Concrete is a material that dates back to the ancient world, famously used by the Romans in their constructions.
The incredibly strong Roman recipe is now lost to history, though refinements over the past few centuries have resurrected the material as central to construction. Still, our modern recipe has its flaws. Small cracks that begin to form as a concrete surface ages soon begin to cause problems; in places that get below freezing, water can seep into these cracks and freeze, expanding and worsening the cracks. Direct sunlight and high temperatures can also contribute to crack formation.
These processes can compromise the strength of the concrete, since cracks allow water to collect around the steel reinforcement bars (more commonly referred to as rebar), causing them to rust. While concrete is exemplary in its compressive strength (the amount of pressure it can withstand), rebar is added to increase its tensile strength (the amount of stretching it can withstand). Therefore, any compromises could cause dire problems for the overall structure.