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2. PRE-CAST BUILDING ELEMENTS MANUFACTURING PROCESS
Manufacture of pre-fabricated concrete building elements is essentially performed in a job shop environment.
That is, each element produced may be very different from all other elements. The manufacture of pre-fabricated
concrete elements differs somewhat from the traditional job shop problem, however. In the traditional job shop,
parts are processed by different machines. In the pre-cast job shop, however, parts are processed by the same
machine (form). In essence, the "machine" has a significant setup time required to create multiple unique parts.
The parts are then batch processed through a "curing operation."
The basic steps to pre-fabricated construction element manufacturing include the following:
1. Form construction. This may include placing wooden frames for architectural openings inside the form.
It may also include installing wooden bulkheads to create elements of different lengths.
2. Set-up.
3. Pull and stress supporting cables (rebar).
4. Pour cement.
5. Lay down insulation (optional).
6. Pull and stress supporting cables for second layer of concrete (if insulation installed).
7. Pour second layer of concrete (if insulation installed).
8. Cure.
9. Strip (remove element from form). This may include sawing through the concrete to create elements of
varying lengths, if wooden bulkheads were not used prior to pouring concrete.
10. Apply finish.
Each of these steps introduces some variability into the manufacturing process. Perhaps most significant source
of variation, however, is customer choice. Each customer may request pre-cast elements of different sizes with
different patterns of openings, different finishes, and different colors. Further, the element's intended use impacts
the depth of concrete poured as well as the number of supporting cables used. A pillar for a parking structure may
need to be significantly stronger than that for a home garage wall.
This variability creates difficulties in scheduling. The time to set-up an element with many architectural features
takes significantly longer than a plain unit. This must be carefully managed to maintain production. A form
requiring many complex pieces and few simple ones may take so long to set-up that the concrete pour is delayed.
This in turn delays the time when the building elements can be removed from the form, possibly delaying the
next pour operation. Thus, each day an effort is made to balance complex pieces with simple ones so that the
complex pieces are distributed over a number of production days.
In addition to the issue of the complexity of a piece, the length of the forms drives decisions. Because it takes 10-
16 hours to cure, regardless of the lineal feet of concrete poured, efforts are made to "fill-out" a bed each day.
This may result in pieces being made before they are ready for installation at the construction site. These pieces
are then stored in a construction yard where they are subject to damage. In addition, inefficiencies are introduced
in the form of double handling of pieces.
Figure 1 displays the general flow of material in this type of manufacturing operation.
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