Frequently Asked Questions

Minimum self cleansing velocities are:

• Foul sewers: 0.75m/sec at one third design flow.
• Surface water sewers: 1.0m/s at full pipe flow.
• What is the maximum permissible velocity in concrete pipes?

Limits are not normally placed on maximum velocities but certain criteria such as the potential for erosion should be considered for steep gradients.

It is recommended that anchor blocks be cast into the ground every 3 or 4 pipe joints behind the socket (also consider installing packers on the face of sockets to prevent concrete to concrete contact).

Roughness values are:

• Foul sewers: 1.5mm.
• Surface water sewers: 0.6mm.

Concrete pipes are designed to operate at low heads of water. The BS performance requirement is set at a minimum of 0.5 bar.

Minimum depth of cover to top of pipes is 0.6m with bedding class S (360° granular bedding). For cover depths less than 0.6m, then the pipeline should be protected with either a full concrete surround, or a reinforced concrete slab should be placed over it (spanning sufficient distance onto trench walls).

In many situations the 'as-dug' trench material is suitable for backfill (should be readily compactable and free from large boulders etc).

It is common practise to carry out air or water testing of pipes normally up to DN750 and a visual inspection above this size.

Common causes of air tests failures include:

• Displaced seals (usually due to use of wrong pipe lubrication during installation).
• Poor laying technique.
• Faulty testing equipment.
• Poor seal between testing equipment and face of concrete pipe.

It is highly recommended to test the pipeline every 3 or 4 pipes laid as an absolute minimum (do not wait until whole line is installed before carrying out such test).

Concrete surrounds to manholes are not always necessary in the majority of situations. However if the sewer is to be adopted, this may be a requirement.

Joint gaps between pipes should be measured on the internal surface and should ideally be between 5-25mm.

Circular concrete pipes are all manufactured to strength class 120 (minimum crushing load for a particular size of pipe is equal to 120 multiplied by DN of pipe divided by 1000).

CPM concrete pipes are generally reinforced in sizes DN675 and above (it is possible to reinforce some of the smaller sizes and also to provide un-reinforced pipes in some of the larger sizes - usually made to order).

The standard access position is 150mm in from the chamber wall below (measured from centre of nearest access edge to inside of chamber wall below). Other positions/access sizes/multiple access slabs can be made to order.

No - they are not required as our standard products are certified under the BS 'Kitemark' system - certification can be viewed via our website. Link to page needed

5000 psi is the generally accepted maximum, although higher pressure can be accommodated by concrete drainage products.

The minimum crushing strength of the concrete is 40N/mm2 (typical crushing strengths are much higher).

The water cement ratio is not more than 0.45.

The water absorption of the concrete is not more than 6% by mass.

Maximum chloride content of the concrete not greater than 1% by mass for unreinforced products (maximum of 0.4% for reinforced products).

Additional assistance can be had via the Concrete Pipeline Systems Association, see www.concretepipes.co.uk