Support the Road with Lightweight Aggregate

From pavement preservation methods to highly trafficked motorways, ESCS offers lightweight aggregate options

The American Society of Civil Engineers (ASCE), in the 2025 Infrastructure Report Card, explained that deteriorated and congested roads still cost the average driver over $1,400 per year in vehicle operating costs and lost time. Poor road conditions also cost counties. Asphalt maintenance experts estimate that every $2 spent on pavement preservation can save $4 to $18 on future costs.

As departments of transportation and municipalities plan maintenance, repair and road improvement projects, it is important to consider paving materials that not only support safer roads, which include preventing damage to constituent vehicles, but also ones that provide long-term resistance to polishing, expected wear and other types of damage.

As demonstrated in projects in North Carolina and Utah, expanded shale, clay and slate lightweight aggregates (ESCS) in asphalt chip seal can be a cost-effective and long-lasting solution to many challenges in asphalt pavement improvement. This material provides a high-friction surface that resists polishing over time to extend the service life of most asphalts by approximately 20 to 40% compared to conventional aggregates, depending on traffic and environmental conditions. It can also reduce damage to constituent vehicles caused by dislodged stones.

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High-friction surfaces that resist polishing create cost-effective solutions

ESCS is produced by heating raw material until it softens and air bubbles form. The bubbles remain as unconnected voids when the aggregate cools. These voids create a surface texture with a higher skid number than common natural aggregates (like limestone, dolomites and gravel), approximately 65 for chip seal (compared to limestone’s 50) and 44 for hot mix (22 for limestone). Further, because the air pockets are found throughout ESCS, the material inherently resists polishing to maintain adequate skid resistance over the pavement’s service life.

In “Pavement Friction for Road Safety,” a primer from the Federal Highway Administration (FHWA), it is noted that high-friction surfaces result in “significant crash reductions in both wet and dry crashes.” Using materials with a high and stable skid resistance, like ESCS, can be a viable option for increased safety along rural and municipal roadways.

For example, the North Carolina Department of Transportation (NCDOT) sought to reduce accidents along Highway 71 near Lumber Bridge. This stretch experienced heavy truck traffic and consistent wet road conditions. To determine if ESCS would be an effective and cost-efficient high friction surface treatment (HFST), NCDOT tested a single layer of ESCS applied at approximately 12 pounds per square yard over CRS-2 emulsion at approximately 0.30 gallons per square yard for 200 yards. The rate of accidents did not increase, demonstrating that ESCS offers an economical HFST alternative to calcined bauxite.

Similar tests to the efficacy of ESCS have been conducted by the Texas Department of Transportation (TXDOT). The results demonstrate this material provides both initial and long-term skid resistance.

Suppress, Collect & Contain Silica Dust at the Conveyor Belt

Improve mechanical and chemical bonding with ESCS

The network of voids provides other benefits in addition to long-term skid resistance. It allows the asphalt sealant to penetrate deep into the aggregate mix.

Because this material is produced, its size is highly regulated, and the amount of dust is reduced. On the one hand, the predictable size helps reduce machine clogging. On the other hand, it allows a more even coat. Being nearly dust-free, ESCS improves contact with asphalt emulsion for better retention; it also supports less dust kicked up during projects. Finally, the almost cubical shape of ESCS aggregate enables it to lock into the stone matrix to further improve its long-term performance.

In addition to the physical properties of ESCS that support a better mechanical bond, ESCS has a slight ionic affinity to many asphaltic emulsions. ESCS typically has a net negative surface charge. When used with a cationic emulsion, such as CSS1 or CRS-2, the ionic charges of these materials allow them to bond on the molecular level. This supplements the mechanical bond between ESCS and emulsion for greater retention rates over the course of a road’s service life. The improved bond strength also reduces the instances of flying rocks dislodged by passing vehicles.

With higher retention rates and better bonding, ESCS represents a cost-effective road repair solution. Photo courtesy of ESCS

Reduced aggregate density minimizes damage to constituent cars

Aggregates can lose adhesion to asphalt emulsions for a number of reasons: over coverage, being too wet or too dusty, as a result of long-term UV damage or simply being at the end of its service life. Without enough adhesion, tires can dislodge stones and send them flying. When this happens with conventional aggregates, their weight and speed of travel can exert enough force to damage windshields and car bodies.

Because they create a strong bond with asphalt emulsion, ESCS is less prone to being dislodged. If stones are dislodged, they are far less dangerous, due to the low density of the material. Thus, windshield damage claims are practically eliminated, reducing liability and helping to stretch road construction budgets.

The Utah Department of Transportation (UDOT) used ESCS on multiple stretches of interstate, totaling approximately 20 miles in length and using over 10,000 cubic yards of lightweight type C/type II chips. Representatives from UDOT reported zero claims or complaints of broken windshields or vehicle damage, which is unheard of when chip sealing a section of interstate. There were also no areas where chips peeled off the road after a year’s time and a season of snow plowing.

Similar results have been reported from the Louisiana Department of Transportation and Development (DOTD), the Kansas Department of Transportation (KDOT) and in other pavement improvement projects across the country.

ESCS’s unconnected voids maintain skid resistance over a pavement’s service life. Photo courtesy of Stalite and ESCS

Less weight can translate into doing more

By reducing the cost of vehicular damage and sidelining expensive HFST material, ESCS can help DOTs do more with less. This alone positions ESCS as a cost-effective option for raising the grade on roads in America.

But it is also important to note that the increased bond strength improves the retention rate of the aggregate to reduce overall costs. In a 2018 report, Carbon County, Utah, found that using ESCS increased the retention rate from 30% to 99%, resulting in a significant reduction of application costs.

Further, since ESCS has a lower density than conventional aggregates, DOTs can expect more yards of coverage per truckload, which can cut fuel and labor costs associated with hauling materials to project sites nearly in half. In addition, because ESCS provides increased durability and skid resistance, it can be specified in smaller aggregate sizes than conventional stone. The Carbon County report indicates that smaller aggregate supports more efficient coverage and shot rates. As the report indicates, the surface area covered with fine chip is often approximately double that of frequently used larger chip sizes. Further, less emulsion is needed to achieve the desired embedment of the aggregate when the particles are smaller.

Both the improved retention rate and a greater per ton coverage rate resulted in over $300,000 in savings annually in Carbon County.

An asphalt chip seal made with ESCS preserves rural roads. Photo courtesy of Stalite and ESCS

Acing ASCE’s Report Card with ESCS

ESCS in asphalt chip seal can be a viable option for improving rural, interstate and municipal roads. With a lower density and the ability to be specified in smaller aggregate sizes, ESCS requires fewer truckloads, less fuel and less emulsion than conventional aggregates. When the total cost of a project is considered, ESCS also represents an economical solution to repairing and replacing pavement across the country. This, in turn, allows DOTs to cover more mileage with their annual budgets.

Between the 2021 Infrastructure Report Card and the 2025 one, road infrastructure did see a slight improvement, moving from a D to a D+. Due to its ability to create longer lasting and more cost-effective asphalt, ESCS can contribute to an even greater improvement between now and the 2029 report card.

Vernon Mortensen is an Aggregate Salesperson at Holcim Lightweight Aggregates–Utelite. He is active in the Expanded Shale, Clay and Slate Institute (ESCSI) Asphalt Committee and has served as the committee chairperson.