I’ve got a bit on a quandry and would appreciate any advice I can get.
I’m working on a 1920’s bungalow, and we added a concrete porch across the rear of the house. The owners wanted it to look original, so we decided on a ‘sanded’ look, sort of like aged concrete. The concrete sub said he could do this easily by troweling the surface smooth, waiting a couple of hours for it to set, and then lightly spraying water on it to dislodge a small amount of the surface skin, leaving a finish somewhat like 80-grit sandpaper.
He poured the steps first, and all went well, but for some reason, when he tried the same technique on the main deck surface, it ended up exposing the gravel, leaving an exposed aggregate look. (I suspect that a contributing factor might have been the torrential rainstorm that occured a few hours after the pour and lasted all night, but the sub says that was not a factor.) At any rate, he said he could fix the problem by trowelling a mixture of sand and concrete glue to the surface, but what resulted was even worse than the aggregate – now we have an uneven surface with globs of sand here and there and several spots where the stuff is peeling off. Owner is upset, sub is not returning phone calls, general (me) is really frustrated.
Any way, short of removal and re-pouring, to salvage this situation? I am already at maximum height for the surface, so there is no way I can pour a layer over top of what’s there. I thought about using a chipping hammer to remove about 1″ of the surface and re-pouring just a finish layer – is this feasible? (The slab is about 4-5″ thick).
I can’t do any exterior finish work on this side of the house until I finish this, so it’s really causing havoc.
Any advice? Thanks!
Replies
Red,
That 'torrential rainstorm' water-blasted the new concrete, so I agree with you there.
You're not going to like the solution, but it's probably cheaper than jackhammering out the existing slab.
First, sandblast off the 'repair' your sub applied. This has the added benefit of roughening the suface of the agregate.
The day before 'Next', thouroughly wet the slab, and immediatly before, mist it again, then wait just till all surface water is gone.
Next apply a grout to bring the suface up to the top of the agregate. The technique and preparation of this is; Since the agregate tops are very close to level, you need a scree width that will bridge across many stones. I think a 5 1/4" width will work. Rip the edges so the corners are square and Tee a 2x4 on top so it doesn't bow. You can make it 3-6 feet long and use it as a giant wood trowel/scree. Cover with visqueen.
As soon as the leveling grout layer resists abrading by your thumb, lightly mist the surface, then sponge float another layer of grout across it. I'm thinking a black sponge will be what you want, but experiment with a red one, too. Black sponge is smoothest, red is a little coarse, and green is way coarse. Cover with visqueen.
The next day, wet the surface and soak the soil around the slab a bit, then recover with visqueen and seal it to the muddy soil. Let cure for at least 7 days. 14 is ten times better. If the underside of the visqueen completely dries out, rewet the concrete and don't count the dried out time as part of the cure time.
Your grout should be between 3:1 and 3:2 sand:portland, with a smidgeon, 1/8 part, lime, for workability, if you like.
If color is important, you can develop a recipe using various proportions of silica and beach sands, and standard and white portlands, and concrete dyes. Because of scalability problems with recipes, always start your experimental mixes with 1 pound of cement and adjust the other parts to suit. Color test by smearing a thin layer on scrap material and letting it dry. The dry color will be very close to the cured color.
SamT
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Or, you could try the Miracote MPC (multipurpose protective coating). http://www.miracote.com
It's a acrylic fortified portland based something or another that comes in either white or greay as well as sanded or not. From there, one can add practically any tint and then trowel, broom, spray, brush, squeegee - just about any type of finishing techique. Holds up quite well to the elements. Can even stain it after.
I just got thru using it on both a vertical as well as overhead application, applied in a stucco texture coat style. Pretty easy to use. But it is sticky!! Wash your tools well, keep a bucket of water close by for spills and cleaning.