Cable Enclosure Theory
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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
Polymer concrete is produced by mixing mineral aggregates with a resin binding agent.

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 life cycle.

Cement Concrete
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 porous.
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FEA moulded designsPlastic
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 laboratory.

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 temperature.

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


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