READY-MIXED CONCRETE

On some jobs, such as large danger of falling highway jobs, it is possible to use a batch plant that contains its own mixer. A plant of this type discharges ready-mixed concrete into transit mixers, which haul it to the construction site. The truck carries the mix in a revolving chamber much like the one on a mixer. Keeping the mix agitated in route prevents segregation of aggregate particles. A readymix plant is usually portable so that it can follow the job along. It must be certain, of course, that a truck will be able to deliver the mix at the site before it starts to set. Discharge of the concrete from the drum should be completed within 11/2 hours.

TRANSIT-MIXED CONCRETE

By transit-mixing, we refer to concrete that is mixed, either wet or dry, en route to a job site. A transit-mix truck carries a mixer and a water tank from which the driver can, at the proper time, introduce the required amount of water into the mix. The truck picks up the dry ingredients at the batch plant, together with a slip which tells how much water is to be introduced to the mix upon arrival at the site. The mixer drum is kept revolving in route and at the job site so that the dry ingredients do not segregate. Transit-mix trucks are part of the battalion's equipment inventory and are widely used on all but the smallest concrete jobs assigned to a battalion.

PRECAST AND TILT-UP CONCRETE

LEARNING OBJECTIVE: Upon completing this section, you should be able to determine projects suitable for and lifting methods necessary for precast and tilt-up construction.

Concrete cast in the position it is to occupy in the finished structure is called cast-in-place concrete. Concrete cast and cured elsewhere is called precast concrete. Tilt-up concrete is a special type of precast concrete in which the units are tilted up and placed using cranes or other types of lifting devices.

Wall construction, for example, is frequently done with precast wall panels originally cast horizontally (sometimes one above the other) as slabs. This method has many advantages over the conventional method of casting in place in vertical wall forms. Since a slab form requires only edge forms and a single surface form, the amount of formwork and form materials required is greatly reduced. The labor involved in slab form concrete casting is much less than that involved in filling a high wall form. One side of a precast unit cast as a slab maybe finished by hand to any desired quality of finishing. The placement of reinforcing steel is much easier in slab forms, and it is easier to attain thorough filling and vibrating. Precasting of wall panels as slabs may be expedited by mass production methods not available when casting in place. Relatively light panels for concrete walls are precast as slabs (figure 6-8). The panels are set in

Figure 6-9.-Precast panels being erected by use of crane and spreader bars.

place by cranes, using spreader bars (figure 6-9). Figure 6-10 shows erected panels in final position CASTING

The casting surface is very important in making with 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 cement poured into forms. These "earth" surfaces are

Figure 6-10.-Precast panels in position.

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 sand and top with a thin coat of mortar that is finished flush with the casting surface.

It is important to remember that any imperfections in the surface of the casting slab will show up on the cast panels. When finishing the casting slab, you must ensure there is a flat, level, and smooth surface without humps, dips, cracks, or gouges. If possible, cure the casting surface keeping it covered with water (pending). However, if a curing compound or surface hardener is used, make sure it will not conflict with the later use of bond-breaking agents.

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