Effect of the presence and size of a real macro-crack on diffuse ultrasound in concrete

In civil engineering structures, the first layer of concrete plays a role in terms of water-tightness, and contributes to the protection of metal frames from corrosion by external chemical agents (water, CO₂, chlorine etc.). In concrete, a crack is comprised of an external part with a surface opening, and a completely or partially closed part below the surface. The aim of this paper is to identify the contribution of a real macro-crack on the diffuse ultrasound parameters. The methods ability to characterize the open part of the crack is confirmed. The results reveal the influence of the closed part of the crack, with regions in partial contacts, on the diffusion parameters.     

田坪特大桥桩基础混凝土力学特性的数值模拟研究

桥梁桩基础混凝土的承载能力是桥梁安全运营的关键.文章以田坪特大桥为工程背景,利用颗粒流数值模拟方法对该桥的桩基混凝土进行双轴压缩数值模拟研究,并通过裂隙体积应变法计算了不同加载速率下混凝土力学特性.结果表明:加载速率对混凝土的力学特性影响显著.随着加载速率的逐渐增大,混凝土的峰值强度、弹性模量均呈逐渐增大趋势,而泊松比...     

W粉粒度对CuW/Cu整体触头结合强度的影响

用自行设计的试样结构研究了W粉粒度对CuW整体触头结合强度的影响.实验发现：随W粉粒度的降低,CuW整体触头的结合强度增加,其机理可用抑制CuW整体触头界面裂纹扩展的W颗粒数目增多解释.     

Gerissener Stahlbeton: Wie korrosionsgefährdet ist die Bewehrung?

Cracked reinforced concrete: What about corrosion risk? The formation of cracks within the tension zone of the concrete is part of the design principle of reinforced concrete structures. The width of these cracks can be limited by means of crack limiting reinforcement. Already by the end of the fifties first examinations have been carried out to determine the influence of cracks and their width on the corrosion risk. Compared to uncracked concrete CO₂ and chlorides as corrosion promoting substances are able to move faster towards the reinforcement within a crack, leading to an earlier depassivation of the reinforcement within the cracks. Before Schießl published his results, from experiments and literature research, it had been assumed that the corrosion rate of reinforcement within cracks increases with increasing crack width. Hence a differentiated maximum crack width linked to exposure classes had to be complied according to the DIN 1045 until the version of 1988. For the first time the DIN 1045 version 1988 did not enclose this differentiation, for durability reasons a maximum crack width of 0.25 mm has been fixed for reinforced concrete. In the version of 2001 the calculated crack width has been raised from 0.25 mm to 0.30 mm. Therewith the findings of Schießl (published in book no. 370 by the Deutscher Ausschuss für Stahlbeton) have been implemented by now. The main contributing factors influencing the corrosion rate of cracked concrete are the concrete quality and the concrete cover and not the crack width, whereas horizontal structural parts exposed to chlorides have to protected generally by additional measures.     

Vertical spectral tomography of concrete structures based on impact echo depth spectra

This paper proposes a vertical spectral tomography for use in nondestructive testing of concrete structures. Impact echo tests are conducted along a test line on the concrete surface. Then, the response curves are transformed into depth spectra. Arranging the depth spectra in parallel and representing the amplitude by color scale, one obtains an image of the vertical cross-section under the test line. Three types of spectral tomograms are proposed. Numerical and experimental tests are performed to verify and compare the tomograms. Although different types of tomograms possess different attributes, all three tomograms can depict the internal crack successfully. The internal crack is represented by bright stripes in the tomograms, which can be used to estimate the size and location of the crack. The vertical spectral tomogram provides a direct image of the concrete cross-section. It may serve as a useful tool in the nondestructive inspection of concrete structures.     

Crack shape and rust distribution in corrosion-induced cracking concrete

This study investigated a reinforced concrete specimen that had deteriorated in an artificial environment for 2 years. The crack width and the rust distribution were observed by digital microscopy. The variation of the total circumferential crack width along the radial direction is presented using a linear function. Observation reveals that rust does not penetrate into the corrosion-induced cracks before concrete surface cracking. After concrete surface cracking, rust fills the cracks, lining the edges of the cracks due to the circulation of the outer solution. A schematic diagram is proposed to describe crack propagation and rust development.     

Scan of surface-opening cracks in reinforced concrete using transient elastic waves

This study develops a method to scan the surface cracks of reinforced concrete using transient elastic wave tests. In the tests, an impact is applied at constant intervals along one side of the crack opening, and the surface response of the concrete due to each impact is measured and recorded. Then the procedure is repeated with the source applied on the opposite side. The method of ellipse intersection is adopted to process the surface response of the concrete structure. An image is then constructed that shows the three-dimensional image of the crack. The image can be used to determine the location of the reinforcing steel bars, the thickness of the covering, and to judge whether the crack penetrates through the rebars. Numerical examples and a model test are presented to verify the effectiveness of this method.     

Modelling of interaction between corrosion-induced concrete cover crack and steel corrosion rate

Chloride-induced corrosion of steel reinforcement in concrete may cause severe damage to RC structures. This paper examines the interaction between corrosion-induced cover crack growth and corrosion propagation. The coupled micro- and macro-cell corrosion process involved in a typical chloride-induced corrosion is numerically simulated. Both oxygen concentration and electrical potential distribution within concrete cover are considered in the electrochemical analysis. A uniform thick-walled cylinder model is formulated to simulate the cover surface crack width evolution. Results show that macrocell corrosion rate may not change so much while microcell corrosion rate increases a lot as oxygen permeability increases with corrosion-induced cover crack.     

Investigation of steel corrosion in cracked concrete: Evaluation of macrocell and microcell rates using Tafel polarization response

Inhomogeneous corrosion in reinforced concrete is investigated using a beam with a flexural crack intersecting the reinforcement. An Evans diagram representation of the macrocell corrosion system is developed. The relationship between the current density and the potentials relative to the crack obtained from the Tafel polarization responses of active and passive steel in concrete compares favorably with the experimental values. When both microcell and macrocell mechanisms contribute to metal loss at the crack, the Evans diagram representation indicates that an increase in the macrocell current density results in a decreasing contribution from the local microcell at the macrocell anode.     

Characterization of surface cracks with Rayleigh waves: a numerical model

In the non-destructive testing of concrete structures, the use of Rayleigh waves shows some advantages to characterise surface cracks: easiness of excitation and recording, access to only one surface of the structure required, great spectral sensitivity to the propagation medium…. But the behaviour of Rayleigh waves on surface defects in concrete is difficult to perceive in the field, even if the dependence of the diffraction pattern on the crack's geometrical features is significant.A numerical model is adapted from earth physics in order to better understand the influence of the crack geometry on Rayleigh-wave propagation. This model, based on an indirect boundary element method, calculates the three-dimensional seismic response of two-dimensional structures. Synthetic seismograms are obtained for the propagation of a Rayleigh wave across various crack geometries. The variations of spectral ratios between the transmitted and incident waves are studied as a function of the crack depth. They are used to design an efficient procedure for the determination of crack depths.     

Elastic and Conductive Properties of Plasma-Sprayed Ceramic Coatings in Relation to Their Microstructure: An Overview

This article focuses on micromechanics-based models that explicitly express the elastic and conductive properties of plasma-sprayed ceramic coatings in terms of relevant microstructural parameters. These parameters reflect, in an integral way, the density and the orientation distribution of microcracks; they apply to strongly oblate pores as well. On the other hand, the porosity parameter usually plays a secondary role. Partial contacts between crack faces—a factor of major importance—are reflected via appropriate reduction of crack densities. The effect of various “irregularities” of crack shapes is discussed. Case studies of YSZ coatings demonstrate how the micromechanics-based modeling can be used and directly interfaced with 2-D image data.     

3D Numerical modelling of steel corrosion in concrete structures

The paper deals with a 3D numerical model for transient analysis of processes after depassivation of reinforcement in concrete, which are relevant for calculation of corrosion rate. The aim of the study is to investigate the influence of the concrete quality, cracking and water saturation in concrete on the current density. The results show that the corrosion rate is higher in poor quality concrete than in good quality concrete. The model predicts that cracks do not influence corrosion rate for the case where the only influence of the crack is on the rate at which oxygen can reach the steel.     

Einfluß einer Feuerverzinkung und PVC-Beschichtung von Bewehrungsstählen und von Inhibitoren auf die Korrosion von Stahl in gerissenem Beton

Influence of galvanizing and PVC-coating of reinforcing steels and of inhibitors on steel corrosion in cracked concrete Cracked concrete beams of concrete quality B15 and B35 with carbonized cracks were exposed in artificial seawater, under frost and deicing salt conditions and in industrial climate. The reinforcement was composed of black steel, galvanized and PVC-coated steel. For the purpose of additional protection the concrete was partly mixed with an inhibitor Ca(NO₂)₂. The concrete cover was 1,5 till 5,0 cm. It was found that the inhibitor protects satisfactorily only in the case of the higher concrete quality, cover 1,5 cm and not to high crack width. A protective effect of galvanizing is given by not to high chloride contents (< 1,5% relative to cement) and crack widths. The chosen PVC-coating failed because of chemical instability in the alcaline medium concrete.     

基于阵列超声成像法的混凝土裂缝深度检测

针对传统超声法检测混凝土裂缝深度时人为影响因素较多,检测效率十分低下的问题,引入了阵列超声成像法,研究了阵列超声成像法检测裂缝深度的原理及其计算方法,并在实验室设计制作了一块预设7条不同深度裂缝的素混凝土试件,进行了跨单裂缝和试件整体成像检测试验。试验结果表明:阵列超声成像法根据缝底反射超声波的传播时间来检测裂缝深度,生成的二维图像可以显示缝底位置,但无法显示缝身;该检测法的检测精度较高,并且随着裂缝深度的加深逐渐提高;检测时应考虑边界效应,否则无法准确检测裂缝深度;检测时存在盲区,当缝底位于盲区内时会出现漏检。     

混凝土裂缝深度超声波检测方法

介绍裂缝深度常用超声检测方法,研究裂缝深度检测新方法,如相位变化法、横波法和冲击回波法等。     

Signal processing procedure for non-destructive test of concrete structure integrity

A signal processing procedure is proposed for testing concrete integrity using impact by shock wave and Laser Doppler velocimetry. The concrete surface vibration signal is first segmented into successive frames. The signal in each frame is analyzed using a linear prediction model to calculate the normalized prediction error. This error indicates the degree to which the signal is modeled in the form of an exponentially decaying sinusoid, and the presence of sub-surface flaws in the concrete can be determined from its value. When a defect in the concrete is detected, the frequency of the decaying sinusoid is calculated to estimate the depth and size of the crack. The proposed procedure was applied to computer-generated signals and actual vibration signals produced by shock waves, and the success of the procedure was verified.     

A comparative study of two non-destructive testing methods to assess near-surface mechanical damage in concrete structures

Concrete specimens displaying surface-breaking cracks are tested using two non-destructive testing techniques: the measurement of electrical resistivity and the transmission of ultrasonic surface waves. The capacity of each method to detect, localize and characterize the induced crack pattern—i.e. its width and depth—is investigated. The specimens are made of concrete with three different mixes. A major crack is induced in the middle of the specimen using a three point bending setup. As the bending strength is increased, three different loading levels are obtained. Both methods were able to correctly localize the main crack and follow its evolution. However, due to the complex crack pattern, the crack depth could not be estimated. Nevertheless, the ultrasonic technique was able to detect a change of state of the material before cracks became visible. As the crack intensity increases, variations of ultrasonic parameters in the apparently undamaged area surrounding the main crack were observed, thus highlighting the sensitivity of ultrasonic waves to distributed damage. The obtained results have also thrown bases for further research to couple resistivity and ultrasonic non-destructive methods.     

油田地面建筑工程混凝土裂缝的原因分析

最近的几年中我国的经济发展速度越来越快,油气资源的需求量也越来越大,油气资源的开采离不开地面建筑工程,油田地面建筑工程质量又和混凝土质量有着直接的影响。本文主要阐述了油田地面建筑工程混凝土裂缝产生的原因,以及油田地面建筑工程混凝土裂缝的预防措施及处理措施分析,促进我国油田地面建筑工程的可持续发展。     

从第15届世界无损检测会议看土木工程无损检测的进展

简要介绍国外土木工程无损检测的进展，包括超声仪器发展、冲击回波法研究，空气耦合超声检测，混凝土裂缝检测以及高强度混凝土检测等。     

Electrical resistivity measurement applied to cracking assessment on reinforced concrete structures in civil engineering

Non-destructive evaluation appears more and more important in the civil engineering economic stakes. In this context electrical resistivity measurements get sensitivity to parameters allowing to assess concrete structures conditions. This article analyses the ability of the resistivity measurement to study cracks in concrete. Its ability to detect and to locate cracks and spalling is shown with on site measurements on a damaged slab. Then specific studies on such disorders allow to distinguish the influence of their characteristics.The sensitivity of the method to cracks depth, according their moisture conditions, is assessed by computation. Experimental works on a reinforced concrete beam, ideally cracked, confirmed these results.Some assumptions based on measurements realised on a size-one structural component allow to assess the general influence of crack opening and bridging degree between crack lips.Qualitative results show the similar effects on measurement of various cracking parameters. Prospective works presented in the paper lead us to say that electrical resistivity method applied to civil engineering structures is a relevant tool for the assessment of structural damage.