Detection of reinforcement corrosion by an acoustic technique

Corrosion of reinforcing steel is a major serviceability issue with reinforced concrete structures, often resulting in significant section and bond loss. However, current non-destructive diagnostic techniques do not allow corrosion to be reliably detected at the very early stages of the process, before damage to the concrete occurs. This research describes the development of an Acoustic Emission (AE) technique as a practical tool for the early detection of corrosion of reinforcing steel embedded in concrete. The study falls into three main areas: (i) determining the influential material parameters of reinforced concrete that affect the magnitude of the acoustic emissions; (ii) investigating the influence of diurnal and seasonal temperature variations on corrosion rate and thus the rate of acoustic emissions; and (iii) developing a testing and evaluation procedure that combines the findings of the first two stages with existing knowledge about corrosion and deterioration of concrete structures. In the first phase of the research material parameters such as cover thickness, compressive strength and rebar diameter were investigated to ascertain the influence of varying these factors on the magnitude of AE Energy obtained per gram of steel loss. The experimental results confirmed that early age corrosion, verified by internal visual inspection and mass loss measurements, can be detected by AE before any external signs of cracking. Furthermore results show that compressive strength was the primary influential parameter, indicating an exponential, empirical relationship between compressive strength and AE Energy. An increase in temperature usually induces an increase in corrosion activity, which should be measurable using the AE technique. Consequently the influences of seasonal and diurnal temperature variations were investigated to determine their impact on undertaking AE measurements. This phase of the research demonstrated that seasonal variations in temperature impart a negligible influence on measured AE Energy. However measurement of AE Energy per hour followed trends in the diurnal temperature and corrosion rate evolution, these being in a state of constant flux. Therefore AE measurements of corrosion in reinforced concrete are more responsive to a change in temperature, and so corrosion rate, as opposed to a specific and constant corrosion rate. In the final phase practical experience with AE from site trials and laboratory work were coupled with leading research and existing knowledge of corrosion in concrete and structural deterioration, to develop a testing and evaluation procedure for on-site application. This rigorous procedure enables reliable corrosion measurements to be undertaken on reinforced concrete structures using AE technology and enabling an assessment of the rate of corrosion induced damage to be made. As far as the author is aware this is the first site testing procedure for detecting corrosion in reinforced concrete using AE. Future research in this area might involve more site testing with a view to improving accuracy and analysis of on-site data, underpinning the developed procedure.