Enthusiasm for cycling is increasing, as is the number of accidents resulting in injury to cyclists. One measure that could reduce injuries is the introduction of shock-absorbing cycle paths to lessen the impact of falls. A new surface coating that mixes rubber particles with asphalt has been developed and tested in Uppsala.
In a fall, asphalt itself is hard on soft cyclists, while cycling on a hard surface is less strenuous; making it a matter of balancing the relative importance of these two parameters. A research project initiated by RISE has explored the possibility of developing and installing a more forgiving type of asphalt.
“Viveca Wallqvist at RISE approached us with the idea,” explains Lars Jansson, laboratory manager at Peab Asphalt’s R&D facility in Stockholm.
“We quickly realised that we could use rubber in the hot aggregate. Shredded, worn tyres are in plentiful supply.”
Initial attempts demonstrated that the mixture gave off far too strong an odour when produced under heated conditions, leading to a difficult work environment for production personnel. Then, the question was if it could be mixed cold instead. Bitumen was mixed with water in the laboratory to produce an emulsion. The problem then was that this process was limited to a 2 cm-thick layer. while a substrate of around 4 cm was required.
“One alternative we considered was mixing while semi-heated; by heating using steam at 80°C we were able to achieve a functional mixture,” says Lars Jansson.
One method for measuring shock absorption in substrata, used for installations such as playground surfaces, is to drop an artificial head equipped with sensors from specified heights, and then calculate the height that is likely to result in head injury. The first year of the project was restricted to laboratory tests in which small sheets of asphalt with added rubber particles were produced and used for field tests. These sheets had a critical fall height of one metre. The critical fall height for standard asphalt is approximately 0.3 metres.
Working life is also critical
There were however other requirements to be met in the design; asphalt ages and cracks, with new layers often being applied to the existing surface. How durable would the new surface coating prove to be? A 200-metre cycle path was asphalted in Uppsala using the new coating in order to evaluate the mixture.
“The surface appeared to be in good shape after the first winter, while cyclists did not indicate that they noticed any difference. Our intention now is to install a new test surface on a more heavily trafficked path, to see how that holds out. As yet, we don’t know how shock absorption will develop over time; this can only be measured in a real-world environment.”
The shock-absorbing coating has other possible applications, such as surfaces outside residential homes for the elderly and at bus stops.
Approximately 23,000 people seek medical attention at Swedish A&E departments each year as a result of injuries sustained while cycling. (Skadade cyklister – en studie av skadeutvecklingen över tid (Injured cyclists – a study of injury development over time), Schyllander, J. and Ekman R., 2013.)