Consider Consolidation in Both Design and Detailing

To minimize honeycombing and other surface and internal imperfections, Section 8.1 of ACI 309R-05 suggests the method of consolidation should be carefully considered when detailing reinforcement and formwork. For example, for internal vibration, the Guide states openings in the reinforcement should be provided to allow insertion of vibrators. Typically, 4-inch by 6-inch openings at 24 inch centers are required. These items require that special attention be directed to member size, reinforcing steel size, location, spacing, and other factors that influence the placing and consolidation of concrete. The mention of member size indicates that the designer must also be involved in the process.

As stated in ACI 309R:

"The designer should communicate with the constructor during the early structural design. Problem areas should be recognized in time to take appropriate remedial measures such as staggering splices, bundling reinforcing steel, modifying stirrup spacing, and increasing section size.

When conditions contributing to substandard consolidation exist, one or more of the following actions should be taken: redesign the member; redesign the reinforcing steel; modify the mixture, in some cases to be self-consolidating; use mockup tests to develop a procedure; and alert the constructor to critical conditions."

Section 18.1 of ACI 309R-05 reiterates the need for obstruction-free vertical runs of 4 inches by 6 inches minimum cross section to permit vibrator insertion. It also provides additional recommendations as follow:

"The horizontal spacing of these vertical runs should not exceed 24 inches or 112 times the vibrator’s radius of influence. Also, these openings should not be more than 12 inches or 34 times the radius of influence from the form. If such runs cannot be provided without compromising structural integrity, the engineer [emphasis ours] should specify construction details and procedures to achieve proper consolidation."

What should be done to avoid consolidation problems?

Structures can be designed to meet ACI 318-11 requirements, but still not allow adequate consolidation during construction. Failure to stagger lap splices, bundle bars, use multiple layers of bars, or increase the structural member cross-section can create consolidation problems that almost guarantee unacceptable voids in the member.

Scale drawings of cross-sections can indicate areas where congestion is severe. Sometimes, moving the bars within tolerance can create openings that permit placement consolidation with an appropriately sized vibrator head. In other cases, it may be necessary to reduce the nominal maximum aggregate size, or use self-consolidating concrete.

When honeycombs or larger voids occur, the contractor’s methods are usually cited as the cause of inadequate consolidation. ACI 309R, however, makes it clear that the engineer is responsible for specifying construction details and procedures for achieving adequate consolidation during construction.