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402 Barry’s Advanced Construction of Buildings
system is that the lightweight planks or beams and filler blocks can be lifted and placed in
position much more easily than the much larger hollow concrete floor units.
Hollow clay block and concrete floor
A floor system of hollow clay blocks and in situ cast reinforced concrete beams between
the blocks and concrete topping, cast on centring and falsework, was for many years extensively
used for the fire-resisting properties of the blocks. This floor system is not much used
because of the considerable labour in laying the floor.
6.8 Precast reinforced concrete frames
Precast concrete has been established as a sound, durable material for framing and cladding
buildings where repetitive casting of units is an acceptable and economic form of construction.
Precast concrete elements are manufactured in highly controlled factory conditions,
which enable high degrees of accuracy and finishing to be achieved consistently and economically.
Most manufacturers stock a range of standard items and can also produce highly
unusual and complex architectural forms simply by adjusting the design of the moulds
used. The shape or profile of a precast unit can be straight or curved, and the level of detail
on the surface finish (known as the degree of ‘articulation’) can be highly complex if
required. The range of colours is also extensive, with different aggregates and additions
used to provide the required colour. Precast units, such as lintels and floor units, can be
reclaimed and reused when buildings are deconstructed, or the units can be crushed and
the aggregates and steel reinforcement recycled and incorporated into new recycled content
products.
The chief challenge with precast concrete framework is joining the members on site,
particularly if the frame is to be exposed, to provide a solid, rigid bearing in column joints
and a strong, rigid bearing of beams to columns that adequately ties beams to columns for
structural rigidity. Where the frame is made up of separate precast column and beam units,
there is a proliferation of joints. The number of site joints is reduced by the use of precast
units that combine two or more column lengths with beams, as illustrated in Figure 6.47.
The number of columns and beams that can be combined in one precast unit depends on
the particular design of the building and the facilities for casting, transporting, hoisting
and fixing units on site. Precast companies usually work with specialist fixing teams to help
ensure that the units are installed correctly. Bar coding strips or e-tags can be embedded
in the units to assist with timely distribution and delivery to site and the accurate identification
and installation of units on site.
The general arrangement of precast structural units is as separate columns, often twostorey
height and as cruciform H or M frames. The H frame unit is often combined with
under window walling, as illustrated in Figure 6.47.
The two basic systems of jointing used for connections of column to column are by direct
end bearing or by connection to a bearing plate welded to protruding studs. Direct bearing
of ends is effected through a locating dowel, which can also be used as a post-tensioning
connection, as illustrated in Figure 6.48. A coupling plate connection is made by welding
a plate to studs protruding from the end of one column and bolting studs protruding from
the other to the plate, as illustrated in Figure 6.48. The studs and plate must be accurately
located or else there will be an excessive amount of site labour in making this connection.