D. M. McCann

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.

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.

Geophysical imaging inside masonry structures

Abstract Geophysical techniques offer the capability to non-invasively investigate engineering structures, both in terms of their spatial variability, and the prediction of properties of interest to the engineer from the ‘pixel’ values of the geophysical tomograms. The combined use of different geophysical methods provides a means of identifying the signature of salient engineering conditions. The response of seismic, radar and electrical resistivity tomography to simple targets within masonry structures is demonstrated. The benefits of combining rapid standard radar surveys with more detailed tomography is discussed and strategies for investigating typical structures developed. The response of the three methods, acoustic, electrical and radar to changes in the condition of masonry, such as reduction in strength and ingress of water, are discussed, with particular reference to a case study utilising the Ribblehead viaduct as an example. The corresponding responses to changes in ground conditions are also presented and shown to be important in understanding possible further applications of these methods, such as investigating behind retaining walls.

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