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.