Durability - Material Properties
Different materials for components used
in enclosures exhibit different physical
and mechanical properties which may
affect performance life cycle.
Cable pit bodies are typically made from
moulded plastic, concrete or polymer
concrete. Other materials e.g. fibreglass
or aluminium are used for very specific applications.
Polymer concrete is produced by mixing
mineral aggregates with a resin binding
The resulting enclosure and its components
are smooth and of high quality as they are
made with matched surface tooling for
consistent quality. The pits are lightweight
and easy to handle ensuring cost effective
installations. Although having thinner side
walls, properly designed polymer concrete
cable pits are strong, durable and possess
mechanical properties typical of bulkier
concrete enclosures. They can also be cut with masonry drills and grinders.
Polymer concrete pits provide excellent
electrical insulation because castings
are homogenous (do not contain steel
reinforcing) and there are no macroscopic
voids (moisture traps).
The material is non porous and offers
a much higher resistance to chemical,
weathering and biological attack than
concrete, ensuring pits have a superior
Precast concrete is occasionally used for
small cable enclosures. However, large
maintenance holes are generally cast-insitu.
Precast concrete pits require thick
walls to provide their structural integrity
and are therefore heavy, cumbersome
and difficult to transport. Higher
installation costs must also be considered
as cranes or other special equipment may
be required on site. Cement concrete
is typically a porous material and can absorb up to 9% water by weight. In
aggressive environments, it is also prone
to deterioration, particularly in alkaline
soils. Therefore, concrete enclosures
require regular maintenance and in some
cases are susceptible to early failure.
Glass Reinforced Concrete
Glass reinforced concrete (GRC) is
sometimes used in the manufacture
of precast pits. GRC is a mixture of
cement, fine aggregate, water, chemical
admixtures, glass fibres and is considered
For a cable pit comparison, click here
Plastic cable pits are generally injection
or rotational moulded and can offer an
effective economic solution for light duty applications.
Product design can be used to overcome
some of the mechanical weaknesses inherent to plastic. Walls are typically
designed with ribs for increased strength
but side walls are often still flexible and
will distort when placed in unstable soils
or load bearing applications.
ACO’s plastic pits have straight sidewalls
with moulded continuous ribs for the
efficient transfer of loads without causing
additional stresses to the structure.
ACO’s plastic pits have undergone a finite
element analysis (FEA) to ensure the most
effective design for their intended use.
Pits have also been physically tested in
an independent test
Lids & Access Covers
Pit lids and/or access covers are
available in precast concrete, polymer
concrete, composite, steel and
aluminium. Cast iron and steel are used in
the manufacture of access covers.
Precast concrete and polymer concrete
lids are not recommended for vehicular
applications whilst steel and cast (ductile)
iron covers have higher tensile strengths
and are best suited for wheel loads.
For optimum corrosion resistance, ACO
recommends steel access covers and lids
are hot dip galvanised to AS 4680.
Effect of Fire on Pits
Polymer concrete is a self extinguishing
material which does not give off toxic
or any other dangerous fumes.
Plastics are combustible. They will burn
and disintegrate when subject to high
In areas which have been subjected to
fire, particular care should be taken in
respect to pit covers. Cement concrete
and polymer concrete will lose their
structural integrity and should not
have loads applied to them.
Steel & cast iron covers may keep
their strength but their seating may be
damaged and subject to collapse.
Click here for material properties table