For an area along a beam or column a linear array of

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For an area along a beam or column, a linear array of transducers can be placed at intervals to monitor the structure under a changing load. In both linear and planar cases, any discontinuity in the structure may be picked up by multiple transducers. Thus, the advantage of a linear or planar array will allow the inspector to analyze the time differences of arrival to each transducer and calculate the approximate position of the active discontinuity. However, determining the type of monitoring and how to use it, will depend upon having the proper knowledge of everything that has occurred over the life of the structure up to the point that the discontinuity or damage was reported. In 1999, a group was established to develop a nondestructive test to detect corrosion in reinforced concrete. The resulting technique is unaffected by carbonated concrete, surface coatings or stray currents. The technique can be used to detect and indicate the rate of corrosion induced damage during the monitoring period. The system detects the microscopic damage created during the formation of expansive oxides at the interface of steel to concrete. This detection can occur long before deterioration is visible, enabling efficient maintenance planning. Corrosion forms oxides that expand to a greater volume than the steel from which they originated. This increase in volume exerts stresses within the concrete that cannot be supported by the limited deformation of the concrete, which results in microcracking and the rapid release of energy in the form of elastic waves. This microcracking not only will weaken the 307 Acoustic Emission Testing of Infrastructure F IGURE 1. In acoustic emission monitoring, waves are detected at the surface of the test object. Stimulus Stimulus Detection Measurement Transducer Instrument Source Acoustic emission wave Recording Interpretation Evaluation
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bond between the reinforcing steel and the concrete but also will create avenues for chloride ingress. Over years or months, corrosion products may accumulate, resulting in the spalling or disbonding of the cover. The result is a reduction in bearing capacity, serviceability and ultimate strength of the structure. When transducers are mounted directly on the surface (Fig. 2), the magnitude and intensity of the elastic wave can be recorded and then correlated with the properties of the concrete and corrosion rate. In some cases, this technique has detected corrosion where measurements of electrochemical potential have indicated an intermediate possibility of corrosion. Thus, acoustic emission testing has demonstrated greater sensitivity than other methods. Risk Based Investigation The success of acoustic emission testing relies on several variables from a protocol known as risk based investigation . Risk based investigation is a decision making technique that gained widespread acceptance in the 1990s among environmental engineers, who used it for future use assessment. There are three kinds of influences on risk based investigation: environmental, electrochemical and material influences.
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  • Fall '19
  • The Land, Nondestructive testing, Acoustic Emission

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