The primary cause of reinforced-concrete bridge deterioration is chloride-induced corrosion of the black steel reinforcement, resulting in expansion forces in the concrete that produce cracking and spalling of the concrete. The chloride comes from either marine exposure or the use of deicing salts for snow and ice removal. Because the use of deicing salts is likely to continue, if not increase, little can be done to prevent bridge structures from being exposed to corrosive chloride salts. Therefore, bridge designs and concrete mixes must be resistant to chloride-induced corrosion. This can be accomplished by: (1) preventing chlorides from getting to the steel surface (physical barriers at the concrete surface, coating the rebar, or low chloride-permeable concrete), (2) making the concrete less corrosive at specific chloride levels (inhibitors or admixtures), or (3) making the rebar resistant to corrosion (corrosion-resistant alloys, composites, or clad materials).
Over the past 20 years, there has been a trend in new construction toward utilizing higher quality concrete and more corrosion-resistant rebars. Longer bridge service life is currently achieved by using epoxy-coated rebars in the majority of new bridge construction, with the limited use of stainless steel-clad or solid rebars in more severe environments. The expected service life of a newly constructed bridge is typically 75 years and up to 120 years for stainless steel rebar construction. Admixtures to the concrete for the purpose of increased corrosion resistance have included corrosion-inhibiting admixtures and mineral admixtures such as silica fume. High-range water reducers permit the use of low water-cement ratio concretes that have lower permeability to corrosive agents and, thus, result in longer times to corrosion initiation of the rebar. Many of these methods are used in combination with each other to obtain a longer service life.
Many rehabilitation methodologies designed to extend the service life of bridges that have deteriorated due to corrosion of the reinforcing steel have been developed and put into practice within the past 25 years. These include cathodic protection, electrochemical chloride removal, overlays, and sealers. Although each of these methods have been shown to be successful, continuing developments are necessary to improve effectiveness and increase the life extension provided by these methods. (reference)