明：在低温环境10℃的养护下，掺加硅灰可极大促进矿渣基地聚物砂浆早期水化反应，生成大量的C-S-H凝胶与水化铝酸钙产物，宏观上表现出更高的早期力学性能和强度增长速率；掺加偏高岭土，对矿渣基地聚物砂浆的早期力学性能有不利影响，但后期强度增长速率较快且强度值较高，养护时间为28d时，矿渣-偏高岭土基(S-MK)、矿渣-硅灰-偏高岭土基地聚物砂浆(S-SF-MK)试件的抗压强度均达60MPa以上；掺入硅灰和偏高岭土后，试件微观结构得到了改善，内部裂缝减少，均降低了地聚物砂浆的干缩率，偏高岭土降低干缩的效果较硅灰更好。研究结果为低温环境下矿渣基地聚物砂浆的制备和寒区抢修工程的应用提供了理论参考。     

粉煤灰对煤矸石混凝土界面过渡区的改性效应

为了促进煤矸石在混凝土生产中的使用，考虑不同粉煤灰掺量(0%、10%、20%、30%、40%)对煤矸石混凝土(Coal gangue concrete, CGC)界面过渡区(Interfacial transition zone, ITZ)结构的影响，从宏观力学和微观力学两个尺度出发，揭示了粉煤灰对CGC的ITZ微观结构及宏观性能的改性机理。研究表明：在宏观力学上，掺加30%的粉煤灰可以提高CGC的劈裂抗拉强度。在微观力学上，加入30%的粉煤灰能显著提高ITZ的薄弱区的硬度值，使其硬度值的分布变得较为均匀。掺加粉煤灰后CGC的ITZ厚度明显减小。这说明掺入的适量粉煤灰在CGC的ITZ上发挥了物理作用及化学作用，改变了CGC的ITZ结构。     

纳米SiO2改性轻骨料混凝土性能

纳米SiO₂（NS）具有极强火山灰活性、晶核作用和填充效应,因此用NS改善水泥基材料性能成为众多学者研究的热点。本课题对不同掺量的NS对轻骨料混凝土强度及耐久性的改性效果进行了研究。通过测试轻骨料混凝土的力学性能（抗压和抗折）和氯离子渗透性能及利用SEM和EDS测试分析了NS对混凝土宏观和微观结构的影响。研究结果表明:在适当的掺量下,NS能够有效地提高轻骨料混凝土的力学性能,其中28 d的抗压强度和抗折强度比空白组混凝土分别提高了21.6%和46.2%。氯离子渗透的结果表明,轻骨料混凝土的抗氯离子渗透性能随着掺量的增加而呈线性增强。混凝土界面过渡区（ITZ）也发生了显著变化,其厚度减小,形貌也更加致密。ITZ的钙硅比随着NS掺量增加而减小,说明该区域内水化产物C-S-H凝胶增多,Ca（OH）₂被消耗,从而形成致密的过渡区,有利于强度提高。      

微生物矿化沉积对再生骨料界面过渡区的影响

骨料-水泥浆体的界面过渡区疏松多孔是导致再生骨料与天然骨料性能差异的重要原因。通过试验研究微生物矿化沉积技术对再生骨料界面过渡区性能的影响。使用水泥净浆包裹不同菌种和不同方式处理的再生骨料,养护后破碎获得骨料-水泥界面过渡区,通过SEM观察界面过渡区的微观形貌变化,利用纳米压痕试验测量界面过渡区的弹性模量和硬度,并结合再生砂浆块的抗压、抗折强度试验结果,分析微生物矿化沉积对再生骨料界面过渡区的改善效果。结果表明:再生骨料经假坚强芽孢杆菌和嗜碱芽孢杆菌处理后所制备的再生砂浆块的抗压、抗折强度及再生骨料界面过渡区的弹性模量和硬度均有明显提升,两菌种矿化生成碳酸钙的能力不同造成了性能提升效果的差异。     

再生砖骨料混凝土力学性能及破坏机理研究

以8种设计配合比的再生砖骨料次轻混凝土为研究对象,以水灰比、最大骨料尺寸和粗骨料种类为试验参数,开展了混凝土干密度、立方体抗压强度和劈裂抗拉强度试验研究,重点分析了骨料界面特征与再生砖骨料混凝土的破坏机理.结果表明:使用再生粘土砖骨料制备的次轻混凝土28 d抗压强度可达36 MPa,干密度不大于2200 kg/m3,能够减轻自重8％～16％;随着水灰比和取代率的降低,混凝土抗压和劈裂抗拉强度均呈显著增加的趋势;最大骨料尺寸对混凝土抗压和劈裂抗拉强度的影响较小;拟合所得混凝土干密度与力学性能换算公式的相关性较好;砖骨料-砂浆界面过渡区微观结构较为密实,混凝土界面性能得到增强;界面过渡区性能和砖骨料强度是影响再生砖骨料混凝土力学性能的两个关键因素,且砖骨料强度是其中最主要的因素.综合考虑,使用再生粘土砖骨料制备次轻混凝土具有良好的经济性和广阔的工程应用前景.     

骨料级配对二维模型混凝土单轴抗拉强度影响的理论研究

混凝土尺寸效应及其宏观力学非线性根源于其材料细观组成的非均质性。结合混凝土细观结构形式,将混凝土看作由骨料颗粒、砂浆基质及界面过渡区组成的复合材料。采用双线性弹性损伤模型来描述砂浆基质及界面过渡区的力学行为,假定骨料颗粒为弹性体而不发生破坏,进而推导并获得了单轴拉伸条件下不同骨料颗粒级配混凝土断裂裂缝扩展路径长度及其抗拉强度的理论解。最后,对比了建立的理论公式结果与细观尺度数值模拟结果,验证了构建的关于裂缝长度及抗拉强度理论解的准确性和合理性。     

纤维增强混凝土氯离子扩散系数的多尺度预测模型

纤维增强混凝土材料属于多相非均质复合材料,其宏观耐久性能由微观和细观的组分占比和夹杂关系共同决定。为考虑纤维增强混凝土材料不同尺度下的非均质性对整体氯离子扩散系数的影响,本文基于从微观到宏观的多尺度方法选取了不同层级代表单元,建立了纤维增强混凝土氯离子扩散系数多尺度预测模型。模型在充分考虑微观水泥水化过程和阈值效应的基础上,分析了细观尺度下纤维、骨料及其与水泥浆体的结合界面对混凝土宏观扩散性能的共同影响,并探究了纤维尺寸、纤维-浆体界面过渡区厚度等因素与扩散系数之间的影响关系,且通过第三方试验对模型的可靠性进行了验证。参数化分析的结果表明,当水灰比大于一定限值(约为0.45),水泥浆体的氯离子扩散系数与水灰比呈指数增长,而在细观层级上,纤维-浆体界面过渡区是影响混凝土整体扩散性能的主要因素：纤维掺量的增加和纤维直径的减小都会增大界面过渡区的体积,而较高的纤维过渡区体积占比和纤维界面扩散系数都会增大纤维增强混凝土的宏观氯离子扩散系数。结果还表明混凝土扩散系数与纤维掺量之间并无直接关系,而需要综合考虑纤维直径、界面过渡区厚度等各种因素的影响。本文所提模型能够有效预测纤维增强混凝土的扩散系数...     

矿物掺合料对地聚合物抗冻性能的影响

以碱激发偏高岭土制备地聚合物混凝土,分别研究了掺入15%的钢渣、矿渣或粉煤灰的地聚合物混凝土的力学抗压强度和抗冻性能,测试了地聚合物混凝土的真空饱水体积吸液率,运用XRD、SEM和DSC-TG等测试方法分析了矿物掺合料对地聚合物微观结构和水化产物的影响。结果表明:钢渣或矿渣能有效提高地聚合物混凝土的抗压强度,而粉煤灰的掺入使其强度稍有降低;地聚合物表观形貌中存在较多的孔洞和微裂缝导致其抗冻性能较差,掺入钢渣或者矿渣后地聚合物形成了新的产物C-S-H凝胶、C-A-S-H凝胶等并填充在结构中形成更加密实的板状结构,降低了地聚合物混凝土冻融破坏速率,五次冻融循环后地聚合物的相对强度均在90%以上,抗冻性能得到提高;粉煤灰降低了制备地聚合物混凝土的用水量且未水化的粉煤灰颗粒镶嵌在结构中增加了其密实性和抗冻性能,五次冻融循环后相对强度为86.9%,基准组的相对强度仅为79.7%。     

硅灰对再生混凝土界面过渡区的影响

为改善再生混凝土的力学和耐久性能,以硅灰为增强材料对再生混凝土进行改良。研究了硅灰对再生混凝土3 d、28 d、90 d抗压强度和28 d、90 d抗氯离子渗透性能的影响。结合扫描电镜、显微硬度等微观观测手段,分析了28 d再生混凝土试样微观结构和性能变化。采用压汞法测试了再生混凝土的孔结构参数,探究硅灰对再生混凝土孔隙性能的影响。结果表明：硅灰可以提升再生混凝土的抗氯离子渗透性能,随掺量的增加提升效果先增后减;掺入硅灰可以改善再生混凝土多重界面过渡区结构,增加界面过渡区(ITZ)显微硬度,降低孔隙率。再生混凝土内部存在较多有害孔隙,硅灰可以细化孔隙结构,降低孔隙率,掺量为6%时效果最佳。     

考虑细观组分影响的混凝土宏观力学性能理论预测模型

混凝土宏观力学性能与其内部细观结构构造密切相关。该文建立了一类能够考虑细观组分影响的混凝土宏观力学性能理论预测模型。首先,采用细观力学数值试验法对理论模型中的参数进行了标定;进而,基于该模型对混凝土断裂能和单轴抗拉强度在材料层次的尺寸效应行为进行了分析。结果表明:混凝土断裂能和单轴抗拉强度均随骨料级配（即最大骨料粒径）发生变化,且受到界面特性的影响。当界面过渡区力学性能相对薄弱时,混凝土强度较低,断裂能和单轴抗拉强度随骨料级配增大而呈现减小的趋势;当界面过渡区力学性能较强时,混凝土强度较高,断裂能和单轴抗拉强度随骨料级配增大亦呈现增大的趋势。计算结果与试验结果吻合良好,验证了该文建立的理论预测模型的准确性和合理性。     

基于CT图像的再生混凝土细观破坏裂纹分形特征

为深入研究再生混凝土的破坏形态和内部裂纹扩展情况与普通混凝土之间的差异,以不同再生粗骨料(RCA)取代率的再生混凝土为研究对象,利用Phoenix v | tome | x s240微焦点工业CT获取再生混凝土加载到90%预估破坏荷载的二维扫描图像,借助Photoshop CS6图像处理软件,对材料内部破坏裂纹进行提取,进而基于分形几何理论,以分形维数及多重分形谱表征裂纹的分形扩展规律,建立分形维数和多重分形谱特征参数与RCA取代率和再生混凝土抗压强度的关系。结果表明:再生混凝土的细观受力破坏模式与普通混凝土不同,其受力破坏形态不仅取决于粗骨料与水泥浆体的界面黏结强度,还取决于RCA自身性能,当裂纹发展至天然粗骨料或强度较高的RCA时会绕过骨料表面继续发展,发展至强度较低的RCA时会贯穿骨料;分形维数可定量描述混凝土材料内部细观裂纹的整体扩展情况,即裂纹越丰富,分形维数越大;多重分形谱可反映从局部到整体不同层次的细观裂纹特征,裂纹分形维数和多重分形谱特征参数均与RCA取代率呈线性下降关系,与抗压强度呈线性增长关系;本研究可为再生混凝土在大型结构工程中的广泛应用奠定理论和实验基础。      

水泥基复合材料界面过渡区体积分数的定量计算

借助邻近函数公式, 给出了水泥基复合材料集料与浆体间界面过渡区体积分数的定量计算公式, 并采用随机点采样方法验证采用该定量公式计算水泥基复合材料界面过渡区体积分数的可行性。在此基础上, 预测了由符合Fuller 分布的集料制备的混凝土中界面过渡区体积分数随集料体积分数和集料粒径范围的变化曲线。讨论了界面厚度变化、集料体积分数变化以及集料粒径分布变化对界面过渡区体积分数影响的差别, 并给出了衡量界面过渡区重叠程度的定性和定量方法。结果表明: 在常规集料体积分数下, 三者中对界面过渡区体积分数以及界面过渡区重叠程度影响的主次顺序均为: 界面过渡区厚度变化带来的影响> 集料体积分数变化带来的影响> 集料细度变化带来的影响。      

Computational homogenization of diffusion in three-phase mesoscale concrete

A three dimensional (3D) mesoscale model of concrete is presented and employed for computational homogenization in the context of mass diffusion. The mesoscale constituents of cement paste, aggregates and interfacial transition zone (ITZ) are contained within a statistical volume element (SVE) on which homogenization is carried out. The model implementation accounts for ITZ anisotropy thereby the diffusivity tensor depends on the normal of the aggregate surface. The homogenized response is compared between 3D and 2D SVEs to study the influence of the third spatial dimension, and for varying mesoscale compositions to study the influence of aggregate content on concrete diffusivity. The computational results show that the effective diffusivity of 3D SVEs is about 40 % greater than 2D SVEs when ITZ is excluded for the SVE, and 17 % when ITZ is included. The results are in agreement with the upper Hashin–Shtrikman bound when ITZ is excluded, and close to the Taylor assumption when ITZ is included.     

Bond and interfacial properties of reinforcement in self-compacting concrete

This paper reports a study carried out to assess the impact of the use of self-compacting concrete (SCC) on bond and interfacial properties around steel reinforcement in practical concrete element. The pull-out tests were carried out to determine bond strength between reinforcing steel bar and concrete, and the depth-sensing nano-indentation technique was used to evaluate the elastic modulus and micro-strength of the interfacial transition zone (ITZ) around steel reinforcement. The bond and interfacial properties around deformed steel bars in different SCC mixes with strength grades of 35 MPa and 60 MPa (C35, C60) were examined together with those in conventional vibrated reference concrete with the same strength grades.The results showed that the maximum bond strength decreased when the diameter of the steel bar increased from 12 to 20 mm. The normalised bond strengths of the SCC mixes were found to be about 10–40% higher than those of the reference mixes for both bar diameters (12 and 20 mm). The study of the interfacial properties revealed that the elastic modulus and the micro-strength of the ITZ were lower on the bottom side of a horizontal steel bar than on the top side, particularly for the vibrated reference concrete. The difference of ITZ properties between top and bottom side of the horizontal steel bar appeared to be less pronounced for the SCC mixes than for the corresponding reference mixes.     

Multi-scale investigation of microstructure,fiber pullout behavior,and mechanical properties of ultra-high performance concrete with nano-CaCO3 particles

The mechanical properties of a fiber-reinforced concrete are closely related to the properties of the matrix, fiber, and fiber-matrix interface. The fiber-matrix bond property is mainly governed by the adhesion between the fiber and surrounding cement materials, as well as the strength of materials at the interfacial transition zone. In this paper, the effect of nano-CaCO₃ content, varying between 0 and 6.4%, by mass of cementitious materials, on microstructure development, fiber-matrix interfacial bond properties, and mechanical properties of ultra-high performance concrete (UHPC) reinforced with 2% steel fibers were investigated. The bond properties, including bond strength and pullout energy, were evaluated. Mercury intrusion porosimetry (MIP), backscattered electron microscopy (BSEM), optical microscopy, and micro-hardness testing were used to characterize the microstructure of matrix and/or interfacial transition zone (ITZ) around an embedded steel fiber. Test results indicated that the incorporation of 3.2% nano-CaCO₃ significantly improved the fiber-matrix bond properties and the flexural properties of UHPC. This was attributed to densification and strength enhancement of ITZ as observed from micro-structural analyses. Beyond the nano-CaCO₃ content of 3.2%, the fiber bond and mechanical properties of UHPC decreased due to increased porosity associated with agglomeration of the nano-CaCO₃.     

Nanoindentation and SEM/EDX characterization of the geopolymer-to-steel interfacial transition zone for a reactive porcelain enamel coating

The increasing use of alternative cementitious materials such as geopolymers as an environmentally-friendly alternative to traditional cements requires an improved understanding of the interfacial transition zone (ITZ) between the matrix and reinforcing steels. In this study, nanoindentation measurements were spatially coupled to images with scanning electron microscopy and chemical composition using energy dispersive X-ray microanalysis. The study focused on the microstructure and chemical composition of the interfacial transition zone (ITZ) for reinforcing steel embedded in a geopolymer mortar. The ITZ was analyzed for uncoated steel and steel coated with a reactive porcelain enamel that improves bonding and corrosion resistance. Results indicate that a more gradual transition of mechanical properties and chemical composition for the coated steel coupled with improved integration to the mortar correlates to increased bond strength measured in macroscale experiments.     

低应力水平下混凝土中氯离子扩散行为多尺度分析方法

混凝土的扩散渗透性能与其微观结构（包括细观尺度上粗骨料颗粒与砂浆之间的界面过渡区及砂浆本身的微观结构等）密切相关。在微观尺度上,砂浆和界面过渡区均可视为由无孔砂浆基质和孔隙水夹杂相组成的两相复合材料,二者的主要区别表现为孔隙率不同。在外荷载作用下,砂浆和界面过渡区的毛细孔隙率及孔隙连通性会发生改变,从而改变混凝土的扩散渗透性能。基于此,该文建立了低应力水平下混凝土中氯离子扩散行为多尺度理论分析方法,获得了混凝土表观扩散系数与外荷载（以体应变表征）及砂浆和界面过渡区当前孔隙率的定量关系。分析所采用的主要参数为砂浆和界面过渡区的毛细孔隙率、无孔砂浆基质和骨料相的力学参数、骨料相和界面过渡区的体积分数、外荷载等。与已有文献数据对比知,该文分析结果与之吻合良好,表明了理论分析方法的合理性与准确性。此外,基于该方法,探讨分析了混凝土微/细观结构对其宏观扩散性能的影响。     

Influence of internal curing using lightweight aggregates on interfacial transition zone percolation and chloride ingress in mortars

The microstructure of the interfacial transition zone (ITZ) between cement paste and aggregate depends strongly on the nature of the aggregate, specifically its porosity and water absorption. Lightweight aggregates (LWA) with a porous surface layer have been noted to produce a dense ITZ microstructure that is equivalent to that of the bulk cement paste, as opposed to the more porous ITZ regions that typically surround normal weight aggregates. This ITZ microstructure can have a large influence on diffusive transport into a concrete, especially if the individual ITZ regions are percolated (connected) across the three-dimensional microstructure. In this paper, the substitution of LWA sand for a portion of the normal weight sand to provide internal curing (IC) for a mortar is examined with respect to its influence on ITZ percolation and chloride ingress. Experimental measurements of chloride ion penetration depths are combined with computer modeling of the ITZ percolation and random walk diffusion simulations to determine the magnitude of the reduced diffusivity provided in a mortar with IC vs. one with only normal weight sand. In this study, for a mixture of sands that is 31% LWA and 69% normal weight sand by volume, the chloride ion diffusivity is estimated to be reduced by 25% or more, based on the measured penetration depths.     

Modeling the elastic properties of concrete composites: Experiment,differential effective medium theory,and numerical simulation

Concrete is a mixture of cement, water and aggregates. In terms of microstructure, besides the cement paste matrix and aggregate inclusions, there is a third phase, which is called the interfacial transition zone (ITZ), which forms due to the wall effect and can be thought of as a thin shell that randomly forms around each aggregate. Thus, concrete can be viewed as a bulk paste matrix containing composite inclusions. To compute the elastic properties of a concrete composite, a differential effective medium theory (D-EMT) is used in this study by assigning elastic moduli to corresponding bulk paste matrix, ITZ and aggregate. In this special D-EMT, each aggregate particle, surrounded by a shell of ITZ of uniform thickness and properties, is mapped onto an effective particle with uniform elastic moduli. The resulting simpler composite, with a bulk paste matrix, is then treated by the usual D-EMT. This study shows that to assure the accuracy of the D-EMT calculation, it is important to consider the increase in the water:cement mass ratio (w/c) of the ITZ and the corresponding decrease in w/c ratio of the bulk matrix. Because of this difference in w/c ratio, the contrast of elastic moduli between the ITZ and the bulk paste matrix needs to be considered as a function of hydration age. The Virtual Cement and Concrete Testing Laboratory (VCCTL) cement hydration module is used to simulate the microstructure of cement paste both inside and outside the ITZ. The redistribution of calcium hydroxide between ITZ and bulk paste regions can further affect the elastic contrast between ITZ and bulk paste. The elastic properties of these two regions are computed with a finite element technique and used as input into the D-EMT calculation. The D-EMT predictions of the elastic properties of concrete composites are compared with the results measured directly with a resonant frequency method on corresponding composites. This comparison shows that the D-EMT predictions agree well with experimental measurements of the elastic properties of a variety of concrete mixtures.