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.     

The effects of various formwork surfaces on the corrosion performance of reinforcing steel in concrete

In this study, the effects of various formwork surfaces on the corrosion performance of reinforcing steel in concrete were examined. For this purpose, seven formwork surfaces from populus nigra, pinus silvestris, steel sheet, and four of plywoods were prepared. Three of the plywood formworks were covered with different geotextile liners and drainage channel and holes were drilled on their surfaces. One of the plywood formworks having no processes on its surface was for control. Reinforcing steels were first embedded in formworks and then the concrete was poured. The samples were exposed to corrosion in a way of settling them in 5% NaCl solution. We tested the corrosion potential and bonding strength of the samples. Later, the mass loss and tensile strength were measured on reinforcing bars that were pulled out from the concrete. The chloride content and pH values were also tested on concrete powders. The results indicated that drained‐lined formwork (F5 with TB50 + F613 geotextile liner) compared to steel sheet (F4 undrained and unlined) gave 16% lower corrosion and 68% higher bonding strength on reinforcing bars embedded samples, 73% lower mass loss and 4% higher tensile strength on reinforcing bars, and 70% lower chloride content and 4% higher pH on concrete powders. In addition, it was found that the corrosion strength of reinforcing steels in concrete could be increased if drained‐lined formworks were utilized.     

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.     

Non-destructive evaluation of cracks in massive concrete using normal dc resistivity logging

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.     

Modelling of reinforcement corrosion – geometrical effects on macrocell corrosion

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.     

Investigation of 3D anisotropic electrical conductivity in TIG welded 5A06 Al alloy using eddy currents

Electrical conductivity undergoes changes in the weld zones of the material. In this study, its anisotropic components along longitudinal, transverse and through-thickness directions have been investigated nondestructively using circular eddy current probe with Hall Effect sensor. The electromagnetic forward model has been solved with material resistivity input profiles corresponding to tensile and compressive residual stresses in a typical welded joint. For reconstruction of 3D anisotropic electrical conductivity, constrained linear least square optimization has been used. Compared with measurements based on calibration with homogeneous specimen or calculated assuming isotropic material properties in forward model, the results indicate that change in conductivity is significantly larger. A quantitative comparison is made between results based on isotropic and anisotropic material models. The conductivity mapping is required for residual stress measurement in components and structures.     

A new sensing skin for qualitative damage detection in concrete elements: Rapid difference imaging with electrical resistance tomography

In this paper we investigate whether a thin layer of electrically conductive materials that is painted to the surface of concrete elements can be used as sensing skin to detect and locate cracking and damage in the concrete substrate. Cracking of the concrete results in the rupture of the sensing skin, thus locally increasing its electrical resistivity. We monitor the local change in the electrical resistivity of the sensing skin using electrical resistance tomography. In this work, we utilize difference imaging scheme. Experiments on polymeric substrates as well as on concrete substrates are performed. The results indicate that the developed sensing skin can be successfully used to detect and locate cracking and damage on concrete and potentially other nonconductive substrates.     

Full surface inspection methods regarding reinforcement corrosion of concrete structures

For reinforced concrete structures a localisation of all significant critical areas can only be done by a full surface inspection. The economic advantages are obvious: uncritical areas have not to be repaired expensively 1 . The first step of the assessment should always be a visual inspection 2 . The range of deterioration causes can be limited and the degree of deterioration may be estimated roughly. The inspection program can be adjusted to the requirements. By means of a full surface potential mapping areas with a high risk for chloride induced reinforcement corrosion can be localised, although no deteriorations are visually detectable at the concrete surface. In combination with concrete cover depth and resistivity measurements areas with corrosion promoting exposure conditions can be localised even if the reinforcement is not yet depassivated. The following publication gives an overview about the essential full surface investigation methods to localise critical areas regarding corrosion of steel in concrete. The selection of methods is based on the inspection procedure given in 2 .     

Theoretical considerations on the supposed linear relationship between concrete resistivity and corrosion rate of steel reinforcement

Traditionally, the assessment of service life of steel reinforced concrete structures has been focused on the prediction of the time required to achieve a transition from passive to active corrosion rather than to accurately estimate the subsequent corrosion rates. However, the propagation period, i.e. the time during which the reinforcing steel is actively corroding, may add significantly to the service life. Consequently, ignoring the propagation period may prove to be a conservative approach. On the other hand the prediction of the corrosion rate may result in a very complex task in view of the electrochemical nature of corrosion and the numerous parameters involved. In order to account for the various influences an essentially empirical model has been introduced in which the electrolytic resistivity of the concrete environment serves as the major parameter. This model will be discussed for carbonation‐induced corrosion based on the commonly accepted theory of aqueous corrosion. An alternative model for microcell corrosion is proposed which is based on the commonly accepted view that anodic and cathodic sites are microscopic and their locations change randomly with time. In line with this view electrolytic resistivity can be incorporated and thus may play a significant role in the kinetics of the corrosion process. For a wide range of corrosion current densities the relationship between corrosion current density, log(i_corr), and concrete resistance, log(R_con), can then be approximated by an almost ideal linear relationship. Assuming a fixed geometrical arrangement of anodic and cathodic sites on the steel surface, this linear relationship is also valid for concrete resistivity, ρ_con. However, from the theoretical treatment of the electrochemical processes underlying reinforcement corrosion it becomes evident that a linear relationship between corrosion current density and concrete resistivity does not necessarily imply that concrete resistance is dominating the overall corrosion cell resistance. In most cases a significant portion of the driving voltage of the corrosion cell will be consumed by the transfer of electrical charge involved in cathodic reactions, i.e. cathodic activation control will dominate.     

接枝共聚-磺化反应改性对木质素磺酸盐减水剂使用效果的影响

目的木质素磺酸盐是一种阴离子型高分子表面活性剂,可以作为混凝土减水剂,但是效果一般。尝试使用接枝共聚-磺化反应,对木质素磺酸盐减水剂进行改性,改善其使用效果。方法同时进行两种改性反应,从混凝土的减水率、凝结时间和抗压强度三个方面,研究了接枝共聚反应和磺化反应顺序及接枝单体和磺化剂用量,对木质素磺酸盐减水剂实际使用效果的影响。结果确定了反应的次序,得到了丙烯酸和亚硫酸钠的最佳用量分别为木质素磺酸盐用量的10%和30%,混凝土各项性能得到了改善。掺加未改性木质素磺酸盐的混凝土,抗压强度明显减小;而掺加改性木质素磺酸盐的混凝土,抗压强度均有一定的增大。结论接枝共聚-磺化反应改性对木质素磺酸盐减水剂的使用效果具有增强作用,可显著提高混凝土的各项性能。     

Fatigue Assessment of Defect-Free and Defect-Containing Brazed Steel Joints

This work aims at the development of lifetime estimation procedures for defect-free and defect-containing brazed joints. Preliminary investigations were performed to measure the influence of specimen geometry on the joint strength. To estimate the influence of defects on the fatigue lifetime, defect-free specimens were compared with specimens containing defects. The experiments show that defect-containing specimens provide considerably lower joint strengths than defect-free specimens. The decrease of the fatigue strength with increasing defect size can be shown, but the direct comparison of different defects is not possible with S-N-curves. Based on the experimental results and on theoretical investigations, a procedure was developed to estimate the lifetime of defect-free and defect-containing brazed joints based on the stress intensity caused by a defect.     

New technique of nondestructive assessment of concrete strength using artificial intelligence

A new technique of nondestructively assessing the compression strength of concrete, which employs artificial neural networks, is presented. A data set based on results of testing concretes (with a 28-day strength of 24–105 MPa) by different nondestructive methods was used to train and test an artificial neural network. The methodology of neural identification of the strength is described. The obtained results, including those of the practical verification of the technique, are reported.     

低碱高强硅铝聚合材料的研究

以煅烧铝土矿选尾矿、矿渣微粉、水玻璃为主要原料,在800℃下煅烧1 h制备得到高强硅铝聚合材料。用XRD和SEM研究了反应产物的相组成和微观结构,并着重比较研究了该材料与特种水泥的抗化学侵蚀性能差异。结果表明：制备得到的胶砂试样,28d抗折、抗压强度分别达到10.2MPa,60.2MPa;试样分别经3%硫酸钠、3%硫酸镁溶液浸泡28 d后,其强度没有下降反而略有上升,在石膏掺量为7%的硫酸盐环境中,其累计膨胀率远远小于中抗硫酸盐硅酸盐水泥的;分别经5%盐酸、5%硫酸溶液浸泡28d后,其质量损失及强度损失均小于铝酸盐水泥和快硬硫铝酸盐水泥的。制备得到的硅铝聚合材料具有优异的抗硫酸盐侵蚀性能、一定的耐稀酸侵蚀性能。     

NDE data fusion to improve the evaluation of concrete structures

A new data fusion application is presented, in the context of a national research project. This project aims to improve the evaluation of indicators or pathologies of in situ concrete structures, combining measurements provided by various non-destructive evaluation techniques. The indicators of interest are: porosity rate, water saturation, modulus of elasticity, compressive strength, chloride content and carbonatization depth. Any non-destructive measurement will tend to provide an estimation of the unknown indicators, with a certain degree of confidence that is a function of its reliability and sensitivity to each of the indicators. When the estimations converge, data fusion improves the final confidence in the estimated values of these indicators. In case of disagreement, conflicts can be managed by specific operators. An adapted solution has been developed, based on the possibility theory and fuzzy sets. Application results are presented for two simultaneous evaluations of two pairs of indicators: water saturation and porosity rate, and water saturation and modulus of elasticity. These results show that the chosen adaptive operator, which is a function of reliability, is appropriate for this type of problem.     

干压成型制备无机聚合物实验研究

无机聚合物是一种性能优良的工程材料，目前主要采用浇注成型方式制备。首次采用干压成型方法制备了无机聚合物材料，结果表明，干压成型方法是可行的，在1d到28d龄期内，样品抗压强度明显提高。干压成型无机聚合物材料样品在1、3、7和28d的抗压强度分别可以达到100、120、140和180MPa，明显优于浇注成型的样品。两种成型方式由于需水量不同，引起了材料反应机理的差别，干压成型样品中生成了莫来石晶体相，有利于强度提高。材料制备过程中，成型压力、样品含水量对无机聚合物材料抗压强度影响有交互作用，两者有一最佳值：样品含水量13．1％，成型压力90MPa时，1d抗压强度达到60MPa。样品中碱含量在5．5％以上时样品抗压强度较高；水玻璃量（15．5％～19.3％）变化对材料抗压强度影响不大；矿渣对无机聚合物材料强度影响很大，矿渣含量增加，材料强度升高。     

Copper-Filled Electrically Conductive Adhesives with Enhanced Shear Strength

In this study, the effects of diethyl carbitol (diluent) and tertiary amines on the electrical, mechanical, and rheological properties of the Cu-filled polyurethane-based electrically conductive adhesives (ECAs) were investigated. Significant difference could be observed in the electrical resistivity and shear strength of ECA prepared with different amount of diethyl carbitol. Reduced electrical resistivity was found in ECAs prepared with addition of tertiary amines, but no obvious change was observed in the shear strength of the ECA joint. Rheological property of the ECA paste was investigated in order to understand the correlation of the viscosity of ECA paste and electrical resistivity and shear strength of ECA joint. Results revealed that decrease in viscosity of the ECA paste reduced electrical resistivity and enhanced shear strength of ECA joint. A Cu-filled polyurethane-based ECA with considerably low electrical resistivity at the magnitude order range of 10^?3 Ω cm, and significantly high shear strength (above 17 MPa) could be achieved.     

三峡地区材料33年土壤腐蚀行为研究

综合分析钢铁、水泥制品、塑料等四类材料在三峡地区埋设33年的土壤腐蚀数据、总结了材料在三峡土壤中的腐蚀规律,讨论了三峡地区土壤的腐蚀性,提出了选材与防护建议     

Critical chloride content for reinforced concrete and its relationship to concrete resistivity

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.     

老混凝土井壁质量综合诊断法

采用回弹、钻芯、超声波相结合的检测方法，对某一泵站沉井井壁老混凝土强度、质量进行综合评估，诊断，得到了较好的鉴定结果。     

铝用冷捣糊颗粒级配的研究

以电煅煤为骨料,以环氧树脂为黏结剂合成铝用冷捣糊.按照最紧密堆积理论,根据Dinger - Funk方程中粒度分布系数(n值)的变化,计算出不同的粒度组成,并以此为依据进行配料,分别测试了不同粒度组成时,糊料的性能指标.结果表明,粒度分布对冷捣糊的各项性能有较大的影响.在n为0.45时,骨料的堆积密度达到最大,焙烧后糊料空隙率、电阻率和抗压强度都达到最佳.