Toward More Sustainable Paving

Posted on February 24, 2022

A holistic approach to sustainable paving is ideal. Analysts increasingly look at a road’s entire lifecycle when calculating its carbon footprint. It is important to consider everything from the acquisition of aggregates and binders to the eventual recycling of road materials.

However, lifecycle assessments must recognize that governments don’t always repair or replace roads when it is ideal. The same is true of the nation's hundreds of thousands of bridges. Budget shortfalls are commonplace.

Infrastructure Spending

On January 28, 2022, Pittsburgh’s Fern Hollow Bridge collapsed in Frick Park. Ten people sustained injuries when the 447-foot-long structure dropped about 100 feet into the ravine below. Rescuers rappelled down to reach passengers in an articulated bus.

The failure occurred only hours before President Biden arrived in Pittsburgh. His trip was part of an effort to promote the federal infrastructure bill. The collapse became a symbol of the need to address the nation’s aging transportation network. The share of federal spending on infrastructure has steadily decreased for decades. It was 30% in 1960 but only 12% in 2018, an all-time low.

The American Society of Civil Engineers (ASCE) estimates that more than 40% of the nation’s roads are “in poor or mediocre condition.” The ASCE observes that “these vital lifelines are frequently underfunded.” As a result, DoTs often apply band-aids when excising the wound is a far better solution.

According to the ASCE, the deficit in road/bridge repair and rehabilitation costs the average American motorist more than $1,000/yr in “wasted time and fuel.” There’s a domino effect. Stop-gap repairs increase the frequency of construction zones and detours. Delays mean wasted fuel, which increases emissions. For multiple reasons, well-maintained pavement enhances long-term sustainability.

Improving Roadway Performance

Longer-lasting roads are more sustainable roads due to decreased maintenance. Delayed replacement reduces concrete and asphalt consumption. In this regard, concrete’s longevity advantage is clear.

The MIT Concrete Sustainability Hub consists of researchers from various departments. They demonstrate how future roadways can be more sustainable. They assert that this is possible even within today’s budget constraints. They cite three innovative strategies for managing pavement networks:

  • Use varied paving materials

  • Mix short- and long-term paving actions

  • Evaluate results over longer periods of time

The average road must last many years as it meets the needs of millions of vehicles. Over the decades, variables often change:

  • Material costs fluctuate

  • Budgets may tighten

  • Traffic levels may intensify

  • Climate change may hasten the need for repairs

Managing uncertainties means looking far into the future while planning for possible changes. Unfortunately, no one has access to a crystal ball. Comprehensive data analysis is the alternative.

Smoother, more durable roads have a lower carbon footprint. Here are some ways to deliver better roads.

Smoother roads

Smoother, more durable roads make a difference. For one MoDOT study, staff drove four dump trucks on a 22-mile loop of interstate highway. Before and after resurfacing, each truck was driven for 50 hours at a constant 60 speed of 60 mph. The same drivers participated in each phase of the experiment.

The pavement was “moderately rough” before repaving. After repaving, the diesel dump trucks averaged a 2.4% improvement in fuel economy. Fuel economy improved approximately 1% for gasoline-powered SUVs.

Self-healing concrete

Self-healing concrete uses microorganisms or enzymes to amplify the process of filling voids. Autogenous formulations use moisture for the healing process. Autonomous formulations use encapsulated healing agents like bacteria spores.

At Worcester Polytechnic Institute (WPI), researchers are working with carbonic anhydrase (CA). This is an enzyme found in red blood cells. In the human body, it quickly transfers carbon dioxide from cells to the bloodstream. In the process, CA reacts with atmospheric CO2 to create calcium carbonate. A matrix of these crystals fills cracks as they occur. Self-healing concrete may last four times longer than regular concrete, according to researchers.

The prompt sealing of micro-cracks prevents the emergence of major cracks. These larger voids reduce a roadway’s lifespan by allowing moisture to reach the steel rebar.

A recent report projects massive growth in the use of self-healing concrete. The authors of the report project a compound annual growth rate (CAGR) of 33% through 2026.

Bendable concrete

Concrete’s strength makes it an excellent choice for roads. More ductile formulations reduce cracking and increase longevity. Dr. Li and his team at the University of Michigan pioneered the development of “bendable concrete.”Their formulation withstands a tensile strain of 3-5%. The trick is to increase tensile strength while maintaining compressive strength.

Ductility comes from the addition of fibers to the concrete mix. Silica fibers, asbestos fibers, glass fibers and steel fibers are all possibilities. An anti-friction coating on the fibers improves results. Ductility is also enhanced by the use of superplasticizers. Examples include lignin, lignosulfonate, naphthalene, melamine-formaldehyde and polycarboxylate ether.

Micro-bar reinforcement

According to a multi-year study, road repairs using micro-bar withstand heavy traffic. Such repairs also resist de-icing chemicals and freeze-thaw cycles. For example, Helix 5-25 is a one-inch-long twisted steel reinforcement. Crews add it on-site or at the mixing plant.

For the study, workers prepared a microbar/rapid-set mix in a revolving drum mixer on site. Patched areas opened to traffic in one hour. One-, two- and three-year test results suggest a 5+ year patch duration.

Materials With a Smaller Carbon Footprint

The use of Portland limestone cement reduces carbon emissions by about 10%. Milled limestone replaces 10-15% of the Portland cement used in traditional formulations. This reduces the amount of energy-intensive clinker required.

Compared to traditional hot-mix asphalt, warm-mix asphalt (WMA) is the more sustainable alternative. Lower-temperature mixes translate to 15% less carbon embodiment. Adoption of this solution varies. In the United States, WMA already accounts for 50% of asphalt paving. In the United Kingdom, WMA use is only 6%.

About PACA

The Pennsylvania Aggregates and Concrete Association (PACA) reports on the latest industry developments. If you have questions about your next concrete project, please contact us.

Concrete vs asphalt cement