Within the European Research project ECOLANES scientists have developed a dry mix concrete which is reinforced with the recycled steel fibres. Their aim was to make the production of the concrete as energy efficient as possible, and using recycled materials is a key factor
There are 5 582 000 kilometres of road network in the European Union that have to be kept up. A never ending task regarding the damages caused annually. Repairing potholes, car ruts and cracks disturbs the flow of traffic. Besides, material for the new road surfaces has to be produced, transported and applied.
At the University of Sheffield in England the department of Civil and Structural Engineering searched within the EU project ECOLANES for ways to create an ecological concrete pavement. Prof. Kypros Pilakoutas, coordinator of the Ecolanes project, and his team of researchers developed an economic, long lasting and energy efficient material.
Which have been the most striking achievements of your research activities?
The main target of ECOLANES was to develop long lasting rigid-pavements for the transport industry, and basically concentrating on roller compacted concrete which is a dry mix concrete, using recycled tyre wire from post-consumer tyres, lower energy cements as well as recycled aggregates. The main innovation is to reuse the steel from waste tyres. In the European Union more than 3.2 Million tons of waste tyres are produced annually. A regulation from 2003 prohibits the disposal of postconsumer tyres in the landfill. They have to be recycled.
Which components can be retrieved from waste tyres?
Machines separate now the components: a typical tyre contains around 50 percent rubber. In the form of granulates it is recycled for floor coverings as for example on sport grounds. Approximately 25 percent in the tyres are textiles which makes a good fuel to heat the ovens of steel mills. The rest are steel fibres. The problem: The metal wires still hold a lot of rubber and plastic from the reinforcement which limits the options to reuse them for new steel products. Most of the cases the way that the steel is extracted it still has a lot of rubber and plastic from the reinforcement and hence the steel mills refuse to take it. So most of the steel ends up being waste that is thrown in landfill. The concrete of the ECOLANES project at the University of Sheffield uses processed steel fibres with rubber content. Typical fibre dosages are at 50 kilograms per cubic meter by mass of concrete.
Which are the advantages of using steel fibres from waste tyres?
The cost of steel fibres from post consumer tyres is at least 50 per cent cheaper than that of the manufactured steel fibre reinforcement. There is no need for raw material to be mined and formed requiring extra energy. Exploiting the short and thin steel fibres enables also to use more of them per square meter so that the concrete is highly interwoven.
After mixing the concrete for two minutes a specimen is casted. Due to a very different consolidation method used, the dry mix requires less cement than conventional concrete: externally it is compacted with 10 ton vibrating rollers. In the lab the compression is done with a suitable vibratory hammer. The frame can be stripped straight away. The specimen is stable enough and the researcher can even stand on it. A rough comparison with conventional concrete shows the big difference on the fresh properties. Roller compacted concrete is ready for light traffic right after laying it. Conventional concrete normally requires between 7 to 20 days before traffic can be allowed on.
Have you also tested the durability performance of the new material?
To test the durability performance of the new material, it was exposed to extreme conditions. It stayed 56 days in a climate chamber with a daily changing temperature from plus 20 to minus 20 degrees Celsius. Results do not indicate any major damage to the concrete, since the fibres help to maintain the integrity of the concrete.
Corrosion leads to an expansion of the metal parts. To investigate the corrosion of the samples the researchers placed them into a basin with salty water for a period of ten months. Every three to four days the specimen were removed and dried to expose them to oxygen. After 10 months of corrosion, the major effects were observed only by rusty appearance. Due to the small size of the fibres, the expansion was not high enough to crack the concrete. The stability of the concrete is not affected.
The scientists used a bending experiment to test how well the material coped with the exposure to extreme temperatures and moisture. It takes 100 metric tons and about one hour to crack the sample. The steel fibres keep the concrete together. Without the fibres it would have taken not more than 10 minutes to crack the concrete.
Have you performed your tests only in labs?
We also had four demonstration projects in four different environments in Europe. First in London at the entrance of a logistics centre; second in Romania on a highway that is exposed to very high frequent freeze and thaw cycles of; third in Antalya in a city with heavy traffic from busses and lorries and problems with rutting especially in the hot summers and fourth in Cyprus in a rural environment in a very unstable slope that is cripping. There are a lot of deformations on the road surface.
Are there any disadvantages?
The only disadvantage so far: for safety reasons a thin layer of asphalt has to be added on top of the concrete. This way steel fibres that stick out cannot damage the tyres of the vehicles or hurt anybody.
Anyway you can state that the results of the eco pavements are impressive
Yes indeed. The new material is about 12 per cent cheaper and the construction times are reduced by 15 per cent. When the material is disused, it can be removed, crushed and recycled for a new pavement. The overall energy balance is the most important factor. It is 40 per cent lower compared to traditional concrete pavement. Besides that we hope the new material will provide a better infrastructure in the future: Fewer potholes, less maintenance required and therefore less impact on the traffic.
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