Black marks in new garage slab? - Contractor Talk - Professional Construction and Remodeling Forum

wackman · 11-10-2007, 05:38 PM

We are finishing up a garage conversion into a living room. We poured a new slab over the old slab to make the floor level and look nicer. The slab is going to be sealed and will be the finished floor. No carpet or anything like that.

Well as the slab is curing it's developed dark spots that have become near black. I have no idea what it is. Some people have told me that it's just water but I mean it's almost black and it's splotchy with real distinct edges.

Any ideas what this is?

I took some picts that I can upload if any one wants to see what it looks like.

Thanks,

Wack

concretemasonry · 11-10-2007, 05:49 PM

How thick was the slab you poured over the old slab?

What kind of concrete or raw materials?

It should not happen with a real concrete slab. If you used a leveler, the colore could be anything, but usually not pretty.

CaliDesigns · 11-10-2007, 07:57 PM

It could be oil migrating up, not for sure. You could buy a gallon of black acid stain for $60 and stain it before you seal it. You may even be able to up sale it.

wackman · 11-10-2007, 11:44 PM

The new slab is about 4 inches thick there. The old slab was cleaned and good for bonding. I don't know where any oil could have come from.

neolitic · 11-11-2007, 12:23 AM

How long since you poured, and where are you?

wackman · 11-11-2007, 05:17 AM

Poured about a month and a half ago. Portland Oregon.

Flyfsh · 11-12-2007, 10:31 AM

Sounds like it could possibly be from the power trowell too. Are the marks circular or curved shape? If Calcium Cloride was used the steel that comes off the trowells will turn black.
Jim

If it was the water causing it (efforence) it would be white.

Taranis · 11-12-2007, 11:02 AM

Wack,
it could be from the sand you used. Could be iron pyrite, Lignite or some vegetable matter contained in it
If it is coming through in black "dots" check out this link and look on page 9

http://www.mortar.org.uk/downloads/lt-aggregates.pdf

If it is, a through wash down and maybe a surface coating is required. Also enough time should have passed so that any migration to the surface should have ceased.

wackman · 11-13-2007, 04:09 AM

Quote:

Originally Posted by Flyfsh

Sounds like it could possibly be from the power trowell too. Are the marks circular or curved shape? If Calcium Cloride was used the steel that comes off the trowells will turn black.
Jim

If it was the water causing it (efforence) it would be white.

The marks look totally natural. They're splotchy with no real shape to them. Definitely no trowel looking marks.

wackman · 11-13-2007, 04:13 AM

Here are the pics. What do you think?

wackman · 11-13-2007, 04:14 AM

The slab isn't cleaned yet so you can see some slight swirl marks but that's just a dusty layer. The actual black marks don't have any pattern but the discoloration you see.

I'm baffled...

Taranis · 11-13-2007, 04:31 AM

Seems strange.
When it was finished could there have been any contamination onto the floor? possibly water? It looks as though, if it is water, that it has come from the back wall in the top picture. Is that an outside wall? If you stand back and look at it it does look as though there is a "flow" from the back wall where it is darkest. Stab in the dark but to get that effect I don't see it being the material used. Not consistent enough.
One thing I would say is it doesn't appear to have damaged the surface so it should be ok to lay onto it with whatever flooring is to be fitted.

CaliDesigns · 11-13-2007, 10:09 AM

You could do a "mat test" to see if is water or not.

neolitic · 11-13-2007, 10:48 AM

Might just be my monitor, but it doesn't look that strange to me.
Slab in enclosed spaces always seem to cure slower in corners/ places with dead air.
Never saw base molding used as concrete form before

I think I would have had reservations about pouring "up" on drywall?

Flyfsh · 11-13-2007, 10:58 AM

I agree with neolithic as far as not being that unusual. I will look at the pics on my home puter ( lot better than this POS at work).
From the pictures it looks like that part may have been getting its initial set. Possibly the finisher waited a bit too long before getting on it with the machine and "burned" it in. The photo shows it in a back corner so the finisher could not have got there until the front part had set enough to get his machine on it.
The biggest part of my job is trouble shooting problems after the fact. I look at 3 to 7 problems a week. Then I get to make someone mad (usually the finisher). I don't have too many friends from work.

Taranis · 11-13-2007, 11:58 AM

The bottom picture is an open corner so it should have dried out at the same time as the rest of the slab. Sometimes the outside edge will dry quicker dependant on the surface it adjoins (more or less porous). There are no trowel marks (or not enough) to show that that part had more or less glut (what we call it) when finishing. That would show in trowel sweeps.
BTW that is skirting on top of the floor, not the floor finished to the board.

CaliDesigns · 11-13-2007, 05:39 PM

A power trowel would not get that close to the edge.

wackman · 11-13-2007, 07:42 PM

The reason I'm concerned about this is because this is supposed to be the finished floor. No coverings. The owner is going for a semi industrial look where everything is simple and clean. He wants just a concrete floor. Barely any moldings at all. The drywall and base went on after the slab. Those are the finished floor and painted walls/trim. If we were just covering it I wouldn't really care about the discoloration. Now he's considering carpet but I can tell he wants the option.

The dark spots are in two places. Both ARE exterior corners but there is a lot of exterior wall with no problems at all and you can see how sudden it is.

There is no water getting in there and the old slab we went over was very clean. Plus if there was any old oil in it it would have been where the cars parked and possibly leaked, not here in the corners.

These ARE also the last places the finisher was doing so he very well could have done a burn in but he did it by hand not a power trowel. How burned in can you get it by hand?

The color wasn't like this at first and has slowly gotten darker and darker. At first we thought the rest of the slab was just drying quicker than the corners but as it got darker and time went by we've realized it's something else.

Could there have been something in the load of crete that was only at the end of the load? Something that would have sunk to the bottom and avoided the mixing action?

wackman · 11-13-2007, 07:45 PM

Quote:

Originally Posted by neolitic

Might just be my monitor, but it doesn't look that strange to me.
Slab in enclosed spaces always seem to cure slower in corners/ places with dead air.
Never saw base molding used as concrete form before

I think I would have had reservations about pouring "up" on drywall?

We didn't pour up. Kinda looks like it in these pics but those are new very simple small base moldings matching the rest of the home. We put a vapor barrier up the walls and blocked in between the studs then poured. Then we did all the new insulation, drywall, moldings, etc...