lightweight aggregates, such as expanded shale, clay, or slag. Concrete containing aggregates like perlite or vermiculite is very light in weight and is  primarily  used  as  insulating  material.  Light- weight concrete is usually classified according to its  weight  per  cubic  foot. Semi-lightweight  concrete  has  a  unit  weight  of 115  to  130  lb  per  cubic  foot  and  an  ultimate compressive   strength   comparable   to   normal concrete.  Sand  of  normal  weight  is  substituted partially  or  completely  for  the  lightweight  fine aggregate. Insulating  lightweight  concrete  has  a  unit weight  ranging  from  20  to  70  lb  per  cubic  foot, and   its   compressive   strength   seldom   exceeds 1,000   psi.   This   type   of   concrete   is   generally used  for  insulating  applications,  such  as  fire- proofing. Structural  lightweight  concrete  has  a  unit weight up to 115 lb per cubic foot and a 28-day compressive strength in excess of 2,000 psi. This type  is  used  primarily  to  reduce  the  dead-load weight  in  concrete  structural  members,  such  as floors,  walls,  and  the  roof  section  in  high-rise structures. Heavyweight  Concrete Heavyweight  concrete  is  produced  with  special heavy  aggregates  and  has  a  density  of  up  to 400 lb per cubic foot. This type is used principally for  radiation  shielding,  for  counterweights,  and for  other  applications  where  higher  density  is desired.  Except  for  density,  the  physical  properties of  heavyweight  concrete  are  similar  to  those  of normal-  or  conventional-weight  concrete. TILT-UP  CONSTRUCTION Tilt-up concrete construction is a special form of  precast  concrete  building.  This  method  consists basically  of  jobsite  prefabrication,  in  which  the walls  are  cast  in  a  horizontal  position,  tilted  to a  vertical  position,  and  then  secured  in  place. Tilt-up construction is best suited for large one- story buildings, but it can be used in multistory structures. Usually, multistory structures are built by setting the walls for the first story, placing the floor  above,  then  repeating  the  procedure  for  each succeeding floor. An alternate method is to cast two-  to  four-story  panels. The wall panels are usually cast on the floor slab of the structure. Care must be exercised to ensure the floor slab is smooth and level and that all  openings  for  pipes  and  other  utilities  are temporarily plugged. The casting surface is treated with  a  good  bond-breaking  agent  to  ensure  the panel  does  not  adhere  when  it  is  lifted. Reinforcement  of  Tilt-Up  Panels The steel in a tilt-up panel is set in the same manner as it is in a floor slab. Mats of reinforce- ment  are  placed  on  chairs  and  tied  as  needed. Reinforcement  should  be  as  near  the  center  of  the panel  as  possible.  Reinforcing  bars  are  run through  the  side  forms  of  the  panel.  When welded  wire  fabric  or  expanded  wire  mesh  is used, dowel bars are used to tie the panels and their   vertical   supports   together.   Additional reinforcement   is   generally   needed   around openings. The panel is picked up or tilted by the use of PICKUP  INSERTS.  These  inserts  are  tied  into the reinforcement. As the panel is raised into its vertical position, the maximum stress will occur; therefore,  the  location  and  number  of  pickup inserts is extremely important. Some engineering manuals  provide  information  on  inserts,  their locations,  and  capacities. Tilt-Up  Panel  Foundations An  economical  and  widely  used  method  to support  tilt-up  panels  is  a  simple  pad  footing.  The floor  slab,  which  is  constructed  first,  is  NOT poured to the perimeter of the building to permit excavating  and  pouring  the  footings.  After  the panel  is  placed  on  the  footing,  the  floor  slab  is completed.  It  may  be  connected  directly  to  the outside wall panel, or a trench may be left to run mechanical,  electrical,  or  plumbing  lines. Another method that is commonly used, as an alternative, is to set the panels on a grade beam or  foundation  wall  at  floor  level.  Regardless  of the type of footing, the panel should be set into a mortar bed to ensure a good bond between the foundation  wall  and  the  panel. Panel Connections The panels may be tied together in a variety of  ways.  The  location  and  use  of  the  structure  will dictate what method can or can NOT be used. The strongest method is a cast-in-place column with the panel-reinforcing steel tied into the column. However, this does NOT allow for expansion and contraction. It may be preferable to tie only the corner  panels  to  the  columns  and  allow  the remaining  panels  to  move. 7-15