Monitoring stress related velocity variation in concrete with a $2.10^{-5}$ relative resolution using diffuse ultrasound (0901.1722v2)

Abstract: Ultrasonic waves propagating in solids have stress-dependent velocities. The relation between stress (or strain) and velocity forms the basis of non-linear acoustics. In homogeneous solids, conventional time-of-flight techniques have measured this dependence with spectacular precision. In heterogeneous media like concrete, the direct (ballistic) wave around 500 kHz is strongly attenuated and conventional techniques are less efficient. In this manuscript, the effect of weak stress changes on the late arrivals constituting the acoustic diffuse coda is tracked. A resolution of $2.10^{-5}$ in relative velocity change is attained which corresponds to a sensitivity to stress change of better than 50 kPa. Therefore the technique described here provides an original way to measure the non-linear parameter with stress variations on the order of tens of kPa.

Monitoring Stress Related Velocity Variation in Concrete Using Diffuse Ultrasound

The paper, authored by Eric Larose and Stephen Hall, details a novel methodology for detecting stress-induced changes in concrete using diffuse ultrasound with exceptional relative resolution. This research uniquely focuses on concrete, which exhibits strong heterogeneity due to components such as cement, sand, gravel, and porosity. Traditional methods, which operate in lower frequency regimes and measure changes via direct ultrasonic wave paths, inadequately resolve the fine structure and micro-damage details due to the attenuation effects in heterogeneous media like concrete. The proposed technique leverages high-frequency data processing to achieve a relative velocity change resolution of $2 \times 10^{-5}$ and sensitivity to stress variations with thresholds as low as 50 kPa.

Experimental Methodology

The experimental setup involved using a cylindrical concrete sample, prepared with specific proportions of cement, sand, gravel, and water. This sample underwent uniaxial loading from an initial stress of 5 MPa, incremented by 50 kPa to reach 5.5 MPa. Ultrasonic measurements were taken at each stage using two transducers, producing data that captured diffuse coda waves rather than direct ballistic waves. Key to this process is the high attenuation and subsequent scattering of waves in concrete at frequencies around 500 kHz, allowing longer paths and higher sensitivity to perturbations.

The received ultrasonic signals were processed using cross-correlation with a known chirp signal, effectively compressing the recorded signals in time. The paper evaluated the impulse responses, highlighting multiply scattered waves through the concrete sample. By employing concepts such as the seismic doublet technique and a stretching correlation method, the researchers precisely quantified the relative changes in acoustic velocity ($dV/V$) induced by stress variations.

Significant Findings

The research demonstrated a linear relationship between stress variation ($\Delta \sigma$) and relative velocity change ($dV/V$), with a calculated non-linear parameter ($\beta$) of approximately -40. This value suggests substantial sensitivity and potential in analyzing micro-damage or structural integrity in concrete. The proposed technique's precision, indicated at 5%, affirms its robustness in distinguishing stress-related velocity changes from other potential influences such as temperature, which was systematically controlled during experiments.

Implications and Future Perspectives

The outlined method presents promising applications for non-destructively assessing the integrity of concrete structures in civil engineering, offering a sensitive probe for gauging micro-damage or stress states. Such capabilities could extend to real-time monitoring, effectively informing maintenance decisions or preempting structural failures. Additionally, there is potential for adaptation in geophysical contexts, where similar acousto-elastic effects could be monitored in earth materials. Future work may focus on further refining the data processing techniques, comparing the stretching approach with traditional methods, and exploring the possibilities of structural health monitoring for various composite materials. This research paves the way for enhanced safety assessments and contributes to a deeper understanding of stress dynamics in complex heterogeneous media.