Fabrication of rebars consists of cutting to length and required bending. The preparation of field placing drawings and bar lists is termed detailing. Ordinarily, the rebar supplier details, fabricates, and delivers to the site, as required. In the farwestern states, the rebar supplier also ordinarily places the bars, In the New York City area, fabrication is performed on the site by the same (union) workers who place the reinforcement. (See ‘‘Details and Detailing of Concrete Reinforcement,’’ ACI 315).
Standard Hooks. The geometry and dimensions of standard hooks that conform to the ACI 318 Building Code and industry practice are shown in Table 9.7.
Fabrication Tolerances. These are covered in ‘‘Standard Specifications for Tolerances for Concrete Construction and Materials,’’ ACI 117.
Shipping Limitations. Shipping widths or loading limits for a single bent bar and an L-shaped bar are shown in Fig. 9.7. Bundles of bars occupy greater space. The limit of 7 ft 4 in has been established as an industry practice to limit the bundle size to an 8-ft maximum load width. (‘‘Manual of Standard Practice,’’ Concrete Reinforcing Steel Institute.)
Erection. For construction on small sites, such as high-rise buildings in metropolitan areas, delivery of materials is a major problem. Reinforcement required for each area to be concreted at one time is usually delivered separately. Usually, the only available space for storage of this reinforcing steel is the formwork in place.
Under such conditions, unloading time becomes important.
The bars for each detail length, bar size, or mark number are wired into bundles for delivery. A lift may consist of one or more bundles grouped together for loading or unloading. The maximum weight of a single lift for unloading is set by the jobsite crane capacity. The maximum weight of a shop lift for loading is usually far larger, and so shop lifts may consist of several separately bundled field lifts.
Regional practices and site conditions establish the maximum weight of bundles and lifts. Where site storage is provided, the most economical unloading without an immediately available crane is by dumping or rolling bundles off the side. Unloading arrangements should be agreed on in advance, so that loading can be carried out in the proper order and bars bundled appropriately. Care must be exercised during the unloading and handling of epoxy-coated rebars to minimize damage to the coating. (‘‘Placing Reinforcing Bars,’’ CRSI.)
Placement Tolerances. The ACI 318 Building Code prescribes rebar placement tolerances applicable simultaneously to effective depth d and to concrete cover in all flexural members, walls, and columns as follows:
Where d in 8 in or less, +-3⁄8 in; more than 8 in +-1⁄2 in. The tolerance for the clear distance to formed soffits is -1⁄4 in. These tolerances may not reduce cover more than one-third of that specified. For additional information on tolerances, see ‘‘Standard Specifications for Tolerances for Concrete Construction and Materials,’’ ACI 117.
Bundling. Rebars may be placed in concrete members singly or in bundles (up to four No. 11 or smaller bars per bundle). This practice reduces rebar congestion or the need for several layers of single, parallel bars in girders. For columns, it eliminates many interior ties and permits use of No. 11 or smaller bars where small quantities of No. 14 or No. 18 bars are not readily available.
Only straight bars should be bundled ordinarily. Exceptions are bars with end hooks, usually at staggered locations, so that the bars are not bent as a bundle (‘‘Placing Reinforcing Bars,’’ CRSI).
A bundle is assembled by wiring the separate bars tightly in contact. If they are preassembled, placement in forms of long bundles requires a crane. Because cutoffs or splices of bars within a bundle must be staggered, it will often be necessary to form the bundle in place.
Bending and Welding Limitations. The ACI 318 Building Code contains the following restrictions:
All bars must be bent without heating, except as permitted by the engineer.
Bars partly embedded in hardened concrete may not be bent without permission of the engineer.
No welding of crossing bars (tack welding) is permitted without the approval of the engineer.
For unusual bends, heating may be permitted because bars bend more easily when heated. If not embedded in thin sections of concrete, heating the bars to a maximum temperature of 1500F facilitates bending, usually without damage to the bars or splitting of the concrete. If partly embedded bars are to be bent, heating controlled within these limits, plus the provision of a round fulcrum for the bend to avoid a sharp kink in the bar, are essential.
Tack welding creates a metallurgical notch effect, seriously weakening the bars.
If different size bars are tacked together, the notch effect is aggravated in the larger bar. Tack welding therefore should never be permitted at a point where bars are to be fully stressed, and never for the assembly of ties or spirals to column verticals or stirrups to main beam bars.
When large, preassembled reinforcement units are desired, the engineer can plan the tack welding necessary as a supplement to wire ties at points of low stress or to added bars not required in the design.