Towards a model of concrete mesostructure

This short paper will present a two-dimensional (2D) model of concrete material, based on probabilistic models: it is a combination of a Voronoi tessellation for the gravel, followed by a Boolean model of spherical grains for the air-voids, with elimination of the air-voids–gravel intersections so that there is no contact between them. The model was tested via crossed-covariance. Results on true and simulated structures are in good agreement. This is a first step towards a 3D model.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

爆炸冲击波通过砾石层衰减规律的试验研究

采用激波管对不同长度、不同空隙率的砾石层消减激波的作用进行了试验研究,并在野外地下坑道进行了化爆试验,两种试验得出了规律性一致的结论。大压力激波衰减更快;砾石层的长度和空隙率对激波的衰减有显著的影响。根据试验数据获得了确定砾石层长度的计算公式。     

爆炸冲击波通过砾石层衰减规律的试验研究

采用激波管对不同长度、不同空隙率的砾石层消减激波的作用进行了试验研究,并在野外地下坑道进行了化爆试验,两种试验得出了规律性一致的结论。大压力激波衰减更快;砾石层的长度和空隙率对激波的衰减有显著的影响。根据试验数据获得了确定砾石层长度的计算公式。     

Evaluation of optimal dispersion conditions for CNT reinforced epoxy composites using cyclic voltammetry measurements

The optimum dispersion time of nanoparticles is important for obtaining uniform dispersion of fillers or other additives in a matrix. In this study, the optimal dispersion time of carbon nanotube (CNT) in a matrix was investigated using cyclic voltammetry (CV), measurement for different dispersion methods and times. In addition, the mechanical properties of CNT composites manufactured using different dispersion methods were evaluated by tensile and flexural tests. The CV and mechanical test results were correlated to the dispersion condition of CNT in the composites. It was found that tip-type sonication resulted in better dispersion than bath-type sonication. Improved CNT dispersion resulted in composites with both enhanced CV measurements and improved mechanical properties. In the study reported here, improvements in dispersion were generally accompanied by higher electrical currents. This suggests that the CV measurement method is an effective tool for determining optimal dispersion times, for different CNT dispersion processes.     

Optimum refractive-index profile of the graded-index polymer optical fiber, toward gigabit data links

The optimum refractive-index distribution of the high-bandwidth graded-index polymer optical fiber (POP) was clarified for the first time by consideration of both modal and material dispersions. The ultimate bandwidth achieved by the POP is investigated by a quantitative estimation of the material dispersion as well as the modal dispersion. The results indicate that even if the refractive-index distribution is tightly controlled, the bandwidth of the graded-index POP is dominated by the material dispersion when the required bit rate becomes larger than a few gigabits per second. It is also confirmed that the material dispersion strongly depends on the matrix polymer and that the use of a fluorinated polymer whose material dispersion [-0.078 ns/(nm km)] is lower than that of poly(methyl methacrylate) [-0.305 ns/(nm km)] allows for a 10-Gb/s signal transmission.     

三种地基处理方法提高某水闸基桩水平承载力效果对比研究

工程建设中经常会遇到软弱地基土，它们必须经过处理才能满足强度和变形的要求。结合曹娥江大闸枢纽工程，对三种地基处理方法下（碎石振动碾压法、振动碾压法和振捣法）基桩的受力特性进行了分析。通过单桩水平静载荷试验得出结论，三种方法对桩水平承载力都有很大的提高，特别是碎石振动碾压法和振动碾压法的效果最显著。     

分散聚合法制备二氧化硅/ 聚苯乙烯单分散复合微球

在醇/ 水混合介质中用分散聚合方法制备二氧化硅/ 聚苯乙烯单分散复合微球。探讨了分散聚合法制备无机/ 有机复合微球的过程, 考察了分散聚合反应的动力学过程。SEM 和TEM 观察结果表明: 在醇/ 水混合介质中, 用PVP K-30 作为稳定剂, 用分散聚合法制备了单分散的以二氧化硅为核、聚苯乙烯为壳的复合微球,成功实现了聚苯乙烯对二氧化硅的包覆, 复合微球的平均粒径为1.25μm , 分散系数ε为0.035 , 达到了单分散水平。还考察了稳定剂的浓度对复合微球粒径的影响, 当稳定剂PVP K-30 浓度增加时, 发现微球的粒径有所减小, 而微球的单分散性提高。      

The suitability of periwinkle shells as coarse aggregate for structural concrete

The results of an exploratory investigation on the suitability of periwinkle shells as coarse aggregate for concrete are reported. The tests show that the strength of concrete made with periwinkle shells are limited by the strength of the shells but high enough for structural concrete. The modulus of elasticity and modulus of rupture are low compared with normal gravel concrete. While the strength of reinforced members in direct compression is low, flexural members have satisfactory strength but with larger deformation than normal gravel concrete.     

Dispersion properties of inhomogeneous nanostructures

We analyze the dispersive properties of inhomogeneous nanostructures (INSs) composed of alternating layers of different materials. Analysis of the interaction between the propagating pulse and the INS provides modified dispersion characteristics. An approximate theoretical model predicting the dispersion properties of the INS is developed and compared with more accurate numeric computation results. It is shown that the dispersion coefficient can be engineered by controlling the spatial distribution of the pulse carrier, the geometry of the INS, and the refractive indices of the materials combined to construct the INS. Specifically, the dispersion coefficient can be engineered to yield various types of dispersion, including normal dispersion, anomalous dispersion, and zero dispersion. As such, the discussed INS can be useful for applications that will benefit from engineered dispersion management and control.     

Magnetic orientation of steel fibres in self-compacting concrete beams: Effect on failure behaviour

The magnetic orientation of steel fibres in transparent silicone oil and in fresh, self-compacting concrete (SCC) beams is studied experimentally. The effect of the generated fibre locations and orientations on the failure response of the SCC beams is determined by means of three-point bend tests. A relatively small coil was designed for the magnetic orientation of single and multiple fibres in the transparent silicone oil. The time required for orienting a single fibre was measured for a range of magnetic fluxes, which showed to strongly decrease with increasing magnetic field strength. The presence of gravel on the fibre orientation behaviour was considered in order to mimic the influence by a concrete aggregate, indicating that the gravel does not prevent rotations and chain formations of fibres. A larger coil was developed for the magnetic orientation of fibres in freshly casted SCC beams. The energy absorption capacity of SCC beams subjected to three-point bending scales approximately proportionally with the number of “well-oriented fibres” bridging the catastrophic failure crack, which emphasizes the importance of adequately orienting steel fibres with the magnetic orientation technique.     

激波驱动下硝酸异丙酯的分散变化规律研究

研究液滴的分散过程对于阻止火灾蔓延、提高内燃机效率、改进云雾爆轰武器和提高云雾爆轰控制技术有着重要的作用。通过高速摄影技术以及压力测量系统,着重研究液态燃料硝酸异丙酯（IPN）的分散过程,分析液膜厚度和激波强度对IPN液膜分散的影响。IPN液膜初始阶段以水平方向的分散为主;随后,以竖直方向的分散为主。水平方向,液膜抛洒先进入减速阶段,随着液膜厚度H的增加,液膜分散效果变差,分散需要的时间更长,其分散表征与石油醚相似。IPN液膜分散所需能量要高于石油醚。激波强度超过某个值之后,超压比ε随液膜厚度H呈线性变化,可以为液膜分散提供足够的能量。当H＜12 mm时,IPN液膜的分散变化过程主要受激波强度影响;当H＞12 mm时,IPN液膜的分散变化过程主要受液膜厚度影响。