Researchers Develop New Test to Measure Chloride Penetration in Concrete - Brief Article

Public Roads, May, 2000

Researchers at the University of Toronto, under an FHWA contract, developed a new method for predicting chloride ion penetration in bridges and pavements. The new method -- the rapid migration test -- will be used to evaluate new mixes, reject or accept new concrete specifications, and evaluate in-place concrete by predicting how resistant concrete is to chloride ion penetration.

One of the current standard test methods used to assess the resistance of concrete to penetration of chloride ions is the rapid chloride permeability test. This test, officially known as AASHTO T 277-93, Electrical Indication of Concrete's Ability to Resist Chloride, measures the charge passed through a concrete specimen subjected to 60 volts dc for six hours. Researchers reported variable results using the rapid chloride permeability test when certain mineral admixtures, such as silica fume, or calcium nitrite (included in some corrosion inhibitors) or reinforcing steel have been present in the concrete mixture.

The rapid migration test is based on a test developed by researchers at Chalmers Technical University in Sweden. The test specimens are 50 mm long and 100 mm in diameter as in the rapid chloride test. The rapid migration test uses the set-up shown in figure 1 with 0.3 molar sodium hydroxide ponded on top of the specimen, and a 10 percent solution (by mass) of sodium chloride at the bottom of the specimen. Similar liquids are used in the rapid chloride test, but they differ in the concentration of the solutions.

For the rapid migration test, the specimen is initially subjected to 30 volts dc, and the resulting current determines the voltage to be applied for the duration of the test. The voltage is applied for three different time periods varying anywhere from two hours to 96 hours. Following the test, the specimen is split in half and a silver nitrate spray is applied to identify the depth of chloride penetration into the specimen.

Field trials of the rapid migration test are under way at FHWA's Turner-Fairbank Highway Research Center (TFHRC) and at the Texas DOT, Ontario Ministry of Transportation, Virginia Transportation Research Council, and at the University of Toronto. All field trials are using concrete from batches that were mixed at TFHRC.