Different materials 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 resin binding agent.
Resulting high quality enclosure is smooth surfaced. The pits are lightweight and easy to handle ensuring cost effective installation. 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 be cut with masonry drills and grinders.
Polymer concrete pits provide excellent electrical insulation because castings are homogenous (no steel reinforcing) and 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 superior life cycle.
Precast concrete can be used for small cable enclosures. Large maintenance holes are usually cast-in-situ. Precast concrete pits require thick walls to provide structural integrity and can be 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 can be susceptible to early failure.
Glass reinforced concrete (GRC) is sometimes used in manufacture of precast pits. GRC is a mixture of cement, fine aggregate, water, chemical admixtures, glass fibres and is considered porous.
Plastic cable pits are generally injection or rotational moulded and offers effective economic solution for light duty applications.
Product design can be used to overcome some mechanical weaknesses inherent to plastic. Walls are designed with ribs for increased strength, but side walls often still flex and distort when placed in unstable soils/load bearing applications.
ACO’s plastic pits have straight sidewalls with moulded continuous ribs for efficient transfer of loads without causing additional stresses to structure. ACO’s plastic pits have undergone a finite element analysis (FEA) to ensure the most effective design for intended use. Pits have been physically tested by an independent laboratory.
Polycrete® Cable Pits v. GRC Cable Pits