F.M. Burdekin

The propagation and attenuation of medium-frequency ultrasonic waves in concrete: a signal analytical approach

The manner in which medium-frequency ultrasonic pulses travelling through concrete are generated, received, digitized and analysed is described. Due to the highly attenuating nature of this medium and its differential effects on the frequency composition of broad-band signals, signal analysis was performed by partitioning the signal into discrete windows in the time domain, corresponding to the emergence of individual wave phenomena within the medium. These windows were then transformed to the frequency domain for subsequent filtering and interpretation. Experimentation combined with theoretical modelling has shown that the appearance and decay of discrete frequency bands depends on both the composition of the concrete, termed the resonance phase, and its external importance with respect to the ultrasonic inspection of concrete and other such inhomogeneous materials.

An inductive scanning system for two-dimensional imaging of reinforcing components in concrete structures

A novel form of imaging steel reinforcing components in concrete is described which employs a scanning inductive sensor. A host computer, linked to the scanning and data acquisition system, generates grey-scale images whose intensities are proportional to the signal strengths produced by the sensor. Simple signal subtraction algorithms have been employed to achieve considerable improvements in image contrast, spatial resolution and the extraction of dimensional information. Models of the sensor response suggest that with further signal manipulation, detailed quantitative measurements on image features will also be possible.

Digital deconvolution analysis of ultrasonic signals influenced by the presence of longitudinally aligned steel cables in pre-stressed concrete

Several experiments are described in which ultrasonic signals were transmitted and received through concrete containing steel rods and cables. Details of how the signals were processed to extract information relating to the condition of the steel itself are presented. In particular, the method deconvolution is employed in a manner which allows the impulse response and frequency response of the embedded steel alone to be uniquely isolated from the effects of the surrounding concrete, whilst still allowing the transducers to be mounted on the surface of the concrete.

Investigation of block filtering and deconvolution for the improvement of lateral resolution and flaw sizing accuracy in ultrasonic testing

Abstract In ultrasonic testing, the accuracy of flaw sizing is normally low because of the use of relatively long wavelengths. In this paper, we investigate two signal processing methods: block filtering and block filtering with deconvolution for the improvement of lateral resolution and flaw sizing accuracy in ultrasonic testing. In block filtering, each returned echo is Fourier transformed, and a band of frequency components is used to build a C-scan image that is based on the estimation of flaw size. In deconvolution, a block-filtered C-scan image is deconvolved with the point spread function of a transducer by a frequency-domain Weiner deconvolution algorithm. The deconvolved C-scan image is then used for the determination of flaw size. A unique feature in our implementation of the deconvolution technique is that the point spread function is obtained based on an ultrasonic scattering model1. By doing so, a separate experiment needed for measuring the point spread function is avoided2. Experiments were conducted on several artificial flaws with different sizes (flat-bottom holes). It was found that the C-scan images, after processing, become sharper; flaw sizing errors were reduced after block filtering and could be further reduced by deconvolution if proper parameters in the deconvolution were selected.

A Novel Inductive System for Two-Dimensional Imaging of Reinforcing Components in Concrete Structures: From Hardware to Image Enhancement

There is now growing concern regarding the safety of many prestressed concrete structures [1]. Of primary importance is the condition of the reinforcing steel, and much research effort has been expended on the development of non-destructive systems for assessing the integrity of internal reinforcing components [2–5]. Concrete is notoriously difficult to inspect, and most civil engineers, when assessing the quality of the internal reinforcing steel, will resort to a battery of tests, since a single test in isolation will rarely provide sufficient information. The system described below is a novel re-bar imaging system, based on an inductive principle, which should eliminate the need for many ancillary tests once fully developed [6]. A sensor which responds to the area and depth of metal in its sensing region is scanned across the concrete under inspection by a system linked to a computer. The signals acquired are stored, processed and displayed as grey level images. The initial images are blurred, due to the point spread function (PSF) of the sensor. These images have been significantly enhanced using a form of processing termed digital deconvolution. This form of processing allows considerable improvements in image quality to be realized from a single scan if the PSF of the sensor is known. In this instance, the PSF has been obtained through the development of an empirical model.

Magnetic Field Imaging of Steel Reinforcing Bars in Concrete Using Portable Scanning Systems

Concrete structures throughout the world face the danger of sudden collapse if the condition of their steel reinforcement is not monitored regularly. Bridges are the most vulnerable to catastrophic failure as they continuously carry dynamic loads. In the United Kingdom, there are more than 150,000 concrete bridges and the annual repair bill is estimated at £500 million. In the United States of America, the situation is even more serious. It was reported in 1977 that more than one in six bridges were unsafe and that it would take more than two centuries to replace the 40,000 deficient structures, given the rate at which funds were currently available [1]. The repair costs for the rest of the world can only be guessed at.

A multi-sensor array inductive scanner for rapid imaging of reinforced and pre-stressed concrete

This paper describes the development of a portable multi-sensor array inductive system for rapidly imaging steel rebars in concrete. Latest research shows that phase images can be generated which reveal areas of surface corrosion and that impedance changes are useful for locating the boundaries of corroded regions. The system comprises a microprocessor controlled acquisition system and a software suite containing algorithms for image restoration. It will enable engineers to perform quick and easy on-site testing.

An ultrasonic inspection system capable of detecting voids and corroded steel components embedded in prestressed concrete structures : Colloquium on Measurements, Modelling and Imaging for Non-Destructive Testing, London (United Kingdom), 27 Mar. 1991. pp. 4/1–4/3. IEE (1991) Digest No. 1991/054

An ultrasonic system has been developed which detect voids, voids in ducts, major cable breaks and severe corrosion damage. Transducers are employed that generate a broad range of frequencies. The system operates by investigating the changes that occur in the different frequency bands as the signal traverses through the concrete. The central hypothesis of the signal processing methodology is that different wavelengths will be reflected with different energies depending upon the size, geometry and acoustic impedance of the defect encountered. By partitioning the reflected signal into discrete frequency bands and by comparing the magnitudes of their energies, it is possible to derive information relating to the position, size and nature of the defect.

An ultrasonic inspection system capable of detecting voids and corroded steel components embedded in prestressed concrete structures

An ultrasonic system has been developed which detect voids, voids in ducts, major cable breaks and severe corrosion damage. Transducers are employed that generate a broad range of frequencies. The system operates by investigating the changes that occur in the different frequency bands as the signal traverses through the concrete. The central hypothesis of the signal processing methodology is that different wavelengths will be reflected with different energies depending upon the size, geometry and acoustic impedance of the defect encountered. By partitioning the reflected signal into discrete frequency bands and by comparing the magnitudes of their energies, it is possible to derive information relating to the position, size and nature of the defect.