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PRESTRESSED  CONCRETE A  prestressed  concrete  unit  is  one  in  which engineered  stresses  have  been  placed  before  it  has been  subjected  to  a  load.  When  PRETENSION- ING   is   used,   the   reinforcement   (high-tensile- strength  steel  strands)  is  stretched  through  the form between the two end abutments or anchors. A  predetermined  amount  of  stress  is  applied  to the  steel  strands.  The  concrete  is  then  poured, encasing the reinforcement. As the concrete sets, it  bonds  to  the  pretensioned  steel.  When  it  has reached a specified strength, the tension on the reinforcement  is  released.  This  prestresses  the concrete,  putting  it  under  compression,  thus creating a built-in tensile strength. POST-TENSIONING   involves   a   precast member  that  contains  normal  reinforcing  in addition to a number of channels through which the  prestressing  cables  or  rods  maybe  passed.  The channels   are   usually   formed   by   suspending inflated tubes through the form and casting the concrete  around  them.  When  the  concrete  has  set, the  tubes  are  deflated  and  removed.  Once  the concrete  has  reached  a  specified  strength, prestressing steel strands or TENDONS are pulled into the channels and secured at one end. They are  then  stressed  from  the  opposite  end  with  a portable  hydraulic  jack  and  anchored  by  one  of several  automatic  gripping  devices. Post-tensioning   may   be   done   where   the member is poured or at the jobsite. Each member may be tensioned, or two or more members may be tensioned together after erection. In general, post-tensioning is used if the unit is over 45 ft long or  over  7  tons  in  weight.  However,  some  types of  pretensioned  roof  slabs  will  be  considerably longer and heavier than this. When  a  beam  is  prestressed,  either  by  pre- tensioning or post-tensioning, the tensioned steel produces a high compression in the lower part of the  beam.  This  compression  creates  an  upward bow  or  camber  in  the  beam  (fig.  7-19).  When  a load is placed on the beam, the camber is forced out,  creating  a  level  beam  with  no  deflection. Those members that are relatively small or that can  be  readily  precast  are  normally  pretensioned. These  include  precast  roof  slabs,  T-slabs,  floor slabs,  and  roof  joists. SPECIAL  TYPES  OF  CONCRETE Special types of concrete are essentially those with  unique  physical  properties  or  those  produced with   unusual   techniques   and/or   reproduction processes.  Many  special  types  of  concrete  are made  with  portland  cement  as  a  binding  medium; some  use  binders  other  than  portland  cement. Lightweight  Concrete Conventional concrete weighs approximately 150  lb  per  cubic  foot.  Lightweight  concrete  weighs 20  to  130  lb  per  cubic  foot,  depending  on  its intended use. Lightweight concrete can be made by  using  either  gas-generating  chemicals  or Figure  7-19.-Comparison  of  plain  and  prestressed  concrete  beams. 7 - 14

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