A Physical Model for the Prediction of Lateral Stress Exerted by Self-Compacting Concrete on Formwork

In this paper, a model is proposed to describe the evolutions of the lateral stress exerted by self compacting concrete (SCC) on a formwork during and after casting. The predictions of the model are compared to pressure drop measurements after the end of casting carried out on real formwork simultaneously with measurements of the evolution of the apparent yield stress of the cast concrete. Then, the predictions of the model during the casting phase are compared to results from the literature and show that the proposed model is able to explain and predict the experimental observations and the quantitative evolutions.     

Factorial design model for proportioning self-consolidating concrete

A factorial design was carried out to model the influence of key mixture parameters on properties affecting the performance of self-consolidating concrete (SCC). Such responses included slump flow and rheological parameters, filling capacity and V-funnel flow to assess restrained deformability, surface settlement to evaluate stability after casting, and compressive strength. Thirty two mixtures were prepared to derive the statistical models and nine others to evaluate their accuracy. The models are valid for a wide range of mixture proportioning. The paper presents the derived models that unable the identification of underlying primary factors and thier interactions that influence the modelled responses of interest for self-consolidating concrete. Such parameters can be useful to reduce the test protocol needed for the proportioning of self-consolidating concrete. The use-fulness of the models to better understand trade-offs between mixture parameters and compare the responses obtained from various test methods are highlighted.     

Thermophysical Properties Necessary for Advanced Casting Simulations

Numerical modelling of casting processes has become an up-to-date standard in industry, aiming at a short term design of modern casting with optimized properties. This paper outlines the present modelling approaches and gives an overview on thermophysical properties (parameters) necessary for such simulations. Mould filling and heat transport simulations, stress and strain predictions, Cellular Automaton and phase-field techniques, and recent multiphase volume average approaches are discussed. The number of necessary material properties is shown to be directly correlated to the amount of complexity considered in the corresponding modelling approach.     

Bond of Reinforcement in Concrete Applied to Concrete Quality Control: The Bottle Bond Test

This paper presents the results of an experimental research dealing with bond strength as a parameter for concrete quality control. To this end, a low‐cost testing technique has been developed: the Bottle Bond Test (BBT). Specimens for the BBT are produced by casting concrete into empty plastic bottles (used as moulds) with a reinforcing bar longitudinally centred. The result is a bottle‐shaped concrete specimen with an embedded rebar, which is pulled out to determine bond strength. Different parameters related to this test setup modify bond strength: their effect has been analyzed. An equation to relate the obtained bond strength values to concrete compressive strength is presented. This equation has been validated with real production data from a readymix concrete plant. Its accuracy and therefore the feasibility of BBT for concrete quality control have been verified. Therefore, the BBT can be an alternative to conventional concrete quality based on uniaxial compression tests.     

Temperature Dependence of Microwave Nano-Oscillator Linewidths Driven by Spin-Polarized Currents: A Micromagnetic Analysis

The dependence of the linewidth on the temperature in a spin valve driven by spin-polarized currents is analyzed by means of full micromagnetic simulations. The results are compared to the recent analytical predictions by Tiberkevic and confirmed by the experiments of Boone In agreement with the Tiberkevic theory and experiments of Sankey , our micromagnetic results point out two regimes. The linewidth increases linearly with the temperature until a threshold value, above which a square root dependence is observed.     

Dynamic behavior of concrete at high strain rates and pressures: II. numerical simulation

The response of concrete and mortar under high-strain-rate impact loading are analyzed using fully dynamic finite element simulations. The analyses concern the load-carrying capacity, energy absorbency and the effect of the microstructure. The simulations focus on the plate impact configuration used in the experimental part of this research, allowing for direct comparison of model predictions with experimental measurements. A micromechanical model is formulated and used, accounting for the two-phase composite microstructure of concrete. Arbitrary microstructural phase morphologies of actual concrete used in impact experiments are digitized and explicitly considered in the numerical models. The behavior of the two constituent phases in the concrete are characterized by an extended Drucker–Prager model that accounts for pressure-dependence, rate-sensitivity, and strain hardening/softening. Model parameters are determined by independent impact experiments on mortar and through a parametric study in which the prediction of numerical simulations is matched with measurements from experiments on concrete and mortar. Calculations show that significant inelastic deformations occur in the mortar matrix under the impact conditions analyzed and relatively smaller inelastic strains are seen in the aggregates. The influence of aggregate volume fraction on the dynamic load-carrying capacity of concrete is explored. The strength increases with aggregate volume fraction and an enhancement of approximately 30% over that of mortar is found for an aggregate volume fraction of 42%. Numerical simulations also show increasing energy absorbency with increasing aggregate volume fraction and provide a time-resolved characterization for the history of work dissipation as the deformation progresses.     

Numerical and Experimental Study of Crack Depth Measurement in Concrete Using Diffuse Ultrasound

This paper presents a combined numerical and experimental study on the diffuse ultrasonic measurement technique for determining the depth of surface breaking cracks in concrete. A finite element (FE) model for the dissipative diffusion in a two-dimensional domain with a surface breaking crack is developed using a commercial FE package; for this purpose, the dissipation term is eliminated by a simple change of variables. Three concrete blocks with a crack depth between 25.4 mm to 101.6 mm are prepared. Diffuse ultrasonic measurements are performed on uncracked and cracked concrete blocks, from which the diffuse energy evolution curves are obtained. The basic material parameters of the hardened concrete, i.e. the diffusivity and dissipation, are retrieved, which are needed for the numerical simulations. The crack depths are then determined by comparing the experimental and numerical arrival times of the average diffuse ultrasonic energy. Various geometrical configurations that arise in real-world concrete structures are simulated including an inclined crack, a partially closed crack, two parallel cracks, and a crack with an underlying reinforcement bar. The objective is to investigate the possible limitations of the diffuse ultrasonic measurement technique when implemented in real concrete structures. Finally, it is shown that the time of flight (TOF) of the average diffuse ultrasonic energy constitutes the theoretical basis of the present diffuse ultrasonic measurement of macroscopic cracks and therefore the present diffuse ultrasonic method forms another kind of TOF technique.     

Simulation studies of methods to delay corrosion and increase service life for cracked concrete exposed to chlorides

The ingress of chlorides in reinforced concrete leads to the onset of steel reinforcement corrosion and eventually compromises a structure’s integrity. To extend its service life and improve safety, it is crucial to develop sound repair strategies for our nation’s infrastructure. In this paper, results are presented for numerical simulations to study the effectiveness of fillers for repair of cracks in concrete, so as to delay the onset of corrosion in reinforcing steel. Concretes without cracks and with either a 50 μm or 500 μm wide crack located directly above the steel reinforcement are simulated, with the addition of silica fume, a corrosion inhibitor, or epoxy-coated reinforcement being considered as additional scenarios. The effectiveness of the crack filler depends not only on its inherent diffusivity with respect to chloride ions, but also on its ability to penetrate and fill the damaged zone or interface between the open crack region and the bulk concrete. Additional simulations indicate that using continuum models instead of models that include details of the rebar placement can lead to underestimating the chloride concentration and overestimating the service life. Experiments are needed to study the ingress of chlorides in damaged (interfacial) regions adjacent to the crack or at the reinforcement surface, as the local transport properties of these regions can significantly influence service life predictions.     

Impact of the initial moisture level and pre-wetting history of recycled concrete aggregates on their water absorption

Initial moisture and pre-wetting method influence on the water absorption of recycled concrete aggregates (RCA) have been studied experimentally. For this purpose, RCA were pre-wetted by three methods: soaking under partial vacuum (simulating long term wetting), soaking under atmospheric pressure, and spraying (simulating short term wetting). The results show that the same initial amount of water in two samples of RCA do not lead to the same total amount of water absorbed by RCA during 5–120 min. The latter depends on the way they have been pre-wetted (either long term or short term). It is suggested that this phenomenon is related to the accessibility filled pores in the different pre-wetting methods. So, the pre-wetting history of RCA could change the amount of total water absorbed by the RCA up to 1%. When mixing and casting concrete are produced with RCA, the corresponding error in the determination of the effective water should have adverse effects on the fresh and hardened concrete characteristics. In this study real applications on mixtures of recycled concrete have been carried out, where the influence of initial moisture content and pre-wetting history of RCA on fresh recycled concrete properties (slump) have been investigated.     

基于统计模式识别的彩色图象分割方法

根据岩芯铸体薄片图象特点,提出了用颜色空间作为特征空间,利用统计模式识别的监督分类方法,最小距法则来对彩色图象进行真彩色二值化分割。实际结果证明该方法具有快,有效和准确的特点,对于彩色岩芯图象的分割处理十分有效。     

Influence of formwork surface on the orientation of steel fibres within self-compacting concrete and on the mechanical properties of cast structural elements

The influences of formwork surface on the final orientation of steel fibres immersed in self-compacting concrete and on the resulting mechanical response of the cast structural elements are investigated. Experimental observations of fibre orientation within cast slabs, obtained via computed tomography, indicate that fibres tend to orient according to the flow patterns during casting, but such tendencies are suppressed near rough formwork surfaces. Fibre orientation, in turn, affects the mechanical properties of the concrete as demonstrated by the load testing of beams extracted from the cast slabs. These processes and results are simulated using a computational fluid dynamics model of the casting process, in tandem with a lattice model of the fracture of the beam specimens. The computational fluid dynamics model determines the coordinates of each fibre within the concrete, which serve as input to the lattice model. Through comparisons with the experimental data, it is shown that these simulations correctly predict the phenomena of interest. We conclude the paper by highlighting a relationship between the number and orientation of the immersed steel fibres crossing the fracture plane and the mechanical response of the structural elements.     

Molecular dynamics and atomistic based continuum studies of the interfacial behavior of nanoreinforced epoxy

In this study, we investigate the interfacial mechanical characteristics of carbon nanotube (CNT) reinforced epoxy composite using molecular dynamics (MD) simulations. The second-generation polymer consistent force field (PCFF) is used in the MD simulations. In particular, we compare MD results with those obtained by atomistic-based continuum (ABC) multiscale modeling technique, which makes use of the appropriate constitutive relations derived solely from interatomic potentials. The results of our comparative investigation suggest that (i) the ABC multiscale model and MD simulation provides almost identical predictions for the interfacial properties of the nanocomposite for smaller diameter of CNTs, (ii) the ABC model slightly over predict the interfacial properties of the nanocomposite for larger diameter of CNTs, and (iii) the MD simulations represents the real nanocomposite structure with the minimum assumptions compared to that of the ABC multiscale model but with much greater computer requirements and limited length scale.     

Application of criterion for A-segregation in steel ingots

A recently developed criterion for A-segregation in steel ingots and castings, based on a dimensionless Rayleigh number, is applied to case studies of two steel ingots. In both cases, experimentally obtained locations of A-segregation are correlated against Rayleigh numbers predicted through casting simulations. It is found that the value of 17±8 previously reported as the critical value for the Rayleigh criterion agrees well with the results of these case studies, but that 6 is an appropriate critical value if a conservative lower bound is sought. Limitations and sources of sensitivity in the application of the criterion are highlighted.     

考虑水泥水化放热与太阳辐射的混凝土路面板温度场数值模拟

为了考察不同强度等级混凝土路面板不同季节、不同浇注时间的温度变化规律,建立了基于有限差分方法的综合考虑水化放热以及太阳辐射、大气温度变化等环境作用下路面板温度场数值计算模型。计算结果显示:混凝土路面板温度受太阳辐射和大气温度影响呈周期性变化,白天主要受太阳辐射影响,夜间主要受大气温降影响,浇注初期的水化放热亦对早期温升有贡献。混凝土路面板顶面与底面温度差以1d 为周期正负循环。其中温度变化幅度及上下面最大温差以夏季最大、秋冬次之、冬季最小。下午2 点浇注和采用较低强度的混凝土可抑制路面板的早期温升和温度梯度。     

Hydrodynamic interaction of two neutrally-buoyant smooth spheres suspended in plane Poiseuille flow: the BEM simulations versus the MoR approximations

The hydrodynamic interaction between two particles suspended in shear flows is fundamental to the macroscopic characterization of suspension flows. Although such interaction in quiescent or linear shear flow is well understood, studies on that in a nonlinear shear field are rare. In this study, the hydrodynamic interaction between two neutrally-buoyant smooth spheres moving at negligible Reynolds numbers in an unbounded plane Poiseuille flow has been calculated by three-dimensional boundary element method (BEM) simulations. The BEM results have been compared with the analytical results obtained with the method-of-reflection (MoR) approximations. The BEM simulations have been found to provide satisfactory predictions if the number of elements on the spheres are more than 200, whereas the MoR approximations provide satisfactory predictions only when the minimum separation between the spheres is relatively large although this MoR method has the advantage to easily calculate the hydrodynamic interaction between two spheres freely moving at negligible Reynolds numbers in unbounded quadratic flow by solving ordinary differential equations. Furthermore, it is found that there is a preferential cross-streamline migration of the center-of-gravity of the sphere-pair in the plane of shear in plane Poiseuille flow which does not arise in simple shear flow. This migration is always directed towards low shear regions when the sphere having larger translational velocity approaches the other sphere, and reverses towards high shear regions when the faster sphere leads the other sphere in the plane of shear. There is also a cross-streamline migration of the center-of-gravity of the sphere-pair in the plane of vorticity, but this migration does not have a preferential direction. These migrations are symmetric about the point where the spheres are at the minimum separation, and are only significant when the hydrodynamic interaction of the spheres is strong. These results show that the migration of the center-of-gravity of the sphere-pair can be attributed to the nonlinearity of the shear field, which agrees with the MoR approximations. The hydrodynamic interaction between the two spheres has been quantified under various conditions by the BEM simulations for both identical and disparate spheres.     

LZ50钢断裂韧度的合理统计模型

试验研究了LZ50车轴钢的断裂韧度的合理统计模型。结果表明,LZ50钢是一种偏脆性的材料,应当严格控制铁道车辆车轴的制造表面质量。同时证实了现有正态分布模型不能合理描述试验数据。为此,通过比较三参数Weibull、两参数Weibull、正态、对数正态、极小值和极大值6种常用统计分布对试验数据的拟合优度、数理一致性和尾部安全性,说明了极小值分布是良好统计模型。建立了断裂韧度的极小值概率测定方法,测定了典型存活概率和置信度下的材料断裂韧度值。     

Optimum design of concrete cable-stayed bridges with prestressed decks

This article presents an optimization-based numerical method for the design of concrete cable-stayed bridges with prestressed decks. This method includes a structural analysis module and a sensitivity analysis and optimization module. The structural analysis considers concrete time-dependent effects, construction stages and geometrical nonlinearities. The discrete direct method is used for sensitivity analysis and an entropy-based approach is used for structural optimization. The design is formulated as a multi-objective optimization problem with objectives of minimum cost, minimum deflections and stresses. Numerical examples concerning the optimization of a real sized cable-stayed bridge are presented to illustrate the applicability of the proposed method.     

Performance of glass fiber reinforced plastic bars as a reinforcing material for concrete structures

The increasing use of fiber reinforced plastic (FRP) bars to reinforce concrete structures necessitates the need for either developing a new design code or adopt the current one to account for the engineering characteristics of FRP materials. This paper suggests some modifications to the currently used ACI model for computing flexural strength, service load deflection, and the minimum reinforcement needed to avoid rupturing of the tensile reinforcement. Two series of tests were conducted to check the validity of the suggested modifications. The first series was used to check the validity of the modifications made into the flexural and service load deflection models. The test results of the first series were also analyzed to develop two simple models for computing the service load deflection for beams reinforced with glass FRP (GFRP) bars. The second series was used to check the accuracy of the modification suggested into minimum reinforcement model.Test results of the first series indicate that the flexural capacity of the beams reinforced by GFRP bars can be accurately predicted using the ultimate design theory. They also show that the current ACI model for computing the service load deflection underestimates the actual deflection of these beams. The two suggested models for predicting service load deflection accurately estimated the measured deflection under service load, and the simpler of the two pertains better predictions than those of the models available in the literature. Test results of the second series reveal that there is an excellent agreement between the predicted and recorded behavior of the test specimens, which suggests the validity of the proposed model for calculating the required minimum reinforcement for beams reinforced by GFRP bars.     

Determining the minimum crack width that can be detected using the impact-echo method Part 2. Numerical fracture analyses

To determine the minimum crack width that can be detected using the impact-echo method, two types of laboratory experiment on concrete plate specimens were conducted. In each experiment, a crack was propagated through the plate parallel to the plate surfaces, while surface displacements caused by the crack opening were measured and impact-echo tests were performed along the crack trajectory. The relationship between the surface displacements and the real crack-opening displacements was established using the results of nonlinear fracture analyses. The minimum crack width was determined by estimating the crack-opening displacements for each impact-echo test location. As a result of this work, the minimum crack width which can be detected was found to be 0.025 mm, and the minimum crack width for which stress waves are not transmitted across the crack faces was found to be 0.08 mm. This paper describes how crack-opening displacements were determined using nonlinear fracture analyses.