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Concrete Construction

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3 major costs of concrete Formwork costs including ... shrinkage If poured same time could stress floors and beams due to shrinkage Cast in Place Floors and ... – PowerPoint PPT presentation

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Title: Concrete Construction


1
Concrete Construction
2
  • 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

3
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

4
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

5
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

6
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

7
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

8
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

9
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

10
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

11
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

12
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

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

14
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

15
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

16
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

17
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

18
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

19
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
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