I am a Gen. Contractor and hired a licensed grading and paving contractor to do a very large drive (14'x65' + 14'x45'). There are two cracks that are the width 14' of the drive and go right through to the soil. Pour is on native soil, compacted and he did use wire in pour. He says "cracks happen". Well they did happen two weeks after the pour. No one drove on them. Either I need a little education on concrete, or he does. So, whats the basic reason for these cracks?
The reason concrete cracks is usually "drying shrinkage".
Concrete is poured wet. As it dries and loses water, its volume decreases (it shrinks). Small amount of shrinkage results in "microcracks" that are not visible to the naked eye and do not create a problem. Larger amounts (which is normal) can result in cracks.
To avoid cracks we try to use concrete with less water (less shrinkage potential). This is done by reducing the water/cement ratio through the use of admixtures, or increasing the cement content (both increase the cost). Higher strength is also achieved by these changes, further reducing the cracking (the concrete is strong enough to resist the stresses).
Try to imagine the concrete is a very stiff rubber band. When it shrinks, the band is pulled and stretched. If it is not strong enough, it will break. If it is stronger, it will stretch without breaking. Keep stretching (drying) and it will eventually break.
Another contributing factor will be the curing of the concrete. Curing is a process of keeping the concrete moist for as long as possible during its early days. As you probably understand now, keeping it moist will reduce the drying part at the early age when concrete is weakest. Drying that occurs later, after it gained more strength, is less likely to cause cracks. A rule of thumb (another one) is that concrete should be kept moist and protected for 7 days or more.
Since cracking may not be avoidable, we must use joints in larger concrete elements. Joints are basically pre-created cracks in the concrete. We place those in straight lines at locations that are acceptable to us (sometimes as decorations).
A rule of thumb for joint locations on a driveway such as yours would be every 10 to 14 feet, depending on the thickness of the slab (the thicker it is - the larger allowed spacing).
When shrinkage is the cause, the cracks will develop between existing joints. If a crack develops near the joint it may indicate that the joint was not done properly and does not relieve the stresses in the concrete.
Wire mesh has very limited ability to reduce cracking, even when it is placed in the center of the slab. When located near the bottom of the slab (as we see often) it can be practically useless.
That's it in a nutshell...