Improved Interpretation of Vibration Responses from Concrete Delamination Defects Using Air-Coupled Impact Resonance Tests

The deteriorating national infrastructure demands improved nondestructive evaluation (NDE) and structural health monitoring methods for existing concrete structures. Vibration resonance tests offer an efficient NDE method to identify and characterize shallow (near-surface) delamination defects that afflict RC structures. However, efficient implementation of effective modal analysis methods for this purpose is hindered by practical testing limitations. This paper studies vibration resonance data from square, rectangular, and circular near-surface delamination defects in concrete using two testing configurations: a coupled source-receiver set configuration (driving point type), which is analogous to the impact-echo (IE) test, and a fixed-source moving sensor configuration, which represents a conventional modal analysis test. All data were collected using contactless air-coupled sensors, which enable efficient data collection from large structures from prepared laboratory samples. An approach to self-normalize each IE data set using surface wave pulse information in the time signal is proposed. The self-normalized multipoint IE data are presented in the form of modal images, where overlap of the first few natural mode shapes accurately predicts the areal size of underlying delamination defects. The mode shapes obtained with the corrected multipoint IE testing configuration, which provides a significant advantage for the application in NDE tests because damage location, size, and shape need not be known in advance, compare favorably with those from conventional modal testing.