Evaluation of interfacial bond condition between concrete plate-like structure and substrate using the simulated transfer function derived by IE

A method incorporating a simulated transfer function is proposed for effectively evaluating loss of bond between concrete and a substrate for plate-like concrete structures. The simulated transfer function is derived from the impact-echo response in which the Rayleigh wave recorded by the displacement transducer used to generate a realistic force–time function. The results of numerical simulations show that the thickness amplitude corresponding to the concrete overlay decreases with increasing substrate stiffness for the case of a semi-infinite substrate. Some unexpected high thickness amplitudes were found for models with finite substrate thicknesses due to a coupling effect between substrate and concrete layers. The experimental thickness amplitudes for bonded composite plates agree well with the ones predicted from numerical simulations.     

Development of non-contact SIBIE procedure for identifying ungrouted tendon duct

For non-destructive evaluation (NDE) of concrete structures, the impact-echo method has been successfully applied to locate defects and voids in concrete. To compensate the difficulty in selecting the resonant frequency and to visually locate a defect, an imaging procedure named stack imaging of spectral amplitude based on impact-echo (SIBIE) has been developed. Combining the SIBIE analysis with the detection by a laser vibrometer, a non-contact procedure is studied to develop an automated system.Since waveforms detected by the laser vibrometer are undistorted, elastic waves generated due to a steel-ball drop were investigated theoretically as Lamb's problem in elastodynamics. It is confirmed that the first motion of detected waves by the impact-echo method is identical to Lamb's solution for a surface pulse, when reflected waves arrive later than the Rayleigh wave.An applicability of a non-contact SIBIE procedure to identify an ungrouted tendon duct of plastic sheath was examined in the impact-echo tests, as well as the duct of metal sheath. In order to study theoretically the SIBIE procedure for identifying ungrouted tendon ducts, the three-dimensional boundary element method (3D-BEM) was applied to synthesize frequency spectra. It is confirmed that frequency spectra detected in the tests are in good agreement with those synthesized. An applicability of the SIBIE procedure is also confirmed by the synthesized spectra. Results of the experiments show that the presence of the ungrouted duct can be visually identified by the non-contact SIBIE procedure in both the cases of plastic sheath and metal sheath.     

Evaluation of simulated transfer functions of concrete plate derived by impact-echo method

In this study, the simulated transfer function of a concrete plate was obtained solely from displacement waveform in an impact-echo test. In the simulated transfer function, the amplitude corresponding to the modal vibration at thickness direction, called the thickness-amplitude, can be consistent during various tests on the same location. An empirical formula was obtained by numerical simulation to predict the thickness-amplitude for given thickness, impact–receiver distance, and P-wave speed. The formula was confirmed experimentally using plates made from various mixed materials with various thicknesses. The differences between the experimental and predicted thickness-amplitude are mostly within 10%. The formula has the potential for quantitative evaluation of the bond between concrete and the substrate layer.     

Simultaneous determination of ultrasonic velocity, plate thickness and wedge angle using one-sided contact measurements

Traditional ultrasonic thickness gauging is based on the assumption that the ultrasonic velocity is known. In actual applications the velocity is often not well known and access is often limited to one side. This paper describes methods for determining the ultrasonic velocity and thickness of plates with parallel or wedged surfaces using contact measurements made on one surface only. For wedged plates the thickness at one point and the wedge angle are determined. Equations are derived for determining the ultrasonic velocity, thickness and wedge angle of the plate based on the times-of-flight measured by two contact transducers coupled to one surface. The time-of-flight to the obliquely reflected longitudinal wave echo was measured as a function of the separation (‘skip distance’) between the two transducers. In addition, the times-of-flight of normal-incident, pulse-echo longitudinal waves from each of the two transducers are also used. Experiments were performed on flat and wedged plates of various materials; the calculated results for the unknown quantities are generally accurate to a few percent.     

Application of empirical mode decomposition in the impact-echo test

This research develops a signal processing method for the impact-echo test based on the empirical mode decomposition (EMD) in the Hilbert–Huang transform (HHT). First, apply the decomposition to decompose the impact-echo signal into several intrinsic mode functions (IMF's). Then, the Fourier analysis is performed on each IMF. Examining the behaviors of the IMF's in the time and frequency domain, one can judge which IMF represents noise, echo wave, surface wave, or modal vibrations. Since the echo IMF has little influence from the other signals, one can locate the echo peak easily. Numerical simulations and model tests show that the proposed method is promising in the detection of internal cracks in concrete even when the vibration and noise signals are strong.     

Use of the normalized impact-echo spectrum to monitor the setting process of mortar

The objective of this paper is to introduce a newly developed nondestructive test technique called the normalized impact-echo spectrum and its application to monitoring the stiffening process of mortar. The impact-echo method is an effective and nondestructive technique for locating flaws in concrete. The depth of an air–concrete interface is determined by identifying the large amplitude peak in the spectrum at the dominant frequency corresponding to multiple wave reflections from the interface. However, the measure of the peak amplitude does not make sense because it changes with different impact forces. To make the peak amplitude meaningful, there is a need to normalize the spectrum on impact force. After normalization, the amplitudes in impact-echo spectra can provide useful information on the intensity of wave reflections at the interface. In this paper, the normalized impact-echo spectra were used to monitor the setting process of mortar. Experimental results demonstrate that the normalization of the impact-echo spectra can make the peak amplitude steady for various impacts and the coefficient of variation is less than 1.5%. The variation in the amplitude of the normalized impact-echo spectrum with the mortar age can indeed reflect the stiffening process of mortar. The results show that the proposed technique is suitable to monitor the setting process of mortar.     

Corrosion and Corrosion Inhibition of Reinforcing Steel: II. Embedded In Concrete

Abstract

Reinforcing steel, whether embedded in Portland concrete or Portland cement-blast furnace slag concrete and mixed with distilled or tap water, becomes passivated. On the other hand, when the concrete is mixed with sea water, the steel corrodes severely, and when Helwan mineral water is used for mixing, a borderline condition is obtained. Portland cement-blast furnace slag concrete had poorer corrosion resistance compared with ordinary Portland cement concrete.

Steel passivity is not impaired when Portland cement concrete or Portland cement-blast furnace slag concrete is admixed with up to 8% of sulphates. However, when cement extract is used, the passivity is impaired when as little as 0·2% of sodium sulphate is added.

Additions of some anodic inhibitors such as benzoate, chromate, nitrite, phosphate and stearate to a corrosive concrete medium are efficient in preventing the corrosion of reinforcing steel. The critical concentration for inhibition is higher in the case of Portland cement-blast furnace slag concrete than that of Portland cement concrete. Coating the steel with Portland cement slurry alone or Portland cement slurry containing inhibitors improves the corrosion resistance of steel to some extent but does not inhibit the corrosion in strong attacking media.     

Detecting flaws in concrete blocks using the impact-echo method

The objectives of this paper are to investigate the impact response of concrete blocks and to study the feasibility of using the impact-echo method for detection of flaws in concrete blocks. Numerical studies were carried out to acquire the transient responses of intact concrete blocks subjected to impact. In addition, the impact responses of concrete blocks containing flaws were explored. Numerical results were verified by experimental studies on concrete blocks with/without flaws. It is shown that the impact response of a concrete block is composed of frequencies corresponding to the modes of vibration of the block. Among these frequencies, there is a predominant frequency and its value depends on the geometry and dimensions of the block for a given P-wave speed in concrete. It is also shown that the presence of a flaw disrupts the modes of vibration. A shift of the predominant frequency to a lower value is a key indication of the presence of the flaw. In addition, multiple wave reflections between the impact surface and the surface of the flaw produce a large amplitude peak in the spectrum at the frequency corresponding to the depth of the flaw.     

Spectral analysis of surface wave response of multi-layer thin cement mortar slab structures with finite thickness

The method of spectral analysis of surface waves (SASW), a nondestructive testing method, has mainly been developed and used for many years in the fields of geotechnical engineering and highway engineering, such as for examining the material properties of pavement systems and soil media under an infinite half-space condition. Extensive research in this area has been focused on understanding the applicability and limitations of the SASW method in recent decades. This method consists of generation, measurement, and processing of dispersive surface waves. During an SASW test, the surface of the medium under investigation is subject to an impact to generate surface wave energy at various frequencies. Two vertical accelerometer receivers are set up near the impact source to detect the energy transmitted through the testing media. By recording signals in digitized form using a data acquisition system and processing them, surface wave velocities can be obtained by constructing a dispersion curve. Through forward modeling, the shear wave velocities can also be found, which can be related to various material properties. This paper presents the relationship between the theoretical and experimental compact dispersion curves when the SASW method is applied to multi-layer thin cement mortar slab systems with a finite thickness. The test results of surface wave velocity obtained from the experimental compact dispersion curve are found to have higher values than the results obtained from the theoretical dispersion curve due to different boundary conditions and reflections from the boundaries. An experimental study was conducted to examine if the dispersive characteristics of a Rayleigh wave exist in the multi-layer cement mortar slab systems. This study can be utilized in examining structural elements of general concrete structures and can be applied in the integrity analysis of concrete structures with a finite thickness.     

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.     

高次驻波谐振定量检测弹性板厚度的方法

基于声波高次谐振的频率偏移与弹性板厚度变化间的关系,提出一种定量检测弹性板厚度的新方法。针对最大有效机电耦合系数(ke2ff(max))对应的谐振模式,研究了弹性板厚度对最大有效机电耦合系数对应模式的谐振频率的影响,给出修正的频率偏移与厚度变化间的关系公式。相关数值计算表明,该方法的检测灵敏度可达50Hz/μm,可用于板厚及其变化的检测和板表面状况的无损评价。     

Non-destructive testing of high strength concrete using spectral analysis of surface waves

Non-destructive testing is essential in the inspection of alteration, repair and new construction in construction industry. Researchers are exploring the performance of non-destructive testing method using spectral analysis of surface waves (SASW) in concrete structures. The method consists of the generation, measurement and processing of dispersive surface waves. In SASW test, the surface of the media under consideration is subject to an impact using, for example, a 12-mm steel ball, to generate surface wave energy at various frequencies. Two vertical accelerometer receivers detect the energy transmitted through the testing media. By recording signals in digitized form using a data acquisition system and processing them, surface wave velocity can be obtained using a dispersion curve. This study is to focus on understanding of the applicability and limitations of the SASW method in a high strength concrete. This study is also to characterize the material property of early age high strength concrete emphasizing compressive strength using non-destructive testing methods. Three high strength concrete slabs of 600, 850 and 1100 kg/cm² compressive strengths were prepared together with cylinders from same batches. Cylinder tests were performed at the ages of 7, 14, 21 and 28 days after pouring concrete. Using the impact-echo method, the compression wave velocities were obtained based on different high strength concrete ages and compressive strengths. The equation to obtain the compressive strengths of high strength concrete has been developed using the obtained compression wave velocities (f′_c=1.083V_p−3816.1143). Using the method, the equation has also been developed to obtain the compressive strengths of high strength concrete based on the surface wave velocities (f′_c=0.253V_p+16.271). This study can be utilized in examining structural elements of high strength concrete structures and be applied in the integrity analysis of high strength concrete structures with a finite thickness.     

Stack imaging of spectral amplitudes based on impact-echo for flaw detection

The impact-echo method recently has drawn a remarkable amount of attention for nondestructive evaluation of defects in concrete structures. Extracting resonance frequencies responsible for the locations of reflectors, the depth and the presence of defects are estimated. So far, however, the technique has some limitations for practical applications. This is because there exist unresolved problems in the application of the impact-echo to concrete structures in service. Consequently, the method is studied theoretically on the basis of the elastodynamics and the signal analysis. In order to circumvent the difficulty to identify peak frequencies in the conventional procedure, a new procedure to evaluate defects in concrete is investigated, applying an imaging procedure. Thus, stack imaging of spectral amplitudes based on the impact-echo is developed. This procedure is applied to a prestressed concrete beam to classify a grouted duct and an ungrouted duct. The location and the presence of the ungrouted duct can be visually identified.     

钢管混凝土质量的无损检测

综合应用超声波法（声时、声频等）、冲击回波法和声音识别法检测构件质量。结果表明,超声波法可以提高检测精度和识别缺陷的能力;冲击回波法能够初步检测钢管混凝土脱层厚度情况;声音识别法能够迅速判断出构件缺陷位置。     

Measuring dispersion curves of acoustic waves using PVDF line-focus transducers

This paper presents a new method for measuring dispersion curves of elastic waves propagating in thin plates and layered media. The measurement is based on line-focus PVDF transducers that have been used successfully to measure surface acoustic wave velocities for both isotropic and anisotropic bulk materials. In this study, the transducer has been further applied to thin plates and layered media for measuring their wave velocity dispersion. In order to obtain the dispersion relation, a new waveform processing method is introduced. A theoretical analysis is given to explain how this method can be used for the determination of velocity dispersion curves. Experimental testing on two samples, a thin glass plate and a glass/silicon layered medium, were carried out using the line-focus PVDF transducer and the new waveform processing technique. The experimental results when compared to theoretical values indicate that the wave dispersion curves can be determined accurately. Discussions on the measurement system and its future application will be detailed.     

The influence of cracks on chloride-induced corrosion of steel in ordinary Portland cement and high performance concretes subjected to different loading conditions

Many reinforced concrete structures are subjected to variable loads, but little information is available on the effect of such loading, and the accompanying opening and closing of cracks, on the initiation and propagation of chloride-induced rebar corrosion. The purpose of this paper is to provide that information for reinforced ordinary Portland cement concrete and high performance concrete exposed to static or cyclic three-point bending and exposed to salt solution.It was observed that: (i) corrosion occurred only at intersections of the rebar with cracks in the concrete; (ii) high performance concrete was more protective than ordinary Portland cement concrete and (iii) the type of loading had less impact on corrosion than the type of concrete and exposure conditions.     

基于CEEMD的冲击回波分析方法研究

冲击回波法检测混凝土结构缺陷过程中由于存在面波和噪声干扰,反射层纵波难以从频谱特征直接识别出来。针对该问题,本研究将完备集成经验模态分解（CEEMD）方法引入冲击回波检测信号分析中,提出基于CEEMD冲击回波信号分析方法。通过对模拟和实验的冲击回波信号分析和研究,结果表明:CEEMD方法从冲击回波信号分解出的特征模态函数IMF1~IMF10,中间模态IMF5、IMF6的频谱反映出混凝土内部缺陷位置和板厚信息,据此频谱能准确确定内部夹层缺陷位置和混凝土板厚。CEEMD方法能分离面波和噪声干扰,可作为处理冲击回波信号的一种常规方法。     

EXPERIMENT AND SIMULATION FOR THICK-PLATE BENDING BY HIGH FREQUENCY INDUCTOR

1.IntroductionSo-called"line--heating"technologyhasbeenemployedintheproductionofcurvedthickshells,whichareutilizedintheouterbodyinships[1].Oneoftheadvalltageofthisbendingprocessingisthattherequiredshapecanbeachievedwithoutdie,sothatwedonotneedanylargescalemachiningequipment.Actuallyitisimpossibletouseapunchpressforsuchlargeplateswithmorethan10mminthickness.Themechanismofthistechniqueisbasedonthethermalstressesarisingfromthedifferenceintemperaturebetweenbothsidesoftheplate.Agasburnerisusuallye…     

Chloride threshold for rebar corrosion in concrete with addition of silica fume

The effect of silica fume on the chloride threshold for the initiation of pitting corrosion of steel in concrete was investigated. Laboratory tests were carried out in concrete specimens made with ordinary Portland cement and with 10% of silica fume. Chloride contents up to 2% by mass of cement were added to the mixes, in order to investigate the corrosion rate of embedded bars made of both strengthened and mild steel. A lower chloride threshold was observed in the bars which were embedded in concrete with silica fume compared to those embedded in concrete made of Portland cement.     

Effect of surface condition on the initial corrosion of galvanized reinforcing steel embedded in concrete

The present work was aimed at determining the effect of coating surface condition on the initial corrosion of hot-dip galvanized reinforcing steel bar (HDG rebar) in ordinary Portland cement (OPC) concrete. During zinc corrosion in OPC concrete, calcium hydroxyzincate (CHZ) formed on untreated HDG steel provided sufficient protection against corrosion. Therefore, it is concluded that treating HDG rebar with dilute chromic acid is unnecessary as a method of passivating zinc. A layer of zinc oxide and zinc carbonate formed, through weathering, on HDG bars increased the initial corrosion rate and passivation time compared with the non-weathered rebar exposed to concrete. HDG steel with an alloyed coating, i.e. containing only of Fe-Zn intermetallic phases, required a longer time to passivate than those with a pure zinc surface layer. The lower zinc content of the surface limited the rate of CHZ formation; hence, delayed passivation. Regardless of the surface condition, the coating depth loss after two days of embedment in ordinary Portland cement concrete was insignificant.