Assessing Early-Stage Concrete Damage Using Thermal Modulation and Coda Wave Interferometry: Influence of Water-to-Cement Ratio

Coda Wave Interferometry (CWI) is known for its high sensitivity in detecting changes in heterogeneous materials like concrete. Originally developed in geophysics, CWI is promising for civil engineering applications to monitor concrete structures. However, its extreme sensitivity to environmental factors, especially temperature, presents challenges: temperature variation impacts often overshadow structural changes like load effects or damage. In current literature, temperature effects on CWI are typically treated as noise to be corrected to reveal true damage indicators. Common correction methods—such as using proxy samples or detrending data—are often challenging to implement in real-world scenarios and may introduce high uncertainty. This study explores an alternative approach: rather than correcting for temperature, using temperature changes with CWI as a diagnostic tool for characterizing samples with varying damage, building on the Thermal Modulation technique for concrete by Sun (2019).Thermal Modulation is in early development, with studies thus far limited to lab-scale material blocks rather than real-life structures. Baseline experiments are needed to understand the conditions under which Thermal Modulation can indicate or quantify damage. While research has shown that thermal modulation coefficients serve as effective proxies for various damage mechanisms, several parameters remain unexplored. This study investigates how different water-to-cement (w/c) ratios in concrete, ranging from 45% to 60%, influence Thermal Modulation results.In this experiment, six concrete blocks with varying w/c ratios were subjected to incremental load levels from 10% to 50% of their maximum compressive strength, inducing controlled cracking. The goal is to assess early damage stages, where CWI's high sensitivity provides a time advantage. The blocks underwent a 24-hour temperature cycle, rising from 10°C to 22°C and returning to 10°C. During this cycle, ultrasound (US) sensors continuously emitted pulses, allowing calculation of relative velocity changes, dv/v, throughout the experiment. By analysing the relationship between dv/v and temperature, we derived thermal modulation coefficients, previously shown to correlate with damage levels. This experiment assesses the combined capabilities of CWI and Thermal Modulation in detecting early-stage concrete damage.This study aims to determine if the w/c ratio significantly affects thermal modulation outcomes. Thermal modulation theory suggests that greater crack density increases the relative velocity change with the same temperature fluctuation, likely due to crack expansion and contraction with temperature. We hypothesize that higher water content may attenuate this effect, indicating that the w/c ratio could amplify or dampen the response.Understanding how Thermal Modulation behaves under varying conditions provides valuable insights into its applicability and limitations. This study lays essential groundwork for expanding the method, potentially enabling future applications in monitoring mid-scale concrete structures.