Concrete Construction

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Concrete Construction
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• 3 major costs of concrete
• Formwork costs including labor, equipment, and
  materials
• Cost of reinforcing steel and placement
• Concrete materials, equipment, and labor for
  placing, curing, and finishing
• F11-1

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Cast in Place

• Traditional method
• Allows concrete to be formed to site conditions
• Used on most construction sites for structural
  members
• Concrete placed in forms to cure
• Forms removed after enough strength in concrete
  to support self and additional construction loads

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Cast in Place

• Walls and wall footings
• Need to avoid voids and separation of materials
  due to large fall
• Special care for areas under windows , pipe
  chases and other formwork installed inside wall
  forms
• Can pump concrete from bottom of form to get rid
  of voids

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Cast in Place

• Floors and roofs
• One way slab only supported in one direction F
  11-3
• Slab and beam
• Slab, beam girder
• Outside beam is called spandrel beam
• Columns poured before slabs shrinkage
• If poured same time could stress floors and beams
  due to shrinkage

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Cast in Place

• Floors and roofs
• One way slab only supported in one direction F
  11-3
• Slab and beam
• Slab, beam girder
• Outside beam is called spandrel beam
• Two directional slabs (waffles) provide support
  in two perpendicular directions
• supported by columns

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Cast in Place

• 2 way
• Basically joists in both directions
• Flat slabs
• Supported directly by columns
• Uses column capitals to distribute load over
  larger area
• More reinforcement needed to carry loads to
  columns
• F 11-6

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Precast concrete

• Cast into desired shape at plant and moved to
  site
• Controlled environment -gt better finish and
  quality
• All prestressed and pretensioned members are
  precast
• F 11-7, 11-8

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Precast concrete

• Types
• Joists and purlins F 11-7
• Roof and floor panels
• Flat 1-4 thick, 15 32 wide, 4-10 long
• hollow core 4 12 thick, 4-8 wide, 15 50
  long
• tee, double tee 4-12 wide span 12 100 feet
• channel slabs 2 5 wide, 15-50 long
• F11-8

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Precast concrete

• Types
• Walls
• Usually curtain wall construction panels fit
  between structural components to form wall
• But used in tilt up construction
• Panels cast horizontally on existing slab and
  tilted upright
• F 11-9

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Prestressed Concrete

• Initial compression load applied to concrete
• Places entire beam in compression
• Makes beam stronger since more force is required
  to induce a tension component
• Reduces deflection
• F11-10

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Prestressed Concrete

• 2 ways to do
• Pretension place prestressing material in
  tension in the form while concrete is poured
• Once concrete hardened remove tension
• Bond between steel and concrete keeps steel in
  place
• Post tensioning steel is placed inside a
  plastic tube during concrete pouring
• Upon placement steel is tensioned and the steel
  is mechanically anchored to the concrete at each
  end
• Load is removed and steel cut flush

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Architectural Concrete

• Appearance effects
• Shape, size, texture, and color
• F 11-12 11-13

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Concrete Construction Practices

• Transporting and Hauling
• Need to avoid segregation of materials in
  concrete
• Wheelbarrows, buggies, chutes, pumps, conveyors,
  buckets, trucks
• Placing and consolidating
• Need to make sure form surfaces and bracing is
  right before placing concrete

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Placing

• Forms must be coated in oil to allow removal
• If placing concrete on ground must moisten
  ground
• If pouring on top of cured concrete must place
  bonding agent between pours
• Shotcrete (gunite) concrete placed
  pneumatically
• Consolidation removing air voids
• vibration

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Finishing Curing

• Finishing bring surface of concrete to its
  final position and surface texture
• Screeding striking off excess concrete
• Floating smoothes and compacts concrete imbeds
  aggregates
• Troweling compacts surface F 11-16
• Brooming surface texture

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Curing

• To get good concrete must cure properly
• Moisture and temperature are key
• Moisture wet straw or burlap, curing compounds
• Vacuum dewatering mat placed on concrete
• Vacuum applied to mat takes out excess water
• Lower W/C ratio -gt denser mix

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Hot weather Concreting

• Curing accelerates when concrete temp above 50
  60 F
• How to lower temp
• Use cold water in mix
• Cool aggregates before mixing
• Use Type IV (low heat cement)
• Add a retarder
• Decrease max time to discharge to 1 hour

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Cold weather Concreting

• Concrete should not freeze for 1st 24 hours
• Min temp for placement is 50F
• Use type III (hi early strength)
• Use accelerator
• Heat water and aggregates before mixing
• Use vented heaters to keep concrete warm