As a Concrete Professional for the last 11 years I have been asked time after time, "Why does concrete crack". There is a multitude of reasons that concrete cracks.
Here is a quick list of reasons that concrete cracks:
1. Too much weight applied to the surface of the concrete via a truck, or a forklift
2. Soil conditions, as you may know clay soil is highly expansive, and can damage concrete, if not engineered properly
3. No crushed rock or A.B (aggregate base) underneath the concrete
4. Tree roots (very common) as trees grow roots grow and then they push up the concrete
5. improper installation of joints in the concrete, if the expansion joints are too far apart, then a crack will want "jump" out of the joint that it is supposed to be in
6. Poor finishing technique, if you do not finish the concrete fast enough or too fast (depending on weather conditions) you can get a crack
7. Dry winds, anytime you have low humidity and even moderate winds can lead to cracking into your concrete
8. The wrong aggregate, too many sharp edges on the aggregatte can lead to cracking
9. Expansion Joints not being deep enough into the slab. You want to be in control of the concrete and not the concrete in control of you. Putting expansion joints where they need to be will help you mitigate most issues of cracking
10. Most important! hire the best, when you hire a concrete contractor take three bids and choose the Medium to highest bid, and expect the best:
Too many times people choose the cheapest price and it only hurts them in the end. Concrete work is for 20 years plus, when you purchase concrete it is a commitment. Go cheep on a meal before you go cheep on concrete. It is better to spend your hard earn money on something that will last for a generation.