Ability of the direct wave of radar ground-coupled antenna for NDT of concrete structures

The research work presented in this paper aims at evaluating the potential of the direct wave of radar ground-coupled antennas for the on site physical characterization of concrete. According to studies highlighting the ability of radar reflected waves to characterize concrete, the potential of the direct wave is studied by systematic comparison of direct and reflected wave attenuations. Among the various features of electromagnetic waves propagating through concrete, attenuation is the most sensitive to moisture and chloride contents. In order to study the sensitivity of the direct wave to the physical condition of the concrete, laboratory experiments were conducted on concrete slabs with various porosities, water contents and chloride contamination levels.The results of this study show very satisfactory correlations between attenuations of direct and reflected waves, indicating that both waves provide similar information regarding the physical state of concrete. This correlation was confirmed on two bridges by comparison of direct wave attenuation maps with reflection attenuation maps. Therefore, radar interpretation method based on the direct wave attenuation should be able to detect, in a very fast way, physical contrasts in concrete structures and, consequently, identify probable pathological areas.     

On the study of elastic wave scattering and Rayleigh wave velocity measurement of concrete with steel bar

This paper presents results on a study of the Rayleigh wave scattering and Rayleigh wave velocity measurement in concrete with a steel bar using transient elastic waves. To study the characteristics of the scattered waves induced by a steel bar in concrete, a three-dimensional heterogeneous finite difference formulation with staggered grids was adopted. The cases for both elastic wave propagation parallel and perpendicular to the steel bar are discussed. The effect of the cover thickness and steel bar spacing on the Rayleigh wave velocity measurement was studied. To confirm the numerical investigations, a concrete specimen containing steel bar was made and the corresponding transient elastic wave experiments were conducted. The numerical results are in good accordance with those of the measured. We note that the result of this study can serve as an important reference in the Rayleigh wave velocity measurement of concrete with steel bar.     

Ultrasonic velocity measurements for characterizing the annealing behaviour of cold worked austenitic stainless steel

Precise measurements of shear wave velocities have been made in 20% cold worked and annealed samples of alloy D9 (Ti-modified austenitic stainless steel) to characterize the microstructural changes during annealing. The variation in wave velocity with annealing time exhibited a three stage behaviour. In the first stage, a slight increase in the velocity during recovery regime has been observed. This is followed by a significant increase in the velocity in the recrystallization regime (second stage) and reaching saturation on completion of recrystallization (third stage). Microstructural observations using optical metallography confirmed these three stages. The maximum variation in velocity is observed only when the polarization or the propagation direction of the shear wave is alingned with the cold working direction. Variation in shear wave velocity during annealing process, in general, is just opposite to that of the variation in longitudinal wave velocity. A number of velocity measurements were made by changing the propagation and polarization directions of the shear waves with respect to the cold working direction. Based on these measurements, a suitable velocity ratio parameter is suggested for determining the degree of recrystallization during annealing of cold worked alloy D9.     

Setting time study of roller compacted concrete by spectral analysis of transmitted ultrasonic signals

The setting time of roller compacted concrete (RCC) is determined by studying the increase in the propagation velocity of ultrasonic waves transmitted through a sample. In the case of a quick setting binder, this method proves unsatisfactory. Concrete setting is modelled by dissociating the chemical kinetics related to the volume transformations from those related to the surface transformations. Calculations show that the latter, and hence the percolation phenomenon, are prevalent in wave propagation and concrete setting. As a result we assume that concrete acts as a time-varying spectral filter. We are developing tests and software to show that the energy and the frequency spectrum of the transmitted ultrasonic signal makes it possible to analyse the setting process and determine the setting time.     

Acoustoelastic effect in concrete material under uni-axial compressive loading

This study deals with the general matter of non-destructive evaluation of pre-stressed structures in civil engineering. Usually such structures are composed of concrete and are steel reinforced. Proposed idea is the evaluation of mechanical stress state of a concrete body (instead of steel cables) via ultrasonic non-destructive evaluation (NDE), by using the link between ultrasonic velocities and mechanical stresses provided by the acoustoelasticity theory. Velocities of the ultrasonic waves (longitudinal and transversal with different polarizations) are observed during propagation through a concrete body submitted to uni-axial loading (compressive testing). Obvious variations in velocity are found depending on the mechanical stress state (e.g. Δc=92 m/s at σ=16 MPa for longitudinal waves). Thus acoustoelastic behavior of concrete is demonstrated. Further analyses provide acoustoelastic coefficients of concrete about ten times higher than the common ones of steel. The feasibility of stress evaluation using ultrasounds in concrete structures is proved under laboratory conditions.     

An ultrasound method for the acoustoelastic evaluation of simple bending stresses

Ultrasonic waves constitute a privileged tool for investigation in the analysis and characterisation of mechanical stress states. Ultrasonic wave velocities depend on many physical properties of the propagation medium such as the second-, third- and higher-order elastic constants, the volume density and the strain. They also depend on whether the wave is longitudinal or transversal. In this last case, they also depend on the wave polarisation direction.

The present paper deals with the classical acoustoelasticity in stressed and elastically deformed media when they are submitted to bending stresses. A numerical and an experimental evaluation of the resulting change in the ultrasound velocities as a function of bending loads are described. Some results have been obtained on the variation of propagation velocities of the longitudinal and transversal polarised waves as a function of mechanical bending loads applied on samples made of S 185 steel. The acoustoelastic evaluations have been achieved in three zones under bending stress (compressed, central and extended fibres) in the case where the path of probing, longitudinal and linearly polarised shear waves are parallel to the sample axis. As additional investigation on acoustoelastic behaviour on bending load, we propose some acoustoelastic responses simulated numerically on materials of known macroscopic properties.

The experimental study was achieved by means of a measurement set-up composed of an ultrasound bench and a mechanical test machine. In order to measure the variations of the propagation velocity, a correlation technique has been used to obtain an accurate estimation of the propagation time. The velocity measurements have taken into account the elastic deformation of the samples made of carbon steel (C 35).     

Longitudinal waves for evaluation of large concrete blocks after repair

The present paper discusses the repair characterization of large concrete blocks based on the propagation of low-frequency elastic waves. A common repair method applied in damaged structures is cement injection. As a result, cracks and voids are eliminated; however, due to the poor initial mechanical properties of fresh cement, pulse velocity does not recover immediately, complicating the assessment. Results of P-wave velocity measured through the thickness of a large structure show that only after a suitable hydration period wave parameters increase. Laboratory tests were also conducted in order to extract information under more controlled conditions. It is concluded that velocity, frequency and amplitude of the ultrasonic waves are sensitive to the repair condition and the hardening of the injection material, while slight dispersion trends are eliminated after proper repair.     

Development of a new algorithm for accurate estimation of GPR's wave propagation velocity by common-offset survey method

The development of a new algorithm to estimate electromagnetic (EM) wave propagation velocity (v) with shielded ground penetrating radar (GPR) antenna with high-frequency of 2 GHz is presented in this paper. Common offset method (fixed transmitter-receiver distance) is used for data collection, the antenna's separation distances and the corresponding triangulation geometry are also considered. The distribution of discrete velocities at each point oblique to the embedded target object is calculated by the proposed method. It is similar to the curve fitting process of hyperbolas in most software. The method has been validated in air and in concrete media and compared with the traditional hyperbola geometry method (ASTM D6432-11). The results show that errors of the new algorithm is significantly lower than the hyperbolic method. Besides, effects of analyzing reflected signals with different object depths are discussed. Based on the result, differentiation of velocity variations in concrete as a result of water seepage and leakage will be studied in next stage.     

Assessment of porosity of mortar using ultrasonic Rayleigh waves

The relationship between Rayleigh wave velocity at ultrasonic frequencies and porosity is investigated in dry and fully saturated mortar. Porosity is varied by changing the water/cement ratio. Rayleigh wave (RW) pulse velocity is measured accurately using air-coupled ultrasonic transducers as transmitter and receiver. The experimental results are compared with two single-phase models. Due to relatively weak correlation between RW velocity and porosity, measured data and the models are compared using statistical hypothesis tests. Although the parameters of the model are not accurately predicted, these tests conclude on a reasonable agreement between measured data and the models. This result can be useful to develop simplified models of wave propagation in concrete, or to estimate porosity in concrete cover from ultrasonic measurements.     

Investigation of the local response of the steel–concrete interface for corrosion measurement

An equivalent circuit representation for the steel–concrete interface is determined from direct potential measurements at the steel surface. The local response of steel–concrete interface to a given polarization applied at the concrete surface is investigated using an Ag/AgCl embedded reference electrode and a test system which allows simultaneous measurements at the steel–concrete interface and on the concrete surface. It is shown that the impedance spectrum on Nyquist plot for the steel–concrete interface comprises of a single arc. The equivalent circuit representation of the steel–concrete interface comprising of a parallel combination of a constant phase element (CPE) and a resistance was found to be suitable for representing the observed frequency response above 10 mHz. The parameters for the equivalent circuit obtained from the frequency-domain impedance measurements are shown to provide close prediction of the transient time-domain response from a linear polarization resistance measurement. The equivalent circuit was found to be suitable for interpreting the transient response of the steel–concrete interface during the linear polarization measurements. Available results indicate that while the response of steel undergoing active corrosion exhibits a distinctive CPE behavior, the passive steel approaches a pure capacitor. The value of resistance when the measured current increases linearly with time during a linear polarization measurement from the concrete surface provide acceptable measurements of the charge transfer resistance, and the Ohmic resistance of the concrete.     

Surface wave dispersion in large concrete structures

The present paper discusses the repair characterization of large concrete surfaces based on the propagation of low-frequency elastic waves. A common repair method is cement injection. As a result, cracks and voids are eliminated increasing the measured average pulse velocity by about 5%. However, the central frequency of the pulses exhibits higher sensitivity to the repair increasing by about 15%. Additionally, the velocity of longitudinal waves exhibits a dependence on frequency. This dispersion is eliminated after repair being an indication of the repair efficiency, while Rayleigh waves are only slightly dispersive due to their long wavelength which enables them to travel below the shallow defects.     

Modelling the propagation of a radar signal through concrete as a low-pass filter

The knowledge of how a dispersive, dielectric medium such as concrete affects a propagating short electromagnetic pulse used in ground penetrating radar (GPR) is helpful both in the interpretation of radar results and in the prediction/modelling of expected radar measurements. Although there are a number of published results on the frequency-dependent, dielectric properties of media such as concrete and soils, the use of this information is still relatively small, primarily due the lack of a reliable method of applying these properties to propagating radar pulses. Modelling the dielectric medium as low-pass filter is one solution to this difficulty. In this paper the propagation of short pulses in concrete with known frequency-dependent properties is studied. The extent of how the pulses are attenuated and distorted is analysed and the implications on GPR applications are also discussed.     

Tomographic reconstruction for concrete using attenuation of ultrasound

The development of attenuation tomography for concrete is discussed. Fundamental ultrasonic measurements of cubic specimens and numerical simulations of wave motion were conducted to examine the decay in amplitude of ultrasound when impinged by defect, and compare its sensitivity to the delay in travel time. An algorithm was then developed to facilitate simultaneous tracing and distribution of amplitude factor in a three-dimensional model by iterative computation. The algorithm was used for reconstruction of attenuation tomographic for visualizing concrete interior. Experimental results indicated successful identification for the location of a defect that was embedded in concrete, thus implying the potential of attenuation tomography as a complementary method to the travel time tomography to enhance soundness evaluation of concrete.     

Frequency-dependent dispersion of high-frequency ground penetrating radar wave in concrete

This paper studies the dielectric dispersion of high frequency radar wave in concrete in early-aged and hardened concrete specimens. Frequency-dependent spectra of phase velocity v(ω) were measured to deduce the spectra of real part of dielectric permittivity ε′(ω). The dispersion was measured by three high nominal ground penetrating radar frequencies (1.5, 1.6 and 2.6 GHz), experimenting on two steel bars with concrete cover 50 and 100 mm. It was found that v(ω) and ε′(ω) dispersed at lower frequency, but became stable at high frequency regions, which agrees with the classical GPR plateau. The same frequency components at different nominal antenna frequencies show a close range of v(ω) and ε′(ω) in concrete of different ages. The results in this paper warrant further investigation of using GPR wave to study material properties.     

Spectral analysis of radar surface waves for non-destructive evaluation of cover concrete

This paper presents the use of spectral analysis of radar surface waves for the non-destructive evaluation of cover concrete. With only two commercial coupled antennas centred around 1.5 GHz, an original measurement mode is proposed for obtaining dynamic wide angle reflection refraction profiles. It consists in taking several profiles at the same place using different distances between the two antennas. After separation of the air wave and surface waves, the wrapped phase spectrum is obtained for a given distance between two receiver positions by means of a Fourier transform. For a given position, the speed is calculated for each frequency from the known wave travel distance. The repetition of the processing for other positions gives the speed variation along a profile. Moreover, the dispersion analysis can provide further information on the humidity of the material. Tests on a dry concrete surface and after wetting provide reliable results which allow the two moisture contents to be distinguished.     

Measurement of residual stresses in austenitic stainless steel weld joints using ultrasonic technique

Abstract

A methodology has been developed using a non-destructive ultrasonic technique for measuring surface/subsurface residual stresses in 7 mm thick AISI type 316LN stainless steel weld joints made by activated tungsten inert gas and multipass tungsten inert gas welding processes. Measurement of residual stresses using an ultrasonic technique is based on the effect of stresses on the propagation velocity of elastic waves. Critically refracted longitudinal L _CR wave mode was employed and accurate transit time measurements were made across the weld joints. Quantitative values of the longitudinal residual stresses across the weld joints were estimated from the measured transit times and predetermined value of acoustoelastic constant for AISI type 316LN stainless steel. The nature of the residual stress profiles and their variations across the two types of weld joints were compared and interpreted.     

激光冲击波传播特性对E690钢表面微凹坑动态塑性变形的影响

为研究激光冲击次数以及多次冲击过程中冲击波的传播特性对微凹坑动态塑性变形的影响,利用ABAQUS有限元软件分析了应力波在E690高强钢中的传播规律及微凹坑动态塑性应变规律,并设计试验验证了仿真的准确性。结果表明,由冲击压力引起的应力波在材料深度方向的衰减呈现先快后慢的特性,第3次和第4次冲击过程中应力波的衰减速度基本接近。受冲击波动态传播过程中表面波与纵波协同作用的影响,微凹坑表面塑性变形深度整体呈现沿光斑中心径向向四周递减,对比冲击1~4次后微凹坑深度方向塑性变形的测试结果与模拟结果,最大误差为4.80%,仿真模型准确可靠。同时,多次冲击后表面硬度增加趋势变缓,4次冲击后微凹坑表面出现硬化饱和现象。     

Ultrasonic measurements of undamaged concrete layer thickness in a deteriorated concrete structure

Ultrasonic wave propagation in deteriorated concrete structures was studied numerically and experimentally. Ultrasonic single-side access immersion pulse-echo and diffuse field measurements were performed in deteriorated concrete structures at 0.5 MHz center frequency. Numerically and experimentally it is shown that the undamaged layer thickness in a deteriorated concrete structure is measurable using pulse-echo measurements when the deterioration depth is larger than the wavelength. The signal overlapping, which occurs in the thin deteriorated layers, can be overcome using diffuse field measurements or a pattern matching technique. The ultrasonic experimental data were shown to be in good agreement with the widely used phenolphthalein test for concrete degradation.     

Advanced NDT methods for evaluating concrete bridges and other structures

Several advances in methods and equipment for sonic and ultrasonic non destructive testing (NDT) of concrete structures are presented, including advances for methods applicable to large structures such as bridges and dams. These new advances and methods greatly increase the speed and accuracy of many NDT investigations, and allow collection of NDT information previously not readily obtainable. The first topic presented is a brief overview of the impact echo (IE) method along with a case history on how is has been employed in testing concrete structures. The IE method is used to locate damage and determine member thickness when access is available from one side only. The next topic is a relatively new N DT method for concrete using surface waves analysis, in a method known as the spectral analysis of surface waves (SASW) technique. This allows the determination of the shear wave velocity profile versus depth for any layered system and is particularly applicable to testing pavements, slabs, tunnels, shaft liners, and massive concrete structures. For bridges and dams, the technique is particularly effective in determining the depth of weathering effects on the concrete. Included is an overview of the method and the principles behind it, as well as some case history examples. The final topic presented is an advance in the technique of ultrasonic plus velocity (UPV) testing. This advance involves the use of rapid scanning techniques to greatly increase the speed and utility of UPV testing. The U PV method uses compression wave energy which is sent through a member and the arrival time, wave amplitude, and velocity are recorded and computed. This new scanning technology allows the near-continuous acquisition of data along any given path over a structure. Included is test data collected using the scanning system to show the great increase in testing speed and utility of scanner-collected data when compared to traditional point-by-point methods.     

Damage detection in concrete using coda wave interferometry

Coda wave interferometry (CWI) is a nondestructive evaluation technique for monitoring wave velocity changes in a strongly heterogeneous medium as demonstrated in previous seismic and acoustic experiments. The multiple-scattering effect in such a medium promotes the rapid formation of a diffuse field, and waves can travel much longer than the direct path, and thus are more sensitive to small changes occurring in the medium. This research applies the CWI technique in conjunction with acoustoelastic measurements to characterize two different types of damage in concrete: damage due to thermal shock and dynamic cyclic loading. The diffuse ultrasonic signals are taken at different levels of compressive stress and then relative velocity changes are extracted using the CWI technique. The relative velocity change (or the material nonlinearity) increases considerably with increasing damage level in most samples for both types of damage. The feasibility and sensitivity of this CWI-based technique in characterizing damage in cement-based materials are demonstrated.