Rapid chloride permeability testing: a test that can be used for a wide range of applications and quality control purposes if the inherent limitations are understood

Concrete Construction, Dec, 2002 by Prakash Joshi, Cesar Chan

Corrosion of reinforcing steel due to chloride ingress is one of the most common environmental attacks that lead to the deterioration of concrete structures. Corrosion-related damage to bridge deck overlays, parking garages, marine structures, and manufacturing plants results in millions of dollars spent annually on repairs. This durability problem has received widespread attention in recent years because of its frequent occurrence and the associated high cost of repairs.

Chlorides penetrate crack-free concrete by a variety of mechanisms: capillary absorption, hydrostatic pressure, diffusion, and evaporative transport. Of these, diffusion is predominant. Diffusion occurs when the concentration of chloride on the outside of the concrete member is greater than on the inside. This results in chloride ions moving through the concrete to the level of the rebar. When this occurs in combination with wetting and drying cycles and in the presence of oxygen, conditions are right for reinforcement corrosion.

The rate of chloride ion ingress into concrete is primarily dependent on the internal pore structure. The pore structure in turn depends on other factors such as the mix design, degree of hydration, curing conditions, use of supplementary cementitious materials, and construction practices. Therefore, wherever there is a potential risk of chloride-induced corrosion, the concrete should be evaluated for chloride permeability.

Testing for chloride Permeability

For specification and quality-control purposes in projects, we prefer a test that is simple to conduct and that can be performed in a short time. The rapid chloride permeability test meets these goals. First developed by Whiting in 1981 (Ref. 1), RCPT has had results that correlate well with results from the classical 90-day salt ponding test.

Standardized testing procedures are in AASHTO T 277 or ASTM C 1202. The RCPT is performed by monitoring the amount of electrical current that passes through a sample 50 mm thick by 100 mm in diameter in 6 hours (see schematic). This sample is typically cut as a slice of a core or cylinder. A voltage of 60V DC is maintained across the ends of the sample throughout the test. One lead is immersed in a 3.0% salt (NaCl) solution and the other in a 0.3 M sodium hydroxide (NaOH) solution (Ref. 2).

Based on the charge that passes through the sample, a qualitative rating is made of the concrete's permeability, as shown in Table 1. Versatile and easy to conduct, the RCPT has been adopted as a standard and is now widely used (Ref. 3). The test, however, has a number of drawbacks:

* The current that passes through the sample during the test indicates the movement of all ions in the pore solution (that is, the sample's electrical conductivity), not just chloride ions. Therefore, supplementary cementitious materials (such as fly ash, silica fume, or ground granulated blast-furnace slag) or chemical admixtures (such as water reducers, superplasticizers, or corrosion inhibitors0 can create misleading results largely due to the chemical composition of the pore solution, rather than from the actual permeability. As a result, some researchers do not recommmend the RCPT to evaluate the chloride permeability of concrete containing these materials (Ref. 4).

* The conditions under which the measurements are taken may cause physical and chemical changes in the specimen, resulting in unrealistic values (Ref. 5). For example, the high voltage applied during the test increases the temperature of the sample, which can accelerate hydration, particularly in younger concretes.

* The test has low inherent repeatability and reproducibility characteristics. The precision statement in ASTM C1202-97 indicates that a single operator will have a coefficient of variation of 12.3%; thus the results from two properly conducted tests on the same material by the same operator could vary by as much as 42% (Ref. 2). The multilaboratory coefficient of variation has been found to be 18.0%; thus two properly conducted tests on the same material by different laboratories could vary by as much as 51%. For this reason, three tests are usually conducted and the test results averaged, which brings the multilaboratory average down to 29%.

Canadian Industry Practice

Despite these drawbacks, this test method has been widely used for specification and quality control purposes. In Canada, the RCPT has been specified on various projects to qualify concrete mixes in bridge deck overlays and parking structures. The RCPT has even been incorporated as a part of the standard in CSA S413-94 Parking Structure Design-C (Ref. 6). Clause 7.3.1.2 defines low-permeability concrete as having "water/cementing materials ratio not exceeding 0.40, and an average coulomb rating not exceeding 1500 based on a test of three specimens tested in accordance with ASTM C1202." The RCPT has also been used to compare the effectiveness and performance of various systems, such as sealers, membranes, and corrosion inhibitors, intended to reduce the ingress of chloride ions or reduce corrosion in concrete structures.