The right cast iron pipe system for every bedding
Cast iron pipes were the basic stock when it came to the installation of urban drinking water supplies 150 years ago. With increasing requirements from users for efficiency and simultaneously increasing chemical and mechanical loads, an innovative foundry industry responded with some sophisticated solutions.
Protection against external chemical influences
Bitumen paints, zinc, zinc-aluminium
In the 1960s the health-related ban on the use of tar became effective and resulted in the use of bitumen paints. In aggressive soils, external coating with “bitumen paint” was no longer sufficient and at the beginning of the 1970s it was replaced by the “zinc plus bituminous finishing layer” system. Initially it consisted of a 130 g/m² layer of metallic zinc with a coating of bitumen paint at least 70 µm thick. Later, this zinc layer was increased to 200 g/m². In around the year 2000, this was supplemented by a zinc-aluminium layer of 400 g/m², with a finishing layer of epoxy paint.
The protective effect is based on the position of iron and zinc in the electrochemical series of metals. This is so in most cases, in particular if the pH value of the soil electrolytes is above 6.5. In boggy and marshy soils with their acidic water, the zinc ions remain in solution and the protective mechanism is inhibited. Thick coatings have been developed for these soils which act as barriers and separate the iron from soil electrolytes with a very high electrical resistance. The Table shows the coatings which have been standardised for cast iron pipe systems in Europe.
Coating numbers 1 to 5 acting as electrochemical barriers can be used in soils of any kind, but they must be free of pores and damage.
A fibre-reinforced cement mortar coating to EN 15542 has a hybrid status. It is 5 mm thick and is applied to the galvanised pipe with an organic adhesive primer. With a polymer-modified mortar, the adhesive primer can be omitted. Both versions can be used in all soils. A cement mortar coating is extremely mechanically robust and has proved its worth above all with the trenchless technique, where sharp obstacles often lie undetected in the ground where the pipe is to be laid. But also, with installation in alpine areas to which it is almost impossible to transport bedding sand, the excavation material from the trench with coarse, sharp-edged stones and boulders can be used again as backfill.
While the fundamental developments of modern corrosion protection systems for buried steel and cast iron pipelines were coming to a temporary end in the middle of the 1980s, experience concerning optimum protection in different soils had also reached the stage that specific rules and regulations could be produced on this range of topics. First and foremost, this includes a determination of the corrosion likelihood for unalloyed ferrous materials depending on the most important soil parameters. This began with DVGW worksheet GW 9 “Evaluation of corrosion risks of buried pipelines and vessels in unalloyed and low-alloy ferrous materials in soils”. After 14 years of experience with the application of this data sheet, DIN 50929-3 “Corrosion of metals – Corrosion likelihood of metallic materials when subject to corrosion from the outside – Part 3: Buried and underwater pipelines and structural components ” was able to be published.
The experiences gained from the application of DVGW worksheet GW 9 are reflected the fact that, of the determining parameters available, only those which have proved able to be determined in practice have been adopted. Soil condition has been more sharply defined and given additional weighting on the basis of experience. Also, parameters which, in themselves, cause very high corrosive behaviour have been given more weight: soils with a high content of organic substances and contamination due to fuel ash, waste, scrap, effluents, coal and coke.
Once the classification of a soil had been established with the help of a type of system analysis, all that was now missing as a link to the type of protection for metallic pipelines mentioned above was a technical rule with which the aggressiveness of the soil surrounding a pipe route could be assigned to a pipe coating to match it. This was DIN 30675-2 for ductile iron pipes. In the revised version of DIN 30675-2 issued in 1993, the areas of application for the different coatings were extended to include the term anode backfill. This documented the fact that, in addition to the coating, the bedding of a pipeline is also part of the passive corrosion protection system and should be taken into account in the areas of application. DIN 30675-2 was revised for the second time in 2019.
The correct choice of external protection for ductile iron pipe systems against chemical attack in a self-contained set of technical rules is relatively simple, particularly if, when considering the local circumstances, it turns out that route is recognisably contaminated with organic admixtures.
Protection against mechanical loads
The system of ductile iron pipes, fittings and valves is, in itself, very robust and does not need any particular mechanically effective external protection unless local circumstances demand a high level of protection against corrosion.
The shortage of sand as a bedding material which has been looming meanwhile has, in the revised version of EN 1610 resulted in recycling materials being permitted for the first time among the construction materials delivered. Also, the reuse of the excavated soil is allowed as long as it does not contain any elements which might damage the pipe.
Cement mortar coating
In this sector, cement mortar coating to EN 15542 has prevailed, being able to be used in all class I to III soils. In addition, this extremely robust coating allows bedding in soils with stones of up to 100 mm in size, as described in Annex G of DVGW worksheet W 400-2.
The use of ductile iron pipes with a cement mortar coating is to be considered sustainable for several reasons:
1. The reuse of trench excavation material saves both its transport away from the site and the delivery of bedding sand to the site, which means that additional HGV traffic, including the CO2 emissions, is avoided. When constructing water pipelines in Alpine areas for turbine pipelines and snow-making facilities, the transport of material is very much restricted and often well-nigh technically impossible. In these cases, the coating according to EN 15542 is so robust that bedding the pipes in the existing rocky screen has become common practice.
2. Bedding in coarse ballast opens up an entirely new application option for the ductile iron pipe with a coating of fibre cement mortar: with the sponge city principle, a pipe trench filled with coarse ballast can be used as linear intermediary storage for rainwater at times of heavy rainfall which is available to trees in urban areas for a longer period. The proven root resistance of the cast iron pipe joint means that a tree can be planted directly on the route of the pipeline. This application offers two simultaneous climate-related effects:
– Avoidance of flooding by the intermediate storage of rainwater.
– Improved growth conditions for street trees with the associated improvement of the microclimate because of the increased evaporation performance of their healthier crowns.
3. The development of trenchless installation and replacement techniques has been significantly influenced by ductile iron pipes with restrained joints. Here one can really talk about a bedding with more or less unknown properties. In a borehole strengthened with bentonite there can actually be anything which a pipe pulled through it might come up against, such as sharp stones, sharp-edged remains on foundations, fragments of grey iron in the case of burst lining etc. The list can easily go on. Ductile iron pipes with cement mortar coating have won through for these techniques with “unknown bedding” right along the pipeline.
The technical solutions for the external protection of ductile iron pipe systems against chemical and mechanical stresses show that sustainable construction of pipeline for water and wastewater is possible with any type of bedding material.
Dr. Jürgen Rammelsberg, EADIPS FGR