High-tech road science: Where nano meets pavement
When people think about the latest technological advancements, images of 3-D HDTVs, smart phones, and other sleek electronic devices are likely to come to mind. Much less likely are images of the road that gets them to school or work each day, or the driveway they leave the car in at night. However, the dark, unassuming pavement that most people only notice when they hit a pothole is the subject of a lot of cutting-edge research; new developments in asphalt pavement could dramatically reduce fuel consumption, environmental pollution, and the frequency and cost of maintenance.
Asphalt is a sticky, black substance that can be naturally occurring (you probably want to avoid going for a swim in Trinidad's Pitch Lake, for example), or refined from crude oil. If you mix asphalt with additives like stone, sand, and gravel, the asphalt binds them together into pavement, also called asphalt concrete or blacktop. Asphalt pavement covers more than 90% of the paved roads in the United States and is the method of choice for many because it is durable, inexpensive, and not very time-consuming to complete.
Standing up to heavy traffic, harsh weather conditions, and constant utilization takes its toll on asphalt pavement. It has been employed for centuries--the earliest record of asphalt paving dates back to Babylon in 625 B.C., according to the National Asphalt Pavement Association--and as you might imagine there have been lots of improvements since then. For example, porous asphalt is becoming increasingly popular for applications like parking lots. Developed in the 1970s, this technology enables water to drain naturally through asphalt pavement into a stone bed below, and then into the soil below the bed. This is preferable in situations where it is costly or hard to build a storm water management system.
Asphalt damaged by heavy traffic or weather can be costly to repair and dangerous. The damage shown here was caused by the freezing of ground water in the soil underneath the asphalt pavement.
Currently, one of the most intriguing areas of asphalt research involves the use of nanomaterials. A research group at Michigan Technological University, led by Dr. Zhanping You, is exploring the use of nanoclays in asphalt pavement. Nanoclays are clay minerals, similar to what you might find in shale or mudstone, that are optimized for mixing in with other materials. Because of their unique structure, adding nanoclays to a material makes the material harder or stronger. You's group found that adding even small amounts of nanoclays to asphalt makes it stiffer. This means that it deforms less under heavy traffic and in hot weather--making the pavement more resistant to ruts (potholes). Ruts are not only costly to repair; they can also be a safety hazard. You and his team have more work to do, refining the mixture and testing how well it stands up to cold weather and other challenges, but the work looks promising.
Another research team, composed of Dr. Franz-Josef Ulm and his student Mehdi Akbarian from the Massachusetts Institute of Technology, modeled what happens to pavement when a vehicle tire rolls over it. They looked at the physical forces involved (like the friction between the tire and the pavement), the characteristics of rubber and pavement, and the motion of a tire to determine how pavement deforms when a vehicle travels on a road. They found that pavement deforms in such a way that a tire is always rolling uphill. Softer, more flexible pavement deforms more under the weight of a tire than stiffer pavement, so the tire has to travel up a steeper slope. The problem is, a tire rolling uphill requires more gas than a tire on level ground. The steeper the slope is, the more gas that's required.
When the tires of a car or truck roll over a roadway, the maximum pavement deflection is just behind the path of travel. This has the effect of making the vehicle’s tires roll continuously up a slight slope (exaggerated in this illustration), increasing the vehicle's fuel consumption.
Image by Mehdi Akbarian
Ulm and Akbarian then gathered information from the Federal Highway Administration to input in their mathematical model for fuel savings from more than 5500 sections of highway: the type of pavement, type of soil beneath the pavement, and usage data, including the number, type, and weight of vehicles that travel on each stretch. The model showed that fuel consumption on roadways in the United States could be reduced by as much as 3% just by using stiffer pavements!
In addition to saving drivers money, the researchers found that using stiffer pavement could reduce carbon dioxide emissions annually by as much as 46.5 million metric tons. In 2010, the carbon dioxide emissions from the transportation sector (highway vehicles, air travel, marine transportation, and rail) totaled about 1.75 billion metric tons, so this reduction would have noticeable impact.
Adding nanoclays to asphalt pavement is not the only way to create stiffer pavement. For example, using a thicker layer of asphalt paving materials or adding a layer of concrete under the asphalt could have a similar effect. In addition, research on the structure of concrete is opening up new possibilities for scientists to nanoengineer concrete that has ideal properties for multiple applications, including paving.
There are about four million miles of public roads in the United States, and decisions about repaving them and paving new projects involve a lot of factors—initial cost, maintenance requirements, estimated use, local weather conditions, and environmental impact, among others. However, research at the nanoscale is likely to lead to the next big technological advancement in paving, an advancement that could have significant benefits even if it's not as flashy as a new TV or smarter than your current phone.
For more information, check out these stories:
Michigan Tech News: Michigan Tech researcher using nanoclays to build better asphalt
Find out more about the work You and his team are doing with nanoclays and asphalt pavement.
MIT news: Civil engineers find savings where the rubber meets the road
Find out more about Ulm and Akbarian's study on pavement deflection and their model for predicting gas savings.
National Asphalt Pavement Association: History of asphalt
Read about the development of asphalt paving, from 625 BC to today.
The Flying Kiwi: Trinidad's Pitch Lake
Experience Trinidad's Pitch Lake through the lens of visitor Richard Seaman.