Civil Engineering Reference
In-Depth Information
Frog
Perforation
F
IGURE
1.23
Types of brick
for hundreds of years as a visit to any of the many cathedrals and churches built in the
Middle Ages will confirm. Some of timber's disadvantages, such as warping and twist-
ing, can be eliminated by using it in laminated form.
Plywood
is built up from several
thin sheets glued together but with adjacent sheets having their grains running at 90
◦
to each other. Large span roof arches are sometimes made in laminated form from
timber strips. Its susceptibility to the fungal attacks of wet and dry rot can be prevented
by treatment as can the potential ravages of woodworm and death watch beetle.
MASONRY
Masonry in structural engineering includes bricks, concrete blocks and stone. These
are brittle materials, weak in tension, and are therefore used in situations where they
are only subjected to compressive loads.
Bricks are made from clay shale which is ground up and mixed with water to form a
stiff paste. This is pressed into moulds to form the individual bricks and then fired in
a kiln until hard. An alternative to using individual moulds is the
extrusion process
in
which the paste is squeezed through a rectangular-shaped die and then chopped into
brick lengths before being fired.
Figure 1.23 shows two types of brick. One has indentations, called
frogs
, in its larger
faces while the other, called a
perforated
brick, has holes passing completely through
it; both these modifications assist the
bond
between the brick and the mortar and help
to distribute the heat during the firing process. The holes in perforated bricks also
allow a wall, for example, to be reinforced vertically by steel bars passing through the
holes and into the foundations.
Engineering
bricks are generally used as the main load bearing components in a
masonry structure and have a minimum guaranteed crushing strength whereas
facing
bricks have a wide range of strengths but have, as the name implies, a better appear-
ance. In a masonry structure the individual elements are the bricks while the complete
structure, including the
mortar
between the joints, is known as
brickwork
.
Mortar commonly consists of a mixture of sand and cement the proportions of which
can vary from 3 : 1 to 8 : 1 depending on the strength required; the lower the amount of
sand the stronger the mortar. However, the strength of the mortar must not be greater
than the strength of the masonry units otherwise cracking can occur.
