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control joints incomposite slab on metal deck /steel beam fl
i have a recently constructed composite slab on metal deck /steel beam floor system that the contractor has sawcut in 1 1/4" control joints. this comprimises the gravity capacity of the floor system.
is there a method in which to treat the control joint to re-establish the contrete section or possibly a retrofit to the beams/stud confirguration to increase its capacity?
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cut control joints where? they might be ok if over beams.
use a repair grout such as those used to patch spalled concrete.
one example is
if they are near mid span of the slab, i would have them grout them. if they are over a beam or joist, i would probably leave them if the slab can span as a simple beam.
sawcut joints locations vary (some between beams, some at girders. occur both parallel and perp to purlin span). composite deck is fairly stout and will likely have enough reserve capacity, primary concern is in regards to the beams.
what about the effect of the control joints relative to the composite beam design? im concerned about the reduction in concrete section relative to stud count, beam deflection etc as the floor see its design live load?
hmmm. reconsidering my post. i was just thinking about the slab. sawcut joints over composite beams seems wrong because of the studs. then again, nobody i know worries about the inevitable slab cracks right over the beams. cjs are bigger and seem different, though.
my above comments apply to the slab only.
considering the composite beam action, i would have 2 main concerns:
1. as mentioned above if the joint is over the beams then it will induce cracking reducing the shear transfer of the stds.
2. if the joint is next to the beam, then it will reduce the effective width of slab that can be taken in composite action.
the repair mortars mentioned above do provide good bond to the concrete if used with the correct surface primers e.t.c, but personally i would prefer not to rely on it for structural stability.
where one of the above 2 points applies, you may want to consider cutting grooves and installing 1' long rebar dowels for shear transfer as well as installing the grout.
regards
csd
check and see if the beams will work with a concrete slab that is h-1.25. i don't see a a problem if you just count on the concrete below the sawcuts. the cracking that occurs below the sawcut will close up when the beam deflects. and all the shear is taken by the webs anyway.
fill the sawed joints with epoxy grout or epoxy concrete. 9000 psi at twenty minutes will let you sleep with no qualms.
an epoxy grout will work better than a cementicious grout i think. filling a 1/4" wide joint might be challenging unless you tape the joint and use injection ports like an epoxy injection repair of a foundation crack.
a potential problem to consider is the fire rating of the slab system, as this is part of the primary gravity load system. most epoxy turns to goo at low elevated temperature. i would research this fire rating issue with the architect and the building official.
are the composite beams controlled by wet concrete dead load or composite dead and live loads? you might be able to "pencil whip" the beams to work with a smaller t-c lever arm and using the actual 28 day strength breaks.
cover plating the beams that are overstressed is an ugly but proven method if things look too dire.

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