When Should The Cables Be Stressed?

How long should the builder wait before framing the house on a new slab?

I am often asked how much time should elapse before the framers start to build the frame structure on a newly placed slab foundation. Sometimes it is a related question: How long should the builder wait before the cables are stressed?

These questions are usually prompted by what the potential new home buyer views as framing too early and cable stressing that has been unduly delayed. 

The basic facts:

1. Normal framing of a single family house does not add that much load to the foundation. I would recommend that nothing beyond basic stick framing and composition shingle roofing be applied before the concrete reaches the specified design (30-day) strength. Builders would be better served not applying any drywall until the 30-day strength is attained.

2. The engineered plans should specify when the cables should be stressed. If cables are stressed too early, it is possible that the dead end anchors being pulled out of the still green concrete. If you wait too long, the concrete is likely to have more and more visible, curing cracks than would otherwise have been the case.

3. The design engineer will usually specify that the cables be stressed sometime between 4 and 7 days after the concrete is placed. 

A concrete example

If the design 30-day strength of the concrete is 3000-psi, then the cables can be stressed as early 4-days and no later than 7-days. Before stressing occurs, an impact hammer should be used to test the strength of the concrete to make sure it is strong enough so the anchors will not fail. A good quality concrete 3000-psi concrete should test at 2000-psi or more by the time 7 days have passed. No drywall should be applied until 30-days after concrete placement. 

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