Disintegrating and deteriorating concrete is a key issue for structures worldwide. A major contributing factor is the presence of chloride ions in the concrete, which causes the reinforcing steel inside to corrode. Recently, a research project1 on chloride limits for reinforced concrete was completed by the National Ready Mixed Concrete Association (NRMCA) (Silver Spring, Maryland, USA) in collaboration with NACE International member Neal Berke, FNACE, vice president of Tourney Consulting Group, LLC (Kalamazoo, Michigan, USA). The research, conducted at NRMCA and Tourney laboratories, established a relationship between the initial calculated total chloride content of the concrete’s constituents and the measured water-soluble chloride content in aged, hardened concrete.
According to the researchers, concrete’s highly alkaline environment protects the reinforcing steel from corrosion. When chlorides concentrate on the steel reinforcement, however, its protective passive layer breaks down and corrosion starts. Rust forms, which takes up more space than the original steel, and causes the concrete cover to crack and spall. While external chloride ions from the environment (e.g., seawater and deicing salts) migrate through concrete over time and eventually reach the reinforcing steel, internal chlorides that originate from materials used to produce the concrete are present when the concrete is cast.
ACI 318-142 prescribes limits on internal water-soluble chlorides for concrete, which are determined based on the weight of Portland cement in the concrete. The reason for the chloride limits, explains Karthik Obla, vice president, technical services with NRMCA, is to protect the reinforcing steel against corrosion initiation. According to NRMCA,3 corrosion initiates when the concentration of chlorides exceeds a threshold concentration at the reinforcing steel. Although these chloride concentration values can vary, they are typically in the range of 0.05 to 0.1% of the weight of the concrete—about 2 to 4 lb/yd3 (1.2 to 2.4 kg/m3), or ~0.4 to 0.8% of the weight of cement (based on the assumption of ~500 lb/yd3 [297 kg/m3] of cement).
Chloride limits set by ACI 318-14 target various atmospheric exposures. For exposure class C1—concrete that is exposed to moisture but not to external sources of chlorides—the maximum internal water-soluble chloride ion content (by weight of cement) in concrete is 0.3% for reinforced concrete and 0.06% for prestressed concrete. For exposure class C2—concrete that is exposed to moisture as well as an external source of chlorides—the maximum internal water-soluble chloride ion content is 0.15% for reinforced concrete and 0.06% for prestressed concrete. Typical sources of water-soluble chloride ions include water, aggregates, cementitious materials, and admixtures.
Obla notes that concrete producers conduct a water-soluble chloride test (per ASTM C12184) on concrete specimens that have cured between 28 and 42 days after casting to determine if concrete mixtures used for buildings comply with the ACI 318-14 chloride limit. If the test results exceed the chloride limit criterion, he says, the concrete producer must adjust one or more of the ingredients in the concrete mixture and then repeat the test. If concrete has been poured prior to the availability of the chloride test results, the viability of the concrete placed may be questionable.
Not all chlorides in concrete contribute to corrosion, says Berke. Some chlorides are chemically bound in the cement matrix and supplementary cementitious materials and cannot be extracted by water. This means they are not available to cause corrosion of the reinforcing steel. Some aggregates also contain chlorides that are trapped and probably unavailable to cause corrosion. Water-soluble chlorides, however, are not bound and can move through the concrete’s capillary pores. Between 50 and 75% of the total chloride content in concrete is estimated to be water soluble and a contributor to reinforcing steel corrosion.3
With an aim to reduce the time and cost added to a concrete job for chloride testing, the researchers implemented the first phase of a project to evaluate chloride limits for reinforced concrete. In this phase, the researchers worked to establish a relationship between the total amount of chlorides in concrete calculated from the chloride levels in the individual concrete constituents and the level of chlorides measured by the ASTM C1218 water-soluble chloride test.
Berke explains that total chloride content in a concrete mixture can be established when the mixture proportions are being developed for a concrete building project. The total chloride content is determined based on measured chloride content of the individual materials used and the proportion of these materials in the concrete mixture. NRMCA’s Technology in Practice (TIP) 133 discusses sources of internal chlorides and calculating chloride content in concrete from the mixture ingredients.
The researchers propose that if the initial calculated total chloride content is less than the chloride limit specified by ACI 318-14, then the water-soluble chloride level measurement for the hardened concrete will be lower than the initial calculated chloride total and comply with the building code—and water-soluble chloride testing may be avoided. If the initial calculated total chloride content exceeds the specified limit, the concrete producer can then adjust the proposed concrete mixture up front to ensure code requirements are met rather than deal with out-of-spec water-soluble chloride level test results during the project.
In the year-long study, the researchers tested more than 500 samples of concrete mixtures, says Obla. Concrete mixtures included numerous combinations of ASTM C1505 Type II and Type V Portland cement, Class C and Class F fly ash, silica fume, and slag cement, with varying water/cementitious material ratios and aggregate types. Chloride in various dosages was also added to the freshly mixed concrete. The initial total chloride content was calculated for the samples and then verified using the acid-soluble chloride content measurement method in accordance with ASTM C1152.6 The water-soluble chlorides were measured when the samples had aged between 28 and 42 days in accordance with ASTM C1218, as required by ACI 318-14.
Study results showed that for most of the mixtures, the ratio of the initial calculated total chloride content and the measured acid-soluble chloride was close to 1.0, which indicated the calculated total chloride content and the measured total chloride content were reasonably consistent. Also, the measured water-soluble chloride content was less than the initial calculated chloride content and the measured acid-soluble chloride content for each mixture, and <60% of the initial calculated chloride content for mixtures that had no chlorides added. This supports the researchers’ proposal that if the initial calculated total chloride content of concrete mixtures is less than the specified limits for water-soluble chloride content in hardened concrete, then the concrete mixture will comply with code requirements for chloride limits. Added time and project costs could be avoided if the requirement for a water-soluble chloride content test could be waived.
This phase of the research work was funded by the RMC Research & Education Foundation and the ACI Concrete Foundation. The complete report can be downloaded at www.nrmca.org/lab.
References
1 K. Obla, C. Lobo, R. Hong, N. Berke, “Evaluation of Chloride Limits for Reinforced Concrete Phase A,” RMC Research & Education Foundation, Project 14-01, July 2017.
2 ACI 318-14, “Building Code Requirements for Structural Concrete and Commentary” (Farmington Hills, MI: ACI, 2014).
3 NRMCA TIP 13, “Chloride Limits in Concrete” (Silver Spring, MD: NRMCA).
4 ASTM C1218/C1218M-17, “Standard Test Method for Water-Soluble Chloride in Mortar and Concrete” (West Conshohocken, PA: ASTM, 2017).
5 ASTM C150/C150M – 17, “Standard Specification for Portland Cement” (West Conshohocken, PA: ASTM International, 2017).
6 ASTM C1152/C1152M-04 (2012) e1, “Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete” (West Conshohocken, PA: ASTM International, 2012).