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Precast Concrete is a form of construction, where concrete is cast in a reusable mould 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 sitecasting 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 sitecast production.
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. 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.
Brief History Edit
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 was adopted all over the world, particularly in Eastern Europe.
Precast Concrete Products Edit
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:
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 troughs, etc...
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.
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.
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 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.
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.
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.
Specialized Products Edit
Underground vaults or mausoleums - calls for quality watertight structures that withstand the tests of time and the forces of nature.
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.
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.
Available in a rainbow of colors, shapes, sizes and textures, these versatile precast concrete pieces can be designed to mimic brick, stone or wood.
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.
Reinforced Concrete Box Edit
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.
Double Wall PrecastEdit
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. 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.
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/Motels, Dormitories/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.
See Also Edit