Michael Forde

Review of NDT methods in the assessment of concrete and masonry structures

Abstract This paper reviews the state of non-destructive testing (NDT) methods as applied to the civil engineering industry in the Millennium Year, 2000. The basic principles of NDT methods are described with particular reference to the five major factors that influence the success of a survey: depth of penetration, vertical and lateral resolution, contrast in physical properties, signal to noise ratio and existing information about the structure. The main NDT methods used in engineering investigations are discussed and illustrated with brief case histories from the literature. The integration of NDT surveys into the investigation of structures is described. The underlying reasons why NDT methods are perceived as ‘not working’ by the structural engineer are identified as: a lack of understanding of variability of both the construction materials used and NDT methods themselves.

Acoustic emission to assess and monitor the integrity of bridges

Abstract Increased axle loads and traffic density necessitate strengthening and widening of traditional masonry arch bridges. The question remains as to how to evaluate the condition of a bridge and assess the effect of modern traffic on its serviceability? The focus of this paper has been the evaluation of the potential for the acoustic emission technique to provide cost effective in-situ long term monitoring of bridge condition. The work was performed on a bridge chosen to be representative of both masonry and reinforced concrete construction methods. The results demonstrate that acoustic emission technique can be applied to condition assessment of bridges. These results confirm that strong acoustic emission signals are obtained from reinforced concrete structures and demonstrate that acoustic emissions can be detected in masonry structures but at lower energy levels. The experimental programme has shown that AE is useful in detecting crack growth and determining the position of the crack tips at a much earlier stage in their development, before they are noticed during visual inspection

Application of sonic and radar tests on the piers and walls of the Cathedral of Noto

Abstract The authors applied systematically sonic tests, radar tests and other diagnostic techniques on the remaining walls and piers of the Cathedral of Noto. The experimental survey was carried out by the Laboratory of the DIS, Politecnico of Milan (Person in Charge: Prof. Binda, Prof. Baronio) in collaboration with the designers (Ing. R. De Benedictis, Arch. S. Tringali) and the experts working on the reconstruction. The Prefettura of Siracusa founded the research. The right aisle and most of the dome of the Cathedral collapsed in 1996. The aim of the research was to verify the state of damage and the possibility of conservation of the walls and piers in view of the reconstruction of the damaged part of the Cathedral. Furthermore, the sonic tests were used to control the effect of grout injection used as a possible technique for repair of the damaged masonry (Proceedings of the Conference on Nondestructive Evaluation of Civil Structures and Materials, Boulder, Colorado; (1992) 329; Proceedings of the Sixth Conference on Structural Faults and Repair, 3 (1995) 195). In the following the results will be reported and the reliability of the tests will be discussed as it was confirmed by the use of other complementary diagnosis techniques (Proceedings of the Sixth International Conference on Structural Studies, Repairs and Maintenance of Historical Buildings, STREMAH 99, Dresden, Germany, (1999) 323).

Electromagnetic properties of railway ballast

Abstract Laboratory experiments were undertaken to identify and characterise the dielectric properties of railway track ballast using Ground Penetrating Radar (GPR). Better results were obtained with lower frequency antennas. Clear distinctions were obtained between wet and dry and clean and spent ballast. The laboratory experiment is described in detail and sample radar scan plots given — the final analysis of dielectric constants is also given. The implications of the findings for radar velocity are discussed. The application to identifying defects in railway track bed is discussed.

Radar measurement of bridge scour

Abstract Scour around bridge piers and abutments has resulted in many structural failures over the past decade and considerable research has been carried out to develop methods which can be used to evaluate the risk of scour affecting the integrity of these major structures. Traditional site investigation methods based on borehole core and samples of the riverbed sediments are expensive and time-consuming and may not always give a complete assessment of the lithological variation in the riverbed sediments. Geophysical methods can be used to determine the riverbed profile beneath the water in a river and may also be of value for obtaining the previous scour history below the riverbed level. Trials of ground penetrating radar (GPR) have indicated that this geophysical method is particularly effective in determining the sub-bottom geological structure in a shallow freshwater environment. In this presentation the results from a number of scour surveys using GPR are presented and discussed. It is concluded that GPR surveys can be effective in determining both the water depth and sub-bottom geological structure near bridge piers and abutments provided that the correct instrumentation and operational procedures are adopted.

The application of time domain ground penetrating radar to evaluate railway track ballast

This paper will evaluate Ground Penetrating Radar (GPR) as a Non Destructive Test (NDT) technique for the assessment of railway track bed ballast. Ballast and tracked deterioration is responsible for the majority of differential settlement of tracks, resulting in poor track geometry. Traditionally ballast is only replaced when it has visually deteriorated, resulting in the planning and programming of maintenance and renewals currently being condition driven. There is a significant potential cost saving if ballast deterioration can be detected earlier. GPR can survey large lengths of track ballast in a relatively short space of time. This paper will show how GPR can be used to quickly identify the degree of track bed ballast deterioration and detect the ballast/formation interface.

AE energy analysis on concrete bridge beams

Concrete bridges were built originally to have a cost-effective maintenance free life of 120 years, however both in Europe and the USA this has proved to the totally optimistic. Concrete bridges require major maintenance after twenty to thirty years in order to extend their life The Acoustic Emission technique has been successfully applied to monitor and provide information on the safety of concrete bridges, but the processing of the AE data is often not trivial. This paper proposes a method of AE analysis to assess concrete bridges based on the fact that the AE energy is one of the effective parameters to evaluate the damage of a concrete structure. Results from experiments on concrete beams with different design and loading configurations are presented. It is argued that in some cases the state of damage of bridge beams can be estimated from the AE energy analysis when approximately 45% of the ultimate load has been applied.

Train speed calculation using ground vibrations

This paper presents a high precision train speed calculation technique based on ground vibration information. This versatile method can calculate speeds for trams, intercity locomotives and high speed trains on any track/embankment arrangement. Additionally, it has high accuracy for sensors located up to 100 m from the track, thus allowing semi-remote, non-invasive monitoring of train velocities. The calculation method combines three separate speed calculation techniques to provide estimates for arbitrary train speeds, even for sensors placed at large track offsets. The first estimation technique involves the use of cepstral analysis to isolate key harmonics for use with speed calculation. The second method is similar; however, the combination of a running rms and a previously developed “dominant frequency method” are used. The third method uses an analytical vibration frequency prediction model in combination with regression analysis to calculate train speed. All three methods are combined into one calculation procedure, resulting in high accuracy estimates. To show the robustness and ability of the new method to calculate a wide range of train speeds, it is used to predict tram, intercity and high speed rail train passage velocities generated from a previously validated vibration prediction numerical model. More importantly, it is used to predict train speeds during field trials performed on operational railway lines in Belgium and in UK. The new method is shown to offer high performance for several train types and track setups (including abutment and tunnel cases).

Infrared thermographic investigation of railway track ballast

Abstract A theoretical study was undertaken to determine if infrared thermography is an appropriate method to identify the condition of railway track ballast. Within this study the optimal conditions for an infrared survey were established. A laboratory experiment was undertaken to identify clean and spent ballast using an infrared camera. By cooling the ballast and watching it heat up to room temperature over time a difference in the rate of heat transfer between the two types of ballast was observed. A field trial of the infrared camera was undertaken over an operational track as part of the work within a normal track maintenance possession. The field trial identified an area of dirty ballast within a section of clean ballast and the findings were also confirmed by using ground penetrating radar and a trial pit. The laboratory and fieldwork are described in detail and sample infrared images as well as visual images of the same area are given — along with calculated values for the emissivity. The findings proved that it is possible to calibrate an infrared camera so that it can determine the condition of the ballast. Also from the success of the field trial it was shown that infrared thermography is a suitable method of identifying the condition of ballast on an operational track

A theoretical evaluation of impulse radar wave propagation through concrete

Abstract It has been shown that one of the most important factors affecting a radar survey of a concrete structure is the operational frequency of the antenna. The operational choice of frequency is difficult as there is always a compromise between signal penetration and resolution. Whilst case histories have proved valuable sources of information, they have not yielded the systematic data needed by structural engineers specifying investigative studies. In the work reported herein, a systematic parametric analysis was undertaken of electromagnetic wave propagation through concrete structures. From this, the use of the simplified expression for wave velocity in relation to concrete structures is seen to be erroneous at lower frequencies - especially at higher conductivities. For a given frequency the skin depth )a measure of penetration( drops as conductivity increases. Also, the frequency of the electromagnetic pulse through concrete is shown to be the order of 75% of the centre frequency of the antenna a...