Concrete Slabs - Center line Cracking

[Inversquare]

I have been experiencing the same crack pattern in most of my garage slabs. A centre line crack from front to back of garage floor. This is fine, and to be expected. But in some cases, the crack has opened wider than I would like to see (~3mm). This results in return trips a year later to epoxy or caulk the line in for rebar protection.

Beam Analysis

Ive begun to analyze the slab treating like a beam. Is this a correct assumption of what is going on? If so, the extreme cases to me seem a rebar problem. Perhaps my finishers are leaving the rebar low, or there is insufficient area of steel in the slab to begin with.

Can anyone carry out the area of steel calculation necessary for this design moment? The slab here is 20' wide, 22' long, 4" thick. I appreciate any help, thanks for weighing in.

Chad

 

[Inversquare]

And yes, a centre line expansion joint is a solution I have worked with. But I would like to ease the stress on that centre crack.

 

[hokie66]

No, it is not "like a beam". In a slab on ground, the force is applied as direct tension to the slab as the concrete shrinks, and the subgrade restrains this shrinkage. There are two ways of dealing with this type of cracking: 1) Use a lot of reinforcement, say 0.5%AG, which doesn't prevent the cracking, but controls the width of cracks; or 2) Provide control joints, which are also cracks, but nice and straight. The second solution is usually used in things like garages.

 

[oldestguy]

Cracking of concrete for some reason scares people. How about cross ways half way into the garage? I usually call for joints there, because shrinkage is expected. Fill the cracks and they look even more obvious.

What do you spec for slump Sacks per yard? High slump and low cement content both contribute to increased shrinkage per unit length.

 

[Ron]

You have a 20'x22' slab, 4" thick. Rule of thumb says you should have joints spaced at between 8 and 12 feet. If your crack occurred at the middle of the 20' width....it's right on target!

As OG says, put joints in cross pattern and you won't have issue again.