Testing concrete for corrosion resistance

Testing Concrete for Corrosion Resistance

Dr J D Bapat

The deleterious agents causing corrosion of reinforcement are water, oxygen, carbon dioxide and salts. They reach reinforcement penetrating concrete surface. The concrete mixtures designed for service exposure environments reduce the opportunities for reinforcement corrosion. The resistance of concrete towards corrosion is measured by its resistance towards the penetration of these agents. The rapid test methods provide a fast and reasonable approximation of the corrosion resistance of concrete. Some of the test methods are given as follows.

Rapid Chloride Permeability Test (RCPT), ASTM C1202: This is quick and the most frequently used test. The RCPT is a measurement of the electrical charge that travels between two sides of a concrete specimen over a six-hour period. This charge is correlated to chloride ions passing through the pore system. Lower values signify a higher resistance to chloride intrusion.

A more rapid test for permeability than the RCPT was recently developed by Florida’s Department of Transportation (FDOT 2004). This procedure uses the Werner probe array method for testing resistivity of concrete on 100x200 mm specimens. ACI 222R (2001) also recommends using this method for assessing the permeability of in-place concrete. The results of the electrical resistivity test have been correlated to the RCPT permeability rating system. The surface resistivity is measured in a matter of seconds thereby allowing for a much larger sample size. The Florida standard requires 8 tests each on 3 specimens, while the RCPT can only provide a single test per specimen because of the inherently destructive preparation requirements.

Determining the Apparent Bulk Diffusion Coefficient of Cementitious Mixtures by Bulk Diffusion, ASTM C1556: The ASTM C1556 is a more rigorous method to calculate concrete permeability. Test results from C1556 typically provide lower variability in test results. The test specimens are moist cured for 28 days and later subjected to unidirectional chloride intrusion. The depth of chloride intrusion is measured over time, beginning at 35 days of soaking period. The successive layers from the specimen are ground. The chloride level of each layer is measured using Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete, ASTM C1152,. This procedure gives a direct correlation to the permeability of the concrete and is considered a useful method for prequalification of concrete mixtures. Unfortunately, C1556 is very time consuming and requires about 3 months to complete.

Standard Test Method for Determining the Penetration of Chloride Ion into Concrete by Ponding, ASTM C1543 (AASHTO T259): The test is commonly used by many highway agencies. A concrete slab (test specimen) is cast and moist cured for 14 days and then air cured to 28 days. The top surface is bermed and ponded with a salt solution for 90 days. Cores are then taken from the exposed surface and sliced into approximately half-inch thick discs. Each disc is crushed and the chloride content of each layer is determined. Unfortunately, this test requires almost six months to complete and there is no clear way provided for interpretation of the results in the method. Transport mechanisms in this test also include undefined components of absorption, diffusion and wick action.

Predicting Chloride Penetration of Hydraulic Cement Concrete by the Rapid Migration Procedure, AASHTO TP 64: In this method a 50-mm long, 100-mm diameter concrete sample is saturated using the vacuum saturation procedure of the RCPT. This test ranks multiple concretes in the same order as ASTM C1202, but has the advantage of not being influenced by strongly ionic admixtures, such as calcium nitrite. The specimen does not experience a temperature rise during the test. The test also has been shown to have a somewhat lower variability than the RCPT.

Question:

Discuss possible ways to protect rebars from corrosion