huangyhg

precast topping reinforcement
have had lots of complaints lately about specifying a minimal welded wire fabric for a 2" hollow core slab topping with contractors preferring no reinf. or fiber mesh. the hollow core manufacturers catalog states that the topping reinforcement is to comply with aci318 as determined by the eor.
the topping is primarily for the composite slab action and a decent finished floor surface. it’s not really needed for the diaphragm and is in a non-seismic region so it could be considered plain (unreinforced) concrete. anyone else putting reinforcment in toppings?

i would always put reinforcement in topping for crack control. i can't imagine you would ever need it for composite action because it doesn't cross the horizontal shear plane and, as a result, won't offer any composite value.
if just for crack control, why can't fibers be used?
someone started a thread asking that question some time ago, and i didn't see (or don't re
fibers are not a replacement for wwf.
if the slab cannot slide (bonded to the relatively rough substrate) how would the cracks form?
i don't like the wwf due to the high degree of difficulty for placement. maybe fibers would help reduce the plastic shrinkage but i'm not sure they're worth the effort either.

jlnj-
the same way that s-o-g's crack is the same way the topping could crack. a sog is just as bonded to the stone beneath it, but because of the drag on that stone, it cracks. if it wasn't bonded at all, then it wouldn't crack (like pre-cast).
the restraining forces not allowing it to shrink is what causes the cracking
in my opinion, the topping slab is the diaphragm and should be reinforced to transfer the horizontal shear forces into the lateral load resisting system and to control the inevitable cracking.
use stiff sheets of wwf not rolls for easier placement, use six inch maximum spacing, i.e. 6 x 6, w10 x w10. equivalent to #4 @ 12" both ways.
i only use wwf if the topping is 4" thick (nominal). any thinner and it's too difficult to place. otherwise i only use fiber. yes the topping slab is the diaphragm, use a bar centered in the slab at the perimeter.
as far as it being for composite action, when you consider the camber in the slab how much topping do you really have? personally i feel that composite topping is a bad idea, there are too many variables - the biggest one being the topping thickness.
i highly recommend using at least a 3" topping. i used to get complaints constantly about "too much camber in the plank". i have not had one complaint since going to 3" topping.
two inch topping can be exactly placed if the the alignment of the cambered hollowcore are pulled into uniformity. hanging weights attached to the individual planks can align the surface. the wwf can be placed on #4 bars for spacers or chairs with 1 1/2" average cover. the addition of structural topping increases the load capacity for very little cost.

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