O.T. New concrete adhering to old

I used to use a product called "Crete-Weld", with good results.

Don't know if they are still around.

It's been a while.

Tom Watson - WoodDorker tjwatson1ATcomcastDOTnet (email)

There is a liquid that is used. I used some on stucco repair and it worked excellent. You could either apply it to the old work to be covered and then stucco, or mix it in with the new stucco and then stucco, or do both. Home Depot might have it but I got mine at the ready mix company which also had mason supplies. I think they call it a "bonding agent". Sam

The type I used is a latex adhesive. I got it at the same location that I bought the bagged cement. IIRC you should wet down the old cement so that it will not wick the moisture out of the new cement too quickly.

I can't give you a recommendation on a specific brand name but when you go to your building/masns supply store, the the product you want to ask for is called a "bonding agent."

Lee

Thank You all for your quick and helpful suggestions

I appreciate it very much.

Fuddzy

We used to call it Moose Milk. It looked like white carpinters glue. Wet the old concrete first, then paint the glue on. Add some glue to the concrete mix (unless you are doing several yards worth), and pour into the forms. robo hippy

Several years ago I did something like that, and there is a liquid adhesive in the same area as the concrete in the store. IIRC, I replaced 1/2 the required water with the adhesive, per some advice from an old guy who had done a lot of it in the past. Seemed to work pretty well, and didn't cause any apparent problems with it setting, but I didn't get much follow-up on it, because the house was sold right after I finished the job.

But in your case, that may not be necessary at all- if you're jackhammering the old stuff out, and there's a rough edge left behind, that may be enough to hold the new "plug" in place. If you're really worried about it, you could get a hammer drill, drill some holes all around the perimeter of the hole, and sink some big lag bolts in so that they protrude into the cavity, sort of like partial rebar.

If you don't use the glue, then a little masonry cleaner (muriatic acid) helps the bond. The bond may not be permanent because a crack is already there, and rebar dowels will hold it there if it does crack (or I should say when) again. It depends somewhat on the size of the patch.

robo hippy

Ardex? Tamspatch?

I think they may be more for resurfacing, but they may have a product for your application.

Prometheus wrote:

code for attaching old concrete to new can in some instances call for rebar pins epoxied into the old with the new poured around it. the epoxy is purpose formulated and dispensed in a double-barrel caulking gun which mixes in the nozzle. it's a bit of a pricey system, but it works well. the advice to leave a jagged mating surface to the old concrete wherever you can is good, even to the point of roughing up smooth surfaces.

Even without the epoxy, rebar pins aren't too easy to get back out once you've hammered them in, especially if you whack them really good with a sledge after they're set in the hole (the end will mushroom a little inside, and act like a masonry anchor) Can work in a pinch if you don't have the double-barrelled gun.