钢纤维自密实混凝土梁抗弯性能的数值模拟方法

钢纤维自密实混凝土梁的抗弯性能与钢纤维的取向有密切关系,而基于钢纤维取向对其抗弯性能进行的数值模拟研究较少。借助Moldex3D软件模拟并分析了浇筑完成后钢纤维的取向概率,在此基础上由Final-v11有限元软件建立钢纤维自密实混凝土梁模型并进行数值计算,通过与试验对比结构的荷载位移曲线、裂缝开展过程等方面,验证了该数值模拟方法的有效性。由变参数研究表明,当浇注位置由梁端向跨中移动时,梁的开裂荷载几乎不变,极限荷载和抗弯刚度的减小幅度不断增大。     

二三维连续介质分析中的等效弹簧元

该文给出了一类将二维和三维连续介质等效成弹簧元体系进行分析的研究思路,并将二维和三维的等效弹簧单元应用于容器结构与内部连续介质相互作用的静力与动力分析中。首先,该文给出了基于刚度等效原理的二维与三维等效弹簧元理论。其次,利用二维等效弹簧元模型作为基本单元分析了连续介质对容器壁的压力分布规律,并利用三维等效弹簧元模型分析了容器壁与内部连续介质共同响应的动力特性。数值算例表明:与常用的连续介质有限元模型相比,等效弹簧元模型的计算效率和精度更高。在容器结构与内部连续介质共同响应动力分析中,等效弹簧元体系与工程中常用的附加质量法相比,不光有更高的计算效率,而且能更正确地反映内部连续介质刚度对共同响应动力特性的影响。等效弹簧元在工程应用分析中具有良好的适应性。     

青藏高原环境下混凝土结构密实性超声无损检测研究

青藏高原的环境条件限制常导致混凝土出现密实性缺陷问题，如何对混凝土的密实性缺陷进行精确的无损检测，是保障结构承载力及耐久性的关键。该文结合此前在西藏雅鲁藏布江藏木大桥与广西南宁实地开展的混凝土超声试验结果，基于不同气压下混凝土实体的超声波速计算模型，建立了以气压、水胶比为参数的混凝土实体部分超声波速计算模型；进一步对混凝土在不同孔隙率、缺陷条件、骨料分布情况条件下的超声波速进行了计算分析。揭示了孔隙率及缺陷条件对混凝土超声波速的影响规律及机理。研究了骨料分布占比对混凝土超声检测结果的影响规律，建立了混凝土整体超声波速的多因素计算模型，并提出了基于超声波速的混凝土密实性统计分析方法，可以为实际工程中混凝土结构密实性的超声无损检测结果分析提供理论依据。     

高抛免振捣自密实混凝土技术

本文提出了对高抛免振捣自密实钢管混凝土性能的评价方法、性能指标以及相应的实验方法。根据上述方法对配置的免振捣自密实钢经凝土拌合物进行实验验证,并进行了两次与现场施工状况完全相同的模拟高抛实验。从模拟高抛实验的检验结果可以证明本文提出的研制高抛免振捣自密实混凝土的一整套技术是切实可行的。     

磁流变液的配制及其流变模型的研究

实验研究了磁流变液的改性配制工艺,并配制出了与美国商品化磁流变液性能相当的试样.其次,实测了磁流变液的剪切速率与剪切应力的关系,并分别用Bingham模型和Herschel-Bulkley模型对磁流变液的流变本构关系进行最小二乘拟合,研究表明后者可以较好地拟合MR液屈服后区的非线性特征.最后,通过阻尼器性能试验研究,进一步验证了磁流变液的本构关系更接近于Herschel-Bulkley体而非Bingham体.     

二次受力下自密实混凝土加固RC梁受弯性能研究

为模拟实际工程中加固构件的真实承载能力,实验在原混凝土构件持续受荷状态下,采用自密实混凝土对构件进行加固、养护。共进行了7根二次受力下自密实混凝土加固钢筋混凝土梁和2根对比梁的受弯性能试验,研究了不同初始受力水平、不同加固厚度及不同界面处理方式对加固钢筋混凝土梁抗弯承载力和截面刚度的影响。试验量测了构件裂缝分布形态、荷载-挠度曲线、钢筋应变发展规律等。结果表明:采用自密实混凝土加固钢筋混凝土梁,能有效地提高钢筋混凝土梁的抗弯承载力、截面刚度等性能;二次受力下自密实混凝土加固梁抗弯承载力随着初始受力水平的增大而降低。在试验结果的基础上,基于平截面假定,提出了二次受力下自密实混凝土加固梁钢筋滞后应变及抗弯承载力计算式,计算结果与试验结果吻合良好。     

基于CT的准脆性材料三维结构重建及应用研究

准脆性材料的结构及其细观组分的力学性质决定了材料的力学性质及破裂机制,在数值模型中尽可能准确的考虑材料的真实结构已成为了数值计算与数值模拟发展的一种趋势。该文以混凝土为研究对象,借助于先进的CT技术获取混凝土内部结构切片图像,利用数字图像处理技术实现了基于CT图像的混凝土材料结构的识别和表征,并针对CT图像具有颜色亮度不均并呈环状分布的特点,提出了环状分区与分割阈值自动识别相结合的CT图像分割算法;在此基础上,建立了基于位图矢量化理论的三维实体材料结构模型的重建方法和三维网格化材料结构模型重建方法,并将三维网格化方法建立的材料结构模型与三维岩石破裂过程分析系统RFPA3D结合进行了初步应用,对混凝土单轴压缩破裂过程进行了数值模拟。通过数值试验与物理实验结果对比发现,考虑混凝土骨料真实分布的数值实验结果与物理实验具有一定可比性,数值试验结果从得到的力学参数和破裂模式方面比较接近于物理实验结果,为深入研究混凝土、岩石、复合材料等力学特征提供了一种可行的研究思路。     

Design-Expert7.0在设计C60高抛钢管自密实混凝土中的应用研究

利用Design-Expert7.0软件对C60高抛钢管自密实混凝土进行配合比优化设计,并对试验结果进行模型拟合。通过扫描电子显微镜(SEM)、X衍射分析(XRD)、超声波法、钻芯法及现场模拟浇筑实验,研究了硬化钢管自密实混凝土性能。结果表明,通过Design-Expert7.0优化设计获得的C60钢管自密实混凝土拌合物工作性能较好,水化产物以球状颗粒的Ⅲ型C-S-H凝胶为主。优化设计有效减少了混凝土的收缩,避免了混凝土与钢管内壁粘结性能不良,完全满足工程应用要求。     

自密实混凝土与老混凝土粘结剪切强度的塑性极限分析

该文假定了自密实混凝土与老混凝土粘结层的直剪破坏机构,选用Mises屈服条件,对新老混凝土的粘结强度进行塑性极限分析,得到粘结剪切强度的上限解τ的理论计算公式,并根据试验结果确定了公式中的主要参数,以供自密实混凝土加固工程设计参考。     

骨料对自密实混凝土性能的影响

近年来自密实混凝土得到了大量应用,自密实混凝土指新拌混凝土具有高流动度而不离析、不泌水和高均匀性,能在不经振捣或少振捣的情况下自流平整并自动通过钢筋间隙充满模具,达到充分密实和获得最佳性能的混凝土。自密实混凝土因其有十分良好的工作性、力学性能与耐久性,而广泛应用于浇筑量大、浇筑深度深或浇筑高度大、钢筋密集、有特殊形状等混凝土工程中。目前,自密实混凝土已经成为高性能混凝土的一个重要研究方向之一。本文着重探讨骨料对混凝土性能的影响。     

Simulation of concrete flow in V-funnel test and the proper range of viscosity and yield stress for SCC

The present paper highlights the flow simulation of self consolidating concrete (SCC) in V-funnel test that is used to determine the concrete filling ability and its resistance against segregation. Simulations were performed using a two-dimensional smoothed particle hydrodynamic (SPH) method to determine the discharge time where SCC was considered as a homogeneous Bingham fluid. The numerical predictions are lower than experimental data because of the assumptions of two-dimensional and homogeneous flow. Having the SPH method employed, SCCs with different viscosities and yield stresses were simulated to compare the discharge time with the suggested criteria in EFNARC (2002) and (2005) guidelines. Based on simulations results, the appropriate range of viscosities and yield stresses as well as a relation between rheological properties and discharge time for SCC taking into account EFNARC (2002) and (2005) guidelines are suggested. Using the suggested relations, one can assess the proper SCC filling ability without conducting the V-funnel test.     

Numerical simulation of self-consolidating concrete flow as a heterogeneous material in L-Box set-up: Effect of rheological parameters on flow performance

A computational fluid dynamics (CFD) software was used to simulate the effect of rheological parameters on the heterogeneous performance properties of self-consolidating concrete (SCC) in the horizontal and vertical directions of the L-Box set-up. These properties consist of flowability, blocking resistance, and dynamic segregation. Different suspending fluids having five plastic viscosity values (10–50 Pa.s), three yield stress values (14–75 Pa), two fluid densities (2000 and 2500 kg/m³), and two shear elasticity modulus values (100 and 1000 Pa) were considered. The suspensions consisted of a number of 135 in total spherical particles with 20-mm in diameter and 2500 kg/m³ density.The results of 25 simulations in total are found to correlate well with the rheological parameters of the suspending fluid. Plastic viscosity of the suspending fluid was shown to be the most dominant parameter affecting flow performance of SCC in the L-Box test. A new approach was also proposed to classify SCC mixtures based on the filling ability properties.     

Rheological properties considering the effect of aggregates on concrete slump flow

Rheology of concrete allows us to understand the flow behavior of concrete and further extend the quantitative evaluation of its construction performance. The use of a concrete rheometer is promising for the purpose, but sometimes limited high associated cost and procedure complexity. This study proposes a simulation-based model that correlates the slump flow test results to a concrete’s rheological properties, allowing quantitative evaluation through this simple method. The proposed model is based on single-fluid simulation using the volume-of-fluid method, with an extension to accommodate the partial segregation of coarse aggregates. Either the channel flow or the L-shaped panel filling of SCC is simulated using the rheological properties obtained by our model. Finally, the rheograph describing the self-compacting ability of SCC is updated.     

A new empirical test method for the optimisation of viscosity modifying agent dosage in self-compacting concrete

Viscosity modifying admixtures (VMAs) are high molecular weight, water soluble organic polymers that are used to stabilise the rheological properties and consistency of self-compacting concrete (SCC). Most studies on VMA focus on the influence of different types and dosages of VMA on the rheological properties and the compatibility issues between superplasticiser and VMA. To obtain the desired rheological properties and to reduce the cost, optimisation of the VMA dosage is essential. In the present study, a simple method was developed for optimising the VMA dosage for the cementitious pastes of different water to powder ratio. The principle of this method was based on Stokes’ law. Rheological properties of SCC pastes with VMA were studied using a viscometer. The studies were also extended to concrete for validation of the optimised dosage obtained from the cementitious paste tests. The results obtained clearly indicate that the optimised dosage obtained by using the empirical method yields SCC without segregation. The lower dosages are not sufficient enough to control the segregation. On the other hand, dosages higher than optimal resulted in reduced slump flow.     

Influence of the rheological properties on the steel fibre distribution and orientation in self-compacting concrete

The interest in potential applications produced with self-compacting fibre reinforced concrete continues to grow, but in practice, problems associated with an uneven distribution and orientation of fibres in the concrete structure occur. It is not clear what exactly influences uneven distribution of fibres in self-compacting concrete (SCC) mixtures, especially during the casting and how different factors influence fibre orientation. The objective of this work was to investigate how rheological properties influence the steel fibre distribution in self-compacting concrete. This work also focuses on the investigation of steel fibre spatial orientation dependence on rheological properties of SCC, while keeping other casting parameters and the proportions of mixture components constant. Mixtures with three different rheological properties were chosen based on slump flow, slump flow time t₅₀₀ and static segregation values. The steel fibre orientation, volumetric concentration and spatial distribution values were determined in separate beam sections using three different non-destructive testing methods: electromagnetic induction, image analysis and computed tomography (CT scan). The comparison of the results is presented. The results show how different rheological properties of SCC affect the steel fibre orientation and distribution for the case of beams produced with the flow-induced casting method.     

Simulation of the three-dimensional non-isothermal mold filling process in resin transfer molding

Numerical simulation of resin transfer molding (RTM) is known as a useful method to analyze the process before the mold is actually built. In thick parts, the resin flow is no longer two-dimensional and must be simulated in a fully three-dimensional space. This article presents numerical simulations of three-dimensional non-isothermal mold filling of the RTM process. The control volume/finite element method (CV/FEM) is used in this study. Numerical formulation for resin flow is based on the concept of nodal partial saturation at the flow front. This approach permits to include a transient term in the working equation, removing the need for calculation of time step to track the flow front in conventional scheme. In order to compare the results of the nodal partial saturation concept with the conventional method, a numerical scheme based on the quasi-steady state formulation is also presented. The computer codes developed based on both numerical formulations, allow the prediction of flow front positions; and pressure, temperature and conversion distributions in three-dimensional molds with complicated geometries. The validity of the two schemes is evaluated by comparison with analytical solutions of simple geometries. In all instances excellent agreement is observed. Numerical case studies are provided to demonstrate the effectiveness of the developed computer codes. The results show that the numerical procedure based on the nodal partial saturation concept, developed in this study, provides numerically valid and reasonably accurate predictions.     

Effects of mix design parameters and rheological properties on dynamic stability of self-consolidating concrete

Stability is a crucial property for successful placement and performance of self-consolidating concrete (SCC). Dynamic stability refers to the segregation resistance of concrete during flow and is not thoroughly described in literature to date. In this research, a newly developed dynamic stability test, the Tilting-box test, was employed to evaluate the effect of different mix design parameters on the dynamic segregation of SCC. Paste volume, water-to-binder ratio (w/b), maximum size of aggregate (MSA), aggregate density, and aggregate grain-size distribution were shown to affect the dynamic stability of SCC, while the coarse aggregate shape did not have a significant influence. The slump flow and V-funnel flow time as well as the rheological parameters were found to be an efficient tool for controlling dynamic segregation. Higher yield stress or plastic viscosity reduced the risk of dynamic segregation. Recommended workability characteristics and rheological parameters of SCC to ensure adequate dynamic stability level were proposed.     

A modified drag model for power-law fluid-particle flow used in computational fluid dynamics simulation

A modified drag model for the power-law fluid-particle flow considering effects of rheological properties was proposed. At high particle concentrations (ε_s ≥ 0.2), based on the Ergun equation, the cross-sectional shape and the tortuosity of the pore channel are considered, and the apparent flow behavior index and consistency coefficient of the power-law fluid at the surface of the particles are corrected. At low particle concentrations (ε_s < 0.2), based on the Wen-Yu drag model, the modified Reynolds number for power-law fluid and the relational expression between drag coefficient for single particle and Reynolds number that considers the effect of the flow behavior index are adopted. Numerical simulations for the power-law fluid-particle flow in the fluidized bed were carried out using the non-Newtonian drag model. The effects of rheological parameters on the drag coefficient were analyzed. The comparisons of simulation and experiment show that the modified drag model predicts reasonable void fraction under different rheological parameters, particle diameters, and liquid velocities in both low particle concentrations and high particle concentrations. The increase in flow behavior index and consistency coefficient increases the drag coefficient between the two phases and decreases the average particle concentration within the bed.     

Regularizing inversion of susceptibility with projection onto convex set using full tensor magnetic gradient data

Traditional magnetic inversion is based on the total magnetic intensity data and solving the corresponding mathematical physical model. In recent years, with the development of the advanced technology, acquisition of the full tensor gradient magnetic data becomes available. We study the inversion of magnetic parameters using the full tensor gradient magnetic data. A Tikhonov regularization model with bounded constraint is established. An improved preconditioned conjugate gradient method with projections onto convex set is used to solve the minimization model. Numerical simulations are performed on two-dimensional (2D) and three-dimensional (3D) synthetic data to show the feasibility of our algorithm.