Consider Consolidation in Both Design and Detailing

While working on construction of a nuclear power plant in the late 1970s, there was a standard joking remark after all of the reinforcing steel had been placed in narrow interior shield walls. The ironworker foreman would ask if we were going to place concrete or just paint the rebars.

It was hardly a joking matter for the concrete placing inspector, whose job it was to ensure there were no large honeycombs, or worse, voids that penetrated the wall thickness. This was prior to the widespread use of high-range water reducers or pea gravel concretes. With a layer of #11 bars at the top of the shield wall, spaced 138 inches apart, there was no room for a vibrator of the specified diameter to fit through the maze of reinforcing steel. So placing ports were cut in the wall forms to allow vibration, and even then, form stripping was a time of great anxiety, primarily due to fear that a void the size of a fist would be a see-through defect requiring repair.

Thirty years later, things haven’t changed much. Consolidation is still a concern in reinforced concrete structures ranging from base mats, heavily reinforced beams and walls, or any earthquake-resistant structure in high seismic risk zones. ACI 318-11 has few requirements related to consolidation. Section 3.3.2 of ACI 318-11 limits the nominal maximum size of coarse aggregate to:

  • One-fifth the narrowest dimension between sides of forms,
  • One-third the depth of slabs, nor
  • Three-fourths the minimum clear spacing between individual reinforcing bars or wires, bundles of bars, individual tendons, bundled tendons, or ducts.

However, these requirements are modified by the following statement:

"These limitations shall not apply if, in the judgment of the licensed design professional, workability and methods of consolidation are such that concrete can be placed without honeycombs or voids."

Section 7.6.1 of ACI 318 also requires that the minimum clear spacing between parallel bars in a layer to be the bar diameter, but not less than 1 inch. The Commentary for this section states that the development lengths became a function of the bar spacings since the 1989 Code and it may be desirable to use larger than minimum bar spacings in some cases. The minimum limits were originally established to permit concrete to flow readily into spaces between bars and between bars and forms without honeycomb, and to ensure against concentration of bars on a line that may cause shear or shrinkage cracking.

The phrase “... between bars and forms ...” is interesting because Section 3.3.2 makes no reference to controlling nominal maximum size aggregate on the basis of the space between bars and forms.

A reference to consolidation in the Commentary for Section 21.1.2.3 of ACI 318-11 deals with earthquake-resistant structures as follows:

"In selecting member sizes for earthquake-resistant structures, it is important to consider constructibility problems related to congestion of reinforcement. The design should be such that all reinforcement can be assembled and placed in the proper location and that concrete can be cast and consolidated properly. Use of upper limits of reinforcement ratios permitted is likely to lead to insurmountable construction problems, especially at frame joints."

Another Code reference to consolidation is in the Commentary for Section 5.10, which deals with depositing concrete. This section states that:

"Recommendations for consolidation of concrete are given in detail in ACI 309R. (This presents current information on the mechanism of consolidation and gives recommendations on equipment characteristics and procedures for various classes of concrete.)"

The ACI 309R-05, “Guide for Consolidation of Concrete,” recommendations are keys to designing and detailing constructible reinforced concrete structures.

Internal vibration recommendations in ACI 309R

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.

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