Specifications for high-performance concrete

Seattle skyline
Tall buildings in Seattle incorporated increasingly higher concrete strengths. Shutterstock image.

When I was a graduate student in the 1980s, papers about high-performance concrete nearly always started out by explaining that “high performance” means more than high strength. Then they went on to report the increasingly high strengths they achieved for the concretes they were using to build taller and taller buildings. Occasionally high strength was a surrogate for something else —high modulus of elasticity or low creep, usually. Still, for practical purposes “high performance” meant high strength, period. Naturally, specifications for high-performance concrete were similarly narrow in scope.

Now, though, there’s more interest in durability. Much of the infrastructure in the United States dates from the 1950s and ’60s, and it’s showing its age. Many of our roads and bridges are well beyond the end of their design service life. If you live in the northern US or near a coast, you don’t have to look far to find rust-stained, cracked, and spalled concrete. As we replace obsolete structures and build new ones, we need to specify durable concrete.

Another aspect of high performance relates to constructability. That is, how easy is it to mix, transport, place, consolidate, and finish the concrete? In a heavily reinforced member, it can be hard to place and consolidate the concrete. In that case, self-consolidating concrete could be the answer. All you have to do is put it in the form; it consolidates under its own weight without any help from you.

Developing specifications for high-performance concrete

The first thing to figure out is what the concrete needs to do. For the Engineer of Record, that usually means compressive strength at 28 days. Precast concrete needs to be strong enough at 16 to 18 hours so the plant can maintain a 24-hour production cycle. Prestressed or posttensioned concrete needs to be strong enough at an early age to withstand the stresses without excess creep. On the other hand, if the concrete has slag cement or fly ash it needs time to gain strength. That calls for a 56- or 90-day strength.

Other mechanical properties may also matter. Prestressed concrete, long-span bridges, and columns in tall buildings benefit from high modulus of elasticity and/or low creep. Both properties correlate with high compressive strength, but it’s best to specify them directly rather than predict them from compressive strength.

On the other hand, if cracking is a concern, creep is your friend. Any material deforms under load. Under a sustained load it continues to deform, a phenomenon we call “creep.” But suppose instead of a constant load we impose constant deformation? In that case, the load needed to keep the deformation constant diminishes over time. This useful in paving. Suppose the temperature falls overnight. The pavement would contract if its base didn’t restrain it. Restraint of volume change induces tensile stresses which could cause cracking. But if the concrete creeps instead of resisting, the stresses are lower —ideally, low enough that it doesn’t crack.

Thermal control plans

Thermal control plans may be a necessary part of specifications for high-performance concrete. In applications such as bridge piers, radiation shielding, and airfield pavements, the warm core concrete may restrain thermal contraction of the cooler surface concrete. Tensile stresses at the surface can cause the concrete to crack through the thickness. These cracks can be wide enough to stick your finger into. Thermal control plans alert the contractor that special measures are necessary to control cracking and provide guidance for doing so.

What about durability?

Unlike strength or modulus of elasticity, durability is not an inherent property of concrete. Rather, it depends on the service environment. Concrete for a bridge deck in the Midwest, say, will have to withstand cycles of freezing and thawing, deicing salts, and abrasion from snowplows. Concrete for a footing in the western US will have to withstand exposure to sulfate-bearing soils. Agricultural concrete has to withstand acids, sulfates, and various salts from animal wastes.

A low water-cementitious materials ratio; judicious use of fly ash, slag cement, and/or silica fume; and good concreting practices —particularly good curing —will go a long way toward producing durable concrete in most environments. However, some environments call for specific provisions. The American Concrete Institute has several publications related to concrete durability in specific applications.

When it comes to specifications for high-performance concrete, you can choose either a prescriptive or a performance approach. Many specifications use a combination of the two, as performance tests aren’t always practical for quality control. The use of concrete exposure classes helps categorize the nature and severity of the service environment.

An example specification

For example, a contractor wanted Beton to develop and test mixture proportions for a concrete for the repair of a marine structure in New England. The specification included such prescriptive requirements as maximum water-cementitious materials ratio, minimum cement content, maximum contents of fly ash and silica fume, and air content. The performance requirements included compressive- and tensile strengths, drying shrinkage, cracking tendency, and resistance to chloride ion penetration.

Interestingly, there were both minimum and maximum limits on compressive strength. The owner was concerned about the association between high strength and the tendency to crack.

With so many requirements, some were not entirely compatible. However, the owner, who had a sophisticated understanding of concrete materials, was willing to listen to reason —so long as we had data to support our position. We selected a water-cementitious materials ratio close to the maximum allowable. Even so, the strength of the concrete exceeded the maximum. However, because we’d used the aggregate suspension method to proportion the concrete, we were able to demonstrate that the concrete had low shrinkage and little tendency to crack. We didn’t want to increase the water-cementitious materials ratio because that would reduce the durability. Had the owner insisted on a lower strength, we would have increased the air content —a way to reduce the strength without reducing durability in this application. In this case the owner agreed with us that this wasn’t necessary.

Beton can help you meet a specification or write one for your project.