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Pavement on stream A concrete road contract in South Wales is providing the UK with its first chance to see a two layer slipformed carriageway with a 'whisper concrete' wearing course mixed in a conti

Whisper concrete has gone through extensive trials on roads throughout the UK, by Britpave, the British Insitu Concrete Paving Association and the Highways Agency, and is now being used on a non-trial basis for the first time. A 45mm thick layer of the exposed aggregate concrete wearing course is being laid on top of a standard lean mix continuously reinforced concrete pavement as an overlay to 12km of life expired concrete dual carriageway in South Wales.

The two layer pavement being laid on the A449 trunk road between Coldra and Usk was designed by Burks Green & Partners for the Welsh Office. SIAC Construction (UK) won the 11.5M contract to lay the new pavement job which, as well as being a first for whisper concrete, also sees the UK's first use of two simultaneously slipformed concrete layers.

The original specification called for the two different concrete mixes to be batched separately to avoid contamination of the wearing course. The whisper concrete uses a single size aggregate/ cement mix to form the hard, thin open layer designed to cut down on traffic noise. This is a very different specification to the standard lean mix which goes into the 250mm thick reinforced base course. Both the Welsh Office and Burks Green wanted two separate batching plants, but SIAC was convinced it could be done with one continuous mixing plant.

The firm had seen these plants working successfully in other European countries - particularly in Germany - and believed the continuous mixing process would provide the right quality concrete with no contamination. The client and consultant understandably insisted on trials, and also visited sites where the machines were in use, before allowing SIAC to go ahead with this method on the A449 contract.

The machine it has bought was supplied by Australian company Aran, and is called a Modumix II-4. Aran has been making continuous mixing plants since the 1970s, and they have always been popular for cement-bound mixes, but it was only in the mid-1980s that they started being developed for use with pavement quality concrete. This is the first time a machine of this type has been used for pavement quality concrete in the UK.

As the name suggests, the plant works on the basis of providing a continuous supply of mixed concrete, rather than the individual batches of a specific size that come out of traditional batching plants. It relies on a constant stream of the correctly proportioned ingredients going into the mix.

Aggregates are delivered via hoppers, and binders like cement and pulverised fuel ash kept in silos with a series of baffles inside to ensure the material leaves the silo at a constant rate. The binder is skimmed off the baffles and on to the delivery belt by a cleated belt feeder. All the ingredients are mixed together in a high intensity twin shaft mixer, where additional water and admixtures can also be added.

In a traditional concrete batching plant, different amounts of each ingredient in the mix - aggregate, sand, binder (ie cement, fly ash etc) and water - go into a hopper and are blended to create a predetermined volume of concrete, according to the capacity of the plant. When the batch has been discharged into a pump, skip or truckmixer, a new set of ingredients is put into the hopper, and the process starts again. So the machine produces individual discrete batches, with the maximum batch size being determined by the capacity of the plant.

A continuous process, however, allows the user to determine what the batch size is, and, provided the ingredients are kept topped up, keeps mixing until enough has been produced. This could be anything from just a few cubic metres to thousands.

It also works much faster than a conventional batching plant, as there is no need to stop and restart every few minutes between batches.

However, with batching plants, the right mix is assured, because the ingredients are measured by weight at the start to the specified ratio of aggregate, sand, binder and water. But if the mixing is continuous, individual ingredients cannot be weighed at the start of the process because they are being added all the time. Instead, proportions have to be checked as they go into the mixing drum.

This is done by measuring the density of each material and calibrating the machine accordingly, and then controlling the speed at which the materials go into the mix.

UK concrete mixes are always specified by weight rather than volume, and on the A449 contract, Burks Green wanted to be sure that the concrete complied with its specification. Aran fitted the machine with a device which weighs each material as it comes out of its hopper or silo and arrives on the belt. Burks Green partner Jonathan Green says: 'Traditionally you're looking at weigh batching the materials, and I was concerned that it must be equipped with full weighing facilities. We wanted to let them mix it by volume but monitor it by weight, so they installed scales that monitored the weight of the constituent materials.'

He says the Aran machine has produced 'consistently good results', but adds that he would always want to see a weighing device fitted: 'It gives a good record of any day's paving, and you can identify where each load went into the carriageway. If there's any problem in the future than we can turn up the data.'

SIAC is using flatbed trucks to take the concrete from the mixing plant to the site, and mixing four loads of base

course to every one load of wearing course. The continuous mixing process allows production to switch automatically from one mix to the other at the touch of a button.

Brian Ingham, SIAC's paving manager, says the most important features of the mixing plant are its efficiency and consistency. 'When you're slipforming, the most important thing is the consistency of the concrete. There are tests they do to check the workability of the concrete, and that's what we're looking for.'

The machine has the capacity to mix between 350m3/h and 400m3/h of concrete, but SIAC is currently running it at about 90m3/h to meet the demands of this particular contract. Ingham says that full width carriageway slipforming on a green field site would require an output of 200m3/h.

The Aran plant is claimed to be the first mobile continuous mixer.

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