Conduit in concrete topping slabs

jgriffvt · Sep 22, 2016

I've got a project that consists of a precast proprietary system with a 2.5" topping slab. As the SER we've design the diaphragm and the precast company designed the precast planks. The plank can span the distance without the topping slab. I've got a request to run up to 5/8"Ø (outside Ø) electrical conduit within the 2.5" slab. I've reviewed ACI and discussed amoungst my office and we're all on the side of not allowing the conduit. ACI requires the conduit to be less that 1/3 of the thickness, so we're ok with that requirement. I also found ACI302.1 requiring 1.5" cover above and below pipes. The slab has welded wire mesh and where the 4 corners of the mesh meet it takes up to 1" of the 2.5". Any thoughts. I've researched online and many forums state they do it all the time with 4" topping slabs which I would agree but with a 2.5" I'm concerned for cracking with such a small section.

BAretired · Sep 22, 2016

I don't recall ever having done such a thing, but if I did, I would place the conduits against the slab with a light welded wire mesh over them to intercept potential cracks, then pour the topping.

BA

darthsoilsguy2 · Sep 23, 2016

it will definitely crack over the conduit and i assume you don't have any details that show that they could use that corridor. if this is a hard money job, the electrician shouldn't have considered the possibility of running plastic conduit in the slab and should have put money in their bid to run rigid conduit. Electricians will act like it is impossible for them to run conduit above ceilings or through walls on "this" project if they think they can sneak into the slab pour.

MotorCity · Sep 23, 2016

Although it would not be my first preference, I am not aware of any codes that prohibit it and I can't think of any reason not to allow it if the topping is not a structural topping.

When the topping cracks over the conduit, the cracks will be prevented from widening by the wire mesh. (You will also probably get cracks in the topping in locations without the conduit). The only caveat is that I would require the conduit to be mechanically fastened to the planks to resist buoyancy when the topping is poured, otherwise they will want to float to the top. And it goes without saying that the method of attachment should consider the location of hollowcores, reinforcing steel, prestressing strands, etc.

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Conduit in concrete topping slabs

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