Evaluation of Delamination in Concrete by IE Testing Using Multi-Channel Elastic Wave Data

Abstract

The main objectives of this study are to develop a non-destructive test method for evaluating delamination defects in concrete by the Impact-echo test using multi-channel elastic wave data and to verify the validity of the proposed method by experimental studies in the laboratory. First, prototype equipment using an eight-channel linear sensor array was developed to perform elastic wave measurements on the surface of the concrete. In this study, three concrete slab specimens (1500 mm (width) by 1500 mm (length) by 300 mm (thickness)), with simulated delamination defects of various lateral dimensions and depth, were designed and constructed in the laboratory. Multi-channel elastic wave signals measured on the three concrete specimens were converted to the frequency-phase velocity image by using the phase-shift method. A data processing method was proposed to extract the dominant propagating waves and non-propagating waves from the dispersion images. The dominant wave modes were used to evaluate delamination defects in concrete. It was demonstrated that the surface wave velocity values were useful for characterizing the shallow delamination defects in concrete. In addition, the peak frequency of non-propagating wave modes extracted from the dispersion images gives information on the lateral dimensions and depths of the delamination defects. This study also discussed the feasibility of combined use of the results from propagating and non-propagating wave modes to better understand the information on delamination defects in concrete. As will be discussed, the multi-channel elastic wave measurements enable more accurate, consistent, and rapid measurements and data processing for evaluation of delamination defects in concrete than the single-channel sensing method.

Keywords: concrete; delamination; lamb waves; multi-channel elastic wave measurement.

Figure 1

Typical source-and-receiver configuration and frequency signals of Impact-echo testing for condition assessment of concrete with delamination defects of various depths: the first column (a,d) represents solid (crack-free) concrete plates; and the second (b,e) and third (c,f) columns represent concrete plates with relatively deep and shallow delamination defects, respectively [5,6,7].

Figure 2

First several modes of Rayleigh–Lamb frequency equations: (a) normalized frequency and normalized wavenumber and (b) normalized phase velocity and normalized frequency [16,17].

Figure 3

Concrete specimens with simulated delamination defects of various sizes and depths: (a–c) for plan views of specimen 1, specimen 2, and specimen 3 with horizontal and vertical distribution of delamination defects, respectively.

Figure 4

Simulated delamination defects which were attached to reinforcing steel mat before casting concrete: (a) DL 1 and (b) DL 3 for simulating shallow and deep delamination defects, respectively.

Figure 5

A prototype equipment for multi-channel elastic wave measurements on concrete specimens.

Figure 6

(a) A photo of elastic wave measurements on the surface of concrete using the multi-channel sensor array developed in this study and (b) a sensing unit composed of an accelerometer, a sensor housing, a compressive spring and a damping rubber.

Figure 7

Test plan on the surface of a concrete specimen: (a) scanning procedure for multi-channel elastic wave measurements and (b) source-and-receiver configuration.

Figure 8

Typical time signals measured by the eight-channel sensor array after applying (a) a broad and (b) a narrow window function, respectively.

Figure 9

Typical dispersion images created by the plane wave transformation method for (a) narrow and (b) broad windowed time signals.

Figure 10

Comparison of spectral amplitude obtained from multi-channel data and single-channel data.

Figure 11

Frequency-phase velocity images created by the plane wave transformation method using multi-channel signals obtained over various delamination defects in concrete specimens: (a) DL 1, (b) DL2, (c) DL3, (d) DL4, (e) DL5, and (f) DL7.

Figure 12

Normalized spectral amplitude obtained by summing the amplitude of the dispersion images for each frequency.

Figure 13

The variation of dispersion curves of A0 Lamb mode over delamination defects with various lateral dimensions and depths.

Figure 14

The delamination map representing the distribution of surface wave velocity values measured on the three concrete specimens. The figures in the first (a–c), second (d–f) and third rows (g–i) represent the results from the concrete specimens 1, 2, and 3, respectively. The first (a,d,g) and second (b,e,h) columns represent the results obtained from the signals generated by the impact source 1 and 2, respectively; and the average of the two results are shown in the third column (c,f,i).

Figure 15

The delamination map representing the distribution of peak frequency values measured on the three concrete specimens. The figures in the first (a–c), second (d–f) and third rows (g–i) represent the results from the concrete specimens 1, 2, and 3, respectively. The first (a,d,g) and second (b,e,h) columns represent the results obtained from the signals generated by the impact source 1 and 2, respectively; and the average of the two results are shown in the third column (c,f,i).

Figure 16

Histograms representing the distribution of peak frequency values measured on the three test regions 1, 2, and 3, each of which represents the test region over solid concrete and deep and shallow delamination defects.

Figure 17

The delamination map representing the depths of delamination defects in the three concrete specimens. The figures in the first (a–c), second (d–f) and third rows (g–i) represent the results from the concrete specimens 1, 2, and 3, respectively. The first (a,d,g) and second (b,e,h) columns represent the results obtained from the signals generated by the impact source 1 and 2, respectively; and the average of the two results are shown in the third column (c,f,i).

Figure 18

Comparison of estimated and as-built depth of delamination defects and thickness of solid concrete: (a) section A-A’ in Figure 17c; (b) section B-B’ in Figure 17c; (c) section A-A’ in Figure 17f; (d) section B-B’ in Figure 17f; (e) section A-A’ in Figure 17i; (f) section B-B’ in Figure 17i.