| The objective of this research is to extend the use of P-wave and S-wave ultrasonic wave reflection (UWR) to monitor stiffening and setting of portland cement paste. The research focuses especially on the very early hydration period, during the first hours after mixing. For such purpose, it is necessary to obtain higher measurement sensitivity, in this case by employing a buffer (high impact resistance polystyrene or HIPS) with acoustic impedance that is close to that of cement paste. The use of the HIPS buffer together with a new signal compensating method provides sensitive, accurate and reliable measurements on the stiffening and setting of early-age cement paste. In order to study the stability of the HIPS buffer, UWR responses from various chemical solutions are tested. The ability to characterize solution properties, such as solution concentration and density, with P-wave UWR is reported. UWR responses from dispersed and flocculated alumina suspensions are measured in order to provide deeper understanding of the UWR response from cement paste. P-wave UWR is affected by amount of segregation, and S-wave UWR is affected by extent of flocculation. Based on the findings of these fundamental experiments, P- and S-wave UWR responses from hydrating Portland cement paste are interpreted. Criteria to define set conditions based on UWR are proposed: initial set is defined as the time at which S-wave acoustic impedance of 0.09 MRayls occurs, and final set by the time at which minimum of the first time derivative of S-wave UWR curve occurs. S-wave UWR is also applied to characterize premature stiffening of cement paste, where premature stiffening is indicated by the rapid linear drop in S-wave UWR in the first hour after mixing. Finally, novel applications of P-wave UWR are proposed for cement paste, including sensing differences in paste w/c, resulting from changes in the solid content (density), and detection of the onset of partial debonding, most likely resulting from autogenous shrinkage of the paste. |
