Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Elevated post-tensioned slab cracks

[Subject Prev][Subject Next][Thread Prev][Thread Next]
As some more general comments:

I would guess that you still have biaxial pre-compression in the slab, despite the cracking.  Even though the cracks are full depth, there is still likely to be alot of interlock between the edges of the two slab section.

If you (or someone else) is concerned, you might want to do some simple load testing - load one side of the crack and see if there is any relative deflection,  load the other side,  see if there is any relative deflection.  See what happens when you drive a heavily weighted vehicle over the crack.  When you actually have full separation, such as an expansion joint that creates a cantilevered slab,  there is quite visible movement of the slab edges as you drive over them.

If you want to use vehicles for loading, there was an ASCE report on parking garages about two years ago that has a table of vehicle weights - if you want to be more precise you can stack pallets of cement or sand.

With respect to the question of why the limit on uncracked prestressed members was raised from 6sqrt(f'c) to 7.5sqrt(f'c) -  this was nominally to "unify" design in that 7.5sqrt(f'c) is typically taken as the modulus of rupture of concrete.  

However, I think it was probably done because noone could remember why it was 6sqrt(f'c) in the first place.  This was in the 63 Code - taken from the original (1958) ACI "Recommendations for Prestressed Concrete", but I'm not sure there was any explanation of where it came from.

Gail S. Kelley, P.E.