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Going with the flow

CONCRETE - Midland Links: Chloride ion attack is still the biggest challenge facing the concrete repair industry. Dave Parker reports on a new answer for one of the UK's worst affected highway structures.

For years the massive Midland Links complex of bridges, flyovers and viaducts has been acting as a forcing ground for new concrete repair technology.

Designed and built before the long term impact of decades of de-icing salt application were fully understood, the hectares of concrete deck and thousands of columns and crossbeams have suffered badly from chloride ion penetration. Now a multi-million pound, long term repair and strengthening operation is well under way. And on the latest section, a new flowing repair concrete is earning its spurs.

Main contractor for the near £20M repair project is Interserve Civils, formerly Tilbury Douglas.

'We're Midland Link veterans now, ' says Interserve project manager Andy Beaney. 'This is contract MM102 - the M6 Gravelly Hill to Thornbridge Major Maintenance contract to give it its full name - and we moved here from MM100 last June. But we've been here for years.'

Gravelly Hill is, of course, better known as Spaghetti Junction.

Nearly 170,000 vehicles a day pass along its six lanes or negotiate its maze of sliproads. From ground level the interchange's serpentine concrete convolutions dominate the skyline, but a closer view reveals the dire state of some of the crossbeams and soffits. Many areas have already been repaired and strengthened.

This time around, Interserve is concentrating almost entirely on repairs to the 230mm thick concrete slab which forms the upper section of the composite viaduct deck.

'In all, around 2,000 individual areas have to be repaired, where there are high chloride levels in the concrete and corrosion of the reinforcement, ' says Interserve sub agent Andy Bond. 'But first we have to plane and scrape off the 100mm of old asphalt and mastic sand carpet. And that's after we've scaffolded up below the deck.'

Three lanes of traffic must be kept open in both directions between 6am and 10pm. This limits the area that can be worked on at any one time: typically each will involve around between five and 10 individual full depth repairs measuring anywhere between 200mm by 300mm to 300mm by 1500mm.

One in 10, typically, will only be part depth.

Repairs start with sawcuts top and bottom delineating the area to be removed. Waterjetting takes out the substandard concrete, then a shutter is installed below before men with angle grinders carefully cut back the top perimeter of the cavity to a 20mm deep square edge. Any badly corroded reinforcement is cut out and new steel welded in.

Then, after a four hour soaking, the area is ready to receive the new concrete.

This, according to the Highways Agency's specification for Class 29F repair concrete, has to be 'a proprietary shrinkage compensated flowable concrete made from Portland cement, natural aggregates, additives and potable water'. Microsilica and chloride based admixtures are banned. The concrete has to pass very tight flowability tests, and achieve a minimum compressive strength in a standard cube test of 29N/mm 2at 10 days after curing at 5'C and in 72 hours after curing at 20'C.

Shrinkage compensated flowable concretes are nothing new, of course, but Beaney says the first generations of these products turned out to have some practical drawbacks. 'In particular, they seemed to be very brittle, ' he explains. 'Because we often have to waterjet concrete right next to a freshly repaired area, we need a tough repair material. But we often found the early flowing concretes would be destroyed in these situations.'

So Interserve was more than willing to work with Tamworthbased product manufacturer Instarmac on the development of a more sophisticated repair concrete, Cemflow MC (see box). This is mixed on site in a forced action mixer, then simply poured into the repair area. A token trowelling produces a smooth finish.

'We then wait until the test cubes from the last pour on that section have achieved the specified strength before we start applying the waterproof membrane', says Bond. Beaney adds:

'On this job the actual repairs have gone very well but the weather has delayed the application of the membrane, which is solvent-based.

'What we really hope someone will develop very soon is a water-based membrane which is much more tolerant of damp conditions than what's available at the moment.'

Secret of strength

Instarmac group sales manager Paul Butcher says that designing a super-fluid high strength repair concrete which expands slightly as it ages, rather than shrinking like traditional concrete, can be tricky. 'It's easy to overdo the shrinkage compensation, ' he explains. 'Or to underdo it, for that matter.

'If you do, you either get surface crazing from the excessive expansion or the classic shrinkage cracking.'

It also seemed that a combination of rapid strength gain and shrinkage compensation could create too much tensile strain at early ages. Instarmac had its own theories on shrinkage and what to do about it, and three years ago was awarded a £100,000 SMART grant by the Department of Trade & Industry to develop a better product.

Cemflow MC is the first fruit of that research, although other products are said to be waiting in the wings. Butcher is understandably reticent about divulging too much of Cemflow MC's secrets. Instarmac's literature reveals only the basic details - a 6mm maximum aggregate size, a total cementitious content of 750kg/m 3and a water/cement ratio of 0.38, which produce a claimed 20'C, 72 hour strength of 38 N/mm 2, for example. He talks only of a 'hydration control system', which 'subtly modifies' the formation of cement hydration products.

This seems to mean that the rate of strength gain curve is smoother and more progressive, reducing internal stresses and strains at early ages. Fibre reinforcement and polymer modification are shunned to ensure the low resistivity required if the material is to be compatible with cathodic protection systems.

A demanding series of trials and tests were needed before Cemflow MC was approved for use on the Midland Link.

Butcher pays tribute to the cooperation received from Interserve - ' they're very receptive to new ideas and were very helpful with the trials', and has some words of reassurance for Interserve project manager Andy Beaney.

'We also see the need for a non-solvented waterproofing membrane - watch this space.'

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