How do I tell if cracks are bad?

I bought a brand new house about 18 months ago. Its foundation is a slab on grade, 10 inches thick, with two layers of rebar (or so I'm told). Before I moved in, two cracks appeared, one down the length of the foundation, and the other through the width. Each crack was about 1/8-inch wide. There are no control joints. The contractor repaired the cracks with epoxy ejection. There are also cracks in the garage slab (which is attached to the house). One runs down the length of the slab, is about 1/8-inch wide, and is spalling here and there. Another 1/8-inch crack runs the width, and is also spalling here and there. Both cracks appeared early on, but I don't exactly when. Since then, more cracks have appeared in the garage slab, mostly parallel along the width, and fairly narrow.

I placed some plaster on the garage cracks to monitor for movement. Several of the "little plaster piles" have disintegrated, but I can't be sure of this is due to the cracks moving, or other factors (e.g., my car, water from rain, etc.). The remaining plaster piles all have hairline cracks that correspond to the underlying cracks in the garage floor.

For what it's worth, the cracks appear to be occurring strickly in the horizontal direction; there seems to be no vertical movement of the cracks. Also, the garage floor shows lots of small "crazy cracks" that look like a tangled spider web.

My question to the experts is how do I tell if I've got a probelm, or if the cracks are normal and nothing to worry about?

Thanks!!!

 

[Ron]

littletim...the cracks are "normal" for poor construction practices. The cracks you described are shrinkage cracks and the crazing or spider cracks result from a high water-cement ratio at the surface (bleed water accumulated at top).

The linear cracks could have been prevented by proper control joints. Epoxy repair is appropriate once they have occurred, but the efficacy of those repairs depends on how well they were done, the amount of shrinkage left in the slab, and the type/integrity of the subgrade, since you have at least some "traffic" on the slab.

The crazing or spider cracks could have been prevented by not allowing excessive bleed water to accumulate and dilute the cement paste at the surface. There was likely some water added to the mix at the site at time of placement, which contributes to both problems.

Make the contractor fix them. They are not structural issues, nor will they cause any other problems, other than allowing ants and other insects (termites, for instance) to bring sand through them. Since you have no faulting of the cracks (vertical movement), then they can be repaired with relative ease.

 

Thank you very much for your response. You answered the question that I really wanted to ask but failed to do so, i.e., whether the cracks in my foundation represent a structural issue.

FYI, the contractor has repeatedly refused to fix the cracks.

Thanks again!

 

[rustbuster1]

The issue is whether the cracks pose a threat to the function of the slab even though everyone agrees cracks are less desirable from an aesthetic point of view. Form the sounds of things the cracked slab will sl till be sitting there providing its main function many years from now.
If the contractor had installed an expension joint in the center of the slab it would serve as a synthetic "crack" so there would still be more than one part to the slab.
Epoxy injection repair is an attempt to glue the pieces together and sometimes works. It also seals the crack so it is not a path for water to go into the concrete.
The poor management of water:cement ratio in the slab mix - discussed above as the likely cause of the network cracks - leaves it of poorer general quality but probably good enough for the expected service. You can play the epoxy card here too. A penetrating epoxy sealer - PROPERLY APPLIED - can make the top surface stronger and far less likely to deteriorate into flakes and powder.

I think the contractor should be asked to do this remediation work. However, unless you can show that specified concrete properties are not met, you have only persuasion as your "weapon" and no valid legal recourse.

 

Rustbuster: Thank you very much for your response!

 

[Inspector807]

To expand on this conversation, i've recently went to an inspection where an object could be inserted a minimun 2-1/4"s into the 1/8 to 3/16" crack of slab and there was some vertical displacement about 1/16" or more, how do you know when this would be a structural problem and what steps should be taken

 

[Ron]

For slabs on grade with no vehicular traffic, cracks are not structural issues for the most part. You have indicated wide cracks with minor faulting. This is indicative of poor construction control and possibly poor compaction of the subgrade.

 

[dirtsqueezer]

I am a special inspector that recently finished a project where exactly the same thing occured. The property was owned by the City of Seattle, and showed extensive cracking very much like what you described above after minimal use. In my case (as I was there for the inspection), the cracking was due to pouring the slab on very uneven ground, as well as the high w/c ratio. The contractor is in business, and these practices saved him time and money. In the end, though, he ended up paying way more in damages than he saved in time.

What the city required of the contractor was two things, the first being the injected epoxy mentioned above, and the second being inspection of the actual injection to certify the installation, all free of charge. The company maybe not be experienced enough to know good construction practice and special inspection will ensure the quality of the work.

As far as persuing damages, your contract should have required the contractor to guarantee their work (your house) for some amount of time, usually a year. I would thoroughly encourage you to look through your records and find this warranty. The contractor is in business, and is not going to just give his money away, unless he has good reason to do it.

Good luck!