Last Saturday my carpenter and I poured a small concrete pad (about 150x70x12 cm) as the base for the stringers of a new stairway (home-mixed concrete). I covered the pad with a plastic sheet, but quite apparently it was insufficent for the conditions 24 hours later. Here’s the conditions and timeline:
Mixture used (weight ratio recommended by mfgr)
1.25 Portland cement
3 gravel
2 sand
Timeline:
Saturday 3PM: finish pouring concrete pad
Saturday 6PM: final smoothing of concrete surface, cover with plastic sheet
Sunday 11 PM, rain begins
Monday 10AM: heavier rain/sleet since early morning, I look under plastic and discover about 4 cm of standing water on concrete surface. I drill a series of 12mm drain holes through the concrete form just at the level of the concrete and let most of the water drain out; the remainder I take out with towels, then recover the form with larger plastic sheets. Rain continues through most of Monday.
Tuesday 10AM: Rain has stopped; a slight amount of water remains on part of the concrete surface so I towel it away and open the surface to the air.
Anybody care to guess what kind of problems I might have down the road with this scenario? After the concrete’s dry should I paint it with sealer or otherwise treat it?
TIA
Replies
Hok
If it rains on a slab, a good chance of the top "peeling", spalled.
Did you tent the form, or just lay the plastic on top of the slab, within the form?
I tented it, but water ran down the board forming the tent peak and dripped under the plastic onto the concrete surface. Some also probably splashed up from around the sides once the rain got heavier (I obviously wasn't expecting that much precipitation). I covered it completely on Monday morning so water couldn't run down the peak board (photo).
The way I read it the concrete was placed by 6PM and the rain didn't begin until 17 hours later. When precisely the slab got wet is hard to guess, I suppose, but it sounds like the concrete probably had a good 24 hours to set before it got wet. The concrete should have been quite well set by that time and reasonably impervious water. In fact, it was almost to the point where you'd want to intentionally wet it to slow curing.
As Calvin suggests, you may have some surface finish issues, but the integrity of the slab would not have been affected.
DAn
Since today is yesterday in Japan, I cornfused myself thinking the slab was a fresh pour.
A day later, I wouldn't even worry about the peel. Should be fine.
Hey, thanks.
Thanks to both Cal and Dan for the responses. I just now went out and checked the pad and while there's substantial efflorescence (see pics), I can't see any spalling.
The efflorescence is likely due both to sitting water and the low, near-freezing temps we had the last couple of nights; it shouldn't be hard to clean off with vinegar or muriatic acid, and at any rate it won't harm the functionality of the pad, much of which won't be visible anyway once the stairway is sitting on it. Thanks again for the tips.
You've encountered one of several reasons why forms are usually set flush with the final top surface of the slab.
I wondered about that when I saw the forms the carpenter had built. Dunno why he decided to use all the plywood!
(I also discovered that the efflorescence comes right off when lightly swept with a deck broom; I'm keeping the surface damp but not innundated.)
Yeah, I figured that the "efflorescence" was really just scum from the water.
(And I kinda wonder how you got the slab flat and level without being able to screed against the forms.)
The carpenter did the trowling, and while the main surface is level, he mentioned I might have to use a grinder on the edges to take off any sharp edge.
--on an off note, Japanese craftsmen are sometimes famed for their precision and dedication to detail, but that's when they're working in their element. Once they leave their area of comfort, they can be unbelievably sloppy. Here's an example:
A year ago November, I had a new heat-pump water heater installed by a local plumbing firm. Part of the installation involved placing a small control unit on the wall of the main bathroom (1F). The heatpump is located on the opposite side of that wall. So they drilled a hole through the wall and on the outside placed a small junction box on the outside surface of the siding I had just installed. The photos show what it looked like when they got done:
I couldn't believe that they'd install the thing overlapping two rows of siding like that, but I realized it's really not so uncommon when you think about it. I took it off and reinstalled it on a mounting block. I still have to take care of the wire running down the wall.
Yeah, getting stuff to fall on the flat on lap siding requires some thought ... before you drill the hole. You see the same problem a lot in the US. The mounting blocks are one solution, but more obstrusive than simply planning well, and probably more expensive/time consuming as well.
"wonder how you got the slab
"wonder how you got the slab flat and level without being able to screed against the forms."
Really?
You have not been around much concrete work have you?
I know it can be done, but it's twice as much work for a result that's only half as good.
No - when you know what you are doing, it is far less work
A carpenter might do that, but a concrete man will not. It is a waste of time and sometimes of materials.
WATER CURED IS ONE METHOD OF
WATER CURED makes for a stronger crete
So stop worrying. You might have a concern if it rained immediately and seeped into the surfface. That could cause scaling or pitting, but yours was set by the tim it got wet.
Thanks to all for the continuing reassurances. The last couple of days have been on-and-off rain (mostly light and scattered), so I've basically left the pad uncovered so it can get wet without further attempts to drain it. I'm having a meeting with the carpenter tomorrow to narrow down how many SS bolts we're going to need on the rest of the project.