The control of concrete quality during slipforming operations

The main objective of this thesis is to study, investigate and introduce new methods for testing and monitoring slipformed concrete and produce a numerical model to simulate the thermal behavior of the concrete during a slipforming operation. The goal is to make the slipforming technique more efficient and economical and therefore potentially more attractive to the construction industry. The attempt is to bring the slipforming operation from an area of experience and empirical science, closer to an area of scientific concrete technology. Of special interest is the monitoring and controlling of the slipform at the time the concrete is filling the form. The time at which the concrete is exposed below the forms must correspond to the concrete initial setting time. An experimental program was performed to obtain the best possible means to control the hardening of the slipformed concrete at an early age. The suitability and the validity of various methods for monitoring and testing the slipformed concrete were evaluated. The most suitable methods for slipforming concrete monitoring are the ones based on the monitoring of the setting time of the concrete. The testing of the development of compressive strength, and static and dynamic modulus did not prove to be practical. The “Rod Test” was confirmed to be a practical method. The ASTM C403 method can be used under controlled curing conditions on the mortar separated from the concrete for testing purposes. The “2°C temperature increase” method was confirmed to be accurate enough for the majority of the ambient temperatures experienced in Canada. However, measurements done at lower temperatures proved to produce less accurate results, and an appropriate improvement of this method has been proposed. A new proposed method for monitoring and testing of the slipforming concrete based on measurement of the electrical conductivity gave very good results due to the fact that the electrical conductivity of the fresh concrete significantly changes as hydration process develops. Electrical conductivity decreases drastically after the precipitation of portlandite. The drop in the electrical conductivity in the mixture or the break in the slope of the conductivity vs. time curves, are clearly visible. (Abstract shortened by UMI. )