Influence of emulsifier content on cement hydration and mechanical performance of bitumen emulsion mortar

The objective of this research was to investigate the influence of a cationic emulsifier on the kinetics of cement hydration and on the overall mechanical performance of bitumen emulsion mortar, as an essential structural component of cold asphalt mixtures, within a 28-day curing period. This influence was investigated as a function of emulsifier content and bitumen grade, selected to cover the whole range of their applicability. The hydration kinetics was evaluated by heat release of cement pastes and bitumen emulsion mortars. The residual w/c was determined at characteristic points during curing. The emulsifier showed a very powerful effect on the complex relation between hydration kinetics, emulsion rheology, and water binding. The presence of the emulsion initially accelerated the hydration, with no main peaks and dormant period observed, but had almost no effect on the ultimate hydration degree. The rate of hydration during the rest of the curing period was critically dependent on the residual w/c which was decisively influenced by the content of emulsifier. The cement could be expected to reach a very high degree of hydration only in mixtures with the highest emulsifier content, considering the specimens as a whole. Together with emulsion breaking at early stage, cement hydration later on was directly reflected on indirect tensile properties of the mortars, which performed from very ductile to very brittle. In conclusion, the emulsifier was recognised as a key factor for a fundamental understanding of the mechanical performance of cold asphalt mixtures.     

Effect of strain rate and saturation on uniaxial dynamic compressive behaviours of mortar

The effect of moisture content on the compressive mechanical behaviours of cement mortar under different high strain rates is studied in this paper. The rapid impact testing, i.e. the strain rates of 80, 100, 150, 200 and 250 s^? 1 by Split Hopkinson pressure bar, on number of specimens with special water/cement ratio of 0.50 and saturations as 0%, 25%, 50%, 75% and 100%, respectively, was executed. The dynamic compressive behaviours were analysed in terms of the maximum stresses, elastic modulus, critical strain at maximum stresses and ultimate strains at failure. Results indicated that similarity existed in the shape of strain–stress curves of mortars with different moisture subjected to different strain rates of impact loading, i.e. the upward section presented bilinear characteristics, while the descending stage was almost linear. As strain rate increases, the dynamic compressive strength, elastic modulus and critical strain at maximum stress increase which can be ascribed to the dynamic fracture effect and the microscope inertia effect. Besides, it was shown that desiccation provokes an increase in mortar strength and deformation behaviour of the studied mortar with different saturation caused by capillary depression and microcracking. Drying effect has to be considered in modelling of the coupling between desiccation and mechanical behaviour of the mortar. Finally, the multi-parametric statistical analysis of water content and strain rate on the mechanical behaviours of cement mortar subjected to dynamic loading is detailed.     

High strain rate mechanical behavior of epoxy under compressive loading: Experimental and modeling studies

Investigations on high strain rate behavior of epoxy LY 556 under compressive loading are presented. Compressive Split Hopkinson Pressure Bar (SHPB) apparatus was used for the experimental investigations. The studies are presented in the strain rate range of 683-1890 per second. It was generally observed that the compressive strength is enhanced at high strain rate loading compared with that at quasi-static loading. During SHPB testing of the specimens, it was observed that the peak force obtained from the strain gauge mounted on the transmitter bar is lower than the peak force obtained from the strain gauge mounted on the incident bar. Further, an analytical method is presented based on variable rate power law for the prediction of compressive strength at high strain rate loading for epoxy LY 556. Using the analytical method, high strain rate compressive stress-strain behavior is presented up to strain rate of 10,000 per second.     

氧化铝陶瓷动态压缩强度的高压和高应变率效应

平面冲击压缩下材料由弹性到非弹性变形转变的临界值对应着材料的动态压缩强度,高压和高应变率强烈影响材料的强度。采用粘塑性比拟法建立了材料发生非弹性变形的动态屈服条件,并引入Drucker-Prager静态屈服准则,可以联合考虑动态压缩强度的高压和高应变率效应,据此提出了新的Hugoniot弹性极限表征形式。利用轻气炮进行了氧化铝陶瓷平板冲击实验,VISAR测试了样品的自由面速度历程,讨论了氧化铝陶瓷动态压缩强度的测定和存在的不确定性问题。     

Dynamic tests of cemented paste backfill: effects of strain rate,curing time,and cement content on compressive strength

This article investigates the compressive strength of cemented paste backfill (CPB) under dynamic loading. To accommodate the low impedance CPB, a modified split Hopkinson pressure bar (SHPB) system is adopted. In contrast to traditional solid steel transmitted bar, a hollow aluminum transmitted bar is introduced to reduce the impedance. With this system, the dynamic stress equilibrium is achieved, which guarantees the valid dynamic material testing condition. The dynamic tests are conducted for CPB with different cement contents and curing time. It is observed that: (1) for CPB with the same curing time and cement content, the dynamic strength increases with the strain rate, (2) for CPB with the same cement content, the dynamic strength increases with the curing time, and (3) for CPB with the same curing time and tested under similar strain rate, the dynamic strength increases with the percentage of cement. This observation can be understood by considering the hydration process of cements.     

A physical phenomenon which can explain the mechanical behaviour of concrete under high strain rates

The free water in concrete underlies the various physical mechanisms that shape the mechanical behaviour of the material. In this article we attempt to show, through an experimental observation and a theoretical assumption, that the mechanical behaviour of concrete under high strain rates could be explained by a coupling between one of these physical effects and the process of cracking in the material. The assumption about the physical mechanism involved is made more to understand what happens inside the material than to lead to quantitative predictions.

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Resume L'eau libre au sein du béton est à l'origine de différents mécanismes physiques qui interviennent dans le comportement mécanique du matériau. Dans le cadre d'une coopération européenne, l'Université de Technologie de Delft et le LCPC avaient étudié le comportement dynamique du béton (c'est à dire, dans le cas présent, sous de grandes vitesses de déformation), en s'intéressant essentiellement à l'influence, sur ce type de comportement, de l'humidité interne du matériau. Les essais étaient réalisés sur la barre de Hopkinson de Delft qui permet de solliciter, en traction directe, des éprouvettes de béton à des vitesses de déformation pouvant aller jusqu' à 10 s⁻¹. Un micro-béton était testé dans deux conditions d'hygrométrie interne: complètement sec, et humide. On a constaté qu'en ce qui concernait le béton humide, la résistance à la traction directe augmentait avec la vitesse de déformation, phénomène connu pour la plupart des matériaux, alors que cette dépendance n'existait pas pour le béton sec. Dans cet article on propose une hypothèse quant au mécanisme physique qui permettrait d'expliquer cette augmentation de la résistance à la traction du béton avec la vitesse de déformation. Il s'agit de l'effet Stefan qui peut être décrit succinctement de la manière suivante: l'existence d'un mince film visqueux (eau ou huile par exemple) entre deux cales parfaitement planes et parallèles, et distantes d'une certaine longueurh, conduit à la création d'une force de rappel lorsque l'on tente d'écarter les deux cales avec une vitesse h. Plus la vitesse est grande, plus cette force de rappel le sera. Dans un béton, les cales séparées par un ménisque d'eau ne sont autres, par analogie, que les parois des micropores et des capillaires.

     

Influence of microstructure and strain rate on the compressive strength of whisker-reinforced ceramic matrix composites

The compressive strength of pyroceramic reinforced with a wide variety of SiC whiskers was characterized at loading rates which range from quasi-static to dynamic. It was found that strength is inversely related to whisker size, and essentially strain rate insensitive. The same strain rate independence was obtained for unreinforced matrix, but the strength of the latter lies below that for small ( 1 m diameter) whisker-reinforced composites, and above that for large ( > 3 m diameter) whisker material. Whisker/crack interaction and (to a lesser extent) whisker pull-out seem to be responsible for the beneficial influence of small whiskers, while the apparently detrimental large whiskers serve as microcrack-nucleating inclusions.     

Behavior of high performance fiber reinforced cement composites under multi-axial compressive loading

Experiments were conducted to better understand the behavior of strain hardening, high performance fiber reinforced cement composites (HPFRCC) when subjected to uniaxial, biaxial, and triaxial compression. The experimental parameters were: type of fiber, fiber volume fraction, and loading path. Two types of commercially available fibers, namely high-strength hooked steel fiber and ultra high molecular weight polyethylene fiber, with volume fractions ranging from 1.0% to 2.0%, were used in a 55-MPa mortar matrix. The selected loading paths consisted of uniaxial compression and tension, equal biaxial compression, and triaxial compression with two levels of lateral compression. The test results revealed that the inclusion of short fibers can significantly increase both strength and ductility under uniaxial and biaxial loading paths, but that the role of volume fraction is rather small for the range of fiber volume contents considered. The results also showed that the confining effect introduced by the fibers becomes minor in triaxial compression tests, where there is relatively high external confining pressure. The experimental information documented herein can serve as input for the development of multiaxial constitutive models for HPFRCCs.     

Relaxation characteristics of cement mortar subjected to tensile strain

Tensile relaxation characteristics are important for the crack resistance of concrete members subjected to restrained contraction, for example bonded overlays and patch repairs. In the experimental research discussed in this paper, relaxation characteristics were measured for mortar specimens subjected to constant strain which corresponded to stresses close to the tensile strength of the mortar. Relaxation was found to relieve a considerable portion of tensile stresses. Ultimate relaxation values ranged from 20 to 45%, depending on w/c ratio and specimen age. As expected, a decrease in specimen age and an increase in w/c ratio resulted in increasing relaxation values. The rate of stress decay was found to be rapid, with approximately 80% of the ultimate relaxation occurring in the first 12?h after loading. A basic equation for the prediction of time development of relaxation is proposed and future research needs are discussed.     

Measurement and prediction of compressive properties of polymers at high strain rate loading

Strain-rate effect is widely recognized as a crucial factor that influences the mechanical properties of material. Despite the acknowledge importance, the understanding of how such factor interact with the sensitivity of the polymers in terms of mechanical properties is still less reported. In this study, an experimental technique, based on the compression split Hopkinson pressure bar, was introduced to perform high strain rate testing, whereas, a conventional universal testing machine was used to perform static compression testing, to experimentally investigate the independent and interactive effects of strain rates towards mechanical properties of various polymers. Based on the experimental results, we parameterized two equation models, which were used to predict the yield behavior of tested polymer samplings. The experimental results indicate that, the yield stress, compression modulus, compressive strength, strain rate sensitivity and strain energy increased significantly with increasing strain rates for all tested polymers. Meanwhile, the yield strain and the thermal activation volume exhibit contrary trend to the increasing strain rates. Interestingly, the proposed constitutive models were almost agreed well with experimental results over a wide range of strain rate investigated. Of the three polymers, polypropylene shows the highest strain rate sensitivity at static and quasi-static region. On the other hand, at dynamic region, polycarbonate shows the highest strain rate sensitivity than that of polypropylene and polyethylene. Overall, both experimental and numerical models proved that the mechanical properties of polymer show significant sensitivity and dependency towards applied strain rates up to certain extent.     

UHMWPE交织双轴向纬编复合材料高应变率压缩性能研究

利用SHPB装置对UHMWPE交织双轴向纬编针织物/乙烯基酯树脂复合材料进行了高应变率压缩实验,研究了该材料的应变率效应和能量吸收.结果表明:等离子体处理后,复合材料的高应变率压缩性能有了较大的提高,放电功率100W是一个较为合适的处理条件.UHMWPE交织双轴向纬编复合材料呈现出一定的应变率敏感性:随着应变率的增加,最大应力、压缩模量、断裂应变能密度相应增大.由于交织双轴向纬编结构中的针织线圈及经纬纱交织作用,其具有较好的抗冲击性能.     

中等应变率下泡沫铝的吸能特性

进行了不同密度、高度和压缩方向下泡沫铝的准静态压缩试验和中等应变率下(＜100 s-1)的冲击试验,研究了具有不同密度的闭孔泡沫铝在准静态压缩和冲击工况下的吸能特性.结果表明,泡沫铝是一种近似的各向同性结构,具有较高的单位质量吸能特性,是一种较好的吸能材料.在准静态和中等应变率冲击条件下,泡沫铝对应变率不敏感,其应力应变关系与应变率关系不大.不同的泡沫铝,其平台应力与密度之间的关系不同,在研究其性能时,必须测量应力-应变关系.泡沫铝的致密区对其吸能特性有很大的影响.     

Experimental characterisation of the drying effect on uniaxial mechanical behaviour of mortar

The main objective of this present study was to evaluate, for a standard mortar, the drying effect on its mechanical behaviour. Numerous uniaxial compression tests were thus performed with loading-unloading cycles. They were carried out on different samples previously preserved under various conditions of conservation: preserved from desiccation, air drying and rapid drying at 60°C. The obtained results showed significant influences of these conditions on the material behaviour (increase in strength, decrease in Young's modulus and Poisson's ratio) and the necessity of taking into account the coupling effects between mechanical—poromechanical behaviours and drying.     

Influence of grain size on the mechanical behaviour of some high strength materials

Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks.In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.     

废水泥浆对水泥胶砂性能的影响研究

研究了废水泥浆澄清液、浆体以及干燥后的粉体对水泥胶砂流动度以及抗压强度的影响,为废水泥浆在预拌混凝土生产中的的回收利用提供技术基础,解决废水泥浆的环境污染和资源化利用问题。     

Effect of strain rate on behaviour of Fe3Al under tensile impact

The effect of strain rate on the behaviour of Fe₃Al has been investigated experimentally in the range 90–1300 s_-1 under tensile impact. The experimental results indicate that Fe₃Al is strain-rate sensitive and its critical strain in maximum stress increases with increasing strain rate. According to the test results for Fe₃Al in air and water, it is confirmed that environmental embrittlement induced the fracture of Fe₃Al under tensile impact. With testing at faster strain rate, further hydrogen embrittlement is suppressed. This is consistent with the micrography of the fracture surface in Fe₃Al specimens, which indicates that these iron aluminides are intrinsically quite ductile. This revised version was published online in November 2006 with corrections to the Cover Date.     

Behaviour of high volume fibre cement mortar in flexure

Tests are reported on the behaviour of high volume percentage steel fibre mortar specimens subjected to flexure. Flexural and cyclic load tests were conducted; in addition, comparison tests were made on conventional fibre mortar and ferrocement specimens. Both strength and deflection characteristics were studied. The results of the investigation indicate that with 8% high volume, steel fibre mortar specimens possess a flexural strength of about 40 MPa.     

Influence of stress state and strain rate on the behaviour of a rubber-particle reinforced polypropylene

This article presents an experimental investigation of a ductile rubber-modified polypropylene. The behaviour of the material is investigated by performing tension, shear and compression tests at quasi-static and dynamic strain rates. Subsequently, scanning electron microscopy is used to analyse the fracture surfaces of the tension test samples, and to relate the observed mechanical response to the evolution of the microstructure. The experimental study shows that the material is highly pressure and strain-rate sensitive. It also exhibits significant volume change, which is mainly ascribed to a cavitation process which appears during tensile deformation. Assuming matrix-particle debonding immediately after yielding, the rubber particles might play the role of initial cavities. It is further found that the flow stress level is highly dependent on the strain rate, and that the rate sensitivity seems to be slightly more pronounced in shear than in tension and compression. From the study of the fracture surfaces it appears that the fracture process is less ductile at high strain rates than under quasi-static conditions.