Time- and stress-dependent elastic properties in a concrete structure; spotting internal damage footprints

Time- and stress-dependency of elastic properties are features particularly observed in a variety of complex solids, ranging from steel, polymers, and cracked structures, to rocks and concrete. Recently, considerable effort has been made to understand the underlying physics of these phenomena commonly regarded as Nonlinear Mesoscopic Elasticity (NME) in laboratory setups.

As a result, various models have been suggested to explain a range of NME phenomena like hysteresis, dynamic softening, and slow dynamics, among others. Due to the high sensitivity of NME to the presence of imperfections or internal damage on solids, there is a growing interest in taking the current models and applying them to construction materials for damage assessment.

Intending to observe and incorporate these models into real-condition structures, we carried out a 1-day multifrequency vibration experiment in a 24-meter-long reinforced concrete test bridge equipped with a pretension system, to investigate the possible presence of internal damage with vibration-based methodologies.

We used the pretension system to subject the specimen to eight compression states in its longitudinal direction (forces of 400kN at the highest, and 280kN at the lowest). At every compression state, we struck the structure in the vertical direction three times on the north and south sides of the bridge with an impulse drop weight. Throughout the whole experiment, we recorded ambient seismic noise at different frequency bands with a 14-six-component sensor array to measure the acceleration in the conventional translational components and the angular velocity (rotation rate), a 14-geophone array of 4.5 Hz of natural frequency, and four pairs of embedded ultrasound transducers were used to estimate relative velocity changes (dv/v) by applying the Coda Wave Interferometry (CWI) stretching technique. internal temperature of the concrete was also recorded to correct our measurements by first-order thermal effects.

At the material scale (ultrasound regime) we observe stress-dependent dv/v at four different locations in the specimen and describe them by using the acoustoelastic effect concept regarded as a classical nonlinear phenomenon. We also analyze the relative velocity drop and the subsequent healing process in the concrete triggered by the action of the drop weight. We used the model of Snieder and Sens-Schönfelder (2017) to numerically describe the relaxation process happening at different time scales in the specimen through a deterministic inversion procedure. The north side of the structure showed to have a higher acoustoelastic effect and higher velocity drops, as well as longer relaxation times, it is important to mention that there is evidence of external cracking in this span of the bridge.

We present preliminary results in the seismic frequency band (structural scale), where we expect to observe the influence of the vertical beams that support the bridge on the spatial distribution of changes in dv/v. Changes in the fundamental frequency of the structure as a function of the stress level are also expected.

With this work, we point towards the development of new nondestructive testing methodologies highly sensitive to small cracks and imperfections using conventional and non-conventional seismic instruments, and linear and nonlinear wave propagation models.