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.

 

[Tractor Seabee]

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.

Waiting that long to do the expansion cuts (proper term is control joints); stand by, larger problems are looming ahead. Expansion cuts have to be made as soon as the saw can get on the slab even if it is midnight. Next day, too late, cracks have already startd where the concrete wants them to not where you want them.

I was a certified concrete inspector in my earlier construction career. There is way more inaccurate information on concrete outthere in the construction world than there is accurate information. A manual you can get on Amazon; "The Contractor's Guide to Quality Concrete Constructio" is a must read for anyone contemplating or doing concrete work.

The first step for slabs on grade is a good solid well and even compacted subgrade (95% density or more). W/O that you are beat before you start.

Ron

 

[Streetcar]

Set timer on sprinklers so that the concrete is always wet.

 

[Egon]

What's happened has happened. Keep the concrete wet may keep the micro cracks from expanding.

They may be surface cracks due to pouring & finishing methods a high temperatures.

The chances are all is fine but do put a proper sealer on thr concrete.

 

[Oldpath05]

("Structurally there is big difference between large crack and micro cracks")<<<<I didn't know that, thanks.

I would think that after one day adding water is to late, mine was poured at 6am finished at 9am by the concrete guy I hired, when he was done told me to keep the slab wet rest of the day with garden hose, couple days after that started building on it.

So I think after one day whats done is done and nothing more can be done except for tarring the slab up and doing it over again. Not sure of your cost but my 6x26x36 slab was just under 3k 5 years ago, not pocket change but I thought it was a fair price.

 

[dmccarty]

Our house is a slab which is not often done in my area of NC. We have a couple inches of insulation between the 67 gravel and the slab. We poured on the hottest day of the summer, , during a very hot season. :shocked: It was so hot, the builder who was up there in years, had to go home a bit early because the heat got to him. :shocked: I think we poured late morning or around lunch time so we were out there during the hottest part of the day. I had drawn up a decorative pattern for the concrete joints and the mason showed up the next morning to cut them. On the day of the pour, the builders son was supposed to show up with a part we needed to run the well, and he was not able to get the right part. :confused3: The neighbors let us fill up some garbage cans to transport water to the house to cool down the concrete and we got the right part the next day for the well. Not optimum but it seems things turned out ok.

No rebar in the slab.

We have a few places were the slab cracked. Interestingly, they are all at "inside" corners but one. The front port is L shaped and at a lower level than the house slab. This means there is an indentation in the house slab. A small crack formed on the inside corner of the indentation. The other "inside" corner cracks are in the master bath that has a walk in shower. The shower floor was to be poured by the tile guy so we blocked off the slab pour. This created stress in the slab at each corner and we have small cracks. What is interesting is that these cracks and the porch crack all have the same angle. The only other crack in the surface of the slab is where the cut in the concrete did not go deep enough. They pulled up on the saw a bit too early when getting near the edge of the slab. There ARE other cracks. Lots of them but they are all down in the cut concrete joints. :thumbsup: The cuts are decorative and did control cracking except as mentioned. The cracks have not moved or changed since they formed 13 years ago.

The cuts are roughly 3'x3' depending on what was requited to fit a room.

Later,
Dan

 

[Jstpssng]

("Structurally there is big difference between large crack and micro cracks")<<<<I didn't know that, thanks.

I would think that after one day adding water is to late, mine was poured at 6am finished at 9am by the concrete guy I hired, when he was done told me to keep the slab wet rest of the day with garden hose, couple days after that started building on it.

So I think after one day whats done is done and nothing more can be done except for tarring the slab up and doing it over again. Not sure of your cost but my 6x26x36 slab was just under 3k 5 years ago, not pocket change but I thought it was a fair price.

Up here the concrete alone would be at least $1800. Add on the rebar and labor, it seems like a good price

 

[Streetcar]

If you can, keep water on slab for a week. Concrete will have 90 percent of ultimate strength by then.
Sealing surface will help with the micro cracks.
Concrete will normally crack at 45 degree angle, due to weakness in tension

 

[EddieWalker]

What are micro cracks?

If I'm understanding this, do you have a lot of tiny, think cracks all over the place?

When too much water is added to the mix, you get large cracks when it cures because the volume taken up in the concrete evaporates and there is now less volume of concrete inside the poured area.

Lots of small cracks are something different. So far, I haven't read any comments on lots of small cracks, they are all talking about big cracks.

Was anything added to the mix? A chemical of some kind to deal with local temperatures? or something else that was supposed to improve the concrete in some manner that might have been done wrong?

Until I knew exactly what was causing the cracks, I would not build on that pad.

The first thing that I would do is hire an engineer who specializes in concrete.

I would also contact the plant where the concrete came from and speak to a manager there. Not the dispatcher. Not a driver.

Take pictures today from a spot that you can take the exact same picture again in 7 days. Put a ruler on the concrete when you take the picture for reference.

 

[ovrszd]

All. Concrete. Cracks. All. Concrete.

 

[Streetcar]

What are micro cracks?

If I'm understanding this, do you have a lot of tiny, think cracks all over the place?

When too much water is added to the mix, you get large cracks when it cures because the volume taken up in the concrete evaporates and there is now less volume of concrete inside the poured area.

Lots of small cracks are something different. So far, I haven't read any comments on lots of small cracks, they are all talking about big cracks.

Was anything added to the mix? A chemical of some kind to deal with local temperatures? or something else that was supposed to improve the concrete in some manner that might have been done wrong?

Until I knew exactly what was causing the cracks, I would not build on that pad.

The first thing that I would do is hire an engineer who specializes in concrete.

I would also contact the plant where the concrete came from and speak to a manager there. Not the dispatcher. Not a driver.

Take pictures today from a spot that you can take the exact same picture again in 7 days. Put a ruler on the concrete when you take the picture for reference.

I am assuming op is calling a series of small interconnected cracks as micro cracks .
Similar to spider web, cracks caused by excess water in surface during finishing