Development of corrosion sensors for monitoring steel‐corroding agents in reinforced concrete structures

Corrosion sensors were devised to develop a system whereby the depth of chloride permeation from concrete surfaces can be monitored non‐destructively on a real‐time basis using such sensors embedded in cover concrete of reinforced concrete structures. The proposed corrosion sensors were subjected to accelerated corrosion in NaCl solutions, mortar specimens intrinsically containing chlorides, and mortar specimens impregnated with chloride solutions, while recording the changes in the resistance readings. The resistance of sensors increased as the degree of corrosion increased. The time to the first change in the resistance decreased and the corrosion degree and resistance increased as the chloride concentration increased and as the distance from mortar surfaces decreased. It was therefore confirmed that the corrosion sensors are capable of monitoring the depth of chloride permeation with sufficient accuracy.     

Einflußeiner Verzinkung auf die Korrosion von mit Zementmörtel beschichtetem Stahl in NaCl-Lösung

Effect of galvanizing on the corrosion of steel in concrete immersed in NaCl solution Galvanized or pickled steel sheet specimens were embedded in portland cement mortar of various water cement ratios and curing conditions and then wholly or partially immersed in 0.5 M NaCl solution for 1 to 5 yrs. Free corrosion potentials and electrical resistances have been measured. Immersion conditions and the presence of zinc have a significant effect on the corrosion resistance of the embedded steel sheets. The potentials of the wholly immersed specimens are very negative. Thus, these specimens cannot act as cathodes in corrosion cells, and the steel sheets within the mortar do not corrode. The partially immersed specimens, on the other hand, show very noble potentials. Also in the case of galvanized steel sheets the potentials are shifted to the same positive values in the course of exposure time. Thus, all these specimens can act as cathodes in corrosion cells. Localized corrosion generally occurs at the water/air line. In the case of pickled specimens the mortar is cracked due to growing corrosion products. In the case of galvanized steels the corrosion is retarded significantly. The test results are discussed in detail with respect to practical problems of cell formation, internal and external protection of pipes as well as the corrosion resistance of reinforced concrete.     

Application of ultrasonic Rayleigh waves to thickness measurement of metallic coatings

The velocity of Rayleigh waves propagating in a layered medium depends on the frequency of the waves, the thickness of the coating and the properties of both the coating and the substrate materials. Rayleigh waves of various frequencies were generated using a broadband pulse and their velocities were measured as a function of the frequency and compared to the theoretical dispersion curve of the specimen. The thickness of the layer was deduced from comparison between these two curves. A special transducer was designed in order to achieve these measurements. Experiments were carried out on AISI 316L specimens coated with vacuum plasma sprayed (VPS) NiCoCrAlY of various thicknesses (190–330 μm) and various surface states. A good correlation was obtained between the thicknesses measured by means of ultrasonics and those obtained using photomicrographs of the cross-section of the specimens. The elastic characteristics of the coating and the substrate materials were measured using a transmission method for parallel-faced plates (disbonded samples of coating material). The velocity of the shear and compression waves propagating in various directions is obtained by changing the angle of incidence on the sample surface. A small anisotropy of the coating material was found. Finally, we demonstrated the efficiency of the use of surface waves for the detection of surface breaking cracks and disbonding at the interface between the coating and the substrate.     

Analysis of plate wave propagation in anisotropic laminates using a wavelet transform

A new approach is presented for the analysis of transient waves propagating in anisotropic composite laminates. The wavelet transform (WT) using the Gabor wavelet is applied to the time-frequency analysis of dispersive flexural waves in these plates. It can be shown that the peaks of the magnitude of WT in a time-frequency domain is related to the arrival times of the group velocity. Experiments were performed using a lead break as the simulated acoustic emission source on the surface of unidirectional and quasi-isotropic laminates. A method was developed to obtain the group velocity of the flexural mode as a function of frequency. Theoretical predictions were made using the Mindlin plate theory, which includes the effects of shear deformation and rotatory inertia. Our predictions on the dispersion of the flexural mode showed good agreement with the experimental results.     

Evaluation of the resistance of mortars coated with silver bearing zeolite to bacterial-induced corrosion

An evaluation of the resistance of mortar specimens coated with silver bearing zeolite to biologically produced sulphate and their antimicrobial characteristics was carried out in this study using Acidothiobacillus thiooxidans. The evaluation was performed based on leaching Ca²⁺ and Si⁴⁺ from the cementitious matrix, rate of bacterial sulphur oxidation, increase in biomass concentration, and acid production. The cumulative concentration of Ca²⁺ leaching were 3.5-folds higher (28.5 mg Ca²⁺/g cement) and 2-folds higher (18 mg Ca²⁺/g cement) in the uncoated and epoxy coated mortar specimens compared to those of zeolite coated specimens (9 mg Ca²⁺/g cement and 8 mg Ca²⁺/g cement for type AC and AK respectively). The cumulative leaching Si⁴⁺ was also 2.6-folds higher in the control mortar specimens (0.65-1.8 mg Si⁴⁺/g cement). Biomass concentration in the control specimens reached 210 mg TSS/L, and that of the zeolites coated mortar specimens was 103 mg TSS/L. The bacterial sulphur oxidation was found to follow the same pattern of the bacterial concentration, 230-270 mg SO₄/g S and 158-182 mg SO₄/g S in the control and zeolite coated mortar specimens, respectively. The resistance of the zeolitic coating was further evidenced by the increase in pH of the control mortar specimens which demonstrated the leaching of Ca (OH)₂ from the cement matrix. The stability of zeolite coated specimens was confirmed by the absence of corrosion products as was examined by FE-SEM and XRPD.     

An acoustic lens to measure wave velocities with the complex V(z) curve method

In this paper, an acoustic lens is designed and built in order to determine wave velocities from measured critical angles of waves, using the complex V(z) curve method. The properties of the new lens are evaluated by the transfer function of the lens and the time-domain response of the transducer. The performance of the new lens is examined using specimens of fused quartz and glass. These results are compared with those of a lens using the conventional V(z) method. It is shown that the critical angles of the waves can be determined clearly and simultaneously using the new lens.     

Fracture detection in concrete by glass fiber cloth reinforced plastics

Two types of carbon (carbon fiber and carbon powder) and a glass cloth were used as conductive phases and a reinforcing fiber, respectively, in polymer rods. The carbon powder was used for fabricating electrically conductive carbon powder-glass fiber reinforced plastic (CP-GFRP) rods. The carbon fiber tows and the CP-GFRP rods were adhered to mortar specimens using epoxy resin and glass fiber cloth. On bending, the electrical resistance of the carbon fiber tow attached to the mortar specimen increased greatly after crack generation, and that of the CP-GFRP rod increased after the early stages of deflection in the mortar. Therefore, the CP-GFRP rod is superior to the carbon fiber tow in detecting fractures. Also, by reinforcing with a glass fiber cloth reinforced plastic, the strength of the mortar specimens became more than twice as strong as that of the unreinforced mortar.     

Model based laser-ultrasound determination of hardness gradients of gas-carburized steel

Gas carburizing is a common industrial process utilized for case hardening of low carbon steels. However, there is a lack of non-destructive evaluation systems for the measurement of hardness-depth profiles. We propose a novel measurement method for the determination of hardness-depth profiles of two-step gas carburized steel specimens. The method is based on the measurement of broadband laser excited Rayleigh waves. Rayleigh waves were generated by a pulsed Nd: YAG laser in the thermoelastic regime and measured with a heterodyne Mach-Zehnder interferometer in the near-field. From two measurements with different source to receiver distances the dispersion diagrams were calculated by means of the phase spectral analysis method. In order to simulate the observed dispersive behavior of the Rayleigh waves, first the two-step gas carburizing process was simulated using solutions of the diffusion equation. The resulting continuous hardness profile was then discretized into up to 100 layers. Thereafter the Rayleigh wave dispersion diagram was calculated from the discretized stack of layers using a delta-matrix formulation of the Thomson-Haskell transfer matrix method. In order to obtain best fitting hardness profiles, the simulated dispersion diagrams were fitted to measurements with a curve fitting algorithm. Comparison of the Rayleigh wave inversion method with destructively obtained Vickers hardness profiles shows good quantitative agreement.     

Characterization of progressive microcracking in Portland cement mortar using nonlinear ultrasonics

This paper presents the successful application of a nonlinear ultrasonic technique, nonlinear wave modulation spectroscopy (NWMS) to quantitatively track the evolution of microcracks in Portland cement mortar samples. The damage type considered in this study is microcracking due to alkali–silica reaction (ASR), a deleterious reaction occurring in concrete structures around the world. Nonlinear ultrasonic measurements are conducted on six different mortar specimens that are monitored from their initial, intact state up to their fully damaged state. The objective of this research is to determine the sensitivity and suitability of NWMS to quantitatively track this damage state throughout an entire life-cycle and to nondestructively identify the initiation time and the extent of microcracking in these mortar specimens. The nonlinear ultrasonic measurements are made with standard laboratory equipment, and the inherent high attenuation of cement-based materials is overcome with a procedure that uses the sideband energy instead of measuring peak amplitudes. The results show that the NWMS method can track the progressive damage in mortar, demonstrating the feasibility of using this nonlinear ultrasonic technique to quantitatively assess the deterioration of cement-based materials.     

Application of X‐ray tomography for the verification of corrosion processes in chloride contaminated mortar

Within the scope of examinations at steel specimens embedded in chloride contaminated mortar for the first time X‐ray tomography was used to analyse the areas, damaged by chloride induced corrosion. Damaged areas with dimensions of a few μm could be detected. The results from the X‐ray tomography were verified by inspection of surfaces of the bars after removing the cover mortar.     

A new non-contacting non-destructive testing method for defect detection in concrete

A new non-destructive testing (NDT) method for defect detection in concrete structures is presented. The method is based on the dynamic response of flawed concrete structures subjected to impact loading. Conversely to similar NDT techniques, such as the impact-echo method, the present method uses non-contacting devices for both impact generation (a shock tube producing shock waves) and response monitoring (laser vibrometers measuring concrete surface velocity). Experimental and numerical (finite element) studies have been carried out for concrete specimens containing artificial defects (penny-shaped cracks parallel to the free surface) with varying length and depth. According to the experimental and numerical results, it appears that the present method enables an effective detection of defects, particularly in the range of shallow defects.     

Investigation of effect of fineness dependent volume of fine aggregate on passive layer formation and corresponding corrosion of steel reinforced concrete using electrochemical approach

This paper aims to determine the effect of the fineness modulus of fine aggregate on the quality and development of passive film and its subsequent effect on the corrosion of reinforced concrete structures. The study uses both electrochemical laboratory controlled experimentation and finite element modelling approach. Corrosion is an electrochemical process that requires an electrolyte for the occurrence of a corrosion reaction. Therefore, it is necessary to calculate the effective corrosion rate with reference to the saturated area of the concrete only when there is corrosion of reinforced concrete structures. Theoretically and numerically, the saturated area depends on the capillary zone porosity, gel zone porosity and degree of saturation in the aggregate free volume of concrete, which is a heterogeneous material. It is thus important to understand the effect of aggregate fineness on the corrosion of steel rebars. Therefore, detailed investigations were conducted using a variety of fine aggregates and combinations thereof. The initiative for this research came from the observation that the interfacial transition zone (ITZ) around the steel bar in concrete does not contain coarse aggregate, but could be surrounded by mortar and not only paste. Hence, the fine aggregate volume may also influence the corrosion rate. Some fine aggregate, which is finer than coarse aggregate, could be present in the vicinity of the steel bar surface, depending on the fineness modulus, which may affect both the development of the passive layer and the corrosion rate. The previous research data in this area were found to be limited. Hence, these factors have been investigated in the present study. Both mortar and concrete prismatic specimens were cast, and the quality of passive layer development and the corrosion severity were compared between specimens to determine the effect of fine aggregate volume with respect to the respective fineness modulus values. The corrosion potential, current and rate values obtained through experimentation showed significant differences in their magnitudes with respect to the variation in the fineness modulus. A directly proportional relation was observed between the fineness modulus of fine aggregate and the corrosion of rebars. Therefore, in the light of the experimental results obtained in this study, a finite element based numerical model incorporating the effect of the fineness modulus of fine aggregate on corrosion of steel rebars in concrete has been developed and successfully verified in this study.     

Korrosionsverhalten von Stahl in Zementmörtel bei kathodischer Polarisation in Meerwasser und 0,5 M NaCl-Lösung

Corrosion characteristics of steel in cement mortar under cathodic polarization in seawater and 0.5 M NaCl solution Steel specimens in cement mortar were exposed to seawater on the VDEh testing stand at Helgoland and to 0.5 M NaCl solutions in the laboratory up to three years. In seawater specimens covered with portland cement were cathodically protected by use of zinc and magnesium anodes. In the laboratory specimens in portland cement and in blast furnace cement were exposed to free corrosion and to potentials ranging from U_H = ?0.53 to ?0.98 V. In all cases small pittings were observed. Those developed in seawater were significantly dependent on potential and accompanied by a slight increase of chloride concentration. Here a normal anodic pitting corrosion must be assumed that cannot be totally prevented by zinc. The process slows down with increasing time. The pitting observed in the laboratory tests showed no systematic dependence on potential and the pits were substantially smaller. In the case of the strongest negative potential U_H = ?0.98 V the specimen in portland cement displayed pits as deep as 0.5 mm. These are assumed to be of cathodic origin. They were full of magnetite and virtually free of chloride. Because that far negative potentials, which lie in the region of hydrogen development, do almost not occur this type of corrosion contrary to literature has no technical bearing.     

聚合物涂层砂浆试样的交流阻抗谱研究

研究两种不同类型的聚合物涂层(丁苯乳液和氯化聚氯乙烯溶液)对砂浆试样的交流阻抗谱的影响及其对氯离子的抗渗性效果．试验发现，涂层砂浆试样的阻抗谱在高频区有一半圆，半圆的大小能够表征涂层的抗渗性．涂层厚度对阻抗谱的影响随涂层种类不同而不同．涂层试样在氯化物扩散前后的阻抗谱的研究表明，丁苯乳液涂层试验后更加密实，适合于混凝土的修补；但CPVC涂层扩散后电阻大大降低，不适合作混凝土的修补材料．     

小口径薄壁管中纵向缺陷的周向超声导波检测

对于小口径薄壁管的检测,通常采用超声水浸聚焦法。该方法灵敏度高、可靠性好,但不适于原位探伤,且检测速度慢、设备昂贵。为此提出了基于周向导波技术的检测方法。在实际检测过程中,观测到一组特殊回波序列,查明了这种回波是由侧向耦合剂液面反射产生,其对保证检测可靠性具有重要价值。通过数值计算,得到管材中周向导波的频散曲线,确定了导波模态,并计算了该导波的位移分布。试验及计算结果表明,该周向导波的能量主要集中在管材的外表面附近,可有效地进行小口径薄壁管的原位探伤,弥补了水浸聚焦法的不足。     

Damage evolution in cement mortar due to erosion of sulphate

Ultrasonic technique is used to detect the velocity change of stress wave propagated in the cement mortar immersed in the solution of sodium sulfate for 425 days. Also the density change of specimens at different erosion time is measured. By curve fitting, the effect of solutions’ concentration and water/cement ratio on the damage evolution is analyzed. The SEM observation on the growth of delayed ettringite is also performed. It shows that the damage evolution of specimens attacked by sulphate solution is dominantly induced by the nucleation and growth of delayed ettringite, and the average size of microvoids in cement mortar affects the damage evolution significantly.     

Non destructive determination of the free chloride content in cement based materials

A non‐destructive chloride sensitive sensor element for use in cement based porous materials is presented. The sensor element determines the activity of the free chloride ions in solutions and in porous cement based materials such as cement paste, mortar or concrete. The calibration in synthetic pore solution showed a response according to Nernst law over three decades of chloride concentration. The sensor element has shown excellent reproducibility and long term stability. The sensor element has been used to monitor the chloride uptake into mortar specimens. The results show a good agreement between the free chloride content determined by the sensor and by pore water expression. Applications in monitoring of reinforced concrete structures and their limitations are discussed.     

Die Möglichkeiten einer Wasserstoffversprödung von Spannstahl bei Einwirkung von Kohlensäure auf sulfidhaltigen Zementstein

The possibility of hydrogen embrittlement of reinforcing steel during carbon dioxide attack ou sulfide containing concrete Cement mortar (furnace cement, alumina-silicate cements) has been carbonated and the H₂S being generated has been quantitatively determined. The gas volumes measured are by orders of magnitude below the values theoretically expected. During the carbonate formation in mortar tubes advancing from the external surface only a small proportion of the H₂S penetrates into the interior of test specimens, e. g. 5·10^?6 H₂S (with reference to the mortar weight) in the case of a high furnace mortar containing 1,19% S, while a H₂S concentration by three orders higher was found in a tube of transformed alumina-silicate cement stone. This goes to show that the mechanism of reinforcing steel embrittlement as described in literature for alumina-silicate cements cannot be correct.     

Thermal–mechanical analysis of the surface waves in laser cleaning

The work described in this paper is to investigate the particle removal mechanism by surface wave in laser cleaning. A mathematical model of laser cleaning processes was established for a flat plate to remove micro-particles from the surfaces by surface waves. The software ABAQUS, based on finite element method, was used to simulate the non-linear stress wave problem for an uncoupled thermal–mechanical system in a two-dimensional domain. The result shows that the cleaned area increases with the laser energy, which led to surface waves with large acceleration in the vertical direction, but the effective cleaned area decreases with an increase of the laser spot size.     

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.