Concrete is strong under compression, but relatively weak under tension. The reverse is true for steel. Therefore, when the two are combined, one makes up for the deficiency of the other. When steel is embedded in concrete in a manner that assists it in carrying imposed loads, the combination is known as reinforced concrete. The steel may consist of welded wire fabric or expanded metal mesh, but, more often, it consists of reinforcing bars, or more commonly "rebar."
WELDED WIRE FABRIC
Welded wire fabric, often referred to as "wire mesh," comes in rolls and sheets. These must be cut to tit your individual application. The individual sections of fabric must be tied together, or "lapped," to form a continuous sheet of fabric.
Specifications and designs are usually used when wire fabric is being lapped. However, as a rule of thumb, one complete lap is usually sufficient with a minimum of 2 inches between laps. Whenever the rule of thumb is not allowed, use the end lap or side lap method.
In the end lap method, the wire mesh is lapped by overlapping one full mesh measured from the end of the longitudinal wires in one piece to the end of longitudinal wires in the adjacent piece. The two pieces are then tied at 1 1/2-foot centers with a snap tie. In the side lap method, the two longitudinal side wires are placed one alongside and overlapping the other and then are tied with a snap tie every 3 feet.
Before placing reinforcing steel in forms, all form oiling should be completed. As mentioned earlier, oil or other coating should not contact the reinforcing steel in the formwork. Oil on reinforcing bars reduces the bond between the bars and the concrete. Use a piece of burlap to clean the bars of rust, scale, grease, mud, or other foreign matter. A light film of rust or mill scale is not objectionable.
Rebars must be tied together for the bars to remain in a desired arrangement during pouring. Tying is also a means of keeping laps or splices in place. Laps allow bond stress to transfer the load from one bar, first into the concrete and then into the second bar.
Methods of Tying
Several types of ties can be used with rebar. Some are more effective than others. The views in figure 7-20 illustrate the six types used by the Seabees: (A) snap, or simple, tie, (B) wall tie, (C) double-strand tie, (D) saddle tie, (E) saddle tie with twist, and (F) cross, or figure-eight, tie. As a Builder, you will probably be concerned only with the snap and saddle ties. However, as a professional, you should be familiar with all six types.
Figure 7-20.-Types of ties.
Figure 7-21.-Carrying tie wire.
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. This can be accomplished by using the reinforcing bar supports shown in figures 7-22, 7-23, and 7-24.
Figure 7-22.-Devices used to support horizontal reinforcing.
Footings and other principal structural members that are against the ground should have at least 3 inches of concrete between steel and ground. 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 No. 5 or smaller. The protective covering maybe 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 7-25. 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. Wood blocks should never be used for this purpose if there is any possibility the concrete might become wet and if the construction is of a permanent type. Bar chairs, like those shown in figure 7-25, are available from commercial sources in heights up to 6 inches. If a height greater than 6 inches is required, 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.
Figure 7-25.-Reinforcing steel for a floor slab.
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. The proper number of ties is slipped over the bars, the remaining vertical bars are added, and then the ties are spaced out as required by the placing plans. A sufficient number of intersections are wired together to make the assembly rigid. This allows it to be hoisted and set as a unit.
After the column form is raised, it is tied to the dowels or reinforcing steel carried up from below. This holds it firmly in position at the base. The column form is erected, and the reinforcing steel is tied to the column form at 5-foot intervals, as shown in figure 7-26.
Figure 7-26.-Securing a column with reinforcing steel against displacement.
The use of metal supports to hold beam-reinforcing steel in position is shown in figure 7-2 7. Note the position of the beam bolster. The stirrups are tied to the main reinforcing steel with a snap tie. Whenever possible, you should assemble the stirrups and main reinforcing steel outside the form and then place the assembled unit in position. Wood blocks should be substituted for the metal supports only if there is no possibility of the concrete becoming wet or if the construction is known to be temporary. Precast concrete blocks, as shown in figure 7-23, may be substituted for metal supports or, if none of the types of bar supports described above seem suitable, the method shown in figure 7-24 may be used.
Figure 7-27.-Beam-reinforcing steel supported on beam bolsters.
Placement of steel in walls is the same as for columns except that the steel is erected in place and not preassembled. Horizontal steel is tied to vertical steel at least three times in any bar length. Steel in place in a wall is shown in figure 7-28. The wood block is removed when the form has been filled up to the level of the block. For high walls, ties in between the top and bottom should be used.
Figure 7-28.-Steel in place in a wall.
Steel is placed in footings very much as it is placed in floor slabs. Stones, rather than steel supports, may be used 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 7-29. Steel mats in large footings are generally constructed in place.
Figure 7-29.-Steel in place in a footing.
Welded wire fabric (figure 7-30) 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 7-30.-Welded wire mesh fabric.
Form construction for each job has its peculiarities. However, certain natural conditions prevail in all situations. Wet concrete always develops hydrostatic pressure and strain on forms. Therefore, all stakes, braces, walers, ties, and shebolts should be properly secured 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, the bars should be lapped not less than 30 times the bar diameter, or not less than 12 inches.
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 (figure 7-31).
Figure 7-31.-Bars spliced by lapping.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015