Day 20 PRECAST AND PRE-STRESSED CONCRETE Pre Cast concrete comprise units that are made under...

Day 20Day 20

PRECAST AND PRE-STRESSED CONCRETEPRECAST AND PRE-STRESSED CONCRETE

Pre Cast concrete comprise units that Pre Cast concrete comprise units that are made under controlled conditions, with are made under controlled conditions, with necessary preparations for assembly, then necessary preparations for assembly, then taken to the project building site and taken to the project building site and incorporated into the construction process.incorporated into the construction process.

There are two types of pre cast There are two types of pre cast concrete elements: concrete elements:

Those that are simply reinforced for Those that are simply reinforced for strength, strength, such as structural floor slabs such as structural floor slabs and wall panels.and wall panels.

Those that have reinforcing installed Those that have reinforcing installed and stressed before concrete is placed. and stressed before concrete is placed. After concrete is cured, the stress in the After concrete is cured, the stress in the reinforcing is released, which places a force reinforcing is released, which places a force in the members that will oppose the forces in the members that will oppose the forces of loading and span.of loading and span.

Slab ElementsSlab Elements Tees (beam and slab)Tees (beam and slab) BeamsBeams ColumnsColumns Wall PanelsWall Panels Decorative UnitsDecorative Units

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

Advantages of Pre Cast ConcreteAdvantages of Pre Cast Concrete

– Production is at ground levelProduction is at ground level– Made in a controlled environment Made in a controlled environment – High quality formwork is used, and High quality formwork is used, and

reused reused – High quality equipment is used.High quality equipment is used.– Controlled concrete curing is possibleControlled concrete curing is possible– Erection time and conditions at the site Erection time and conditions at the site

are considerably better. are considerably better.

Hollow Core SlabHollow Core Slab

Size of Units are limited only by ability of transportation

Double TeeDouble Tee

Depth12” 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 Special high strength reinforcing cable is placed in each leg of the bed, then stretched to a placed in each leg of the bed, then stretched to a specified stress according to the structural specified stress according to the structural design. Steel is placed across the top of the bed design. Steel is placed across the top of the bed to reinforce the top flange of the double tee. to reinforce the top flange of the double tee. High strength concrete (5,000 psi) is placed in High strength concrete (5,000 psi) is placed in the bed and vibrated into the legs around the the bed and vibrated into the legs around the steel.steel.

When the concrete reaches a specified When the concrete reaches a specified curing strength, the stretched reinforcing steel is curing strength, the stretched reinforcing steel is released from the hydraulic machine. The steel released from the hydraulic machine. The steel has a tendency to shrink back to its original has a tendency to shrink back to its original length before stress, which exerts a compressive length before stress, which exerts a compressive force on each end of the beam toward the center.force on each end of the beam toward the center.

The force of the reinforcing steel causes the The force of the reinforcing steel causes the beam to bow upward along its length, which will beam to bow upward along its length, which will settle to a horizontal position when the beam is settle to a horizontal position when the beam is set in place with its intended load. 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 Pre stressed tees are placed on pre stressed beams. The units are secured by welding plates beams. The units are secured by welding plates that were cast into the points of contacts in the that were cast into the points of contacts in the beams and the tees. beams and the tees.

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

not pre stressednot pre stressed