Technical Questions

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

Consolidation of concrete—required or not?

Q. We always require concrete slabs to be vibrated, regardless if they are cast on ground, temporary formwork, or metal deck. I know that ACI 301 provides provisions for vibrating normal-weight concrete and lightweight concrete, but does it have a section that explicitly states that a concrete slab must be vibrated? Lately, contractors have questioned the need for consolidation of slabs, and it has become an issue on-site.


A. Prior to consolidation, freshly placed concrete contains irregularly distributed entrapped air voids. So if a typical concrete mixture is allowed to harden without consolidation, the final product can have large voids, referred to as honeycomb. Although the voids may not be visible with slabs-on-ground and slabs-on-metal-deck, poorly consolidated areas will have lower strength and higher permeability than well-consolidated areas. So, durability may be compromised. For example, Commentary Section R19.3.2 in ACI 318 states: “In addition to the proper selection of cementitious materials, other requirements for durable concrete exposed to water-soluble sulfate are essential, such as low w/cm, strength, adequate consolidation, uniformity, adequate cover of reinforcement, and sufficient moist curing to develop the potential properties of the concrete.”

Section of ACI 301 states that unless otherwise specified, consolidate concrete by vibration. This applies to concrete slabs as well. In the Optional Requirements Checklist in ACI 301, Section permits other methods of consolidation, too: “Specify if consolidating of concrete by methods other than vibration will be permitted.”

It’s important to determine the proper consolidation method for the concrete mixture specified on a project, as well as placing conditions, formwork (if used), and type and amount of reinforcement. Recommendations for consolidation of concrete are given in detail in ACI 309R, including information on the effects of mixture proportioning on consolidation and consolidation mechanisms. ACI 309R also gives recommendations on equipment and procedures for various classes of construction.

Of course, the required degree of consolidation will vary inversely with the workability of a mixture. A stiff mixture will require the greatest vibration effort, a highly plastic to flowing mixture will need minimum vibration effort, and self-consolidating concrete won’t require any vibration.

In closing, to ensure desired strength and durability, all concrete placements require a proper degree of consolidation. Consolidation methods and equipment should be included in project specifications. Except for projects that include self-consolidating concrete, consolidation should be an important part of every concreting job.


References: ACI 318-19; ACI 301-20; ACI PRC-309.5-22

Topics in Concrete: Consolidation; Placing; Slab; Specifications

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