Location for Reinforcing Steel
The proper location for reinforcing bars is given on the drawings. To ensure that the structure can withstand the loads it must carry, place the steel in exactly the position shown. Secure the bars in position so that they will not move when the concrete is placed. Accomplish this by using the reinforcing bar supports shown in Figures 43, 44, and 45.
Footings and other principal structural members that are against the ground should have at least 3 inches of concrete between steel and ground.
CAUTION
If the concrete surface is to be in contact with the ground or exposed to the weather after removal of the forms, the protective covering of concrete over the steel should be 2 inches for bars larger than No. 5 and 1 1/2 inches for bars No. 5 or smaller. The protective covering may be reduced to 1 1/2 inches for beams and columns and 3/4 inch for slabs and interior wall surfaces, but it should be 2 inches for all exterior wall surfaces.
The clear distance between parallel bars in beams, footings, walls, and floor slabs should be a minimum of 1 inch, or one and one third times the largest size aggregate particle in the concrete. In columns, the clear distance between parallel bars should be a minimum of one and one half times the bar diameter, one and one half times the maximum size of the coarse aggregate, or not less than 1 1/2 inches.
The support for reinforcing steel in floor slabs is shown in Figure 46. The height of the slab bolster is determined by the concrete protective cover required. Concrete blocks made of sand-cement mortar can be used in place of the slab bolster. Never use wood blocks for this purpose. Bar chairs, like those shown in Figure 46, are available from commercial sources in heights up to 6 inches. If you require a height greater than 6 inches, make the chair of No. 0 soft annealed iron wire. Tie the bars together at frequent intervals with a snap tie to hold them firmly in position.
Steel for column ties can be assembled into cages by laying the vertical bars for one side of the column horizontally across a couple of sawhorses. Slip the proper numbers of ties over the bars, add the remaining vertical bars, and then space out the ties as required by the placing plans. Wire a sufficient number of intersections together to make the assembly rigid. This allows you to hoist and set it as a unit.
After raising the column form, tie it to the dowels or reinforcing steel carried up from below. This holds it firmly in position at the base. Erect the column form and tie the reinforcing steel to the column form at 5 foot intervals, as shown in Figure 47.
The use of metal supports to hold beam reinforcing steel in position is shown in Figure 48. Note the position of the beam bolster. The stirrups are tied to the main reinforcing steel with a snap tie. Whenever possible, assemble the stirrups and main reinforcing steel outside the form and then place the assembled unit in position.
Placement of steel in walls is the same as for columns except that the steel is erected in place and not preassembled. Tie horizontal steel to vertical steel at least three times in any bar length. Steel in place in a wall is shown in Figure 49. Remove the wood block when the form has been filled up to the level of the block. For high walls, use ties in between the top and bottom.
Steel is placed in footings very much as it is placed in floor slabs. You may use stones, rather than steel supports, to support the steel at the proper distance above the subgrade. Steel mats are generally preassembled and placed in small footings after the forms have been set. A typical arrangement is shown in Figure 50. Steel mats in large footings are generally constructed in place.
Welded wire fabric (WWF) as shown in Figure 51 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. Form construction for each job has its peculiarities. Certain natural conditions prevail in all situations. Wet concrete always develops hydrostatic pressure and strain on forms. Be sure to properly secure all stakes, braces, walers, ties, and shebolts before placing concrete.
Splicing Reinforcing Bar
Because rebar is available only in certain lengths, it must be spliced together for longer runs. Where splices are not dimensioned on the drawings, lap the bars not less than 30 times the bar diameter, or not less than 12 inches, whichever is more.
The stress in a tension bar can be transmitted through the concrete and into another adjoining bar by a lap splice of proper length. The lap is expressed as the number of bar diameters. If the bar is No. 2, make the lap at least 12 inches. Tie the bars together with a snap tie as shown in Figure 52.
