I have a question on cracks in slabs. I have a home that has not been tiled yet and it has a couple (2 ) cracks across the slab from on side of the home to the other. (see picture) I can put a quarter in the crack in most places. Is this a problem? How big does a crack have to be to be of concern? If there is a crack what is the repair procedure, if any?
thanks
The crack looks like a typical stress, shrinkage crack which does not exhibit horizontal movement (normal) found on all slabs without the proper control joints.
Here is a link explaining how to cover the crack with a membrane before installing the tile. I have done it numerous times with tar paper or thin metal flashing so the slab can move without affecting the tile.
http://www.rd.com/content/openContent.do?contentId=19399
Confirming Dales’ advice.
10/4, Ditto, OK.
Second the motion.
That OK?
Marcel:) 
These are very typical and even happen in slabs with control joints. A lot depends on how fast the concrete dries. Slower drying usually equates with fewer cracks.
For everyone’s information, it has been proven by the ACI that any interior slabs poured today, on the interior, should not have air entrainment and should be of at least 3000PSI content.
Now we know this is not happenning in the residential market, and therefore, cracks in the slab as such will occurr most often then not.
I won’t even go into expansion control, and contraction, due the unaffectiveness of the Building Community Building Contractors knowledge out there, that are constructing these dwellings.
The most uncontrolled building standard of performance of building out there in the industry right now is the Residential Market. That must be why our industry of inspections are climbing.
The consumer is not getting what they paid for and that includes my Daughters spec house that I inspected last year while it was being built. It is unfortunate that people are in such of a hurry to get something for the first time, that they will even forget what their Father is telling them what is wrong.
Now they are paying for it, and why does everyone have to learn the hard way???
Why can not people learn to heed the warnings brought forth by the Elders that have already gone through that scenario.??
Marcel
Ignorance is bliss? Just look how many new homes are not fully inspected if at all by municipal officials. It is a repetitive theme.
A quarter is between 1/16" and 3/32", so the cracking in not that wide … but a little wider than I would have expected for initial shrinkage cracking. Perhaps the slab was poured in very hot weather with way too much water, or calcium chloride added on-site (a real no-no) to help with set time?
The best repair for excessive initial shrinkage cracking is epoxy filling (similar to epoxy injection, but gravity fed). But if the cracking if from some ongoing movement the slab may crack again … in which case you may select a less ideal/permanent repair. Sika makes some excellent crack repair products, and they have very good tech/field support for contractors.
P.S. I prefer to use pennies to get an indication of crack widths, as a penny is almost exactly 1/16" thick … :idea:
http://www.usmint.gov/about_the_mint/index.cfm?action=coin_specifications
JMO & 2-nickels … 
I recommended sealing a slab crack similar to the one shown in the picture in a garage, as it was a post tension slab. Good call?
When ya get into recommending specifics on repairs that goes beyond a home inspection, and may or may not be the right/best course of action. In some cases it’s better/necessary to structurally repair a crack, and in other cases it’s better to seal/overlay the cracks. A lot depends on the on-site conditions and circumstances. Let someone who has experience with the various methods and materials look at it and make the call on the appropriate repair.
I think the best thing to do with minor cracking is recommend further evaluation & repair by a qualified specialty contractor, and monitor for future movement. For significant cracking I think recommending an SE evaluate the situation is best.
JMO & 2-nickels as an engineer also …
Ok I’ll try the question this way, will moisture entering small shrinkage cracks have adverse effects on post tension slabs?
At the time Robert I did not think that the all encompasing “sealing” to be directing the repair. I am giving that some more thought.
Funny Brian;
Not knowing much about Post Tensioning Slabs, I would have to say that it might be good there is a crack, it would allow the moisture to get out, right?
Just my theory. ha. ha.
Just thought I would note that some forms of cracks usually occur due to improper design and construction practices, such as:
Omission of isolation and contraction joints and improper jointing practices.
Improper subgrade preparations.
The use of high slump concrete or excessive addition of water on the job.
Improper finishing.
Inadequate or no curing.
Concrete, like other construction materials, contracts and expands with changes in moisture and temperature, and deflects depending on load and support conditions. Cracks can occur when provisions to accommodate these movements are not made in design and construction.
Most random cracks that appear at an early age, although unsightly,m rarely affect the structural integrity or the service life of concrete.
Marcel
Well with post tensioned garage slabs I think it’s a good idea to repair anything other than fine hairline cracking in areas with significant snow, as the melting road salt laden snow on vehicles can aggressively attack post tension cables.
In areas without much snow it’s not really necessary to repair minor cracking as there shouldn’t be much moisture that can get in the cracks. But a significant amount of moisture getting into the slab (and to the cables) can accelerate deterioration, even without the chlorides from road salts.
JMO & 2-nickels …
I also recommend sealing cracks in any slab here, if their accessible.
Termites make a freeway out of them…:shock:
I agree with cracks in the garage to be sealed properly to not let anymore moisture in the cracks. I would also reccomend that as well.
Cracks in concrete slabs also sometimes let air in and with the moistness under the slab, can and will over time cause slab curling. This is an other reason to seal them properly.
Marcel