PURPOSES AND TYPES OF REINFORCING STEEL
Reinforced concrete was designed on the principle that steel and concrete act together in resisting force. Concrete is strong in compression but weak in tension. The tensile strength is generally rated about 10 percent of the compression strength. For this reason, concrete works well for columns and posts that are compression members in a structure. But, when it is used for tension members, such as beams, girders, foundation walls, or floors, concrete must be reinforced to attain the necessary tension strength.
Steel is the best material for reinforcing concrete because the properties of expansion for both steel and concrete are considered to be approximate] y the same; that is, under normal conditions, they will expand and contract at an almost equal rate.
NOTE: At very high temperatures, steel expands more rapidly than concrete and the two materials will separate.
Another reason steel works well as a reinforcement for concrete is because it bonds well with concrete. This bond strength is proportional to the contact surface of the steel to the concrete. In other words, the greater the surface of steel exposed to the adherence of concrete, the stronger the bond. A deformed reinforcing bar adheres better than a plain, round, or square one because it has a greater bearing surface. In fact, when plain bars of the same diameter are used instead of deformed bars, approximately 40 percent more bars must be used.
The rougher the surface of the steel, the better it adheres to concrete. Thus steel with a light, firm layer of rust is superior to clean steel; however, steel with loose or scaly rust is inferior. Loose or scaly rust can be removed from the steel by rubbing the steel with burlap or similar material. This action leaves only the firm layer of rust on the steel to adhere to the concrete.
NOTE: Reinforcing steel must be strong in tension and, at the same time, be ductile enough to be shaped or bent cold.
Reinforcing steel can be used in the form of bars or rods that are either plain or deformed or in the form of expanded metal, wire, wire fabric, or sheet metal. Each type is useful for different purposes, and engineers design structures with those purposes in mind.
Plain bars are round in cross section. They are used in concrete for special purposes, such as dowels at expansion joints, where bars must slide in a metal or paper sleeve, for contraction joints in roads and runways, and for column spirals. They are the least used of the rod type of reinforcement because they offer only smooth, even surfaces for bonding with concrete.
Deformed bars differ from the plain bars in that they have either indentations in them or ridges on them, or both, in a regular pattern. The twisted bar, for example, is made by twisting a plain, square bar cold. The spiral ridges, along the surface of the deformed bar, increase its bond strength with concrete. Other forms used are the round and square corrugated bars. These bars are formed with projections around the surface that extend into the surrounding concrete and prevent slippage. Another type is formed with longitudinal fins projecting from the surface to prevent twisting. Figure 7-1 shows a few of the types of deformed bars available. In the United States, deformed bars are used almost exclusively; while in Europe, both deformed and plain bars are used.
Figure 7-1.-Various types of deformed bars.
Eleven standard sizes of reinforcing bars are in use today. Table 7-1 lists the bar number, area in square inches, weight, and nominal diameter of the 11 standard sizes. Bars No. 3 through 11 and 14 and 18 are all deformed bars. Table 7-2 lists the bar number, area in square inches and millimeters, weight in pounds per foot as well as kilograms per meter, and nominal diameter of the 8 standard metric sizes. At various sites overseas, rebar could be procured locally and could be metric. Table 7-3 is given for comparison. Remember that bar numbers are based on the nearest number of one-eighth inch included in the nominal diameter of the bar. To measure rebar, you must measure across the round/square portion where there is no deformation. The raised portion of the deformation is not measured when measuring the rebar diameter.
Table 7-1.-U.S. Standard Reinforcing Bars
Table 7-2.-Metric Reinforcing Bars
Table 7-3.-Comparison of U.S. Customary and Metric Rebar
Reinforcing bars are hot-rolled from a variety of steels in several different strength grades. Most reinforcing bars are rolled from new steel billets, but some are rolled from used railroad-car axles or railroad rails that have been cut into rollable shapes. An assortment of strengths are available.
The American Society for Testing Materials (ASTM) has established a standard branding for deformed reinforcing bars. There are two general systems of bar branding. Both systems serve the basic purpose of identifying the marker size, type of steel, and grade of each bar. In both systems an identity mark denoting the type of steel used is branded on every bar by engraving the final roll used to produce the bars so as to leave raised symbols between the deformations. The manufacturer's identity mark that signifies the mill that rolled the bar is usually a single letter or, in some cases, a symbol. The bar size follows the manufacturer's mark and is followed by a symbol indicating new billet steel (-N-), rolled rail steel (-I-), or rolled axle steel (-A-). Figure 7-2 shows the two-grade marking system.
The lower strength reinforcing bars show only three marks: an initial representing the producing mill, bar size, and type of steel. The high strength reinforcing bars use either the continuous line system or the number system to show grade marks. In the line system, one continuous line is rolled into the 60,000 psi bars, and two continuous lines are rolled into the 75,000 psi bars. The lines must run at least five deformation spaces, as shown in figure 7-2. In the number system, a "60" is rolled into the bar following the steel type of mark to denote 60,000 psi bars, and a "75" is rolled into the 75,000 psi bars.
Expanded Metal and Wire Mesh Reinforcement
Expanded metal or wire mesh is also used for reinforcing concrete. Expanded metal is made by partly shearing a sheet of steel, as shown in view A figure 7-3. The sheet steel has been sheared in parallel
Figure 7-2.-American standard reinforcing bar marks.
lines and then pulled out or expanded to form a diamond shape between each parallel cut. Another type is square, rather than diamond shaped, as shown in view B, figure 7-3. Expanded metal is customarily y used during plastering operations and light reinforcing concrete construction, such as sidewalks and small
Figure 7-3.-Expanded or diamond mesh steel reinforcement.
concrete pads that do not have to bear substantial weight, such as transformer and air-conditioner pads.
Welded Wire Fabric
Welded wire fabric is fabricated from a series of wires arranged at right angles to each other and electrically welded at all intersections. Welded wire fabric, referred to as WWF within the NCF. has various uses in reinforced concrete construction. In building construction, it is most often used for floor slabs on well-compacted ground. Heavier fabric, supplied mainly in flat sheets, is often used in walls and for the primary reinforcement in structural floor slabs. Additional examples of its use include road and runway pavements, box culverts, and small canal linings.
Four numbers are use-d to designate the style of wire mesh; for example, 6 by 6-8 by 8 (sometimes written 6x6x8x8or6x6-W2.1xW2.1).The first number (in this case, 6) indicates the lengthwise spacing of the wire in inches; the second number (in this case, 6) indicates the crosswise spacing of the wire in inches; the last two numbers (8 by 8) indicate the size of the wire on the Washburn and Moen gauge. More recently the last two numbers are a W number that indicates the size of the cross-sectional area in the wire in hundredths of an inch. (See table 7-4.) WWF is currently available within the Navy stock system using the four-digit system, 6 by 6-8 by 8, as of this writing, but if procured through civilian sources, the W system is used.
Table 7-4-Common Stock Sizes of Welded Wire Fabric
Light fabric can be supplied in either rolls or flat sheets. Fabric made of wire heavier than W4 should always be furnished in flat sheets. Where WWF must be uniformly flat when placed, fabric furnished in rolls should not be fabricated of wire heavier than W 2.9. Fabricators furnish rolled fabric in complete rolls only. Stock rolls will contain between 700 to 1,500 square feet of fabric determined by the fabric and the producing location. The unit weight of WWF is designated in pounds per one hundred square feet of fabric (table 7-4). Five feet, six feet, seven feet, and seven feet six inches are the standard widths available for rolls, while the standard panel widths and lengths are seven feet by twenty feet and seven feet six inches by twenty feet.
Sheet-metal reinforcement is used mainly in floor slabs and in stair and roof construction. It consists of annealed sheet steel bent into grooves or corrugations about one-sixteenth inch (1.59 mm) in depth with holes punched at regular intends.
Tension in Steel
Steel bars are strong in tension. Structural grade is capable of safely carrying up to 18,000 psi and intermediate, hard, and rail steel, 20,000 psi. This is the SAFE or WORKING STRESS; the BREAKING STRESS is about triple this.
When a mild steel bar is pulled in a testing machine, it stretches a very small amount with each increment of load. In the lighter loadings, this stretch is directly proportional to the amount of load (fig. 7-4, view A). The amount is too small to be visible and can be measured only with sensitive gauges.
At some pull (known as the YIELD POINT), such as 33,000 psi for mild steel, the bar begins to neck down (fig. 7-4, view B) and continues to stretch perceptibly with no additional load.
Figure 7-4.-Tension in steel bars.
Then, when it seems the bar will snap like a rubber band it recovers strength (due to work hardening). Additional pull is required (fig. 7-4, view C) to produce additional stretch and final failure (known as the ULTIMATE STRENGTH) at about 55,000 psi for mild steel.