Staining/Discoloration on Recently Poured Slab on Grade 1

[Gopher13]

I have an interior slab on grade that is showing staining and discoloration. In some places it looks as though mud was smeared on it. It was recently cleaned, so it is not dirty. It was poured about 3 months ago. The weather at the time was slightly below freezing, however the roof was on the building and they where pumping heat in through the two doors so it was pretty warm in there. It was finished with a walk behind power driven steel trowel after it appeared as though the bleed water had come out. After finishing, a combination curing/sealing compound was applied. Any ideas as to what caused this staining and discoloration? Thanks!

 

[Gopher13]

Here is a second photo

 

[dik]

Did you cover the slab with poly? or was calcium chloride added to the mix?

Dik

 

[Gopher13]

Neither Dik.

 

[dik]

Was there any noticeable water on the slab after it was finished? Any signs of 'dusting'; this may be caused by the addition of CO2 via the heaters. Were the heaters properly vented?

Dik

 

[hokie66]

Don't worry about it. It's just concrete.

 

[NorthCivil]

I'm with dik. probably carbonation - the C02 exhaust fumes from the heaters cause the surface to carbonate and become "chalky". can ruin the first 1/4" of your slab if the place is really poorly ventilated - or can just leave a chalky film if the slab can "breathe" a bit.

http://www.pennradiant.com/library/CARBONATION OF CONCRETE.pdf

http://lmcc.cyberlaunch.net/q_and_a/concrete-dusting-2.asp

for a slab on grade, I'm sure its nothing to worry about.

 

[dik]

That's why I asked if there was any 'dusting'. This can be a weaker area and 'spall off' over a period of time... likely not an issue for a normal slab. If concerned, it is necessary to take a small core and take it into a lab... a report will cost a few hundred dollars, but, you may have a better handle on the problem.

Added: Funny how you notice these things after you post. Discolouration can occur if there is a non-uniform surface of bleed water left on the slab.

Dik

 

[Ingenuity]

Looks like the slab has a 'burnished' finish from the power steel troweling.

 

[Gopher13]

Thanks for the info. everyone. That's exactly it! There are 5 bays to this building and they poured every other bay. On the second series of pours they moved the heaters indoors without proper (pretty much no) ventilation. Thanks again!!!

 

[Ron]

Common finishing issue. It can be corrected with a light citric acid wash. That will make the surface more consistently uniform in appearance. Check the depth of carbonation before acid washing. To properly do this, take a small core of the concrete (minimum 1" diameter, minimum 1" deep). When the core is removed, clean it with a brush then break it down the middle (do not sawcut). On the broken face apply phenolphthalien indicator. The depth of carbonation will be clearly shown by color change.