It was during the 1960s that the serious problem of corrosion of embedded steel in concrete became associated with the action of chlorides and in particular with the use of the accelerating admixture calcium chloride. Extensive research indicated that, with carefully controlled dosages of up to 1.5 percent anhydrous calcium chloride by weight of cement, over 95 percent of chloride is chemically bound in the hydrated cement paste and the free chloride at this level would not be detrimental to reinforced concrete in the long term. However, it is now accepted that if the concrete is at all porous, so permitting the passage of moisture and oxygen, then all the chloride could possibly migrate and cause accelerated corrosion of the embedded metal, especially in warm conditions.
As a consequence, in 1972 severe restrictions were imposed by setting a 1.5 percent limit on the anhydrous calcium chloride used in concrete containing embedded metal. In addition, an impermeable barrier was required and the use of extra-rapid-hardening cements was not permitted. Similarly, calcium chloride was not to be used with high alumina cement, sulfate-resisting cement or supersulfated cement.
There appear to be a few misgivings among manufacturers on the banning of calcium chloride and manufacturers are quick to point out that there are chloride-free accelerators available that do not have a corrosion problem. However, one manufacturer calls attention to the fact that the long-term performance of chloride-free accelerators is not yet proven and also that the corrosion problem will not be completely solved until concrete cover is sufficiently improved in quality to arrest the ingress of moisture and oxygen.