Civil Engineering Reference
In-Depth Information
each human body can only withstand a certain amount of neglect and abuse, and
evidence indicates that mine is by now well over its quota.
A starting point to develop the sand and powdered clay render
There is still no sure fire render recipe even with this approach, as the clay content of the
brickies sand will vary from quarry to quarry so you will still have to make up test
recipes. Start with three part sand to one part clay for the first coat and four part sand to
one part clay for the second. If you want to add a little lime to the final coat replace about
one-third of the clay with lime for the initial test. The introduction of lime is unnecessary
in the first coat and is questionable in the second coat. If you are using polyester
compression strapping, you should not include lime in any render that comes in direct
contact with the strapping, as the strapping is vulnerable to high alkaline materials.
The sand represents the greater bulk of the material in the render, so you should vary
the amount of clay for the alternative recipes rather than adjusting the sand content.
Mixing of earthen renders
I have seen people mix the render using a hoe in a pit or even their feet. This would no
doubt be a lot of fun on a warm day, although with my family I doubt that much of the
render would end up on the walls. I have found that a common cement mixer works well.
If you are hiring a mixer it is best to get the larger mixer rather than a small one. This is
not like mixing cement. When mixing cement if you add too much cement the primary
outcome is a change in colour and an empty pocket. It would end up more brittle but
when put into a path, the best place for concrete, it will have little or no effect, as it dries
primarily by chemical reaction rather than evaporation. Cement will set under water,
earthen render will not.
Consistency in adding materials
The materials added to an earthen render must be consistent. If the chosen recipe is not
adhered to you will not be producing the desired recipe but a bastardisation of the recipe,
which is likely to fail. It is not sufficient to add shovels full of the different material as this
can give anything up to a 30 per cent variation in the mix. You need to take the same
approach to earthen render as you do to making a cake. Too much flour and the cake will
crack. Too much clay and the render will crack.
Using a bucket
Add material to the mixer via buckets to ensure you have a consistent render. Use a shovel
to fill buckets much the same way as filling a cup measure with flour, as this will give you
control over the contents of the render. When mixing in a cement mixer you will be able
to gauge the viscosity of the mix by watching the paddles in the back of the mixer. Watch
to see when the paddles separate from the mix. With render fed through the pump we
normally find that it is best when the paddles break from the mix at about the one o'clock
position.
