Takahisa Okamoto

Damage assessment of reinforced concrete beams qualified by acoustic emission

Acoustic emission (AE) techniques have been widely studied in concrete engineering and are presently being applied to monitoring concrete structures in service. In order to assess the damage levels of the structures, a criterion based on the Kaiser effect of AE is proposed in the recommended practice recently published by the Japanese Society for Non-Destructive Inspection. New AE parameters of load and calm ratio are defined for qualification of the damages. Accordingly, the feasibility of the damage qualification is experimentally examined using reinforced concrete (RC) beams damaged under incremental cyclic loading. It was found that the damages qualified by the 2 ratios were in good agreement with actual damages of the beams. This suggests that the damages of such RC structures in service as bridges, docks, and buildings are quantitatively assessed by simply applying cyclic loading and monitoring AE activity.

MOMENT TENSOR ANALYSIS OF ACOUSTIC EMISSION FOR CRACKING MECHANISMS IN CONCRETE

A quantitative analysis of acoustic emission (AE) waveforms, which was developed as a Simplified Greens Function for Moment Tensor Analysis (SiGMA) code, is revised. By the analysis, cracks of AE sources are located, classified into a tensile crack and a shear crack, and their orientations are determined. For practical application to concrete, both experimental and analytical procedures are reexamined on the basis of the background theory of moment tensor. To apply the SiGMA analysis to AE waveforms recorded, a basic experimental procedure is discussed and established. In the tensile test of an L-shaped reinforced concrete model, nondestructive evaluation (NDE) of cracking mechanisms is investigated. To improve the accuracy of SiGMA solutions, a postanalysis is developed. Unreliable solutions are screened out, based on the discrepancy between SiGMA solutions of the experiment and those of synthesized waveforms. The mechanisms of the fracture process zone are studied in the bending test of a notched mortar beam. These results demonstrate the applicability of the moment tensor analysis to both NDE of concrete structures and to experimental fracture mechanics in concrete.

A Proposed Standard for Evaluating Structural Integrity of Reinforced Concrete Beams by Acoustic Emission

A series of studies has been performed to evaluate the structural integrity of reinforced concrete (RC) beams by acoustic emission (AE). Cyclic loadings were applied to RC beams with a single reinforcing bar, large repaired beams, beams deteriorated due to corrosion of reinforcement, and two beams with different damage levels in an aging dock. The test results demonstrated that the Kaiser effect starts to break down when shear cracking starts to play a primary role. It has been also shown that high AE activity is observed during unloadings after serious damage (slips between the concrete and the reinforcement or those between the original concrete and the repaired part) has occurred. A standard for evaluating structural integrity of RC beams by AE is proposed, based on these results.

Acoustic emission evaluation of an arch dam during construction cooling and grouting

Abstract Acoustic emission (AE) monitoring was conducted in order to ensure safe construction of an arch dam under severe climate conditions. The AE was detected during the secondary cooling in the winter time while the construction was suspended due to low temperature and heavy snow. In the spring time just before the construction restarted, AE was monitored during grouting into a joint of dam blocks. A multi-channel digital signal processing system was used to perform a quantitative AE waveform analysis based on the moment tensor analysis as well as the conventional AE parameter measurement. Low frequency AE sensors (15 kHz resonant) were employed to detect AE signals since the signals with ordinary frequencies attenuate greatly during the propagation in concrete of the dam. Twelve sensors were buried from the dam surface and four were mounted on the corridor floor inside the dam. Neither critical AE activity nor intense AE cluster were observed in the AE monitoring. Thus, it was confirmed that the dam was safe during both the secondary cooling and the grouting. The AE monitoring was proven to be very useful for the safety inspection of a dam under construction in severe conditions.

ACOUSTIC EMISSION FROM REPAIRED REINFORCED CONCRETE BEAMS

The acoustic emission behaviors of repaired reinforced concrete beams under flexure load was studied. Useful correlation was established between the acoustic emission activity and crack growth in the repaired reinforced concrete beams. Three different acoustic emission activities were observed during the flexural loading. Flexural crack initiation, flexural crack propagation and slip between repair material and old concrete were identified by monitoring the acoustic emission. The zone of slip initiation and development can be identified from the acoustic emission event-location plot, showing major acoustic emission concentration areas. Repetitive loading showed that the Kaiser effect was not valid after slip was happened at the connected face between repair material and old concrete. Flexural crack growth acoustic emission burst event signals were found to be of higher amplitude compared with those due to the slip. The accumulated number of burst events of these highamplitude emissions, monitored by the use of a high threshold, showed good correlation with the crack growth in the beam.