Cast in Place 2 way Flat slabs Basically joists in both directions 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 F11-8

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 F11-10

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 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