Lateral Pressures of Fresh Concrete on Wall Formwork under High Rates of Placement

Abstract

Current ACI guidelines for the design of formwork contain equations for the prediction of the lateral pressure of concrete exerted on the form which consider the rate of vertical rise of the concrete in the form, the temperature of the concrete, and the effect of various admixtures. These equations contain limitations on the acceptable slump of the concrete and the maximum rate of vertical rise during placement. However, in an ever increasing effort to increase productivity in the construction field, many concrete contractors place concrete in wall forms under conditions that exceed the equation limitations. The focus of this study is to create a field measurement system that can be used by future researchers to accurately measure formwork pressures in a wide range of applications and to gain a better understanding of the lateral pressure behavior of fresh concrete.

For the experiment, two different data collection systems were created, one utilizing center-hole load cells which could measure tension forces in a variety of form ties, and the other consisting of a group of pressure cells that can easily be screwed into many concrete form facings. A total of nine forms were instrumented in the field with some combination of this data collecting equipment and lateral pressures were monitored during the concrete placement.

A lateral pressure distribution was plotted for each placement based on the field measurements, noting the rate of vertical rise, slump, and temperature of the concrete mixture. Measured pressures were then compared to those that would be predicted by various ACI and other equations ignoring the limitations on rate of placement, to test the applicability of those equations when the concrete is placed at very high rates.