Monitoring the efficiency of plasticizer in fresh cement using ultrasound

The importance of evaluating the quality of cementitious materials at an age as early as possible is well known. High performance cementitious materials usually include chemical admixtures like accelerators, plasticizers or air entraining agents in order to tailor the properties of the material at the fresh or hardened state. Ultrasonic methods have been developed for monitoring the hydration process. Measuring the transit time and amplitude of the pulse at regular intervals, the elastic modulus development with time can be calculated and therefore, the contribution of the admixtures can be evaluated. However, this requires monitoring for a period of at least a few hours, which although fast compared to the standard compressive test, still is not fast enough to allow decisions about accepting the material before placement in the construction forms based on the suitability of concrete. The general quality of concrete and specifically the contribution of the admixtures should be assessed immediately. In the present paper fresh cementitious material is examined by ultrasound. The excited frequency is varied in order to apply different wavelengths, which are influenced differently by the constituent materials. The effect of chemical plasticizer is examined through the release of air bubbles and the change in viscosity it imposes. Wave velocity vs. frequency curves for different mixes show that the existence of sand plays an important role due to interaction with different wave lengths. The possibility to characterize the effectiveness of chemical admixtures by a single dispersion and attenuation measurement just after mixing is discussed.