The question as to when the proper time to apply the stressing force to the post tensioned cable in this type of reinforced foundation is of considerable interest. If you apply the stressing load too soon you have the danger of pulling cable anchorages out of the green concrete. If you wait too long there is a problem of excessive loads being applied to the foundation, due to the super structure framing proceeding and, probably most significant, the increase in number and width of shrinkage cracks which could occur in the un-reinforced foundation.
Post tensioned slabs should be stressed when the concrete strength designated by the engineer’s plan is reached. The practice of MLAW has been to apply stress when the concrete reaches 2000 psi. Good quality concrete should reach 2000 psi in approximately 4 days under normal curing conditions, and certainly within 7 days. If 2000 psi is not reached within 7 days, the concrete will probably not reach 3000 psi in 28 days.
Loadings should not be placed on the slab beyond the basic framing and possibly composition shingle roofing. Certainly no sheetrock or any other heavy loads should be applied to the structure before stressing the cables. Remember, the foundation slab is in a totally unreinforced condition prior to stressing. The best time to apply the stressing would be between 4 and 7 days and certainly before significant loads are applied to the foundation. The strength of the concrete at the time of the stressing should be verified by some type of indirect concrete testing procedure such as an impact hammer or other technique.
Kirby T. Meyer, P.E.
MLAW Consultants & Engineers
Originally published in MLAW Newletter, January 2006
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