Concrete Permeability Testing

  

                   AASHTO T 259                                   AASHTO T 277                                    AASHTO TP 64   

 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. 

 

           ASTM C1585                                                            ASTM C1556                                        

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
ASTM 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.