Monitoring Early Age Setting of Flyash Modified Concrete Using NDT Techniques

Abstract

Strength monitoring is important in concrete structures to determine the structure’s readiness for service. In this study suitability of non destructive testing (NDT) techniques of acoustic emission and ultrasonic guided waves methods for monitoring early age setting properties of flyash modified concrete is explored. In ultrasonic guided waves method, guided wave is transmitted and received through a waveguide that is embedded in early age concrete. As the cementitious material sets and hardens, the strength of the received waves changes, indicating the transition from semi-fluid to solid state. This approach monitors the attenuation of the fundamental guided longitudinal wave mode, resulting from the leakage of energy from the cylindrical steel rod to the surrounding flyash modified concrete. The evolution of the material’s properties is related to the energy leakage or attenuation of the guided wave. Whereas in Acoustic Emission, waves arising out of the solidification of the concrete are recorded using Acoustic Emission (AE) sensors mounted at different locations. Both the techniques are different in their nature of application, i.e., Ultrasonic Guided Wave (UGW) is active in nature and Acoustic Emission Technique (AET) is passive in nature. Ultrasonic Guided Wave depends on the transmitted signal whereas the Acoustic Emission Techniques depends on the stress changes occurring inside the system. Experiments were performed on concrete mixtures with varying flyash contents in concrete i.e. 0%, 5%, 10%, 15%, 20% and 30%. For AET and ultrasonic guided waves, cube specimens of size (150mmx150mmx150mm) with embedded mild steel rods were used to monitor early setting and hardening. For AET, the system was paused while UGW reading was taken to in order to prevent the recording of noise. The focus of the study was to track every aspect of the setting and hardening process of flyash concrete immediately after pouring of fresh concrete using these newly developed NDT Techniques of AE and UGW.