Realistic expectations with concrete - cracks

[DougWil]

Ugh.. so your saying design every slab based on bad sub-base prep and based on non uniform loads and expect water, earthquake, and all other unknown issues.

Awesome.

No, I am saying that putting typical mesh in a slab will significantly shorten the life of the slab because it is inadequate for the stresses a slab will be subjected to.

That is compounded by how poorly the mesh frequently is placed as the OP pics shows. And that any cracks that do occur and have water or road salt water (not unusual in a garage or parking lot) seeping through will rust away a small wire much faster then a #4 bar.
^Then you have 2 slabs, or more.

The OP's slab already has cracks, it will have plenty more when loaded and it is doubtful that all the slab runoff or adjacent roof runoff will be directed away from what we hope is a theoretically perfectly compacted soil and base.
The soil will then become soft, lose load bearing capacity or expand.
Either is far from ideal but is realistic.
Which incidentally is the title of this thread.

So any slight benefit of reducing shrinkage cracks by using mesh to minimize the effects of thermal expansion in the uncured stage are insignificant compared to the costs of a much shorter slab life.

 

[brownbagg]

no control joint and they poured it too wet

 

[tn_popo]

So once the cracks start does anyone recommend doing anything to fill or seal the cracks? My newly poured garage floor has some hairline cracks and 1 that's bigger.

 

[woodzy]

When I have all my concrete done last year, they formed and poured my driveway which was 12' wide by 170' long, and approach to the garage that was 33' wide and 48' long, stamped sidewalk in front, and patio out back. They started on a Tuesday morning and was done at the end of Wednesday. They came back on Thursday to cut the rest of the control joints in the freshly poured concrete. They did put control joints (The rubbery stuff) about every 30 ' and at the intersection of the driveway and approach, they did this I'm sure to help control cracking. They had a crew of about 10 people and if I had to arrange this work, I would still be working on it a year later.

 

[Falcon67]

I'll throw this in - the drag strip here has about 800' of concrete. It's 12" thick, with lots of bar poured in two continuous 25 x 800' pours. It was saw cut - DEEP - every 20' in both lanes all the way down. Cut enough that they put backer rod and epoxy in the cuts to start with a fairly level surface.

The 2nd year after the pour, the lanes had running cracks at 10' intervals between the saw cuts, almost like you'd gone back and curt some more joints. That was 12 years ago, still the same. It's been surfaced (ground) twice, both times the company said it was some of the hardest crete they'd ever run down.

 

[wssix99]

So what do I do from here. Do I pay the man and just '**** it up'? Should I request some type of fix??

The fact that you got any cracking before your control joints were cut is proof that your concrete finisher was late to the game in cutting your joints. I would definitely not let anything go unsaid about it!!!

The feature you have is called a Reentrant Corner. If not properly accounted for, they are guaranteed to cause a radiating crack: http://www.concreteconstruction.net...a-reentrant-corner-crack-ruin-your-image.aspx

No amount of normal reinforcing, mesh, or proper control joints can guarantee a stop to the radiating crack, but as the article and lakeroadster point out, proper control joints can encourage the crack to form where one wants it to. In order to guarantee that this crack will not happen, an expansion joint to isolate the slab in to non-reentrant sections would be needed. If you look at woodzy's post and pictures, that what is on the sidewalk at the reentrant corner, there. (Although the sidewalk will also have expansion joints for other reasons, in addition to this one.)

I had a reentrant corner in my basement slab where I could not put an expansion joint or control joint for aesthetic reasons. I did the 45 degree reinforcing, as illustrated in the article above, and I still got the crack. (Fortunately, it was very small and my epoxy coating hides it fully.)

 

[joes169]

The fact that you got any cracking before your control joints were cut is proof that your concrete finisher was late to the game in cutting your joints. I would definitely not let anything go unsaid about it!!!

The feature you have is called a Reentrant Corner. If not properly accounted for, they are guaranteed to cause a radiating crack: http://www.concreteconstruction.net...a-reentrant-corner-crack-ruin-your-image.aspx

I can't believe it took 45 posts for the correct answer to be typed, or at leat together.

This has nothing to due with the theraml expansion rates of steel vs. concrete, this is elementary concrete 101. Inside corners are the most prone area to crack in a slab like that. If you give a darn, you don't wait 45 or 48 hours to saw control joints. You saw them the same day, or as early as possible the next morning. There's a reason almost every decent size concrete flatwork contractor has at least one "Soff-cut" saw, they save your rear end from things like this.

Another thing looked over is the "wet curing" procedure the OP mentioned. If you insist on wet-curing, you need to keep it completely wet the entire process, like for 3 days on the minimum end, longer is even better. The first time the water dries from the slab, you're not doing it any good. This is concrete, not grass. As a matter of fact, depending on the weather, you could actually be doing more harm with intermittent watering of fresh concrete. Look up the term "thermal shock" as it pertains to concrete. ANything more than 50 degrees difference between concrete & water temps can create random cracking as well.

Lastly, I can't believe that 4-4.5" is recommended for commercial parking slabs in your area. Here, 5" is the absolute minimum, with 6"+ being far more common.

 

[joes169]

When I have all my concrete done last year, they formed and poured my driveway which was 12' wide by 170' long, and approach to the garage that was 33' wide and 48' long, stamped sidewalk in front, and patio out back. They started on a Tuesday morning and was done at the end of Wednesday. They came back on Thursday to cut the rest of the control joints in the freshly poured concrete. They did put control joints (The rubbery stuff) about every 30 ' and at the intersection of the driveway and approach, they did this I'm sure to help control cracking. They had a crew of about 10 people and if I had to arrange this work, I would still be working on it a year later.

Just to be clear, those are either expansion joints, construction joints, or both. Control joints are different. Either way, the work looks nice!

 

[Joemctag]

It was almost guaranteed to crack there. It won’t grow. Your rebar was not a waste. They iucould have tooled a joint or sawcut the slab across that part and the hairline crack would have followed the joint or cut. You’ll have a good strong slab after you cure it. Just quit worrying about the hairline crack. Doesn’t look too soupy to me, either. Maybe the fiber wasn’t worth it, but for sure the extra thickness and stronger mix will be worth it over the life of the slab. They should have put in a joint there or sawed earlier, but that one thing is probably the only thing. Not too much perfect concrete. The finish and whether or not they overworked it (probably not if mild weather) is much more important than that sawcut.

 

[Walter_TA]

What you have is shrinkage cracks. Too much water was added to the mix. As the water evaporated the concrete got smaller. The concrete did not have the strength it needed not to crack. The water has to be held in the mix till it gains strength as it cures. Did the concrete have a vapor barrier underneath? If not that is the second mistake. It takes 7 days for concrete to cure. What the fiber does is give concrete strength until it can cure. This will help to stop cracks. The forms should not be removed for a week. They keep water in.

Concrete should not be allowed to dry out as it cures. There is a chemical reaction as is sets up that requires water. But not too much water in the mix. The concrete will gain strength as it cures. Do not let the top or bottom of the slab dry out, that will stop curing. I used burlap and a sprinkler on a timer to keep mine wet.

Concrete need to be pored at a slump of 4 or greater not to crack. Yours was not. I bet your concrete guy added water the on site. There is a additive called Super P that will let the concrete be positioned and still have a high slump. No water has the be added.

In a slab rebar or remesh holds the concrete together after it cracks. It does nothing else.

Unless you want to jack hammer what you have, it is way to late to fix it.

 

[p_mori7]

What you have is shrinkage cracks. Too much water was added to the mix. As the water evaporated the concrete got smaller. The concrete did not have the strength it needed not to crack. The water has to be held in the mix till it gains strength as it cures. Did the concrete have a vapor barrier underneath? If not that is the second mistake. It takes 7 days for concrete to cure. What the fiber does is give concrete strength until it can cure. This will help to stop cracks. The forms should not be removed for a week. They keep water in.

Concrete should not be allowed to dry out as it cures. There is a chemical reaction as is sets up that requires water. But not too much water in the mix. The concrete will gain strength as it cures. Do not let the top or bottom of the slab dry out, that will stop curing. I used burlap and a sprinkler on a timer to keep mine wet.

Concrete need to be pored at a slump of 4 or greater not to crack. Yours was not. I bet your concrete guy added water the on site. There is a additive called Super P that will let the concrete be positioned and still have a high slump. No water has the be added.

In a slab rebar or remesh holds the concrete together after it cracks. It does nothing else.

Unless you want to jack hammer what you have, it is way to late to fix it.

^^^ This is my opinion as well.

If it hadn't cracked yet, it would have eventually. Mine developed some similar cracks after a few years (there were probably some hairline cracks that developed during the first winter). Everything is still perfectly level and held close together. i don't fret over it.

 

[wssix99]

This thread was answered correctly 4 years ago. Unfortunate that wrong "answers" and incorrect information have popped up now.

Mods - please lock this thread.