D. Stephen Lane, Virginia Transportation Research Council
Recent attention to the durability of concrete used in bridge construction has focused on the development of service life prediction models. These models attempt to make use of the fundamental properties of concrete that govern its response to deterioration mechanisms. The principal mechanisms involved in the movement of chloride ions through concrete are sorption and diffusion. Sorption pulls the fluid carrying chloride ions into the concrete while diffusion moves the chloride ions from regions of high concentration towards regions of low concentration. For concretes exposed to the air and subject to wetting and drying, the capillary system of the cementitious paste is usually only partially saturated, and thus sorption (wicking) plays an important role in the penetration of fluids into the concrete. For concrete in which the pore system remains saturated, sorption becomes negligible and the primary mechanism for chloride penetration is through concentration driven diffusion. The water sorptivity of concrete can be determined using ASTM C1585,(1) and its chloride diffusion coefficient using ASTM C1556.(2) This article describes and discusses the sorptivity and diffusion tests. An article in HPC Bridge Views, Issue No. 58 discussed the ponding and electrical tests used to assess the chloride penetrability of concrete.
Sorptivity Test
ASTM C1585 measures the sorptivity of a concrete specimen that has been conditioned at a constant relative humidity and then allowed to equilibrate to a presumed stable internal relative humidity. The specimens are 4-in. (100-mm) diameter, 2-in. (50-mm) long cylinders. Prior to testing, the specimens are stored in a chamber at a temperature of 122°F (50°C) and a relative humidity of 80% for 3 days. The target relative humidity of 80% was chosen since this is a common value observed for in-service bridge decks. The specimens are then sealed in individual containers and stored in the laboratory at 73°F (23°C) for 2 weeks to allow the internal relative humidity of the specimens to come to equilibrium. The sides of the specimens are then sealed with tape and the ends of the specimens opposite the absorbing surface are covered to impede evaporation from this surface during the test. The specimens are then weighed, and the absorbing surfaces are exposed to water, either by immersion into a reservoir or by ponding. At increasing time intervals, the specimens are removed from exposure to water, the surfaces blotted to remove excess surface water, and the specimens reweighed. Frequent measurements are made during the first 6 hours of testing, followed by daily measurements for at least 8 days. The change in mass over time is used to calculate the sorptivity. Typically, the rate over the first 6 hours is higher than the rate over the succeeding days. These are expressed as initial and secondary rates, respectively.
Chloride Diffusion Coefficient
The chloride diffusion coefficient of concrete can be determined using ASTM C1556. Test specimens with a minimum dimension of 3 in. (75 mm) across the finished surface and a minimum length of 3 in. (75 mm) are used. Prior to final preparation for testing, specimens should be in a state of saturation to minimize the influence of transport mechanisms other than concentration driven diffusion. The specimens are then allowed to surface dry and the sides and one end of the specimens sealed. The specimens are then immersed in lime-saturated water for 6 days to complete re-saturation. The specimens are removed from the lime water, rinsed free of lime and immersed in salt solution. The standard solution is 15% by mass sodium chloride (NaCl), but other concentrations can be used. Specimens remain immersed in the salt solution for a minimum of 35 days, with longer periods necessary for high performance concretes with low permeability. Following exposure to the salt solution, the specimens are rinsed and allowed to dry for 1 day under laboratory conditions. If the sampling for chloride analysis is delayed more than 48 hours, the specimens should be sealed in a plastic bag and stored in the laboratory. If longer than 7 days, the bagged specimens should be frozen until sampling to prevent continued migration of chloride ions. Samples for chloride analysis are obtained by profile grinding in incremental depths of 0.04 to 0.08 in. (1 to 2 mm) parallel to the exposed surface. A sample of the concrete is also obtained prior to the salt exposure to provide its background chloride content. The samples are analyzed for total acid-soluble chloride content using either AASHTO T 260(3) or ASTM C1152.(4) The results of the chloride analysis tests are used to calculate the apparent chloride diffusion coefficient by fitting an equation to the data using non-linear regression analysis.
Test Results
The table below contains sorptivity values and chloride diffusion coefficients for four concretes reported by Lane.(5) Concretes C1, C2, and C3 were portland cement concretes, whereas C4 contained 6% silica fume as a portion of the cementitious material.
| Concrete | C1 | C2 | C3 | C4 | |
| w/cm | 0.58 | 0.48 | 0.38 | 0.38 | |
| ATM C1585 | Initial Rate mm/s1/2 x 10-4 | 35.2 | 23.5 | 12.7 | 4.8 |
| ASTM C1585 | Secondary Rate mm/s1/2 x 10-4 | 15.3 | 11.2 | 6.5 | 2.3 |
| ASTM C1556 | Diffusion Coefficient m2/s x 10-12 | 10.6 | 10.2 | 7.5 | 1.9 |
References
1. “Standard Test Method for Measurement of Rate of Absorption of Water by Hydraulic-Cement Concretes,” ASTM C1585, ASTM International, West Conshohocken, PA.
2. “Standard Test Method for Determining the Apparent Chloride Diffusion Coefficient of Cementitious Mixtures by Bulk Diffusion,” ASTM C1556, ASTM International, West Conshohocken, PA.
3. “Standard Method of Test for Sampling and Testing for Chloride Ion in Concrete and Concrete Raw Materials,” AASHTO T 260, American Association of State Highway and Transportation Officials, Washington, DC.
4. “Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete,” ASTM C1152, ASTM International, West Conshohocken, PA.
5. Lane, D. S., “Laboratory Comparison of Several Tests for Evaluating the Transport Properties of Concrete,” VTRC 06-R38, Virginia Transportation Research Council, Charlottesville, 13 pp. 2006.