Surface layer measurements of early age mortar investigated by ultrasonic guided waves and finite element analysis

A pulse-echo ultrasonic guided wave approach that monitors the development of early age mortar during the initial stages of setting and hardening is presented. The method transmits the fundamental torsional wave mode on one end of a cylindrical steel rod partially embedded in mortar and then monitors the reflected signals. Both the reflection from the end of the rod and the reflection from the point where the waveguide enters the material are monitored. The development of the material's mechanical properties is related to the change in signal strength of the reflections. The reflection at the location where the waveguide pierces the material is essentially a surface measurement. It is known that the material properties of concrete vary with depth; specifically, the material in the surface layer possess different material properties than the core material of the specimen. Finite element analysis was used to analyze these differences and to provide a clearer understanding of the entry-reflection. The analysis shows that at early stages, it takes more time for the surface layer to achieve the same properties as that of the core of the specimen. However, after the mortar has achieved measurable strength (e.g. 12-14 hours), the material properties of the surface layer can reasonably approximate those of the core of the specimen.