NRMCA research finds cause to revisit ACI 318 chloride limits

Sources: National Ready Mixed Concrete Association, Silver Spring, Md.; CP staff

In collaboration with Dr. Neal Berke of Kalamazoo, Mich.-based Tourney Consulting Group, NRMCA has completed research to a) establish a relationship between the calculated total (initial) chloride content and measured water-soluble chloride content in hardened concrete; and, b) potentially give ready mixed producers latitude in mixture adjustments on projects bound by ACI 318 Building Code Requirements for Structural Concrete.

Testing for “Evaluation of Chloride Limits for Reinforced Concrete Phase A” was performed at NRMCA and Tourney labs. In a report on findings, authors observe: “Current ACI 318 chloride limits are based on research in the 1980s with primarily portland cement mixtures. These are conservative limits to ensure that the available chlorides in concrete will not exceed the threshold concentration that can initiate corrosion. There is no clear agreement on the actual chloride threshold concentration that initiates corrosion and additional uncertainty for mixtures that contain SCMs [supplementary cementitious materials]. In general, the threshold concentration is assumed to be between 2 to 4 lbs. per cubic yard (0.05 to 0.1 percent by mass of concrete). It is useful to evaluate whether ACI 318 chloride limits are appropriate for the different Exposure Category C [classes] and concrete mixtures that contain SCMs.”

NRMCA and Tourney staff evaluated specimens with Type II and Type V portland cement, Class C and Class F fly ash, silica fume and slag cement. Mixtures with varying water-cementitious materials ratio were evaluated; initial chloride content was varied between the background amount up to 2 percent by weight of cementitious materials by adding calcium chloride. Initial chloride content was verified by measurement in accordance with ASTM C1152, Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete. Per ACI 318-14, water-soluble chlorides were measured at an age between 28 and 42 days in accordance with ASTM C1218, Standard Test Method for Water-Soluble Chloride in Mortar and Concrete.

In each case, the measured water-soluble chlorides were less than the initial calculated and measured acid-soluble chloride contents. The estimated initial chloride content—calculated by materials’ chloride content and the concrete mixture proportion—can be used to ensure that proposed mixtures will conform to ACI 318 limits. This permits concrete producers to make pre-submittal mixture adjustments rather than dealing with measured test results during the project. The NRMCA/Tourney findings support a proposed code change permitting calculated total chloride to document conformance with ACI 318 chloride limits.

“Evaluation of Chloride Limits” was funded by the RMC Research & Education Foundation and the ACI Concrete Foundation. The complete report is posted here.