Common question always being asked is " Why concrete cracks"
Here is some information and a few common reasons to explain why concrete cracks.
One of the reasons is, in residential concrete application to much water is added to the concrete on the job site. This water is added to make the concrete easier to install.Concrete does not require much water to get its strength, but when to much water is added, it reduces the strength of the concrete.
In addition, shrinkage is the main reason for cracking. When concrete dries it shrinks, because all the water is evaporate. The wetter the concrete,the more shrinkage will be. When concrete shrinks it pulls the slab apart and you see the cracks as a result of it.
Another reason for cracks, is fast drying. There is a chemical reaction, that makes concrete to go from mix form to solid form and this chemical reaction, continues to occur for days and weeks after the placement of concrete, so watering and keeping the concrete wet after it becomes hard will help make concrete stronger and prevent cracking.
Few more important factors, is having the proper concrete strength mix. Sometimes, some people like to cut corners and they order improper mix and it will crack.
Very important factor is having a proper placement of expansion joints. They help concrete crack where you want it to crack. The joints should be of the depth of the slab and no more than 2 or 3 times in feet of the thickness of the concrete in inches. Let's use a sidewalk as an example. The sidewalk is usually 4" thick so expansion joint placement should be around 8/12' apart.
There is other factors that are being neglected during installation i,e. ground should be hard and compacted,grade must be done to meet to soil conditions. Sometimes you need 6"of stone base and rebar,etc.
I hope you find this helpful and use some of the information for when you hire a Contractor to replace your sidewalks, patio or a driveway.