Why use precast concrete tanks?

Written by Stephen Casey, Project Engineer, Whites Concrete

Precast concrete tanks answer many current industry needs, offering longevity and sustainability while minimising on-site construction time. With ever demanding safety and environmental concerns, as well as the water industry’s drive towards total expenditure (Totex), correct storing and handling of liquids prompts all kinds of important questions. Metal, GRP, membrane or concrete? And if it’s the latter, then should it be precast – away from the construction site – or cast in-situ?

What are the pros and cons of different tank materials?
Metal (steel) tanks will always be a cheaper option, but depending on the requirements of your application, they are not really permanent structures, which therefore may fail to meet the requirements set out by Ofwat for Totex. With concrete, standard longevity is still considered by many as up to 60 years, but more and more companies are designing and building walls, tanks and structures for a 100 year design life. Metal also cannot be used for underground storage, whereas high quality concrete will provide excellent load bearing for stormwater tanks.
As the anaerobic digestion (AD) market continues to gain momentum in the UK we have started to see membrane tank covers in operation, as well as concrete and glass-fused-to-steel tanks. Membranes offer cost savings, but it is a matter of choice in an area where safety is critical as to whether you feel comfortable with site personnel peeling back lids, or are happier with a far more solid structure.
Meanwhile, GRP can be cheaper for certain applications as it is lighter, although it can be limited on span, and will therefore only be able to sustain restricted loadings.

What is the advantage of precast concrete compared to cast in-situ?
Even on the most practical level, where space is invariably restricted on site, nobody wants materials taking up room when they’re not needed.
Having your concrete panels delivered, ready for commencement, ticks all the boxes for minimising time and cost, with speed of construction greatly accelerated. At the 800,000m3 Doe Park Reservoir in Yorkshire, for example, we reduced the upgrade of a wave wall from 12 months to just six by minimising the amount of steel fixing required on site. Precast concrete also provided another benefit by (instead of traditional masonry) having its exposed external faces finished with masonry-effect formliner to blend in with the local environment. Alone, this more sustainable and aesthetically-pleasing option saved £70,000.

How green is precast concrete?
Precast is extremely sustainable, made of natural raw materials (stones, gravels, sand, cement), available almost everywhere and in an enormous quantity. Compared with other construction materials, precast concrete minimises the whole life cycle impact on the environment and can be re-used or recycled almost entirely.
Wastage is also very much reduced. Before anything arrives at a construction site, reinforcement placing and curing methods are all carried out under finite, undercover factory-controlled conditions for manufacture, where a specified mix can be monitored and varied at the flick of a switch. In addition, rigorous structural testing by independent laboratories can verify specification and strength, whilst reusable moulds and a big reduction in transport costs also help meet green credential requirements with ease.

Is precast preferable to post-tension concreting?
Post-tension relies on the integrity of its internal steel tines which can be susceptible to rust and catastrophic failure. The well-documented ‘daisy’ effect, whereby panels have collapsed to form such a circular flower pattern is usually caused by factors such as walls cracking, tendon damage, ingress of water into tendon ducts, as well as drilling and cutting of existing sections. There are many of these types of tanks already in operation throughout the UK, and the simultaneous occurrence of these factors is rare. The problem is, though, that where the steel cables are enclosed it is difficult to monitor corrosion within the units. With precast units, during the development phase, destructive testing can eliminate this, and if there is ever a problem with a precast concrete tank, the evidence is apparent and repair work can be carried out immediately, eliminating the possibility of total failure.

Is precast concrete suitable for anaerobic digesters?
Yes. Concrete is now being used extensively for tanks within the AD industry, capable of adopting a circular or square/rectangular format, as well as the ability to work below ground.
With the added requirement of feed storage for digesters, precast walls can again meet these needs. However, design is the key. The footprint on some schemes can appear unnecessarily large. This may not seem like a major issue but space can soon be at a premium and it seems unwise to create a problem so easily avoided at the design stage. We recently received an enquiry for a feed storage width of 48m. To achieve this, the clients’ plans showed an overall construction width for the walls of 64m using almost 35% more material space and cost than a conventional precast vertical wall.
Precast can be applied to meet numerous design criteria and, in addition to tanks, can be used in a wide variety of applications: walls, roofs and end-walls to name but a few. The modularity of precast means installation is faster and there is no waiting for it to gain its design strength. As with any construction, early involvement is always an advantage, but precast will always offer more options – and will reduce the overall cost of a project.