Form Design

Forms for concrete construction must support the plastic concrete until it has hardened. Stiffness is an important feature in forms. Failure to provide form stiffness may cause unfortunate results. Forms must be designed for all the weight to which they are likely to be subjected. This includes the dead load of the forms, the plastic concrete in the forms, the weight of the workmen, the weight of equipment and materials, and the impact due to vibration. These factors vary with each project, but none should be ignored. Ease of erection and removal is also an important factor in the economical design of forms.

Displacement of forms due to loading and impact shock from workmen and equipment can be avoided by using platform and ramp structures independent of the formwork.

When concrete is placed in forms, it is in a plastic state and exerts hydrostatic pressure on the forms. The basis of form design is the maximum pressure developed by concrete during placing. The maximum pressure developed depends on the placing rate and the temperature. The rate at which concrete is placed affects the pressure because it determines how much hydrostatic head builds up in the form. The hydrostatic head continues to increase until the concrete takes its initial set, usually in about 90 minutes. At low temperatures, the initial set takes place much more slowly. This makes it necessary to consider the temperature at the time of placing. By knowing these two factors and the type of form material to be used, you can calculate a tentative design.