Concrete pad has micro-fractures - that bad?

[JFoy]

Just had the pad poured for our 24x60 building yesterday. Today, the concrete guy came back to finish the edges and said the pad is extremely hot and has micro-fractures in it. We did add 1in EPS rigid foam board below the concrete and the board has a foil side that was faced down. So, the order was gravel, poly, insulation (foil face down), and concrete.

Metal building going up on this pad in two weeks. They'll anchor the building by drilling into the pad and screwing in some kind of lag bolt. Footings on four sides.

Should I be concerned with the "very hot and has micro-fractures" issue?

JFoy

 

[Deere Dude]

I would sprinkle water over the whole floor and try to slow the drying out time. Cool it down in other words. Msny times I would cover it with visqueen and keep water on it to slow curing time down.

I am sure if the base was packed down good and rebar or mesh was used micro cracks won't hurt anything. I wouldn't like it but nobody will tear up a floor for micro cracks.

 

[dodge man]

I agree the slower the cure the better. Usually either curing compound is sprayed on it or it is kept wet. When curing concrete hydrates it generates heat so that is what he meant by hot. If the air temp is high it痴 that much worse.

 

[Steppenwolfe]

Just had the pad poured for our 24x60 building yesterday. Today, the concrete guy came back to finish the edges and said the pad is extremely hot and has micro-fractures in it. We did add 1in EPS rigid foam board below the concrete and the board has a foil side that was faced down. So, the order was gravel, poly, insulation (foil face down), and concrete.

Metal building going up on this pad in two weeks. They'll anchor the building by drilling into the pad and screwing in some kind of lag bolt. Footings on four sides.

Should I be concerned with the "very hot and has micro-fractures" issue?

JFoy

In the olden days, fresh curing concrete would be covered in burlap and kept moist.

 

[RNeumann]

The concrete was too wet when poured. You/they didn’t account for the foam under the slab- a lot of the moisture would naturally settle out below concrete while finishing.
What you are seeing is the concrete now shrinking as the moisture is coming out.
This is the classic tug of war with any concrete job that has an engineer involved. The engineers want dry concrete for strength and the finishers, pumpers and folks spreading the concrete would prefer to have something wetter that will flow.
My guess is the finishers were worried about a big pour on a warm day but didn’t want to pay for retarder. When the truck arrived (with a good mix) someone climbed up with a hose and added several gallons of water.
Their is not much you can do now-it’s already happened! More than likely it will be fine.
If someone were the research/learn something type do a search for “slump test.” I sure the stuff yesterday was way down there- just a few inches max!

Heat is a natural part of curing. The Hoover dam is a shocking example of that. It still has the cooling pipes embedded in the dam. They pumped refrigerant through the concrete to slow the cure rate. But on a slab their isn’t much to do at this point. A retarder mixed in at the plant or applying a sealer/curing agent on the wet finished slab are the conventional solutions now. Burlap, tarps etc aren’t used much anymore as it is thought they trap the heat.....

Go have a beer- it’s done (in a good way)!

 

[Oldpath05]

As long as theres rebar in concrete I dont see a problem. I had my 6''x26'x36' slab poured in 2013 then 2 years after the pour I have a long crack in it, probably should be ground out and repaired but with rebar all through the slab I'm in no hurry.

 

[Streetcar]

Some bad advise and some ok advise so far.
Concrete does not cure by drying, curing is chemical reaction that creates heat
Adding water to the concrete surface during curing will help control the cracks.
Micro cracks can be sign that excess water was added to the concrete mix and is leaving too quick, if so there can be a durability problem with concrete surface.
Hot concrete is not a problem, variation in temperatures in the slab can be a problem.
Water cure with burlap is still the preferred method for construction joints. Tarps are used to help regulate temperature variations

 

[Streetcar]

As long as theres rebar in concrete I dont see a problem. I had my 6''x26'x36' slab poured in 2013 then 2 years after the pour I have a long crack in it, probably should be ground out and repaired but with rebar all through the slab I'm in no hurry.

Structurally there is big difference between large crack and micro cracks

 

[JFoy]

What I decided to do was put an impact sprinkler on it with a controller. The controller turns on the sprinkler every 6 hours and runs for 15 mins. I set the timing of the controller so it comes on during the hottest time of the day. Hopefully, that will help a bit.

The expansion cuts were made this afternoon, so about 24hrs after the pour.

Thanks for all the advice.

 

[dodge man]

I work for an engineering firm as a land surveyor but have also tested thousands of yards of concrete. The above statement about the slump is correct. There is a device called a slump cone to test it along with air test and cylinders to test the strength.

Without reducer we like the slump to be less than 4”. Which is really pretty stiff. With reducer it can be 7”. They can also add a retarder that also makes it easier to finish, of course this all adds cost. You can also over finish the concrete, which is more of a problem if it’s to wet. This means to much cement floats to the top. All this might not mean a thing or worse case the top might break up or flake.