Abstract
Structural health monitoring (SHM) is a term used in the last decades to describe a range of systems implemented on constructed facilities (including reinforced concrete (RC)) with the purpose of assisting and informing owners/operators on the condition of structures under gradual or sudden changes to their state of serviceability. At the simplest level and with reference to RC, recurrent visual observation and assessment of structural condition (e.g., corrosion, cracking, spalling and deformations) could be viewed as SHM activities. Nowadays, the aim of research efforts is to develop effective and reliable means of acquiring, managing, integrating and interpreting structural performance data. By taking full advantage of the progress in modern technologies, there is an untapped potential to make the assessment of existing RC structures more accurate and cost efficient. Among the various nondestructive techniques (NDT), acoustic emission (AE) monitoring is arguably based on the simplest physical concepts (nearly everyone has heard audible AE in the form of popping and cracking noises from materials under stress), but is one of the most difficult techniques to practically implement. A formal definition of the AE phenomenon is often given as the release of transient elastic waves in solids as a result of rapid localized redistributions of stresses which accompany the occurrence of damage mechanisms. Examples of AE events related to civil engineering materials include yielding of steel, crack growth in steel and concrete, corrosion for metals, fiber breakage, and matrix debonding for composites. In this dissertation, AE technology is applied to RC members in order to identify and evaluate damage. The dissertation is articulated into three studies. The first and the second study (Study 1 and Study 2) focus on the identification of damage and more specifically in the detection of the onset of corrosion by means of AE. The last study (Study 3) presents an innovative AE methodology to assess damage in RC members during load testing. Study 1 describes the AE monitoring of an experimental campaign on small-scale RC specimens under accelerated corrosion. This study was conducted to investigate the effectiveness of an alternative AE monitoring approach to detect the onset of corrosion that is well suited to the low power requirements typical of long-term detection in the field. Results show that the proposed AE approach, coupled with well-established electrochemical techniques, is a promising tool to develop an early warning alarm system for corrosion in RC structures. Based on the experience gained in Study 1, Study 2 presents laboratory tests on a second batch of small-scale RC specimens monitored with AE while the corrosion process is accelerated without imposed current. This study was conducted to investigate frequency spectrum of the AE signals before and after onset of corrosion. Results show that the breakage of the protective layer of reinforcing steel, and thus the onset of corrosion, can be localized to a small portion of the AE frequency spectrum. In Study 3 two identical strips of the one-way RC slab of the first floor of a building scheduled for demolition were saw cut and load tested. In parallel to the well-established measurements of load and deflection, an AE monitoring system was implemented. The results demonstrate the suitability of AE technology for assessing damage in the practice of in situ load testing. In particular, AE intensity analysis can be used as a graphical acceptance criterion.