"Concrete is a common building material used in fire-resistive and noncombustible construction. To enhance concrete's tensile strength, steel is embedded in the mixture while the concrete is being made. Concrete in building construction is subject to a condition called spalling. 'Concrete spalling is usually caused by corrosion of the steel reinforcement bars embedded in the concrete matrix, but can be caused by other ferrous elements either fully or partially embedded in the structure. Steel frame window systems, handrails, structural I-beams, metal pipes and conduits are among the most common of the damage causing building components. Corrosion of the reinforcement steel however, is by far the most common cause of spalling and splitting in older concrete structures.'"
Coffee Break Training - Fire Protection Series No. FP-2014-18
United States Fire Administration: http://www.usfa.fema.gov/