There are a few explanations as to why concrete begins to crack and crumble overtime.
Below is a breakdown on what causes concrete to corrode.
Pop-out: A pop-out is a small piece of concrete, which has broken away from the surface leaving a hole in the concrete. A pop-out can be caused by the expansion of a porous rock after it absorbs water and freezes. A pop-out can also be the result of a chemical reaction when alkalies in the cement react chemically with the silica found in some fine sands, also causing an expansion. Pop-outs do not affect the life expectancy of concrete.
Concrete Scaling: Scaling is the localized flaking or peeling of a concrete surface exposed to freezing and thawing.
Problem: Cracking Cause: Structural Prevention: Concrete should set for a minimum of seven days before vehicles are driven on the new surface. In cooler weather, more time should be allowed.
Problem: Cracking Cause: Moisture Prevention: Prevent rainwater from collecting under concrete by using drain-tile rain gutters and good landscaping.
Problem: Surface blemishes Cause: Water freezing and thawing numerous times on the surface Prevention: Apply a protective, penetrating concrete sealer and caulk on all saw cuts and joints. Keep surface clear of snow and ice.
Problem: Surface deterioration Cause: Use of de-icers and chemical fertilizers Prevention: Avoid de-icing chemicals including salts which can cause surface deterioration of your concrete.