Concrete Slab Can anyone explain this?

[JayJones]

Just had a concrete slab poured and everything was looking OK. Kept the forms on and put the sprinkler on it for 11 days and the slab stayed a darker gray color. There wasn't any type of sealer applied to cure, just the water from the sprinkler. On the 12th day, we started to setup some corner blocks, and my 8 y.o. daughter was playing with roller blades.

Everywhere she passed over created white lines on the slab. Now, the following day, the lines have "swelled" out and are getting much larger.

Is this something to worry about? Is it de-laminating from the surface or something? Or was there some type of surface layer that has been broken and now the surface is drying out much quicker? Or maybe the whole surface is going to end up this light color anyways?

Please let me know,

Thanks!

 

[LXCam]

I think you have bigger issues to deal with. Like how you're gonna pay Photobuckets ransom so we can see the picture and figure out what your concern is

 

[PWC Repair]

Yep, all my pics on various forums are gone too. I'm not paying anything for their super slo-mo webspace.

 

[starquestMM]

When you say the lines "swelled" do you mean they are actually raised bumps now?

Was there a curing compound applied? They are basically a wax coating that forms a temporary seal to prevent moisture from evaporating from the concrete. If you daughter's rollerblades disturbed that curing compound seal, the concrete would start to dry out and turn a lighter color and the lighter color patches would grow as the concrete died out more. If this is the case, its good news as your concrete will evenually all be the light color and you won't see the marks.

 

[Richard Cranium]

And welcome to our group,
Please update your profile to include your city and or at least your state.

 

[JayJones]

Pictures visible now? Hosted on Google Photos:

 

[JayJones]

When you say the lines "swelled" do you mean they are actually raised bumps now?

Was there a curing compound applied? They are basically a wax coating that forms a temporary seal to prevent moisture from evaporating from the concrete. If you daughter's rollerblades disturbed that curing compound seal, the concrete would start to dry out and turn a lighter color and the lighter color patches would grow as the concrete died out more. If this is the case, its good news as your concrete will evenually all be the light color and you won't see the marks.

No, not raised bumps, just increased in area size. Hopefully the pictures show up as they can explain much better than I can
There wasn't a curing compound applied, but the rest sounds exactly like you're explaining it.

 

[readhead]

Have you talked to the guy that poured it?

 

[The Tool Tyrant]

Very interesting...also looks like you had the same reaction from another source creating the light spots that look like droplets of something created them. I'm sure there is some scientific reasoning behind this phenomenon...and I'd love to know the cause.

 

[blair683]

Wow that looks awful. Can't believe that is over 11 days old. I just had one poured about a month ago. The same day it was poured the guy walked in his socks and sprayed a sealant on the slab with a pressurized pump sprayer. My slab was dark gray for about a week, slowly turning light gray. The day after it was poured the guy came and cut the control joints. I could walk on it without leaving marks the day after it was poured. But within 11 days it was light grey and I was driving on it.

 

[jhelrey]

Tell your daughter to make sure she covers every square inch and no more issue...

I'm also curious to know what the contractor says...

 

[ssdave]

I don't think you have a problem, just something on the surface was keeping the slab holding moisture and now it's not. It will dry out and be a great slab for your use. The 11 days of curing will give you a sound, strong slab. Much better than if you had allowed it to dry out quickly and have a great appearance, but much lower strength and durability.

 

[wssix99]

+1 on talking to your contractor and finding out what surface treatments they applied. Did they tell you to sprinkler the slab?

 

[Gittgo]

Have you talked to the guy that poured it?

Go to the top. Ask the Co. to send an engineer from the concrete producer. It may be a bad batch.

 

[gungatim]

I'm no expert, but I've never heard of putting a sprinkler on fresh concrete for 11 days. around here they put plastic or hay down for a day or two to help delay evaporation, but I've never heard of spraying water on it that long.

obviously I know little about concrete but it does make me wonder...could it be minerals from the water? or dry spots from the roller blades that leached minerals out of the concrete? kind of looks like calcium maybe...

 

[JayJones]

I don't think you have a problem, just something on the surface was keeping the slab holding moisture and now it's not. It will dry out and be a great slab for your use. The 11 days of curing will give you a sound, strong slab. Much better than if you had allowed it to dry out quickly and have a great appearance, but much lower strength and durability.

I'm really hoping that this is the case. I'll give it a few more days and see if it continues to blend out. Might need to get her to skate all over the whole thing

 

[JayJones]

+1 on talking to your contractor and finding out what surface treatments they applied. Did they tell you to sprinkler the slab?

Nothing was applied to the surface. It was just steel-trowel finished with a helicopter. They did say to keep the slab wet/cool by using the sprinkler. I think the surface was just really holding the moisture keeping it dark gray this whole time, now it's drying out. It just looks terrible though...

 

[MagKarl]

Give it some time, it will lighten and even out.

 

[Balor]

Your 8 y.o. daughter pushed out the moisture/water just under the surface, the very top of the concrete was sealed by the troweling so water could not escape as fast where your daughter pushed/broke the surface. Let it finish drying then seal it, I like the look. rngr1

 

[nut_buster2017]

My first thought was that the heat generated by the curing perhaps wore material off the wheels, but then I found this:

http://www.concreteconstruction.net/how-to/construction/curing-concrete-in-hot-weather_o

 

[sberry]

Only thing better than the sprinkler would be submersion. They actually do that in ultra high strength applications.

 

[Falcon67]

My first thought was that the heat generated by the curing perhaps wore material off the wheels, but then I found this:

http://www.concreteconstruction.net/how-to/construction/curing-concrete-in-hot-weather_o

It was over 100 when my slab was poured. That was on a Friday morning. I could keep it wet - not constantly, but a lot - until Sunday night then it was on it's own. Surface temp if the water evaporated was 135F. I didn't have any surface problems with the cure. The house next door was poured in summer. 92 yards. They started at 7 AM and had the forms off the slab by 2 PM. No water cure or anything else. Just pour, set, lunch, check, pull most of the forms, buh-bye. 7 days or so later, framers all over it.

 

[Chadwilliam1]

Interesting...

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[matt_i]

My opinion is that it does look like a curing sealer was used.

My slab was uniform when cured with a sealer (needed no water) in the dark grey color.

However as winter set in and the building was not framed, were dragging framing timbers and wall sections around, running a forklift to set the trusses, the sealer flaked off but not uniformly.

As I recall, the directions on the curing sealer were to recoat with same stuff after all other trades work was done for a uniform finish.

 

[blair683]

After the pour they didn't add any water to my slab. But as stated earlier they did spray it with a sealer. Mine was dark gray for several days but walking on it the very next day did not cause it to look like yours. It was about 80 degrees when they poured mine at 6:30 am.

 

[JayJones]

I'm positive a sealer was not applied after the pour, and I'm not aware of anything being added to the concrete mix, other than air entrainment.

I was really happy when I turned the water off and it was still nice and dark gray. Figured it had retained a bunch of moisture and cured properly. Now I'm second guessing that...

I'm going to keep an eye on it and see what happens.

 

[JimNC]

Have you tried pouring a cup of water on a light spot?

 

[nes999]

My opinion is that it does look like a curing sealer was used.

My slab was uniform when cured with a sealer (needed no water) in the dark grey color.

However as winter set in and the building was not framed, were dragging framing timbers and wall sections around, running a forklift to set the trusses, the sealer flaked off but not uniformly.

As I recall, the directions on the curing sealer were to recoat with same stuff after all other trades work was done for a uniform finish.

Matt is correct, they used a hardener to speed up the process so they can get moving to the next job. You can apply sealer over in in a month or so and it will hide the grey marks. This is super common and on most site can happen so quick you wouldn't have noticed.

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[ard]

The house next door was poured in summer. 92 yards. They started at 7 AM and had the forms off the slab by 2 PM. No water cure or anything else. Just pour, set, lunch, check, pull most of the forms, buh-bye. 7 days or so later, framers all over it.

Still standing?

 

[RPH]

When the Davison freeway was poured in Detroit many years ago, it was the first freeway in the USA. They actually flooded the concrete and it was under water for a month.

 

[850xpeps]

I'm no expert, but I've never heard of putting a sprinkler on fresh concrete for 11 days. around here they put plastic or hay down for a day or two to help delay evaporation, but I've never heard of spraying water on it that long.



obviously I know little about concrete but it does make me wonder...could it be minerals from the water? or dry spots from the roller blades that leached minerals out of the concrete? kind of looks like calcium maybe...

Lots of spec require us to cure for a week with water and burlap. The longer you water the slab the stronger it will be. Moisture causes a chemical reaction. The slower it leaves the cement the longer it cures and stronger it will be.


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[wssix99]

Matt is correct, they used a hardener to speed up the process so they can get moving to the next job. You can apply sealer over in in a month or so and it will hide the grey marks. This is super common and on most site can happen so quick you wouldn't have noticed.

Right, it could look like they were spraying the slab with water or something while it was being poured. The tanks could even look just like the ones they used for form release agent/oil.

My opinion is that it does look like a curing sealer was used.

My slab was uniform when cured with a sealer (needed no water) in the dark grey color.

Sealer will give the concrete a darker, richer color. The look of the slab really looks like a layer of thin ice over the concrete that has fractured and let water out from underneath it.

I'm going to keep an eye on it and see what happens.

When you spray water on the slab right now, does it bead?

 

[JayJones]

Right, it could look like they were spraying the slab with water or something while it was being poured. The tanks could even look just like the ones they used for form release agent/oil.


Sealer will give the concrete a darker, richer color. The look of the slab really looks like a layer of thin ice over the concrete that has fractured and let water out from underneath it.


When you spray water on the slab right now, does it bead?

As mentioned, sealer was definitely not sprayed on after the pour, just ran the sprinkler keeping it nice and wet.

The water doesn't bead on the surface.

Have you tried pouring a cup of water on a light spot?

It just rained yesterday and some of the light spots re-darkened, but I'm sure they will lighten up again once it dries in the sun. The whole slab is looking much lighter in color as well.

Up close, the light streaks look more like crazing cracks now that they're wet...I might be seeing it cure in super slow motion or something.

 

[JayJones]

Here's a closeup attached:

 

[Falcon67]

Still standing?

All 3200 sq/ft. They do everything like that here, I never see blankets, hay, water, whatnot. Form, pour, float, build shortly after you can stand on it. No foooter, etc either - scrape the ground, set forms, dump sand, set pipes, sometimes vapor barrier/sometimes not, call the trucks.

My shop was poured on 7/24 and I had sticks on it 8/8.

 

[ard]

Your 8 y.o. daughter pushed out the moisture/water just under the surface, the very top of the concrete was sealed by the troweling so water could not escape as fast where your daughter pushed/broke the surface. Let it finish drying then seal it, I like the look. rngr1

Sounds reasonable. As time went on, more water migrated out of the 'skate paths', and they appeared to grow.

 

[wssix99]

As mentioned, sealer was definitely not sprayed on after the pour, just ran the sprinkler keeping it nice and wet.

Your pictures are consistent with a delaminating sealer caused by excess moisture: https://www.concretenetwork.com/vapor-barriers/moisture.html

Either way, sealer or not, this is something you should be concerned about. When concrete cures in the first two days, there is a chemical reaction and the material physically changes. Water doesn't get "under" the surface or anything like that.

There must either be some non-concrete product on the surface that is delaminating or the concrete was finished incorrectly (given the finishing technique, admixtures, water content, water application, environment, etc.) and the layer of cement that raises to the surface during that process is loosing it's bond. Either issue is not self-correcting and would require a potentially uncomfortable consultation with your concrete contractor.

 

[6768rogues]

As concrete cures, a matrix forms and pores open up. Those pores allow more moisture to evaporate and the process goes on. Curing sealer closes pores so it slows the cure similar to the way misting with water will slow the cure. More pores continue to open, but sealing some slows the process down. In our temperate climate, it is common to put curing sealer on as soon as the surface can support foot traffic, usually the next morning, and then no misting with water.
For those hot weather jobs, I would add retarder. Otherwise the cure would be too fast to get a good finish and durable product.

 

[blair683]

OP, keep us updated as time passes. I am curious to how this turns out.

 

[kb1982]

My bet is that the streaks are calcium pushed to the top by water.

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