1. Fabrication and Placement of Drilled Shaft Reinforcing Cages
AASHTO Subcommittee on Construction
Virginia Beach, Virginia
August 2, 2011
Barry D. Siel, P.E.
Senior Geotechnical Engineer
FHWA-RC

2. Attributes of Rebar Cages to Assure Good Constructability

3. Longitudinal Reinforcing
A rebar cage, which is normally fabricated on-site, must be kept free of dirt, oil, or anything else that will impair bonding between the steel and concrete.
This photo shows good practice for keeping the cage clean. Note how the cage being fabricated is elevated above the ground surface. The steel bars on the left are also stored above ground.

4. Selecting Bar Sizes: A Practical Compromise
Make Bars as Large As Possible for Better Flow Paths for Concrete (Constructability)
Make Bars as Small As Possible to Minimize Crack Size (Durability)
Splices
Use Only Lap or Mechanical Splices
Locate Splices As Deep As Possible
Bullet 1: If a large amount of steel is required (e.g., high bending moment), the use of small-diameter reinforcing bars makes for small openings between bars, restricting the free space for concrete flow. This can be addressed by using bars with larger diameters, which provide more clear spacing for the same amount of steel.
Bullet 2: High bending stresses also increase the likelihood of cracking in the concrete on the tension side of the shaft. Tension cracks, especially large ones, makes the steel more vulnerable to corrosion by allowing chlorides (salt) or other corrosive agents to reach he steel. The size of tension cracks can be minimized by using smaller diameter bars, conflicting with the use of large rebar sizes desired for constructability. A good compromise is the use of relatively large steel bars, but not bars with extraordinary diameters (such as No. 18 bars) unless absolutely necessary.

5. Splice 50 ft Below Top of Shaft
Splicing of cage sections must be done in a way that does not sacrifice the ability of the steel to resist bending stresses.
This is a photo of a spliced rebar cage lifted to the vertical and ready to be placed in the hole. Note that the splice begins about 50 ft below the top of the shaft. The splice will not be in a zone of high bending stress. Consideration can also be given to staggering the splices around the cage from bar to bar when the splice must appear in a zone of high bending stress. This is somewhat more difficult for the contractor to accomplish than non-staggered splices and also wasteful of steel, but it results in a more structurally sound cage. Note that for this project the longitudinal bar lap splices are 50 ft below the top of the cage, but they are not staggered. It is recommended that no more than 50% of the splices be at any one level (that is that the splices be staggered longitudinally).

6. Transverse Reinforcing
Typical transverse reinforcing bar sizes for structural requirements, may need to be larger to prevent distortion
Resist shearing, compressive and flexural stresses, tie longitudinal rebars together, provide confinement of concrete

7. Transverse Reinforcing
Anchor ends of transverse steel with either hooks or with a lap joint defined by ACI and is (f)(Ab, fy and f’c)

8. Cage Design for Constructability
Clear Spacing Between All Bars  5 D of Largest
Coarse Aggregate or 5 inches, (whichever is
larger) to Allow Placement of Concrete
Bundle Bars if Necessary
Tie Transverse Steel Every Lap
The concrete must be able to flow through the rebar cage into the annulus between the cage and the borehole wall. This is the reason for the “5D” criterion. If structural loads are so high that rebar must be closer than 5D (clear), then the designer should consider either bundling the rebar or using a smaller coarse aggregate in the concrete. While a 5D spacing should produce adequate passage of concrete, the FHWA suggests the greater of 5D or 5 inches. On rare occasions, two or more concentric cages have been constructed in zones where extremely high rebar density is required structurally. This is very costly.
Tying transverse steel, every lap adds stability to the cage during handling. The FHWA also suggests that all laps (junctions) between the transverse and longitudinal steel be double tied.
Generally, butt-welding is not used on rebar cages, nor is flash welding, so splices must be mechanical. Avoid having splices at levels where bending stresses are high.
Photos illustrating these issues are shown in the next two slides.

9. Photo illustrates the use of bundled bars in a reinforcing cage and ties for spiral reinforcing.
Point out the bundled longitudinal rebar. Bonding problems do not occur between the rebar and concrete because the rebar is vertical, or very nearly so, in the hole. Note that this cage has been tied at every lap and is well-designed to allow free flow of fluid concrete for a concrete mix with a 3/4-inch coarse aggregate.

10. Concentric Rebar Cages
Increases steel but also increases resistance to lateral flow of concrete

11. External Centralizer
Centralizers, when properly designed and attached to a cage, allow the cage to be centered in the hole and avoid contamination of the steel by soil.
Centralizers should be made of materials that do not conduct small electrical currents, such as plastic or concrete. (Metal centralizers can promote corrosion of the steel). These can be purchased from suppliers who specialize in this equipment or (if concrete) made directly by the contractor.

12. External Centralizer
Centralizers should be made of materials that do not conduct small electrical currents, such as plastic or concrete. (Metal centralizers can promote corrosion of the steel). These can be purchased from suppliers who specialize in this equipment or (if concrete) made directly by the contractor.

13. External Centralizer
These photos show centralizers (wheels are the best so they do not scrape the sides of the borehole and causes soil to fall to the base), both external (to keep the cage centered in the borehole) and internal (to keep a tremie or pump line centered within the cage).
The vertical spacing of centralizers is also a consideration. Experience indicates that if this spacing is more than about five cage diameters, the cage may bend and contact the side of the borehole, producing a possibility for later corrosion of the steel.

14. Concrete Cover and Location Tolerance
Most codes call for three-inch cover over the rebar below the ground surface. To accommodate tolerances in the FHWA guide specification for drilled shafts, it is recommended by the FHWA that shafts be designed with a six-inch cover, so that if the cage position has to be adjusted there will always be at least three inches of cover.

15. Covers aspects of reinforcing cage design that relate to constructability, as opposed to structural design.

16. Partial-Length Rebar Cage
Usually, below a depth diameters, very little shear and moment occur in the shaft, and the density of longitudinal steel can be reduced by cutting off perhaps half of the bars, with due consideration for maintaining development lengths below the theoretical cut-off points. Here, we see a large cage. In the bottom third of the cage we can clearly see a less dense rebar layout and widely spaced hoops.
A final consideration can be mentioned while viewing this slide. For ease of construction (handling cage, placing concrete, maintaining alignment in drilling), the aspect ratio (length / diameter) of the drilled shaft should be kept to about 30: 1.

17. Essentials of Cage Design for Constructability
No Out-Hooks if Casing Method is Possible
Make Cages Full Depth if Casing Method is Possible
Do Not Allow Welding Unless Weldable Steel is Specified
Key Message:
These bullets are continued from the three previous slides .
Instructional Strategies:
Obviously, out-turned hooks will make it impossible to remove casings, so other alternates need to be developed. This is mainly a problem with abutment foundations.
Note that it is physically difficult to hold a cage in position while pouring concrete and pulling the casing simultaneously. Except in unusual cases, the cage should be self-supporting if it is possible that the casing method will be used.
Welding is permitted by some agencies near the bottom of the shaft where the only function of the cage is to support the structural steel above while the concrete is being placed. No welding should be allowed where the steel serves any structural purpose. It is noted that some contractors will tie cages over-length and cut them off at the bottom as necessary when the exact length of the cages cannot be determined with certainty before the boreholes are drilled (sockets in highly variable rock, for example). This will speed the construction process. Normally, doing so is not a pay item, however.

18. Support Cage Above Bottom
This slide illustrates the use of steel plates to act as “feet” at the bottom of the cage in order to prevent the bars from punching into soil at the base

19. Internal Bracing

20. Internal Bracing

21. Cage with Internal Bracing
Internal bracing provides a means to prevent distortion of the cage under its own weight during fabrication and storage, and to minimize distortion when the cage is lifted into place.
Although it is not a cage design issue, the contractor should be required to keep the cage from egging or otherwise distorting permanently when it is handled by installing temporary internal braces. These can be helixes, heavy hoops, Z bars and similar devices. They should be removed as the cage is lowered into the borehole.

22. Lifting Cages
This cage on the right is picked up at four points (using the ground as one point of support) and remains quite straight and undistorted. The cage on the right also used several internal stiffeners to assist in the pick-up process. These were removed as the cage was placed in the borehole.

23. Lifting Cages
The cage is picked up essentially at one point and is permanently distorted by the pick-up maneuver. The cage on the right is picked up at four points (using the ground as one point of support) and remains quite straight and undistorted. The cage on the right also used several internal stiffeners to assist in the pick-up process. These were removed as the cage was placed in the borehole.

24. Lifting Cages
Some elastic distortion of the cage during lifting is acceptable

25. Lifting Cages
Prefab cage being delivered to site and off loaded using multiple lift points

26. Lifting Cages
Cage being lifted with a tipping crane

27. Lifting Cages
Fabrication over the hole, “wind chime” suspended over hole, transverse reinforcing added as longitudinal is lowered into hole

28. Practical Tips on Storing and Handling Rebar Cages
Storage
Keep it clean
Cage Pick Up:
No permanent distortion
Use of temporary stiffeners
Use of Centralizers: External & Internal
NDT Access Tubes: Straight & Parallel

29. QUESTIONS? AASHTO Subcommittee on Construction
Virginia Beach, Virginia
August 2, 2011
Barry D. Siel, P.E.
Senior Geotechnical Engineer
FHWA-RC