| Repeated failures of concrete structures during construction reveal the urgent need for a reliable testing method to perform in-situ monitoring of concrete properties at early ages. A new nondestructive testing method called ultrasonic wave reflection (UWR) method was developed at the Center for Advanced Cement-Based Materials. This method measures the shear wave reflection loss at an interface between the hydrating cement paste and a buffer material. The objective of this research is to evaluate the possibility of using this method to predict the mechanical properties of early-age concrete.; This research was conducted in three phases. First, the UWR method was used to monitor four macrostructural parameters: setting, viscosity, dynamic shear modulus, and compressive strength of cement pastes. The results showed that the UWR method can determine the viscosity and the dynamic shear modulus of cement pastes accurately. The relationship between the reflection loss and the compressive strength of cement paste is independent of both the curing temperature and the water/cement-ratio.; The evolution of microstructure during hydration was studied during the second phase. The volumetric properties of the solid and the pore phases, and the connectivity of the solid phases were studied using a continuum model called HYMOSTRUC, which was developed at Delft University in The Netherlands. It was found that the UWR measurement is dominated by the percolation and the intensity of bonding of the solid phase in the microstructure.; A three-phase model based on the differential effective medium theory was established to predict the elastic properties of concrete from measured properties of the paste matrix in the last phase of this study. In the model, concrete was viewed as a bulk paste matrix containing composite inclusions, which consisted of aggregates and interfacial transition zone (ITZ). CEMHYD3D, a digital-image-based model that was developed at the National Institute of Standards and Technology, was used to simulate the microstructure of ITZ. The research showed the importance for considering the properties of ITZ separately from the bulk paste matrix. An empirical model for strength prediction, based on the predicted dynamic shear modulus, was also proposed for the field application of the UWR method. |
