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Cracks in New Home's garage slab


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I performed two home inspections on new homes this week, both had cracks in the garage slabs. These cracks are starting to separate and radiate out from the cent of the garage floor towards the outer foundation footings. There are 4-5 cracks in each home's garage slabs and some travel toward the slabs raised portions and run along the edge of the raised slab for several feet.

I have seen these in older homes and they always seem to eventually transfer into the home's slab. I was not able to upload any photos from the report. The cracks average 3-5 feet in length and split into 3-4 different directions. One of the homes had what appeared to be settlement cracks in just about every corner of the home's interior wall-ceiling sheet rock joints.

The builder's always tell the buyer that these are superficial curing cracks, but these appear to be deeper that curing cracks and already have separation and the edges are chipping on a micro level.

Any comments on these cracks would be appreciated -

Robert,

http://www.atexinspects.com

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All concrete shrinks and cracks; that's a fact of life. If memory serves, it's either 1/16" or 1/8" for every 10 linear feet of a placement.

The guy that placed that slab knew it and should have planned for it.

Around here, they're placing strips of flashing on edge in the concrete so that the slab cracks exactly where the flashings are and nowhere else. It works most of the time but not all of the time.

ONE TEAM - ONE FIGHT!!!

Mike

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sounds like a curing problem probably due to incorrect installation or mix was off.

Robert,

Was this on a monolithic slab or was the garage a separate pour from the house? If it is a one piece slab with no expansion or control joints, cracks could be normal curing cracks depending on the size.

Movement or cracks in the walls is not normal and is likely associated with slab movement, not normal.

What was the width of the cracks and was there any vertical displacement?

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When were the slabs poured? If the concrete was not protected, it may have frozen (and weakened) before the completion of the chemical reaction needed for it to properly cure. This can cause all kinds of issues with cracking. (Later Edit-Just re-read your post and saw that you are in Texas-I am not familiar with the winter weather there- is freezing an issue where this was built?)

I also see welded wire mesh poorly installed and pushed down to the bottom of the slab when they are walking around in the wet concrete. This basically renders the mesh useless for holding the slab together as designed.

Another common problem is when they prepare the ground under the slab, the edges are backfilled, and the center is more stable because it was not excavated for footings. The soil may be poorly compacted and then the edges settle more than the center. This causes cracks to occur.

What does it mean when you say the edges are cracking on a "Micro Level"?

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Steve,

Honestly, do you think 6X6 mesh offers any kind of protection against cracking or any structural support?

My opinion is it's just there to torture the demo guys.

And to torture the installers. Have you ever placed concrete over that stuff? As soon as the mix hits it, the wire leaps up in the air or lies flat on the base. Then, as you're trying to float the finish, the wire ends poke up out of nowhere.

It makes great tomato cages though.

- Jim Katen, Oregon

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Steve,

Honestly, do you think 6X6 mesh offers any kind of protection against cracking or any structural support?

My opinion is it's just there to torture the demo guys.

Yes- but only if installed properly. The mesh has to be installed so it does not sit at the bottom of the slab.

I actually prefer the fiberglass add mix because it requires less precision during the pour. It helps hold the slab together.

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When I was a super, we used fiber-mesh on several jobs. These were 5 story office building with composite slabs. I too, thought it was wonderful. No more tripping on the rusting mesh, no more hiring a crane to load the building. Great stuff; I think it was about $10.00 a yard back then.

During tenant fit-up, the usually questions were 'How come the floor is so hairy?'

Never used it on the exterior; wonder what a garage floor would look like with that stuff?

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I use fiber mesh every time I do flat work. The hairs wear off pretty quick, especially if it's outside and sunny. My barn floor has a "super mesh"; the fibers are quite long, 3/4 to an inch so. I disagree w/Mike that concrete will crack. A 4500 lb mix placed stiff, with appropriately sized large aggregates over a properly prepared and compacted base is unlikely to crack anywhere but at a control joint.

The problem described at the beginning of this thread is something I see all the time. The excavations for the footings around the perimeter are almost never properly compacted and the perimeter settles leaving a series of cracks that appear to originate in the center of the floor and radiate to the perimeter. Insult the condition with a parsimonious 3 inches of concrete spec'd by the architect who is trying to keep costs down and who also has a fantasy that if the base is prepared well, three inches will be enough. Three inches of depth pretty much precludes the use of #2 stone. Finally, install concrete that's literally wet enough to pour in place and we have the beginning of a belief that all concrete cracks.

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Iron mesh (6x6) doesn't do anything to stop cracks. Nothing. It's almost impossible to place concrete over the stuff anyway. It does what Katen said; it flies up, or goes to the bottom. It's almost impossible to get it in the middle of the placement unless the dobs are on 6" centers, which isn't ever going to happen on a residential gig.

I have the misfortune of having placed a few thousand yards of concrete in a series of grain storage facilities in central Michigan. The engineers assured me that the mesh does nothing to prevent cracking. All published information that I've ever read indicates mesh does nothing to prevent cracking. Our work couldn't crack. Couldn't, because pests could find the cracks and get in. And, it didn't. We prepped for a month before the mud even showed up.

Cracks happen due to inadequacies in soil preparation, subgrade soil conditions, support (footings, etc.), final placement and finishing work, slab thickness, and the mix. It's almost ALWAYS, the mix, and the problem is almost aways too much water.

There's a reason they use mechanical screeds for pulling mud on big jobs; if the mix is right, the stuff doesn't flow. At all. It's an almost dry mound that will kill crew members trying to pull it manually.

Placing concrete that doesn't crack isn't hard. It's just hard work.

Brutally hard.

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As home inspectors we have different opinions about this. We are not alone. Here is an engineering website that discusses this issue:

http://www.eng-tips.com/viewthread.cfm? ... 441&page=6

Yeah, but even the people who think WWM is a good thing all say that it has to be installed properly. Have you ever seen that done?

It's like using staples on asphalt roof shingles. They work fine when they're installed just right. Unfortunately, they're nearly impossible to install just right. The same goes for barrier EIFS -- Great system if installed perfectly. Never seen it installed perfectly.

- Jim Katen, Oregon

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Mesh doesn't stop cracking even when it's placed in the material correctly. Doesn't. Engineers that specify it to stop cracking should stop engineering and go place concrete on a job so they'll understand what it is they're trying to do.

Dozens of things can cause cracking. A few smart things will prevent cracking. The presence or absence of mesh has no effect on cracking. I'm amazed there are engineers still arguing about this.

Apologies to Walter, but this isn't about anything other than nerd brains excising some of the nerdiness so they can function in the world. I'd never talk about this with my customers. I'd tell 'em they have a cracked slab and if they don't like it, it's a do over.

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Landfill houses? Heck, we've got some landfill multifamily buildings here in the ChiTown.

There's thousands of condo buildings that are leaking and rotting like you wouldn't believe.

I've been opening up some walls and ceilings lately to reveal clear span load bearing truss joists that are black with rot, galvanized gusset plates rusted out, and bearing ends that are nearly gone.

It's going to be front page news in a few more years, i.e., building failures. Not boatloads of defects.

Failures. Maybe catastrophic.

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As home inspectors we have different opinions about this. We are not alone. Here is an engineering website that discusses this issue:

http://www.eng-tips.com/viewthread.cfm? ... 441&page=6

I get that it's good for HIs to have some knowledge of good and bad practice of pouring concrete slabs. And it's good to know a little engineering.

However, if the HI is working the usual residential job, it's highly unlikely that the homeowner is going to have any knowledge of slab-pouring at all. Even if the homeowner (or seller, buyer, etc.) had such knowledge, it would be useless to him once the slab is in place. If the HI handed the (naive) customer a stack of engineering texts, or sent him to the best engineering websites available, it wouldn't help him at all.

So, wouldn't the homeowner benefit most from the HI simply explaining that the slab wasn't poured correctly, and there's no practical solution for the problem other than a do-over?

I bring this up because we often seem to talk "inside baseball" amongst ourselves, figuring out how and why a concrete slab failed, the nature of Texas soils, etc., even though the customer likely won't ask for, understand, or apply that information.

Of course, I could be wrong, and customers could be just dying to learn concrete chemistry. But I think they might get more usable info from, say, Family Handyman. Because it's written for regular homeowners, not scientists.

WJ

Walter,

I agree with you. I was just responding to the initial question about some of the possible reasons for the garage slab cracking. Some of my clients get confused when I explain the difference between return and supply registers for their forced air system. I can only imagine the reaction if I started to discuss concrete slab reinforcing.

Steve

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I disagree w/Mike that concrete will crack. A 4500 lb mix placed stiff, with appropriately sized large aggregates over a properly prepared and compacted base is unlikely to crack anywhere but at a control joint.

Sorry, but a crack at a control joint IS STILL A CRACK.

Concrete gets hard and it cracks is a valid statement.

If concrete did not crack there would be no need for expansion joints and control joints.

Yes there are lots of reasons for it to crack but EVERY piece of concrete "I" have ever seen has cracks.

The reason I asked the question about what type of slab is because there are NO control joints or expansion joints in the vast majority of slabs in this region 1500-3000 square feet and not one control joint means there will be visible cracks in the concrete, the physics of the process demand it.

If you can design out the cracks, come to Texas and make a fortune.

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Landfill houses? Heck, we've got some landfill multifamily buildings here in the ChiTown.

I've been opening up some walls and ceilings lately to reveal clear span load bearing truss joists that are black with rot, galvanized gusset plates rusted out, and bearing ends that are nearly gone.

Failures. Maybe catastrophic.

At least we don't have to worry about the split-face block buildings.

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I disagree w/Mike that concrete will crack. A 4500 lb mix placed stiff, with appropriately sized large aggregates over a properly prepared and compacted base is unlikely to crack anywhere but at a control joint.

If you can design out the cracks, come to Texas and make a fortune.

Don't the Buddists or some such folk use fly ash concrete 'cause it doesn't crack? They can't have their prayer meetings or whatever on a cracked floor, or so I heard...bad juju.

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sounds like a curing problem probably due to incorrect iinstallation or mix was off.

curring cracks are longitudinal to the longest span and they are parrallelle to one and another. The kind of cracks that you are talking about are more of the differential stifness kind, ( bad compaction or big debris on top of the fill that created the radial craks). Have the house setteled a bit? with pictures i could tell you what is the probable cause of the cracks.

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Properly installed wire mesh will add strength to a floor slab.

But it is rare to find it properly installed on a residential job and the added strength can't overcome the stress created by the typical improperly prepared base.

In practice, the wire mesh serves to hold the pieces together after the slab cracks.

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