Concrete Discoloration

[bigmig]

I have a project where we poured this architecturally exposed concrete wall. The contractor took 2 days to pull forms.
For some reason, the forms taken off during the 2nd day have left the concrete a unique "darker" color.

Everything, except the form removal, is identical. Same truck, same mix, same forms.
The wall points North.
The south side, curiously, is perfect. No checker board discoloration.

The concrete is about at day 15 in terms of curing process.

The owner is flipping out. No one knows what is going on. The end goal, is that all the concrete needs to match (same color).
Has anyone ever seen this?

PHOTO IS ATTACHED.

 

[jayrod12]

How long ago did they pull the forms to when the picture was taken? If it was literally an hour ago, maybe give it a day to dry out and see if it's not just moisture and the fact that heat of the day hasn't had a chance to dry it out yet.

 

[TehMightyEngineer]

Tell him it's a feature (there's a reason our company runs away from any project with the term "color" in the specifications).

As far as seeing different color between form panel removal times; not really. I've seen concrete hold water in the pores and evaporate between form removal giving the appearance of different colors but after drying it goes away.

Only thing I can think of is some sort of reaction between something on the forms? Is it just surface deep? What color is the concrete when you grind away the surface with a rubbing brick?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[sticksandtriangles]

Not helpful in the conversation, but I think this looks pretty cool.
Sell the "feature" route and continue the pattern on up hahah.

 

[bigmig]

The forms have been off for about 2 weeks, hence the concern that hey, this might not be going away.
At first glance, everyone was like "let it dry". Now everyone is getting worried.

 

[bigmig]

The other unique thing about this is that the forms were brand new. Like never used, right from the factory. We did this in an effort to get the "texture" the owner wanted.
The only thing that discounts a form flaw (something reacting) is that the only ones it happened on where the forms that were left on until the 2nd day.

 

[TehMightyEngineer]

What form release agents were used? Can we get a picture of the concrete surface up close of both the good and discolored areas?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[MotorCity]

Regardless of when the forms are pulled off, and even if all work was perfectly mixed, placed, cured, etc. it is not reasonable to expect a uniform color be achieved. Its just not a reasonable or practical expectation. If the concrete is exposed and will not be painted or covered, you get what you get.

That said, I think your odds of a uniform color improve if all of the formwork is removed at the same time, and all panels have been given the same time, temperature, wind, and humidity to cure. If the client (and the contract) demand a uniform look, I might give it a few more weeks and then consider some sort of abrasive removal of the skin surface (sand or water blasting??) It might fix the problem if the discoloration is mostly at the surface. Of course then the owner would have to live with a different surface profile (i.e. not a smooth finish).....pick your poison.

 

[jayrod12]

I'm surprised any contractor wouldn't of just pushed to remove all of the forms in the one day. They were essentially almost there as it was.

Also surprising is that they didn't start top down with the removal.

 

[bigmig]

I couldn't get photos up close because of my lack of ability to fly, but will inquire about the form release agent.
Regarding pulling all the forms, I'm not clear on why they didn't pull them all down, and how in the world they removed the bottom forms first.

 

[TehMightyEngineer]

Zoom lenses are useful. I imagine I looked very silly but I always had a decent sized camera with my when I did industrial field inspections/measurements. Not needing a ladder to see something in the roof 30 ft up is indispensable for saving time.

(Random side note; I once did a reasonably complex pipe routing 20 ft overhead using nothing but a portable self-leveling laser, a crayon to mark the floor slab, a magnetic tape, and a camera with a good zoom. 3D modeled all the existing pipes, cable trays, conduits, and what not. Didn't have any field install issues that I heard of.)

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[oldestguy]

Any temperature difference each day effects that might have cooked factory applied form oil out and doing the staining? I'd get back to the supplier of the new forms with this question.

 

[bigmig]

Alright....some progress. Note 4 page 2 of this document is pretty identical to what is happening....varying hydration of cement caused by uneven curing times.
More hydration = darker color of grey.

 

[TehMightyEngineer]

Wow, I could see that happening to a small degree but concrete is concrete, I would have assumed that this would not have such a big effect on color and more just on the internal strength of the concrete and so on. Can't argue with your apparent results and that document though; this might be your answer.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com