Precast Concrete Overview

Precast Concrete Overview

• Precast concrete is manufactured off-site, commonly referred to as pre-fab or pre-made.

Minimum Thickness Specifications for Precast Slabs

• Solid flat slabs range from 4 inches (100 mm) to 8 inches (205 mm) in thickness.
• Hollow core slabs have a thickness of 6 inches (150 mm) to 12 inches (305 mm).
• Single tees measure between 20 inches (510 mm) to 48 inches (1220 mm).
• Double tees typically have a thickness of 12 inches (300 mm).

Precast Tees and Applications

• Precast tees are pre-stressed concrete components designed for parking structures, roofs, and floors, capable of spanning large distances and bearing heavy loads.
• Walls made from precast concrete can function as load-bearing structures, needing to resist lateral loads.
• Exterior walls must shield against external forces, while doors and windows must support vertical loads.
• Concrete and masonry walls are reinforced designs that effectively withstand forces, with surfaces typically remaining exposed.

Construction Techniques and Material Handling

• Dowel systems utilize alternating placements for stability.
• Sandblasted plywood is a treatment applied to surfaces achieved once formwork is removed, enhancing aesthetic quality.
• The precast method reduces onsite panel requirements, although additional reinforcement is necessary for transportation.

Roof Systems

• Precast roof systems are designed to efficiently shed rain and snow.
• They need to be resistant to UV radiation to ensure longevity.

Principles of Pre-stressing

• Pre-stressing involves using steel and concrete to prevent tension cracks that often originate from the center/bottom of components.
• Compression forces enhance concrete’s strength, while steel provides tensile strength.
• The bottom portion of beams is consistently supported, with measurements taken for accurate specifications.

Material Standards and Specifications

• Metal components must adhere to British standards, ensuring quality and safety.
• Inner wires must be 2% larger in diameter than outer wires.
• Indented bars or wires are classified as deformed bars.

Pre-tensioning and Post-tensioning Techniques

• Pre-tensioning involves securing the wire, which is then stretched using a hydraulic jack (28 N per mm²), and is typically released after 24 hours.
• Transfer lengths for pre-tensioned wires range from 80 to 120 meters.
• Post-tensioning occurs after the concrete has hardened, allowing for modifications in tension.

Common Applications of Pre-stressing

• Useful in deep excavations and ground anchor systems.
• Injection anchors are developed by boring holes into the soil for secure fastening.

Economic Considerations

• Pre-stressing is particularly economical for creating longer-span concrete structures and was pioneered by four Swiss engineers.

Construction Systems

• Two primary systems exist for the implementation of pre-stressing techniques, catering to various engineering needs.

This quiz covers essential information about precast concrete, including its definitions, thickness specifications for different slab types, and applications in construction. Test your knowledge on the properties and uses of precast components in modern building practices.