Radiant Heated Floor Slab

[campbelltc]

We're designing a floor slab for an airport hanger where the concrete slab on grade has radiant floor heat in the slab. The pipes for the heat are supposed to be in the top 5" of the slab. The design software for concrete pavement tells us that it needs to be 8.5" thick, so we went with a 10" slab with #4 @ 12 ew top and bot (before knowing the pipes need to be high, as we usually use radiant heat in thinner slabs and just pour over the pipes). A radiant heat specialist said they cannot hang the pipes from the top steel (my preference). They asked if the slab can be poured in two 5" thick sections, laying the mat on the lower section before placing the upper section. Obviously that creates a plane of weakness, but if up to you, how would you detail that slab? Pour the lower portion with a heavy broom finish? Bonding agent on the first pour? Is there another way to finish to get a better bond? Dowels between the two layers?

I was putting CJ's in the slab at 25' on center. Go to 15' max?

Thanks.

 

[ANE91]

Quarter-inch peak-to-valley scarification should be sufficient. Broom doesn’t get you that. Need a meat-tenderizer bit attached to a hammer-drill. Bonding agent will likely cause more problems than it’s worth, on an application like this. (I’m normally a big fan of bonding agents and would prefer to use it here, if the contractor can be trusted to do it right.) Dowels between the two layers will be a nightmare to install, so I wouldn’t spec it. Also, my preference would be that any anchors into this slab (e.g., racking) need to be embedded beyond this intentional, horizontal cold joint.

I’d be concerned with losing water to the underlying 5 inches, while the top half is curing. Presumably you have poly under the bottom half.

 

[dik]

A radiant heat specialist said they cannot hang the pipes from the top steel

Did he give a reason for that. If the rebar is properly chaired, there is no reason for that requirement. Alternatively you can support the hydronics from an independent support system. Definitely do not split the slab thickness. Find a different hydronics supplier expert.

 

[CarlB]

Are 2 layers of reinforcing necessary? Aircraft pavement is typically thicker, and unreinforced (or at least used to be). The steel was not necessary for support/structural reasons. Use of appropriate contraction joints addressed crack control.

 

[rlmengineer]

Since most airplane hangers are large what are your plans to protect the PEX tubing from gas and oil spills at construction and expansion joints?

 

[wth]

Did he give a reason for that. If the rebar is properly chaired, there is no reason for that requirement. Alternatively you can support the hydronics from an independent support system. Definitely do not split the slab thickness. Find a different hydronics supplier expert.

I've faced similar objections, it's most likely due to the costs increase/inconvenience for the contractor.

 

[campbelltc]

Did he give a reason for that. If the rebar is properly chaired, there is no reason for that requirement. Alternatively you can support the hydronics from an independent support system. Definitely do not split the slab thickness. Find a different hydronics supplier expert.

I believe it was purely constructability. Thanks for the reply.

 

[campbelltc]

Since most airplane hangers are large what are your plans to protect the PEX tubing from gas and oil spills at construction and expansion joints?

This one isn't huge. 120' x 100' at a regional (county) airport.

Sealing the joints. The tubing will also be 5" below top of slab.

 

[campbelltc]

Are 2 layers of reinforcing necessary? Aircraft pavement is typically thicker, and unreinforced (or at least used to be). The steel was not necessary for support/structural reasons. Use of appropriate contraction joints addressed crack control.

Pavement calculator said 8.5 inches of unreinforced concrete, but it's on fill. Even with good inspection, I'm leery of using unreinforced concrete, even with closely spaced joints. Thanks for the reply.

 

[campbelltc]

Quarter-inch peak-to-valley scarification should be sufficient. Broom doesn’t get you that. Need a meat-tenderizer bit attached to a hammer-drill. Bonding agent will likely cause more problems than it’s worth, on an application like this. (I’m normally a big fan of bonding agents and would prefer to use it here, if the contractor can be trusted to do it right.) Dowels between the two layers will be a nightmare to install, so I wouldn’t spec it. Also, my preference would be that any anchors into this slab (e.g., racking) need to be embedded beyond this intentional, horizontal cold joint.

I’d be concerned with losing water to the underlying 5 inches, while the top half is curing. Presumably you have poly under the bottom half.

Thanks for the input. Yes. We would have poly under the slab.