is better than a plain round or square one. In fact, when plain  bars  of  a  given  diameter  are  used  instead  of deformed  bars,  approximately  40  percent  more  plain bars must be used. The  adherence  of  the  concrete  depends  on  the roughness of the steel surface: the rougher the steel, the better the adherence. Thus, steel with alight, firm layer of rust is superior to clean steel, but steel with loose or scaly rust is inferior. Loose  or  scaly  rust  may  be removed from the steel by rubbing the steel with burlap. The requirements for reinforcing steel are that it be strong in tension and, at the same time, ductile enough to be shaped or bent cold. Reinforcing steel may 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 a different purpose, and engineers design structures with these purposes in mind. Plain reinforcing bars are usually round in cross section.  They  are  used  as  main  tension  reinforcement for concrete structures. They are the least used of the rod  type  of  reinforcement  because  they  offer  only smooth, even surfaces for the adherence of concrete. Reinforcing  bars  or  rods  are  commonly  referred  to  as rebars. Deformed bars are like the plain bars except 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-corrugate d 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 7-10 shows a few of the various 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. There are 11 standard sizes of reinforcing bars. Table 7-1 lists the bar numbers, weight, and nominal diameters of the 11 standard sizes. Bars No. 3 through No.  18,  inclusive,  are  deformed  bars.  Remember  that bar numbers are based on the nearest number of 1/8 in. (3. 175 mm) included in the nominal diameter of the bar. To  measure  rebar,  you  must  measure  across  the roundsquare portion where there is no deformation. Table  7-1.-Standard  Reinforcing  Bars The  raised  portion  of  the  deformation  is  not  considered in measuring the rebar diameter. BENDS.—  Frequently,   it   is   required   that reinforcing bars be bent into various shapes. There are several reasons for this. First, let us go back to the reason  for  using  reinforcing  steel  in  concrete—to increase  the  tensile  and  compressive  strength  of concrete. You might compare the hidden action within abeam from live and dead loads to breaking a stick over your knee. You have seen how the splinters next to your knee push toward the middle of the stick when you apply force, while the splinters from the middle to the opposite side pull away from the middle. This is similar to what happens inside the beam. For  instance,  take  a  simple  beam  (a  beam resting  freely  on  two  supports  near  its  ends).  The dead load (weight of the beam) causes the beam to bend  or  sag.  Now,  from  the  center  of  the  beam  to the   bottom,   the   forces   tend   to   stretch   or 7-7