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# Bonding Concrete to Existing Concrete

## Bonding Concrete to Existing Concrete

(OP)
Hey there folks, I have a relatively simple quick question here.

My firm has been asked to redesign an outdoor concrete patio type area. This is no big deal, but adjacent to the patio area is a 5" deep concrete slab.
The 5" concrete slab is not very aesthetically pleasing and does not line up well with the patio, so I have recommended to tear it out and replace the entire area. You would think this would be a simple task for a relatively small area, however the client has pushed back saying they cannot do that. Their response was to scrape out the top 3 inches of concrete and essentially just re-pour the top half of the slab. Has anyone heard of any type of coating that would allow placing a couple inches of fresh concrete over existing concrete and bonding them properly? It sounds absurd to me to just re-pour half the depth of the slab, but I wanted to make sure I wasn't missing anything.

Thanks!

### RE: Bonding Concrete to Existing Concrete

Years ago, we did exactly what your client has requested on concrete sidewalks that crossed and were flush with the asphalt in a parking lot. Sika made and their technical representative recommended a product for this application. I don't recall which product it was. Scabbling the existing concrete is a lot of work, and the end result was poor... would never do it again.

On your project, removing the top 3" of a 5" slab would leave (theoretically) 2" of concrete. IMHO, the 2" would never survive the removal process. I suppose you could use hydrodemolition, instead of mechanical scabbling, but either way the result could be just very expensive removal of the exiting 5" slab.

### RE: Bonding Concrete to Existing Concrete

I second SlideRuleEra. There's no way that can be cheaper.

Have they spoken to a contractor about this? I can't imagine any contractor would think it was cheaper to chip out 3" from a 5" slab and somehow manage to leave the bottom 2" intact.
You should have them reach out to a contractor. They'll soon agree to your original approach...

### RE: Bonding Concrete to Existing Concrete

If you had a thick slab on a good, compacted base, I would be ok with removing the top few inches and pouring a new, unbonded 5" slab. Since this is a patio, the cracking clearly did not occur due to excessive load but probably poor base material and/or compaction. The quality of the base is 99% of a good slab on grade. Convince your client to remove and replace it (removing a few inches of the slab would probably be more costly than removing the whole thing anyway). If you do as the client suggests, you may get reflective cracking in your new slab and be in the same predicament.

### RE: Bonding Concrete to Existing Concrete

Sikatop123... works well and is a bit pricey and may be difficult to finish... best to remove...

Dik

### RE: Bonding Concrete to Existing Concrete

I've used Sikatop123 before. It's pricey but hard and bonds well with a scrub coat and properly moistened base material. Great if you have to feather down to almost nothing. However, as dik stated it's not going to finish the best and it comes out a darker color than normal concrete.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

### RE: Bonding Concrete to Existing Concrete

5 Star makes some excellent (and strong) epoxy grouts. Not sure how aesthetically pleasing it would be however.

### RE: Bonding Concrete to Existing Concrete

5" - 3" concrete = 2" rubble.

But you can demo less concrete thickness, and pour new concrete topping.

### RE: Bonding Concrete to Existing Concrete

Sometimes we just have to stick to our guns. Maybe you need support from a contractor in order to convince your client that his idea is silly.

### RE: Bonding Concrete to Existing Concrete

Talk to a Contractor. I have serious doubts they can remove 3” from a 5” layer without damaging the remaining concrete. I think it’ll be much cheaper to demo the entire slab and pour new. It might help your case to back that up with hard numbers to show the client which way is cheaper.

### RE: Bonding Concrete to Existing Concrete

I agree with the above opinions in that removing the entire 5" of concrete will be cheaper, and better quality, in the end. Keep in mind that bigger (i.e. faster) equipment can be used to demo the entire 5" thick slab quickly, scoop it up with a bobcat and take it to a dumpster, whereas if you were only to remove the top 3" than you would be restricted to 30lb jackhammers and moving the debris away by hand as you wouldn't be able to drive a bobcat on the 2" of concrete without breaking it up.

Yes, it is technically possible to remove the top 3" ($$), although expect lots of damage to the 2" that remains. Then, in order to bond properly, you should have the concrete sandblasted ($$), then a bonding agent applied (), then reinstate the concrete. Bonding agents such as Sika HiMod 32, Sika Armtec 110, and many others are applicable.

You are correct that partial demo is absurd. Letting the client know that it will cost them at least double to do partial demolition will likely be your best argument.

### RE: Bonding Concrete to Existing Concrete

(OP)
Thank you for all the input. I reached out to the client yesterday and explained the situation and how the risk versus reward was not worth it in my opinion. He is still resistant but has agreed to discuss with his typical contractor first before making a final decision. So it looks like I should be all good on this one, can't imagine the contractor recommending anything but removing the entire slab.

I'll take this as a win for the good guys!

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