Discard criteria for twin wall concrete pumping pipe
Issued: 14/05/2024
Last Updated: 15/05/2024
Purpose
The purpose of this safety alert is to provide guidance on the discard criteria for twin wall steel pipe used for concrete pumping. This alert only discusses the pipe thickness at which twin wall pipe should be discarded. There are other issues that also need to be considered when determining whether pipe should be discarded.
Background
In late 2023, a straight section of steel twin wall pipe on a mobile concrete placing boom burst. The incident sprayed concrete over a wide area and smashed a car window but fortunately no people were near the pipe and no one was injured. In this incident, the inner wall had been totally worn away in places with only the outer wall still remaining. The pipe that failed had been powder coated by the owner after being purchased.
Rupturing of concrete pumping delivery lines has the potential for major injuries to workers and other people due to the high pressures required to pump concrete. In addition, if the inner wall is worn away, large sections of inner wall can detach and enter the flow of concrete potentially creating pipeline blockages.
Contributing factors
The circumstances of the incident are being investigated but there are a range of factors that can lead to failure of pipes. Notwithstanding this, reduced wall thicknesses and blockages are known to be contributing factors in the failure of pipes. Allowing twin wall pipe to remain in service after the inner wall has been worn through, reduces the burst capacity of the pipe and leads to an increased risk of blockage.
When the inner wall of twin wall pipe fails, relatively large pieces of the inner wall can detach and enter the flow of concrete. In some cases, these pieces of steel can be more than 0.5 metres long. The likelihood of blockages can be high especially around bends and where reducers are used.
Section 4.3.5 of the Concrete Pumping Code of Practice 2019 includes a number of causes for blockages in concrete delivery lines, including fracture of the inner wall in twin wall pipe.
Action required
Manufacturers of twin wall pipe should ensure that any guidance on discard criteria does not permit the inner wall to be totally worn away. Discard procedures should ensure the outer wall maintains its full thickness and remains intact, and that there is also still a consistent thickness of the inner wall. Figure 1 illustrates a diagram of twin wall pipe and unacceptable discard criteria for twin wall pipes.
Figure 1 - unacceptable discard criteria
Manufacturers, importers, and suppliers of concrete pumping pipe all have duties with respect to the supply of appropriate information on the safe use and maintenance of pipes. This information should take into account common practices and level of competency typically found in the concrete pumping industry.
Owners of concrete placing booms should:
- Ensure that pipes are discarded before the inner wall has been totally worn away.
- Obtain guidance from manufacturers or suppliers on the inner wall thickness at which the pipe should be discarded. Owners may also seek guidance from a competent person on the inner wall thickness at which the pipe should be discarded. A competent person should be from a technical background and be able to demonstrate extensive experience in the inspection and use of concrete pumping pipe.
- Ensure that maintenance and inspection tasks are updated to reflect the guidance provided by manufacturers or suppliers.
- Provide the updated information to operators and technicians responsible for maintenance.
- If pipes have had additional paint or powder coating applied, in addition to that provided at manufacture, ensure additional allowance is made for the thickness of the coating (e.g., if the powder coating adds 0.5mm thickness, and the manufacturer’s discard wall thickness is 2.3mm, the new discard thickness will be 2.8mm).
- Ensure discarded pipes are marked as being defective and preferably damaged in such a way that they cannot be re-used.