Factors Affecting the Monitoring of the Early Setting of Concrete by Ultrasonic P-Waves

Ultrasonic P-wave measurements are widely used to monitor concrete setting. Although the largest wave velocity increase occurs during setting, the earliest increase is rather caused by other factors. Air bubble migration, internal settling, formation of ettringite and early C-S-H, workability loss and thixotropy might affect the velocity change in time. Tests on mortar in which cement was replaced by bentonite, confirmed the possible influence of thixotropy on the measurements. The effect of air bubble migration, internal settling and workability loss was proven to be restricted by testing a mixture in which the cement was replaced by inert material. In a cement mixture, the precipitation of hydration products might however accelerate settling and workability loss. During cement hydration simulations, the change in porosity due to the formation of early C-S-H and ettringite was considered for the calculation of the elastic properties of the granular framework. Nevertheless, the calculated velocity hardly increased before percolation and thus could not confirm that the first velocity increase is attributed to formation of early hydration products. Thus, apart from thixotropy, none of the other factors could unarguably be indicated as the cause of early velocity increase.