Civil Engineering Reference
In-Depth Information
Ashlar
The most highly worked stone, fully
rectangular and coursed, is termed
ashlar (Figure 2.38). Just how much
stone is dressed in ashlar walls of
course depends on the thickness of
the wall; solid stone walls often
consist of two wythes filled with
rubble or undressed stone (Figure
2.39). Frequently too, stone walls
were backed by brick masonry,
sometimes with a waterproofing of
painted bitumen incorporated
between the two materials.
Mortars
Wherever possible, the fine
aggregate for masons' mortars ought
to consist of crushed stone of the
same kind as the rest of the wall. This
will ensure a reasonable match. The
mix might best consist of 12 parts of
finely crushed stone, to 3 parts of
hydrated lime (or lime putty if
available), to 1 part of Portland
cement, though some mortars have
successfully used less fine aggregate
and more cement. In essence, the
mortar should be less strong than the
stone, so that it is sacrificial.
Random, or uncoursed, rubble
Figure 2.39
The wythes have parted in this solid stone
wall, due mainly to the absence of through
stones. Part of it has already collapsed,
together with the hearting
originate from the way they were laid
down, and these planes place a limit
on the quarried sizes of useful stone
blocks. Some building stones are
from metamorphic or igneous
formations, such as the granites, and
do not have these bedding planes -
size tends to be restricted mainly by
handling and positioning criteria.
Stones may need to be accurately
identified for replacement purposes,
and specialist advice and access to
reference collections may be needed.
In choosing stone for replacement
purposes, even if the original stone
has been accurately identified, it is
important to verify that the
replacements are of acceptable
standard since quality can vary, even
within the confines of a small
quarry
(114)
.
Flint does not fit neatly into any
formal classification of natural stone.
Even in facing work, in Norfolk for
example, it is sometimes found used
straight from the chalk bed with thick
mortar joints, but in better work,
especially from the mid-sixteenth
century onwards, is knapped into
rough cubes and either roughly
coursed or laid in a species of stack
bond or in chequer pattern (Figure
2.40).
Irregular coursed, snecked or square random
rubble
Main performance
requirements and defects
Choice of materials for structure
Stone used for building purposes is
mainly from sedimentary rocks, for
example the limestones and
sandstones. These can have
pronounced bedding planes which
Coursed rubble
Figure 2.37
Common rubble walling (after Davey)
Figure 2.38
Part of Prior Park College, built by Ralph Allen in Combe Down stone
