SCC Evolution of Formwork Hydraulic Pressure and Rheological Properties

This paper describes how an experimental investigation was performed in order to determine the hydraulic pressure variation of cementitious based materials (cement paste, limestone paste, concrete, Self compacting concrete (SCC), etc.) during the plastic phase. A method based on measurements of both total lateral pressure and hydraulic pressure, using a novel device was investigated. Just after mixing, a simultaneous drop of both the hydraulic and the total lateral pressures was recorded, followed by a cancellation of total lateral pressure and a negative value of hydraulic pressure. Compared to other standard methods (Vicat, calorimetry, ultrasonic pulse-echo, etc …), the device was able to give simple and direct information about the mechanical state of the material, in situ. The kinetic variation of the hydraulic pressure occurring during the plastic phase of cement pastes using two portland cement fractions and a limestone filler was investigated. To relate the hydraulic pressure measurements with workability, a study on the evolution of the rheology of the cement paste was conducted. The experiments on standard concretes, which had the same free water content as an equivalent cement paste, show a similar hydraulic pressure variation as long as the pressure is positive. This is not the case for SCC where the observed hydraulic pressure variation is slower and the time of zero pressure is delayed compared to the equivalent cement paste. The presence of limestone and the HRWRA is the main reason for this retardation effect. In addition, as soon as the pressure becomes negative, due to the presence of aggregates, a delaying effect on the pressure variation was observed. In the end, field test show that the hydraulic pressure device could be used to monitor the field schedule of successive pouring, setting and demolding.