As the paving industry seeks to become more sustainable across all practices, some interesting possibilities are being explored. Waste products from other industries could be reused as additives in asphalt and pavement. These techniques could be applicable in the not-too-distant future, and the pavement industry could become a leader in using recycled materials in paving from other industries and creating more sustainable products.
Waste Engine Oil
Waste engine oil is the oil removed from cars and trucks during oil changes. This oil is typically collected for re-refining. Too much engine oil on asphalt can damage the pavement, but engine oil in the right amount can actually be beneficial when used in the right way. Reclaimed Asphalt Paving (RAP) is a common practice in the industry and an excellent way to reuse asphalt at a lower cost to consumers. This is often done through milling or Full Depth Reclamation. However, that recycled asphalt endured years of ultraviolet radiation and damage. This creates a more brittle pavement than new asphalt. The solution to brittle pavement is adding a new bitumen to the recycled asphalt. A recent study revealed using waste engine oil may be feasible as a way to restore the pavement’s flexibility in the same way as a new binder. More research needs to be conducted, but the study is promising.
Plastics
Plastics are in everything. They are difficult to recycle and don’t biodegrade on their own. The United States produced 35.7 million tons of plastic waste in 2018 alone, a stunning amount. The idea of recycling plastics into asphalt has been proposed and explored in the last several years. The benefits of this practice are that using plastic as aggregates could be more economical, in addition to recycling the material. However, there are still drawbacks preventing this from becoming a common practice. Plastics haven’t produced a consistent asphalt yet, the use of plastics in asphalt in low temperatures is still not proven, and there is concern the plastic could break down into microplastics and flow into water, further contaminating our water sources with microplastics. The theory is interesting, but the practice is still one to wait and see what develops.
Bottom Ash from Municipal Solid Waste
Municipal Solid Waste (MSW) is the garbage sent from households and businesses to landfills and incinerators. The amount of MSW in the United States has increased by 93% from 1980 to 2018. We are drowning in trash. To cope, some of this waste is incinerated in plants, and this incineration process creates bottom ash, an inorganic mixture containing particles of glass, minerals, metals, and ceramics. Bottom ash is typically sent to landfills, but the material could be used to create a more sustainable pervious concrete. In a study from Lebanon, bottom ash replaced coarse aggregate in pervious concrete. The concrete made from bottom ash had lower flexural strength, so its use could be limited to concrete for lower traffic areas. Still, the use of bottom ash in this study opens the way for investigating its use in pervious concrete and possibly provides a green solution to a previously unusable waste product.
Glass
Glass is a recyclable material but large quantities of glass are still being deposited in landfills. When glass is ground down to fine particles, it can be used as a replacement for Portland cement up to a 30% ratio. When glass particles are used, they become an effective binder because of the pozzolanic reaction that occurs. A pozzolanic reaction converts a silica-based material to calcium silicate, which results in a stronger cement. The Romans used this method in their cement building, and it is still used today in some Portland cement. By substituting finely ground glass into the mix, cement makers lower their carbon emissions and effectively recycle waste glass otherwise headed for landfills.
Bio Oil
Bio oil is waste oil derived from any organic industry waste. This includes cooking oil and oil from timber waste, rice husks, sugar cane, and coconut fibers. This oil has a history of use as a fuel, but now it’s being considered as an alternative source for bitumen in asphalt. Using bio oil as a binder offers several advantages. It is more sustainable than crude oil products, and some studies show bio oil could reduce the mixing time and stiffness of reclaimed asphalt, similar to the way waste oil functions. Further study is still needed to see how the oil interacts with the aggregate and if there are environmental risks to long-term use.
Finding ways to reuse waste products is a core part of finding our way to a more sustainable future. The paving industry can play key roles in recycling previously unusable products and keeping these waste products out of landfills while offering more sustainable paving practices.