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
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 L- shaped girders.
Part Ⅰ Illustrated Words and Concepts Figure 8-2 Hollow-core Slabs Supported on Precast Concrete Wall Panels
Part Ⅱ 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.
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.
Part Ⅱ 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.
Part Ⅱ 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.
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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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,
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
Part Ⅱ 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.
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.