Early age ultrasonic pulse monitoring on cement paste, mortar and concrete

There have been many studies on the use of the ultrasonic pulse velocity (UPV) technique for assessing hardened concrete. Many of these have been aimed at establishing a correlation between UPV and other concrete properties, such as strength and dynamic elastic modulus. These were extended to early age monitoring of paste and mortar using a system designed to provide UPV data from almost immediately after mixing. Due to the high attenuation of the pulse at early ages small path lengths were used. The initial experimental work in this study was aimed firstly at investigating the performance of the early age monitoring system for paste, mortar and concrete with varying path length. This led to establishing an optimum arrangement for the "standard" Pundit transducer system and also an investigation of improved transducer design, in particular replacing the stainless steel casing material with aluminium or brass. In all the investigations measurements were made of UPV, the amplitude of the first received wave (pulse strength) and the time to reach the peak of this first wave (rise time). As well as supplementing the UPV data the pulse strength and rise time measurements were used to estimate the Pundit error time used in the analysis for optimum path length. The data collection began immediately after mixing and continued for up to 28 days. The UPV data were measured continuously using a computer with a pre-installed data acquisition package. To assist in interpreting the results, path length effects in hardened samples were also investigated with the UPV measurements supplemented by dynamic elastic and shear modulus tests. The results of these investigations showed that at very early ages (up to 24 hours for concrete, 10 hours for mortar, and 72 hours for paste) higher UPV values are obtained for lower path lengths, and beyond that the opposite is the case. Pulse strength increases and rise time decreases with reducing path length throughout maturity. At early ages the preferred transducer material for testing paste is aluminium, and stainless steel for concrete. All materials performed equally well on mortar. An optimum test system was established for paste, mortar and concrete The study continued with an investigation of ultrasonic pulse measurements in early age concrete with varying mix proportions, cement type and replacements, curing temperatures and admixtures. The measurements consisted of UPV, pulse strength, rise time, setting (by penetration resistance) and compressive strength. Continuous, as well as manual, UPV measurements were made. This work was aimed at establishing UPV as a predictor of setting and strength development in concrete for the various parameters tested It was found that rise time provides a better indication of setting than UPV and pulse strength. Relationships of UPV, pulse strength and rise time with compressive strength have been established for all the variables, and ultrasonic pulse measurements corresponding to the start of strength gain and a strength of 5 N/mm2 have been determined.