Agriculture Reference
In-Depth Information
was, not surprisingly, weaker than the other bricks tested, failing at four tons
(Rivington, 1901, 115), which was significantly lower than the other types of
brick tested. This was further substantiated in the results from similar tests on
the crushing strengths of various types of brick, undertaken a few years later
(Mitchell and Mitchell, 1904, 327-8) (see Table 3).
The low crushing strength demonstrated by rubbing bricks is due to a
combination of factors - their fine washed structure, low-fired temperature,
and characteristic large voids volume, typically around 35%, which defines
their porous nature. Yet it would be wrong to categorise these soft bricks as
being constructionally weak and non-durable in the context they were used.
Historical use has proved that cut and rubbed and gauged brickwork, prop-
erly detailed, will last as long as standard facework. This is due, in part, to the
soft lime mortar that was used with these bricks. Structural engineer Lachlan
McDonald writes:
In Victorian brickwork red rubbers were commonly used to form arches with
panels of brickwork over. The construction involved the use of lime mixed with
fine sand to reduce shrinkage. The joint width restricted to around 2-3 mm max-
imum for gauged arches, and as fine as 1 mm, depending on the available budget
and skill of the craftsman; and up to 6 mm in ashlar work. Despite the low com-
pressive strength of the mortar and the brickwork, it is a general view that if con-
structed with thin joints this brickwork performed well; even if it was significantly
overstressed by modern standards.
The soft lime mortar can distribute the forces within the brickwork over
several courses, with much more efficiency than a modern hard cement mortar
(L McDonald, 2002). Additionally, due to the plastic nature of the mortar, small
movements can be accommodated in the joints without cracking the bricks, and
cracks within the lime mortar itself, following movement, are often re-sealed
due to the so-called 'autogenous healing'. Although this mechanism is not well
understood, it is likely to involve continuous carbonation, or re-carbonation, of
deposits of lime out of solution, or free lime. Furthermore, the porous nature of
both the bricks and the mortar is more favourable to water movement, allowing
wetting and drying-out to occur, leading to less trapped moisture than occurs
with cement construction. This improves the weathering characteristics of the
brickwork.
John Addison (2006), another structural engineer with vast experience
on traditionally built brick and stonework, has this to say about the overall
strength of gauged work:
The ideal brick, from a structural point of view, is one which has the usual
102 mm (4.ins) thickness but blessed with unlimited dimensions. This mega-brick
could match the height and length of your wall and you would need no joints
