S.G. Millard

Dielectric properties of concrete and their influence on radar testing

Sub-surface radar is becoming increasingly popular as an inspection method. Interpretation can be enhanced if uncertainties about the dielectric properties of the concretes under investigation are removed. The need for reliable data to identify possible variations of the dielectric properties of different concrete mixes and their condition on site has led to a systematic laboratory based experimental programme under the auspices of a major European Commission (Brite-Euram III Framework 4) funded project. Some key results from this recently completed work are presented in this paper with practical implications related to field surveys of structural concrete.

Measurement of Concrete Resistivity for Assessment of Corrosion Severity of Steel Using Wenner Technique

The Wenner technique is becoming increasingly popular for measuring resistivity of reinforced concrete to assess the rate of corrosion of the steel reinforcement once it becomes depassivated by chloride attack or carbonation. This paper summarizes work carried out over several years in assessing the efficiency of this technique. Several sources of measurement error are identified, and recommendations are put forward to enable engineers to obtain more accurate results when using this technique in practical situations to evaluate the severity of ongoing corrosion problems.

Environmental influences on linear polarisation corrosion rate measurement in reinforced concrete

Abstract The linear polarisation resistance (LPR) method can be used to measure the active rate of corrosion of steel reinforcement in concrete structures. However a single measurement may be sensitive to the ambient environmental conditions and thus may not be representative of the mean annual rate of corrosion. The LPR measurement may be dependent upon the temperature at the time of sampling and on any wetting/drying effects resulting from rainfall and wind or sunshine. Recent studies have examined the behaviour of a series of reinforced concrete specimens subject to chloride contamination or carbonation induced corrosion in a controlled laboratory environment. LPR measurements taken at frequent intervals have been related to a wet/dry cycle imposed upon the specimens to promote corrosion activity. In addition, similar studies have been carried out, of an in situ reinforced concrete specimen exposed to the ambient weather conditions. The study is only partly completed, but interim results show the variability of LPR measurements and the influences the environment may exert on single spot measurements.

Assessing bridge pier scour by radar

Detection of river-bed scour around bridge piers represents a major current problem for engineers both in the UK and internationally. Techniques using boat-mounted impulse radar survey equipment have been suggested as a potentially viable solution to the problem. A programme of systematic laboratory model tests has been undertaken to examine the influences of pier and hole geometry upon measured radar response. These tests have been supplemented by field trials on real scour holes. Results from both laboratory and field studies have been used to validate numerical models developed using ray-tracing and finite difference techniques.

Field pattern characteristics of GPR antennas

Sub-surface radar has become increasingly popular for carrying out completely non-invasive integrity tests on concrete structures. Commercial systems are available with a range of antenna frequencies that may be selected for an investigation. The choice of a particular antenna frequency is often a compromise between the physical size of the antenna and the penetration and resolution capabilities provided by the antenna. However, there is an interaction between a surface contact antenna and the substrate under investigation that will alter the field pattern being transmitted that may have significant implications for interpretation of results. An experimental programme is described in which the characteristics of commercially available 900 MHz and 1 GHz antennas are compared in air and when in contact with concrete and water surfaces, together with an emulsion used in previous studies to simulate the radar properties of concrete. Results are presented showing the influence of the material being measured upon the signal divergence. These effects are considered in terms of the practical implications for field-testing using radar.

Programmable linear polarisation meter for determination of corrosion rate of reinforcement in concrete structures

AbstractA programmable linear polarisation meter has been developed principally for on site measurement of the corrosion of steel reinforcement in concrete. The special features of this instrument are its programmable potentiostatic mode of operation. a simultaneous 300 Hz ac measurement facility to evaluate the resistance due to the concrete, and an instrument earth connection to maintain a monitoring bar at the same potential as the main reinforcement when they are uncoupled to perform linear polarisation. The features of the instrument are experimentally investigated and assessed.

Radar assessment of structural concrete using neural networks

An assessment of the capability of artificial neural networks to interpret radar images of reinforced concrete is presented. The capability of three-layer, fully connected networks to detect the presence of a bar, the size of a bar, and the depth of a bar is examined. The study demonstrates that the use of a neural network approach to interpret complex sub-surface radar results of embedded reinforcing bars is promising. The neural network approach is successful in locating reinforcing bars over a range of embedment depths on almost all occasions. In addition it is shown to be possible to place each bar into a depth category with a high degree of success.

Linear polarisation resistance measurements using a potentiostatically controlled guard ring

The use of a sensor controlled guard ring has been developed in recent years to enhance the accuracy of linear polarisation corrosion rate measurements on reinforced concrete structures. The established method of measurement uses a galvanostatically controlled guard ring device. In the method reported in this article the reinforcing steel is polarised potentiostatically by an inner auxiliary electrode and the real time plot of the current response is displayed on a laptop computer which controls the guard ring device. The area of steel polarised is confined by a current applied from an outer guard ring electrode which is controlled by two sensor electrodes positioned between the inner auxiliary and outer guard ring electrode. The potential between the two sensor electrodes is continually monitored, and the current output from the guard ring electrode varied to maintain a constant potential difference between the two sensor electrodes. The method has been validated on an electrical test circuit simulating active and passive reinforcement corrosion. Further testing has been conducted on a reinforced concrete specimen and the results compared with those of commercially available galvanostatically controlled equipment and those of standard unconfined linear polarisation resistance measurements.

Galvanostatic Pulse Measurements of Passive and Active Reinforcing Steel in Concrete

Results of galvanostatic pulse transient response experiments were reported to determine the corrosion parameters associated with passive and actively corroding reinforcing steel in concrete. Galvanostatic pulse measurements were conducted on a number of short sections of steel-reinforcing bar embedded in concrete. The duration of the applied galvanostatic pulse varied from 45 s to 600 s. The bars displayed a range of corrosion rates, from passive steel to highly active corrosion. Analysis of the galvanostatic pulse transient enabled separate components, which made up the measured transient, to be isolated. These components displayed a range of resistances and associated capacitances, dependent upon the corrosion state of the reinforcing steel, which may be attributed to the corrosion process or other effects within the bulk concrete. The corrosion rate was calculated from the summation of the separate resistive components and compared with the corrosion rate determined from linear polarization resistance (LPR) measurements on the same specimens. Results from the galvanostatic pulse transient analysis showed that significant errors in evaluating corrosion rates using the LPR method may be incurred if the usual 30-s equilibrium period was used for measuring actively corroding steel bars.

Fast-Track Construction with Slag Cement Concrete: Adiabatic Strength Development and Strength Prediction

The early-age strength development of concrete containing slag cement has been investigated to give guidance for its use in fast-track construction. Measurements of temperature rise under adiabatic conditions have shown that high levels of slag cement-for example, 70% of the total binder-are required to obtain a significant reduction in the peak temperature rise. Despite these temperature rises being lower than those for portland cement mixtures, however, the early-age strength under adiabatic conditions of slag cement concrete can be as high as 250% of the strength of companion cubes cured at 20 °C (68 °F). The maturity and, hence, strength development were calculated from the adiabatic temperature histories based on several maturity functions available in the literature. The predicted strength development with age was compared with the experimental results. Maturity junctions that take into account the lower ultimate strengths obtained at elevated curing temperatures were found to be better at predicting the strength development.