Categories: Issue 26 March/April 2003

Question:

What value of chloride permeability should I specify for a bridge deck?

Answer:

Chloride permeability or, more appropriately, chloride penetrability is being widely specified to ensure longevity in our transportation structures. There has been considerable debate regarding appropriate applications and interpretation of test results, and most importantly, which values to use.

The relationship between results from the rapid chloride penetrability test (RCPT) as measured using AASHTO T 277 and diffusion coefficients calculated from ponding tests using AASHTO T 259 is quite good even though test conditions and ages are different.(1-3) If comparable curing and longer ages are used in each test method, the relationships can be improved. For example, Virginia Department of Transportation (VDOT)(4) has adopted accelerated curing of specimens consisting of one week at 73°F (23°C) and three weeks at 100°F (38°C) for the RCPT while extending the duration of the ponding test from 90 days to one year or more. It should also be noted that values of RCPT become lower with time. Consequently, the concrete age at time of testing is important.

The specified value of chloride penetrability should be selected based on the importance of the bridge element, the exposure conditions, expected service life, and practical achievability with acceptable materials. For example, lower values can be achieved more readily in precast, prestressed concrete beams than in cast-in-place concrete decks. Concretes containing only cement as the cementitious material may require very low water-cement ratios to achieve sufficient resistance to chloride penetration.(5) The resistance of concretes to chloride ion penetration increases with the use of latex, pozzolans (Class F fly ash, silica fume, or calcined clays), or slag.(6) Based on extended ponding tests, concretes made with these materials can have chloride contents at the depth of the steel reinforcement below the threshold value for corrosion initiation.

A penetrability value should be selected and a mix design developed to minimize cracking potential. For example, the use of a lower water-cementitious materials ratio as a means of reducing the penetrability of concrete results in a high strength concrete with a high modulus of elasticity and reduced creep. As a result, the deck concrete is more likely to crack from shrinkage, particularly if the concrete is not adequately cured.

Based on the above considerations, a maximum value of 2000 or 2500 coulombs at 56 days with standard curing represents a good starting point for bridge deck concrete. In harsher climates, lower RCPT values may be appropriate and should be considered. If test results at 28 days are required, accelerated curing may be specified. Selection of the specified values should consider the importance of the structural component, local experience, exposure conditions, curing of the test specimens, and achievable results.

References

  1. Hooton, R. D., Pun, P., Kojundic, T., and Fidjestol, P., “Influence of Silica Fume on Chloride Resistance of Concrete,” Proceedings, PCI/FHWA International Symposium on High Performance Concrete, New Orleans, October 20-22, 1997, pp. 245-256.
  2. Hooton, R. D., “Discussion of Durability Aspects of Precast, Prestressed Concrete – Parts 1 and 2,” PCI Journal, Vol. 42, No. 3, May-June 1997, pp. 65-66.
  3. Hooton R. D., Thomas M. D. A., and Stanish, K., “Prediction of Chloride Penetration in Concrete,” FHWA-RD-00-142, Federal Highway Administration, October 2001, 412 pp.
  4. Ozyildirim, C., “Permeability Specifications for High-Performance Concrete Decks,” Transportation Research Record 1610, Concrete in Construction, Transportation Research Board, 1998, pp. 1-5.
  5. Pinto, R. C. A. and Hover, K. C., “Frost and Scaling Resistance of High-Strength Concrete,” PCA Research and Development Bulletin RD122, Portland Cement Association, Skokie, IL, 2001, 74 pp.
    6 Ozyildirim, C., “Resistance to Penetration of Chlorides into Concretes Containing Latex, Fly Ash, Slag, and Silica Fume,” ACI SP-145, Durability of Concrete, American Concrete Institute, 1994, pp. 503-518.

Answer contributed by H. Celik Ozyildirim of the Virginia Transportation Research Council at [email protected] or 434-293-1977.

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