are placed on chairs and tied as needed. Reinforcement should be as near the center of the panel as possible. Reinforcing bars are run through the side forms of the panel. When welded wire fabric or expanded wire mesh is used, dowel bars are used to tie the panels and their vertical supports together. Additional reinforcement is generally needed around openings.
The panel is picked up or tilted by the use of PICKUP INSERTS. These inserts are tied into the reinforcement. As the panel is raised into its vertical position, maximum stress will occur; therefore, the location and number of pickup inserts are extremely important. Some engineering manuals provide information on inserts, their locations, and capacities.
TILT-UP PANEL FOUNDATIONS. - A n economical and widely used method to support tilt-up panels is a simple pad footing. The floor slab, which is constructed first, is NOT poured to the perimeter of the building to permit excavating and pouring the footings. After the panel is placed on the footing, the floor slab is completed. It may be connected directly to the outside wall panel or a trench may be left to run mechanical, electrical, or plumbing lines.
Another method that is commonly used, as an alternative, is to set the panels on a grade beam or a foundation wall at floor level. Regardless of the type of footing, the panel should be set into a mortar bed to ensure a good bond between the foundation wall and the panel.
PANEL CONNECTIONS. - The panels may be tied together in a variety of ways. The location and the use of the structure will dictate what method can or can NOT be used. The strongest method is a cast-in-place column with the panel-reinforcing steel tied into the column. However, this does NOT allow for expansion and contraction. It may be preferable to tie only the corner panels to the columns and allow the remaining panels to move.
A variety of other methods of connecting the panels are also used. A BUTTED connection, using grout or a gasket, can be used if the wall does NOT contribute any structural strength to the structure. Steel columns are welded to steel angles or plates secured in the wall panel. Precast columns can also be used. Steel angles or plates are secured in both the columns and plate and welded together to secure the panel.
When panel connections that do not actually hold the panels in place are used, the panels are generally welded to the foundation and to the roof by using steel 3-38 angles or plates. All connections must provide waterproof joints. This is accomplished by the use of expansion joint material.
Precasting is done either in central prefabrication plants or on-site prefabrication plants, depending upon the product and its application. On-site or temporary prefabrication plants are generally more suitable for military operations. These plants are without roofing and, therefore, are subject to weather and climate considerations. The prefabrication yard is laid out to suit the type and quantity of members to be processed. It must be on firm, level ground, providing ample working space and access routes. Bridge T-beams, reinforced concrete arches, end walls, and concrete logs are typical members produced at these plants. A schematic layout of a prefabrication yard suitable for producing such members is shown in figure 3-21. A prefabrication unit of this size can be expected to produce approximately 6,000 square feet of precast walls per day. The output will vary according to the experience of the personnel, equipment capabilities, and product requirements.
CASTING. - The casting surface is very important in making precast concrete panels. In this section, we will cover two common types: earth and concrete. Regardless of which method you use, however, a slab must be cast in a location that will permit easy removal and handling.
Castings can be made directly on the ground with cement poured into forms. These "earth" surfaces are economical but only last for a couple of concrete pours. Concrete surfaces, since they can be reused repeatedly, are more practical.
When building casting surfaces, you should keep the following points in mind:
The subbase should be level and properly compacted.
The slab should be at least 6 inches thick and made of 3,000 psi or higher reinforced concrete. Large aggregate, 2 1/2 inches to 3 inches maximum, may be used in the casting slabs.
If pipes or other utilities are to be extended up through the casting slab at a later date, they should be stopped below the surface and the openings temporarily closed. For wood, cork, or plastic plugs, fill almost to the surface with sandContinue Reading