Concrete Construction 3 major costs of concrete Formwork

Concrete Construction

• 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 – F 11 -1

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

Cast in Place • Walls and wall footings – Need to avoids 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

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

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

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

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

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 – F 11 -8

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

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 • F 11 -10

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

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

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

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

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

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 -> denser mix

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

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