Day 20 PRECAST AND PRE-STRESSED CONCRETE

Pre Cast concrete comprise units that are made under controlled conditions, with necessary preparations for assembly, then taken to the project building site and incorporated into the construction process. There are two types of pre cast concrete elements: Those that are simply reinforced for strength, such as structural floor slabs and wall panels. Those that have reinforcing installed and stressed before concrete is placed. After concrete is cured, the stress in the reinforcing is released, which places a force in the members that will oppose the forces of loading and span.

 Slab Elements  Tees (beam and slab)  Beams  Columns  Wall Panels  Decorative Units

Solid and hollow slabs are pre cast, but not pre stressed. Solid slabs are heavy and inefficient. Hollow slabs are designed to incorporate areas of concrete with reinforcing steel, which simulates the arrangement of small beams side by side.

Advantages of Pre Cast Concrete – Production is at ground level – Made in a controlled environment – High quality formwork is used, and reused – High quality equipment is used. – Controlled concrete curing is possible – Erection time and conditions at the site are considerably better.

Hollow Core Slab Size of Units are limited only by ability of transportation

Double Tee Depth 12” to 36” Pre stressed double tees have replaced single tees Units are 10’ wide Can be made any length re: transportation.

Steel casting bed for concrete double tees The height of the legs can be adjusted, as can the length of the bed.

Special high strength reinforcing cable is placed in each leg of the bed, then stretched to a specified stress according to the structural design. Steel is placed across the top of the bed to reinforce the top flange of the double tee. High strength concrete (5,000 psi) is placed in the bed and vibrated into the legs around the steel. When the concrete reaches a specified curing strength, the stretched reinforcing steel is released from the hydraulic machine. The steel has a tendency to shrink back to its original length before stress, which exerts a compressive force on each end of the beam toward the center. The force of the reinforcing steel causes the beam to bow upward along its length, which will settle to a horizontal position when the beam is set in place with its intended load.

Stressed Cable

The stress in the reinforcing steel inside the beam causes a force at each end of the beam, directed back toward the center, causing an upward bend in the length of the beam – called Camber.

Steel hooks are cast into the top of the tees at strategic points to facilitate transporting the units and lifting into place

 Pre stressed tees are placed on pre stressed beams. The units are secured by welding plates that were cast into the points of contacts in the beams and the tees.

 Pre cast and pre stressed beams are made in various configurations, pending the condition of strength and support. The AASHTO beams are primarily for bridges and overpasses in highway construction. (American Association of State Highway and Transportation Officials)

AASHTO beams used for a bridge structure

Pre cast concrete units installed for arena seating

Arena Seating – pre cast concrete but not pre stressed

Connection of spandrel beam to column in a parking garage

Connection of a pre stressed double tee to a wall in a parking garage

Double tee support connection to a column – the projection on the column is called a “haunch.”

Lifting units into place with a crane

ARCHITECTURAL PRE CAST WALL PANELS not pre stressed