Minimum Stripping Time for Slab Forwork

[ajk1]

Generally we specify to strip and re-shore when concrete in slab has reached 75% of its specified 28-day strength. The contractor for a 10 storey residential building is asking if we can reduce the stripping time if he uses high early strength concrete. My answer would be that he has to meet the 75% criterion but the thought occurs to me that perhaps there should also be a minimum age for stripping/reshoring even if the 75% criterion is met. For example, what if the 75% of specified strength is achieved at a very early age (say 24 hours, as determined by a reliable method such as Lok tests)...can the forms be stripped at very early age? Might that cause problems such as excessive deflection and creep, additional cracking, etc. if the modulus of elasticity does not develop as quickly as the strength developes? Any thoughts on that? Does ACI SP-4 say anything on that?

 

[rapt]

akj1,

Curing affects more than durability.

It affects strength gain, creep and shrinkage properties.

Your specs will define a minimum curing time. This includes the soffit.

If you strip earlier, creep deflections will be worse unless the re-shoring is very efficient and allows no deflection. No stripping whole bays and then re-shoring as they never take out the SW deflection!

 

[ajk1]

Thanks for all the good points. Much appreciated.