From Sulfate Attack

By Rachel J. Detwiler, Ph.D., PE

Concrete exposed to sulfates is vulnerable to deterioration. In the field, this deterioration usually takes the form of loss of adhesion and strength, but may also be seen as expansion, cracking and spalling. Concrete can be designed for resistance to sulfate attack. The first step is to characterize the severity of sulfate exposure. Once that is done, the concrete can be designed for durability.

Where do sulfates occur?

Sulfate-bearing soils are found in the northern Great Plains states primarily the Dakotas and Montana and in many locations in the western United States, as well as the prairie provinces of Canada. Concrete in contact with these soils buried pipelines, foundations, slabs on grade is vulnerable to sulfate attack. The use of fertilizers can artificially increase the sulfate content of soils. Sanitary sewers and sewage-treatment plants can also be subject to sulfate attack. Industrial environments where sulfate attack could occur include coal-fired power plants and fertilizer plants.

How severe is the sulfate exposure?

In the 1930s the Bureau of Reclamation developed a way of characterizing the severity of sulfate exposure based on their experience with concrete dams in the western United States. Their classification of sulfate exposure (as shown in Table 1) based on the concentration of sulfates in the soil or water is still used today.

The method for analyzing the sulfate content of soils may cause confusion, as there are several methods available. Calcium sulfates have limited solubility in water, so variations in the test method such as the dilution rate (how much water is used with how much soil), moisture condition of the soil, extraction time and temperature will give very different results. Ideally, everyone would use the Bureau of Reclamation test method, which is the basis of the original sulfate table. However, this method is not readily available, and the published sulfate tables do not specify a test method. This situation has left most engineers ignorant of the importance of the test method and laboratories free to select the method most convenient for them.

How do we get the right test for soil?

In 2005 ASTM published a test method, ASTM C 1580, "Standard Test Method for Water-Soluble Sulfate in Soil," which is based on the Bureau of Reclamation test method. It is specifically directed at classifying the severity of sulfate exposure for concrete. Braun Intertec has recently begun offering this test to our clients. Having this test available will help them make informed decisions about protecting the concrete that will be in contact with these soils.

Thin section photomicrograph of concrete subjected to sulfate attack. Plane polarized light. Field width, left to right is 700 micrometers.

Figure 1. Formation of ettringite in microcracks (pink arrow) and the air void (yellow arrow). The sulfate front has advanced into the concrete from the upper left, and the cracks have formed perpendicular to it. Total width of field is 700 micrometers. Cross-polarized light with 530 micrometer accessory plate.

Figure 2. Same field as in Figure 1. The dark areas (arrows) Indicate where calcium hydroxide has been either leached out of dissolved to provide calcium for the formation of ettringite.

Figure 3. Formation of ettringite in microcracks (pink arrow) and the air void (yellow arrows). The sulfate front has advanced into the concrete from the upper left, and the cracks have formed perpendicular to it. Total width of field is 700 micrometers. Cross-polarized light with 530 micrometer accessory plate.

How do we get the right cement?

Many engineers are aware that they can specify ASTM C 150 Type II cement for moderate sulfate resistance or Type V for high sulfate resistance. This seems simple enough, but it may not be. Type II cement has two specialized properties, moderate sulfate resistance and moderate heat of hydration. The engineer must specify which of these characteristics is required, since both are optional physical requirements according to ASTM C 150. Simply ordering Type II cement does not guarantee that the cement will provide any sulfate resistance at all.

The availability or cost of sulfate-resistant cement, whether Type II or Type V, may also present complications, particularly where sulfate-bearing soils do not naturally occur. Fortunately there is more than one way to meet the requirement. Blended cements or mixtures of portland cement with one or more supplementary cementitious materials can be formulated to provide as good sulfate resistance as Type V or even better. ACI Committee 201's Guide to Durable Concrete gives the criteria for testing the desired combination of materials according to ASTM C 1012, "Standard Test Method for Length Change of Hydraulic-Cement Mortars Exposed to a Sulfate Solution." Table 2 summarizes these criteria.

Braun Intertec now offers ASTM C 1012 for clients wanting to find acceptable alternatives to Type II or Type V cement.

How do we get sulfate-resistant concrete?

As can be seen in Table 1, it is not enough to have the right sulfate-resistant cement. Limiting the permeability of the concrete is even more important. Maximum limits on the water-cementitious materials ratio, combined with good concreting practices especially good curing are even more important to sulfate resistance than the right cement. Even Type V cement concrete will deteriorate eventually under sulfate attack. High-quality concrete lasts longer.

About the author:

Rachel Detwiler is Associate and Senior Materials Engineer at Braun Intertec Corp., 11001 Hampshire Ave. S., Minneapolis, Minnesota 55410. Dr. Detwiler provides concrete-related consulting services, does construction troubleshooting and conducts forensic investigations. She has an extensive background in consulting, research, evaluation of concrete materials, concrete durability, supplementary cementitious materials, and test methods. She is a registered professional engineer in Illinois, Minnesota, Wisconsin, North Dakota and Hawaii. She is also a fellow of the American Concrete Institute.

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© 2008 L&M Construction Chemicals, Inc. | ConcreteNews January 2008.

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