Thermal Modulation (TM) is a nonlinear approach that has shown success in detecting damage-related variations in uniform materials such as steel and aluminum. Its underlying principle suggests promising use in ultrasonic (US) monitoring systems by exploiting natural temperature fluctuations to evaluate structural integrity rather than compensating for them.

Despite its potential, the application of TM to concrete remains relatively unexplored and not yet fully understood given the intrinsically complexity of concrete. The technique’s response is sensitive to several parameters, including the range and rate of temperature change, humidity, temperature gradients, and the inherent properties of the material. To contribute to this developing field, the present study investigates the influence of one key material property — the water-to-cement (w/c) ratio — on the TM response of concrete. Expanding upon previous work that considered two ratios, this research examines four (0.45, 0.50, 0.55, and 0.60). Twelve concrete specimens were casted, three for each mix. One specimen per group was maintained as a reference, while the remaining samples were subjected to incremental uniaxial compression ranging from 5% to 95% of their estimated compressive strength, in 5% steps. Between each loading stage, all the specimens experienced at least two 24-hour thermal cycles (10°C–25°C) at 60% relative humidity and no applied load.

Throughout these cycles, ultrasonic measurements were continuously collected and processed using Coda Wave Interferometry (CWI) to calculate relative velocity variations (dv/v). The resulting data were used to determine Thermal Modulation Coefficients (TMC). Initial observations reveal that TMC values tend to rise just before the samples reach their elastic limit (approximately 50–60% of ultimate strength), with more pronounced effects in mixes with higher w/c ratios. This pattern indicates that both material composition and the progression of damage play significant roles in shaping the TM response.

In summary, this study provides preliminary evidence of the potential of combining TM and CWI techniques for assessing damage in concrete. The results shed light on how the w/c ratio influences TM behaviour and offer valuable insights for refining the approach. Continued research is essential to enhance understanding and confirm the viability of TM as a diagnostic tool for Structural Health Monitoring (SHM) in concrete infrastructure.

The propagation of acoustic waves in a finite medium with low attenuation results in long-duration measured signals (reverberation). In conventional non-destructive testing and imaging techniques, only the first wave packets are exploited, and the information potentially contained in the reverberation codas is thus lost. The work presented here aims to exploit the overall behavior of the codas recorded in plate-like structures in order to extract useful information from a limited number of sensors. In previous works, we developed a statistical model that relates the scattering properties (in particular, the scattering cross-section) of a local heterogeneity in a thin elastic plate to the statistical properties of the scattered and reverberated flexural waves [1]. In particular, we showed that the theoretical expression for the average reverberation envelope of the differential signals (with/without defect) is obtained as a function of the scattering cross-section of the heterogeneity σ. Furthermore, Chehami et al. [2], developed an imaging algorithm for defect localization in reverberating plates, taking into account the dispersion of Lamb waves. The work presented here complements these achievements and shows that it is possible to characterise defects using only two characterizing parameters: their scattering cross-section obtained from [1] and the contrast of the localization image obtained from [2]. Numerical validations were performed on a cylindrical defect of few millimeters size, exhibiting anisotropic scattering characteristics depending on the scattering direction. To enable imaging, ultrasonic signals (10-30 kHz, A0 Lamb mode) were also collected at N points placed at arbitrary but known positions on the plate surface. First, the cross section was estimated from the statistical model by fitting the mean of the envelopes of the reverberated signals and compared to a theoretical formulation derived from the Norris and Vemula’s works [3] as well as the classical approach using only direct paths. Next, the contrast was calculated from the localization images. Finally, a characterisation approach is proposed and addresses the inverse problem by exploiting the scattering response of the defect and the corresponding imaging properties (contrast) by defining a contrast ratio relative to an isotropic defect. Encouraging preliminary results were obtained for several defects of different diameters.

References

1] Hossep Achdjian, Emmanuel Moulin, Farouk Benmeddour, Jamal Assaad, Lucie Dupont, and Lynda Chehami. Reverberation of flexural waves scattered by a local heterogeneity in a plate. The Journal of the Acoustical Society of America, 140:157–164, 07 2016.

[2] Lynda Chehami, Emmanuel Moulin, Julien Rosny, Claire Prada, Olivier Bou Matar, Farouk Benmeddour, and Jamal Assaad. Detection and localization of a defect in a reverberant plate using acoustic field correlation. Journal of Applied Physics, 115:104901–104901, 03 2014.

[3] A.N. Norris and C. Vemula. Scattering of flexural waves on thin plates. Journal of Sound and Vibration, 181(1):115– 125, 1995.

The Zeeland Bridge (Zeelandbrug) is a 5 km multi-span prestressed-concrete bridge in the Province of Zeeland, the Netherlands, completed in 1964. To reduce the uncertainties inherent in structural re-assessment, a dedicated two-year field lab has been established, focusing on long-term monitoring and targeted load testing. Within this field lab, four cross-sections across two typical spans have been instrumented with Smart Aggregates (SA) (embedded piezoelectric transducers), Fiber Bragg Grating (FBG) strain sensors, and numerous temperature and humidity sensors. The latter enables compensation for environmental effects. Static load tests were conducted using a 50-tonnes truck at different positions across the spans. Coda-Wave Interferometry (CWI) was applied to the SA signals to extract wave velocity changes; these changes are potential indicators of microstructural changes or stress redistribution. The preliminary results obtained during the static load tests at different loading conditions revealed changes in wave velocity in the concrete. These findings are compared with local strain measurements from co-located FBG sensors. This enables combined analysis of both wave velocity changes and strains under static loads. The outcomes of this research will not only improve the understanding of the structural behavior of typical spans of the Zeeland Bridge under static load conditions, but also assess the feasibility of using SAs and CWI technology for long-term structural health monitoring of concrete bridges.