Detection, Protection, Repair and Rehabilitation of Corroded Prestressed Concrete Structures

Abstract

Corrosion is a natural and unavoidable process and its control is a global challenge. The civil engineers of 21st century are facing a major problem for corrosion of prestressed concrete as they maintain an aging infrastructure. It affects various public and private economic sectors including infrastructure, transportation, production, manufacturing and utilities. Corrosion of prestressing steel is much more severe than corrosion of mild steel reinforcement. This is due to higher strength of the prestressing steels, and the high level of stressing in the steel. Usually prestressing steels are stressed about 70%-80% of their ultimate strength which is much lower in mild steel reinforcement. The loss of cross-sectional area of the reinforcing steel due to corrosion is likely lead to tensile failure. The cross-sectional area of prestressing steel is less than mild steel reinforcement due to its higher strength. As a result, the loss of one prestressing strand or bar will have a tremendous effect on the capacity of the member than the loss of an equivalent size mild steel bar. The corrosion of prestressing steel in concrete is an electrochemical reaction that is influenced by various factors including chloride-ion content, pH level, concrete permeability, and availability of moisture to conduct ions within the concrete. Normally steels in concrete are protected from corrosion by a passive film of iron oxides resulting from the alkaline environment of the concrete. For the corrosion process to be initiated, the passive oxide film on the prestressing steel must be destroyed. Passivation of the steel may be destroyed by the carbonation or by the presence of the chloride ions. In Canada, one of the reasons of this problem is due to the huge amount of deicing chemicals to combat the cold climate. Once corrosion occurs, the corrosion products occupy up to six times as much volume as steel, leading to cracking and disruption of the concrete. The ACI limit on chloride in prestressed concrete members is half of that for conventionally reinforced concrete. Prestressing steel is also more inclined to other forms of corrosion related deterioration that do not occur in mild steel reinforcement. These forms are stress corrosion cracking, hydrogen embrittlement, fretting fatigue and corrosion fatigue. These types of deterioration are very difficult to detect, and can lead to brittle failure with little or no sign of warning. This report presents the mechanisms, causes and effects of corrosion in North American design and construction and the proper detection and protection systems.