Many elastomers (including natural rubber), both in their virgin form and as recycled materials (mainly from scrap tyres) have been, and are being added to bitumen (asphalt) to enhance the properties of road surface dressings. There has been interest in rubber as a paving material for a long time. Rubber has been admixed with bitumen to improve the durability of roads since the 1930s and there have been many programmes instigated mainly by elastomer producers. The elastomers have been evaluated in the form of latices and as both vulcanized and unvulcanized powders (fine particles) and crumbs (larger particles). Rubbers range from the very cheap (various off-grade materials) to the relatively expensive, such as polychloroprene and thermoplastic elastomers.

The viscoelastic properties of the bitumen are modified by the addition of elastomeric materials. Claims made by Mullins for modified bitumens include:

• longer life for the surface dressings,
• reduction in cracking and fatting up,
• improved resistance to flow or deformation,
• improved adhesion and resistance to stripping,
• reduction in aquaplaning (the water drainage capacity may be increased) and road noise,
• the low temperature properties (Shim-Ton) of the surface dressing are also improved,
• in many cases it is possible to achieve a reduction in the pavement thickness,
• it is possible to lay the material under a wider range of climatic conditions,
• in countries such as India, which lack an adequate domestic supply of bitumen, the use of rubber crumb from scrap may be regarded as a means of limiting imports.

Tests by Smith showed that unvulcanized materials (latices or powders) are about twice as effective as vulcanized virgin materials. Tyre scrap only showed a very marginal enhancement in the properties of the bitumen in comparison with these materials. Nevertheless, it is at least questionable whether it would be detrimental. An inherent problem is that bitumen is a very cheap material, and most elastomers are very expensive in comparison with it. Furthermore, the laying of surface dressings tends to be highly competitive, and any modification of the process is bound to lead to a major distortion of the costs. The incorporation of an elastomer may lead to an increase in costs of well in excess of 10%. Dunn estimates that the pavement life may be approximately doubled, although the cost is increased by between 50 and 100%. Thus any enhancements to the durability of the dressing have to be very large to cover the increase in preparation costs. Within this type of environment there is a need for state intervention.

Ultimately, more durable roads are advantageous to users as less road space is wasted on repairs and journeys are less liable to delay. Similarly, a reduction in road noise offers considerable environmental benefits which can only be achieved otherwise by lower speed limits. Rubberized roads are safer and should reduce accidents.

The United States introduced, but did not implement, legislation which would have required individual states to order scrap tyres to be incorporated within federally financed road surfacing projects. As originally drafted, from 1994 5% of federally funded roads which are currently paved with asphalt should have contained rubber from scrap tyres, and that percentage should have increased by 5% annually until 1997. Pressure from individual states and from the bitumen suppliers (who would lose some of their market) led to the postponement of the measure. There appears to be little European interest in the use of discarded tyres in surface dressings for roads. The natural rubber producing countries remain highly interested, however.

References

• Dunn J, Recycling/reuse of elastomers - an overview, Rubber Division, American Chemical Society, Orlando, Florida, Fall 1993
• Mullins L, NR Technology, 1971 (3)
• Shim-Ton J, Rubber Chemistry and Technology, 1980, 53, 88
• Smith L, Journal of Applied Chemistry 1960, 10, 296
• Eastman A, United States scrap tyre alternatives, markets & solutions, Rubber Division, American Chemical Society, Orlando, Florida, Fall, 1993