Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.

Why is consolidation needed?

Q. Why is consolidation needed?


A. A mass of freshly placed concrete is usually honeycombed with entrapped air voids that are larger than the intentionally entrained air voids. If allowed to harden in this condition, the concrete will be nonuniform, weak, porous, and poorly bonded to the reinforcement. It will also have a poor appearance. The mixture needs to be consolidated if it is to have the properties normally desired and expected of concrete (ACI 309R). Consolidation is the process of inducing a closer arrangement of the solid particles in freshly mixed concrete or mortar during placement by the reduction of voids, usually by vibration, centrifugation, rodding, tamping, or some combinations of these actions.

Some uses of concrete do not allow adequate access for vibration to effect consolidation. For such circumstances, self-consolidating concrete can be produced. This concrete contains a high-range water-reducing admixture and flows under the action of gravity. Rather than slump, the measurement used for this concrete is slump flow, which is the diameter of spread. Many high-slump concretes will benefit from light vibration to expel residual entrapped air, but truly “self-consolidating” concrete, where nearly all the entrapped air escapes without consolidating effort, can be made when needed.


References: SP-1(02); ACI 309R-05; ACI CT-18

Topics in Concrete: Concrete Fundamentals; Consolidation; Placing

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