Extending Lifespans, Improving Safety

By Steven Hoard, Contributing Author

In March, the American Society of Civil Engineers (ASCE) released the 2025 Report Card for America’s Infrastructure. The report states that 39% of the country’s roads are in poor or mediocre condition.

The fatality rates in rural areas are disproportionately higher than on all other roadways. Considering that the ASCE’s 2021 Report Card indicated that roadway design is a factor in approximately one-third of traffic fatalities, it is important for each Department of Transportation (DOTs) to consider how rural road and bridge repair can improve the condition of infrastructure and increase the level of safety.

These considerations include selecting road surfaces with significant skid resistance and materials that support long-lasting infrastructure.

Lightweight aggregate can improve the durability, resilience and safety of roads and bridges. For example, expanded shale, clay and slate aggregate (ESCS) is a lightweight material that provides long-term skid resistance when used in asphalt chip seal.

This material can be used to facilitate internal curing in concrete bridge decks and structures. ESCS also can be added to soils to increase bearing capacity in conjunction with mechanically stabilized earth (MSE).

Understanding this material can help DOTs plan longer-lasting roads and bridges.

Road Rehabilitation

ESCS aggregate is prepared by expanding raw materials in a rotary kiln at temperatures over 1830 degrees Fahrenheit. Expanding the shale, clay or slate up to 40% of its original size creates air bubbles within the material that remain as the aggregate vitrifies. These voids reduce the material’s density, which lowers transportation costs. The air pockets also imbue ESCS with qualities that are beyond what traditional quarried aggregate can provide.

For instance, ESCS has a high internal friction angle that reduces lateral forces when used as a geotechnical backfill. Because these voids are found throughout the aggregate, they support a pavement surface with a long and stable skid resistance rating. When used in concrete, this type of lightweight aggregate can facilitate internal curing and improve the bond between aggregate and cement mix by creating more surface area.

After expansion and cooling, ESCS is crushed and sorted into consistent aggregate sizes. The ability for aggregate producers to highly regulate aggregate size allows concrete engineers and contractors to have more control over the paste requirements, tensile strengths and curing processes.

Consistent aggregate size also impacts road surfacing. For asphalt chip seals, aggregate size, if inconsistent, can clog equipment and lead to patchy coverage. This forces crews to go over road sections, wasting time, material and increasing costs for equipment maintenance.

While the production of ESCS aggregates is more energy-intensive than traditional quarried materials, it requires less material to be mined from the ground. It also can reduce the environmental impact of transportation and increase the resilience and durability of infrastructure. These qualities can offset its initial ecological cost to support more sustainable infrastructure construction and repair.

Road Resurfacing

ESCS aggregates can improve the cost-effectiveness of maintaining asphalt roads. This material is lighter than conventional rock, providing more square yards of coverage per ton. For rural highways, this can contribute to more efficient maintenance operations. For residential applications, this minimizes jobsite congestion.

In addition to its lighter density, ESCS aggregates provide more area for the aggregate to adhere to the liquid asphalt emulsion due to the network of voids. This results in higher retention rates when compared to natural aggregates.

Internally cured concrete bridge components have higher durability and longer service lives than traditional cured concrete.

In fact, the Carbon County Road Department documented the retention rates of ESCS aggregates. The department’s Return on Investment (ROI) report states chip retention rates increased up to 99% with ESCS lightweight aggregates. This report also indicates that the amount of liquid asphalt used in these projects was reduced by 12% when they switched to lightweight aggregate. This helped the Utah county save $300,000.

Lightweight aggregates contribute to reduced chip dislodgement immediately and over a pavement’s surface life. ESCS aggregates also provide long-term skid resistance. As a result, this material can contribute to an approximately 40% longer service life than chip seals that use conventional aggregates. And in residential areas, the service life may extend up to 14 years.

Road Safety

Stable skid-resistance and reduced dislodgment improve road safety, as well. For instance, because ESCS has a superior hold, it reduces damages to headlights, windshields, paint and vehicle undercarriages.

After using ESCS lightweight chip seal aggregate for approximately 20 miles of interstate, representatives from the Utah Department of Transportation (UDOT) reported zero claims and complaints in terms of broken windshields and vehicle damage caused by dislodged pavement. This is especially true for rural applications, where speeding vehicles are at a greater risk of damage from dislodged aggregate.

The voids within ESCS create a surface with a skid resistance that is above limestone aggregate, approximately 65 for chip seal (compared to limestone’s 50) and 44 for hot mix (22 for limestone). Low-volume roads are recommended to provide skid resistance numbers greater than 30 and high-volume greater than 35.

In addition, these lightweight aggregates do not polish as they wear—unlike limestone, which can polish to below recommended skid resistance numbers over time. This creates a stable amount of skid resistance over the pavement’s service life. While some degree of polishing is inevitable in application, ESCS lightweight aggregate resists polishing to extend the level of safety the surface can provide.

Research conducted by the National Transportation Safety Board and Federal Highway Administration (FHWA) indicates that about 70% of wet pavement crashes can be prevented or minimized by improved pavement friction. Thus, the long-term stable skid resistance provided by ESCS aggregates can contribute to safer road conditions.

Although pavement surfaces are crucial to improving road safety, they are not the only element. Bridge structures also play a critical role in maintaining safe road conditions.

Bridge Decks and Structures

According to the FHWA, “minimizing cracking of concrete bridge decks is key to a long service life.” Because cracks give water and other ions access to steel reinforcements, they can reduce the structural capacity of bridge decks and structures.

Lightweight aggregate in asphalt chip seal improves surface skid resistance.

It is estimated that a bridge’s service life “can be reduced by 50% depending on the depth and width of cracking.” The risk of corrosion is increased in areas near salt water or locations that use salt-based deicers as both have increased chloride content in water, which raises the potential for chloride attack.

Internal curing minimizes instances of early-age cracking for less permeable and more resilient concrete. Less permeable concrete can ward off structural failures for longer—offering safer conditions than traditionally cured concrete. Engineers can replace a small percentage of fines with prewetted ESCS aggregates to induce internal curing. This creates a concrete that is more resilient and longer lasting.

The Indiana Department of Transportation (INDOT) constructed four bridges with internally cured concrete and compared the performance of these bridges to traditionally cured ones. The internally cured concrete bridges show the potential to more than triple the service life of a typical bridge deck due, in part, to the material’s ability to reduce early-age autogenous shrinkage by more than 80%. Similar findings have been reported in New York, Utah, Kansas and Texas.

The improved resilience due to internal curing supports more structurally sound bridges with longer service lives. This not only keeps motorists safe and minimizes lane closures for repair, but it also reduces expected maintenance costs over the service life of a bridge.

The FHWA reports that when best-practice recommendations are followed, internal curing typically increases total project costs by less than 5%. The report also indicates that internal curing can reduce the life-cycle cost of concrete by 29-70%.

Infrastructure Improvements

ESCS can be used in fill behind MSE walls to improve the bearing capacity of soil as well as its structural integrity. Furthermore, because ESCS has a high internal friction angle and low density, it significantly reduces lateral force and supports structurally efficient road and bridge repair.

The aggregates themselves are free draining and so contribute to flood mitigation to further improve infrastructure repair.

When a road experiences heavy traffic, harsh weather, or both, ESCS lightweight aggregates can provide a holistic solution for repairing and improving infrastructure.

Incorporating this material from the pavement surface to the soil beneath a road or bridge abutment can help engineers create cost-effective, long-lasting roads and bridges. RB

Steve Hoard is the Southwest Sales Manager for Arcosa Lightweight.