D. Stephen Lane, Virginia Transportation Research Council
For concrete bridges, chloride-induced corrosion of reinforcement has long been the major durability problem and tests developed have attempted to measure, directly or indirectly, the penetrability of chloride ions into concrete. Such tests include the salt ponding methods of AASHTO T 259 and ASTM C1543 and the electrical methods of AASHTO T 277, AASHTO TP 64, and ASTM C1202 for rapid assessment of concrete’s resistance to chloride ion penetration. Of these, the electrical resistance tests of AASHTO T 277 and ASTM C1202 have gained the widest use and are often found in specifications for concrete materials when chloride-induced corrosion is a concern. With the advent of service-life prediction models, an emphasis has been placed on methods that measure the more fundamental properties of concrete such as chloride diffusion (ASTM C1556) and water sorptivity (ASTM C1585). This article will describe and discuss the ponding and electrical tests. A future article will focus on the diffusion and sorptivity tests.
Salt Ponding Tests
AASHTO T 259 and ASTM C1543 were designed to simulate the mechanism by which chloride ions penetrate into concrete bridge decks. The test specimens consist of a concrete slab with a minimum thickness and a minimum surface area. A dike is constructed around the top perimeter to hold the ponding solution. The slabs are typically moist cured for a length of time followed by a period of drying at 50% relative humidity before ponding with a 3% sodium chloride solution. AASHTO T 259 calls for 14 days moist curing followed by 28 days of drying, while ASTM C1543 specifies moist curing either until a specified strength is reached or 14 days, followed by 14 days of drying. Prior to ponding, the sides of ASTM C1543 slabs are sealed to prevent evaporation from those surfaces and impose directional control of the chloride penetration. The ponded slabs are stored to allow air circulation around the slabs in a room at 50% relative humidity. A cover is placed over the solution pond to prevent evaporation of water from the solution. AASHTO T 259 calls for a ponding period of 90 days. For low-permeability concretes, this is typically found to be too short for significant penetration of chloride ions into the concrete, and ponding is often extended for longer periods. For this reason, ASTM C1543 allows the user to select the ponding period based on the materials under test, recommending initial sampling at 90 days with subsequent sampling at 6 and 12 months, and 12-month intervals thereafter.
Slabs are sampled by coring or drilling with hollow-stemmed bits to obtain samples for chloride analysis at approximately 0.5-in. (13-mm) incremental depths. The samples are analyzed for total acid-soluble chloride using either AASHTO T 260 or ASTM C1152. Sampling at 0.5-in. (13-mm) depth increments provides a rather gross indication of chloride penetration into the concrete. If a more detailed profile is desired, the slab should be cored and the core carefully milled to obtain samples at increments of 0.04 to 0.08 in. (1 to 2 mm).
Electrical Tests
Because of the length of time needed to directly measure the chloride penetration into concrete with ponding tests, Whiting developed what has come to be known as the rapid chloride permeability test (RCPT).(1) This test is standardized as AASHTO T 277 (ASTM C1202). The electrical charge in coulombs passed through a water-saturated concrete specimen over a 6-hour period is measured. The 4-in. (100-mm) diameter, 2-in. (50-mm) thick specimens are placed between two cells, one containing a sodium hydroxide solution, the other a sodium chloride solution. Each cell contains an electrode and an electrical potential of 60V DC is imposed across the electrodes. The method simulates diffusion flow accelerated by the driving force of the electrical potential as opposed to a concentration gradient and it correlates fairly well with concentration-induced chloride diffusion.(2) This has led to its use as a specification tool for controlling concrete quality by a number of agencies. An example of performance limits based on the RCPT is given in the table.
Virginia DOT Criteria for Low-Permeability Concretes using AASHTO T 277
| Concrete Class | Maximum Value at 28 days, coulombs |
| Prestressed and other special designs (e.g., low-permeability overlays) | 1500 |
| Posts & rails | 2500 |
| Paving | 3500 |
AASHTO TP 64, the rapid migration test (RMT), operates under the same principle as the RCPT, but is designed to actually drive chloride ions into the concrete specimen so their depth of penetration can be measured. Test specimens have the same dimensions as used for the RCPT. The test apparatus is fairly simple. The concrete specimen is sealed in a neoprene sleeve and placed on plastic strips resting on the electrode immersed in sodium chloride solution in a tub. The second electrode is placed in the sleeve with the sodium hydroxide solution. The potential across the specimen is set based on its conductivity and then maintained for the 18-hour period. Alternatively, the RCPT apparatus can be used. Major differences between the RMT and the RCPT are that a higher (10% versus 3%) concentration sodium chloride solution is used in the RMT; the voltage across the electrodes is adjusted to one of three levels based on the conductivity of the specimen and decreases with increasing conductivity; and the test duration is 18 hours rather than 6 hours. Following the test, the specimen is split and silver nitrate solution is sprayed on the surface to determine the depth of chloride penetration. The test results are also reported to correlate well with long-term ponding tests.(3)
References
1. Whiting, D., “Rapid Determination of the Chloride Permeability of Concrete,” FHWA RD-81-119, Federal Highway Administration, Washington, DC, 1981, 173 pp.
2. McGrath, P. F. and Hooton, R. D., “Re-evaluation of the AASHTO T259 90-Day Ponding Test,” Cement and Concrete Research, Vol. 29, 1999, pp. 239-248.
3. Hooton, R. D., Thomas, M. D. A., and Stanish, K., “Prediction of Chloride Penetration in Concrete,” FHWA-RD-00-142, Federal Highway Administration, Washington, DC, 2001, 412 pp.