Thursday, November 12, 2009

Re: Cracking in new PS concrete parking deck

                I design precast/prestressed parking decks.
                You are correct, the cantilever portion of the double tee flange is not as strong as the section between stems. Both can be cracked. Reasons for flange cracks include:
Tension in the flange due to high prestress in the stem bottoms and the extra force required to “break suction” as the tee is first lifted from its mold – the precaster should know what it takes to minimize these;
Warping the tee deck in place: one end not sloped to drain the same as the other end – if the difference amounts to 2” or more from one end to the other on a ~60’ long tee, there is an increasing chance of a crack between stems that angles from one stem at one end to the other stem at the other. This should be eliminated during design. Warping can also occur from the truck hauling the tee turning  sharply and crossing a high curb to enter a jobsite. This should be noted and eliminated before tees are delivered.
Welding heat cracks that arise from the reentrant corner of the pocket for the embed and the slight expansion of the embed from welding heat. The welders are supposed to monitor the energy used in welding. But higher weld production – which their foreman is always requesting is enhanced by higher energy use.
Overload cracks: it is more common than you might think for a larger than pickup/Excursion size vehicle to be driven on a deck while it is being built. This usually results in a continuous longitudinal crack on the outside of the stem or the corner of the flange breaking. The precaster should make the GC aware of the capacity and the other trades should comply. A couple of weeks ago I was in a car following a dual-wheel truck loaded to within an inch or so of the available headroom in a deck not yet open. Fortunately, it was carrying insulation for the roof of a storefront top to a staircase.
                Finally there are just hairline cracks that are wide enough to permit seeping water but have no obvious cause.
                All of these cracks can have a cement slurry smeared on to help fill them or epoxied. The choice partly depends on crack width.
                If you have more questions I get the digest, so will respond the next day or you may email me off-list at
        Jim Getaz
        Precast Concrete Engineer