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

Comparison between two ultrasonic methods in their ability to monitor the setting process of cement pastes

This paper presents the comparison between ultrasonic wave transmission (USWT) method and ultrasonic wave reflection (USWR) method in their ability to monitor the setting process of cement pastes. The velocity of ultrasonic longitudinal waves and shear wave reflection coefficient were measured simultaneously on cement pastes with different hydration kinetics. Even though both methods are able to reliably monitor the hydration process and formation of structure of an arbitrary cement paste, they monitor the setting process in different ways. The relationship between the velocity of longitudinal waves and shear wave reflection coefficient can be simplified into three characteristic phases and the end of the first phase can be used to define the beginning of the setting process of cement paste. (C) 2009 Elsevier Ltd. All rights reserved

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Relating ultrasonic velocity measurements on fresh concrete to CEMHYD3D microstructure simulations

Ultrasonic measurements can be used to monitor concrete setting. To relate these measurements with fundamental structural changes in the cement paste, the results are compared with the microstructure development as simulated with an adjusted version of the pixel model Cemhyd3D. Mixtures in which the Portland cement was replaced by different dosages of blast-furnace slag and fly ash were tested. The fraction of percolated solid particles is het most important microstructure parameter determining the change of the wave velocity during setting. However, the measured wave velocity starts to increase earlier than the solid percolation, which is by other researchers assumed to be caused by the formation of early-age- C-S-H and AFt. The simulations could not confirm the latter. After percolation, the formation of additional hydration products increases the elastic moduli and thus the wave velocity. This becomes especially visible for the high-volume slag mixture. After approximately 24h, the presence of slag causes a second acceleration of both the hydration product formation and the wave velocity increase. The pore space depercolation seems unimportant in the study of the setting of concrete or mortar