Concrete
tanks are generally required to withstand force both from the contained effluent and
backfill material.
Concrete
is strong in compression but to withstand the tensile stresses in tank walls reinforcing
steel is essential. The exact size and length of reinforcement is determined primarily
from the tank depth - see Dept. of Agriculture FDS specification for varying reinforcement
requirements.
An
arrangement of U-shaped bars in both the base and lower wall is a very effective method of
reinforcement at the critical wall-floor joint (see diagram).
The
reinforcement should be protected by a cover layer of dense concrete 40-50mm thick to
prevent corrosion. This is achieved using proprietary spacers to ensure the cover is
maintained.
Construction
joints may be required in long walls. See FDS specifications for design options.
Readymixed concrete producers employ a standard mix designation, which
should be used when ordering.
The correct mix for effluent tanks is given in Table I. The initial figure
of 30N or 40N indicates the strength grade and the latter figure of 20 indicates the
maximum size of aggregate.
Adequate durability is achieved by specifying a cement content appropriate
to the demands made on the concrete. This is particularly important for silage efluent
tanks.
