Kamal H. Khayat, University of Sherbrooke, Quebec, Canada
Self-consolidating concrete (SCC) is a new class of high performance concrete based primarily on the properties of the concrete during placement. When properly proportioned and controlled, SCC can flow significant distances and consolidate to normal density without the application of compactive effort, as illustrated in Fig. 1. The concrete maintains sufficient resistance to segregation to remain homogeneous during and after placement.
The use of SCC can accelerate the filling of formwork, especially when casting densely reinforced elements and sections with restricted access. It reduces labor demand and noise on construction sites and in precasting yards. SCC can result in high-quality, smooth surfaces that are free of honeycombing and signs of bleeding.
This “vibration-free” concrete was initially used in the casting of large civil engineering structures. Subsequently, SCC has been used for a variety of applications, including building construction, short-span bridges, concrete repairs, and precast, prestressed concrete members. Some properties of the hardened SCC relative to those of conventional concrete are described in References 1 and 2.
Quality Control
From a rheological point of view, SCC exhibits a low yield value, which ensures that the concrete will flow, and has a moderate viscosity, so the concrete remains homogeneous during placement. Consequently, different test methods are needed with SCC. A number of field-oriented test methods have been proposed for quality control of SCC. The ease of flow is often determined by measuring the slump flow as shown in Fig. 2. Special tests are employed to evaluate the resistance to segregation during placement, ability of the concrete to flow through restricted spaces, resistance to bleeding, settlement, and segregation.(3,4)
Mixture Proportioning
Material selection and mixture proportioning should be aimed at reducing interparticle friction among the solid particles. SCC typically incorporates fly ash, blastfurnace slag, or limestone filler to enhance both deformability and stability of the fresh concrete. Deformability is the ability of the concrete to undergo a change in shape under its own weight, even in the vicinity of obstacles that interfere with its flow. Proper stability can be obtained by reducing the water-cementitious materials ratio, increasing the concentration of solids finer than 80 μm, and/or incorporating a viscosity-modifying admixture. The use of a highrange water-reducing admixture can disperse cement grains and reduce interparticle friction. This allows a reduction in water content while maintaining the required levels of flowability and viscosity.
Conclusion
In recent years, the interest in SCC has spread widely. With further training of personnel, more experience gained with the design and proportioning of SCC using readily available materials, specification changes, and the use of proven quality control tests, this new class of HPC has great potential for use in bridges.
Selected References
- Khayat, K. H., Paultre, P., and Tremblay, S., “Structural Performance and In-Place Properties of Self-Consolidating Concrete Used for Casting Highly Reinforced Columns,” ACI Materials Journal, Vol. 98, No. 5, September-October 2001, pp. 371-378.
- Khayat, K. H., Manai, K., and Trudel, A., “In Situ Mechanical Properties of Wall Elements Cast Using Self-Consolidating Concrete,” ACI Materials Journal, Vol. 94, No. 6, November-December 1997, pp. 491-500.
- Khayat, K. H., “Workability, Testing, and Performance of Self-Consolidating Concrete,” ACI Materials Journal, Vol. 96, No. 3, May-June 1999, pp. 346-353.
- Okamura, H., “Self-Compacting High-Performance Concrete,” Concrete International, Vol. 19, No. 7, July 1997, pp. 50-54.
- Japan Society of Civil Engineers Proc., International Workshop on Self-Compacting Concrete, Kochi, Ed. Ozawa, K., Ouchi, M., JSCE Concrete Engineering Series 30, August 1998, 400 pp.