The water-soluble chloride ion content is limited by specifications in order to provide corrosion protection of the reinforcing steel. Limits for the chloride ion content are predicated upon the exposure conditions of the concrete element to water and/or external chlorides. ACI 318-19 Building Code Requirements for Structural Concrete [1] has the following requirements:

Exposure Class	Description	Maximum Water-Soluble Chloride Ion
		Non-prestressed	Prestressed
C0	Dry in service	1.00	0.06
C1	Exposed to moisture, no external chlorides	0.30	0.06
C2	Exposed to moisture and external chlorides	0.15	0.06

Accordingly, the concern is for chloride ions that are capable of going into solution in water (water-soluble) that can then participate in corrosion of the reinforcing steel. As a result, the specification limits are based on the water-soluble chloride ion content of the concrete. Chlorides are generally present in all concrete making materials to some degree. Chloride testing of individual ingredients, particularly cementitious materials, is most often based on determination of the chloride ion content after the sample is digested in acid; therefore referred to as the “acid-soluble” chloride. The acid-soluble chloride will represent essentially the total chloride ion content of the material, much of which is not water-soluble. Of the total (acid-soluble) chloride content only about 50% will be water-soluble [2]. An alternate to the direct testing for the water-soluble chloride ion by ASTM C1218 [3] has been proposed by NRMCA [4,5]. In this approach a preliminary assessment of the chloride ion content of the concrete is made by calculating the total chloride ion content based upon the weighted average of total (acid-soluble) chloride ion of the individual mixture ingredients. Since the water-soluble chloride ion content will always be lower than the acid-soluble chloride content, this will provide an overly-conservative estimate of the chloride ion content of the concrete. Provided that the calculated chloride ion content is below the threshold requirements for water-soluble chloride ion then there is no need for further testing to determine the water-soluble chloride ion content of the specific concrete mixture. However, should the calculated total (acid-soluble) chloride ion content be above the required threshold this does not constitute a failure but would then require testing to determine the water-soluble chloride ion content in accordance with ASTM C1218. ACI 318 now acknowledges this approach in the 2019 edition of Building Code Requirements for Structural Concrete. ACI 318-19 section 26.4.2.2 provides two alternatives for determining the chloride ion content of concrete: (1) calculate the chloride ion content based upon the total chloride ion content of the concrete materials and mix proportions, or (2) test for water-soluble chloride ion content in accordance with ASTM C1218. If the calculated chloride ion content determined by option (1) meets the prescribed maximum limites then no further testing is required. However if the calculated chloride ion content exceeds the prescribed limits then testing for the water-soluble chloride ion content per option (2) would be required at a test age of 28 to 42 days. ACI 318-19 also further clarified the method of calculating and reporting the chloride ion content of concrete. In previous versions of ACI 318 [6] as well as in ASTM C1218, the water-soluble chloride ion content is to be reported as a percentage of the mass of “cement”. The use of the term “cement” has long been a source of confusion; is this intended to mean “Portland cement” or “cementitious material”. Now this has been clarified in ACI 318-19. ACI 318-19 Table 19.3.2.1 specifically states that the limits for maximum water-soluble chloride ion are expressed as a “percent by mass of cementitious material”. Although seemingly minor, these changes for determining the chloride ion content of concrete can greatly simplify the acceptance process of concrete mixes minimizing the time and cost needed for testing while still safeguarding the integrity of concrete construction. Courtesy of Robert E. Neal, FACI Technical Services Engineer with Lehigh Cement Co.

References

• [1] ACI 318-19 Building Code Requirements for Structural Concrete, American Concrete Institute, Farmington Hills, MI, 2014.
• [2] Kosmatka, Steven H. and Wilson, Michelle L., Design and Control of Concrete Mixtures, EB001, 16th Edition, Portland Cement Association, Skokie, IL, 2016.
• [3] ASTM C1218-17 Standard Test Method for Water-Soluble Chloride in Mortar and Concrete, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 2017.
• [4] NRMCA, “Standard Practice for Rapid Determination of Water Soluble Chloride in Freshly Mixed Concrete, Aggregate, and Liquid Admixtures,” NRMCA Technical Information Letter No. 437, National Ready Mixed Concrete Association, March 1986.
• [5] NRMCA, “Evaluation of Chloride Limits for Reinforced Concrete Phase A” National Ready Mixed Concrete Association, July 2017.
• [6] ACI 318-14 Building Code Requirements for Structural Concrete, American Concrete Institute, Farmington Hills, MI, 2014.