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Fundamentals of Building Construction, Materials & Methods, 5thEditionCopyright 2009 J. Iano. All rights reserved.

Plant-Cast Precast Concrete

Concrete elements, cast and cured in a manufacturing plant, thentransported to the construction site.

Plant casting allows increased efficiency and higher quality control. Durable, permanent steel forms are reused many times, reducing

formwork costs compared to sitecast concrete.

Use of Type III, high early strength cement and steam curing allowconcrete members to be cast and cured in as little as 24 hours.

Controlled casting conditions and high quality forms allow for greater

control of surface finishes.

15 PRECAST CONCRETE FRAMING SYSTEMS


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Plant-Cast PrecastConcrete

Structural elements arecommonly reinforcedwith tightly stretchedpretensioned steelstrands, which provideincreased structural

efficiency.

Conventional steelreinforcing is added forresistance to thermaland other secondary

stresses.



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Plant-Cast Precast Concrete

On the construction site,precast concrete elementsare lifted into place andassembled into structuralassemblies in a processsimilar to that used forstructural steel.

Compared to sitecastconcrete, precast concreteerection is faster and lessaffected by adverseweather conditions.

A vacuum lifting device is used to lift and place precastconcrete pranks.



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PRECAST,PRESTRESSEDCONCRETE

STRUCTURAL

ELEMENTS



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Precast Concrete Slabs

Used for floor and roof decks.

Deeper elements (toward the rightbelow) span further than those that areshallower (toward the left).

Right: Hollow core slabs stacked at theprecasting plant.

PRECAST PRESTRESSED CONCRETE STRUCTURAL ELEMENTS


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Precast Concrete Beams and Girders

Provide support for slabs.

The projecting reinforcing bars will bond with concrete cast on site. Right: Inverted tee beams supported by precast columns.



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Precast Concrete Columnsand Wall Panels

Provide support for

beam and slabelements.

Since these elementscarry mainly axial loadswith little bending force,they may beconventionallyreinforced withoutprestressing.

Or, long, slendermultistory elements

may be prestressed toprovide resistance tobending forces duringhandling and erection(columns at right).



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Precast Concrete Columns and WallPanels

Precast concrete wall panels may

be solid (right), hollow, orsandwiched (with an insulatingcore).

Wall panels can be ribbed, toincrease their vertical spancapacity while minimizing weight,or formed into other specialshapes (below).



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Other Precast Concrete Elements

Precast concrete stairs (below)

Uniquely shaped structuralelements for a sports stadium (right)

Etc.



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Assembly Concepts forPrecast Concrete Buildings

Vertical support can be

provided by precastcolumns and beams(above), wall panels(below), or a combinationof all three.

The choice of roof andfloor slab elementsdepends mainly on spanrequirements.

Precast slab elements arefrequently also used with

other vertical loadbearingsystems such as sitecastconcrete, reinforcedmasonry, or steel.



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Assembly Concepts forPrecast Concrete Buildings

Above:Precast concretestructure consisting of solidwall panels and hollow coreslabs.

Below:A single storywarehouse consisting ofdouble tees supported byinsulated sandwich wallpanels.



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Assembly Concepts forPrecast ConcreteBuildings

A parking garagestructure consisting ofprecast double teessupported by invertedtee beams on

haunched columns.



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MANUFACTURING OFPRECAST CONCRETE

STRUCTURALELEMENTS



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Casting Hollow Core Planks

Precast elements aremanufactured in castingbeds, 800 ft or more inlength.

High-strength steelstrands are strung thelength of the bed andtensioned.

Conventional reinforcing,weld plates, blockouts,lifting loops, and otherembedded items are

added as needed.

Concrete is placed.

MANUFACTURING OF PRECAST CONCRETE STRUCTURAL ELEMENTS

Untensioned prestressing strands can be seen in the left-mostcasting bed. In the bed second from the right, low-slumpconcrete for hollow core slabs is being formed over tensionedstrands using an extrusion process. A completed hollow corecasting is visible at the far right.


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Prestressing and ReinforcingSteel

Many precast elementscontain both prestressingstrands and conventionalreinforcing.

Right: The prestressingstrands for an AASHTOgirder are depressed intoa shallow v-shape to mostefficiently resist tensileforces in the beam. Shearstirrups are formed from

conventional steelreinforcing.



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Once the concrete hascured to sufficientstrength, the castings arecut into sections ofdesired length (above).

In some cases, transversebulkheads are inserted todivide the casting bedinto sections beforeconcrete is placed. In thiscase, only theprestressing strands need

to be cut to separate thesections (below).



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Individual sections arelifted from the castingbed (right) and stockpiledto await shipping to theconstruction site.



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Precast concreteelements are shipped tothe construction site bytruck and erected on siteby crane.



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Casting Hollow CorePlanks

Sample hollowcore slab sectionsof varying depths.

At bottom left,note the insulatedsandwich floor

panel.



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JOINING PRECASTCONCRETE ELMENTS



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Example Column-to-Column Connection

Metal bearing plates and embedded anchor bolts are cast into the ends of thecolumns.

After the columns are mechanically joined, the connection is grouted to providefull bearing between elements and protect the metal components from fire andcorrosion.

JOINING PRECAST CONCRETE ELEMENTS


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Example Beam-to-ColumnConnection

Beams are set onbearing pads onthe columncorbels.

Steel angles are

welded to metalplates cast into thebeams andcolumns and thejoint is grouted

solid.



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Example Slab-to-BeamConnection

Hollow core slabs

are set on bearingpads on precastbeams.

Steel reinforcingbars are in insertedinto the slabkeyways to span the

joint.

The joint is groutedsolid.

The slab may

remain untopped asshown, or toppedwith several inchesof cast in placeconcrete.



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Sitecast ConcreteToppings overPrecast Slabs

Greater floorstrength andstiffness

Greater fireresistance

Greater acousticisolation

Allow easyintegration ofelectrical servicesinto floor system

Create asmoother, flatterfloor surface.



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