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Re: crack slab on grade

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Cracked concrete is more common than un-cracked concrete.  Lay-folk don't understand cracks -- they are the ones who worry about them.  Structural Engineers should not worry about cracks, they can evaluate them, and they can design crack-control measures into their structures.  When evaluating cracks in concrete, don't study the cracks; study what moved to open the cracks.
A principal reason that concrete cracks is that it shrinks as it cures and as it dries.  Significant cracks in large slabs on grade can be controlled with control joints at reasonable intervals.  Alternatively,  reinforcing steel can be used to limit the size of cracks by forcing the cracks due to shrinkage to be distributed as small cracks along the lengths of the reinforcing rather than occurring as a few large cracks at widely spaced intervals.  [Figure that the sum of the widths cracks that will occur in a given length of slab will always be about the same for a particular concrete mix -- with reinforcing, you can control whether the total of the widths is made up of a few wide cracks or many fine cracks]. 
Cracking of slabs on grade can also occur because of poor curing practice: if the top surface is allowed to dry more quickly than the bottom surface, non-uniform shrinkage, with the top shrinking more than the bottom during the curing process.  This can result in a pattern of cracks at high edges around bowl-shaped areas.
Another cause of cracking of slabs on grade is concentrated loads, such as heavy wheel loads on a garage floor.  Slabs can be designed to resist cracking due to heavy concentrated loads.
Once you figure out what moved to open a crack and when, you can begin to figure out whether its cause needs to be mitigated, or whether it can be explained and ignored.
There are some good references that you can refer to to minimize cracks in slabs on grade: PCA's "Concrete Floors on Ground" is one.
In my work on old buildings, I sometimes need to determine whether a crack is old and stable, or currently moving.  After an earthquake people see cracks that they have never seen before.  Sometimes debris and spider webs [and even paint] in them make it clear that those never-before-noticed cracks are not new.  A set of right-angle cross marks made with a straight-edge across the crack marked with a sharp pencil and dated can be monitored periodically to determine whether or not the crack is currently active, and in which direction it is moving.
Nels Roselund
Structural Engineer
South San Gabriel, CA