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1.
Discontinuities are one of the most harmful damage to the durability of concrete structures. Currents approaches are limited to assessments of surface damages, even if non-destructive methods appear to be effective for the detection and the location of cracks. That is why, these methods might be unsuitable for investigation of massive concrete body as dams and locks. Present works aim at the presentation of new results with electrical resistivity to study damages within concrete structures. In this article, the electrical resistivity focused on the study of cracks and discontinuities (concrete joints, interfaces…) in massive concrete structures by measurements done in preexisting boreholes. The used array is the normal dc resistivity logging. The study presents numerical and experimental results. First, modeling allows correcting experimental data. Then, development shows the sensitivity relatively to the discontinuity characteristics (aperture and the resistivity contrast between the discontinuity and the concrete). The tests on a real structure are carried out to define the methodology for on-site measurement. Results support the modeling.  相似文献   

2.
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.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
圆形截面梁广泛应用于机械设备与道路桥梁等领域中,裂纹的存在及扩展直接危及梁的安全可靠性,进而引发灾难性事故。旨在研究一种基于频率等高线的裂纹识别方法。使用一种新方法推导了裂纹梁在轴力、剪力和弯矩共同作用时的应力强度因子,给出了相应的柔度系数计算公式。基于有限元法分别推导了含裂纹单元和无裂纹单元的柔度矩阵,推导了梁的振动方程,探讨了裂纹对梁模态频率的影响。在此基础上,将固有频率作为裂纹识别的判断指针,研究了圆形截面梁裂纹识别频率等高线法。最后,基于频率误差函数对固有频率等高线法进行改良,提高了裂纹识别精度。结果表明:改良后的频率等高线法弥补了其在一些工况下无交点甚至无三角形区域的缺陷,使它更具工程应用可行性,并且该方法对裂纹位置和深度的识别也较为精准,相对裂纹深度识别最大误差为77.5%,相对裂纹位置识别最大误差仅为12.6%。  相似文献   

7.
A procedure is proposed for the prediction of the compressive strength of concrete based on the use of NDT measurements. Prior to formwork removal, electrical resistivity can be assessed and based on an electrical resistivity versus compressive strength correlation curve, the strength of concrete can be estimated. This procedure ensures, for example, that the formwork is only removed when the concrete reaches a predefined compressive strength, dispensing with the calculation of concrete maturity. On the other hand, the whole surface of the concrete structure can be systematically measured so as to estimate 28-day compressive strengths and detect areas that could potentially not conform to the specified requirements. In this manner, a realistic overview of the in situ compressive strength of the whole structure can be obtained.The procedure proposed predicts the 28-day strength of concrete based on two models. The first model is based on an empirical equation used for the maturity assessment of concrete. The second model is based on a theoretical equation for the nucleation process of cement hydration. The effect of temperature and different curing conditions (concrete maturity) can be effectively taken into account by the conversion of all electrical resistivity measurement to the same referential. The models are used to estimate 28-day compressive strength based on the 7-day electrical resistivity measurements. The results show that the errors in estimation of the 28-day compressive strength are less than 22% and 10% for these models, respectively. If 28-day electrical resistivity measurements are used then the error in estimation is lower, circa 9% and 5% for these models, respectively.  相似文献   

8.
This communication analyses information supplied by the electrochemical parameters related to rebar corrosion in reinforced concrete structures (RCS). Corrosion potential and corrosion current density are determined for different sets of conditions. Tests are performed to gather data on conflictive aspects of the two electrochemical parameters, with regard to evaluating the results of RCS inspections. Consideration is made of the limitations of potential measurements if not accompanied by complementary indications such as concrete resistivity, rebar surface area involved in the measurements, or the instrumentally determined corrosion current. The capacity of galvanostatic pulses applied directly on large RCS to offer a reliable guide to corrosion rate of steel reinforcements is discussed.  相似文献   

9.
The critical chloride content for initiation of reinforcement corrosion is an essential element in service life design and modelling of concrete structures. The critical content is laden with questions regarding its definition, experimental assessment and practical aspects. It should be addressed by a statistical approach. The paper discusses such issues, presents experimental evidence and considers its relationship to the electrical resistivity of concrete. A low concrete resistivity can be theoretically argued to relate to a low critical chloride content. However, only part of the experimental data supports this. The resolution of available data does not allow distinction with regard to the critical content between cement types.  相似文献   

10.
采用能谱、扫描电镜和X射线衍射等测试方法,研究了湿度对铝合金接头裂纹敏感性的影响规律及机理. 结果表明,在40% ~ 80%相对湿度的条件下,改良的铝合金鱼骨试件接头表面可见结晶裂纹和液化裂纹;随着湿度的增大,接头裂纹敏感性逐渐提高,其中结晶裂纹的敏感性逐渐降低,液化裂纹的敏感性逐渐提高;湿度增大,铝合金焊缝晶粒逐渐细化,裂纹扩展阻力升高,焊缝组织细化导致结晶裂纹敏感性降低;同时提高湿度,电弧温度降低,焊接热输入量减少,液态金属越发不易在晶界处形成连续的网状薄膜结构,也导致结晶裂纹敏感性降低.  相似文献   

11.
Reinforcement corrosion is still the most frequent reason for damage of concrete structures. It can be caused by carbonation or the ingress of chlorides. In cases of localized contaminations with chlorides, macrocells with very high corrosion rates can be established. Thereby the resulting macrocell current is dependent on many different boundary conditions like driving voltage, concrete resistivity and the geometrical arrangement of anode and cathode. In order to investigate macrocell corrosion, the herein presented research work was carried out by laboratory experiments and additionally by numerical analyses. First the numerical simulations were calibrated by laboratory measurements and thereafter, a numerical parameter study was carried out to increase the available database and identify the impact of changes in single parameters. As the focus is on geometrical effects, all laboratory specimens and numerical models were designed to represent practical conditions with diverse geometrical arrangements, e.g., slabs or beams with localized depassivations. In addition, parameters like concrete resistivity, driving voltage and cathode to anode surface area ratios have been varied. Thereafter, all results were used to derive cell factors for a simple macrocell current estimation. The present status of the project will be presented and discussed.  相似文献   

12.
The effects of Cu content on the electrical and structural properties of Ag-Cu thin films were studied before and after microwave processing. Copper was chosen as the alloying element because of its low solubility in Ag, which enables it to segregate at the surface. After microwave annealing, enhanced electrical and structural properties were observed. The results from Rutherford backscattering spectrometry, x-ray diffraction, and four-point probe measurements suggested that the resistivity is controlled by the residual copper concentration and increased grain growth during the low-temperature microwave anneals. For each particular copper concentration, microwave annealing resulted in highest Hall mobility and lowest resistivity. These findings opened the possibility for novel materials structures based on low-temperature microwave processing schemes.  相似文献   

13.
The paper reports the results of numerical simulations carried out with FEM and aimed at evaluating the corrosion conditions of steel bars in concrete elements subjected to chlorides. Two case studies were analysed: a reinforced concrete element subjected to de‐icing salt in the presence of a crack and a concrete tunnel in a chloride‐contaminated, water saturated soil. Attention was focused on the selection of proper values of concrete resistivity and of the parameters suitable to describe the electrochemical behaviour of steel in the different conditions of exposure. The results allowed to quantify the effects of the galvanic coupling between active and passive areas on the corrosion rate of steel.  相似文献   

14.
In Germany in recent years failures of more than 30‐year‐old prestressed concrete structures have been observed. In all these cases a quenched and tempered steel type, strength class St. 1420/1570 was used. In order to assess the corrosion risk, i.e. crack initiation and propagation, under depassivating conditions lifetime tests in environments relevant for building practice were carried out using different prestressing steels of this quenched and tempered type of material. From the results it can be concluded that the specific susceptibility of certain prestressing steels is the determining factor with respect to the occurrence of cracks rather than environmental factors.  相似文献   

15.
Concrete structures typically have cracks due to the brittle characteristics of the material. In order to check their severity in an existing structure, the crack depth should be evaluated with nondestructive testing such as the wave transmission method. The method determines the crack depth using the energy of the Rayleigh wave propagating over the crack and needs a reliable technique to evaluate the wave energy with measured surface waveforms. This paper proposes a self-compensating frequency response function to improve isolating the Rayleigh wave components from the accompanying noise. A rigorous procedure using the spectral estimator for noisy measurement is derived for self-compensating. The proposed technique was verified with experimental application to structural cracks as well as artificial notches.  相似文献   

16.
于波  宋凯 《失效分析与预防》2023,40(6):351-355, 373

涡流检测技术在航空铝合金的表面或近表面裂纹检测中具有较高的检测灵敏度,且速度快、无需耦合剂,但粘胶层会降低检测灵敏度。飞机在服役期间因应力集中萌生的疲劳裂纹,容易导致机体结构失效,故承力部件的完整性是保障飞机飞行安全的关键因素之一。针对承力部件虎口位置的疲劳裂纹,提出基于双线测长法缺陷尖端定位方法;设计虎口裂纹缺陷专用双拉线式涡流屏蔽探头,可自适应定位螺栓的安装误差。结果表明:利用自适应定位螺栓将测量装置固定于虎口,裂纹尖端双线测量装置可以有效定位裂纹;当激励频率为30~40 kHz时,研制的双拉线式涡流屏蔽探头能有效检出粘胶层下半径为7.0 mm、宽度为0.2 mm的扇形人工刻槽。

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17.
常用的工程陶瓷电阻率较高,直接采用电火花加工,性能较差。以ZnO/Al_2O_3陶瓷为例,研究了电阻率和脉冲宽度、脉冲间隔、工具极性等工艺参数对电火花加工工艺性能的影响。结果表明:采取合适的放电加工参数,在ZnO/Al2O3的电阻率达到3410Ω·cm时仍可对其进行有效的电火花加工。  相似文献   

18.
Cathodic protection (CP) is considered to be the only rehabilitation method for chloride-induced rebar corrosion in reinforced concrete structures. The protection current distribution depends on several parameters, such as the geometry and number of rebars and the concrete resistivity. In order to investigate the influence of concrete resistivity on the possibilities and limitations of rebar protection, this paper presents a numerical approach based on the finite element method (FEM) in conjunction with laboratory results to determine its impact on the CP of alkali-activated slag mortar. An ordinary Portland cement was also tested for comparative purposes.  相似文献   

19.
Many processes and systems require hot surfaces. These are usually heated using electrical elements located in their vicinity. However, this solution is subject to intrinsic limitations associated with heating element geometry and physical location. Thermally spraying electrical elements directly on surfaces can overcome these limitations by tailoring the geometry of the heating element to the application. Moreover, the element heat transfer is maximized by minimizing the distance between the heater and the surface to be heated. This article is aimed at modeling and characterizing resistive heaters sprayed on metallic substrates. Heaters were fabricated by using a plasma-sprayed alumina dielectric insulator and a wire flame-sprayed iron-based alloy resistive element. Samples were energized and kept at a constant temperature of 425 °C for up to 4 months. SEM cross-sectional observations revealed the formation of cracks at very specific locations in the alumina layer after thermal use. Finite-element modeling shows that these cracks originate from high local thermal stresses and can be predicted according to the considered geometry. The simulation model was refined using experimental parameters obtained by several techniques such as emissivity and time-dependent temperature profile (infra-red camera), resistivity (four-probe technique), thermal diffusivity (laser flash method), and mechanical properties (micro and nanoindentation). The influence of the alumina thickness and the substrate material on crack formation was evaluated.  相似文献   

20.
The development of fatigue cracks at fastener holes represents a common maintenance problem for aircraft. High frequency guided ultrasonic waves allow for the monitoring of critical areas without direct access to the defect location. During cyclic loading of tensile, aluminum specimens fatigue crack growth at the side of a fastener hole was monitored. The changes in the energy ratio of the baseline subtracted reflected guided wave signal due to the fatigue damage were monitored from a stand-off distance using standard ultrasonic pulse–echo measurement equipment. Good sensitivity for the detection and monitoring of fatigue crack growth was found.  相似文献   

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