Concrete cracks either because of settling or shifting in the earth below from the time when the concrete mix was originally laid
Read more: How to Mend Cracks in Concrete | eHow.com
Is there a way to tell if the cracks are caused by geology or if the driveway was not properly packed before the concrete?
The tennis courts at our recreation department also have some pretty significant cracks.
---------- entropy sucks
Average concrete that has no joints is going to crack roughly every 15 feet due to normal shrinkage. How wide are the tennis court slabs between construction joints?
Metspitzer wrote in news: snipped-for-privacy@4ax.com:
It's probably both. Plus poor drainage.
Most of the time driveway cracks are caused by poor bed-preparation, often combined with poor drainage (who has ditches on either side of his driveway?).
Residential driveways basically consist of whatever ground the home- building equipment drove over while the house was built, plus a couple of inches of gravel spread around afterwards. And the lawn will be level with the pavement.
Nobody builds a driveway the way a road is built -- unless he works in road-construction, and takes two months to prep and build his driveway (I've seen this done, many times!).
Poor drainage and/or poor bed-prep. What's the drainage like? Drainage is probably more important than bed-prep.
Mine increases as the week goes on. Always.
Lets not forget that in many cases the concrete that is supplied is crap, far far below the quality that would be used in any commercial project.
If I were building a driveway, I would pour it 8 inches thick instead of
6 or 4 and put two layers of rebar in; one an inch below the top of the slab and the other an inch above the bottom of the slab.The reason why drywall is as strong and rigid as it is (considering what it's made of) is because paper is very strong in tension. Try folding up a sheet of paper so that it's 11 1/2 inches long an 1 inch wide and pulling it apart by straight tension (rather than by tearing it) and you'll see how strong paper actually is.
In order for drywall to bend, the paper on one side of the gypsum or the other has to stretch to conform to the new shape, and it's the strenth of the paper on each side of the drywall and it's resistance to stretching that gives drywall it's strength and rigidity.
Reinforced concrete is designed exactly the same way. "Reinforced" concrete consists of ordinary concrete with rebar inside it. If you imagine a slab of concrete with two layers of rebar inside the concrete; one near the top of the concrete and one near the bottom, then for that slab to bend, the rebar on either side would have to stretch to conform to the new shape.
But, since steel is very strong in tension, it's the steel's resistance to stretching that would prevents the slab from bending, and therefore cracking. That is, the slab could support very much MORE weight or force on it before it bent far enough for the concrete to crack. The steel rebar carries the load so that the reinforced concrete slab DOESN'T bend far enough for the concrete to crack, and that's why reinforced concrete is so much stronger than unreinforced concrete. It's exactly the same reason why drywall is considerably stronger than the gypsum core itself.
The problem is that when they pour a typical garage pad or drive way, THEY DON'T put the rebar where it's needed, which is at the top and bottom surfaces of the slab so it's in the proper location to resist bending of the slab. Instead, they just put a layer of rebar in the MIDDLE of the slab; equidistant from the top and bottom of the slab where it's unable to prevent the slab from bending. So, the slab bends easily to the point where the concrete cracks, and for $50 more for a second layer of rebar and $100 more in labour to install that second layer of rebar before pouring the concrete, you could have prevented that driveway slab from cracking.
Really, when they put a layer of rebar in the middle of a slab, the purpose is NOT to strengthen the concrete like it would if you had two layers. The purpose of that single layer of rebar in the middle of the slab is simply to hold the pieces of slab together AFTER it cracks. The idea is just to prevent any cracks from spreading. TWO layers of rebar would also prevent any cracks from spreading, but two layers properly located would go a far sight further to prevent the slab from cracking in the first place.
If you made drywall the same way they pour your average driveway, what you'd have is a layer of paper sandwiched between two 1/4 inch thick layers of gypsum. Something like that is gonna break if you breathe on it too hard. Putting the paper on the outsides makes the drywall strong and rigid because it takes a lot of force to stretch paper. Ditto for steel, and putting steel on the outsides of a concrete slab would make for a very much stronger and more rigid slab of concrete; one much less likely to bend to the point where it cracks.
The above is true. Plus expansion/movement joints at intervals. Easiest to put in when the job is finished with a diamond disk cutter. It only takes one delivery truck on your driveway to crack it.
If I were using 8" thick concrete for a driveway I would not put any rebar in it. You don't need it unless you are planning on landing commercial jet aircraft and the rebar will eventually corrode and crack the concrete. Just saw joints every 12 feet about 12 hours after it's poured.
If it were me, I'd still put some rebar in it just for any possible flexure or tension. Besides, the cost of rebar is minimal when added to concrete costs.
interstate hiwghways are built with multiple layers, gravel bed well drained, a 6 inch concrete surface, that will never be driven on, then more gravel etc, then the 12 inch or so finished driving surface, with rebar.
for driveways this is expensive overkill.
even interstates crack......
So you're spamming for ehow now, Metspitzer? Shame on you, I thought you would have known better.
Jon
Read more intelligently:
Yes (see above) ;)
Additional reading: Concrete basics and more ("Outer view of the Roman Pantheon, still the largest unreinforced solid concrete dome."):
Susan
You are correct of course but I'd still use some rebar if it were me. Actually I might use steel wire mesh instead if the rebar doesn't have enough cover. Overkill probably but I'd do it.
As a side note that agrees with you from many years ago....... my dad was a builder / carpenter and when I was young he poured a long driveway with no rebar except near the end of the driveway where it met the street and from the driveway connecting on an incline to the garage slab. This driveway was built upon Long Island sand and per Google Maps, I can still see the driveway slab exist today .
Thank you for the laugh.
There is no such thing as "ordinary" concrete. Concrete is specified or "spec'd".
Concrete is never poured, it is placed.
Relief/control joints are made to "crack". You are only partially correct about single sticks of rebar. You are absolutely wrong about your hypothetical reasoning.
Where in the heck did you get your ideas from?
University of Manitoba Dept. of Mechanical Engineering
They're not my ideas. They're common sense.
If you're trying to convince me, this is what they teach for residential concrete placing, you can go piss up a rope. Maybe you're an unemployed engineer, and I can understand why.
Now if you would say you were ACI certified, we may have something in common. But then, you would have never made the ludicrous statements you made, and I wouldn't have had the laugh that I did.
...and when they do, I've yet to see one repaired in such a manner that I'm not subjected to thump...thump...thump...for miles.
Most of the time they cut out a section and fill it with asphalt. It's never level and the asphalt usually comes out after awhile leaving bigger holes and sharper edges than the original crack.
I understand the concept of a concrete roadway lasting longer than asphalt, but once they need repair they seem a lot worse than a repaired asphalt roadway.
Just curious where in this link? I can't speak for Canada but here in the states, it would be under Civil / Structural Design.
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