Remove/replace a portion of and existing SOG

[SteelPE]

I am currently putting a proposal together for an architect client. The project involves an existing 10” slab that contains a radiant heating system. The system is outside a hangar and is used to reduce/eliminate the need for snow removal. Somewhere along the line the heating system has failed and they are looking to replace the system (I was told this is normal). Other than the failed system, the slab itself is fine. The existing slab has #5’s at 12” o.c. each way mid depth. I have been told that the existing system is tied to the existing reinforcing.

Method 1: Demolish the entire slab and reconstruct.
Method 2: Remove a portion of the existing slab and casting a new system in heating place (chip” say 4” off the top of the slab, place a new heating system down and cast a new 4” slab on top). The final depth of the chip would be determined.

Method 2 seems extremely labor intensive, putting that aside, would there be any negatives to this method? Obviously, the surface would not be smooth (which would work to our advantage as we would want the existing and new slabs to act as one).

Would the existing and new slabs act as a single unit once construction is complete?

What type of bonding agent would you apply to the existing slab to get to two slabs to work together?

 

[thoughtofthis]

You would be doubling the voids in the slab. Also, there's some recommendations in ACI 302.1 that you would want to consider that may make it impractical.

 

[SteelPE]

If you are referencing doubling the voids as meaning we have 2x as many radiant heating ducts then I am not overly worried about that. The pipes are 3/4" diameter at 12" o.c. This would equate to a loss of .88 square inches per foot of width, or .88 inches in 120 square inches.

I had to dig out my old ACI 302, yes, they have some good discussion on construction procedures for deferred bonded slabs (which this would be similar to).... and a good warning that even the best care bonding may not be successful. However, I think we have the advantage of creating a rougher surface then what is described in the ACI 302.

 

[jayrod12]

I believe OG or BA typically respond with the portland cement paste application to dry concrete.

Essentially, base concrete needs to be dry and clean. Rub in portland cement mixed to paste consistency. Pour topping slab.

 

[epoxybot]

Between the reinforcing & the radiant heating system, it sounds too crowded to expect a best effort at using a cementitious bonding agent. Regardless of the bonding agent, I'd be concerned about delamination. If you went with an epoxy, you would have to be concerned about a rising moisture condition, which the epoxy would effectively stop. That might be a problem over time, also causing delamination.

 

[hokie66]

I can't imagine that chipping away half the depth of a 10" slab would be more practical and economical than removing it all and replacing. Labor intensive, likelihood of damaging the ducts without knowing, inadequate bond, restraint shrinkage cracking in the new topping...I could go on.

 

[Ron]

Have it bid both ways. You'll find that it is cheaper to remove and replace than to selectively demolish part of the thickness.

 

[SteelPE]

Ron, while I agree with that approach, I do not want "what's cheaper" entering the contractors mind. If they do find chipping to be cheaper then it will be a fight for proper surface preparation as discussed above and ACI 302. While I am 99.9% sure remove and replace will be quicker and cheaper, I don't want to have that fight if I don't have to.

 

[JStructsteel]

Remove and replace. They will need access to the existing tubes. During demolition of a 4" topping they are going to damage the rest.

 

[chicopee]

"...Somewhere along the line the heating system has failed..." can you elaborate about the failure as you stated that you were told that it is a normal occurrence.

 

[SlideRuleEra]

Method 2 seems extremely labor intensive, putting that aside, would there be any negatives to this method?

1) Remove a portion of the existing slab and casting a new system in heating place (chip” say 4” off the top of the slab, place a new heating system down and cast a new 4” slab on top). The final depth of the chip would be determined.

2) The system is outside a hangar and is used to reduce/eliminate the need for snow removal.

3) The existing slab has #5’s at 12” o.c. each way mid depth. I have been told that the existing system is tied to the existing reinforcing.

4) Would the existing and new slabs act as a single unit once construction is complete?

5) I do not want "what's cheaper" entering the contractors mind. If they do find chipping to be cheaper then it will be a fight for proper surface preparation...

1) Will the depth to be removed be determined before bids are received. Does this depth guarantee the bidders that rebar will not be encountered anywhere? If rebar is encountered, who pays for extra work? Who pays if the remaining concrete "cracks" or is otherwise damaged during demolition?

2) Method 2 puts the piping in the upper part of the slab... the worst location for snow removal. A hydronic system for snow removal should be positioned as low in the slab as practical, but no higher that the middle. The idea is to heat all of the concrete so that snow melt is uniform, not primarily where the pipes are located.

3) Piping is tied to the the rebar mat to keep pipes from moving during concrete placement. Without a rebar mat in the overlay, another anchorage system will have to be used... which should be spelled out in the bid documents.

4) No, Epoxybot explained why. Anchored piping (See #3, above) will limit the area of the existing slab that can be prepared for bonding the overlay. To work, the bonding has to be essentially continuous.

5) Qualified Contractors are always aware of what is "cheaper". Ron's proposal for comparative bids is accepted practice to get real pricing for an alternate designs. For projects that I have worked on, the Owner/Engineer pick which alternate is the basis for contract award... not the Contractor. Also, if the bid documents are well prepared, there is no "fighting" to make a qualified Contractor do the work correctly. Bidders understand what is required and price the work to do it right the first time.

http://www.SlideRuleEra.net

 

[radiocontrolhead]

Another strong vote for Method 1 here.

 

[Chickenhawk6451]

Strong vote for Option 1 as well. I don't think this is a good application for bonding concrete to concrete, especially in a snow-prone area - the freeze-thaw action would probably result in the slab cracking up quite quickly