The biggest complaint when it comes to concrete is a concrete crack. Every driveway, patio, sidewalk, etc., will crack sooner or later. In fact, when talking with potential customers, I always explain the two guarantees of concrete: It will get hard and it will crack.
But don't let that scare you away from taking advantage of concrete's many benefits.
The reason concrete cracks is because it has relatively low tensile strength. Tensile strength is the compacity to resist tension or being pulled apart. This is why you can't build a bridge with just concrete. It would collapse under it's own weight.
One of the causes of cracks is shrinkage. Concrete shrinks as cures or hardens. As the cement hardens it bonds with the aggregates to form a rocklike mass. Often times the bonds created are not yet strong enough to withstand stress of shrinkage resulting in cracking. This can be minimized by using a concrete mix with a low water:cement ratio.
Movement or shifting of the ground under concrete also causes cracking. This shifting creates stress that usually results in cracks. Concrete that is spanned out horizontally is very susceptible to the stress. Rebar and wire mesh reinforcement can help hold the concrete together but won't prevent cracks from occurring.
Notice in the image above the severe cracking and displacement of the slab due to tree roots. This is a prime example of how concrete, with it's weak tensile strength, cannot withstand the upward force of the root growth. Steel reinfocement would not have prevented this. The same goes with water freezing under a slab. Avoiding situations like this is the best way to protect against severe cracking down the road.
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