Muhammad Zaid

Image reconstruction of steel reinforcing bars in concrete using Fourier-domain interpolation applied to a sparsely populated data set

High-resolution image generation of bars in concrete using a single coil scanning inductive sensor is a time consuming process. This paper presents a method of generating the image using a sparsely populated data set (SPDS), obtained from a reduced number of scan lines, whose use is justifies by an analysis of the sensor spatial frequency response. Three methods are discussed: linear and cubic spline interpolation in the spatial domain, and Fourier interpolation, all of which are applied to an SPDS. Data for the SPDS are provided from a widely spaced scanning regime implemented in both the x and y-axes. Interpolated values are then synthesized to obtain high-resolution images. The Fourier-based method employs zero padding in the spatial frequency domain and inverse Fourier transformation to obtain higher resolution data in the time/spatial domain. In general, the results obtained by the linear interpolation algorithm are unacceptable since they do not represent the point-spread function of the sensor. The results obtained by the cubic spline and Fourier methods are very satisfactory, with the deviation from the results obtained by the standard high-resolution image generation process being very small. However, the cubic spline method is cumbersome to implement, requiring the computation of a large number of unique polynomials. In contrast, the Fourier algorithm is efficient, straightforward to code and yields an ideal band-limited interpolation. Experiments show that this new methodology is faster than the traditional scanning protocol by at least a factor of 10; a large area scan of 0.5 m2 can thus be produced in 12 min, rather than 2 h. In the future, this technique could be applied to widely spaced solid-state arrays, requiring a fraction of a second for image synthesis.

Inductive and magnetic field inspection systems for rebar visualization and corrosion estimation in reinforced and pre-stressed concrete

Inductive and magnetic field inspection systems are becoming increasing popular for the nondestructive imaging and condition assessment of reinforcing components, such as steel reinforcing bars (rebars) and tendons in reinforced and prestressed concrete structures. In this article, we review the principles of this nascent technology, the research and commercial instruments that are now available, and the directions of future research. Magnetic field imaging technology has in general many potential benefits; it is truly nondestructive and non-invasive, it is non-hazardous, cost-effective and, most important, ignores the concrete matrix in which the reinforcing components are embedded. Most significantly, by analysing the impedance change in an excitation coil, it is also possible to obtain quantitative information (and image data) in relation to regions of corrosion. However, the testing industry has traditionally been reluctant to apply this methodology, for the important reason that the detection range is limited by the rapid attenuation of the magnetic field with increasing distance from the source. This limitation is now being addressed with research into novel coil arrangements, new, more stable and sensitive solid state sensors, and reconstruction algorithms that allow virtual three dimensional reconstruction of embedded components.

Detection of Magnetic Fields Highly Attenuated by the Skin Effect Through a Ferrous Steel Boundary Using a Super Narrow-Band Digital Filter

A complete instrumentation system is described that is capable of launching alternating magnetic fields through a large mild steel plate that is 2 mm in thickness and detecting them on the face opposite from the transmitter with remarkable signal-to-noise ratios. Results for signal frequencies ranging between 4.5 and 13 kHz are reported. The skin depth at 9 kHz for the steel used is approximately 137 mum. The detection of the minute fields arriving at the receiving coil is made possible by the use of digitally synthesized input signals, low-noise amplification, and, in particular, the use of a powerful real-time digital signal processing system that isolates the signal of interest using a super narrow-band filter and very high levels of distortion-free gain. Although traditional methods of weak signal detection, such as lock-in amplification, may also be applied in this context, the digital approach discussed here is both more cost effective and flexible, allowing the simultaneous detection of multiple frequencies.

The use of magnetoresistors for imaging steel bars in concrete

This paper reports on the design of a new system for the detection and imaging of steel bars in concrete. It is based on the magnetoresistive effect, whereby the resistance of a material changes due to the presence of a magnetic field. An innovative way of processing and separating the steel characteristics is presented. Here, the combination voltage, frequency and distance parameters generate a unique signature for each metal, profoundly enhancing the analysis.

A Transformer Coupling Method for Imaging Defects in Concentrically Arranged Steel Tubes

This paper describes the development of a system that uses a transformer coupling method for imaging defects in the outer tube of a concentric steel tube pair. Transformer coupling is employed where a receiver is mounted opposite to the transmitter. Magnetic field coupling is used, in which the field propagates through the inner steel tube towards the outer steel tube. Defects in the outer section distort the field and the change is detected by the receiver.

Assessment of Corrosion Activity in Reinforcing Steel Bars Embedded Within Concrete Using the Inductive Scanning Technique

Corrosion processes with time may cause cracking, delamination and finally catastrophic failure of concrete structures. From a safety aspect as well as economic reasons, it is necessary to monitor concrete structures to detect corrosion at its early stages utilizing non‐destructive testing techniques. In this paper, a new monitoring technique using inductive scanning technology is presented. Results obtained using this technique are compared with the potential mapping data. The advantages and limitations of both techniques are discussed.

Preliminary results: a three coil balanced system for imaging steel bars behind steel boundaries

This paper describes preliminary results obtained from a modified transformer coupled system for detecting and imaging steel objects on the distal side of a ferrous steel boundary up to 2 mm thick, using a balanced three-coil system. Typically, it may be applied for the imaging of steel reinforcing bars embedded within concrete, which includes a sheet steel layer located between the bars and the upper surface of the concrete. A transmitter, receiver and a dummy coil are placed on the upper surface of the specimen and moved across it. The dummy coil is used to cancel the common mode signal (cross-talk) between the transmitter and receiver, thereby enhancing the signal emanating from the steel bars. By exciting the transmitter with signals ranging between 125 Hz and 1 kHz, it is possible to generate a magnetic field that is not severely attenuated by the sheet, enabling the detection of a ferrous target located below the sheet. In addition to signal cancellation, a real-time digital signal processor is employed to perform very narrowband signal identification, amplifying the signal-to-noise ratio considerably. Both good quality and defective steel bars with diameters of 16 mm have been successfully imaged below mild steel sheets up to 2 mm in thickness.

A high sensitivity transformer coupled system for the detection and imaging of breaks in prestressing tendons in concrete pipes

A system has been developed, described here, which can detect and image localised faults and breaks in the prestressing tendons of large bore concrete pipes, intended for the transport of water. The system exploits the principle of magnetic field transformer coupling, in which a low frequency time varying field is transmitted over the region being inspected. Perturbations in the amplitude and phase of the signal that arise from variations in the coupling conditions are detected by a receiving system and fed to instrumentation for processing, image reconstruction and interpretation. The system has been field trialled in 4 m diameter pipes in the Sahara desert. Results confirm that it is capable of detecting and imaging as few as two adjacent tendon breaks, in which the diameter of the tendon was 7 mm and the pitch was 20 mm.

Finite Element Analysis for Imaging Steel Bars Placed Under a Mild Steel Boundary Using Eddy Current Techniques

This paper reports on recent modelling results obtained using finite‐element analysis for penetrating a magnetic field through a 2 mm steel boundary. The object is to detect 16 mm steel bars placed under mild steel boundaries at different operating frequencies. To penetrate thicker steel boundaries and increase the depth penetration, a different configuration based on remote field eddy currents (RFEC) has been modelled.

Automatic Corrosion Classification and Quantification of Steel Reinforcing Bars Within Concrete Using Image Data Generated by an Inductive Sensor

This paper presents a methodology to automatically distinguish and quantify the corrosion of reinforcing bars within concrete using images generated by an inductive sensor. The methodology comprises three stages; image generation using the inductive sensor, image segmentation and feature extraction and neural network object classification. Preliminary results have shown that the methodology has correctly classified all the corroded parts on the testing samples while estimated the corrosion rate correctly on 80% of the testing samples.