Detection of cracks in concrete composites using acoustic microscopy

We demonstrate that a scanning acoustic microscope (SAM) operating in the frequency range 100–400 MHz can detect cracks at the aggregate/paste interface and subsurface cracks in aggregates with high sensitivity. A complete interpretation of the obtained results has been made by combining high and low frequency acoustic microscopy, SEM, optical microscopy, and energy dispersive (X-ray) spectroscopy.     

Non-destructive thermal-wave-radar imaging of manufactured green powder metallurgy compact flaws (cracks)

Thermal wave radar imaging (TWRI) was developed to detect manufacturing cracks in automotive powder metallurgy components (transmission sprockets) in their green (unsintered) state. The crack detection capability of the TWRI phase was validated by two sets of cracked/crack-free green and sintered sprockets which were sectioned after TWRI measurements. An automatic defect recognition (ADR) TWR image processing method was also developed to differentiate cracks from local defects. Measurement results demonstrated that TWRI is superior to conventional lock-in thermography imaging (LITI) in both flaw detection resolution and speed, and thus is a viable green-sprocket manufacturing flaw imaging technology.     

A simple device for detecting impact time in impact-echo testing of concrete

The objective of this paper is to describe a simple device for detecting the time of an impact and to demonstrate its efficiency in measuring the depth of surface-opening cracks in concrete. Surface-opening cracks in concrete are relatively deep so that a mechanical impact instead of an ultrasonic transmitter is used to generate stress waves with enough energy for successful evaluation. A technique called the time-of-flight diffraction technique with impact-generated stress waves is an efficient method for detection of surface-opening cracks in concrete. Because the initial time of the impact is unavailable in a mechanical impact device, two receivers are employed in the test. One of the receivers is used to determine the time of the impact indirectly. The present study developed a simple device to record the time of impact initiation. The device comprises a conductive impact device, a sensing film, and an auxiliary circuit. With the device, stress waves are generated and the time of impact is detected so that the evaluation work needs only a single receiver. Laboratory studies were carried out on concrete specimens containing cracks with depths of 0.158 and 0.90 m. Field studies were performed on a dam damaged by an earthquake. Experimental results show that the new device can be used to accurately determine the crack depth in concrete. Using the device, the cost and effort for evaluation and signal analysis are reduced, which largely enhances the evaluation efficiency.     

Non-destructive testing in the high-temperature regime by using a magneto-optical film

The plane of a polarized light beam is rotated when the beam is transmitted through a magneto-optical film (MOF). Magnetic flux leakage (MFL) occurs around a crack when the specimen is magnetized and affects the magnetic domains. Therefore, a crack can be detected using a polarized lighting system and MOF. The crack detection ability depends on the domain width and saturation magnetization of the MOF. Also, crack detection at high temperatures is essential. The physical properties of the MOF at high temperatures were examined, and a nondestructive testing (NDT) method for detecting cracks at high temperatures was proposed in this paper.     

Analysis of the relation between water and resistivity isotherms in concrete

Concrete is a porous material, that contains an alkaline solution, whose pore network enables mass transport from the exterior. The concrete as composite material is quasi an insulator when it is dry and presents relatively low resistivities, in the order of few kΩ cm when it is fully saturated. Concrete resistivity reflects the degree of saturation and is therefore a useful indicator of the risk of chemical attack and of reinforcement corrosion. The dependence of resistivity on the degree of saturation has been studied, yet this subject has still not been fully elucidated. The present paper studies the evolution of concrete drying from the end of curing in order to analyze the water and resistivity isotherms. Four concrete mixes (w/c = 0.4 and 0.7 after 3 and 7 days of curing wrapped in plastic sheets) have been prepared and submitted to conditions of isothermic water desorption in controlled atmospheres with relative humidity (RH) of 55, 65, 75, 85, 95%. Resistivity measurements were simultaneously carried out which enabled to identify for the four mixes the water content and the RH which gives resistivity values higher than 10⁵ Ω cm. The radii of water‐filled pores calculated from Kelvin's law were found to be smaller than those calculated by integrating the mercury intrusion porosimetry (MIP) accumulated intrusion curve with the water lost during conditioning at different RHs. The water content measurement that seems to be most adequate for comparison with resistivity values is the volumetric fraction.     

Small crack detection in cementitious materials using nonlinear coda wave modulation

This paper presents an ultrasonic method, based on the nonlinear acoustic mixing of coda waves with lower-frequency swept pump waves, for providing an efficient global detection of small cracks in cementitious materials. By simultaneously comparing, for both uncracked and cracked mortars, the ultrasonic velocity variations and decorrelation coefficients between the unperturbed and perturbed signals with pump amplitude, this method makes it possible to accurately detect cracks with widths of around 20 µm in correlation with velocity variations of approximately 0.01%. The potential influence of certain material parameters such as microscopic damage is also discussed.     

Feasibility of detecting embedded cracks in concrete structures by reflection seismology

Reflection seismology has been widely used in the petroleum exploration industry for decades. To improve the capabilities of detecting the depths and lengths of cracks inside a concrete element, an ultrasonic common depth point (CDP) reflection technique was employed in this study. The effectiveness and accuracy of applying the reflection seismology method to detect cracks embedded in concrete structures are discussed. Test results show that simulated blind cracks with certain lengths, dip angles and depths inside concrete specimen can be successfully imaged and identified. This study shows that when the signals reflected from the crack are obscure, the CDP signal stacking method is very useful and provides an opportunity to look into the object, which is not possible with traditional ultrasonic methods.     

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.     

Advanced monitoring of cracked structures using video microscope and automated image analysis

Traditionally, the monitoring of cracks in buildings is performed by very simple techniques yielding only limited information. By means of a video microscope in combination with automated image analysis, an advanced and more precise analysis method of cracks in structures is obtained. The system was evaluated during laboratory tests on concrete beams and slabs. Furthermore, on site monitoring has been evaluated by measurements in a historical building in Ghent. The results were verified by means of traditional measuring techniques. For in situ monitoring of cracked structures, the combination of video microscope and automated image analysis seems to be a user-friendly and very accurate technique.     

Influence of the depth and morphology of real cracks on diffuse ultrasound in concrete: A simulation study

The aim of the present paper is to simulate the propagation of diffuse ultrasonic energy in concrete in the presence of a real crack. The numerical model is presented and validated by the comparison with experimental data from the literature. Unlike most of the studies which consider a crack as a notch, a realistic crack morphology exhibits partial contacts along its lips. These contacts are modeled in order to study their influence on the diffusion parameters. The feasibility of determining the contact density of the crack is shown, revealing practice implications for non-destructive crack sizing and imaging in concrete.     

Non-destructive evaluation of coating thickness using guided waves

For thin coating thicknesses, techniques for thickness characterization requires a high level of precision. On composite structures, actual bulk wave methods can be impaired by the presence of a periodic metallic mesh on its surface, thus reducing their performance. In this paper, a technique based on guided waves is proposed for coating thickness characterization. This method relies on the interpolation of the linear behavior of the S₀ mode at low frequency with regard to thin coating thicknesses. It is demonstrated numerically and experimentally that coating thickness can be estimated within 10 μm of the actual coating thickness.     

A comparative study of two non-destructive testing methods to assess near-surface mechanical damage in concrete structures

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.     

Non-invasive characterisation of ancient foundations in Venice using the electrical resistivity imaging technique

Electrical resistivity is a physical parameter linked to both texture and moisture content of the investigated bodies; therefore, its study provides reliable information about the internal structure and eventual degradation state of the elements generally used for construction purposes, like walls, columns, etc. Recent advances in hardware and software allow to acquire resistivity information of the investigated body in a ‘tomographic’ manner. This technique, called Electrical Resistivity Tomography (ERT), furnishes detailed two (2D) and three (3D) dimensional models of electrical resistivity distribution of the portion of the subsurface being investigated.In this work, the application of the ERT technique, to evaluate restoration quality of the building foundations after being consolidated, is described throughout a test site, located in Venice (Italy). The reconstructed 3D images of the resistivity distribution before and after consolidation provided details about the internal structural of the foundation itself as well as the volumes which have been occupied by the mortar after consolidation. Direct field test confirmed the diffusion of the mortar in the foundation and further laboratory analyses, conducted on a specimen taken at 80 cm, showed that cracks of 40 mm width had been completely filled with mortar.     

Modelling reinforcement corrosion – usability of a factorial approach for modelling resistivity of concrete

As part of the macro cell corrosion process of reinforcement steel, the resistivity of concrete plays a crucial role. In order to investigate the influence parameters on the resistivity of concrete, the results of a discrete quantification were presented and implemented into a factorial approach for modelling corrosion propagation. First results delivered significant deviations from values obtained by measuring. Using Gauss' method of least mean squares provided a decrease in deviations. The hereby obtained deviations were lower than the scatter of the measuring results. A usability of the proposed factorial approach could therefore be approved.     

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.     

Scan of surface-opening cracks in reinforced concrete using transient elastic waves

This study develops a method to scan the surface cracks of reinforced concrete using transient elastic wave tests. In the tests, an impact is applied at constant intervals along one side of the crack opening, and the surface response of the concrete due to each impact is measured and recorded. Then the procedure is repeated with the source applied on the opposite side. The method of ellipse intersection is adopted to process the surface response of the concrete structure. An image is then constructed that shows the three-dimensional image of the crack. The image can be used to determine the location of the reinforcing steel bars, the thickness of the covering, and to judge whether the crack penetrates through the rebars. Numerical examples and a model test are presented to verify the effectiveness of this method.     

On‐site investigations on concrete resistivity – a parameter of durability calculation of reinforced concrete structures

For determination the electrolytic resistivity of concrete, resistances of realistic designed probes are measured during a one and two year period, respectively. Measurements are made with a commercially available instrument normally used for measuring earth resistances. Comparison to other usually applied methods show not only that the used method GEOHM is qualified for measuring the electrolytic resistance of concrete but also that the magnitude of measured values is realistic. To obtain electrolytic resistivities from these measurements the individual cell constant for the investigated system was derived from experimental research. First results of the in‐situ stored specimens are presented, intended future activities are sketched.     

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.