Water & Wastewater Treatment

28 | SEPTEMBER 2015 | WWT | www.wwtonline.co.uk In the know Getting to grips with… concrete tanks 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.

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