The use of re-shores

[jheidt2543]

A concrete sub-contractor poured a structural deck over an accelerator room scheduled to house radiation equipment for cancer treatment. The deck is approximately 3’ thick with an integral concrete beam that is 6’ deep. Based on 3 day and 7 day cylinder breaks, both of which were higher than the design strength, the concrete sub wants to strip the forms without using re-shores. My contention is the cylinder breaks provide only an indication of the concrete strength and may not reflect the actual strength of such a thick block of concrete.
The concrete sub says that a thicker block of concrete hydrates at a faster rate and therefore should be stronger than the test cylinders indicate and since the concrete is stronger than the specified design strength, there is no need for re-shoring. While it is true that thicker sections of concrete hydrate faster due to the generation of more heat (kind of a self-reinforcing loop), I would maintain that re-shores should still be used to make sure there is no deflection due to the sudden application of loads. Any one care to comment?

 

[BAretired]

What do the specs say? I tend to lean towards re-shoring. Another alternative would be to leave the forms on a bit longer.

BA

 

[JAE]

Maybe check this ACI document - specifically section 4-1

http://www.concrete.org/BookstoreNet/ProductDetail.aspx?SACode=347205

 

[Ingenuity]

What is the magnitude of the span/s involved? A well designed 3' thick slab probably does not have any deflection issues.

Are there any additional floor/s to be constructed over, placing temp construction loading/s on the 3' thick slab?

 

[SlideRuleEra]

jheidt2543 - Under the conditions you describe, I would not have any problem with stripping the forms and not reshoring if the span / depth ratio is reasonable and no significant loading is applied to the deck.

As a bridge contractor, we (and our competitors) would routinely remove false work from 30' span, 2' thick flat slab highway bridge decks (essentially one-way slabs) after 7 days of wet curing and not do any reshoring. Of course this was done with full concurrence of the State DOT. Have not heard of this ever causing a problem.

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