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Precast Structural Components

July 22, 2011
Precast concrete is used throughout construction. As a structural system, it provides a quick cost-effective and  is most useful in buildings that have a consistent repetitive design. Once a form is created for one component it can be replicated easily.  Precast structural components can are fabricated off site and shipped to the site and with the use of steam curing the fabrication schedule can be decreased to as quickly as 24 hours for a component. Their limitation is largely governed by transportation restrictions; maximum lengths that can be shipped and the maximum weight roadways can withstand.

There are several different types of precast structural components; columns, beams, plank (solid core and hollow core), T plank sections (single and double ) and wall panels. All precast components are use the same elements, Concrete, steel rebar reinforcing, bearing pads, and steel angles and plates. Concrete has a high resistance to compression forces, but is poor in tension. The use of steel rebar reinforcing allows concrete to perform better in tension. Bearing pads help for expansion and contraction allowing the components some flexibility. Steel plates and angles are cast into components for making connections to other components.  Steel anchoring points are also cast into the components to help with transportation and assembly in the field.

 

Flooring Systems:
Hollow core and solid core planks come in depths of 6” to 16” and can span smaller lengths 25’ to 40’. Single and double T’s are fabricated in depths of 36” to 48”  and can span longer lengths of 85’ to 105’. After the precast elements are installed as a flooring system, grout is installed in between the planks and a topping slab of light weight concrete is installed to make a cohesive slab.

 

Columns & Beams:
Columns and beams can be created using precast concrete. Columns typically are fabricated with corbels to help support the beams. While the beam rests on the corbel, separated by a bearing pad, steel plates and angles are used to form solid connections between them. Columns are typically square and are sized by the structural engineer. Concrete beams can come in four different shapes; rectangular, L shape, inverted tee or the AASHTO girder.

To make concrete beams and planks stronger prestressed and posttensioning is used. Prestressed concrete is cast around a stressed steel tendon, after the concrete has cured, the tendons are cut. Posttensioning concrete contains steel tendons are cast into concrete structural sections and are tensioned after the concrete has hardened. Both methods put the concrete in tension producing a camber allowing the concrete section to carry more load.

 

Precast Walls:
Precast walls can be as simple as an 8” thick wall, typically used for interior walls, or as complex as a an exterior wall with insulation. Thin brick can also be cast into concrete walls to create a more aesthetically pleasing walls. Door and window openings are cast into the walls during  fabrication.

The final step in using precast concrete structural components is fabrication. After they are shipped to the site the components are put in to place using a crane. Temporary supports and shoreing is used until the final connections can be made.
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