Coda Wave Interferometry for Monitoring Bridges with Embedded Ultrasonic Transducers – Lessons Learned at the Gänstorbrücke Bridge Ulm, Germany

Abstract

Ultrasonic Coda Wave interferometry has the potential to detect minute changes in scattering materials like concrete. By permanently installing ultrasonic transducers in concrete, DFG Research unit CoDA aims to develop methods for concrete damage assessment in Germany's ageing infrastructure. To test the methods developed in simulations and laboratory experiments on a large scale, we have implemented several ultrasonic transducers, both embedded and externally attached at the Gänstorbrücke Ulm, one of Germany's most monitored road bridges. Since fall 2020 we are monitoring parts of the centre of the Bridge, as well as an abutment, and compare the results to the commercial monitoring system. All data is recorded with a self-made data collection device, the so-called W-Box, and analyzed with different coda wave-based algorithms to detect signal and volumetric velocity changes for several transducer combinations. The long-term measurements show that the influence of temperature changes on strains and therefore ultrasound velocity changes calculated with coda waves can be monitored. Maps of velocity change show that parts of the bridge react differently to environmental heating or cooling which indicates material property differences comparing those areas. The capabilities and limitations of the coda wave-based monitoring system are tested in a controlled experiment. Static loading using a truck with varying load at several positions allows the calibration of the system to improve the detectability of possibly damaging loads and changes induced by this loading versus the influence of the environment.

Viability of Coda Wave Interferometry for Fatigue Monitoring of Asphalt Mixtures

“…For this study, the t99.99% was selected as the starting point of the window for Method 2. Typically, in studies using the cross-correlation technique, the doublet phase-shift technique or the stretching interpolation techniques can be used [23,38]. In the stretch interpolation method, the window length is determined using the Coda velocity rate of change, dV/V.…”

“…where E is the spectral energy density (SED) as a function of time, t, distance, r, and frequency, f. P0 is the magnitude of the source pulse, D is the diffusivity, and σ is the dissipation rate. As presented by [38], when taking the logarithm of Eqn. 5, the resulting linear function can be used as: , and C = −σ.…”

“…5, the resulting linear function can be used as: , and C = −σ. After solving for the coefficients, A, B and C, by fitting the Log(SED) curve, P0, D and σ can be back calculated to plot the diffuse envelope in the time domain [13,17,38]. Below in Figure 4, an example spectral energy density plot is prepared for the initial signal of an undamaged specimen before loading has begun.…”

Combining Active and Passive Stress Wave Monitoring Techniques: Opportunities for Condition Assessment of Concrete Structures

“…Additionally, AE can be used during the structural load testing of in-service bridges, providing useful information regarding load-level and cracking [3,4]. Separately, active ultrasonic stress wave (USW) monitoring has been explored recently to monitor loading of and deterioration processes in real structures [5,6]. While AE monitoring can capture ongoing internal fracture processes such as cracking, reinforcement-concrete interaction, and reinforcement fracture, USW monitoring, in particular combined with coda wave analysis, can document minute and slowly varying condition changes from, e.g., temperature variation [7], changes in the stress field due to service-level loads [8], and degradation processes [5].…”