In the late 1970s ACI Committee 201, Durability of Concrete, was finishing work on its "Guide to Durable Concrete" to update a 1962 report on the subject. It came to our attention that during the interim the corrosion of reinforcing steel had become a major problem particularly in bridge decks but also in marine construction, parking decks and certain other concrete construction. The corrosion is detrimental to the steel and causes an expansion that severely spalls the concrete. In most cases a principal cause of the corrosion is chloride in the concrete, which comes from deicing agents as well as the concrete ingredients. Poor quality of the concrete and inadequate cover over the steel are also involved sometimes.

Because of the effect of chloride on steel corrosion, some might recommend that all chloride in concrete be outlawed. This would be a difficult and costly approach, however, because chlorides are present in varying amounts in all of the ingredients of concrete. Furthermore, calcium chloride is our cheapest and probably most reliable set accelerator and therefore has been used considerably in concrete and chemical admixtures. Committee 201 concluded that the proper approach was to set limits on the chloride content of the entire mix for various categories of construction.

Maximum water-soluble chloride ion content, percent by weight of cement, recommended for prestressed concrete: 0.06; for reinforced concrete which will be exposed to chlorides in service such as bridge decks, parking garages, marine construction, and certain industrial plants: 0.15; for reinforced concrete which will not be exposed to chloride but may be wet in service: 0.30; and for reinforced concrete which will be completely dry, or suitably protected, in service: No limit for corrosion. It is expected that these limits will be further adjusted as the corrosion problem becomes better understood and more data become available.