Concrete wall cap pour in place questions
I recently installed concrete caps on top of some pony walls made of CMU (8x8x16 block size). I used a combination of 2x4s and 2x6s to make the forms for these caps. The 2x4s were attached via ram set to the top both sides of the block wall. 2x6s were then screwed into these 2x4s to create a height of 4-inches (from the top of the 2x4s, which were flush with the top of the wall to the top of the 2x6s). The dimensions of the caps ended up being approx. 11-inches wide (8-inches (the width of the wall and a 1.5-inch overhang on each side of the wall). I installed rebar and applied acrylic bonder to the tops of the blocks before pouring the concrete – which was a 4000psi crack resist. type.
After finishing the tops and carefully unscrewing the 2×6 forms and removing them from the 2x4s (about 2 or so hours later), the bottom 90 edge (where the 2×6 attached to the 2×4 which was ram set attached to the wall) ended up being quite jagged – despite all efforts to finish/float/work the concrete. There were a few other places where the forms stuck to the concrete – which made it a heck of a challenge to clean up. Despite the effort, these places need some fixing.
My questions to anyone out there are:
1. What can I do to clean up these edges? I was thinking of a plastic cement/sand mix, but worried about the bonding of this material to the concrete.
2. I was also considering a smooth stucco finish on top of these caps….or possibly a skim coat. However, I’m not very familiar with specific products that might be suitable for this, and also worried about bonding to the concrete.
Replies
M
I'd use Sandmix-easily avaiable. Use a bonding agent, keep the surface of the concrete damp so it doesn't suck the water out of your repair.
Cover with pretty damp wrags and keep them wet for a few days to a week. Try not to do this in the hot sun.
To help the corners from breaking away-put a chamfer strip in the corners of the form and coat well with a bond breaker.
use form release agent not bond breaker, bond breaker is for something like a tilt up, release agent is for wood forms or steel forms
Calvin-
I've been doing some searching of different products and I think your approach is what I'm going to try. I picked up a few products that I'm going to try: Wunderfixx, Sika 223 and Tamms (thin patch) and Tamms patchII. I'm curious if you've worked with any of these products before. I was turned onto them because their a polymer fortified product that can be featheredged. The Wunderfixx can be smooth sanded too. I'm thinking how I would do this is 1. fix the edges and sides that have gone off on me when I pulled off the forms and then 2. come back and put a skim coat of the same material over the entire cap - similar to a smooth stucco finish. Any thoughts on this approach or the products I've listed?
Thanks for the input!
M
Nope, never tried any of those products.
The finished look is the important thing. That and a long lasting repair. If you skim coat the lid, it all should look fine.
Best of luck.
I've used several different products over the years -- bagged stuff from Menards that says "fortified" or whatever and claims to be able to feather edge. Mostly it works pretty well, but I've had the best luck, in terms of ease of application, smooth finish (almost too smooth), and good adhesion, with plain old thinset. The only problem is that thinset is really dark, so if you want a color match you need to then apply a thin layer of something else -- a product intended for coating vs filling.
We often use Sika 31 Hi-Mod Structural Epoxy when making repairs. Simply mix the epoxy and brush on a medium/heavy coat just prior to troweling on your patch, mortar, or stucco mix. We have never had an bond failure using this method.
Another option would be to use a wet diamond grinder/polisher if you want to go with an exposed aggregate look. Can have a nice look but time consuming and makes one heck of a mess.
Not being an expert at this stuff, I really want to avoid using any epoxy if I can. I've heard these can be really challenging to work with. But thanks for mentioning the company - Sika. In one of my replies to Calvin, I mentioned a few products I've been looking at - one of them a Sika product - Repair 223. Have you had any experience with this one, or any of the others I listed: Wunderfixx, Tamms thin patch and Tamms patchII?
Sika has excellent products, and support
Check with your local sources, and check out the Sika webpage, and if you need to contact their tech support to be sure you get their recommended product for you application.
They definitely will have at least one product that will do what you need to. Some of their products are expensive, but I have never known anyone to have problems with any of their products when using the correct product, as they recommend it be applied.
Brick
Did you cut the radius off the 2X4s? If not that is why you have a ragged edge.
You could have bricked the edge while it was still green an knocked it off pretty easily.
Now that is is curing out, you can still brick the edge, but it will be harder to do.
I would put the forms up with tap cons, then they are a lot easier to strip and if you strip them while the concretye is still green (stiff enough not to fall but still fairly soft) you can tool the mold marks off. We do a lot of stucco over block and concrete here. If you are worried about it, paint some link on but they usually don't. Usually they use stucco mix. It is not much different than type S mortar but it has agents in it that sand mix doesn't.