… I was always under the opinion that you can NOT skim coat exterior concrete because it will eventually seperate because of changes in temperature (freezing and thawing).
Is this correct? Or, do the polymer resin/polyester based skim coats work well enough to not seperate or crack under exterior conditions? And, if skim coating will work… any ideas as to what would be the best product/method to use?
Joe Moya
Please explain why you would want to do this.
The problem is getting a bond between the old and new layers. You can buy bonding agents, which are basically glue and moisture control that help get the bond reasonable.
If the problem is the rough surface, you would probably be better off grinding it smooth.
I don’t want to do this… but, the contractors poured some concrete for a new addition… it rained very hard that evening (about 7-8 yrs. later). Lots of imperfections on the surface… in fact, it looks like a really bad exposed aggregate attempt (rain run off from the slab caused this). This is specially true with the section of concrete that to be on the exterior (i.e., sidewalk area).
The interior portion of the concrete is simular and the concrete actually has small rolling-like buckles in parts plus some very rough surface (bad exposed aggregate).
The heavy 4-5 hrs. of steady rain certainly screwed things up.
Joe
This is a standard problem with pouring concrete. If it rains too soon in the curing process you get surface problems. The solution is to get a machine that will grind the surface smooth. The process is a mess, but it does work. The end result is actually pretty good.
Putting a skim coat would scare the hell out of me if it were a building I owned and expected to continue to own. I don’t want to say that it is impossible to do correctly, but that is a path I wouldn’t trust.
Grinding… thought of that… but, we are talking up to 3/4 in. deep rough sections in certain areas. This is specially true along the edge for a 1-4 ft inward distance from the foundation edge. The portions of the concrete closes to the existing foundation (i.e., home) is relatively even and smooth and get less smooth and even as you go toward the edge. Even if they grounded it down to a compromising depth between the existing foundation and the new poor, that would leave a significant discrepancy between the two concrete foundations. The portion of foundation would be anywhere from 3/8 in. or more lower than the existing foundation if the concrete is grinded. The two foudations wouldn’t match very well… extending the existing floor would be not be very easy to achieve.
Joe
BTW, were talking about a 280 sq. foot foundation with 18 in. deep (12 wide) footing below the areas that are most eroded to the gravel.
I don’t know what to tell since I can’t see the job, and I am not a concrete expert by any means. I can only say you are right to be concerned about putting in a skim coat. Even with the best expertise, I would not go down that path given any workable alternative.
My reaction would be if you can work with the existing concrete, find a way to do that. Otherwise, take the problem sections up and start over.
Joe,
I am a little confused. What is the application for this slab? You seem to indicate that it is exterior, and obviously it was at the time of the pour. Obviously you are looking for it to be a smooth finish, correct? I have a hard time believing that the contractor would pour something like that on a day that was even threatening rain. Personally, I would have had him pull it out the next day and redo it. But that is water under the brodge now. Let’s see if we can help.
First, is cutting out the offending sections and repouring an option. I know it sounds like a lot of work, but it really is not that bad. Especially if it is outside. Of course you will have some gaps where there will be expansion at the places you cut. You could use shingles to make the expansion joints very thin, and then fill them with a self-levelling polyurethane caulk.
Second, is it possible to use a latex floor leveler to smooth it down, and then go over the whole thing with epoxy paint (the kind that they sell to paint garage floors)? Again, this really depends what the application of theis slab is. That epoxy paint. if put on correctly, is nearly indestructible. We used it on the floor in our fire station, and the fire engines and squads don’t seem to hurt it. Even with the salt in the winter. Cleaning it is a breeze too. My concern is that it can be a little slippery when it is wet, so you will need to throw some kind of grit in the mix for some traction.
I hope this helps. Concrete can be a cruel mistress. There are only two guarantees, it will crack and it won’t blow away. Please let me know if you have any more questions.
Bernie
The slab is for an additon…
They also poured the small sidewalk…
Both are screwed up. I was expecting a smooth finish… but, looks like I’m going to have to settlel for a self-leveling pour over the slab.
…you can’t believe that the contractor poored concrete with rain on the horizon? …Well, believe it… he poured knowing it was going to rain… I come home… it’s done… and started raining cats and dogs a few hrs. later. …total set time was about 7-8 hrs. tops. I called the general contractor and told him what was happening… he was on his way to Houston at the time… and blew some smoke up my ass by saying everything would be OK.
The only options seems to be breaking up the sidewalk portion and starting over. However, the slab portion is going to be tough to breakup… were talking about 8+ inches at one end of slab and about 3+ inches of slab closes to the house (with a 18 inch footing at the outer section of the slab.
Cutting the section that is the worse includes the 8 plus inches thick of slab with 18" footing. We are talking alot of concrete for about 275 sq. ft. (11 yrds.).
Leveler is an option… My question is this… does that stuff last - specially outdoors? As best I can tell, it’s OK for indoors (where temps are relatively constant and freezing doesn’t occur. However, side walks are another matter… Does floor leveler work on side walks in the worse of conditions and over a long period of time - say 15-20 yrs. min.?
We are talking about a 1+ inch leveling pour(s) on large sections of the foundations… and the sidewalk looks like a total mess.
When I looked up the sites about leveling pour products… I see a lot of limitations for depths deeper than 1 inch. And, quite honestly anything over 1/2 inch looks problematic.
Quikrete makes a floor leveler that doesn’t require a latex primer… But, I don’t know if that’s a good idea. Other products use a latex primer then the floor levelor. Hell if I know which will be better… In fact, I wished I had other options but the more I looked at that crappy concrete pour, the more I realized that a skim coating made of a concrete leveler product is the only option.
Cutting the really bad sections (mostly exposed aggregate) sections seems a really tough option since the section that needs to be removed is about 8-10" thick IF you don’t include the 18 inch thick footing (12 inch wide).
FWIW Joe Moya
BTW, the concrete slab will be finished out by laying an extension of a the wood flooring that exists in the adjacent rooms.
If it was me, there would be no question about the sidewalk. Yank it and re-pour. I would be hesitant to try to use floor leveler for more than a 1/2 inch. I don’t know if 15-20 years is realistic or not. I would really consider replacing the parts of the slab that are rough. I know it may seem daunting, but the hardest part will be getting the first hole in it.
Bernie