Standardization and adjustable moulds

Economic use of precast concrete can be achieved only if there is a high degree of standardization in the design of the units. Costs are inevitably higher when greater numbers of panel types are used. There is not only the additional cost of extra moulds, but if moulds have to be continually altered to cast'specials', the daily casting cycle is disrupted. Moreover; special units require separate stacking and delivery at a particular time. On site, special lifting and erection procedures may be required for non-standard units. The Cement and Concrete Association's (1977) Technical Report No 14 gives the main advantages of standardization as follows:

- lower production and erection costs;

- less time for detailing, mould making and production periods;

- reduced risk of detailing and production errors;

Standardization and adjustable moulds 13

- reduced risk of delays due to units being damaged;

- speedier erection.

Architects should therefore make every effort in the early design stages to reduce the number of panel types. Usually it is easier to standardize panel widths than panel heights, because of the frequent requirement to have different storey heights and parapet levels. In this case it is sometimes possible to adjust the design of the supporting in situ concrete frame to avoid the use of non-standard panels (Fig. 1.3). Any additional costs in terms of the shuttering of the main structure can be offset against the savings in greater standardization of precast units. Part of the skill of designing with precast units is this choice between standardizing precast and in situ construction. A good example is the student residence for Christ's College, Cambridge (architects: Denys Lasdun & Partners), where the architects have used a basically precast aesthetic in situ construction using a limited range of panel forms (Fig. 1.4).

Adjustable moulds

It may also be possible to provide some variety of panel size by adjusting the mould during manufacture. For example, Fig. 1.5 shows an adjustable mould for face-up casting, in which the stop end can be repositioned in the mould.This is more easily done with a panel of uniform thickness than with a coffered panel, where the coffer former will also need to be repositioned. Early consultation with the manufacturer will establish whether or not an adjustable mould is possible for any particular project.

Size of panels

The size of the precast concrete unit is influenced by three factors:

1.3 Different storey heights A and B allow standard panel height C.

Adjustable

Adjustable mould {face-up casting) end stop

Adjustable

Adjustable mould {face-up casting) end stop

Extension piece for coffer former

Extension piece for coffer former

1.5 Adjustable mould for a variety of panel sizes.

1.4 Student residence for Christ's College, Cambridge (architects: Denys Lasdun & Partners).

- ease of manufacture;

- method of transportation;

- weight of lifting.

The width of the unit is related to the method of manufacture, depending upon the type of finish required. For a polished smooth finish the width is

1.6 Cladding unit in course of finishing by hand.

restricted to about 1.2 m to allow a man to lean over the unit to polish. Figure 1.6 shows a typical width of unit being polished by hand. If large aggregate is placed by hand, then units should not exceed 2 m in width, unless a moving gantry is used.

Transportation will also limit the size of units. If units are placed flat on a trailer the width is related to payload restriction.There is no restriction on trailers up to 2.895 m (9 ft 6 in) wide. Trailer widths of 2.895-3.500 m (I I ft 6 in) need police permission, and loads exceeding 3.500 m wide need police escort. Thus 1.2-3 m seems to be an economical size for the width of units.

Wider units can be transported on A-frames supported on their long edge. Figure 1.7 shows two methods of transporting units on their edge, where the maximum height when loaded should not exceed 4.880 m. Allowing for the height of the trailer and bearers, panels of up to 4.130 m width are possible.

Standardization and adjustable moulds 15

1.7 Width of panel related to safe height during transportation: (a) unit transported on A-frames supported on edge; (b) large unit held vertically

Figure 1.8 shows wall panels being loaded on an A-frame trailer

Lengths of units are normally 3 x width, and will also be affected by the maximum lengths of the trailers, normally no longer than 12 m.The thickness

1.8 Wall panels being loaded onto A-frame trailer

of the panels is related to the required cover for the reinforcement in the panel web (Fig. I. I 8). As a rough guide to deciding the typical thickness of panel related to the longest side of the units,Table 1.1 can be used. Once the panel width, length and thickness have been determined, the cubic volume x the density of concrete will determine the overall weight of the unit.

In most cases wall panels should not exceed 7 t to allow easy handling during manufacture and on site. However, some manufacturers have facilities for handling units up to 20 t in weight.

Table I. I Rule of thumb for panel thickness related to its length

Panel length

Approximate thickness

(m)

(mm)

2.0

75 (using stainless steel reinforcement)

3.0

90

4.0

100

5.5

125

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