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.