The concrete specimen shown here has a w/c ratio of 0.3. Unhydrated cement is white, porosity is black, calcium hydroxide is light gray, and calcium silicate hydrate gel and other hydration products are dark gray.
Credit: National Institute of Standards and Technology The concrete specimen shown here has a w/c ratio of 0.3. Unhydrated cement is white, porosity is black, calcium hydroxide is light gray, and calcium silicate hydrate gel and other hydration products are dark gray.

Durable concrete is concrete that performs well for a long time. It doesn’t scale from freeze/thaw cycles and the surface doesn’t erode. The concrete industry has come a long way in developing more durable mixes over the last 30 years. But sometimes, for a decorative concrete contractor, high durability mixes can be too much of a good thing.

Defining durable concrete

Durable concrete is primarily the result of three factors: the design of the concrete mix, the amount of mix water, and the amount of entrained air for concrete in freeze/thaw climates. Procedures, such as placing and finishing practices, ambient conditions at the time of placement, and curing, also affect durability.

Using supplementary cementitious materials (SCM), such as fly ash, slag, silica fume, and metakaolin, as partial replacements for portland cement, can greatly enhance concrete durability. Low water-cementitious materials ratios are very important as well, made possible by the addition of water-reducing admixtures.

The amount of water added to fresh concrete directly affects its permeability in the hardened state. When the permeability of concrete is high, fluids such as water or solvents can readily pass through. The porosity of concrete, sometimes confused with permeability, is defined by the air void spaces in concrete, while permeability relates to the size and degree in which voids or pore spaces are interconnected with one another, forming channels or capillaries through which fluids can move. Concrete mixes with low water-cementitious material ratios produce few interconnected capillaries but can have significant porosity, such as with air-entrained concrete.

Highly durable concrete generally is considered to have a water-cementitious materials (w/cm) ratio of 0.45 or less. Dale Bentz, a chemical engineer for the National Institute of Standards and Technology, Gaithersburg, Md., says concrete above 0.45 won’t depercolate (cement paste fills capillaries, making them discontinuous); concrete with a w/cm ratio above 0.50 isn’t considered durable.

The scanning electron microscope image of the surface of a concrete specimen with a well-developed pore/capillary structure, visible as black dots.
Credit: National Institute of Standards and Technology The scanning electron microscope image of the surface of a concrete specimen with a well-developed pore/capillary structure, visible as black dots.

Manufacturing process

Today’s superplasticizers make it possible to reduce the water-of-convenience and slump of concrete—thus increasing durability while still achieving good flowability. Often this is combined with the use of SCMs, which combine with water and calcium hydroxide­—a product of the hydration process—to produce more calcium silicate hydrate that expands into and fills capillaries, reducing permeability and increasing durability.

Cemstone, Mendota Heights, Minn., only sells durable concrete. Kevin MacDonald, vice president of engineering services, says they produce concrete for exterior use with water-cementitious ratios less than 0.45, and contractors aren’t allowed to add water on the jobsite. “You need a water-cementitious materials ratio of 0.45 or less to have proper resistance to freeze/thaw and deicing salts,” he says. “The lowest water-cementitious mixes we sell for exterior use are about 0.30.”

Accounting for durability

High durability mixes can create problems for decorative contractors. Any post-placement decorative treatment, such as acid etch staining, dye staining, or water-based staining, must penetrate the surface of the concrete by moving into the capillaries. When there are few capillaries, or their diameters are very small, stains remain on the surface with minimal penetration and can be washed away easily.

Once it does penetrate into the capillaries, acid etch stains must combine with calcium hydroxide to produce permanent metallic colors. No color production occurs if the two don’t come together. Because SCMs also react with calcium hydroxide, less is available to react with acid etch stain. This should also be taken into consideration.

The best mixes for acid staining are those with good capillary structures and plenty of calcium hydroxide. Work with your ready-mix producer to develop the best mix for the job.

Another potential problem is that sealers and coatings mechanically bond to concrete surfaces by penetrating into pores and capillaries. Penetrating sealers also require a capillary network to penetrate into, so very durable concrete with few capillaries can inhibit sealer performance.

Being mindful during curing

Many decorative concrete contractors don’t cure their work. This is because many curing procedures can result in altering color or because post-decorative processes require open concrete surfaces. Is durable concrete with a water-cement ratio of 0.45 or less affected by curing? Scott Wolter, a petrographer with Petrographic Services, St. Paul, Minn., says it’s more difficult for mix water to move around in dense concrete. “At 0.40 water-cement ratios, only the approximate top 1/32 inch of a slab could be affected.” However, concrete that isn’t cured with water-cement ratios greater than 0.45 can exhibit dusting, carbonation, and scaled surfaces.