Understanding depth-amplitude effects in assessment of GPR data from concrete bridge decks

The variation of concrete cover thickness on bridge decks has been observed to significantly affect the rebar reflection amplitude of the ground penetrating radar signal. Several depth correction approaches have been previously proposed in which it is assumed that, for any bridge, at least a portion of the deck area is sound concrete. The 90th percentile linear regression is a commonly used procedure to extract the depth-amplitude relationship of the assumed sound concrete. It is recommended herein that normalizing the depth-dependent amplitudes be divided into two components. The first component takes into account the geometric loss due to inverse-square effect and the dielectric loss caused by the dissipation of electromagnetic energy in sound concrete. The second component is the conductive loss as a result of increased free charges associated with concrete deterioration. Whereas the conventional depth correction techniques do not clearly differentiate the two components and tend to incorporate both in the regression line, they are separately addressed in this research. Specifically, while the first component was accounted for based on a library of GPR signals collected from sound areas of twenty four bare concrete bridge decks, the conductive loss caused by an increased conductivity is linearly normalized by the two-way travel time. The implementation of the proposed method in two case studies showed that, while the method significantly improves the accuracy of GPR data analysis, the conventional methods may lead to a loss of information regarding the background attenuation that would indicate the overall deterioration of bridge decks.     

A novel method for FDTD numerical GPR imaging of arbitrary shapes based on Fourier transform

Ground penetrating radar (GPR) systems are very powerful tools with a wide range of advantages in non-destructive testing. Target detection is one of the serious results of GPR application. The landmine detection in the soil and reinforcement bars and holes in the concrete is some of the GPR object detection examples. In this research, the targets are modeled in different cases, an air hole or a conductive material. A cracked surface is also modeled for both conductive and dielectric media. For modeling, the three-dimensional (3D) finite difference time-domain (FDTD) method is applied. Perfectly matched layer (PML) absorbing boundary condition is used for simulating the physical absorbers and free space. The results were shown in time domain. The difference between results helps to distinguish the target depth and electrical properties. For better specification of the target shape, a transform in frequency domain is used. This transform contains a Fourier transform in a selected frequency. The results show a sudden change in the frequency response over the hole or target scanning. By this idea, the shape of any arbitrary hole or crack can be extracted.     

GPR signal de-noising by discrete wavelet transform

Ground penetrating radar (GPR) is a non-destructive investigation tool used for several applications related to civil infrastructures; including buried objects detection and structural condition evaluation. Although GPR can be effectively used to survey structures, signal analysis can be sometimes challenging. The GPR signals can be easily corrupted by noise because the GPR receiver has usually an ultra-wide bandwidth (UWB). The noise collected by the system can easily mask relatively weak reflections resulting from the inhomogeneities within the surveyed structure; especially when they are at a relatively deep location. This paper presents the use of discrete wavelet transform (DWT) to de-noise the GPR signals. Various mother wavelets were used in this study to de-noise experimental GPR signals collected from flexible pavements. The performance of wavelet de-noising was evaluated by computing the signal-to-noise ratio (SNR) and the normalized root-mean-square error (NRMSE) after de-noising. The study found that wavelet de-noising approach outperforms traditional frequency filters such as the elliptic filter. At the same level of decomposition, the Daubechies order 6 and Symlet order 6 outperform the Haar and Biorthogonal mother wavelets when de-noising GPR signals by soft thresholding.     

Simulation of direct wave propagation by numerical FDTD for a GPR coupled antenna

This article describes a numerical model of a GPR antenna used in the context of civil engineering to test structures. The model proposed was obtained by comparisons between simulations and experiments made with different cementitious materials. This model does not represent the real sensors exactly because some components are not known due in part to the manufacturing secrets, but it gives a good approach to the fields measured in the experiments.     

Time-frequency analysis of enhanced GPR detection of RF tagged buried plastic pipes

Ground Penetrating Radar (GPR) is a well-received non-destructive technique for the detection of underground utilities, such as water/gas pipes, sewers, power cables, and telecommunication ducts. However, radar signatures of plastic pipes are generally weak when the surrounding soils are attenuative and/or the pipe and soils have similar electromagnetic characteristics. In order to increase the radar visibility of these pipes, attaching Radio-Frequency (RF) tags to them is a useful method. In this paper we designed two types of Bowtie-Omega shaped RF tags. The finite element method (FEM) simulation and measurement of the designed tags show strong resonances in the GPR spectrum, and by conducting GPR B-scans stronger radar signatures are observed when the RF tags are buried in our 2.0$\times$1.5$\times$0.75 m tank filled with dry sand. Furthermore, we implement a simple processing procedure based on Short-time Fourier Transform (STFT), by which strong time-frequency domain response of the tags is clearly seen at those designed resonant frequencies, and disappear when tags are not inserted. The resulting detection of the hollow plastic pipe in both time and frequency domains due to the co-located tag is evidently enhanced.     

Use of electromagnetic two-layer wave-guided propagation in the GPR frequency range to characterize water transfer in concrete

The objective of this paper is to adapt a recent innovative technique for extracting and exploiting the Electromagnetic (EM) waveguide dispersion of civil engineering materials by means of GPR, and allowing to monitor the water ingress front during the absorption process for various concrete mixes. This technique is based on an inversion procedure that applies the Electromagnetic Waveguide Model (WGM) to invert phase velocity dispersion curves in their modal form. A Parallel homogenization model, derived from the Lichtenecker-Rother equation, has been employed to extend the waveguide model from a one-layer to a two-layer medium. The WGM outputs are then used to estimate the geometric parameters of the propagation medium and offer a primary application to water transfer monitoring in concrete through capillarity effects. The initial WGM validation is carried out on FDTD-simulated propagation signals, while the second validation relies on GPR data acquired from homogeneous materials. Then, a broad-based experimental study is conducted for the purpose of correlating electromagnetic waveguide dispersion parameters with both the geometric and hydric characteristics of various concrete mixes. Results obtained using the two-layer WGM serve to monitor the water ingress front during an absorption process. These results are then compared to the moisture gradients generated on cores using gammadensimetry, which is set as the reference. This procedure yields a number of trends, which in turn provide key information on the conditioning state of the studied concretes.     

GPR evaluation of the damage found in the reinforced concrete base of a block of flats: A case study

Ground-penetrating radar (GPR) is a non-destructive geophysical technique that is used to analyse soils and structures by providing continuous images of the interior of the media being analysed. In this paper, we present the methodology and results of a study of the reinforced concrete base of a large block of flats, where flow water and flooding are common in the wet and dry seasons. There are also a large number of cracks and fissures in the soil under the base. The damaged building is located in Valencia (Spain). The measurements were taken during two different seasons to include a period when flooding occurred and a period without flooding. The aim of the study was to determine whether the base would remain watertight in future and to analyse the depth of the damage observed on the surface. The results provided us with a detailed map of the damage and the zones that require more detailed analysis. Such damage affects the whole concrete slab. In addition to this analysis, several measurements were carried out to determine the possible causes of the inundations by locating potential wet areas and zones with high water content. A water table and a well were detected, which could be the cause of the humidity damage.     

Effect of water and chloride contents and carbonation on the electromagnetic characterization of concretes on the GPR frequency band through designs of experiment

Water content, chloride content and carbonation are three durability indicators that affect not only concrete properties and integrity, but also the ground penetrating radar (GPR) response. This article characterizes the effect of water and chloride contents as well as carbonation on the high-frequency electromagnetic (EM) properties of concretes using a coaxial-cylindrical cell operating within the large frequency band from 50 MHz to appprox. 1000 MHz, thus making possible to study dispersion effects on the complex relative permittivity of various concretes.During a second step, three specific designs of experiment (DoE), involving water and chloride contents, and then carbonation, are designed and studied in order to establish the relative sensitivity of these parameters compared to others in describing the mixes at three different frequencies.The complex dielectric properties are evaluated in the laboratory on many concrete specimens (numbering 24, 36 and 24 for the moisture, chloride and carbonation studies respectively). For these evaluations, the composition parameters consist of nature of aggregate, nature of cement, cement content and water-to-cement ratio (W/C) while conditions are described by degree of water saturation, chloride content or carbonation. In the case of the chloride and carbonation DoEs, the mixes are tested under both dry and saturated conditions.The models describing complex relative permittivity, in association with the designs of experiment, are multi-linear polynomials of the 4 or 5 identified engineering parameters, to be chosen depending on the study. The interpretation of datasets entails in independent calculation of model coefficients for the real and imaginary parts of relative permittivity, at the three chosen frequencies. This approach reveals the significant relationships existing between dielectric properties and composition parameters as well as their evolution versus frequency.The major results of this work can be summarized as follow:

• -The degree of water saturation amounts to about 80% on ε′_r variability and 60% to 15% versus frequency on ε″_r variability, for the first campaign.
• -The chloride content, while only visible in saturated mixes, represents approx. 40% and 50% with porosity (via the W/C ratio) accounting for some 30% and 40% on ε′_r and ε″_r variabilities, respectively.
• -Lastly, carbonation, while also only visible in saturated mixes, corresponds to a weight of about 14% in low frequencies and vanishes at high frequencies for ε′_r variability; this amount rises to roughly 24% for ε″_r variability.

     

The application of shakedown theory to pavement design

Shakedown theory for rate-independent materials has been successfully applied to discrete structures for many years, but has only recently been applied successfully to continua. A notable success is the use of the upper-bound theorem (Koiter’s theorem) to analyse different types of wear mechanisms of surfaces subjected to repeated sliding or rolling contacts. The present paper is concerned with the analogous geomechanics problem of analyzing wear mechanism of roads and pavements. In many of the less densely populated parts of the world pavements are of the “unbound type” where the top asphaltic layer is very thin, has no structural role in the response of the pavement, and serves only as a weatherproofing layer. In such pavements, the structure can be modelled as a rate-independent, pressure dependent, elastic-plastic material, using Mohr-Coulomb, critical state or other similar standard geomechanics model. The ongoing research described in this paper is concerned with computing the critical shakedown load associated with various failure mechanisms, such as subsurface and surface slip and rut formation. The optimal design, is obtained using various nonlinear optimization techniques including quasi-Newton and simulated annealing. Whilst the techniques involved have some resemblance to classical limit analysis methods, the optimal solutions are shown to be strikingly different.     

Ultrasonic imaging using signal post-processing for a flexible array transducer

In this study, we used a flexible array transducer to obtain images of flaws in materials with irregular surfaces. The transducer was manufactured from a 1–3 composite made of piezoelectric zirconate titanate and epoxy resin, and covered with neoprene rubber layers. An approach that involved combining a flexible array transducer and full matrix capture (FMC) is effective, because the surface geometry and flaw signal data can be separately measured and merged during post-processing. The disadvantage of the flexible array transducer is its narrow frequency band due to the thin damping material. To enhance the spatial resolution, we used the scattering amplitude extracted from raw signal data. We also introduced a numerical apodization technique to suppress the influence of side lobe. We validated the performance of the proposed method by measuring the signals from artificial flaws in aluminum specimens, performing high-speed FMC imaging using graphic processing unit computation.     

An application of thermography for detection of delaminations in airport pavements

Since delaminations in airport pavements can lead to failure, cost effective ways must be suggested to inspect the quality of pavements. One of these cost effective ways is the infrared thermography technique. This research paper covers the determination of delaminations in asphalt pavements situated at one of the busiest airports in Greece, the International Airport of Athens, by means of infrared thermography. The infrared thermographic technique is used efficiently in the detection of cracks, flaws and other imperfections appearing on airport pavements. The outcome of this investigation leads to the proposition of a forecasting non-destructive technique intended for assessment of efficient airport pavements.     

Field pattern characteristics of GPR antennas

Sub-surface radar has become increasingly popular for carrying out completely non-invasive integrity tests on concrete structures. Commercial systems are available with a range of antenna frequencies that may be selected for an investigation. The choice of a particular antenna frequency is often a compromise between the physical size of the antenna and the penetration and resolution capabilities provided by the antenna. However, there is an interaction between a surface contact antenna and the substrate under investigation that will alter the field pattern being transmitted that may have significant implications for interpretation of results. An experimental programme is described in which the characteristics of commercially available 900 MHz and 1 GHz antennas are compared in air and when in contact with concrete and water surfaces, together with an emulsion used in previous studies to simulate the radar properties of concrete. Results are presented showing the influence of the material being measured upon the signal divergence. These effects are considered in terms of the practical implications for field-testing using radar.     

三维曲面板类件的连续柔性成形技术研究

文章通过对三维曲面板类件的不同成形方法的比较与分析,介绍了该技术的成形原理,开发出基于多点调整式连续柔性成形原理的实验设备,并进行了三维曲面板类件的成形实验。实验结果表明,板类件横向形状(曲率)取决于柔性辊的弯曲形状,纵向形状(曲率)由上辊的压下量决定。得出结论:使用可弯曲柔性辊的连续柔性成形技术是一种成形三维曲面板类件快速而经济的方法。     

Estimating the hotmix asphalt air voids from ground penetrating radar

Traditionally, in-place air voids are obtained based on field cores. Coring is a destructive and time-consuming process. This study presents a high speed Non-Destructive Testing (NDT) technique with Ground Penetrating Radar (GPR) to characterize the in-place air voids. A total of 92 cores were retrieved from three field projects to establish relationship between the air voids and the measured dielectric by GPR. A statistical model was developed to express the air void value as a function of dielectric and other variables. Contour air void maps could also be produced for the entire pavement sections. The results from the underlying studies have been used as the basis for the repair strategy selections.     

In situ measurements of hot-mix asphalt dielectric properties

Twelve different flexible pavement sections, which comprised different layers/materials, are incorporated in the Virginia Smart Road test facility. These sections provide a good opportunity to explore the feasibility of using ground penetrating radar (GPR) to assess pavements and to verify its practicality. Thirty-one copper plates, serving as a reflecting material, were placed during construction at different layer interfaces throughout the pavement sections. Results show that enough radar energy is reaching the subgrade, but due to low dielectric contrast between some pavement materials, energy is not reflected back. In these cases, the copper plates indicate where the interface between each two layers occurs. Reflections from the copper plates are also used to determine the dielectric constant of pavement materials over the GPR frequency range. This paper presents an overview of the Virginia Smart Road test facility, data obtained from different sections using two GPR systems, and a method to calculate the complex dielectric constant of hot-mix asphalt over the frequency range of 750–1750 MHz using an air-coupled GPR system.     

FMS激光切割机柔性生产线

对FMS激光切割机柔性生产线进行整体介绍,系统分析构成整条生产线的各个组成部分。本生产线由国际先进的全自动立体仓库、高性能激光切割机等关键设备组成,包括三菱触摸屏、PMC轴同步运行控制、全自动上下料装置等多种技术,立体仓库与激光切割机采用总线数据通讯,通过自动编程软件为客户实现完整的加工流程。     

柔性化生产在摩托车架批量焊接生产中的应用

为了适应现代工厂级多任务、多品种的生产需求,开发并设计具备先进加工生产能力的柔性化生产设备就变得十分必要.分析了传统作业方式与柔性化生产方式各自的优缺点以及在现代工厂中的应用情况,并结合现代工业控制系统的发展现状,针对多任务、多品种的生产要求,以实际生产需求为设计对象,进行了"专用成套设备制造方案"和"柔性化制造单元集成控制方案"的技术分析和方案论证.以柔性化生产为设计理念,选用三菱PLC为控制核心,在电气控制系统中集成机器人柔性控制单元,开发一套具备CC-Link网络数据交换、人机交互功能的综合电气控制系统,充分满足用户在摩托车车架多品种生产制造中柔性化生产的要求.     

轴类件的柔性辊轧成形工艺与实验研究

柔性辊轧成形是成形轴类件的有效方法之一,具有柔性、快捷和低成本的特点,文章简要介绍了该工艺的成形原理和主要工艺参数,制作了实验装置,采用实验方法进行了实际轧制实验,应用电测法测量了轧制力能参数,结合理论计算分析力能参数与工艺参数间的关系,分析结果为成形设备的开发打下了理论基础。