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2020 / 2021 Edition

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Roadbuilding Roughage

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Asphalt

Proving that, like humans, even roads can benefit from a little more fibre, The University of Sheffield has passed a major milestone in its EcoLanes project which seeks to use steel fibre from post-consumer tyres in road-building. Here the project’s Kyriacos Neocleous (left) offers readers a background to the project and what it may mean for roadbuilding.

The environmental impact of waste is high on the EU agenda and since 1975 has led to increasingly specific environmental legislation aiming to reduce transport and treatment of waste. Today, that legislation is being incorporated into a new thematic strategy which stresses the prevention and recycling of wastes. Post-consumer tyres are one of the many wastes covered under the strategy.

Post-consumer tyres arise from different sources, and until 2006 many tyres were disposed of in landfills. However, today the majority are managed in environmentally sound ways such as reuse, re-treading, recycling or energy recovery.

Tyres are recycled by utilising diverse treatments and technologies. The materials produced include rubber as well as steel tyre-cord fibres. While there are many applications for the recycled rubber, there are limited applications for steel tyre-cord.

The steel is rich in tensile properties and with the right shredding and sorting of tyre material a yield of around 70% can be achieved.

The University of Sheffield identified the potential use of these recycled steel fibres as a concrete reinforcement in construction since industrially produced steel fibres are replacing conventional reinforcing bars in a range of wet-mix concrete applications, such as industrial floors.

In laboratory studies, steel fibres have also been successfully used to reinforce roller-compacted concrete (RCC) – a dry consistency concrete which has zero slump and does not require side formwork – which is used in the construction of pavements and reservoirs.

Conventional asphalt pavers are used to place and initially compact the RCC up to 90%. It is further compacted by using conventional asphalt rollers. Thus, this construction process does not require the development of any new equipment. In addition, RCC pavements require less time for curing than conventional wet-mix concrete pavements so may be opened to the traffic much earlier – typically two to three days after compaction.

Despite the improved mechanical properties of steel fibre-reinforced (SFR) RCC, up to this point there have been no commercial applications of SFR-RCC pavements. The main obstacles being the cost of fibres and the practical problems associated with adding the steel fibres in in-situ RCC.

However, research and development currently being performed at the University of Sheffield and Aggregate Industries UK under the FP6 STREP project 'EcoLanes’, aims to develop economical and sustainable infrastructure for surface transport by using the RCC technology and recycled steel tyre-cord fibres.

The new construction concept promises to reduce construction costs by 10- 20%, reduce construction time by at least 15%, reduce the energy consumption in road construction by 40%, and use post-consumer materials in road construction, including recycled aggregates. The ultimate goal is for a recyclable construction system for surface transport.

Through its nine workpackages (see picture below), the project will deliver new processes, models for life-cycle assessment and costing, and design guidelines. The results of the project will be validated by constructing full-scale demonstration projects in four diverse European climates and economies – Cyprus, Romania, Turkey and the United Kingdom.

The trial in Romania is scheduled for construction late this year, while the other three will be constructed in early 2009. The UK demonstration will replace an existing pavement and it is expected that the aggregates, recycled from the existing concrete pavement, will be used in the construction of the new SFR-RCC pavement.

But to achieve its aims and objectives, the project has to overcome scientific and technological barriers in fibre processing, concrete manufacture and road design. And a main barrier, as has been mentioned, is the difficulty in incorporating the fibres in the dry RCC mix.

To eliminate this technological barrier, the EcoLanes consortium is examining various industrial processes and equipment which could be used to successfully disperse steel fibres in dry mixes, and maximise the amount of fibre content added to the mix without attaining balling.

And in June, at a predemonstration trial carried out in the UK at Greenwich, London, to assess some of the technologies planned to be used at the EcoLanes project, we stepped a lot closer to overcoming barriers.

Two of the main aims of the trial were to check the suitability of ready mix pan mixers for producing SFRRCC and to test the suitability of existing laying machinery for laying SFR-RCC. The trial was also designed to test some other technologies regarding the use of special thin asphalt mixes as overlays and their bond onto RCC.

The monitoring of the trial was divided into four phases in order to effectively assess the performance of the different technologies used. The three phases were:

  • Mixing of SFR-RCC at the ready mix plant
  • Transport of SFR-RCC from mixing plant to site
  • Laying and compaction of SFR-RCC
  • Surface treatment, laying and compaction of asphalt

The first phase of the trial involved dispersing the fibres into the conventional RCC mix, mixing and then discharging the SFR-RCC onto tipper trucks for transport to the site. Due to the nature of the ready mix plant, it was decided to disperse the fibres manually through an opening of the pan mixer.

A special chute was constructed to facilitate the dispersion of the fibres. The results of the trial (see picture below right) were satisfactory as the fibres were evenly distributed in the dry mix with minimal fibre balling. No problems, such as segregation or mix clogging, were encountered either during the mixing or discharge of the mix.

The second phase of the trial, which involved transport, was aimed to check whether any segregation or drying of the mix would occur over long distances. During this phase of the monitoring no problem occurred either. The mix was to the optimum moisture content when delivered on site and no segregation was present.

The third phase of the trial also ran smoothly. The SFRRCC was placed through the paver without any issues of segregation or clogging of the mix in the paver or balling. The mix behaved exactly the same as an ordinary RCC.

Discharge from the paver was smooth and the levels were achieved without any problems. The flow of the mix was constant with no patches or compacted material, and there was no effect on the speed of laying.

All these factors are very important to be achieved. They are what make RCC construction efficient and competitive compared with other pavement construction methods. The compaction of SFR RCC and the joint cutting was also achieved to the design values without any problems.

The laying and compaction of the thin asphalt overlay was the final phase of the trial. A specialist mix was chosen and then different bonding techniques were used during the overlay. Again, there were no problems. The thin layer was placed without any difficulties. Bonding is going to be tested by taking cores. This phase is still underway.

In general, the trial was a big success and all the technologies and methodologies on trial proved to be effective. Lessons learned from this will help create a methodology of using SFRRCC in the EcoLanes project and beyond.

And while this trial dealt with the use of recycled steel tyre-cord fibres in concrete pavements, these fibres could be used in other concrete construction products – such as crack resistent factory floors – for which the University of Sheffield (USFD) has patents granted.

As well as a sustainable future for road building and other concrete applications, this approach to using steel fibres in concrete construction offers the tyre-shredding and recycling industry
serious added value.

Current tyre derived steel is a difficult product to shift. Recyclers either shoulder the costs of dumping or secure a low price as scrap. However, this final route is closing with rubber contamination being a major issue.

Using the material in the UK road network is a win-win all round.

If you would like to get involved or learn more about the opportunities offered by the product call Kyriacos in Sheffield on 0114 222 5729.

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