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The only way to know how much a new form will shorten is to measure it.

Q. We just bought a new set of prestressing forms for a project with very tight dimensional specifications. We want to limit the potential for any problems. Are there any guidelines in setting up a new casting area?

Also, how much shrinkage should we expect to experience with the forms when we place it under load?

A Whenever a producer introduces a new form into the production plant, it's important to consult with the manufacturer before the first pour. The engineers who designed the form can offer advice on the amount of stress the form can safely handle. With their help, the producer can then develop a stress envelope for the application.

It's important to understand what happens when strand is tensioned in a vessel type, self-stressing form. Engineers design self-stressing steel forms to withstand the hydrostatic force of concrete pushing the form outward, as well as the compressive force introduced through prestressing pushing inward.

When strand is tensioned in a self-stressing bed, the compressive force from prestressing causes the bed to shorten. When this happens, it is important to measure the amount of bed movement to properly calculate strand elongation.

Even with the best engineering help, it's important to remember the only way to know how much a new form will shorten under tension is to measure it. Engineers from Hamilton Form suggest that under many circumstances, a bed will move about 1 inch for each 100 feet of length. But the form and the operating conditions along with a production plant's processes are unique. These unknowns can influence shrinkage.

There are several other factors that can affect shrinkage:

  • How is the bed anchored? Beds should be anchored at the center point and then fastened to the floor toward each end to allow free movement. If the bed is anchored or welded in place, movement will be restricted. Under some circumstances, an anchored form can also cause the form to buckle under a load.
  • How smooth is the floor's surface? If the bed is secured to a smooth surface where it can move freely, shrinkage will be greater. If movement is restricted by a rough or bumpy surface, shrinkage will be reduced.
  • How clean in the floor? Debris or material buildup around the base of the form may restrict movement.
  • How much stress is being applied to the bed?
  • What are the thermal conditions? Heat from either high ambient operating conditions or excessive heat or hydration will expand a steel bed.
  • With all of these considerations, engineers insist that the only sure method of knowing how much your bed is shortening is to measure it. They propose this method:

    Put a reference mark on something stationary at end of the bed not attached to the device. After tensioning the bed, measure the distance between the reference mark and the end of the bed. Record the change and include this measurement as part of your casting/quality control daily report. Keeping a log will help you understand variables such as temperature, bed set-up and other factors that affect bed shortening.

    Along with the care of the bed, pay close attention to using proper casting procedures during tensioning and detensioning to help mitigate tolerance problems. Here are some suggestions: