Frequently Asked Questions
Q. What is Rubberized Asphalt?
A. Rubberized Asphalt is a mixture of up to 2% by weight, of rubber placed in an otherwise conventional asphalt mix. Two main types of Rubberized Asphalt processes are known. One is the Wet system in which the rubber crumb is mixed with the hot aggregate and then the asphalt cement is added to the final product. The second is the Dry system in which the rubber crumb is mixed with the hot aggregate and then the asphalt cement is added to the final product. Another method that Grey County Highways Department has used is the Cold in-place Asphalt system. With this system a coarser rubber crumb is spread on the road and mixed into the newly recycled road by grinding up the existing asphalt surface and adding emulsion and laying the recycled surface back down on the road. The other process that we have used the last three years, is our own Wet/Dry system. This system that we have developed can use up to 4,000 tires per kilometre and is giving us Asphalt pavement which has very minimal cracking after the eight years that it has been down.
Q. What is Crumb Rubber?
A. Crumb Rubber is a granular form of rubber usually in the size range of 2 to 2.5 mm (4 to 19 mesh) down to less than 0.6 mm (30 mesh). The rubber is typically made from used tires either by grinding at room temperature (ambient crumb), or after freezing with liquid nitrogen, (cryogenic crumb).
Q. What is the whole procedure from start to finish?
A. (i) Wet System
With the wet system a finer rubber crumb (30 mesh) is added either at the asphalt cement terminal or with an on-site mixer set up. A finer crumb is preferable as it will stay in suspension better than a coarser crumb. Usually when we off load asphalt cement with the rubber crumb, we circulate continuously at the asphalt plant. We add up to 12% of this rubber crumb into the asphalt cement (this is by volume and not by weight). If on-site mixing is done, the temperature of the asphalt cement should be at 1770C rather than the normal temperature of 1600C. It should be noted that with the addition of rubber into the asphalt cement, the viscosity changes from 500 centi-poise to around 35 centi-poise. This asphalt cement mixture is then added at the asphalt cement into the aggregates in the same method as conventional mixes.
(ii) Dry System
With the dry system, rubber crumb 2 to 2.5 mm (4 to 10 mesh size) is added at the asphalt plant. This rubber crumb is added directly to the pub mill of the plant with a separate conveyor system. The percentage ranges from 1.5 to 3% by weight or around 8% by volume. The important thing with the dry system is to increase the mix cycle by 6 to 10 seconds as the lighter rubber crumb will tend to float up in the pug mill before burying itself in the mix. The rubberized asphalt mix that comes to the road looks the same as conventional asphalt only the colour will stay much blacker than conventional asphalt. It is also much stickier when rolling with the rubber and steal wheeled rollers. Therefore, you will be required to use a soap solution in the water solution that goes right onto the roller wheels. Another product that we have used is Vestenamer. It will help break the bond between the rollers and the road and gives a very good integration of the rubber crumb and the aggregate mixture. This rubberized mix also stays hotter when laid on the road which will mean that the rollers will have to stay back from the asphalt spreader before rolling. This retention of temperature is an advantage in Grey County as it allows us to pave roads at much colder temperatures. With conventional asphalt mixes the temperature should be well above the freezing mark when paving is done for good results.
(iii) Wet/Dry System
The wet/dry system which Grey County uses is done the same way as the other mixes only the rubber crumb is incorporated both into the asphalt cement and also mixed with the aggregate. We like this system as it uses much more rubber and it gives us the qualities of both the mixes. The added benefit with the dry system is also a much quieter road.
Q. What is the benefit of mixing crumb rubber into the asphalt?
A. It has a much higher skid resistant surface than conventional asphalt which is a plus because of our climatic conditions in Grey County. The surface is blacker, it retains the heat better and we are now using fewer amounts of salt and sand in the winter months. We believe the skid resistance is also helping to reduce accidents.
A rubberized asphalt mix (dry method) also reduces the amount of aggregate which is used to manufacture the hot mix asphalt mix by 8% or more. This is a plus as aggregate resources are scarcer and more expensive each year.
A rubberized asphalt mix adds flexibility to the pavement which is reducing reflective cracking and in turn will extend the life of the pavement by 20% or more.
A rubberized asphalt mix reduces wheel rutting in the roads.
A rubberized asphalt mix allows you to reduce the layer equivalency when you are paving. Instead of using a 2 inch lift, we use a 1 1/2 inch lift. This greatly reduces our paving costs.
The roads are much quieter to travel on by the motoring public. Joggers and horse owners enjoy the benefit from reduced impact on joints and muscles while traveling on these portions of road.
The most important advantage of rubberized asphalt is that it aids in the reduction of the large environmental problem which exists in our County and the Province of Ontario with the stockpiling and storing of scrap tires
Q. What is the percentage of rubber in Rubberized Asphalt?
A. The Wet processes are generally in the range of 0.5-1.0 percent by weight or up to 12 percent by volume. The Dry process is usually in the range of 1.5-2.0 percent rubber by weight but this works out to 7-8 percent by volume. Rubberized asphalt for running tracks and tennis courts can be as high as 10 percent rubber by weight.
Q. Where do the scrap tires come from?
A. Scrap tires come mainly from tire retailers such as Canadian Tire, Goodyear, and so on where most of the public gets their old tires exchanged for new. In our County, local landfills are also a source. Other used tires come from auto wrecking yards where cars are scrapped.
Q. Does the Crumb Rubber melt when it gets mixed in the asphalt plant?
A. No, most of the rubber can be recovered from cores taken from the cold pavement. However, there is still some integration of the rubber into the asphalt cement binder.
Q. Does the metal stay in the tire rubber when it is mixed with asphalt?
A. No, the metal and the fiber is removed before we get delivery of the rubber crumb.
Q. How is the metal removed from the tires?
A. In the ambient process, this is done mainly by grinding and using magnetic separators and air blowers. To increase throughput, many processors with ambient equipment use scavenged rubber, i.e. rubber tread stripped from the tire without reaching the steel layers. In this case the production rate goes up but the yield of rubber goes down. The cryogenic process has no problem with steel. The rubber is made brittle by the freezing process and when broken by a hammer mill, the rubber falls away from the steel and is readily separated through screening and magnets.
Q. Approximately how many tires are used per kilometre of road?
A. Because the rubber content varies, the number of tires used is different for different processes. The wet process recycles 1 to 2 tires per ton of asphalt and the dry process 4 to 6 tires. Since about 800 tonnes of asphalt are used per kilometre, rubberized asphalt processes can use about 600 to 4,000 tires per kilometre.
Q. How many tires are generated in Grey County, population 80,000?
A. The regeneration of used tires is more or less constant across the whole of North America at about one tire per person per year. A population of 80,000 will generate about 80,000 tires per year.
Q. What are the types of Rubberized Asphalt projects that Grey County is involved in?
A. The dry system where the rubber is incorporated in the mix as an aggregate. The Cold In-Place Asphalt process where the rubber crumb is laid on the road and mixed with the existing ground up asphalt. The wet system, where the rubber crumb is incorporated into the asphalt cement. This has been done with on site mixing and terminal mixing. The combination wet and dry system where the two systems are combined and used at the same time. This combination system seems to be showing very promising results as we are incorporating the two systems into one and are receiving the combined benefits of both systems into the one mix The dry system where we mix Vestenamer with the rubber crumb to cross link the rubber crumb with the asphalt cement.
Q. Does Rubberized Asphalt Pavement last longer in comparison to conventional asphalt pavement?
A. The performance of Rubberized Asphalt pavement using the different process which we have used over the last 13 years, shows a noticeable reduction in cracking and rutting compared to conventional asphalt. Some projects have not done as well in other parts of Canada but I blame this on lack of quality control. There is no difference in Rubberized Asphalt mixes compared to conventional asphalt in that there has to be quality control checks. In Grey County, I have always had good quality control. I also have good comparisons on my projects because on every project I always do a section of road using conventional asphalt. This gives us a good comparison when comparing rubber to conventional asphalt mixes.
Q. What are the extra costs per kilometre for Rubberized Asphalt?
A. For most Rubberized Asphalt processes, the extra cost ranges from 15 to 20 percent more than conventional asphalt mixes. This includes the main cost item which is the purchase of rubber crumb. Grey County has paid between 0.25 per kg to 0.339 per kg for rubber crumb. This price goes down based on quantity, size of the rubber crumb, and whether or not the product is in bags or bulk form. In Grey County, our costs are not that much different than conventional asphalt mixes because of our Closed Loop System. With our Closed Loop System, we collect $2.50 per tire for all tires collected. This $2.50 offsets our price of shredding and crumbing the tire. When we call our asphalt projects we then supply the rubber crumb free of charge to the contractor. This in turn allows him to save on the aggregates that go into the mix because of the rubber crumb that the County is supplying. We also get the larger benefit with our asphalt pavements lasting 20% longer and the cost savings of reduced salting in the winter months. With rubberized asphalt mixes you can also reduce the thickness of the mix which also saves dollars per kilometer laid.
Q. Can Rubberized Asphalt mixes be recycled?
A. Grey County has had no problem recycling Rubberized Asphalt mixes. A test in Thamesville, Ontario, Canada in 1991 recycled a mix of 30% Rubberized Asphalt laid the previous year. They mixed it with 70% virgin Rubberized Asphalt. The recycled asphalt was placed as a surface course. Grey County tried recycling a section of Rubberized Asphalt using a Cold In-Place recycling machine and also as a R.A.P. in a hot mix asphalt plant. Both projects went well with no problems.