Following my work on
the Hockley mill sluice gate I thought some details
on the more general aspects of using recycled
plastic in and around mills would be of interest.
It is clear to me that this material has a lot to
offer millers as long as it is used in a sensitive
way within its physical capabilities. The case
study in the last newsletter shows that the material
is not suitable for significant load bearing duties
as, by itself, it has limited strength and is too
flexible to be useful in most load bearing
situations – which will probably have many purists
giving a sigh of relief!
Its properties can be
best utilised in and around water, especially where
there is a constant cycle of wetting and drying.
The main beneficial attributes of recycled plastic
are that it is easily workable and is not
susceptible to fungal or insect attack. I
understand that for some time it has been
successfully used by the Environment Agency, Water
Supply Companies, and other organisations with
similar issues.
The material is
available in a limited number of colours, sizes, and
formats. The formats available can be divided into
three classes – sheets, boards, and posts. Sheets
are available in a variety of thicknesses; boards
of various sizes are manufactured with differing
surface textures some of which include steel
reinforcing bars; and posts of differing sizes and
characteristics are available and can include
internal reinforcing steel cores.
The most obvious and
common use in water-related environments is for
decking (or other walkways) and bank support. Many
examples can be found and reports indicate that the
material has performed well. The main drawbacks
seem to be an
aesthetic one
(it’s plastic!) and that it is not cheap, certainly
when compared with treated softwood. The
manufacturers acknowledge this and provide tables to
show the Cost of Ownership of recycled plastic is
lower than standard wooden decking as the
maintenance is virtually zero and it lasts several
times longer.
In addition to these
more conventional uses it seems to me that the
milling environment has situations that would
significantly benefit from utilising the properties
of recycled plastic, especially in mills that are
only operated occasionally and are subject to the
accelerated levels of decay that this leads to.
Examples that come to mind include flumes, sluices,
small sluice gates or boards, float boards, and even
starts in suitable water wheels.
From the evidence
available it is clear that this material should
perform well in most water channelling situations as
long as its more flexible nature is taken into
account and adequate support is provided. I think
the most interesting area of use will be in more
dynamic flow areas. Aspects of its use in the
construction of sluice gates has been discussed
elsewhere but this was in the context of rebuilding
an existing gate. Building a new sluice and gate
from scratch could well generate a different result.
Another use which I
would like to see tested is its viability for starts
and float boards in metal rimmed water wheels. This
would require a trial to be set up that comprised a
few float boards on a suitable wheel. The biggest
single issue would probably be how a start is
connected to the rim. The start will need a metal
core to provide enough strength and the attachment
to the rim must be some sort of fixing which does
not damage or require modification of the rim, all
at an acceptable cost. The float board itself
should not be an issue as the loading on an
individual board is not that high. The other
constraint that may become critical is the
availability of suitable standard profiles from the
recycled plastic manufacturers – I am sure no one
will be prepared to finance the tooling to make a
special profile.
Clearly to take this
forward a trial in a real environment is needed
which would require the availability of a suitable
water wheel. The first activity would be to develop
an outline design for the starts which is
economically viable. Following the completion of
the design the real work of building a prototype
would begin, assuming there is funding for the
materials and any necessary tooling. The final
activity would be to fit the parts into the target
wheel and test the performance.
I would be interested to hear what others feel on
this subject.
Editor: I would also
be interested in your comments on this controversial
subject. Below
is a letter from Peter Mobbs, which coincided with
Ivor sending me this article.
Recycled
plastic boards
a response from Peter Mobbs
It would be possible
to overcome the lack of bending stiffness of this
material using "Sandwich Technology”. This can be
achieved by cladding the core material (in this case
RPB) with thin stiff skins. A more detailed
explanation of this technology is available at
https://en.wikipedia.org/wiki/Sandwich-structured_composite
. Since the RPB material is thermoplastic (melts
when heated) it could be possible to hot melt a
suitable surface reinforcement medium into the
boards. Alternatively many builders merchants stock
a variety of suitable ‘off the shelf’ galvanised
steel strips. These could be used as a low cost
surface reinforcement material secured in place
using conventional fasteners.
Ivor also mentioned
that the thrust of the water pressure is highest at
the bottom of the gate and quoted a force of one
third of a ton in magnitude. This would indeed
generate a significant bending moment on the sluice
gate. However at the water surface level the
bending moment is negligible. So there would be
some design merit in tapering the gate profile
(similar to a dam): thick at the bottom and thinner
at the top.
So it can be seen
that there are indeed several novel solutions that
could be used to overcome the perceived limitations
of RPB. However, on the downside, turning workable
design concepts into reality could have been a too
advanced project for the team to undertake. Also
this radical use of modern materials might not
appeal to the more traditional mill enthusiast.
