article Fall 2019

Creating Greener Asphalt

Recycled asphalt provides a platform for a sustainable future within civil infrastructure.

The demand for an improved quality of civil infrastructure, particularly concerning cross-country roads and parkways, is encouraging global research of novel engineering solutions for a sustainable paving material. The sheer quantity of asphalt production required to maintain optimal road conditions invites an environmental burden. Because asphalt production is a major contributor to material waste, civil engineers are increasingly concerned about conserving resources in its production. The aggregate used in roads cannot be reproduced and the practice of breaking down mountains for the necessary aggregate is not sustainable. The aggregate found in asphalt roads can and should be reused as much as possible to replace existing roads.

A major advantage of using recycled asphalt mix is that the original asphalt does not need to be broken down to aggregate. As a result, the material is already prepared to be processed on-site rather than having to be hauled long distances to be disposed of. The fumes and traffic congestion created from transporting the material are inconvenient and potentially hazardous to the public. The use of existing asphalt to replace major roads could reduce the environmental impacts by limiting the transportation of materials and may also negate the inconvenient impact of construction on public traffic. According to Hussain Bahia, a distinguished civil and environmental engineering professor and asphalt expert at the UW-Madison, adding higher recycled asphalt to mixes does not reduce the life of asphalt roads and parkways, nor does it increase the amount of required maintenance. In fact, with the correct engineering, Bahia says it may be possible “to produce recycled mixes that exceed the quality of the original material.”

The main obstacle to transitioning to higher percentages of recycled asphalt in the mix is the binder — the adhesive component of asphalt roads. As the binder is organic and oxidizes with time, asphalt roads become brittle and fragile as they age, losing their optimal viscoelastic properties such as durability, resistance to fatigue, and workability. To keep asphalt fresh and ensure that it performs for the duration of the road’s life, a recycling agent is used to rejuvenate, or soften, the old oxidizing roads. The recycling agent is a petroleum or plant-based oil that is selected by engineers. The makeup of an optimal recycling agent is still being discussed and is hindered by the inconsistent quality definition for the agent.

In fact, with the correct engineering, it may be possible “to produce recycled mixes that exceed the quality of the original material,” — Hussain Bahia

Another obstacle preventing the use of recycled asphalt mix from gaining popularity in practice is that only up to 20% of asphalt can be composed of recycled material. According to Bahia, to achieve 30–60% recycled asphalt within the mix, project specifications should be updated to inform the contractors of how to specify the quality in a scientific way.

Ideally, drivers will not even notice a difference between traditional asphalt roads and those constructed with recycled asphalt mixes. Highway engineers can ensure that the material performs as well as the original or better. Local communities will benefit financially once asphalt roads are produced with a higher percentage of recycled material as it encourages a sustainable future for the environment. Taking advantage of existing resources will help communities by limiting their environmental impact. By informing the public of this simple alternative for a material in high demand, the acceptance and practice of implementing recycled material for international infrastructure will hopefully gain popularity and become the standard.

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