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Rapid Chloride Permeability Test (RCPT)

Rapid Chloride Permeability Test: This test method is used to provide an indication on the penetrability of concrete. It is used for evaluation the resistance of a concrete sample to the penetration of chloride ions.

Today, several concrete buildings are constructed to requirements of low-permeability concrete. RCPT is widely accepted in the construction industry as a means of measuring chloride permeability.

Currently, there are multiple standards that can be followed in order to perform the testing. These include:

AASHTO T277, “Standard Method of Test for Rapid Determination of the Chloride Permeability of Concrete”
ASTM C1202, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration”

Why RCPT was Developed?

RCPT was developed as part of a Federal Highway Administration research initiative. The initiative was established to develop non-destructive methods for determining the chloride penetration of in-place concrete. Prior to the creation of the test, concrete’s chloride permeability was determined using a ponding test. Ponding testing generally lasts for 90 days or more and entail the collection of concrete samples at different depths in order to establish the chloride profile. The FHWA desired a test that could be performed in situ and had a high degree of correlation with data derived from chloride ponding experiments.

Chloride migration within concrete is a fairly slow phenomenon, especially in high water/cement ratio concrete. As a result, researcher needed a test procedure that would expedite this migration. They discovered that applying an electrical current to a concrete specimen enhanced and increased the rate of chlorine migration into the concrete. Additionally, the researchers discovered a strong association between the coulombs (the integral of the current vs. time plot) traveled through the sample and the results of a ponding test. The researchers used these data to design the test techniques that are now described in AASHTO T277 and ASTM C1202.

How Rapid Chloride Permeability Test is Performed?

The test procedure begins with the extraction of a 100 mm (4 in.) diameter core or cylinder sample from the tested concrete. From the specimen, a 50 mm (2 in.) specimen is cut i.e. the sample is 50 mm thick. The cylindrical specimen’s side is covered with epoxy, and after drying, it is placed in a vacuum chamber for three hours. After one hour of vacuum saturation, the specimen is allowed to soak for 18 hours. It is subsequently inserted into the test apparatus. The test cell’s left-hand side (–) is filled with a 3 percent NaCl solution. The test cell’s right-hand side (+) is filled with 0.3N NaOH solution. After connecting the system, a 60-volt potential is applied for six hours. Every 30 minutes, readings are obtained. After six hours, the material is taken from the cell and the coulombs passing through it are determined.

How are the Test Results Interpreted?

The test results are compared to the values shown in the following chart. Five categories were established in which coulomb test results from comparable test samples were deemed similar.

It is critical to realize that these values were obtained using the test procedure mentioned above on laboratory concrete. The ranges should be used only for the purpose of comparison. The test is intended to indicate how the tested concrete compares to the values on the chart or to other concrete being tested according to the test protocol.

Factors that Influence Test Results

Numerous variables may alter the test procedure’s accuracy. The age and curing of the test specimen has a significant impact on the result. Coulombs often decrease with age, if the material has been adequately cured. Additionally, research indicates that the presence of admixtures containing ionic salts in concrete may alter the findings obtained. The presence of ionic salts results in an increase in the number of coulombs passed. It is hypothesized that the ionic salts serve as an extra conduit for charge transmission. This results in an increased coulomb level despite the fact that the permeability of the concrete remains same. Ionic salt-containing mixes are mainly accelerators made up of the following materials:

Calcium Nitrite
Calcium Nitrate
Calcium Chloride
Sodium Thiocyanate

If concrete containing these admixtures is to be evaluated using this technique, it is highly advised that tests be conducted with and without the admixture to determine the influence of the admixture on the findings. Additional variables that may alter test findings include the following:

Cement factor
Air content
Water/Binder ratio
Curing
Aggregate source/type

Precision and Accuracy

There has been much disagreement concerning this test procedure due to the wide range of findings obtained on companion test specimens. According to AASHTO T277, the findings of adjacent samples examined by the same operator shall not differ by more than 19.5%. This is an exceptionally wide range of permissible outcomes. According to the ASTM standard, the results of two correctly completed tests on concrete samples from the same batch by the same operator might vary by up to 42%. This proportion is increased to 51% when companion samples are analyzed by various labs. This wide range of test results reveals the relative inaccuracy of the test procedure while implying that concrete samples within this wide acceptable range are substantially identical in quality.

Conclusion

It is critical to realize that:

this test approach does not simulate actual field circumstances. No circumstance exists in which concrete is exposed to a 60-volt potential.
This test technique does not determine the permeability of concrete. It does, however, measure concrete resistivity. Volts divided by current equals resistance. It has been shown that there is a reasonable link between the resistivity and permeability of concrete.

At the moment, this is the only commonly acknowledged test procedure in the concrete industry. As more expertise is acquired with this and other test techniques, new processes for measuring concrete permeability may be created.