Agriculture Reference
In-Depth Information
The first, and perhaps the more easily believable, is that if a clay body is of great
density and therefore has very low water absorption, water cannot enter the fabric
in sufficient quantities to cause sufficient force by the expansion of ice formation.
Furthermore, such dense bodies generally have high strength that resists what
force is exerted. Water absorption is relatively easy to measure and characteristic
figures vary across the range of different types of bricks from less than 2% to over
35%! But there is no correlation to frost resistance. Generally bricks of very low
water absorption (below approximately 5%) are frost resistant, but some are not;
there is no consistent correlation.
The second characteristic is very difficult to measure or otherwise define. If
the clay body absorbs water, the nature and continuity of the pores in the clay
body seems to be significant. Studies reveal that the behaviour of freezing water
in microscopic spaces is complex and the state of saturation produced by 'prac-
tical' exposure, i.e. in the field, is not always absolute and so tiny air voids exist
behind the surface saturation that allows pressure of freezing water to be dissi-
pated within the body.
Recent scientific testing of both historic and modern rubbers supports this
latter point. It is believed that one of the main contributing factors to the
resistance, and hence durability, of cutters and rubbers is their extensive pore
structure, which makes them very efficient at transporting moisture (Pavia and
Lynch, 2003, 19) (Colston, 2004) See Case Study. Hammett (2004), however,
adds that, '…this characteristic has proved impossible to define in any practi-
cal way and the supposition remains a hypothesis'.
Frost resistance is traditionally declared by experience in firing to a semi-
vitreous state and in use of the products. Testing to assist definition of frost resist-
ance of clay bricks in the UK, since the mid 1980s, centres on replication to con-
ditions as experienced in the field, of exposure to wetting and freeze/thaw cycles
of brickwork. The vast accumulated data indicates a good correlation between
long-term field experience and developed test methods of the products.
Under BS EN 771-1:2003 Specification for Masonry Units: Clay Masonry Units ,
there are three categories of frost resistance, classified by declaration as a
result of observation after a number of years in use, which are:
F0 - bricks suitable for passive exposure.
F1 - bricks suitable for moderate exposure.
F2 - bricks suitable for severe exposure.
Future Prospects for Rubbing Brick Producers
Modern rubbing-brick manufacture has over-concentrated on producing the
fully-washed, late Victorian type of homogeneous bodied brick. This has been
 
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