Precast concrete

Precast concrete is a construction product
produced by casting concrete in a reusable mold
or "form" which is then cured in a controlled
environment, transported to the construction
site and lifted into place. In contrast, standard
concrete is poured into site-specific forms and
cured on site. Precast stone is distinguished
from precast concrete by using a fine aggregate
in the mixture, so the final product approaches
the appearance of naturally occurring rock or
stone.
By producing precast concrete in a controlled
environment (typically referred to as a precast
plant), the precast concrete is afforded the
opportunity to properly cure and be closely
monitored by plant employees. Utilizing a
Precast Concrete system offers many potential
advantages over site casting of concrete. The
production process for Precast Concrete is
performed on ground level, which helps with
safety throughout a project. There is a greater
control of the quality of materials and
workmanship in a precast plant rather than on a
construction site. Financially, the forms used in
a precast plant may be reused hundreds to
thousands of times before they have to be
replaced, which allows cost of formwork per
unit to be lower than for site-cast production. [1]
Many states across the United States require a
precast plant to be certified by either the
Architectural Precast Association (APA), National
Precast Concrete Association (NPCA) or Precast
Prestressed Concrete Institute (PCI) for a precast
producer to supply their product to a
construction site sponsored by State and Federal
DOTs.
There are many different types of precast
concrete, forming systems for architectural
applications, differing in size, function, and cost.
Precast architectural panels are also used to clad
all or part of a building facade free-standing
walls used for landscaping, soundproofing, and
security walls, and some can be Prestressed
concrete structural elements. Stormwater
drainage, water and sewage pipes, and tunnels
make use of precast concrete units. The New
South Wales Government Railways made
extensive use of precast concrete construction
for its stations and similar buildings. Between
1917 and 1932, they erected 145 such
buildings. [2]
Brief history
Ancient Roman builders made use of concrete
and soon poured the material into moulds to
build their complex network of aqueducts ,
culverts, and tunnels. Modern uses for pre-cast
technology include a variety of architectural and
structural applications featuring parts of or an
entire building system.
In the modern world, pre-cast panelled
buildings were pioneered in Liverpool , England
in 1905. A process was invented by city
engineer John Alexander Brodie, whose
inventive genius also had him inventing the
football goal net. The tram stables at Walton in
Liverpool followed in 1906. The idea was not
taken up extensively in Britain. However, it was
adopted all over the world, particularly in
Eastern Europe. [3]
Precast Concrete Products
The following is a sampling of the numerous
products that utilize precast/prestressed
concrete. While this is not a complete list, the
majority of precast/prestressed products can fall
under one or more of the following categories:
Agricultural Products
Precast concrete products can withstand the
most extreme weather conditions and will hold
up for many decades of constant usage.
Products include bunker silos, cattle feed bunks,
cattle grid , agricultural fencing, H-bunks, J-
bunks, livestock slats, livestock watering trough,
feed troughs, concrete panels, slurry channels,
and more. Prestressed concrete panels are
widely used in the UK for a variety of
applications including agricultural buildings,
grain stores, silage clamps, slurry stores,
livestock walling, and general retaining walls.
Panels can either be used horizontally and
placed either inside the webbings of RSJs ( I-
beam ) or in front of them. Alternatively panels
can be cast into a concrete foundation and used
as a cantilever retaining wall.
Building and Site Amenities
Precast concrete building components and site
amenities are used architecturally as fireplace
mantels, cladding, trim products, accessories,
and curtain walls. Structural applications of
precast concrete include foundations, beams,
floors, walls, and other structural components.
It is essential that each structural component be
designed and tested to withstand both the
tensile and compressive loads that the member
will be subjected to over its lifespan. [1]
Precast concrete wall veneer formed
to replicate brick.
Building construction using precast concrete
walls and floors
Retaining Walls
An example of a precast concrete
retaining wall.
Precast concrete provides the manufacturers
with the ability to produce a wide range of
engineered earth retaining systems. Products
include: commercial retaining wall, residential
retaining walls, sea walls, mechanically
stabilized earth (MSE) panels, modular block
systems, segmental retaining walls, etc.
Retaining walls have 5 different types which
include: gravity retaining wall, semigravity
retaining wall, cantilever retaining wall,
counterfort retaining wall, and buttress retaining
wall. [4]
Sanitary and Stormwater
Sanitary and Stormwater management products
are structures designed for underground
installation that have been specifically
engineered for the treatment and removal of
pollutants from sanitary and stormwater run-
off. These precast concrete products include
stormwater detention vaults , catch basins , and
manholes . [5]
Transportation and Traffic Related Products
Precast concrete transportation products are
used in the construction, safety and site
protection of road, airport and railroad
transportation systems. Products include: box
culverts, 3-sided culverts, bridge systems,
railroad crossings, railroad ties, sound walls /
barriers, Jersey barriers , tunnel segments,
precast concrete barriers, TVCBs, central
reservation barriers and other transportation
products. Used to make underpasses, surface-
passes and pedestrian subways, so that traffic in
cities is disturbed for less amount of time. [6]
Utility Structures
For communications, electrical, gas or steam
systems, precast concrete utility structures
protect the vital connections and controls for
utility distribution. Precast concrete is nontoxic
and environmentally safe. Products include:
hand holes, hollowcore products, light pole
bases, meter boxes, panel vaults, pull boxes,
telecommunications structures, transformer
pads, transformer vaults, trenches, utility
buildings, utility vaults , utility poles, controlled
environment vaults (CEVs,) and other utility
structures. [7]
Water and Wastewater Products
Precast water and wastewater products hold or
contain water, oil or other liquids for the
purpose of further processing into non-
contaminating liquids and soil products.
Products include: aeration systems , distribution
boxes, dosing tanks, dry wells , grease
interceptors, leaching pits, sand-oil/oil-water
interceptors, septic tanks, water/sewage storage
tanks, wetwells, fire cisterns and other water &
wastewater products. [7]
Specialized Products
Cemetery Products
Underground vaults or mausoleums - calls for
quality watertight structures that withstand the
tests of time and the forces of nature.
A precast concrete hazardous
material storage container.
Hazardous Materials Containment
Storage of hazardous material, whether short-
term or long-term, is an increasingly important
environmental issue, calling for containers that
not only seal in the materials, but are strong
enough to stand up to natural disasters or
terrorist attacks. [8]
A precast concrete armour unit
(ACCROPODE).
Marine Products
Floating docks, underwater infrastructure,
decking, railings and a host of amenities are
among the uses of precast along the waterfront.
When designed with heavy weight in mind,
precast products counteract the buoyant forces
of water significantly better than most materials.
[9]
Modular Paving
Available in a rainbow of colors, shapes, sizes
and textures, these versatile precast concrete
pieces can be designed to mimic brick, stone or
wood. [10]
Prestressed/Structural Products
Prestressing is a technique of introducing
stresses of a predetermined magnitude into a
structural member to improve its behavior. This
technique is usually found in concrete beams,
spandrels, columns, single and double tees, wall
panels, segmental bridge units, bulb-tee girders,
I-beam girders, and others. "Prestressed
member are crack-free under working loads
and, as a result, look better and more
watertight, providing better corrosion protection
for the steel." Many projects find that
prestressed concrete provides the lowest overall
cost, considering production and lifetime
maintenance. [4]
Reinforced Concrete Box
a reinforced concrete box being
used in a storm drain
RCC Magic Box , used to build an
underpass at Madiwala at the
junction of Hosur Road and Inner
Ring Road , Bangalore City.
A reinforced concrete box , referred to as a
box culvert in the UK, is a square or rectangular
"pipe" made of concrete with rebar or wire
mesh fabric strewn throughout for the addition
of extra strength. Multiple such boxes are
arranged sideways to make a pipe or tunnel like
structure.
It is often used for sanitary sewer trunks, storm
drain spillways, pedestrian subways, utility
tunnels, catch basins, and other similar
underground passage ways. Due to the
enormous strength of reinforced concrete , it is
often used in sewers or tunnels that have little
cover above them which means they will be
subjected to the stress of the road atop them. In
India, pre-cast concrete boxes known as "Magic
Boxes"" are used for the construction of flyovers
and underpasses . [11]
Double Wall Precast — Concrete
Sandwich Panels
The double wall process has been in use in
Europe for many years. The walls consist of two
wythes of concrete separated by an insulated
void. The most commonly specified thickness of
the wall panels is 8 inches. The walls can also be
built to 10 and 12 inches thick if desired. A
typical 8-inch wall panel consists of two wythes
(layers) of reinforced concrete (each wythe is
2-3/8 inches thick) sandwiched around
3-1/4 inches of high R-value insulating foam.
The two wythes of the interior and exterior
concrete layers are held together with steel
trusses. Concrete sandwich panels held together
with steel trusses are inferior to those held
together with composite fibreglass connectors.
This is because the steel creates a thermal
bridge in the wall, significantly reducing the
insulative performance and reducing the ability
of the building to utilise its thermal mass for
energy efficiency. There is also the risk that
because steel does not have the same expansion
coefficient as that of concrete, as the wall heats
and cools, the steel will expand and contract at
a different rate to the concrete, which can cause
cracking and spalling (concrete "cancer").
Fibreglass connectors that are specially
developed to be compatible with concrete
significantly reduce this problem. [12] The
insulation is continuous throughout the wall
section. The composite sandwich wall section
has an R-value exceeding R-22. The wall panels
can be made to any height desired, up to a limit
of 12 feet. Many owners prefer a 9-foot clear
height for the quality of look and feel it affords a
building.
A single-family detached home
being built up from precast concrete
parts
The walls can be produced with smooth surfaces
on both sides because of the unique
manufacturing process, which form finishes
both sides. The walls are simply painted or
stained on the exterior surface to achieve the
desired color or textured surface. When desired,
the exterior surface can be manufactured to
have a wide variety of brick, stone, wood, or
other formed and patterned appearances
through the use of reusable, removable
formliners . Interior surfaces of the double-wall
panels are drywall quality in appearance right
out of the plant, requiring only the same prime
and paint procedure as is common when
completing conventional interior walls made of
drywall and studs.
Window and door openings are cast into the
walls at the manufacturing plant as part of the
fabrication process. Electrical and
telecommunications conduit and boxes are
flush-mounted and cast directly in the panels in
the specified locations. The carpenters,
electricians, and plumbers do need to make
some slight adjustments when first becoming
familiar with some of the unique aspects of the
wall panels. However, they still perform most of
their job duties in the manner to which they are
accustomed.
Double-wall precast concrete sandwich panels
can be used on most every type of building
including but not limited to: multi-family,
townhouses, condominiums, apartments, hotels
and motels, dormitories and schools, and
single-family homes. Depending upon building
function and layout, the double-wall panels can
be easily designed to handle both the structural
requirements for strength and safety, as well as
the aesthetic and sound attenuation qualities the
owner desires. Speed of construction, durability
of finished structure, and energy-efficiency are
all hallmarks of a building that utilizes the
double-wall system.