Civil Engineering Reference
In-Depth Information
Table 10.9:
The durability of exposed components made of plants
Type of plant
Unfertilized/fertilized
Artificially
naturally (years)
fertilized (years)
Reed
50-100
30
Straw from rye/wheat
20-35
10-12
Eelgrass
200-300
Bracken
8-10
Heather
More than 25
from plants such as flax, stinging nettles and hemp is carried out by retting on
the ground. The first stage of the cleaning process is left to mould, bacteria, sun
and rain for two to three weeks until the fibres loosen from the stalk and can be
harvested for the final cleaning process. Certain plant products are chemically
impregnated to increase durability. Jute, produced in Bangladesh, is often
impregnated with a copper solution for shipping to Europe. When producing
building boards, it is quite usual to add glue even if many of the plants used con-
tain natural glues which are melted out when the board is heated under pressure.
Building chemicals from plants
Plants can be the source of many building chemicals, which can be pressed out
or distilled through warming in the absence of oxygen, a process known as dry-
distillation. The main chemicals are as follows.
Wood vinegar
This is extracted from trees and can form a raw material base for methanol and
acetic acid. It has a disinfectant effect on timber that is beginning to rot, and can
form the basis of the production of synthetic substances. Other plants can form
alcohol through fermentation. This can be used as a solvent for, amongst other
things, natural resin paints and cellulose varnish.
Wood tar
Wood tar can be distilled to any consistency, from a thin transparent liquid to a
thick black viscous liquid. One liquid, 'real' turpentine, is used as a solvent for
paints.
The amount obtained from distillation depends upon the speed of the process
and the type of wood used. Rapid distillation produces more gas and less liquid.
Deciduous trees such as beech and birch produce the most wood vinegar, where-
as coniferous trees contain more tar.
