Active-Passive Monitoring of Concrete under Uniaxial Pressure: Relating Acoustic Emission and Ultrasonic Velocity for Elastic Property Assessment

Concrete is the most commonly used building material in the world – therefore to ensure the durability, safety, and long-term serviceability of concrete structures, quantitative non-destructive monitoring and testing techniques are crucial. However, the techniques currently applied have limitations, e.g. to detect, characterize, and quantify deterioration and damage in an early state. Active-source ultrasonic monitoring using ultrasonic pulse velocity measurements and coda wave interferometry (CWI) can provide information about changes in ultrasonic measures (i.e., wave velocity and attenuation) and consequently, the evolution of elastic properties of concrete with increasing damage. The acoustic emission (AE) technique is a widely used passive structural health monitoring (SHM) method based on "listening” to the radiation of elastic waves in solids caused by irreversible changes within the material structure, such as crack formation. The occurrence of AE events, so-called AE activity, gives the first indications of microstructural changes and microcracking. The question is how the registered microcracking activity alters the elastic properties of concrete. In our study, we seek to quantify the connection between the evolution of AE events and the overall elastic properties of concrete measured using active-source ultrasonic testing. We determine the spatiotemporal evolution of AE events and changes in the wave propagation speed of a concrete specimen during stepwise quasi-static loading/unloading. The sample (7.5 x 7.5 x 22.5 cm3) was loaded in 3.5 MPa steps, during which AEs were recorded by eight sensors mounted on the upper, middle, and lower sections of the sample. Between successive loading steps, there were 205-second-long pauses, during which active-source ultrasonic measurements were taken. During these intervals, each of the eight sensors sequentially acted as a source, while the remaining seven sensors functioned as receivers. We present the CWI results from four pairs of sensors installed on opposite sides of the sample compared with the spatiotemporal evolution of located AE events. During the loading/unloading phases, the most significant velocity changes, as estimated by CWI, occurred within the upper part of the compressed concrete sample, where most AE events were localized, especially in the early loading stages. As damage progresses, however, the velocity changes become too large to be captured by CWI, which works best if the velocity changes do not exceed a few percent and if the irreversible damage between measurements is limited. By integrating CWI with AE analysis, we achieve a comprehensive assessment of the damage state of the sample throughout the entire loading process.