SECTION IV. PRECAST CONCRETE
DEFINITION AND CHARACTERISTICS
6-35. Nearly all prestressed concrete is precast. Prestressed concrete is subjected to compressive stresses before any external loads are applied. The compressive stresses counteract the stresses that occur when a unit is subjected to loads.
6-36. Precast concrete is concrete cast in other than its final position. Unlike cast-in-place concrete, precast concrete requires field connections to tie the structure together. The connections can be a major design problem. Although precasting is best suited to a manufacturer's factory or yard, job-site precasting is common on large projects and in area's remote from a precasting source. Prestressed concrete is a combination of high-tensile steel and high-strength concrete. Prestressing is accomplished by stretching the high-strength steel wire, wire strands, or bars, either by pre-tensioning or post-tensioning (stretched before or after pouring the concrete).
6-37. Long-span, precast floor and roof units are usually prestressed with tongue-and-groove edges and special clips to anchor them to supporting members. Short members, 30 feet or less, are often made with ordinary steel reinforcement. Available types of precast floor and roof units include solid or ribbed slabs, hollow-core slabs, single and double tees, rectangular beams, L-shaped beams, inverted T-beams, and I-beams. Hollow-core slabs are usually available in normal or structural lightweight concrete. Units range from 16 to 96 inches wide and from 4 to 12 inches deep. Hollow-core slabs may have grouted shear keys to distribute loads to adjacent units over a slab width as great as one-half the span. Precast-reinforced slabs are also available for curtain walls in buildings having either steel or concrete frames. Such wall panels include plain panels, decorative panels, natural stone-faced panels, sandwich panels, solid panels, ribbed panels, load-bearing and nonload-bearing panels, and thin-section panels. Prestressing these panels makes it possible to handle and erect large units and thin sections without cracking. Two other convenient forms of precast concrete are tilt-up and lift-slab that save formwork expense since wall and floor surfaces are cast horizontally at a convenient level. In tilt-up, the wall is cast horizontally, then tilted up to its final, vertical position. Lift-slab involves precasting the floor and roof slabs of a multistory building horizontally, then jacking or lifting them one upon another to their final positions.
6-38. Precast concrete tends to be a local business. Although there are many manufacturers, few deliver more than 200 to 300 miles from their facility. For this reason, there is a wide variation in the types and sizes of precast items available in different areas. Check those manufacturers who will deliver to the project area to find out what is available. Many firms do not maintain facilities for prestressing or special surface treatments that you may require. In general, precast concrete products fall roughly into five groups.
6-39. Form costs are much less with precast concrete, because you do not have to support the forms on scaffolding; simply set the forms on the ground in a convenient position. A thin concrete wall is very difficult to construct if cast vertically, because the concrete must be poured high up in the narrow opening at the top of the form. But a thin wall is easily precasted flat on the ground. Thus, the large-side forms are eliminated, as well as the braces that hold a vertical form in place. In some types of precast construction, you lose no time waiting for concrete to gain strength at one level before you can place the next. Such delays are common in cast-in-place construction. In addition, permanent precast units can often be used as a working platform, thus eliminating the need for scaffolding. Precast units can be standardized and mass-produced. Great savings can result from repeated reuse of forms and assembly line production techniques. You can also maintain high quality control of precast products. Precast units poured on-site require minimal transportation, but any off-site precast concrete requires transportation, usually by truck. Precast concrete buildings are built so that all the structural concrete (except footings) is precast and put together piece by piece. The variety of shapes and designs available is limited only by the imagination and cost. Most are available in either reinforced or prestressed conditions.
6-40. The decision as to whether concrete is cast-in-place, purchased precast, or precast on-site is made by the design engineer, architect or construction supervisor. However, precasting on-site is not recommended unless experienced precast personnel are available to supervise the work. In making the decision, the engineer must consider the following:
Special requirements. Precast concrete requires large placing equipment such as cranes, derricks and a specialty crew to set the pieces. Precasting also takes plenty of space. Is enough space available on-site or should the material be precast off-site and transported to the job-site? If precasting is accomplished off-site, the necessary facilities and transportation costs must be added to the cost of the items precast.
Manufacture. Should forms be made from steel, wood, fiberglass, or a combination of materials? Who will make the forms? How long will manufacturing take? How much will it cost? The cost of the forms must also be charged to the precast items.
Supervision. Is a specialist available to supervise manufacture? Someone also has to coordinate the work and prepare the shop drawings. These costs must also be included, along with the following:
Extra materials. Materials must be purchased for:
Extra labor. Labor is required to:
Extra equipment. Necessary equipment may include mixers, lift trucks, cranes, derricks, equipment to cure the concrete, and miscellaneous equipment such as hoes, shovels, wheelbarrows, hammers, and vibrators. Special equipment is required to prestress concrete. All equipment requires storage space.
6-41. The design of a structure composed of precast members requires both cleverness and engineering skill. There are inherent problems to resolve in this type of construction such as provisions for seating members and tying the structure together. When using precast members, the accuracy of the building layout is extremely important. The precast pieces must fit together like a jigsaw puzzle leaving little room for error. Because the purpose of this chapter is only familiarization with the end products and the materials and methods used to produce them, neither the engineering nor design of precast concrete is discussed.
6-42. You can precast structural members either in central off-site or on-site prefabrication plants depending upon the product, the numbers required, and its use. On-site or temporary prefabrication plants are suitable for military operations. They are subject to prevailing weather and climate conditions because they have no roofing. Lay out the prefabrication yard to suit the type and the quantity of members you will precast. Bridge T-beams, reinforced-concrete arches, end walls, and concrete logs are precast structural members. The yard must be on firm, level ground that has ample working space and access routes. Figure 6-13 is a schematic layout of a prefabrication yard suitable for producing typical members. Table 6-5 below recommends the personnel requirements. You can expect such a prefabrication facility to produce approximately 6,000 square feet of precast walls per day, although output will vary according to personal experience, equipment capabilities, and product requirements.
Figure 6-13. Schematic layout of on-site or temporary prefabrication yard
Table 6-5. Recommended precasting team personnel requirements
6-43. Precast members can withstand farther hauling and rougher treatment then either plain or reinforced members; all members require adequate support so that they do not undergo either strain or loading greater than the design loading. Various types of hauling and handling equipment are needed in precast concrete operations. You can transport heavy girders long distances on tractor trailers or with a tractor and dolly arrangement in which the girder acts as the tongue or tie between the tractor and the dolly. Smaller members can be transported (columns, piles, slabs) on flatbed trailers, but must be protected from too much bending stress caused by their own dead weight.
6-44. Erecting precast members is similar to erecting reinforcing steel and requires cranes or derricks of sufficient capacity. Use spreader bars to handle the elements at their correct pickup points. Sometimes two cranes or derricks are needed to lift very long or heavy members.
|David L. Heiserman, Editor||
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Revised: June 06, 2015