Metal-wood   forms   are   just   like   metal   forms except for the face. It is made with a sheet of B-grade exterior plywood with waterproof glue. Wood Wooden forms are by far the most common type used   in   building   construction. They   have   the advantage  of  economy,  ease  in  handling,  ease  of production,  and  adaptability  to  many  desired  shapes. Added  economy  may  result  from  reusing  form  lumber later  for  roofing,  bracing,  or  similar  purposes. Lumber should be straight, structurally sound, strong, and only partially seasoned. Kiln-dried timber has a tendency to swell when soaked with water from the concrete. If the boards are tight-jointed, the swelling will   cause   bulging   and   distortion. When   green lumber  is  used,  an  allowance  should  be  made  for shrinkage, or the forms should be kept wet until the concrete is in place. Soft woods, such as pine, fir, and spruce,  make  the  best  and  most  economical  form lumber since they are light, easy to work with, and available  in  almost  every  region. Lumber   that   comes   in   contact   with   concrete should be surfaced at least on one side and both edges. The surfaced side is turned toward the concrete. The edges of the lumber may be square, shiplap, or tongue and  groove.  The  latter  makes  a  more  watertight  joint and tends to prevent warping. Plywood can be used economically for wall and floor forms if it is made with waterproof glue and is identified for use in concrete forms. Plywood is more warp  resistant  and  can  be  reused  more  often  than lumber.  Plywood  is  made  in  1/4-,  3/8-,  1/2-,  9/16-, 5/8- and 3/4-inch thicknesses and in widths up to 48 inches.  Although  longer  lengths  are  manufactured, 8-foot  lengths  are  the  most  common.  The  5/8-  and 3/4-inch  thicknesses  are  most  economical;  thinner sections require additional solid backing to prevent bulging.  However,  the  1/4-inch  thickness  is  useful  for forming  curved  surfaces. Fiber Fiber forms are prefabricated from impregnated waterproofed  cardboard  and  other  fiber  materials. Successive  layers  of  fiber  are  first  glued  together  and then  molded  in  the  desired  shape.  Fiber  forms  are ideal   for   round   concrete   columns   and   other applications  where  preformed  shapes  are  feasible since  they  require  no  form  fabrication  at  the  job  site. This  saves  considerable  time  and  money. Fabric Fabric  forming  is  made  of  two  layers  of  nylon fabric. These layers are woven together, forming an envelope.  Structural  mortar  is  injected  into  these envelopes,    forming    nylon-encased    concrete “pillows.” These are used to protect the shorelines of waterways,  lakes  and  reservoirs,  and  as  drainage channel  linings. Fabric forming offers exceptional advantages in the  structural  restoration  of  bearing  piles  under waterfront structures. A fabric sleeve with a zipper closure is suspended around the pile to be repaired, and mortar is pumped into the sleeve. This forms a strong  concrete  jacket. FORM DESIGN Forms  for  concrete  construction  must  support  the plastic concrete until it has hardened. Stiffness is an important feature in forms. Failure to provide form stiffness may cause unfortunate results. Forms must be designed for all the weight to which they are likely to  be  subjected.  This  includes  the  dead  load  of  the forms,  the  plastic  concrete  in  the  forms,  the  weight  of the workmen, the weight of equipment and materials, and the impact due to vibration. These factors vary with each project, but none should be ignored. The ease  of  erection  and  removal  is  also  an  important factor  in  the  economical  design  of  forms.  Platform and  ramp  structures  independent  of  formwork  are sometimes preferred to avoid displacement of forms due to loading and impact shock from workmen and equipment. When concrete is placed in forms, it is in a plastic state and exerts hydrostatic pressure on the forms. The basis of form design, therefore, is the maximum pressure developed by concrete during placing. The maximum  pressure  developed  depends  on  the  placing rate  and  the  temperature.  The  rate  at  which  concrete is placed affects the pressure because it determines how  much  hydrostatic  head  builds  up  in  the  form. The hydrostatic head continues to increase until the concrete  takes  its  initial  set,  usually  in  about  90 minutes. At low temperatures, however, the initial set takes   place   much   more   slowly.   This   makes   it necessary to consider the temperature at the time of 7-2