Cemar (mortar lining)
The standard internal protection for PAM pipes consists of a cement mortar lining. Applied by a centrifugal process, this lining provides:
- excellent hydraulic flow conditions,
- preservation of carried water’s quality,
- effective protection of pipe wall against aggressive waters.
Standards: GB/T 13295, GB/T 17457.
The cement mortar lining is applied by a centrifugal process. With this process, chosen by PAM, the mortar is introduced into the pipe and rotated at high speed, ensuring good lining compaction.
The lining is then treated under controlled temperature and humidity conditions, giving it the optimum mechanical strength. The spinning process has the advantage of producing a smooth internal surface layer consisting of very fine particles (laitance). The process gives the following properties:
- high mortar compaction,
- low roughness,
- good mortar key.
Flow Hydraulic performance
The cement mortar has a very smooth inner surface, which favours flow, reduces head losses and guarantees long term hydraulic efficiency.
The roughness coefficient of a single pipe (COLEBROOK formula) is k = 0.03. However, PAM recommends the use of the value k = 0.1 in designing a pipe system, to take any exceptional head losses into account.
The cement mortar lining is an active coating. It is not merely a barrier coating but provides chemical protection through the phenomenon of passivation. When the pipe is filled, the mortar gradually absorbs water, which becomes enriched in alkaline substances ; it is therefore non-corrosive when it reaches the proximity of the metal wall.
Crack plugging is recognized and is covered in the standards. Crazing (shrinkage), and indeed small cracks formation, often occur during transport, storage or laying, but it heals up under the combined effect of two reactions:
- swelling (rapid) of the mortar in water,
- hydration (slow) of the cement compounds.
The linear coefficient of thermal expansion of cement mortar linings is approximately 12 x 10–6 m/m/°C, a value virtually identical to that of ductile iron (11 x 10–6 m/m/°C), thus eliminating the risk of cracks formation through differential thermal expansion.
Cement mortar mechanical performance
The quality of the adhesion to the ductile iron wall gives to the lining two important
- a good resistance to vacuum,
- a good resistance to longitudinal bending and to ovality.
Longitudinal bending tests on small diameter pipes have demonstrated the ability of the cement mortar lining to withstand limited pipe diametral deflection.
For large diameter pipes, which are more susceptible to ovality effects, ring bending tests have confirmed the good performance of the mortar lining under high top loads.
Cement mortar has good abrasion resistance, which allows the pipes to be used for carrying raw waters with high abrasive particle content.
GB/T 13295: Ductile iron pipes, fittings, accessories and their joints for water pipelines. Requirements and test methods.
GB/T 17457: Centrifugal cement mortar lining. General requirements.