On the behaviour of concrete under water jet impingement

It is important from the economics point of view to understand the behaviour of concrete under loading due to plain water jets. Investigations which are based on mass removal measurements, grain size analysis, and mercury intrusion show that the performance of failure can be described using fracture mechanics methods. The reactions of hardened cement paste and concrete differ widely from each other because of the influence of interfaces between the cement paste and the aggregate.     

Quecksilberporosimetriemessungen an durch Druckwasserstrahlen beanspruchten Betonproben

Mercury Penetration Technique and Grain Size Analysis of Concrete The paper describes the behaviour of hardened cement paste and concrete under high velocity water jet attack. Using mercury penetration technique and grain size analysis method, the failure performance is identified as a fracture mechanical one. The behaviour of the materials is influenced due to aggregate grains.     

Influence of interface properties on fracture behaviour of concrete

Hardened concrete is a three-phase composite consisting of cement paste, aggregate and interface between cement paste and aggregate. The interface in concrete plays a key role on the overall performance of concrete. The interface properties such as deformation, strength, fracture energy, stress intensity and its influence on stiffness and ductility of concrete have been investigated. The effect of composition of cement, surface characteristics of aggregate and type of loading have been studied. The load-deflection response is linear showing that the linear elastic fracture mechanics (LEFM) is applicable to characterize interface. The crack deformation increases with large rough aggregate surfaces. The strength of interface increases with the richness of concrete mix. The interface fracture energy increases as the roughness of the aggregate surface increases. The interface energy under mode II loading increases with the orientation of aggregate surface with the direction of loading. The chemical reaction between smooth aggregate surface and the cement paste seems to improve the interface energy. The ductility of concrete decreases as the surface area of the strong interface increases. The fracture toughness (stress intensity factor) of the interface seems to be very low, compared with hardened cement paste, mortar and concrete.     

Experimental study of creep-damage coupling in concrete by acoustic emission technique

In order to design reliable concrete structures, prediction of long term behaviour of concrete is important by considering a coupling between creep and damage. An experimental investigation on the fracture properties of concrete beams submitted to creep bending tests with high levels of sustained load is reported. The influence of creep on the residual capacity and the fracture energy of concrete is studied. The progression of fracture is followed by the measurement of the crack mouth opening displacement during a three-point bending test. The sustained loading seems to increase the flexural strength of concrete, probably because of the consolidation of the hardened cement paste. The acoustic emission (AE) technique is used to perform the characterization of the influence of creep on the crack development. Results give wealth information on the fracture process zone (FPZ) and the propagation of the crack. A decrease in the amplitude distribution of AE hits is observed in the post-peak region for creep specimens. The width of the FPZ also decreases in this later indicating that the material has a more brittle behaviour which may be due to the development of microcracking under creep and the prestressing of the upper zone of the beam.     

Effects of transition zone densification on fiber/cement paste bond strength improvement

This paper investigates the effectiveness of transition zone densification on the fiber-cement paste system. By controlling the water: cement (w:c) ratio and the condensed silica fume content, environmental scanning electron microscopy studies confirm that transition zone densification can be achieved in all brass, steel, and polyethylene fiber-cement systems. However, single fiber pullout tests indicate that densification only enhances the brass-cement paste interface bond strength and not the other systems. Further microscopy investigation of the surface of fibers peeled off from a composite fracture surface and of the groove left by the fiber on the cement paste suggests that bond failure for the brass-cement system is of a cohesive type, whereas bond failure for the other two systems is of an adhesive type. It is concluded that the transition zone densification technique should be effective in fiber-cement systems in which bond strength is controlled by cohesive failure of the transition zone material.     

Pore structure of hardened cement paste and mortar

The paper describes an experimental investigation into the pore structure of hardened cement paste and mortar. The pore structure was studied using mercury porosimeter. Ordinary portland cement and natural river sand were used. Pore structure determination was carried out for both the cement paste and mortar mixes over four hydration periods and five water-cement ratios. The threshold radius which was found to be prominent in the hardened cement paste, flattens out as the fine aggregate content increased.     

高速水流下空蚀热效应对水泥水化产物的破坏

随着坝工技术的提高和水力资源的深入开发利用,高坝建设发展迅速,其泄量大、流速高时的泄洪消能和抗空蚀保护问题是目前建设中的主要技术难题。区别于传统对高速水流引发空蚀的力学破坏定义,本研究从热学角度阐述了空蚀引起破坏的机理。本研究利用超声波空蚀仪进行不同类型水泥净浆空蚀试验,综合空蚀破坏面表面分析和对空蚀破坏后的水泥水化产物微观成分分析,对空蚀在硬化水泥表面产生的热效应进行测定和研究分析,初步讨论空蚀热效应对混凝土结构的破坏机理。试验结果表明,空蚀对水泥净浆表面的破坏不仅是简单的力学破坏,同时空蚀热效应还可以造成水泥水化产物的部分分解。具体而言,经过6 h超声空蚀试验后的试样,水化凝胶损失量不低于40%,氢氧化钙的损失量在25%以上,这对硬化水泥浆体的劣化有较为显著的影响。     

Resolution of fine fibrous C-S-H in backscatter SEM examination

Secondary electron mode SEM images of hardened cement paste (obtained on fracture surfaces) have been published since the early 1970s. Most such images emphasized fine fibrous or dendritic C-S-H structures, which are commonly found in porous areas preferentially exposed by fracturing. Such fibrous structures are not generally seen in backscatter-mode SEM images obtained on polished surfaces. Examination with a high-resolution field-emission SEM (FE-SEM) instrument in the backscatter mode resolves these fibrous C-S-H structures, and suggests that failure to image them in conventional backscatter SEM is primarily due to inadequate resolution.     

Crack modelling: A novel technique for the prediction of fatigue failure in the presence of stress concentrations

Finite element (FE) analysis and other computational methods have developed rapidly in recent years, allowing accurate predictions of elastic stresses in components of complex geometry. However, the prediction of fatigue failure in these components is still a non-trivial problem; one reason for this is the difficulty of assessing stress concentrations and regions of high stress-gradient. This paper describes a new technique, called “crack modelling”, which addresses the problem through a modification of linear-elastic fracture mechanics (LEFM). LEFM is designed to deal with cracks in nominally elastic stress fields, using elastic analysis to derive a characteristic stress intensity, K or, for cyclic loading, a range ΔK. This methodology is modified in two ways. Firstly it is shown that LEFM can be extended to predict the fatigue behaviour of bodies containing notches of standard geometry, instead of cracks. Secondly, FE analysis is used in conjunction with a modelling exercise in order to extend the method to include bodies of arbitrary shape subjected to any set of loads. The method was first tested using standard notch geometries (blunt and sharp notches in beams), where accurate predictions of fatigue limit could be achieved. It was then applied to an industrial problem, giving a prediction of high-cycle fatigue behaviour for an automotive crankshaft. The method requires only simple mechanical-property data (the material fatigue limit and stress-intensity threshold) and uses only linear-elastic FE modelling. It allows fracture mechanics theory to be used without the need to specifically model the presence of a crack and uses far-field elastic stresses to infer behaviour in the region of a stress concentration.     

The effect of the longitudinal stiffness of the testing machine on the strength of hardened cement paste

Tests were carried out to see whether the longitudinal stiffness of the testing machine had any effect on the strength and fracture toughness of hardened cement paste. Six different testing machines were studied; in addition, machine stiffness was varied by inserting proving rings in series with the specimens. It was found that machine stiffness has no significant effect on the compressive strength, the flexural strength, or the critical stress intensity factor.     

An explanation for the negative effect of elevated temperature at early ages on the late-age strength of concrete

Elevated curing temperature at early ages usually has a negative effect on the late-age strength of concrete. This article aims to study the mechanism of this phenomenon. The results show that elevated curing temperature at early ages has a negative effect on the late-age strength of hardened cement paste, but it has a greater negative effect on the late-age strength of cement mortar. After elevated temperature curing at early ages, the late hydration of cement is hindered, but the late reaction of fly ash is not influenced. Owing to the continuous reaction of fly ash, the late-age pore structure of cement–fly ash paste under elevated curing temperature is finer than that under standard curing temperature, and the late-age strength of cement–fly ash paste under elevated curing temperature is higher. However, the late-age strength of cement–fly ash mortar under elevated curing temperature is lower. Apparently, there are differences between the effects of elevated curing temperature on hardened paste and mortar. It is the deterioration of transition zone between hardened paste and aggregate that makes the negative effect of elevated curing temperature on the mortar (or concrete) be greater than the hardened paste. As the water-to-binder ratio decreases, the negative effect of elevated curing temperature on the transition zone tends to be less.     

Investigation into engineering properties and strength mechanism of grouted macadam composite materials

To further understand engineering properties of grouted macadam composite materials (GMCM) used as a surfacing layer in pavement, the mechanical properties and durability characteristics of GMCM were evaluated, and the relevant strength mechanisms were investigated at the micro level. Results indicate that GMCM has better high-temperature stability, fatigue performance and moisture stability than that of conventional asphalt mix, while it shows an acceptable decrease in low-temperature crack resistance due to the relative brittleness of hardened cement paste. The hardened cement paste also generates a spatial network crystalline lattice in asphalt mix skeleton to form a three-dimensional integral coagulation-crystalloid structure. This facilitates the asphalt mix skeleton and hardened cement paste to bear loads in unison and increase durability of the GMCM. Further, the fibre-like hydrated products of fresh cement slurry on the bitumen film surface increase the interfacial strength between bitumen and hardened cement paste due to toughening and bridging effects, which plays an important role to enhance mechanical properties and durability of GMCM. Finally, GMCM strength is from the internal friction of asphalt mix skeleton, the network structure of hardened cement paste and the adhesion between porous asphalt mix and hardened cement paste. It is concluded that GMCM can better meet the requirements of mechanical properties and durability characteristics than the conventional asphalt mix.     

Framework for a generalized four-stage fracture model of cement-based materials

In cement-based materials the full range from brittle to ductile fracture can be achieved by changing the material structure, the loading conditions, the specimen size and/or the boundary conditions. Considering just the material, at one side of the spectrum hardened cement paste behaves brittle, whereas at the other side, new fibre reinforced cements may behave ductile. Structural conditions affect the brittleness/ductility as well, and by simply changing the loading (uniaxial tension, uniaxial and confined compression, etc.), the specimen/structure size or by changing the boundary conditions the full range from brittle to ductile response can be observed. Basically there is no difference in behaviour between the various loading cases and the same four-stage fracture process can always be identified. The four ‘universal’ stages are the linear elastic regime, the microcrack regime (before the maximum load is reached), the macrocrack regime (viz. the first, usually steep part of the softening curve), and the bridging stage. Microcracks are defined as cracks that can be arrested by elements in the material structure, whereas macrocracks can only be delayed/ arrested by means or structural measures at a larger scale than the material structure. In this paper it is tried to develop a unified view on fracture of materials belonging to this broad class, which may be seen as conceptual framework for an all encompassing fracture model for cementitious materials.     

Fluid jet erosion of tension-softening materials

Fluid jets are increasingly used to process and machine tension-softening materials. Typical applications are drilling, cutting, fragmentation, hydrodemolition and 3D-machining. Tension-softening materials, such as concrete, reinforced ceramics, most rocks and solidified impurities, are quasi-brittle behaving materials characterised by a so-called fracture process zone. Therefore, linear elastic fracture mechanics does not suitably cover these materials. It is also known that conventional strength parameters, such as compressive strength, cannot describe the resistance of this group of materials against fluid jet erosion. In this paper, the behaviour of cement paste, mortar and concrete during the erosion by high-speed waterjets with velocities up to 470 m s^–1 is investigated. The results of SEM-studies are presented that clearly illuminates features of quasi-brittle behaviour, including grain bridging, microcracking and crack branching. It is also found that the erosion process is strongly determined by the size of the aggregate (inclusions) in the material. It is concluded that a non-linear fracture resistance parameter may be suitable to estimate the erosion resistance of the material. It is found that the brittleness B _M – the inverse of the characteristic length of a material – is a suitable resistance parameter. A very good relationship between the brittleness and the volumetric erosion rate estimated from 24 different types of concrete could be obtained.     

Influence of surface charge on ingress of chloride ion in hardened pastes

The amount of free chloride content in concrete is one of major factors in initiating the corrosion process. The material and environmental factors play a key role in diffusing the chloride ion through the cover concrete to reinforcement. Thus, the electrochemical study is indispensable to understand the mechanism of chloride ingress into concrete. Determination of surface charge and its influence on diffusion of chloride ion into cement matrix of concrete are researched for Ordinary Portland Cement (OPC) paste and cement paste containing Ground Granulated Blastfurnace Slag (GGBS). Different kinds of experiments such as measurement of membrane potential, determination of porosity and pore size distribution, determination of pore solution concentration, and steady state diffusion coefficient of chloride and sodium ions are employed to understand the mechanism of chloride ingress. The obtained results show that the positive surface charge on the pore walls of hardened paste regardless of GGBS’s presents. The surface charge of hardened paste mainly depends on pore solution concentration and cement composition. The physiochemical characteristics of the pores are affecting on transporting ions through it. Hardened paste has greater resistance to diffusing sodium ions than chloride ions. Moreover, there is a strong interaction between transport of chloride ion and surface charge in matured hardened paste.     

Gültigkeits- und Anwendungsbereich der Rißbruchmechanik

Fracture Mechanics Range of Validity and Application. The range of validity and application of the linear-elastic and elasto-plastic fracture mechanics is determined according to practicable quantitative criteria. The former is applicable to the low stress brittle fracture in brittle materials, the latter is applicable to the higher stress brittle and tear fracture in tougher materials. The real continuum mechanics fracture criterium is not yet determined. The linear-elastic fracture mechanics is widely applicable to fatigue fractures. Taking into account the interaction and sequence effect of low and high load amplitudes is necessary. The fracture mechanics approach to brittle and fatigue fracture is compared with traditional testing techniques.     

Crack propagation and fracture in silicon carbide

Fracture mechanics and strength studies performed on two silicon carbides — a hot-pressed material (with alumina) and a sintered material (with boron) — have shown that both materials exhibit slow crack growth at room temperature in water, but only the hotpressed material exhibits significant high temperature slow crack growth (1000 to 1400? C). A good correlation of the observed fracture behaviour with the crack growth predicted from the fracture mechanics parameters shows that effective failure predictions for this material can be achieved using macro-fracture mechanics data.     

Cohesive zone models of localization and fracture in bone

Previously published experimental data for the fracture of bone are analysed using cohesive zone models to deal with the non-linear processes of material failure. Non-linear effects dominate tests; linear-elastic fracture mechanics cannot give an internally consistent account of the data. In contrast, the same cohesive traction law can account accurately for substantial differences in the fracture data for normal (non-diseased or aged) adult human humeral cortical bone taken at two laboratories, where different specimen configurations were used. Further model calculations illustrate more general characteristics of the non-linear fracture of bone and demonstrate in particular that the fracture toughness of bone deduced via LEFM from test data is not a material constant, but will take different values for different crack lengths and test configurations. LEFM can be valid only when the crack is much longer than a certain length scale, representative of the length of the process zone in the cohesive model, which for human cortical bone ranges from 3 to 10 mm. Naturally-occurring bones and the specimens used to test them are not much larger than this dimension for most relevant orientations, implying the necessity of non-linear fracture models. The analysis of fracture data also requires proper representation of the approximately orthotropic elasticity of the bone specimen. The fracture test data show that human humeral cortical bone is much more compliant in shear in the plane of the test specimen than would be inferred from the relevant Young’s moduli, if the material were isotropic in that plane, as is often assumed. If the specimen is incorrectly assumed to be isotropic in that plane, the initial measured compliance cannot be reproduced to within a factor of four and the fracture toughness deduced from the measured work of fracture will be overestimated by ∼30%.     

冻融作用下硬化水泥浆体和混凝土结构的变化

用压汞法和扫描电子显微镜研究了冻融循环作用下硬化水泥浆体和混凝土宏观与细观结构的变化.结果表明,冻融以后硬化水泥浆体和混凝土中砂浆的孔隙率,特别是大于50 nm的毛细孔体积和微裂纹增大较多,导致硬化水泥浆体由密实体向松散体发展,最后导致粗骨料与砂浆分离.提出了快速冻融破坏机理与自然条件下缓慢冻融的破坏机理存在差异的观点.