CHLORIDE PERMEABILITY

One of the important factors for long-term durability of a reinforced concrete structure is its resistance to penetration of chloride ingress, which leads to corrosion of reinforcing steel in concrete. Chloride penetration, however, is a slow process, which cannot be determined directly in a time frame that would be useful as a quality control measure. Hence, in order to evaluate chloride penetration, a test method that accelerates the process is needed to allow the determination of diffusion values in a reasonable time frame. 

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Mechanisms of penetration of chloride in concrete are by capillary absorption, hydrostatic pressure, and, diffusion, of which diffusion is the most common method where chloride ingress is driven by a concentration gradient through a continuous liquid phase. Chloride ingress by permeation is driven by a hydrostatic pressure gradient from an applied hydraulic head on one face of the concrete, which, however, is a rare situation in many concrete structures.  A more common transport mechanism is absorption driven by moisture gradients, which occurs during wetting and drying cycles where water containing chloride ions encounters a dry surface and drawn into the pore structure though capillary suction. Due to the shallow depth of drying (unless the concrete is very porous or has a shallow depth of cover over reinforcement), absorption, by itself cannot bring chlorides to the level of the reinforcing steel. Diffusion is the main mechanism that brings chloride to the level of rebar.  

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The rate of chloride penetration into concrete depends on the pore structure of the concrete, which is affected by factors including concrete materials, mix proportions, construction practices, curing, exposure conditions, and age. The penetrability of concrete is related to the pore structure and connectivity in cement paste, which, in turn, is influenced by the water-cement ratio of the concrete, use of supplementary cementing materials, and the degree of hydration of cement. The rate of chloride penetration is also affected by the chloride binding capacity of the concrete. 

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There are various standardized test methods available in the industry to determine chloride permeability in concrete, e.g., AASHTO T 259 Standard Method of Test for Resistance of Concrete to Chloride Ion Penetration (Salt Ponding Test), NordTest NT Build 443 Bulk Diffusion Test, AASHTO T 277 Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration (Rapid Chloride Permeability Test), and ASTM C 1202 Standard Test Method for Electrical Induction of Concrete’s ability to Resist Chloride Ion Penetration. Of all these standard and many other non-standard test methods, CMC follows the most popular method, which is the ASTM C 1202 method, which consists of monitoring the amount o electrical current passed through cylindrical concrete specimens for six hour durability where the totl charge (in coulombs) passed through the concrete directly relates to the resistance of concrete to chloride ion penetration. CMC uses Germann Instruments’ very popular Proove’iT equipment for rapid chloride permeability tests. Germann Instruments’ PROOVE'it rapid chloride permeability test (RCPT) system is used extensively in many laboratories to accurately determine the resistance of concrete to the ingress of chloride ions. 

 

CMC has multiple units of PROOVE'it to determine chloride permeability of multiple samples simultaneously. The test involves preparation of a 4 in. (100 mm) diameter and 2 in. (50 mm) thick cylindrical sample of a hardened concrete core or a cylinder that is subjected to an electrical potential across the sample according to the procedures of ASTM C 1202, ASTM C 1760, AASHTO T 277, and Nordtest Build 492. The following excerpt from Germann Instruments explains the test procedures that CMC follows for determination of chloride permeability of concrete. Also provided below is the Standard Operating Procedures (SOP) for ASTM C 1202 test that CMC follows.

The PROOVE´it system is used to evaluate the resistance of concrete to the ingress of chloride ions in three ways:

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• By determining the total electrical charge that passes through a saturated concrete specimen by applying an electrical potential across the specimen in accordance with AASHTO T 277 or ASTM C1202. This is known as the “Coulomb Test” or the “Rapid Chloride Permeability Test (RCPT).”

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• By measuring the penetration depth of chloride ions, after an electric potential has been applied to the specimen in accordance with Nordtest Build 492 to determine the “Chloride Migration Coefficient,” which can be used to estimate the chloride diffusion coefficient for service life calculations.
   

• By measuring the current passing through a saturated concrete specimen and determining the bulk conductivity in accordance with ASTM C1760.

ASTM C1202 “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration” is actually a test of electrical conductance, rather than chloride permeability as is often stated. Electrical conductivity is related to the diffusion coefficient. In this test, a water- saturated concrete specimen, nominally 100 mm diameter and 50 mm thick, is positioned in a test cell containing fluid reservoirs on both ends of the specimen. One reservoir is filled with a 3% NaCl solution and the other with a 0.3N NaOH solution.

 

An electrical potential of 60 VDC is applied across the cell. The negative terminal of the potential source is connected to the electrode in the the NaCl solution and the positive terminal is connected to the electrode in the NaOH solution. The negatively charged ions will migrate towards the positive terminal resulting in current through the specimen. The current is measured.

 

The more permeable the concrete, the more negative ions will migrate through the specimen, and a higher current will be measured. The current is measured for 6 hours. The area under the curve of current versus time is determined, which represents the total charge or Coulombs passed across the specimen. Test results are corrected for a standard specimen diameter of 95 mm.