Currently in California the main type of Rubber Asphalt paving material is RHMA-G which means Rubber Hot Mix Asphalt-Gap Graded. It is gap graded so that the small chunks of rubber can fit into the mix. Yes you read correctly, small chunks of rubber.
It is a common misconception that rubber paving mixes have a smooth liquid asphalt just like a traditional hot mix asphalt (HMA) however if you take a look at the asphalt binder for a rubber paving mix you will find a very different type of mixture. Imagine that normal asphalt has the consistency of pancake batter. A traditional rubber asphalt has the consistency of pancake batter with mini chocolate chips. I think that all of you could figure out that rubber goes into rubber asphalt but most people, myself included, figure that they melt down the rubber before it is shipped to us or that the rubber bits melt down in the batching process at the hot mix asphalt plant. For traditional asphalt rubber binder, both of these scenarios are false.
The reason that RHMA is gap graded is so that these small pieces of rubber, called crumb rubber, can fit into the mix without leaving undesired gaps in the pavement which can lead to raveling or a variety of other pavement failures. To be honest I'm not sure what the initial reasoning behind leaving the chunks of rubber in the mix was, but for me it seems like a chicken and the egg scenario. You see, one of the biggest benefits to rubber asphalt pavements is that it reduces road noise. You know when you are driving on the highway and you can barely hear yourself think and then all of a sudden its 10x quieter? Chances are you just transitioned from traditional asphalt pavement or concrete to rubber asphalt pavement. So I feel that it is definitely possible that some smart engineer out there figured out that we could reduce road noise from leaving in the chunks and designed what we now call RHMA for just that purpose. Of course, it could also be that the rubber requires a special, expensive process to reduce it down to a liquid form...and no one wanted to pay for it at the time. If any of you know the truth I would be extremely interested in what the real reason is.
The crumb rubber in the asphalt is able to reduce noise because it is flexible. If you're having trouble imagining this just remember that most of the rubber chunks in rubberized asphalt actually come from old car tires. It is one of the biggest sources of recycling in the United States and an amazing way to recycle tires since, lets face it, aside from being good weights for tent poles, they are relatively useless after they wear out and just take up space in our landfills. The flexibility of the crumb also helps the pavement's resistance to cracking. It's like if you compared an uncooked piece of spaghetti to a cooked one. Traditional asphalt pavements are generally made of two ingredients, traditional asphalt and rock. Since rock is not flexible it cracks when too much pressure is applied to it. Rubber asphalt pavements generally consist of traditional asphalt, rock, and crumb rubber which gives it flexibility, making it slightly malleable and resistant to more pressure than a traditional asphalt.
Another theory for why asphalt rubber pavements are more flexible and quiet is because of their higher asphalt binder content. The crumb rubber has the ability to absorb more asphalt binder than rock and as such called for a higher content of asphalt. Unfortunately, that brings me to what a lot of people would consider the biggest downfall to rubberized asphalt pavement, it's much more expensive than traditional asphalt pavement, almost double the price. This additional cost is partially due to the crumb rubber pieces and higher asphalt binder content but a large portion of the additional cost is due to the process in which the crumb has to be mixed with the traditional asphalt binder. Because you want to keep the crumb rubber evenly distributed in the traditional asphalt binder it is not a good idea to ship the asphalt rubber binder in a traditional oil tanker where the crumb rubber will sink to the bottom of the tank. Instead, all of the components in the asphalt rubber binder are shipped to the asphalt plant separately and mixed on site before being fed into the asphalt plant and mixed with the rock. The process includes an additional piece of equipment (called a rubber plant) to heat and mix the rubber asphalt binder components together as well as at least 1-2 additional employees. Many companies do not own their own asphalt rubber plant and rent the services as they need to produce asphalt rubber mixes, usually only a few weeks per year. However, with asphalt rubber pavement becoming more and more prevalent in California due to its longer life span, I would not be surprised if it finally becomes financially feasible for companies to purchase their own rubber plant.
If you'd like to learn more about asphalt rubber pavements you might want to visit http://www.rubberpavements.org/. I'll also try to break down the topic a little more over the next couple of weeks to give you a more detailed look into what I think is one of the best investments California is making right now.
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