Concrete is a rigid building material that is subject to freezing in winter and thawing in summer. As a result of these temperature changes, concrete will expand and contract to some degree.  This means it can crack.

The rebar in concrete keeps it together, however if cracks in concrete are exposed, water will slowly work its way in and begin to corrode the reinforcing steel. The rebar will expand as it corrodes, causing spalling to take place. This will expose the rebar even further, accelerating the corrosion process and potentially causing further damage to the structure.