Every 10 days a shipload of concrete tunnel segments leaves Gravesend bound for Denmark. In all, Taywood's Southall plant will supply the Copenhagen Metro project with more than 15km of 4.9m internal diameter lining over three and a half years. A grand total of more than 63,000 individual segments each weighing up to 2.5t will be needed.
Winning the contract against competition from UK and Europe was a major coup for Taywood, which has chalked up an impressive
list of UK projects since the facility was set up in 1991.
'But getting the metro contract wasn't easy,' admits Taywood Precast contracts manager Sam Simons. 'Normally, tunnel contractors in Europe buy a set of moulds and set up segment production on site. We had to convince the client we could deliver the product he was asking for - in terms of programme as well as quality.'
One key factor, Simons adds, was the ability to begin production much sooner than a site-based facility. Taywood's Southall yard has years of records of concrete performance, enabling the tunnelling contractor to begin work without any doubts about meeting the onerous Danish specification. So the first consignment could be dispatched in August last year, only six months after the initial order was placed and long before the Danish authorities completed their mix evaluation programme.
'Nearly all our production uses the C80 mix we developed for the Channel Tunnel,' Simons explains. 'Most specifications call for C50 like the Metro, so we have plenty in hand for the most onerous conditions - which occur during construction
rather than in service.'
A large tunnel boring machine's shove rams can generate 5,000t of thrust on occasions - all of which has to be resisted by the tunnel lining. Poor quality linings often crumble under these levels of stress, causing major delays, so the high strength of Taywood's segments has to be a major selling point.
Simons adds: 'By the time they actually get into the tunnel concrete strength will be approaching 100MPa. This is because we use a 70:30 blend of cement and pulverised fuel ash in the mix, which keeps on gaining strength way past the 28 day point.'
Scottish granite aggregates from Glensanda, a water/cement ratio of only 0.34 and a superplasticising admixture produce a very low permeability concrete - if not the most aesthetically perfect surface finish. Taywood also claims that its high-tech vertical casting production process minimises thermal shock and allows it to achieve very high levels of dimensional accuracy.
'Making our own moulds helps,' says Simons. 'But it is the integration of mould design, concrete mix performance and manufacturing techniques that enable us to work to tolerances of as little as 0.2mm.
'It's also a question of knowing just which dimensions have to be really accurate to produce a really efficient segment design.'
Getting the design right means segments which fit together easily but without any loss in geometrical accuracy. Traditionally each lining ring was made up of as many as 20 or more relatively small segments whose size, and weight, were determined by the limitations of segment handling techniques. 'And the multiplicity of joints gave elasticity to the ring, allowing it to absorb and limit stresses,' Simons goes on.
'Fewer segments means higher concrete stresses - but faster erection.'
Taywood's current ring designs feature six or seven segments weighing up to 3.5t each, which take maximum advantage of the segment handling capacity of modern TBMs. Reinforcement is conventional - although the cages have to be produced to tolerances of +/-2mm - but the company has developed a revolutionary alternative for applications where reinforcement corrosion could be a major problem.
Simons explains: 'When our standard mix is demoulded, it already has a compressive strength of at least 30MPa, which means a potential tensile strength of 3MPa.
'And segments are free of the microcracks caused by thermal shock that can reduce tensile strength to effectively zero. So the unreinforced units are strong enough to be handled immediately, and once in place, of course, the risk of tensile stresses developing is extremely low.'
In practice tensile strength could top 7MPa in mature concrete. Simons admits that promoting such steel-free segments 'requires a lot of confidence in the production process', but points to the success of the 5.5km Brighton stormwater project as an example of what is now possible. This 6m diameter undersea storage tunnel is exposed to seawater both internally and externally, and was built completely from unreinforced segments.
Some clients will always find it hard to accept the concept of steel- free linings, even if they are concerned about the risk of chloride ions penetrating to the reinforcement. Taywood can point to the very low chloride ion permeability of the standard mix, but if this is not enough, then yet another option is available, Simons says.
He explains: ' My view is that protection to the reinforcement starts with very high quality concrete. However, adding a high performance coating can produce concrete which can truthfully be described as impermeable.'
Trials at Southall have used Sikagard-65W, a water-based epoxy coating. Segments were coated immediately after leaving the mould at a temperature of 40degreesC. 'This high temperature accelerated the curing of the coating, while the coating acted as a curing membrane to the concrete,' Simons reports.
He is confident that such impermeable segments could be produced economically. More trials will be needed, however, to finalise the mix design for another new product - fire resistant linings.
Pressure to develop this option comes obviously from the disastrous fire in the French-built section of the Channel Tunnel nearly two years ago. Simons was able to inspect the damage after the blaze, and also witnessed the series of fire tests carried out in advance of finalising the design of the Channel Tunnel Rail Link tunnels. He reports that these trials produced some surprising results.
'Sintered lightweight aggregates failed miserably. Limestone and granite aggregates performed very similarly up to 600degreesC, but in a 1,200degreesC hydrocarbon fire limestone breaks down chemically.
'And concrete with granite aggregate and plastic fibres went through the hydrocarbon fire unscathed.'
Plastic fibres may not feature in any future Taywood fire resistant products, however. Simons says he is not convinced that the type of fibre actually makes that much difference.
'I believe fibres increase fire resistance, especially on very dense high strength concretes, simply by melting away and leaving voids which let steam escape. We'll probably go for the cheapest available fibre - maybe even a natural material.'
This sort of ongoing product development is a key element in Taywood's march away from just being the supplier of 'a lump of concrete'. 'We have deliberately developed the ability to deliver quality equal to or greater than any project specification', says Simons. 'Our objective is to make tunnels easier to build through the application of an integrated engineering approach. We offer a much
more engineered higher performance product - at a competitive price.'
Claims like these are backed by a roll call of major project successes, including the London Ring Main and the Heathrow Express. The Copenhagen Metro contract dwarfs even the 14M Ring Main job, but even so it ties up only less than a third of the Southall plant's capacity. Segments for a number of other projects sit in the yard, while Taywood is involved in tendering for projects all over the world.