Piling contractor Westpile has just completed commissioning its new slipforming process for its precast Hardrive piles at its Hoveringham works in Nottinghamshire.
Developed with Italian equipment manufacturer Nordimpianti, the slipform machine has taken over two years to reach fruition, explains Westpile contracts director Pat Deighan.
Traditionally, precast piles are formed in moulds. Reinforcement cages are placed in troughs that are filled with concrete, vibrated and allowed to cure. The main drawback of the method is that it is slow, with piles only ready the next day. Furthermore only four piles of a set cross-section and length can be formed in each mould, and different sized piles require moulds to be changed over. Firms often do not have all the necessary moulds .
To overcome this, precast piling companies have typically produced and stockpiled a large number of different piles sizes and lengths to allow new contracts to start without waiting for piles to be cast. This is fine if work is steady and demand for the entire range of piles is constant. However this is not the case and so companies often create a build-up of piles that sit in the yard unused, tying up finances. 'And you can only store so many,' says Deighan.
So, when Westpile's moulds were coming to the end of their working lives, rather then simply replace them, the company decided to look at different ways of forming precast piles. 'We had to decide whether to replace the moulds, refurbish them or try something different,' says Deighan.
There were a number of options: longer moulds with a concrete train to increase production rates, extrusion or slipforming, which had not been attempted before. Extrusion would only work if the piles could be cut so the firm decided to throw out the idea of moulds altogether and look at slipforming.
Westpile first approached Nordimpianti, which manufactures bespoke slipformers. The firm was initially reluctant, says Deighan but after a successful trial of the principle in August 1998, the plan was able to go forward.
The new slipform machine arrived at the end of September and work to get it up to speed could start in earnest, involving minor adjustments to the machine and trialling a number of different concrete mixes. The machine's arrival coincided with the move of the firm's plant operations and northern office from Newark-on-Trent to Hoveringham.
The slipformer runs along a metal bed, onto which the reinforcement cages, complete with joint plates are laid. Four, 110m lengths of 250mm or 270mm square piles, or three, 110m lengths of 320mm or 360mm square piles can be formed side by side in one pass of the machine.
Westpile has two machine lower sections (containing the formers, hoppers and the vibrators), one to produce the smaller and one the larger piles. The pile size is achieved either by adjusting the machine to switch between 250mm and 270mm or between 320mm and 360mm or by removing the top half of the machine and placing it on the other forming section.
Instead of being restricted by the length of available moulds, up to a maximum of 13m, in theory any length of pile section can be made using the new method, the only restriction being the limit of the machine driving them and the logistics of transporting longer piles to site. Currently the limit is 15m. Stop ends are placed between each pile, fixed to the bed by magnets.
After being loaded with concrete, carried from the mixing plant in a specially designed skip loader suspended from a hoist running the length of the factory, the machine begins to move slowly along the bed. A front hopper feeds two thirds of the concrete into the formers and is densified by vibrators at the back of the machine. Concrete from a second rear hopper completes the pile. The piles emerge from the back of the machine to be cleaned by hand, leaving the lifting eyes standing proud. Once cured, the stop ends are removed, and the piles are lifted into a lorry driven alongside the bed. Deighan says that eventually the lifting eyes will be removed from the design, with the piles being lifted by grabs.
A great deal of work has gone into perfecting the concrete for the piles. The final mix is very dry and achieves a high strength in a very short space of time - strong enough to support a person almost immediately the piles are formed. 'We have achieved 45N/mm2 in 24 hours,' Deighan confirms. The cement content is also low (although within specifications), helping to keep the cost of the piles low and reducing the chances of cracking. Key to the entire process is a constant supply of concrete so that the machine does not have to stop. This is achievable because all the concrete is produced on site, with the aggregate supplied from Tarmac, which quarries aggregate next door to Westpile's factory.
The machine can complete a run in just 1.5 hours, equating to 440m of the smaller section piles and 330m of the larger ones. With a potential output of 4,400m a week - some 30% to 50% more than is possible with moulds - and the ability to change the sizes of piles produced, the benefits of the system speak for themselves.
Westpile estimating manager John Gedge says there is currently a demand for low load bearing piles generally 250mm and 270mm section for housing and low rise commercial construction. At the moment, around 20% of the firm's business is precast driven work and the new technique is a result of the firm looking to increase its share of the market. He adds that the advantage of the new system is that only a relatively small investment is needed to change the sizes of piles to react to demand, rather than having to buy in new moulds.
'The method is much cleaner, faster and more flexible,' confirms Deighan, who adds there is potential for increasing pile section lengths, removing construction joints and lowering cost. It will also be easy for more beds and machines to be housed in the factory, further increasing production.
And while Westpile will keep its moulds for the moment, particularly for larger contracts, it is obvious why things had to change. 'The price per linear metre for precast driven piles is the same as it was 20 years ago,' says Gedge. 'Unfortunately all the other costs have gone up.