Q. Occasionally, we having problems stripping product from self-stressing, vessel-type forms. When we strip it from the form, the product is spawling or cracking. What can we do to prevent this?
A. First, check that the form is installed straight. Next check that your form is in good condition. You can't strip product from a form that is warped or bowed. If there are any noticeable dips or bows in the form, they need to be straightened.
How the form is installed, the substructure, and how the form is used all affect if the form stays straight and true.
Next, check that the form is properly anchored. When you tension a self-stressing form, it shortens. Conversely, when you detension, the form elongates due to the release of force. Steel forms also expand and contract with temperature change. To help control the movement of your form, the form should be:
- Anchored at centerline. This doesn't stop the form from moving; it splits the movement from the center to both ends.
- Secured to the bed using anchors 30 inches, every 10 feet on each end and 5 feet apart for the remainder of the bed.
- Secured to hold the form in line and resist upload force where necessary.
You also should check your detensioning method. During detensioning in a vessel type form, the form elongates due to the release of the prestressed force. When detensioning, try to minimize product movement in the form; keep the product where it was cast. To do that, balance the shortening of the product against the elongation of the form by evenly dispersing the release of force when you detension the bed.
Here's an example of a detensioning pattern for a double tee form:
- First, cut one or two top strands or the first 20% of the strand on both stems, simultaneously at both ends of the bed.
- Move in to the next header on each end and cut the same strands, continuing in to each header until you reach the center of the bed.
- At the center, cut the same strands, plus another 20% of the strand.
- Move out across the bed again until you reach the ends.
- When you cut about 40% of the strand, release strand depressors.
- Now, start again at both ends of the bed and release another 20% of the strand.
- At center, move out again and continue this pattern until all of the strands are cut.
Precast producers sometimes release strand depressors or hold-downs as a first step in detensioning. This creates uplift on the product while still in a stressed (tensioned) mode. Normally, you should detension about 40% of the longitudinal strand before releasing the depressing load.
When detensioning other vessel forms, use the same method of cutting simultaneously from each end and moving in to the center of the bed and then out again. But cut the strand in the center of the strand pattern first. Then move to opposite ends of the pattern as you continue detensioning.
The key to successfully detensioning a bed is to ease the pressure off of the form to minimize the movement of the product in the form and make stripping easier.
Before making any changes to your detensioning method, be sure to check with your engineer.
This originally appeared in the Summer 2006 issue of Hamilton Form's newsletter, Form+Function. This is reprinted with Hamilton Form's permission. Visit www.hamiltonform.com