Form Pressure and Relaxation in Formwork

Form pressure distribution curves are presented based upon an analytical model that incorporates an instantaneous hardening transition of the concrete from a liquid-like initial mix into a rigid solid and the representation of relaxation effects in the formwork by means of a viscoelastic analogy. The time for the hardening transition appears to vary with the rate of placement. Such a variation in the time of setting provides excellent agreement between experiment and analysis, and indicates the need for further study of the physical and chemical basis for the interaction between the time of setting and the rate of pour. The release of pressure against the forms subsequent to hardening is shown to be a function of dimensionless ratios that incorporate the stiffness parameters of the formwork, the setting time of the concrete, and the relaxation time of the formwork.