When is My Concrete Floor Gonna Quit Cracking?

[danieldd]

Just completed a 24x24 detached garage (pictures coming in a later thread..) They poured a monolithic slab the 1st couple of weeks in March. I'm in North Alabama, so they dug the appropriate footer and installed 5/8 inch rebar, gravel, vapor barrier, and concrete with fiber mesh. No wire mesh was installed prior to the pour. Seems fiber mesh has taken the place of wire mesh - now I'm not too sure.

Within 72 hours of them finishing the slab (and it looked fantastic), I found a micro crack about 5 feet long and of course, became concerned. The contractor had both the concrete finisher and the concrete company out to inspect the crack. Both assured me it was normal and if I was going to exoxy the floor, than it was nothing to be concerned about.

I should also mention at this point that due to the location of the garage, I had to have the concrete pumped in, which they tell me has a different consistency than a normal pour. Additionally, I elected not to have any control joints due to the location of the MaxJax I have yet to install. I was assured that since it was a free standing structure (not attached to an adjoining structure), we could get away without having control joints cut in the concrete.

As the weeks progressed during the formation of the garage, I noticed more and more cracks. It has been close to 3 months now and there are several cracks in the concrete, some 12 feet long and intersectiing some of the other cracks. The width of probably the worse crack is 1/16 of an inch. Fortunately, there are no cracks within 2 feet of where I'm installing the MaxJax - yet.

It is quite evident now that there is nothing I can do to fix the situation. The garage is finished and I don't think I can fault the contractor. Even if I could, its beyond the point where you could start over.

So, I figure I can do any of the following:

A.) Leave the lights off when I go in the garage so I don't see the cracks.
B.) Wear dark sunglasses in the garage.
C.) Install carpet
D.) Epoxy the floor and get on with my life.

Any advice would be welcomed.

 

[Justanoldguy]

When is My Concrete Floor Gonna Quit Cracking?

When hell freezes over it will still crack some more.

 

[Justanoldguy]

Seems fiber mesh has taken the place of wire mesh - now I'm not too sure.

Malicious rumour for sure.

 

[6768rogues]

Concrete always shrinks as it cures and that is why cracks appear. Pumped concrete is soupier and has an additive to make it go through the pipes. With a slightly higher water content, it could crack more. You should have had some control joints put in the floor to control the cracking.

 

[cj7365]

Seems fiber mesh has taken the place of wire mesh - now I'm not too sure.


I should also mention at this point that due to the location of the garage, I had to have the concrete pumped in, which they tell me has a different consistency than a normal pour. Additionally, I elected not to have any control joints due to the location of the MaxJax I have yet to install. I was assured that since it was a free standing structure (not attached to an adjoining structure), we could get away without having control joints cut in the concrete.

You made the decision, now you have to live with it, control joints are usually done with a slab minimum of 12x12, Who assured you that you could get away with it. Fiber is not a replacement for wire mesh.

 

[Shadowdog500]

I understand the installers like the fiber mesh because it is easy to install and "kind of works" I had metal mesh put in my floor and figured the extra fiber couldn't hurt so I had them put that in too. Kind of like wearing a belt and suspenders.

My installer was 83 years old and did concrete his entire life (family business) (You could almost see the knowledge of concrete radiating from his brain). When asked about possible cracking his response was "concrete always cracks, hopefully it will crack where we want it to." Lucky for me , it did(in the control joints). Cant believe you didn't get any saw cuts in a 24' by 24' slab. :

Here is a quote from the Max Jacks install page "If your concrete has cracks or other defects, please consult with a certified contractor before proceeding to install the MaxJax™ lift." You may want to give them a call with floor specs and photos.

 

[Charles (in GA)]

Rebar would have been desirable. Fiber in the mix is a waste of money, does little to nothing. I have wire mesh, I wished we had used rebar, but I was depending on a contractor that I thought knew all the tricks.

Did you use sheet plastic under the slab? Or was it poured directly on the dirt?

I used metal keys so as to not saw the slab, reality is that my sections are too big at 20x30 ft. They need to be 20x20 or less for best effect.

After 12+ years, I have a couple of hairline cracks in my 60x60 slab, radiating off of where anchor bolts are located for the building.

It is said that concrete takes 50 years to fully cure, and so it will continue cracking for years to come.

Charles

 

[Galaxie]

As said you should have had it cut, we have a 60x40 done in two halves and the only cracks are against the walls where they couldn't get the saw cut all the way to the end. No rebar just good old fiber mesh, haven't used rebar for years now.

 

[gc427]

You elected to not have any control joints and now you are wondering why your floor is cracking?


Mount your lift and epoxy the floor and do whatever you are going to do.

 

[W-Cummins]

I can tell you the exact date YOUR concrete is going to stop cracking! The day you sell the place and it's no longer yours! The old saying about there only 2 types of concrete, cracked and about to crack is true.

William....

 

[rasit]

Cracks? What cracks? At this point ignore them......As others have said, you really needed some reinforcment and relief joints. You didn't mention how thick the floor was poured. If it's any less than 6" where the lift is going in I think I would saw cut out the required pad areas for the MaxJax lift and repour them per manufacturer's recommendation.

 

[dirttracker18]

It's not for lack of control joints. They are not needed. No one here uses them except in large industrial applications.

I am no expert but I will give you my $.02.

Did you cover the concrete and try to keep it moist?
If it dries too fast it will crack just as you explained. Your contractor should have told you about this. With the additives and likely extra water for pumping cracking is a danger for sure.

 

[Full Throttle]

It's not for lack of control joints. They are not needed. No one here uses them except in large industrial applications.

I am no expert but I will give you my $.02.

Did you cover the concrete and try to keep it moist?
If it dries too fast it will crack just as you explained. Your contractor should have told you about this. With the additives and likely extra water for pumping cracking is a danger for sure.

agreed 100%

I watered my floor for 2 weeks prior to the structure being erected.

also 2 gauranteed things to happen with concrete. #1 Its gonna get hard, and #2 it's gonna crack. Broomed finishes you don't see cracks in the portland but machine trowled finish will have hairline portland cracks.

 

[darkk]

First, concrete guys, as knowledgeable as they are, rarely ever tell you anything unless you ask. I don't know why that is. It's like their giving away trade secrets. My guys were very knowledgeable but said nothing about the plastic under the concrete and didn't tell me to keep it wet or to cover it or anything. I got no instructions at all. They didn't tell me anything unless I asked specifically. I researched concrete pours very well before ours was done. We had a 30x30 floor poured for our garage before building it so the carpenters would have a solid floor to work on. I had the ground area prepared, poured the footings and within 8 days had the floor poured. We did sand, 6 mil plastic,wire mesh and fibermesh. 3500 psi 6" thick no control joints. I kept that slab soaking wet for 8 days. That was 3 years ago and no cracks. Fibermesh is only to help prevent surface cracks while the concrete is curing, nothing more. Proper preparation is the key to a nice floor. Hope mine stays the way it is....

 

[Justanoldguy]

It's not for lack of control joints. They are not needed. .

Sure, don't put them in.
But don't complain when the concrete cracks and it will.
Control joints will "control" where it cracks.

 

[wssix99]

No wire mesh was installed prior to the pour. Seems fiber mesh has taken the place of wire mesh - now I'm not too sure.

Both are used to combat shrinkage cracking, which you are experiencing now. The cracks could get larger for up to a year.

Given your direction on control joints (control joints just cause shrinkage cracking along them so the floor looks nicer - the cracks still happen) the contractor should have gone with both.

I should also mention at this point that due to the location of the garage, I had to have the concrete pumped in, which they tell me has a different consistency than a normal pour. Additionally, I elected not to have any control joints due to the location of the MaxJax I have yet to install. I was assured that since it was a free standing structure (not attached to an adjoining structure), we could get away without having control joints cut in the concrete.

It sounds like your contractor didn't know concrete very well. That the point you elected not to have control joints, they should have given you better advice or taken extreme measures to limit the cracking. (Even though you don't want control joints in the middle of the pad, you can still put them at 1/3 and 2/3 of the slab and keep clear of the jacks.)

As others have posted, the wet pumpable mix probably makes things worse, but its inexpensive. In order to make a slab with limited control joints and cracking, a dry mix is required - which may mean that the costs go up to get mix-on-site concrete, a team of workers with wheelbarrows, etc.

I would advise leaving the pad alone for 1-2 years until the cracking tapers off and then re-assessing what type of coating you want to place over it. Putting a coating down now will just cause you to have cracks in the coating later on. (A professional coating company should be able to give you some additional advice on this. Do-it-yourself coatings will probably not make you happy here.)

 

[KCarGuy]

I also had to have my slab poured with a "Pumper" Truck.
They went over my house from the street and poured my Slab about 15 years ago.
But I used lots of Rebar and Had My Slab "Cut" the next day.
Friends and Family have all told me that I should see cracks sooner or later.
To this day...no cracks.
(I also tied in some Old leftover chain link fencing that I had laying around, before the pour)
I am sorry to hear about your floor, I would contact an expert about it.

 

[danieldd]

Thanks for the feedback, guys. Yeah, I wish I had been better informed by the contractor and the concrete guys. Like I mentioned, I had them inspect the 1st crack within 3 days of the pour. You would have thought that these experienced people would have recommended to me to put in control joints, but they didn't.

The concern I have now is whether to install the MaxJax or not. I'll give them a call and see what they have to say.

 

[aka Larry]

ALL concrete will crack there is simply no way to avoid it. The trick is to control where the cracks will occur. Hence why they are called 'control joints'.

In my recently poured 40'x40' slab, I used keyway-type control joints in a grid and it's cracking nicely right along those joints and no place else.

 

[Daniel Dudley]

The difference between mesh, rebar and fiber is that mesh and rebar will help to hold concrete together after it cracks, and can help to minimize cracking and increase load bearing of concrete in tension. Concrete is only strong in compression with steel.

I rarely fail to use rebar or wire in a pour, and I always try to establish a good base of compacted material under a slab. If the base material settles, concrete can go from compressing into the material to tension due to settling.

If your base material is good, your settling will be minimal, and your cracks will be stable.

You want to have solid, crack free concrete under your lift posts. If you installed your lift and experienced cracks under it in the future, it would be a simple matter to cut out a section of concrete and put in a proper reinforced pad to support your lift. In such a case I would make the slab thick enough to go under the original pad around the edges, to help support the transition. It has been mentioned, but it should be noted, that concrete only continues to cure if the initial cure time was long enough. Concrete should be kept wet a minimum of three days, and at least seven if you actually care about longevity. Concrete cures by the formation of limestone crystals around the aggregate. Water is needed to form these crystals. If concrete drys prematurely, it will not form enough of these crystals, and will be only as strong as the initial set.

If your concrete was in the shade, it is probably OK.

 

[Kevin54]

As most have already stated....CONCRETE CRACKS. If you find a way to keep it from cracking, you'll be a millionaire. Secondly..wire mesh does NOT keep concrete from cracking. Fiber Mesh does NOT keep concrete from cracking. Rebar does NOT keep concrete from cracking. CONTROL JOINTS control the direction of the cracks that you are going to get. Control Joints do NOT keep concrete from cracking.

What helps to prevent it though is site preparation. Clean undisturbed soil, gravel compacted, keeping the slab wet while the concrete cures. Short of that, your stuck with what you have. It's a little late for control joints. Before you epoxy the floor, I would let it go through one winter and let everything settle to where it's going to. Then next year, fill the cracks, grind, and epoxy.

 

[Falcon67]

I had several cracks in mine within a week of the pour. I kept the slab wet for 3 days, but it was last summer and the top would hit 130F+ in the sun if you didn't constantly hose it down. I had to go to work on Monday, so I let it fend for it's self after that. We were damed hot last year. Some of the cracks were about what you mentioned - 4~6', some places approaching 1/16" wide. It was about 6 months until I got the slab weathered in and painted the floor. I squished extra epoxy in the cracks and rolled over them a little thick. They pretty much disappeared. There is one in the work area that I can still see a little. The other "big" one has a car sitting over it, so I really don't know. What I DO know is that in the last year they have not grown, so it's all good IMHO.

So "D" is my answer.

FWIW - the slab next door has thicker footers and more bar in it than mine. It's set up for a 30x50 metal building. It has fine surface cracks all over it. So it's pretty normal.

 

[buening]

It sounds to me like plastic shrinkage cracks, which develop shortly after a pour. Typically fiber mesh minimizes the PSCs, but the lack of reinforcement worked against the fiber mesh. Steel reinforcement is used to control cracking (full width cracks) that occur from temperature changes. As the temperature fluctuates, your slab will expand and contract. Concrete is rigid and doesn't handle expansion well (tension in slab), thus reinforcement is added to help with the tension induced from expansion. A few good reads if you are curious about this topic:

http://nrmca.org/aboutconcrete/cips/05p.pdf

http://www.cement.org/tech/faq_fibers.asp

Did you see large temperature swings in the weeks/months after your pour? Did they cover your slab after pouring with either a burlap or plastic? This retains the moisture in the slab and prevents the upper surface of the slab to dry out before the remainder of the slab hardens.

Also, a pump truck adds air to the concrete mixture so I'd assume the quality control manager at the concrete plant knew it was being pumped and dropped the air in the concrete leaving the plant.

In regards to blanket statements of all slab crack, that is false. There are numerous warehouses that require very large joint spacing (which are more for constructability rather than cracking), and fees/penalties are often put into the construction documents for cracks that would occur after construction. Proper mix design, reinforcement size and placement, subbase preparation and thickness, and construction practices all contribute to a slab that will not crack. 99.9% of the time for a garage, the budget does not allow for this and thus the slab is either too thin, under-reinforced slab, thin or improper subbase preparation, and/or poor construction practices occur and eventually leads to an upset homeowner. Too often I see the concrete finisher take the water hose from the mixer truck and start hosing down the concrete to make his job easier, but fails to realize the quality of concrete suffers due to too much water.

 

[buening]

Oh and regards to when it'll quit cracking, you will likely see a drop in new cracks after the first year. The large temperature swings (depending on your location) of late spring/early summer and late fall/early winter will be the true test since you don't have any reinforcement.

 

[dirttracker18]

ALL concrete will crack there is simply no way to avoid it. The trick is to control where the cracks will occur. Hence why they are called 'control joints'.

In my recently poured 40'x40' slab, I used keyway-type control joints in a grid and it's cracking nicely right along those joints and no place else.

Sure, don't put them in.
But don't complain when the concrete cracks and it will.
Control joints will "control" where it cracks.

You are welcome to come up north here where we routinely get 3 plus feet of frost and see my floor.

I have a 6 to 8 inch hairline crack where the concrete extends over the foam board at the service door. No other cracks anywhere and no control cuts.

1 It is all in the prep. The foundation must be compacted correclty as it is laid down. No more than 6 inches of without compaction.

2 Keep the concrete covered and moist for at least 72 hours after, longer if you can.

3 Make sure your concrete guy knows his stuff. There is a science and art to concrete in pour, mixture and finish.

 

[jhelrey]

The fiber **** they put into concrete does hold well. Try breaking up a sidewalk with that **** in it. Figure double if not triple your time sledging away.

 

[djkeev]

The heck with the cracks!

For me the bigger problem would be the fiberglass fibers when you lie on the floor!

I've seen floors not floated properly that looked like hairy concrete!

Itchy itchy itchy!

Dave

 

[pop pop]

There is more myth about concrete installation than science, thus more bad pours than good ones. But then, you don't need a PhD to float concrete either.

 

[brihvac]

I have personally never seen a garage floor with control joints. I just had my 16X26 floor done last week. Undisturbed ground, 4" of stone, wired, 6" of concrete and no control joints. My fathers garage was poured in 1994 its 30X30 with no control joints and there are no cracks anywhere.

 

[ConCretin]

It never ceases to amaze me how much misinformation is out there when it comes to concrete slabs. With all due respect to some who have posted, there is no reason at all that you can't construct a good sized residential garage slab without cracks.

Leaving aside the whole issue of reinforcing, which is largely irrelevant, the biggest single issues are mix water and curing. If you want to avoid random shrinkage cracks, place your concrete at a painfully low slump and maintain it in a continuously moist condition for at least 7 days.

To the OP's question, you are probably the victim of poor craftsmanship. Your floor guy likely ran up the slump to get it through the pump (and make his job easier), which resulted in shrinkage cracks.

The damage is done. Cracks that formed early may open up and become more noticeable but assuming there aren't additional issues, the problem is primarily aesthetic and will not continue to get worse.

 

[ConCretin]

Also, a pump truck adds air to the concrete mixture so I'd assume the quality control manager at the concrete plant knew it was being pumped and dropped the air in the concrete leaving the plant.

buening, your post was very well written and accurate. If i were to quibble it would be your statement on a pump adding air.

Maybe it's just semantics but in my experience, a pump knocks down the air in concrete. In addition, you should never use air entrained concrete in hard troweled slabs. It causes de-lamination.

 

[danieldd]

The difference between mesh, rebar and fiber is that mesh and rebar will help to hold concrete together after it cracks, and can help to minimize cracking and increase load bearing of concrete in tension. Concrete is only strong in compression with steel.

I rarely fail to use rebar or wire in a pour, and I always try to establish a good base of compacted material under a slab. If the base material settles, concrete can go from compressing into the material to tension due to settling.

If your base material is good, your settling will be minimal, and your cracks will be stable.

You want to have solid, crack free concrete under your lift posts. If you installed your lift and experienced cracks under it in the future, it would be a simple matter to cut out a section of concrete and put in a proper reinforced pad to support your lift. In such a case I would make the slab thick enough to go under the original pad around the edges, to help support the transition. It has been mentioned, but it should be noted, that concrete only continues to cure if the initial cure time was long enough. Concrete should be kept wet a minimum of three days, and at least seven if you actually care about longevity. Concrete cures by the formation of limestone crystals around the aggregate. Water is needed to form these crystals. If concrete drys prematurely, it will not form enough of these crystals, and will be only as strong as the initial set.

If your concrete was in the shade, it is probably OK.

Good information and thanks. We had a good solid base of gravel and the footers were more than adequate. It was in the shade maybe part of the day, there is a pine tree that shadows in the morning and a pear tree that shades it in the afternoon..

Footers:

http://i67.photobucket.com/albums/h299/fromthefarside/New Garage/IMG_2821.jpg

Concrete:

http://i67.photobucket.com/albums/h299/fromthefarside/New Garage/IMG_2822.jpg

BTW, it rained the afternoon after they poured and then I think it warmed up a bit afterward. We had a very mild winter and spring came early..

 

[danieldd]

Did you see large temperature swings in the weeks/months after your pour? Did they cover your slab after pouring with either a burlap or plastic? This retains the moisture in the slab and prevents the upper surface of the slab to dry out before the remainder of the slab hardens.

.

Nope. They didn't cover it because when they poured, the temp was cool, then it rained and it warmed up a bit, maybe in the 70's if I were to guess..

 

[danieldd]

The heck with the cracks!

For me the bigger problem would be the fiberglass fibers when you lie on the floor!

I've seen floors not floated properly that looked like hairy concrete!

Itchy itchy itchy!

Dave

I've already been on my back on the floor trying to get my exhaust put back on my 928. No problems from itching..

 

[danieldd]

It never ceases to amaze me how much misinformation is out there when it comes to concrete slabs. With all due respect to some who have posted, there is no reason at all that you can't construct a good sized residential garage slab without cracks.

Leaving aside the whole issue of reinforcing, which is largely irrelevant, the biggest single issues are mix water and curing. If you want to avoid random shrinkage cracks, place your concrete at a painfully low slump and maintain it in a continuously moist condition for at least 7 days.

To the OP's question, you are probably the victim of poor craftsmanship. Your floor guy likely ran up the slump to get it through the pump (and make his job easier), which resulted in shrinkage cracks.

The damage is done. Cracks that formed early may open up and become more noticeable but assuming there aren't additional issues, the problem is primarily aesthetic and will not continue to get worse.

It is a shame these days but it is simply difficult to find people willing to perform at a quality craftmanship level these days. I was bitching to the contractor about how they installed my hardiboard and I got the song and dance that his crew was the best I was going to get in this area. He said most people don't aspire to be in construction - that most of them are either on meth or have other substance abuse problems. I don't quite believe his rationale as I believe there are good quality people in the construction industry - just maybe not around here..

 

[Frank The Plumber]

I have fiber crete, wire mesh and suspended triangulated rebar in my 10" thick pour. None of that stops shrink cracks. None of them go all the way through. Epoxy it and let it be.

 

[LegacyIndustrial]

If these are hairline cracks on the surface it is likely crazing. This can be controlled to a degree by limiting (slowing it down) the dehydration of the slab.

Two ways to do this....

Plastic over the top for the first week or so, traps the moisture keeps the surface wet.

or...

Apply a densifier directly after finishing. Same principle, however this also gives protection down the road against abrasion and dusting.

If these are deep cracks a grid of control joints would have helped guide the cracks but again this is concrete and no guarantees.

 

[Al Bundy]

If you can't finish school, you can finish concrete.

 

[Justanoldguy]

To this day...no cracks.

You can bet the cracks are there alright.
But deep into the bottom of the cuts where they should be.

 

[Justanoldguy]

Concrete is only strong in compression with steel.

Not sure which comic book you read that in?