Self-Consolidating High Performance Concrete – SCC – Self-Compacting & Highly Flowable Concrete

Current ACI provisions for formwork pressure (e.g. ACI 347R, Eq. 2-2) were developed many years ago. These empirical expressions related pressure to the rate of placement and the temperature of the material. In recent years there has been an effort to update the equations to account for different kinds of cement and the density of the concrete through the C_c coefficient and the C_w coefficient. These still do not address many of the issues related to SCC where the increased thixotropic nature causes SCC to produce far different pressures than would otherwise be expected using the current equations.

New models are in development to predict maximum pressure values with SCC mixtures. One such model has been proposed using the Janssen model and is a step forward in that it incorporates a measurement of the time-dependent behavior of the material where earlier work ignored time dependent affects. The unique feature of this model is that it measure friction affects and factors the friction between the concrete and formwork walls into the calculation of formwork pressure. The test set up for this experiment uses a vertical load on the top, similar to the system used by the Northwestern University researchers, as well as a metal or wood blade which is pulled through the material. The horizontal pressure is monitored along with the applied vertical load as well as the force necessary to move the blade through the sample of material. Two time-dependent parameters are determined, one for the friction coefficient and one for the horizontal pressure.

An alternative model for formwork pressure has been proposed by Khayat that relates pressure to rheological parameters. This model was developed by measuring lateral pressure on a cylindrical column and a rheological parameter called “break down area.” Pressure and breakdown area were compared for three different times. It was found that break down area and lateral pressure as a function of hydrostatic pressure were nearly linearly related. In addition it was found that the three different values for break down area for each mixture were also linearly related. This resulted in a model that used the initial breakdown area, determined during the first 30 minutes after mixing, to predict lateral pressure as a function of hydrostatic pressure and time.

A third model developed at the UIUC relates formwork pressure to the pressure decay recorded in a short test column (3 ft). The test column is rapidly filled, and then the formwork pressure is recorded while the SCC is at rest. The decay curve is fit to a mathematical expression, C(t). This pressure decay curve is used to extrapolate pressure drop for concrete pours at varying rates and varying heights of formwork. The model predicts pressure in a given element where concrete is to be poured based on element height and desired filling rate. Thus, the maximum pressure generated at a particular point in the wall element can be predicted for any arbitrary casting rate. An example is shown in Figure 7 that compares the formwork pressure for three different casting rates (4, 8, and 16 ft/hr). The slowest casting rate limits the pressure to under 5 psi while the rapid casting rate reaches 20 psi. Given that a typical industrial formwork is rated at 1000 pcf (~ 7 psi), construction at the slow rate would proceed with no problems, while construction at the 16 ft/hr could overload the formwork and lead to form failure.

Figure 7. Example results of UIUC formwork pressure model