Martin McGovern, Portland Cement Association
Self-consolidating concrete can flow between congested reinforcement without requiring vibration.
The construction industry has always longed for a high-performance concrete that can flow into tight and inaccessible spaces without requiring vibration. This desire has grown over the years. More designers are specifying concrete members that are heavily reinforced and require complex formwork.
Until recently, the closest the industry came to developing "self-consolidating" concrete was to add a superplasticizer to a conventionally proportioned concrete mix. Superplasticizers allow for the use of concrete with a slump of 200 mm (8 in.) or more. Still, such concrete still requires some vibration for adequate consolidation. High doses of superplasticizer create a very fluid concrete. However, the mix often segregates because the mortar is too thin to support the coarse aggregate.
Today, advances in admixtures and mix proportioning are making self-consolidating concrete a reality.
Balancing flowability and stability
The key to creating self-consolidating concrete (SCC) is to produce a very flowable mortar (low yield value) that still has a high enough viscosity to support the coarse aggregate. The desired flowability is best produced with superplasticizers based on polycarboxylate ethers. Recently developed in the 1990s, they produce better water reduction and slower slump loss than conventional superplasticizers based on sulfonated melamines and naphthalenes. However, these latter, more conventional superplasticizers can be used for SCC as well.
To increase the viscosity of the mortar, self-consolidating concrete contains more fine material-but essentially the same amount of water as conventional concrete. The total content of materials (including cementitious materials) finer than the 150 Ám (No. 100) sieve must be high. Usually, this means about 520 to 560 kg/m3 (880 to 950 lb/yd3). In some cases, a viscosity-modifying admixture can be used instead of, or in combination with, an increased fine content.
Fresh concrete properties
Self-consolidating concrete has unique rheology. Consequently, the standard slump test (ASTM C 143) is not an adequate method for measuring its workability. Still, the most common way to assess workability has been to perform a modified slump test. The difference is that instead of measuring the concrete's slump, you measure its spread is measured-the diameter of the concrete "puddle" formed after lifting the cone. (A conventional slump cone base is too small for this test.) A spread of up to 700 mm (28 in.) is common for self-consolidating concrete.
A German standard (DafStb 2001) combines the spread test with a J-ring, which simulates reinforcement. Well-proportioned self-consolidating concrete should be able to flow between and behind reinforcement. It should also have about the same spread with and without the J-ring.
Segregation resistance is a critical property for self-consolidating concrete. Unfortunately, it is also difficult to measure objectively. AFGC, a French civil engineering association, has developed the Screen Stability Test in which concrete is poured onto a 5-mm screen to see how much of the mortar falls through (AFGC 2000). The less mortar that falls through, the less likely the concrete is to segregate.
In June 2001, ASTM created a self-consolidating concrete committee © 09.47). Martin Vachon, Technical Services Manager of Axim Concrete Technology and ASTM committee chairman, says that a goal of the committee will be to establish standard test methods to measure the relevant properties of fresh self-consolidating concrete. In so doing, ASTM will also set performance requirements for the material.
SCC in action
In April, Fihoff Concrete, Johnstown, Pa., supplied self-consolidating concrete for construction of a 12 x 12-m (40 x 40-foot) turbine table at the Seward Power Plant in New Florence, Pa. The heavily reinforced elevated table had 1.5-meter (5-foot) deep grade beams and was poured in 0.3-m (1-foot) lifts.
The mix proportions for the concrete are shown in the box (below). "We essentially switched the amount of coarse and fine aggregate that you'd add to a normal concrete mix," said Von Parkins, president of Fihoff.
The superplasticizer was added at the jobsite. According to Parkins, no special batching sequence was required at the plant. Fihoff delivered the concrete to the jobsite at a 25-mm (1-in.) slump. It then added the superplasticizer, revolved the drum 100 times, then measured the spread of the concrete. The spread averaged 635 mm (25 in.).
Rick Huss, quality control manager for Fluor Constructors, Seward, Pa., was pleased with the fresh concrete properties. "The concrete traveled like they said it would, and it carried the coarse aggregate with it," he said. Huss also said the concrete pumped well without segregating.
For more information regarding Self-Consolidating Concrete, visit: www.cement.org or e-Mail Martin McGovern at firstname.lastname@example.org, Telephone 847-966-6200.
For specific product information contact Axim Concrete Technologies at 800-899-8795, ext. 2045 or visit Axim's website: www.aximconcrete.com.
© 2002 L&M Construction Chemicals, Inc. | ConcreteNews Fall 2002.