Submitted:
22 September 2026
Posted:
23 September 2026
You are already at the latest version
Abstract
Bond strength between a repair material and a concrete substrate is a principal parameter for assessing repair quality, most commonly evaluated by the semi-destructive pull-off test. Impact-echo (IE), a stress-wave based non-destructive method, is sensitive to interface quality, but the amplitude of characteristic frequency-domain peaks alone is statistically insufficient for reliable bond-strength estimation, motivating more advanced signal processing such as wavelet analysis. This paper presents a discrete wavelet transform (DWT) analysis of IE signals recorded on two groups of concrete repair systems differing in substrate class and surface-preparation technique. Statistical parameters of the detail-coefficient distributions - mean absolute deviation (MAD), standard deviation (SD), range (RG) - and detail energy (EN) were computed at the detail levels corresponding to the interface echo; finite-element simulations confirmed their sensitivity to interfacial voids. Multiple regression showed that detail-coefficient statistics and detail energy correlate with pull-off bond strength markedly better than frequency-peak amplitude, and that adding substrate roughness and near-surface tensile strength further improved the correlation. A multicollinearity and leave-one-out cross-validation analysis showed, however, that the amplitude-distribution statistics - mean absolute deviation, standard deviation and range - are severely collinear and do not validate as predictors at this sample size, whereas detail energy, and particularly the summed energy of details D2 and D3, is comparatively robust and yields the only model in this study that is both statistically significant and positively cross-validated. Regression equations are proposed accordingly, with detail energy identified as the recommended basis for bond-strength estimation. Discrete wavelet analysis - and specifically detail energy at the interface-echo pseudo-frequency - proves a useful and comparatively robust tool for evaluating bond quality and estimating repair-layer bond strength on concrete substrates.
Keywords:
concrete repair
; bond strength
; interface quality
; impact-echo
; discrete wavelet transform
; non-destructive testing
; pull-off test
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