Early-age ultrasonic testing of concrete and shotcrete using embedded sensors

Abstract

This paper presents the results of preliminary laboratory testing of early-age concrete properties using disposable, embedded ultrasonic sensors. These tests were carried out during the development of a field system for monitoring the early-age physical properties in concrete and shotcrete. The design is particularly suitable for applications in Fibre Reinforced Shotcrete (FRS), commonly used for ground support in underground mine tunnels in Australia, where the determination of early-age properties is critical for establishing safe re-entry times. The embedded system used in these experiments comprises a pair of piezoelectric transducers mounted to an open frame, which is designed to hold them within the concrete at a fixed offset separation. The probe is implanted at the time of placement, and connected via wire leads to an external control system. The implanted transducers are con fi gured to excite longitudinal (P) waves, at a nominal resonance frequency of 40 kHz, and P-wave transmission is detectable soon after the initial set time. The data presented herein includes evolution of early-age P-wave velocity. This data is compared to conventional unconfined compressive strength (UCS) and dynamic (low strain) elastic modulus, as specifi ed in ASTM C215, for equivalent batches and curing conditions. The embedded P-wave measurement is functionally equivalent to the conventional dynamic modulus testing procedure, and these results may be further used to infer UCS during the early stages of hydration. The ability to perform in-situ, real time, nondestructive testing offers significant advantages to the safe and ef fi cient use of FRS in underground mining applications. © RILEM 2013.