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Calcium chloride was used as an accelerating admixture in concrete up until the mid 1970s. In the hydrate form it contains about 25% by weight of water of crystallisation. Since this can vary the code of practice current at the time (CP 110: part 1: 1972, Structural use of concrete) specified the amount of anhydrous calcium chloride i.e. the material containing no water. The Code stated that the amount should never exceed 1.5% by mass of cement where the concrete contains embedded metal. This value included that chloride associated with the marine aggregate if used.
An amendment to the Code in May 1977 effectively outlawed the use of calcium chloride as an accelerating admixture because "experience shows that corrosion of prestressing tendons, reinforcement and embedded metal usually results from the combination of factors including excess addition of calcium chloride…departure from specified mix proportions, poor compaction, inadequate cover and poor detail design".
Subsequent codes of practice and specifications have reiterated this and restrict chloride ion contents. BS 8500-1:2015, Concrete, section A.4.2, Table A.8 limits the chloride ion content from all sources, by mass of cement, to 0.4% for normally reinforced concrete and to 0.1% for prestressed reinforced concrete. Calcium chloride was often used as a flake and was poorly mixed into the concrete. This produced differential chloride levels and encouraged the formation of corrosion macro-cells on the reinforcement leading to pitting corrosion. Liquid forms were less problematical in terms of dispersion, although the chloride levels are higher than modern Standards permit.
Note: The numerical conversion of anhydrous calcium chloride to chloride ions is 0.64
Further information on the use of calcium chloride as an accelerator, and on other aspects of the use of concrete over the years, is given in Concrete Society Technical Report 70, Historical approaches to the design of concrete buildings and structures.
Acknowledgement: The Concrete Society
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