J.H. Bungey

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.

SUB-SURFACE RADAR TESTING OF CONCRETE: A REVIEW

Abstract The background and current state of the art of radar testing of concrete is briefly reviewed. This encompasses developments of equipment, procedures, applications, analysis, interpretation and presentation of results. There has been substantial progress in all these areas over the past ten years and reference is made to recent and ongoing research. Key documents from the literature are identified including authoritative guidance reports on practical usage.

Using compression wave ultrasonic transducers to measure the velocity of surface waves and hence determine dynamic modulus of elasticity for concrete

Abstract A new method of measuring the velocity of surface waves and hence determining the dynamic modulus of elasticity and Poissons ratio for concrete or other solid materials is presented. When a pair of conventional compression wave ultrasonic transducers are placed on the surface of a solid, surface (Rayleigh) waves will be generated and propagate along that surface. Due to the marked difference between the amplitude of compression waves and surface waves, the leading edge of the surface waves can be identified on the waveform and their arrival time can be measured. Using the relationship between the dynamic modulus of elasticity, Poissons ratio and the velocity of surface and compression waves, the dynamic modulus of elasticity and Poissons ratio can be calculated. In the research project described in this paper, concrete specimens were made to permit measurement of dynamic modulus of elasticity and Poissons ratio using the proposed method and results were compared with values obtained by the conventional resonance method. These experimental results show that the proposed new method is reliable. Because specimens of specific shape and size are not required, the dynamic modulus of elasticity can be measured directly on in situ construction using this approach.

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.

Reinforced concrete design to Eurocode 2

Preface Notation Properties of Reinforced Concrete Limit State Design Analysis of the Structure Analysis of the Section Shear, Bond and Torsion Serviceability, Durability and Stability Requirements Design of Reinforced Concrete Beams Design of Reinforced Concrete Slabs Column Design Foundations and Retaining Walls Prestressed Concrete Composite Construction

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.

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.

Reliability of partially-destructive tests to assess the strength of concrete on site

Abstract The range of available tests for assessing the strength of insitu concrete based on measurements of surface zone properties is examined, together with developments in supporting documentation. Attention is concentrated upon a number of recent research programmes, including work undertaken as part of the European Concrete Building Project. These focus primarily upon pull-out and pull-off techniques, and encompass applications to early age strength assessment, lightweight and high strength concretes, and testing of repairs.

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.