New garage floor cracking... 1

[maniskov]

Hi. First time post.

I've read some of the threads on problems with slabs
cracking. I'd like some advice on whether or not what
I'm experiencing is normal and, if not, how I can test
the product to determine the source of the problem.

I had a two-care garage built, 24' wide x 30' deep.
I live in Southern New Hampshire.

Slab was poured about a month ago. After a week or so,
I noticed two cracks, each starting at about the middle
of each garage door and shooting straight back, each
probably 5-6' in length. A few more days later, we
left for a family vacation for 6 days, and closed
everything up.

When we returned (July 5th), I didn't notice much
different, but then taking a closer look about 5 days
ago, I noticed the cracks had spread. The initial
cracks started branching right and left. New cracks
started appearing in various places in the garage. No
particular pattern. Some very thin, some now approaching
1/32".

It seems to be getting worse daily.

I contacted the guy who poured the floor, and he says
cracks are normal, but I think there are far too many
at this point. It's quite noticeable. I believe
something was not done properly.

I don't think there's a problem with the packing of the
base, since the builders who put up the garage worked in
there for a couple of weeks, while we also had a good
deal of rain which helped to settle the dirt.

Sub-flooring was what looked to be wire mesh (no re-bar).

I don't know anything about the type of concrete he
poured, what the temp was, whether it was "in the field"
or whether it was a poor mix of various compounds.

My guess is I'm in for a lot of trouble if I let it
stay the way it is. I have yet to even put a car in
there (although that's what they're for, right?).

I'd love any advice on how to proceed, how to determine
the source of the problem, and how to make a case
for myself. This seems unacceptable to me.

Thanks for your time.

-MikeA.

 

[boo1]

Mike I would classify your case as common. That sized slab (assume 3 1/2" to 4" thk) with no control or expansion joints would crack. But it should have been preventable (Assume good compaction and no frozen soils).

Shrinkage is a main cause of cracking. As concrete hardens and dries it shrinks. This is due to the evaporation of excess mixing water. The wetter or soupier the concrete mix, the greater the shrinkage will be. Concrete slabs can shrink as much as 1/2 inch per 100 feet. This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces. Subcontractor adds to much water.

Also, rapid drying of the slab will significantly increase the possibility of cracking. The chemical reaction, which causes concrete to go from the liquid or plastic state to a solid state, requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete. (your code requires 7 day continous moisture cure (sprinkler or covered). Hot, windy conditions increase this dange.

Control joints help concrete crack where you want it to. The joints should be ¼ of the depth of the slab and no more than 2-3 times (in feet) of the thickness of the concrete (in inches). So 4"concrete should have joints 8-12' apart ACI code recomends these ratios.

The low sear strenght and crack resistance of concrete indicate once a crack starts it will "run its course" to the opposite wall. Hopefully the concrete sub pulled the wire mech up in the slab and not just walked on it, this contols the relative movement. The cracks have done what they are going to do. But in the winter the gap will increase.

Was a soil compaction test conducted to insure the code require 95% proctor was met? Is there any concrete core sample to do strenght test on to insue the proper mix? Was there slump test taken while the slab was pored? Was there a continous moisture cure for seven days? prob no to all

So do u have a beef, yes but it is not structural related you will have difficulty is having the slab replaced (capping will recrack). Remember the sweetness of low price is forgoted long after poor quality is remembered!

 

[maniskov]

I don't know whether a soil compaction test was taken. I assume a core sample is something done ahead of time? I don't know what a slump test is, or whether one was done. For all of these, however, I will find out.

As for 7-day continuous moisture cure, it was NEVER mentioned to me, else, I certainly would have done it.
In fact, we kept the doors open because we thought it would help, rather than hurt.

One other thing I've noticed-- from the get-go, there were obvious circular swirl marks visible on the surface, which leads me to believe the stuff was too dry, rather than too wet. Does that make sense?

The building inspector hasn't been brought in to do a final sign-off on the job, either. We're waiting for (in fact, postponed) the driveway until we get this slab thing figured out.

 

[Ron]

Mikea...the swirl marks you see are power trowel marks. The fact that you see them indicates the concrete was too wet when they did the finishing.

The cracks you've described are shrinkage cracks. Boo1 is correct in that there should have been control joints placed in this slab. For a 24x30 slab, you should have had at least one joint each way down the center of each side, or preferably two joints each way dividing each dimension of the slab into thirds.

Compaction is not an issue with the cracking, since the cracks are not external load induced. The cracks result from, as Boo1 said, internal shrinkage stresses resulting from the hydration process. The continuing branch cracking concerns me and indicates a lower strength concrete and/or a slow strength gain concrete, such as a mix with fly ash or ground blast furnace slag cement (NewCem or similar).

Your contractor has breached acceptable practice (ACI recommendations)by not providing proper control jointing. I suspect you will find inconsistencies in the thickness of the slab as well, which will also contribute to more "randomization" of the cracking. If you accept what you have, you will not be satisfied with it for the long term. Not only have the contractor's procedures resulted in cracks, but you will likely see some durability problems start to occur. These will be in the form of "crazing" or very fine "map" cracking at the surface and probably some dusting or scaling. Further, when you put loads on the slab (truck, car, etc.) you will find that some of the cracks might start to "fault" or move down on one side relative to the other. This is because the cracks are growing in width and there is not sufficient load transfer capability across the cracks.

One crack in each direction of this slab would be tolerable. More than this would give potential to a claim against the contractor.

 

[maniskov]

Ron, Boo1-- thanks much for your information. It's been very helpful.

Now, I'm thinking of how I proceed... and I'd like your counsel on this if you don't mind.

If I want to pursue a claim, and to get my general contractor to convince the sub-contractor to re-do the job, what is the best way to build my case, in terms of the data I need as backup, whether recommendations from the ACI, material tests, on site inspection, etc?

And, if I can't convince based on getting the best data I can, then what would be the next step?

Again, thanks much for your feedback. I really appreciate it.

-MikeA.

 

[Ron]

Mike...
There are lots of data available on this subject. I suggest the following to start:

ACI (American Concrete Institute)
ACI 302.1R "Guide for Concrete Floor and Slab Construction"
ACI 360 "Design of Slabs on Grade"
ACI 332 "Guide to Residential Cast-in-Place Concrete"

PCA (Portland Cement Association)
"Concrete Floors on Ground", 2nd edition

ASTM (American Society for Testing and Materials (old name))
ASTM C94 "Ready Mixed Concrete"

I would also suggest you take several cores (at least 3) from the slab to check thickness. If you core through one of the cracks, you can tell a lot about when the cracks occurred, and/or the concrete strength. The cores can also be used to check the air content, water-cement ratio, and re-tempering (adding water at the site), through a process called concrete petrography. This is a bit expensive (about $800 per sample), but gives you a lot of info if you are pursuing litigation.

As for thickness....if you were contracted for a 4-inch thick slab (hopefully, even thicker design), then there is a tolerance on the thickness that the contractor should meet. The tolerance is -1/4 inch, + 3/8 inch for the thickness. Having overly thick concrete next to overly thin concrete creates "random" cracking. This means the subgrade must be well prepared and well compacted to achieve the tolerances. It must be flat, as well.

Good luck. Keep us posted on your progress. Arm yourself with the info noted. Most of these publications are available in technical libraries (hopefully you have a University nearby with an engineering college), or you can contact a nearby ACI local chapter. The New England Chapter is located in Boston. Check the ACI website (

http://www.aci-int.org)

for contact info. You can also order publications mentioned through their website, if you can't find them locally, though ACI's publication prices are a bit steep!

 

[boo1]

Mikea
The local building code for your area is BOCA or UBC which requires the 7 day continous moisture cure (sprinkler or covered), your local building dept will have the correct ref. Look under the Foundation chapter. The required strenght of your slab is 2500 psi which the bore test can confirm (see the same ref)

Ron listed the ACI codes better than I could have.

Caution must be said the litgation route is very expensive and slow. Research the references above. Meeting with the contractor armed with facts the slab was not placed inaccordance with code. Tell him that you are not satisfied with that part of the project. Negotiate to have the slab replace. Allow every one too win.

Cracking is the most common complaint I hear of concrete slabs. Most residential contractor/concrete subs answer is, "we have been doing it like this for twenty years" or "it would be too stiff of a mix to work if we don't add water". GOOD LUCK