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 maneuvered into place; examples include precast beams, and wall panels, floors, roofs, and piles. In contrast, cast-in-place concrete is poured into site-specific forms and cured on site. Recently, lightweight expanded polystyrene foam is being used as the cores of precast wall panels, saving weight and increasing thermal insulation. Precast stone is distinguished from precast concrete by the finer aggregate used in the mixture, so the result approaches the natural product.
Overview Precast concrete is employed in both interior and exterior applications, from highway, bridge, and high-rise projects to parking structures, K-12 schools, warehouses, mixed-use, and industrial building construction. 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. Using a precast concrete system offers many potential advantages over onsite casting. Precast concrete production can be performed on ground level, which maximizes safety in its casting. There is greater control over material quality and workmanship in a precast plant compared to a construction site. The forms used in a precast plant can be reused hundreds to thousands of times before they have to be replaced, often making it cheaper than onsite casting in terms of cost per unit of formwork. Precast concrete forming systems for architectural applications differ in size, function, and cost. Precast architectural panels are also used to clad all or part of a building facade or erect free-standing walls for landscaping, soundproofing, and security. In appropriate instances precast products – such as beams for bridges, highways, and parking structure decks – can be prestressed structural elements. Stormwater drainage, water and sewage pipes, and tunnels also make use of precast concrete units. Precast concrete molds can be made of timber, steel, plastic, rubber, fiberglass, or other synthetic materials, with each giving a unique finish. In addition, many surface finishes for the four precast wall panel types – sandwich, plastered sandwich, inner layer and cladding panels – are available, including those creating the looks of horizontal boards and ashlar stone. Color may be added to the concrete mix, and the proportions and size aggregate also affect the appearance and texture of finished concrete surfaces.
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 – including individual parts, or even entire building systems. In the modern world, precast panelled buildings were pioneered in Liverpool, England, in 1905. The process was invented by city engineer John Alexander Brodie. 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 Central and Eastern Europe as well as in Million Programme in Scandinavia. In 1918, to address a shortage of housing for steel workers following the rapid wartime expansion of its Campbell Works, the Youngstown Sheet and Tube Company constructed a pre-cast planned workers' estate in Youngstown, Ohio. Designed by the St. Louis firm Conzelman, Herding and Boyd, the development comprised nearly 250 dwellings built from factory produced Precast concrete panels, making it by 1920 one of the earliest and most ambitious large scale applications of Prefabricated home housing in the United States. In the US, precast concrete has evolved as two sub-industries, each represented by a major association. The precast concrete structures industry, represented primarily by of the Precast/Prestressed Concrete Institute (PCI), focuses on prestressed concrete elements and on other precast concrete elements used in above-ground structures such as buildings, parking structures, and bridges, while the precast concrete products industry produces utility, underground, and other non-prestressed products, and is represented primarily by the National Precast Concrete Association (NPCA). In Australia, The New South Wales Government Railways made extensive use of precast concrete construction for its stations and similar buildings. Between 1917 and 1932, it erected 145 such buildings. Beyond cladding panels and structural elements, entire buildings can be assembled from precast concrete. Precast assembly enables fast completion of commercial shops and offices with minimal labor. For example, the Jim Bridger Building in Williston, North Dakota, was precast in Minnesota with air, electrical, water, and fiber utilities preinstalled into the building panels. The panels were transported over 800 miles to the Bakken oilfields, and the commercial building was assembled by three workers in minimal time. The building houses over 40,000 square feet of shops and offices. Virtually the entire building was fabricated in Minnesota.
Reinforcement Reinforcing concrete with steel improves strength and durability. On its own, concrete has good compressive strength, but lacks tensile and shear strength and can be subject to cracking when bearing loads for long periods of time. Steel offers high tensile and shear strength to make up for what concrete lacks. Steel behaves similarly to concrete in changing environments, which means it will shrink and expand with concrete, helping avoid cracking. Rebar is the most common form of concrete reinforcement. It is typically made from steel, manufactured with ribbing to bond with concrete as it cures. Rebar is versatile enough to be bent or assembled to support the shape of any concrete structure. Carbon steel is the most common rebar material. However, stainless steel, galvanized steel, and epoxy coatings can prevent corrosion.
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 typically fall under one or
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