New Concrete Overlay Existing Pad

[gtg597r]

Good Morning,

I have a project where an existing reinforced concrete transformer pad (20'-6"L x 13'-0"W x 2'-6"Th) has some significant spalling damage from an equipment fire on top. The spalling/pitting is 3" in some areas on top. This was a hot fire that went for hours. My question is instead of trying to sandblast and patch the damaged areas with some cementitious/polymer product, can a full concrete overlay be installed over the pad? Possibly a 4" overlay with WWF for reinforcement?

My thoughts on both approaches:
Patch Option:
-May not bond well to the existing pad which could lead to deterioration in the future
-Outdoor exposure to the elements may cause the patch to deteriorate
-may require more time to sandblast and chip away damaged material to reach good concrete. (Schedule is important) Also, don't want to damage rebar in the process.
-May require forensic analysis by a third party engineer to select appropriate product. (Schedule)

4" concrete overlay Option:
-Will the overlay bear the heavy load (total 325k) from the transformer or will it crush/be pulverized on top of the existing pad? (This is my question) If the overlay has a solid bearing surface (top of pad) then will it not support the load?
-Must check if new transformer can be set higher. Check connections.
-Must check additional dead load from add'l material, although I suspect that this won't cause the pad to settle.
-Must sandblast and chip away damaged material anyway.
-Confidence in contractor to form and pour the overlay is higher than finding a specialty contractor familiar with patching. (Schedule)

Overall I believe the mass concrete pad is still fine to support the heavy transformer (325k). Lastly, this is a static load not a dynamic vibrating load.

Any feedback or comments would be great! Thank you!

 

[Nor Cal SE]

I think your overlay option sounds good, presuming the following:

- The existing concrete appears to be in decent condition to where the original compressive strength has maintained (despite the spalls, etc.).
- You can get at least one mat of reinforcing bars into the new concrete overlay.
- You get good transfer of new concrete to existing concrete via epoxy dowels.
- You can resolve all lateral loads on the transformer anchors, down into the existing pad. If your scenario uses post-installed anchors, I would think about embedding them deep enough such that their depth below the cold joint is equal to the depth that your calc says you would need into a concrete base (therefore, total anchor depth would be at least 4 inches more than the design depth).

 

[SlideRuleEra]

The spalling/pitting is 3" in some areas on top.

To get a successful relatively thin structural overlay, the new concrete has to have an essentially constant thickness. This would mean removing at least 3" everywhere, even if some of the existing concrete removed is "good".

Agree with NorCalSE that a rebar mat is needed, not WWF. To me, this would mean an overlay at least 6" thick.

With proper surface preparation, the bond between the new and old concrete should be all that is needed to tie the two layers together.

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[gtg597r]

To get a successful relatively thin structural overlay, the new concrete has to have an essentially constant thickness.

Why does the constant thickness relate to a successful overlay?

Would the patch option be as viable as the overlay? Are there any other considerations to the patch option that I didn't think of?

thank you,

 

[SlideRuleEra]

A heavily loaded transformer pad that is 2'6" thick almost certainly has a top layer of rebar. From the description of the fire and damage, I assume this top rebar mat has been compromised.
A bonded overlay would contain the replacement top mat.
The rebar probably needs to be fairly sizeable, say at least #6.
In a 6" thick overlay, the rebar would take up the center 1 1/2", or so. Since the pad is outdoors, use rebar cover of at least 2"; I would say 3". Concrete between the bottom of the rebar mat of, say 1 1/2" to 2 1/2".

The bonded overlay has a continuous cross section (thickness) so that the composite structure has uniform structural properties.

You ask about patching, that's your call... depends on the damage. Patches scattered around the surface are not going to strengthen the overall pad, even if they are bonded to it. If you are convinced that the pad is structurally adequate "as-is", then patches should be fine. You mention that the schedule is important. That's understandable, the transformer is important...probably essential. Making marginally acceptable repairs is false economy.

This may be of help: Portland Cement Association - Resurfacing Concrete Floors.

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