Study of phase dispersion in concrete by image analysis

The scope of this paper is to investigate by an automatic image analysis technique the dispersion of phases in concrete. Three morphological and statistical tools were used: co-occurrence matrices, and simple and crossed-covariance. It was shown (1) that there is a repulsion between gravel, (2) that gravel and air-voids are surrounded by matrix (cement paste and sand), and (3) that the dispersion of gravel and air-voids is perfectly uniform.     

Some fields of applications of automatic image analysis in civil engineering

This paper illustrates the use of automatic image analysis technique to investigate the morphology of cement, concrete and fibre-reinforced concrete. First the methods to be used for powders and secondly for mortar and concrete are introduced. The dispersed phases are characterized by classical morphological parameters: these also enable to accede to the hydration process. The covariances give quantitative information on the homogeneity and dispersion of the different components: gravel, air-voids and cement paste. Air-voids are characterized by granulometric distributions and their mean free paths. Rose of directions gives information on feature orientation: fibres, microcracks for fibre-reinforced concrete, etc. Finally probabilistic models can be used to simulate the microstructure of such materials.     

Measurement of entrained air-void parameters in Portland cement concrete using micro X-ray computed tomography

The entrained air-void system in concrete is closely related to freeze-thaw durability in concrete pavements or other structures. For either research or forensic purposes, reliable and economical methods for the quantification of entrained air are desirable. This study explores the potential of using micro X-ray computed tomography (μCT) to measure entrained air-void parameters in concrete. A series of small cores (6 mm dia.) were retrieved from larger (100-mm-dia.) cores from two different concrete pavements, representing both adequate and marginal air contents, and scanned at a resolution of 7.5 μm/pixel. A systematic procedure based on image processing is proposed to address practical difficulties such as void/solid thresholding, air-type discernment (entrained air-voids vs. voids in aggregate) and the separation of bubbles within close proximity to each other (e.g. clustered air-voids). Air content and specific surface were measured directly from the three-dimensional (3D) reconstructed X-ray images, while values for paste content were derived from manual point counts performed on two-dimensional (2D) slices obtained from the 3D images. The derived values for air content, specific surface and paste content were used to calculate Powers’ spacing factor. To assess the issue of local fluctuations of material constituents and the limited dimensions of the small cores, uncertainty associated with the sample volume of concrete under measurement was also estimated. Based on the results in this study with regard to the work involved in sample preparation, data analysis and uncertainty bounds, μCT has been found to be a viable option for measurement of spacing factor and specific surface, but due to limitations imposed by the dimensions of the sample size (6-mm-dia. cores), the method is not appropriate for bulk air content determination.     

Numerical simulation of gravel deposits using multi‐circle granule model

Abstract

This study proposes a new Multi‐Circle Granule Model (MCGM), based on overlapping circles with various diameters to create an irregular particle shape, to simulate the geometry‐dependent behavior of gravel particles for two‐dimensional mechanical analyses of gravel deposits. The MCGM model is implemented in a Distinct Element Method (DEM) program for numerical simulation. Two numerical examples are presented to study the effects of gravel shapes on the angles of repose in free falling tests, and on the stability of gravel tunnel excavation in trap door tests. Additionally, the numerical study reveals that the shapes of gravel significantly affect the angle of repose. Furthermore, interlocking, anisotropy, and contact numbers among the gravels are found to be key factors governing the stability of gravel tunnel excavation. These numerical simulation results of geometry‐dependent behavior are quite encouraging for the MCGM model.     

Frost susceptibility of sub-base gravel used in Pearl-Chain Bridges: an experimental investigation

This study investigates frost susceptibility of sub-base gravel determined by the ASTM D5918-13 standard as a conservative estimate of the frost heave risk of fill in overfilled arch bridges, particularly in Pearl-Chain Bridges. Frost heave of granular materials has been of great research interest from the end of the 1920s until the present day. Most new literature relates to empirical results that are several decades old. This is also the case for Danish tender specifications according to which the frost susceptibility of a sub-base gravel is solely assessed from its fines content. However, no actual frost tests have been carried out to verify this assumption. In the present study, the frost susceptibility of four different Danish gravel materials is categorised from their heave rate. We test two Danish sub-base gravel materials, with particle size distributions of 0–8 mm and 0–31.5 mm, respectively, and also two modified sub-base gravel materials with increased and reduced fines contents. The fines content of the gravel materials is analysed by laser diffraction, and compared with two common frost susceptibility criteria, Casagrande’s and Schaible’s, and with Danish tender specifications. Even though the two sub-base gravel materials are expected to be frost safe, 0–31.5 mm sub-base gravel shows medium frost susceptibility, whereas 0–8 mm sub-base gravel shows negligible frost susceptibility. The gravel materials with increased and reduced fines content are categorised as having low to medium frost susceptibility and low frost susceptibility, respectively. The permeability of the gravel materials is determined, and the permeability coefficient of 0–31.5 mm sub-base gravel is five times greater than that of 0–8 mm sub-base gravel. The results suggest that the criterion used to classify the frost susceptibility of Danish sub-base gravel materials based solely on their fines content is insufficient, and that the permeability coefficient should also be considered.     

半正弦循环交通动载下加筋砾性土动力特性研究

为研究交通荷载下加筋砾性土的动力特性,以土工格栅为加筋材料,开展了多级长期循环荷载下无筋、平铺、环形竖向及环形立体组合加筋等不同加筋形式的半正弦循环荷载下加筋饱和砾性土动三轴试验,同时也进行了半正弦循环荷载下土体阻尼比计算公式的推导,并研究了加筋层数和围压对环形立体组合加筋砾性土阻尼比、骨干曲线等动力特性的影响规律。试验结果表明:多级循环荷载下砾性土试样的阻尼比呈现前期增大随后减小的规律,阻尼比在动应变达到1%左右达到峰值,且加筋能够减小砾性土试样的阻尼比;Hardin-Drnevich模型适用于描述加筋砾性土的骨干曲线,且模型参数a、b与围压和加筋层数之间分别存在线性关系和指数关系,基于此提出适用于不同围压、不同加筋层数下环形立体组合加筋砾性土的骨干曲线方程。     

半正弦循环交通动载下加筋砾性土动力特性研究

为研究交通荷载下加筋砾性土的动力特性,以土工格栅为加筋材料,开展了多级长期循环荷载下无筋、平铺、环形竖向及环形立体组合加筋等不同加筋形式的半正弦循环荷载下加筋饱和砾性土动三轴试验,同时也进行了半正弦循环荷载下土体阻尼比计算公式的推导,并研究了加筋层数和围压对环形立体组合加筋砾性土阻尼比、骨干曲线等动力特性的影响规律。试验结果表明:多级循环荷载下砾性土试样的阻尼比呈现前期增大随后减小的规律,阻尼比在动应变达到1%左右达到峰值,且加筋能够减小砾性土试样的阻尼比;Hardin-Drnevich模型适用于描述加筋砾性土的骨干曲线,且模型参数a、b与围压和加筋层数之间分别存在线性关系和指数关系,基于此提出适用于不同围压、不同加筋层数下环形立体组合加筋砾性土的骨干曲线方程。     

Preparation and characterization of 3D porous ceramic scaffolds based on portland cement for bone tissue engineering

There is a constant need for bone substitutes. This work was focused on developing a porous substrate based on Portland cement with air-voids introduced by outgassing reaction product from lime and aluminum powder. The structures were obtained through two routes of raw-materials and processing. Water absorption and compressive strength measurements and scanning electron microscopy, X-ray diffraction, and Fourier Transformed Infrared Spectroscopy assays were conducted in order to characterize the porous substrates. The substrates have shown pore size structure compatible with bone tissue colonization. Also, the mechanical strength exhibited by the scaffolds fall in the normal ranges for trabecular bone. These characteristics indicate potential use of the developed porous scaffold for bone tissue engineering which was endorsed by in vitro experiments via cell culture.     

A study of the interaction between a guardrail post and soil during quasi-static and dynamic loading

A roadside guardrail system is anchored in gravel beside a roadway to eliminate the risk of fatal accidents during off-road crashes and collisions with hazardous roadside objects. The desired safety behaviour is ensured not only by the guardrail structure itself, but also by the interaction between the gravel and the guardrail post. The interaction of gravel with a Sigma-post of a standard Swedish guardrail was studied in experiments and numerical analysis. The aim was to measure the strength of the single post embedded in gravel and use the data to validate a computer model for the investigation of the soil–post interaction. A quasi-static and dynamic test series were designed and carried out. Two corridors were formed by the test data for the quasi-static and dynamic loading conditions, respectively. A parametric study was subsequently conducted to investigate the influence of the gravel stiffness on the soil–post interaction through computer simulations using LS-DYNA. The numerical results showed that the LS-DYNA soil and concrete model and the Cowper–Symonds steel model effectively captured the soil–post interaction since the calculated strength of the post agreed with the corridors of the test data. The input parameters for the soil and concrete material model were recommended for roadside gravel in crash analyses.     

Semi-probabilistic design of rockfall protection layers

Increasing rockfall activity in the European Alps raises the need for designing systems protecting Alpine infrastructure. So far, layout of rockfall protection layers was carried out in a quasi-deterministic manner. This paper is concerned with the extension towards a semi-probabilistic design of the thickness of gravel layers covering steel pipelines. Quantities with little scatter such as geometric dimensions and elasto-plastic material constants of steel and gravel are treated as deterministic. By contrast, strongly scattering quantities such as the indentation resistance of gravel, R, and rockfall characteristics including boulder mass m and height of fall h _f are considered as probabilistic variables. While 5 and 95% quantiles of R (obtained from statistical evaluation of a series of real-scale impact tests onto gravel) represent probability-based interval bounds for designing the gravel layer thickness, the lack of statistical data from rare rockfall events motivates to follow the philosophy of EUROCODE 1, i.e., to define a design rockfall: m = 10,500 kg and h _f = 80 m. Based on this input, a standard burying depth of steel pipelines (H = 1 m) is assessed, by comparing estimates of (i) boulder penetration depth into gravel and of (ii) the maximum impact force, respectively, with corresponding quantities related to a suitable real-scale impact test. This comparison shows the need to increase the height of the gravel overburden. In order to prove that a gravel layer thickness H = 2.7 m is sufficient to prevent the pipeline from inelastic deformations when the structure is hit by the design rockfall, several structural analyses with different values for R are carried out. This is done by means of a validated Finite Element model. As a by-product of the proposed semi-probabilistic design procedure, three different deformation modes of the hit pipeline are identified.     

Strength Evaluation of Concrete Made with Dug-up Gravel from Southwestern Nigeria

Large expanses of natural deposits of gravel and sand are known to be available in many parts of the Southwestern Nigeria, which are being used for concrete production but with no standards. This study investigates the effects of using raw and washed dug-up gravel on the strength properties of concrete with a view to provide some standards for their usage. Field survey of quarry sites around Ile-Ife identified two commonly used dug-up gravel (Tiwantiwa), gravel A and (Majeroku), gravel B, which were selected for investigation. Prescribed mix proportions (measurements by weight) of 1:2:4 (cement:fine aggregate:washed/raw gravel) and 1:6 (cement:raw gravel) were used to produce the concrete cubes and beams for compressive and flexural strength tests. The results showed that using gravel A in raw form without sand and in washed form with sand have almost the same compressive strength (CS), while washing gravel B before use and with sand improved the CS of concrete. At 28 days, the CSs of concretes made from raw with no sand (RNS) of gravels A and B were 17.07 and 12.47 N/mm², while those of raw with sand (RWS) and washed with sand (WWS) were 14.73 and 16.80 N/mm² for gravel A and 11.93 and 16.00 N/mm² for gravel B, respectively. Characteristic strength of 15 N/mm² is recommended for concrete made with RNS and WWS of gravel A, while 11 and 14 N/mm² are recommended for those of gravel B, respectively.     

Methods for threshold optimization for images collected from contrast enhanced concrete surfaces for air-void system characterization

Several automated procedures for the characterization of the air-void system of hardened concrete rely on a contrast enhancement step to make air-voids appear white and aggregates and paste appear black. Pixels in the digital image darker than a selected threshold level are classified as non-air, pixels brighter are classified as air. Laboratories that perform air-void testing typically have a large number of samples with corresponding results from manual operators. Proponents of automated methods often take advantage of this fact by analyzing the same samples and then comparing results. A similar iterative approach is described here where scanned images collected from a significant number of samples are analyzed and the threshold optimized to best approximate the results of the manual operator. The results of this calibration procedure are compared to an alternative approach based on more rigorous digital image accuracy assessment methods employed by the remote sensing/satellite imaging community.     

强夯沉降模型及参数特征分析研究

基于承德机场高填方土石混合料的大型强夯试验,采用激光扫描、3D建模的手段,得出单击夯沉量与夯击次数呈指数型衰减的关系,并将其分为快速下沉段和平缓发展段;由此构建3参数强夯沉降量模型;根据参数演化规律,定义α为起始夯沉量因素参数,β为待加固区的场地因素参数,δ为稳定夯沉量参数;将该模型推广至涵盖碎石土、砂土、黏土、粉土等类型地基加固中均有较好的适用性,并得出模型参数取值与场地条件的相关规律。该模型的运用能够指导强夯的设计及辅助施工效果检测,具有较强的工程实际价值。     

糯扎渡高心墙堆石坝坝料特性研究及填筑质量检测方法和实时监控关键技术

系统研究了糯扎渡水电站高心墙堆石坝坝料特性,提出了静动力本构模型修正方法和水力劈裂发生的物理机制;研究了掺砾土料的压实特性、压实标准及填筑质量的检测方法;采用PDA、GPS、GPRS技术,通过传输网络,实现大坝填筑碾压各项参数的全面、实时、在线、自动监控的新技术。     

重型设备-桩箱基础-砂卵石地基体系动力稳定性振动台试验

设计并完成了重型设备-桩箱基础-砂卵石地基体系地震反应的大型振动台模型试验,分析了设备结构位移和转角反应特征以及桩箱基础和砂卵石地基的动力稳定性。试验结果表明:设备结构位移反应峰值呈上大、下小的三角形分布,位移和转角反应受结构振动方向第一振型控制;振动过程中基底两侧土反力增量不均,由上部重型结构水平惯性力引起的基础转动效应明显,这也是基础发生倾斜的主要原因;箱形基础振动对周围地基土产生挤密作用,基础转动显著影响侧墙土压力分布形态;强震作用下,砂卵石地基持力层振动孔压比能达到0.3~0.5,振动孔压的累积和消散可引起基础附加沉降。试验结果可为砂卵石地基箱形基础抗震设计提供参考和借鉴。     

Incipient motion of gravel and coal beds

An experimental study on incipient motion of gravel and coal beds under unidirectional steady-uniform flow is presented. Experiments were carried out in a flume with various sizes of gravel and coal samples. The critical bed shear stresses for the experimental runs determined using side-wall correction show considerable disagreement with the standard curves. The characteristic parameters affecting the incipient motion of particles in rough-turbulent regime, identified based on physical reasoning and dimensional analysis, are the Shields parameter, particle Froude number, non-dimensional particle diameter and non-dimensional flow depth. Equations of critical bed shear stress for the initial movement of gravel and coal beds were obtained using experimental data. The method of application of critical bed shear stress equations is also mentioned.     

多桩型组合桩复合地基承载力的可靠度分析

基于优化桩型的观点,将柔性桩和刚性桩组合,刚性桩和散体材料桩组合,形成多桩型组合桩复合地基,充分发挥了柔性桩、刚性桩和散体材料桩的优点,弥补了单一桩型在地基处理中的不足。通过对现有多桩型组合桩复合地基承载力计算方法进行分析,建立了多桩型组合桩复合地基极限承载力可靠度概率分析模型。用JC法进行可靠度计算。通过刚柔组合桩复合地基实例,分析了随机变量的变异性对可靠性指标影响。     

风沙两相流条件下沟槽形态对混凝土壁面抗风蚀磨损性能的影响

采用RNGk-ε湍流模型结合DPM离散相模型,在不改变材料性能条件下数值模拟了混凝土壁面的抗风蚀磨损性能,并进一步分析了槽壁倾斜角度和槽体深度变化对壁面抗风蚀磨损性能的影响。研究结果表明:在低冲蚀角度条件下,沟槽形态壁面较易出现风蚀破坏的集聚分布,而在高冲蚀角度条件下,壁面整体的抗风蚀磨损性能得到提升;槽壁倾角的改变会影响沟槽轮廓对槽体区域的遮蔽效能和沙砾首次撞击该区域的角度,进而改变壁面抗风蚀磨损性能;沟槽倾斜角度一定时,沟槽加深可增大沟槽区域占壁面面积的比率,从而增强壁面整体的抗风蚀磨损性能,但此手段不能有效扩展用于沙砾减速降能的低速流场。     

Nonintrusive coupling of 3D and 2D laminated composite models based on finite element 3D recovery

In order to simulate the mechanical behavior of large structures assembled from thin composite panels, we propose a coupling technique, which substitutes local 3D models for the global plate model in the critical zones where plate modeling is inadequate. The transition from 3D to 2D is based on stress and displacement distributions associated with Saint‐Venant problems, which are precalculated automatically for a simple 3D cell. The hybrid plate/3D model is obtained after convergence of a series of iterations between a global plate model of the structure and localized 3D models of the critical zones. This technique is nonintrusive because the global calculations can be carried out using commercial software. Evaluation tests show that convergence is fast and that the resulting hybrid model is very close to a full 3D model. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluation of the compactability of bituminous emulsion mixes: experimental device and methodology

Laboratory asphalt mix design methodologies are aimed at predicting behaviour and determining characteristics of mixes. The French laboratory methodology for bituminous mixes is partially devoted to a compactability assessment. Samples are characterised by tests using the gyratory compactor. In this test, loose material is submitted to simultaneous compressive and shear forces, which lead to an internal aggregate reorganisation. The current French experimental device, called ‘PCG3’, was devoted strictly to hot mix asphalt characterisation. At present however, compactability properties of emulsion-treated gravel also appear as critical information due to the growing interest in bituminous cold mixes within an environmentally friendly context. A research project has been conducted in order to adapt the current ‘PCG3’ to allow for the compaction of cold mixes by adding the functionality of collecting extruded water due to emulsion breaking. This article is intended to describe this new device and determine the influence from adjusting parametric sensitivity on gravel treated with emulsion and compactability behaviour.