Technical Questions

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How is concrete consolidated?

Q. How is concrete consolidated?

 

A. The consolidation method should be compatible with the concrete mixture, placing conditions, form intricacy, and amount of reinforcement. Methods of consolidation are explained in ACI 309R. Some consolidation is caused by gravity as the concrete is deposited in the form. This is particularly true for well-proportioned flowing mixtures where less additional consolidation effort is required. Self-consolidating concretes provide the necessary properties without the need for additional consolidation effort.

The manual consolidation methods are generally used only on smaller nonstructural concrete placement. Plastic or more flowable mixtures may be consolidated by rodding. Spading— in which a flat tool is repeatedly inserted and withdrawn adjacent to the form—is sometimes used at formed surfaces. Hand tamping may be used to consolidate stiff mixtures. The most widely used consolidation method is vibration. Vibration can be internal, external, or both. Vibration consists of subjecting freshly placed concrete to rapid vibratory impulses that liquefy the mortar and drastically reduce the internal friction between aggregate particles. While in this condition, concrete settles under the action of gravity (sometimes aided by other forces). When vibration is discontinued, friction is reestablished (ACI 309R).

 

References: SP-1(02); ACI 309R-05

Topics in Concrete: Concrete Fundamentals; Consolidation; Placing

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