A report into Sweden’s water and wastewater networks anticipates a long-life for the next generation of pipes. The authors of Sustainable water and wastewater pipe systems of the future, which has been published by the Swedish Water & Wastewater Association (SWWA), argue that the networks currently being installed should have an operational life of at least 100 years and that pipes laid from 2020 should have an operational lifetime of 100-150 years.
The report also argues that the current renewal rate needs to increase by 40% to maintain the current condition of the network. However, when renewal is carried out, it should be done to such a standard that the new pipes have an average operational lifetime of 100-150 years.
Concrete pipes account for 69% of the waste- and storm water pipelines in four of the main water utilities – NSVA (six southern municipalities), Kretslopp och vatten (Gothenburg), VA Syd (Malmö) and Höganäs. This is representative of the country as a whole, though some have been rehabilitated or replaced in the 10 years since this figure was calculated.
A survey in Malmö found that many concrete pipes are still operating successfully 100 years after installation. Unreinforced concrete pipes manufactured in Sweden today are generally 150-1000mm diameter, with the larger reinforced concrete pipes coming in at 400-3000mm. The main method for condition assessment of gravity sewer lines is CCTV, particularly for assessment of pre-stressed steel reinforcement pipes.
Concrete pipes laid in the 1940s generally have a shorter life of 50-100 years due to the shortage of cement during the Second World War and its substitution with finely ground limestone filler. They currently require replacement. It was during the 1960s and 1970s that most concrete pipes were installed and, given a pipe-life of 100 years, a major replacement programme will be required around 2050.
The various causes of deterioration of underground concrete pipes are already fairly well known. Sulphuric acid attacks from the hydrogen sulphide (H2S) in sewage can reduce the thickness of concrete, especially where the sewage flow slows down or becomes stationary.
The durability of concrete pipes can be improved in a number of ways and cured-in-place pipe (CIPP) lining is common in Sweden. Resistance to acid and sulphate degradation can be improved by mixing a number of alternative binding agents into the concrete, particularly a ground granulated blast-furnace slag (GGBS) such as Alfarör, which uses a 15% slag mix and fly ash.
Some of the older pipes in Sweden are believed to have been made using 100% Portland cement. In the UK almost all concrete pipes are manufactured to exposure class DC-4, which includes 30% fly ash or GGBS to 70% Portland cement.
Other additives, such as limestone filler or polymers, can be effective, as can a number of surface treatment methods including internal centrifugal spraying.
Recent innovations include mixing bactericidal additives, such as a cationic polymer, into fresh concrete. The polymer is particularly effective in binding and rendering H2S bacteria, while avoiding harm to other bacteria. It has been used successfully in North America since 1996 and became available to the Swedish market in 2010.
In parts of the world where bacterially-induced H2S formation appears as a result of exposure to an optimum temperature of around 30°C, calcium aluminate cements such as Ciment Fondu Lafarge are used. They have greater chemical resistance than most Portland cement and are used for both the manufacture of concrete pipes and also as cement mortar insulation.
The report’s authors recommend that more research is undertaken on how to prevent, control and forecast the degradation of concrete in ageing sewer pipe systems. Particular regard needs to be paid to developing innovative non-destructive methods for condition assessment and online surveying, which can provide valuable data to support maintenance planning and prevent unexpected disruption to operations.
New non-destructive techniques and sensors need to be developed, along with forecasting tools that can enable proactive maintenance and minimise leaks, bursts and network failure. For concrete sewerage this involves the generation of a model for pipe degradation based on the key factors of H2S, temperature, soil movement, reinforcement and corrosion.
Selected information translated from the SWWA’s report The Sustainable Water Management System of the Future (Framtidens hållbara VA-ledningssystem, 2018). Authors: Helena Mårtensson, Annika Malm, Bror Sederholm Jan-Henrik Sällström, Jan Trägårdh (original article published at the Summer issue of the BPDA Newsletter).