Combined Passive and Active Ultrasonic Stress Wave Monitoring of UHPFRC Properties on a Structural Level

Ultra-High-Performance Fiber-Reinforced Cementitious Composite (UHPFRC) is becoming popular in designing lightweight and durable structures. UHPFRC structural elements remain crack-free and waterproof under service conditions, significantly improving durability compared to designs made of conventional reinforced concrete. Due to its unique composition, UHPFRC has specific mechanical properties. In particular, the early-age development of UHPFRC properties, such as the elastic modulus, is difficult to monitor as it occurs while elements are in the formwork. Monitoring of the very early-age behavior of UHPFRC is thus challenging, which is why the hydration process of UHPFRC on a structural level is still not yet fully understood. This paper proposes a novel combined passive and active ultrasonic stress wave monitoring (USW) approach involving a network of ultrasonic transducers and thermocouples. The scheme was applied to a laboratory beam with a span length of 4.0 meters. The monitoring network consisted of 24 embedded ultrasonic transducers and 15 thermocouples and allowed data recording from the beginning of the UHPFRC hardening process. From the beginning, a set of ultrasonic pulses were emitted every 30 minutes and recorded (active USW monitoring), while acoustic emissions (AE) were recorded continuously (passive USW monitoring). After 28 days, the beam was moved into a load frame and tested under four-point bending. Continuous monitoring using this unique passive and active monitoring approach enabled accurate characterization of the evolution of material properties during the very early age of UHPFRC, as well as the structural behavior and degradation processes of the beam during structural load testing.