新拌混凝土的匀质性对其性能的影响

良好的匀质性是制备高性能混凝土的基础,本文通过调整外加剂的种类和掺量,设计了一系列匀质性不同的混凝土,通过对混凝土的流变性、力学性能、氯离子渗透性、界面结构等进行了测试分析,研究了匀质性对普通混凝土和高强混凝土的影响.结果表明,改善混凝土复合体系的匀质性可以提高混凝土的整体性能.     

负温防冻剂混凝土的界面显微结构与性能

研究了不同种类的防冻剂、矿物掺合料对正、负温下水泥净浆、砂浆及混凝土力学性能的影响.通过对比实验表明:水泥净浆、砂浆及混凝土在正、负温条件下抗压强度的变化规律存在显著差异,这取决于其内部显微结构,尤其是界面显微结构的差异.利用显微硬度计、扫描电镜、能谱分析、X射线衍射研究了防冻剂混凝土界面过渡区的范围、元素分布、钙硅比及氢氧化钙的取向度.结果表明:防冻剂能够改善负温下混凝土的界面显微结构,防冻剂和矿物掺合料的复合使用可为负温混凝土提供理想的界面显微结构.     

纳米级SiOx与硅灰对水泥基材料的复合改性效应研究

用TEM测试评价了纳米级SiOx与硅灰的颗粒形貌，在分别研究纳米级SiOx硅灰的活性和其对水泥基复合材料改性的基础上，将纳米级SiOx与硅灰复合作为高活性的组合料，探索了对水泥基材料的复合改性效应，除进行宏观力学性能测试评价之外，用DTA，XRD和SEM分析了水泥基材料的组成、显微结构。研究表明，纳米级SiOx和硅灰具有很高的活性和表面效应，水泥石基体相的显微结构致密，CSH凝胶交织成致密的网状结构，结构缺陷显著降低。DTA，XRD分析表明，水泥石中Ca(OH)2明显减少，由Ca(OH)2带来的砂浆和混凝土集科界面过渡区的缺陷将大大改善。此外，纳米级SiOx与硅灰复合后对砂浆的力学性能改善效果比水泥净浆更为显著。     

塔城地区白杨河引水工程输水管道高性能混凝土耐久性试验

运用YK-Ⅲ高耐久性矿物外加剂和大掺Ⅱ级粉煤灰,通过高性能混凝土配制技术,改善混凝土水泥石以及界面过渡区微结构,从而提高混凝土的密实度。在满足塔城地区白杨河引水工程输水管混凝土道所要求强度的同时,满足混凝土在抗渗、抗冻、抗硫酸盐侵蚀等耐久性指标的要求。试验结果及现场使用效果均表明,与传统混凝土相比,运用YK-Ⅲ高耐久性矿物外加剂和大掺Ⅱ级粉煤灰的通过高性能混凝土配制技术配置的高性能混凝土,不仅有效提高了混凝土的配置强度,而且在抗渗、抗冻、抗硫酸盐侵蚀试验中均取得了优异的效果。     

纳米CaCO3对超高性能混凝土性能影响的研究

利用水化放热分析、差热分析、收缩仪、扫描电镜、流动扩展度、力学试验等手段研究了纳米CaCO3对超高性能混凝土体系的水化放热特点、结合水含量、收缩、水化产物特征、流动性、力学性能的影响;分析了胶凝材料体系水化硬化后的微观结构及其对宏观性能的影响规律.结果表明:超低水胶比的超高性能混凝土的水化放热速率低,但掺入纳米CaCO3能促进水化反应,提高水化开始时的放热速率.同时,掺入纳米CaCO3会使得超高性能混凝土的流动性下降,自收缩增加,但能提高超高性能混凝土的抗压强度及抗折强度,改善水泥浆体的微观结构,使得超高性能混凝土的结构更加均匀、密实.     

CPE对纳米CaCO3增韧PVC复合材料界面和性能的影响

研究了CaCO3/CPE(氯化聚乙烯)/PVC(聚氯乙烯)纳米复合材料的结构和性能,探讨了CPE对纳米CaCO3/PVC复合材料界面作用和力学性能的影响. SEM结果显示,引入CPE可明显改善纳米CaCO3颗粒在PVC基体中的分散性和相容性,提高其界面作用. 引入界面作用参数定量表征纳米CaCO3颗粒与基体之间的界面结合作用,证实随着CPE加入量的增大,基体和颗粒之间的界面作用逐渐增大. 力学性能研究表明,相对于仅用纳米CaCO3增韧PVC,在CPE加入量为PVC的0~8%(w)范围内,用CPE和纳米CaCO3协同增韧可以更好地提高复合材料的冲击强度. 复合材料的冲击强度在CaCO3/CPE/PVC质量比为25/8/100时达到纯PVC的5.6倍,是纳米CaCO3/PVC(25/100)体系的2倍.     

高性能再生集料混凝土的研究及在世博项目的应用

<正>0前言本文提出一种科学化、定量化的混凝土配制技术体系,并开展示范工程应用,提高了混凝土的"绿色度",既节约资源、利用固废、降低生产成本;又实现了混凝土的高性能化,即提高混凝土的和易性、体积稳定性、力学性能和耐久性。     

纳米碳酸钙性质对聚氯乙烯复合材料界面及性能的影响

选择了不同的表面处理剂对纳米CaCO₃进行表面改性. 研究了不同表面处理剂对CaCO₃/PVC纳米复合材料微观结构、界面结合强度、力学性能及加工性能的影响.研究表明,钛酸酯偶联剂处理可使纳米CaCO₃颗粒在PVC基体中达到良好分散,明显改善纳米CaCO₃颗粒与PVC基体之间的界面结合,并提高其界面结合强度.力学性能和流变性能研究表明,钛酸酯处理的纳米CaCO₃填充PVC具有更高的拉伸强度、冲击强度以及更低的平衡转矩, 而且CaCO₃/PVC复合材料的冲击韧性在填充量为20%(mass)时达到最大值26.5 kJ&#8226;m^-2,是纯PVC的4倍.     

高性能再生骨料混凝土的性能与微结构

通过制备系列配合比的普通再生骨料混凝土(recycled aggregate concrete,RAC)和高性能再生骨料混凝土(high performance recycled aggregate concrete,HPRAC),研究再生骨料取代量对混凝土性能和微结构的影响.结果表明:RAC的力学性能和耐久性随着再生骨料取代量的增加而有所降低,但HPRAC仍具有良好的力学性能,并且抗渗性较高,具有抵抗300次冻融循环的耐久性.HPRAC水泥石基体较为密实,界面过渡区被致密的水化产物填充,孔隙变小,氢氧化钙和钙矾石含量均较少.     

纳米纤维路面混凝土配合比及耐久性实验研究

为研究纳米纤维对路面混凝土的抗冻性和力学性能的影响,分别选取纳米二氧化硅、纳米氧化铝、聚丙烯纤维掺杂的混凝土进行抗压强度和抗冻性研究。通过实验得出：纳米二氧化硅的掺杂对混凝土抗压强度的提高效果最为明显,纳米颗粒的掺杂比聚丙烯纤维的掺杂对提高混凝土抗冻性的作用更显著,掺杂5%纳米二氧化硅和0.2%聚丙烯纤维的混凝土在冻融循环后长度损失和质量损失最少。     

Fracture of model concrete: 1. Types of fracture and crack path

The paper presents experimental data that would serve to verify theoretical models of concrete fracture, particularly the effect of aggregate strength and mortar-aggregate interface on concrete strength, fracture paths, and deformation properties. Six types of concrete were designed and tested. All the concretes were made with the same matrix. We used two types of aggregates (spheres of the same diameter but of different strength) and three kinds of aggregate-matrix interfaces (debonded aggregates, strongly bonded, and intermediate bonded). Fracture behaviour was investigated by testing notched beams. All in all, 87 tests were performed. Detailed load-displacement and load-CMOD curves for the six types of concrete and two beam sizes are given. The different types of fracture—intergranular, through the matrix or the interface, or transgranular—are specified for the different aggregate-matrix interfaces. The properties of the matrix, aggregate, and interfaces needed to reproduce these tests numerically are provided in this paper, permitting others to check numerical models of concrete fracture.     

Experimental and analytical investigations of creep of epoxy adhesive at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations on the long-term behavior of epoxy at the interface between the concrete and the fiber-reinforced-polymer (FRP). Double shear experiments under sustained service load were performed on nine specimens composed of two concrete blocks connected by FRP sheets bonded to concrete using epoxy. The primary investigation parameters included the ratio of shear stress to ultimate shear strength, the epoxy thickness and the epoxy time-before-loading. Loading was sustained for periods up to nine months. We show that the magnitude of shear stress to ultimate shear strength and the epoxy time-before-loading could be the most critical parameters affecting creep of epoxy at the concrete–FRP interfaces. It was also found that the creep of epoxy can result in failure at the interfaces due to the combined effect of relatively high shear stress to ultimate shear strength and thick epoxy adhesive. This can have an adverse effect on the designed performance of reinforced concrete (RC) structures strengthened with FRP. Based on the experimental observations, rheological models were developed to simulate the long-term behavior of epoxy at the concrete–FRP interfaces. It is shown that the long-term behavior of epoxy at the interfaces can be properly modeled by analytically for both loading and unloading stages.     

不同引发剂对应马来酸酐接枝改性橡胶混凝土力学性能影响研究

引入不同引发剂对马来酸酐接枝改性废旧橡胶粉制备橡胶混凝土,对其坍落度、抗压强度、动弹性模量等性能进行研究,并通过SEM对其界面结合进行分析。试验结果表明,以DCP、BPO作为引发剂马来酸酐在橡胶表面均有接枝现象,但DCP效果明显,且接枝率最高。 DCP作为引发剂对应改性橡胶混凝土28 d抗压强度明显优于另外两种改性剂,且橡胶最佳掺量为15%时,强度达到最大值45.1 MPa。 DCP作为引发剂的改性橡胶混凝土中界面周围有很多针状和片状水泥水化物穿插嵌入橡胶,两物相界面结合牢固紧密,效果最佳。     

Fracture of model concrete: 2. Fracture energy and characteristic length

The specific fracture energy G_F was measured in six types of simple concrete: all from the same matrix. The aggregates were spheres of the same diameter (strong aggregates, that debonded during concrete fracture, and weak aggregates, able to break); three kinds of matrix-aggregate interface (weak, intermediate and strong) were used. All in all, 55 test results are reported. These results are intended to be used as an experimental benchmark for checking numerical models of concrete fracture.A meso-level analysis of these results showed a correlation between the measured G_F values and the properties of the matrix, aggregates and interfaces, particularly with the actual area of the fracture surface. The strength of the matrix-aggregate interface correlates quite well with G_F, and concrete ductility, measured by means of the characteristic length, correlates also with the strength of the matrix-aggregate interface.     

Rheological modeling and finite element simulation of epoxy adhesive creep in FRP-strengthened RC beams

We have shown that a significant creep occurs at the concrete–fiber reinforced polymer (FRP) interface based on double shear long-term test. The primary test parameters were the shear stress to ultimate shear strength ratio, the epoxy curing time before loading as well as the epoxy thickness. The test results showed that when the epoxy curing time before loading was earlier than seven days the shear stress level significantly affected the long-term behavior of epoxy at the interfaces, and in particular the combined effect of high shear stress and thick epoxy adhesive can result in interfacial failure if subjected to high-sustained stresses. In this paper, based on the previous experimental observations, an improved rheological model was developed to simulate the long-term behavior of epoxy adhesive at the concrete–FRP interfaces. Furthermore, the newly developed rheological creep model was incorporated in finite element (FE) modeling of a reinforced concrete (RC) beam strengthened with FRP sheets. The use of rheological model in FE setting provides the opportunity to conduct a parametric investigation on the behavior of RC beams strengthened with FRP. It is demonstrated that creep of epoxy at the concrete–FRP interfaces increases the beam deflection. It is also shown that consideration of creep of epoxy is essential if part or the entire load supported by FRP is to be sustained.     

再生混凝土单轴受压下多重界面过渡区的破坏机理研究

为研究再生粗骨料混凝土在单轴受压荷载下的破坏特性,使用数字图像相关技术记录混凝土立方体在破坏过程中的应变变化规律,并结合显微硬度测试、背散射电子成像技术等观测手段对界面过渡区的宽度和孔隙率进行定量表征。试验结果表明,分别经骨料强化和砂浆强化后,再生混凝土的抗压强度较未处理试样提升了17.86%和35.55%,说明砂浆强化更有利于提升再生混凝土的抗压强度。骨料强化可以提升老砂浆显微硬度值,降低老骨料-老砂浆界面过渡区孔隙率;砂浆强化可以提升新砂浆显微硬度值,降低老骨料-新砂浆界面过渡区以及新-老砂浆界面过渡区的孔隙率。经强化处理后的特定界面产生贯通裂缝的概率降低。     

Properties of fly ash-modified cement mortar-aggregate interfaces

This paper investigates the effect of fly ash on strength and fracture properties of the interfaces between the cement mortar and aggregates. The mortars were prepared at a water-to-binder ratio of 0.3, with fly ash replacements from 15 to 55%. Notched mortar beams were tested to determine the flexural strength, fracture toughness, and fracture energy of the plain cement and fly-ash modified cement mortars. Another set of notched beams with mortar-aggregate interface above the notch was tested to determine the flexural strength, fracture toughness, and fracture energy of the interface. Mortar-aggregate interface cubes were tested to determine the splitting strength of the interface. It was found that a 15% fly ash replacement increased the interfacial bond strength and fracture toughness. Fly ash replacements at the levels of 45 and 55% reduced the interfacial bond strength and fracture toughness at 28 days, but recovered almost all the reduction at 90 days. Fly ash replacement at all levels studied increased the interfacial fracture energy. Fly ash contributed to the interfacial properties mainly through the pozzolanic effect. For higher percentages of replacement, the development of interfacial bond strength initially fell behind the development of compressive strength. But at later ages, the former surpassed the latter. Strengthening of the interfaces leads to higher long-term strength increases and excellent durability for high-volume fly ash concrete.     

Quantitative analysis of the microstructure of interfaces in steel reinforced concrete

This article reports the results of a backscattered electron imaging study of the microstructure of the steel– and aggregate–cement paste interfaces in concrete containing 9 mm ribbed reinforcing bars. The water to cement (w/c) ratio, hydration age, steel orientation, and surface finish were varied. For vertically cast bars, there was more calcium hydroxide (CH) and porosity and less unreacted cement at both the steel– and aggregate–cement paste interfaces when compared to the bulk cement paste. As the hydration age increased, the porosity near the interfaces decreased, and the CH increased with more CH close to the steel than to the aggregate. Horizontal bars had more porosity and less CH under them than above. An increase in the w/c ratio produced interfaces of higher porosity and lower levels of CH. Wire-brush cleaned bars had higher levels of CH at the steel–cement paste interface at 365 days when compared to uncleaned bars.     

Characterising aggregate surface geometry in thin-sections of mortar and concrete

Measurement of microstructural gradients at the aggregate/cement paste interfacial transition zone (ITZ) in hardened mortar and concrete is commonly performed via quantitative image analysis of multiple micrographs of specimen surfaces, using a scanning electron microscope. However, due to the random orientation of interfaces sectioned by the specimen surface, measurements of the microstructural gradients at the interface have an unknown angular component, and thus have an unknown error. We present a method for the identification of interfaces that are perpendicular to the specimen surface, and therefore, are more suitable for accurate ITZ analysis. This method employs simple optical and electron imaging techniques on petrographic thin-sections. Use of 3D laser scanning confocal microscopy helped to validate the method. Quantitative 2D image analysis of backscattered electron micrographs, captured over three angular classes of interface gives an indication of this error in the determination of interfacial porosity and anhydrous cement content.     

A way for improving interfacial transition zone between concrete substrate and repair materials

An attempt to modify the repair interfacial transition zone by introducing fly ash into a primer between concrete substrate and repair materials was proposed. A comparison test was carried out for five different bond interfaces coated with five kinds of primers, namely neat cement paste, expansive paste, cement mortar, water-dispersible epoxy resin, and fly ash-modified mortar. The test results showed that the fly ash-modified primer made the microstructure of the repaired interface zone more dense and uniform. As a result, the splitting bond strength of the interface coated with the fly ash-modified primer was significantly higher than those coated with the other kinds of primers.