Unit 8
Precast Concrete
Part ⅠIllustrated 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 Ⅱ Passages
Passage A Precast Concrete Structural Elements and Assembly Concepts
Passage B Precast Concrete and Its Uniqueness
Unit 8
Precast Concrete
Part Ⅰ Illustrated Words and Concepts
Figure 8-1 Precast Slabs on a Frame of Precast Columns
Double-tee slab elements supported on a frame of precast columns and Lshaped girders.
Unit 8
Precast Concrete
Part Ⅰ Illustrated Words and Concepts
Figure 8-2 Hollow-core Slabs Supported on Precast Concrete Wall Panels
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage A
Precast Concrete Structural Elements and
Assembly Concepts
Precast Concrete Slabs
The most fully standardized precast concrete
elements 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.
Unit 8
Precast Concrete
Part Ⅱ Passages
Passage A
Four kinds of precast slab elements are
commonly produced: for short spans and minimum slab
depths, solid slabs are appropriate. For longer spans,
deeper elements must be used, and precast solid
slabs, like their sitecast counterparts, become
inefficient because they contain too much deadweight
of nonworking concrete. In hollow-core slabs, precast
elements suitable for intermediate spans, internal
longitudinal voids replace much of the nonworking
concrete. For the longest spans, still deeper elements
are required，and double tees and single tees
eliminate still more nonworking concrete.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage A
For 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 a
smooth 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage A
The 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 camber
that 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.
Unit 8
Precast Concrete
Part Ⅱ Passages
Passage A
There 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 particular
situation. 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-tee
design because they do not need to be supported
against tipping during erection.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage A
Precast 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 direct
support 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 rest
on top.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage A
AASHTO 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 be
prestressed or simply reinforced.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage A
Precast 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 are
located 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 shear
ties between the inner and outer wythes of concrete.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage A
Assembly Concepts for Precast Concrete Buildings
Figure 8 1 shows a building whose precast slab
elements (double tees in this example) are supported
on a skeleton frame of L-shaped precast girders and
precast columns. The slab elements are supported on
precast load bearing wall panels. Sometimes, the slabs
of the building are supported on a combination of wall
panels and girders. These three fundamental ways of
supporting precast slabs—on a precast concrete
skeleton, on precast load bearing wall panels, and on a
combination of the two—occur in endless variations in
buildings.
Unit 8
Precast Concrete
Part Ⅱ 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 elements
may be solid, hollow-core, or double tee, topped or untopped. 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage A
The Construction Process
he construction process for precast concrete
framing is directly parallel to that for steel framing. The
structural drawings for the building are sent to the
precasting 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage A
Then 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
Precast Concrete and Its Uniqueness
Structural precast concrete elements—slabs,
beams, girders, columns, and wall panels—are cast
and cured in factories, transported to the construction
job 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
The 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 generally
better than on the construction job site. The concrete is
cast in permanent forms made of steel,
Unit 8
Part Ⅱ
Precast Concrete
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. The
forms 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
The 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 concrete
elements. 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
For the fastest possible curing, the concrete for
precast concrete elements is made with Type Ⅲ
Portland cement, high early strength. And the elements
are usually steam cured. Steam furnishes heat to
accelerate 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 hour
cycle.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
When 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 is
often 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
Erection 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 place
under some types of adverse weather conditions,
such as extremely high or low temperatures, that would
not permit the sitecasting of concrete.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
When 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
This 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 usually
precludes utilization of the efficiencies of two-way
structural action in precast slabs. And the fully threedimensional sculptural possibilities of sitecast concrete
are largely absent in precast concrete.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
Uniqueness of Precast Concrete
Precast, prestressed concrete structural
elements are crisp, slender in relation to span,
precise, repetitive, and highly finished. They combine
the 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.
Unit 8
Part Ⅱ
Precast Concrete
Passages
Passage B
Because 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 coming
to 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.
Unit 8
Precast Concrete
Part Ⅱ Passages
Passage B
Engineers and architects have long been
comfortable with precast concrete in longer-span
building types, especially parking structures, warehouses, and industrial plants, where its awesome
structural potential and efficient serial production of
identical elements can be fully utilized and openly
expressed. Now we are becoming increasingly
successful in creating public buildings of the highest
architectural quality that are built of precast concrete
both inside and out. It is reasonable to expect that the
most innovative of buildings in the coming years will be
built of this sleek, sinewy, rapidly developing new
material of construction.