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precast technolgy

Apr 14, 2018

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    Unit 8 Precast Concrete

    PartIllustrated Words and Concepts

    Figure 8-1 Precast Slabs on a Frame of Precast Columns

    Figure 8-2 Hollow-core Slabs Supported on Precast Concrete Wall Panels

    Part Passages

    Passage A Precast Concrete Structural Elements and Assembly Concepts

    Passage B Precast Concrete and Its Uniqueness

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    Unit 8 Precast ConcretePart Illustrated Words and Concepts

    Figure 8-1 Precast Slabs on a Frame of Precast ColumnsDouble-tee slab elements supported on a frame of precast columns and L-

    shaped girders.

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    Unit 8 Precast ConcretePart Illustrated Words and Concepts

    Figure 8-2 Hollow-core Slabs Supported on Precast Concrete Wall Panels

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    Unit 8 Precast ConcretePart Passages Passage APrecast Concrete Structural Elements andAssembly Concepts

    Precast Concrete Slabs

    The most fully standardized precast concreteelements are those used for making floor and roof

    slabs. These may be supported by bearing walls of

    precast concrete or masonry or by frames of steel,

    sitecast concrete, or precast concrete.

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    Unit 8 Precast ConcretePart

    Passages Passage A

    Four kinds of precast slab elements arecommonly produced: for short spans and minimum slabdepths, solid slabs are appropriate. For longer spans,deeper elements must be used, and precast solid

    slabs, like their sitecast counterparts, becomeinefficient because they contain too much deadweightof nonworking concrete. In hollow-core slabs, precastelements suitable for intermediate spans, internallongitudinal voids replace much of the nonworking

    concrete. For the longest spans, still deeper elementsare requiredand double tees and single teeseliminate still more nonworking concrete.

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    Unit 8 Precast ConcretePart Passages Passage AFor most applications, precast slab elements of

    any of the four types are manufactured with a rough top

    surface. After the elements have been erected, a

    concrete topping is poured over them and finished to asmooth surface. The topping, usually 2 inches (50 mm)

    in thickness, bonds during curing to the rough top of the

    precast elements and becomes a working part of their

    structural action.

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    Unit 8 Precast ConcretePart Passages Passage AThe topping also helps the precast elements to

    act together as a structural unit rather than as individual

    planks in resisting concentrated loads and diaphragm

    loads, and conceals the slight differences in camberthat often occur in prestressed components. Structural

    continuity across a number of spans can be achieved

    by casting reinforcing bars into the topping over the

    supporting beams or walls. Underfloor electrical

    conduits may also be embedded in the topping.

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    Unit 8 Precast ConcretePart Passages Passage AThere is considerable overlapping of the

    economical span ranges of the different kinds of

    precast slab elements, allowing the designer some

    latitude in choosing which to use in a particularsituation. Solid slabs and hollow-core slabs save on

    overall building height in multistory structures, and their

    smooth undersides can be painted and used as finish

    ceilings in many applications. For longer spans, double

    tees are generally preferred to the older single-teedesign because they do not need to be supported

    against tipping during erection.

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    Unit 8 Precast ConcretePart Passages Passage APrecast Concrete Beams, Girders, and Columns

    Precast concrete beams and girders are made

    in several standard shapes. The projecting ledgers on

    L-shaped beams and inverted tees provide directsupport for precast slab elements. They conserve

    headroom in a building by supporting slabs near the

    bottoms of the beams, as com-pared to rectangular

    beams without ledgers, where slab elements must reston top.

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    Unit 8 Precast ConcretePart Passages Passage AAASHTO girders were designed originally as

    efficient shapes for bridge structures, but they are used

    sometimes in buildings as well. Precast columns are

    usually square or rectangular in section and may beprestressed or simply reinforced.

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    Unit 8 Precast ConcretePart Passages Passage APrecast Concrete Wall Panels

    Precast solid slabs are commonly used as

    load bearing wall panels in many types of low-rise and

    high-rise buildings. The prestressing strands arelocated in the vertical mid-plane of the wall panels to

    strengthen the panels against buckling and to eliminate

    camber. Rigid foam insulation can be cast into wall

    panels for thermal insulation, with suitable wire shearties between the inner and outer wythes of concrete.

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    Unit 8 Precast ConcretePart Passages Passage AAssembly Concepts for Precast Concrete Buildings

    Figure 8 1 shows a building whose precast slabelements (double tees in this example) are supportedon a skeleton frame of L-shaped precast girders andprecast columns. The slab elements are supported onprecast load bearing wall panels. Sometimes, the slabsof the building are supported on a combination of wallpanels and girders. These three fundamental ways ofsupporting precast slabson a precast concreteskeleton, on precast load bearing wall panels, and on acombination of the twooccur in endless variations inbuildings.

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    Unit 8 Precast ConcretePart Passages Passage A

    The skeleton may be one bay deep or many

    bays; the load bearing walls are often constructed of

    reinforced masonry, or of any of a variety of

    configurations of precast concrete; the slab elementsmay be solid, hollow-core, or double tee, topped or un-

    topped. One of the principal virtues of precast

    concrete as a structural material is that it is locally

    manufactured to order and is easily customized to an

    individual building design, usually at minimal additional

    cost.

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    Unit 8 Precast ConcretePart Passages Passage AThe Construction Process

    he construction process forprecast concrete

    framing is directly parallel to that for steel framing. The

    structural drawings for the building are sent to theprecasting plant, where engineers and drafters prepare

    shop drawings that show all the dimensions and details

    of the individual elements and how they are to be

    connected. These drawings are reviewed by the

    engineer and architect for conformance with their

    design intentions and corrected as necessary.

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    Unit 8 Precast ConcretePart Passages Passage AThen the production of the precast components

    proceeds, beginning with construction of any special

    molds that are required and fabrication of reinforcing

    cages, then continuing through cycles of casting,curing, and stockpiling as previously described. The

    finished elements, marked to designate their positions

    in the building, are transported to the construction site

    as needed and placed by crane in accordance with

    erection drawings prepared by the precasting plant.

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    Unit 8 Precast ConcretePart Passages Passage BPrecast Concrete and Its Uniqueness

    Structural precast concrete elementsslabs,

    beams, girders, columns, and wall panelsare cast

    and cured in factories, transported to the constructionjob site, and erected as rigid components. Precasting

    offers many potential advantages over sitecasting of

    concrete: The production of precast elements is carried

    out conveniently at ground level.

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    Unit 8 Precast ConcretePart Passages Passage BThe mixing and pouring operations are often

    highly mechanized, and frequently, especially in difficult

    climates, they are carried out under shelter. Control of

    the quality of materials and workmanship is generallybetter than on the construction job site. The concrete is

    cast in permanent forms made of steel,

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    Unit 8 Precast ConcretePart Passages Passage B

    concrete, glass-fiber-reinforced plastic, or wood

    panels with smooth overlays, whose excellent surface

    properties are mirrored in the high-quality surfaces of

    the finished precast elements that they produce. Theforms may be reused hundreds or thousands of times

    before they have to be renewed, so that formwork costs

    per unit of finished concrete are low.

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    Unit 8 Precast ConcretePart Passages Passage BThe forms are equipped to pretension the steel

    in the precast elements for greater structural efficiency,

    which translates into longer spans, lesser depths, and

    lower weights than for comparable reinforced concreteelements. Concrete and steel of superior strength are

    used in precast elements, typically 5 000 psi (35 MPa)

    concrete and 270 000psi (1 860 MPa) prestressing

    steel.

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    Unit 8 Precast ConcretePart Passages Passage BFor the fastest possible curing, the concrete for

    precast concrete elements is made with Type

    Portland cement, high early strength. And the elements

    are usually steam cured. Steam furnishes heat toaccelerate the hardening of the concrete and moisture

    for full hydration. Thus, a precasting plant is able to

    produce fully cured structural elements, from the laying

    of the prestressing strands to the removal of the

    finished elements from the casting bed, on a 24 hourcycle.

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    Unit 8 Precast ConcretePart Passages Passage BWhen the elements produced by this

    expeditious technique are delivered to the construction

    job site, further advantages are realized: The erection

    process is similar to that of structural steel, but it isoften faster because most precast concrete systems

    include a deck as an integral part of the major

    spanning elements, without the need for placing

    additional joist or decking components.

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    Unit 8 Precast ConcretePart Passages Passage BErection is much faster than that of sitecast

    concrete because there is no formwork to be erected

    and stripped, and little or no waiting for concrete to

    cure. And erection of precast structures can take placeunder some types of adverse weather conditions,

    such as extremely high or low temperatures, that would

    not permit the sitecasting of concrete.

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    Unit 8 Precast ConcretePart Passages Passage BWhen choosing between precast and sitecast

    concrete, the designer must weigh these potential

    advantages of precasting against some potential

    disadvantages. The precast structural elements,although light in weight as compared to similar

    elements of sitecast concrete, are nevertheless heavy

    and bulky to transport over the roads and hoist into

    place.

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    Unit 8 Precast ConcretePart Passages Passage BThis restricts somewhat the size and proportions

    of most precast elements: They can be rather long, but

    only as wide as the maximum legal vehicle width of 12

    to 14 feet (3.66 4.27m). This restricted width usuallyprecludes utilization of the efficiencies of two-way

    structural action in precast slabs. And the fully three-

    dimensional sculptural possibilities of sitecast concrete

    are largely absent in precast concrete.

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    Unit 8 Precast ConcretePart Passages Passage BUniqueness of Precast Concrete

    Precast, prestressed concrete structural

    elements are crisp, slender in relation to span,

    precise, repetitive, and highly finished. They combinethe rapid all-weather erection of structural steel framing

    with the self-fireproofing of sitecast concrete framing to

    offer economical framing for many kinds of buildings.

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    Unit 8 Precast ConcretePart Passages Passage BBecause precast concrete is the newest and

    least developed of the major framing materials for

    buildings, having been brought to market only a few

    decades ago, its architectural aesthetic isjust comingto maturity. Solid and hollow-core slabs have become

    an accepted part of our structural vocabulary in

    schools, hotels, apartment buildings, and hospitals,

    where they are ideal both functionally and

    economically.

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    Unit 8 Precast ConcretePart Passages Passage B

    Engineers and architects have long beencomfortable with precast concrete in longer-spanbuilding types, especially parking structures, ware-houses, and industrial plants, where its awesome

    structural potential and efficient serial production ofidentical elements can be fully utilized and openlyexpressed. Now we are becoming increasinglysuccessful in creating public buildings of the highestarchitectural quality that are built of precast concrete

    both inside and out. It is reasonable to expect that themost innovative of buildings in the coming years will bebuilt of this sleek,sinewy, rapidly developing newmaterial of construction.