SECTION III. REINFORCING STEEL
GRADES, DESIGNATIONS, AND METHODS
6-19. The main types of reinforcing steel are vertical and horizontal bars (see Figure 6-4) wire for tying columns (see Figure 6-6), and wire mesh for shrinkage and contraction reinforcement of slabs and walls. Bars are available in 11 sizes designated by numbers (see Table 6-1) that range in size from 3/8 inch to about 2 1/4 inches in diameter. Reinforcing steel is specified in ASTM A615, A616, and A617. Minimum-yield strengths are: 40,000 psi, 50,000 psi, 60,000 psi, and 75,000 psi.
Table 6-1. Standard steel reinforcing bar
6-20. The engineer specifies the sizes, amounts, and classifications of reinforcing steel to be used. Wire mesh is cold-drawn from hard-grade steel; but the steel in reinforcing bars is ductile so that you can generally cold-form hooks of relatively small radius on the job without breakage. However, the improved deformed bars (rebars) that conform to ASTM specifications are so superior in bonding value that hooking their ends does not add much strength. The latest ASTM specifications require permanent rolled-on identification because so many grades of steel reinforcing bars have the same deformation pattern. The two adopted grade-marking systems are shown in Figure 6-7.
Continuous Line System. The bars have a small, continuous, longitudinal grade line running between the main longitudinal ribs to identify 60,000 psi.
Number System. A rolled-on grade number follows the producing mill, bar size, and steel type symbols. The number 40 identifies 40,000 psi, the number 50 identifies 50,000 psi, and the number 60 identifies 60,000 psi.
Figure 6-7. Reinforcing bar grade-marking systems
BOLSTERS OR HIGH CHAIRS
6-21. Bolsters or high chairs (see Figure 6-5) are available in many heights to fit almost any requirement. They support the reinforcing steel and hold it a specified distance away from the exterior concrete shell poured around them, called protective concrete. Table 6-2 gives the minimum concrete cover that must be provided for different types of reinforcement under varying conditions. Cover must be provided for the bottom, sides, and tops of rebar.
Table 6-2. Minimum concrete cover requirements for steel reinforcement
6-22. Stirrups (see Figure 6-4) help bent bars to resist diagonal tension, as well as to reinforce the beam web to keep cracks from spreading. At least one stirrup must cross every potential diagonal tension crack. Vertical stirrups pass underneath the bottom steel and are perpendicular to it to prevent lateral slippage. They are one of the most practical methods of web reinforcement because they are easy to arrange and set in the forms with the other bars. Vertical stirrups are anchored in different ways: by welding to or hooking tightly around longitudinal reinforcement or by embedding them sufficiently above a beam's midline so that they develop the required bond. Hooks should be fabricated to meet dimensional requirements stated in Table 6-3 below. Welded stirrups are more efficient than vertical ones, because they can be welded at any angle parallel to the diagonal stress. However, practical considerations offset this advantage. Inclined stirrups must be welded to longitudinal reinforcements on-site to prevent slippage and displacement during concrete placement. This work is not only expensive, it is somewhat troublesome.
6-23. Reinforcing bars are available only in certain lengths; splice them together for longer runs. A common way to splice bars is to lap them. If one bar is not long enough for the span, never butt reinforcing bars.
6-24. Lapping the bars (see Figure 6-8 below) allows bond stress to transfer the load from one bar to a second bar. Although you could hook the bars, it is not always practical nor even desirable to bend them. Engineering design dictates the actual length of the lap after considering the anticipated beam stresses, but the length is about 24 to 36 bar diameters, depending on bar size. Table 6-4 below lists the recommended lap for particular bar designation numbers. Note that the minimum lap for plain bars is twice that for deformed bars. Except as shown on plans, do not splice steel reinforcement without the approval of an engineer, and never splice bars at the points of maximum bending. It is usually best to locate splices beyond the center of the beam. When possible, stagger the splices so that they all do not fall at the same point. Every bar requires at least two supports with bolsters spaced at about 5-foot intervals.
Figure 6-8. Method of splicing reinforcing bars
Table 6-4. Minimum splice overlap
6-25. The splicing method shown in view 1 of Figure 6-8 above is satisfactory when bar spacing is large. Do not use this method:
Lapping bars in a horizontal plane, shown in view 2 of Figure 6-8, is the most practical arrangement, if the spacing provides enough clearance for the aggregate to pass. Both this method and the one shown in view 3 of Figure 6-8 facilitate tying the bars to hold them in position during concreting. However, tying does not add much to splice strength and creates possible problems of air pockets and poor bond in the space under the bar overlap. Although lapping bars in a vertical plane, shown in view 3 of Figure 6-8, permits better encasement, the top bars will not fit the stirrups properly, and the beam will have a smaller effective depth at one location than at another. In practice, these bars would probably be locked into the positions shown in view 2 of Figure 6-8.
6-26. Care must be taken to prevent reinforcing steel from rusting too much during outdoor storage. Before storing outdoors, remove any objectionable coating from the steel, particularly heavy corrosion caused by previous outdoor storage. Other such coatings include oil, paint, grease, dried mud, and weak dried mortar. If the mortar is difficult to remove, leave it. A thin film of rust or mill scale is not seriously objectionable either; in fact, it can increase the bond between the steel and concrete. Remove all loose rust or scale by rubbing the steel with burlap or other similar means. Remember, anything that destroys the bond between the concrete and the steel can create serious problems by preventing the stress in the steel from performing its function properly. After cleaning, cover the steel to protect it from weather.
6-27. Large numbers of reinforcing bars can be prefabricated on the job to the varied lengths and shapes shown on the drawings. You can usually cold-bend stirrups and column ties that are less than 1/2 inch in diameter, as well as steel bars no larger than 3/8 inch in diameter. Heating is not usually necessary, except for bars more than 1 1/8 inch in diameter. Make bends around pins whose diameters are not less than six times the bar diameter (see Table 6-1), except for hooks and straight bars larger than 1 inch in diameter. For these bar diameters, the minimum bending pin diameter should be eight times the bar size. If possible, it is better to bend steel bars greater than 3/8 inch in diameter on a bar-bending machine; use a hickey (a lever for bending bars) or a bar-bending table like the one shown in Figure 6-9. However, be aware that the bends these devices make are usually too sharp and weaken the bar. It is possible to improvise a hickey by attaching a 2- by 1 1/2- by 2-inch pipe tee section to the end of a 1 1/4-inch pipe lever that is 3 feet long, then sawing a section from one side of the tee.
Figure 6-9. Bar bending table
6-28. All steel reinforcement must be accurately located in the forms and held firmly in place both before and during concreting. To do this, use built-in concrete blocks, metal bolsters and chairs, spacer bars, wires, or other devices that prevent steel displacement during construction, as well as retain it at the proper distance from the forms.
6-29. Be sure to use enough supports and spacers to hold the steel firmly, even when subjected to construction loads. Never use rocks, wood blocks, or other unapproved supports. Wood blocks deteriorate, eventually creating a pathway for water to penetrate and degrade reinforcing rebar. Support horizontal bars at minimum intervals of 5 or 6 feet, and secure all bars to supports and other bars using tie wires not smaller than 18 gage. Twisted-tie ends should project away from an interior surface to avoid contact with the concrete surface.
6-30. Some specifications state that no metal be left in concrete within a certain distance of the surface. Generally, the minimum clearance between parallel bars in beams, footings, walls, and floor slabs is not less than 1 1/3 times the largest size aggregate particle in the concrete, but in no case less than 1 inch. To meet such specifications, make spacer and supporting blocks from mortar having the same consistency as the concrete, but no CA. Spacer blocks are usually 1 1/2 inch square or larger, varying in length as required. Cast tie wires into the blocks to secure them to the reinforcing bars. Do not remove this type of spacer block when placing the concrete.
6-31. The minimum clearance between parallel bars in columns is not less than 1 1/2 times the bar diameter. First, tie the column steel together and position it as a unit. Then erect the column form around the unit and tie the reinforcing steel to the form at 5-foot intervals. Minimum clear space between reinforcing steel and concrete forms must also be checked with the MSA in the concrete mix design. The aggregate must not become lodged between the form and reinforcing steel, preventing passage by any further concrete.
6-32. Figure 6-10 shows a typical arrangement of reinforcing steel in a floor slab. The required thickness of the concrete protective cover determines the height of the bolster. Instead of the bolster, you can use spacer blocks made from sand-cement mortar as described above. Hold the bars firmly in place by tying the intersections together at frequent intervals with one turn of wire.
Figure 6-10. Reinforcing steel arrangement for a floor slab
6-33. Figure 6-11 shows the position of bolsters and stirrups with the reinforcing bars in a reinforced-concrete beam. Note that the stirrups pass under the main reinforcing bars and are tied to them with one turn of wire.
Figure 6-11. Beam reinforcing steel
WALLS AND FOOTINGS
6-34. Unless you are using wire-mesh fabric, erect the reinforcing steel for concrete walls in place. Do not preassemble it as you do for columns. Use ties between the bars, from top to bottom, for high walls (see view 1 of Figure 6-12). You can remove the wood blocks after you fill the forms to block level. Place reinforcing steel for footings as shown in view 3 of Figure 6-12. Use spacer bars to support the steel above the subgrade to the proper distance. Welded-wire fabric (see view 2 of Figure 6-12) is also used as limited reinforcement for concrete footings, walls, and slabs, but its primary use is to control crack widths due to temperature changes.
Figure 6-12. Wall and footing reinforcement
|David L. Heiserman, Editor||
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Revised: June 06, 2015