Dec 5, 2020
Will we be Driving on our Future Garbage?
Roads made of Recycled Plastic, Glass, and Tires
Asphalt concrete (the black stuff that most roads are made of) has been utilizing old ground up tires and glass for many years now. Due to the collapse of the Asian plastic recycling market, plastics are about to be used in asphalt as well, although it is not clear if this is the ultimate plastic waste solution. Will you someday be driving on your garbage instead of taking it to the dump?
Earlier, I wrote an article about the collapse of the plastic recycling market (mostly the refusal of countries like China to accept our plastic waste) and how the plastics industry needs to streamline and reduce its product (likely under pressure from consumers and government). Enter the road building industry, which it turns out, can make use of recycled plastic, though on a currently limited basis. The road building industry is able to utilize old rubber tires to great effect (the crumb rubber produced is a great binder), and has been using ground-up glass as part of fine aggregate used in asphalt-concrete (the name of the finished product). There are also pure plastic road applications (in which plastics have to be separated), since dissimilar plastics might cause the asphalt concrete to delaminate (this restriction does not exist in asphalt-concrete roadways). Even though this reuse of plastics is encouraging, there are still several reasons to reduce plastic use, including the use of CO2 producing fossil fuels and the discovery that plastics take centuries to decompose, creating a danger to living organisms. Though doubtless there will be cries of “overregulation”, there needs to be disincentives for plastic use (mostly favoring reusable cloth, paper and glass products).
The present plastics recycling system is clearly not working, as globally more than 425 million tons of plastic are produced annually, and up to 75% gets thrown away, where it contaminates the environment. Over 13 million tons of plastic enters the world’s oceans annually, where it does harm to over 700 marine animals. The idea behind plastics utilization in asphalt concrete is to create plastics pellets of three or more categories (depending on the desired strength of the road) and to combine them with coarse and fine aggregate as a binder using heat. According to a UK-based company MacReber, who has been utilizing this process, the plastic road product then becomes inert and not subject to microplastic degradation or contamination, although others have disputed this assertion. Microplastic degradation is compounded by the tendency of these particles to pick up other pollutants. Pure plastics roads have been placed in Sweden, and are thought to require one type of plastic (usually HDPE or PETE) to prevent phase-separation and structural weaknesses. The use of plastics in asphalt concrete is thought to be less sensitive to dissimilar plastic types, has better wearability than traditional asphalt concrete, and can be made at a lower temperature. Plastic infused paving materials are potentially cheaper than traditional asphalt concrete, and can be made to incorporate channels and conduits for drainage or utilities. Plastic pavement is currently a sort of competitor to rubberized asphalt (using old rubber tires), but there may be a place for both in future roadway paving. Both products can utilize ground up glass as a portion of fine-aggregate.
The two most common types of plastics are PETE plastic (the kind you see in soda bottles), and HDPE plastic (the milk bottle type). Sadly, these two may not be going away any time soon, as they are favored by the industry and grocery stores/farmers hate the alternative (glass). We may be resigned as a society to the usage and recycling of these two types for many years to come, but the good news is that if we separate them properly, there are many secondary uses (in addition to asphalt substitution), such as the manufacture of new containers, carpet, furniture, clothing, and other construction materials. PVC plastic is heavily used to make water lines and toys, and will also not be replaced any time soon. PVC should not be used for beverage containers (it is confused with PETE and thus becomes a recycling contaminant) and should be a third recycling category, primarily for construction materials. LDPE (used primarily for plastic shopping bags) should not be used at all, and California has successfully banned its use in favor of cloth and paper bags. Polypropylene should be banned, as it is in limited usage (mostly margarine and soup containers, plus the insidious plastic straw) and not easily recycled. Polystyrene (Styrofoam) should similarly be banned, as there are superior hot beverage containers (ceramic and cardboard) and superior insulating materials. Miscellaneous plastic (such as that used in glasses, CDs/DVDs, wire insulation) should find a “miscellaneous” recycling solution. The proliferation of “take out” food has increased the use of LDPE, Polypropylene, and Polystyrene. Responsible restaurants should either use returnable glass/ceramic/metal containers (with a deposit) or paper/carboard containers. McDonalds and others have done a great job of replacing plastic in take out containers, but there is still a ways to go.
Like this article? Read more in Vern Scott’s new book “Civil (Engineering) Disobedience”, available on Amazon.com
