By Dr. Jon Epps, Ph. D., P.E. and Dave Johnson, P.E.
Much has transpired in the asphalt industry over the past five decades. Advancements in binder, mix design and construction technology have all worked together to improve the quality of flexible pavements.
Moreover, advances in testing methods (laboratory and field) and equipment now can offer to technicians and engineers more precision and greater confidence in test results than had once been possible. Judicious incorporation of recycled asphalt products as well as non-asphaltic materials has improved the environmental footprint of asphalt and aided in the global movement towards improved environmental stewardship.
Advancements in management systems, maintenance techniques and a better understanding of when to apply reconstruction, rehabilitation and maintenance alternatives have assisted in improving asphalt pavement life.
Yet while these and other advancements are to be commended, the asphalt industry is not satisfied to sit on its laurels. Rather, we continue to strive to improve the product and to prepare for the challenges of the future.
To best prepare for those challenges, the industry must anticipate what those challenges will be, and proactively engage in meeting them. It is the goal of the authors of this article to offer their perspectives on what they currently see as significant challenges and to add to the dialogue on how they might be addressed. Areas that will be discussed include:
• asphalt binders
• asphalt mixtures
• recycling and environmental stewardship
• management systems
• workforce challenges
The advent of the Performance Graded asphalt binder specification in the 1990s, developed during the Strategic Highway Research Program (SHRP) research program, and the improvements made primarily through activities of the Federal Highway Administration (FHWA) Asphalt Binder Expert Task Group and its related research significantly improved specifications as compared with the penetration and viscosity-based specifications. As a result of these efforts, asphalt binder properties are determined over a wider temperature range and under both short-term and long-term aging conditions. In the 2010s the Multiple Stress Creep Recovery (MSCR) grading system was developed to improve the characterization of the contribution of polymer-modified binders to rutting resistance.
Further work is needed to determine the presence of various additives and modifiers and improve the ability of test methods to predict short-term and long-term age hardening of asphalt binders in service in a variety of climates and mixtures. Chemical characterization of asphalt binders remains elusive and might not be possible because of their complex chemical nature. Additionally, various non-traditional materials, such as plastics, continue to be explored for incorporation into asphalt pavements. Research into the appropriateness of any proposed material needs to be explored to ensure that they do not hinder the performance of our pavements, at the very least, or to show that they can improve their performance.
Rejuvenators or recycling agents, both bio-based and petroleum-based, have been developed to modify the aged asphalt binders contained in RAP and RAS, enhancing their physical and chemical properties to encourage performance that emulates virgin materials. Continued work in this area will be needed to keep up with an unknown number of additional materials that will alter the properties of base binders.
Nanotechnology shows some promise to help solve asphalt binders’ challenges including aging and water sensitivity. New materials that alter the loading rate sensitivity of binders and the fracture properties of asphalt binders and the asphalt binder–aggregate interface are needed.
Most asphalt mixtures designed today continue to follow traditional principles that originated years ago. While these have provided roadways that have performed admirably, increasing traffic loading, constrained budgets and a keener awareness of environmental stewardship are begging for alternative considerations. Compacted air voids, most commonly four percent, and controls on the rheological properties of the asphalt binders and physical properties of the aggregates continue to be the primary mixture design controls.
Perhaps every state department of transportation, along with other agencies, are researching or adopting a balanced mix design (BMD) approach. BMD seeks to address various asphalt distresses during the mix design phase. Primarily rutting and the different forms of cracking are investigated to design a mixture that has the right balance of materials to resist these distresses. Moisture sensitivity and aging continue to be considered as well.
Continued refinement of current and future fundamental property tests is needed to accomplish the goals of BMD. These tests should be performance-related and include parameters that will control rutting and cracking (fatigue, low temperature, volume change, and reflection) under various moisture and aging exposure conditions. Sample preparation for these tests is key. These mixture property tests should be suitable for mixture design, pavement thickness design and quality assurance. A willingness to consider moving away from traditionally held beliefs of acceptable volumetric ranges may be needed to maximize the benefits of changing materials.
Recycling and environmental stewardship
Given the asphalt industry’s long history of recycling, many of today’s green initiatives fit well with the widespread use of asphalt pavements. To continue this legacy of environmental stewardship, improved asphalt binder modifiers are needed that will allow for recycling higher percentages of materials without compromising performance. The improved modifier needs to resist age hardening in service and improve the chemical properties of the aged binders in the recycled material.
Renewed interest in cold central plant recycling and in-place recycling, both hot and cold, is occurring. With the improvements in equipment and materials, performance characteristics continue to improve. Research is underway and will need to continue to develop improved mixture design, structural design parameters, water sensitivity, and methods for curing and conditioning laboratory-prepared samples to duplicate field curing conditions.
Environmental Product Declarations (EPDs) are a growing need. An EPD is defined by FHWA as, “a transparent, verifiable report used to communicate the environmental impact associated with the manufacture or production of construction materials” including asphalt binders and mixtures. EPDs are commonly thought of as an environmental nutrition label addressing the environmental impact of material as related to global warming potential, ozone depletion, acidification, eutrophication and smog. Although not currently required by federal law, the state of Colorado has mandated EPD usage for its agency’s projects. Other states, and perhaps federal agencies, are expected to follow suit with several already moving towards EPD requirements.
On the mixture side of our industry, the National Asphalt Pavement Association (NAPA) has developed an EPD tool, Emerald Eco-Label, and the Asphalt Institute is actively developing a similar tool for binders. Both the current NAPA tool and the future AI tool will need to be continually refined as changes in refining technology and materials used for both binders and mixtures occurs. This is vitally important as EPDs are seen to be a potential part of future green procurement processes for pavement design and pavement rehabilitation alternatives.
Pavement and maintenance management systems and quality assurance systems are key to providing quality pavements. Pavement and maintenance management systems are mature and useful to those involved in scheduling maintenance operations in the short term and tracking the condition of a public agency’s pavements. The use of the information contained in these systems by decision-makers and planners for allocating financial resources for various budgeting horizons needs improvement. Integration of these ‘‘big data’’ systems with construction, materials, environmental and other public agency data systems is needed.
Rapid pavement evaluation equipment capable of operating at highway speeds and collecting continuous data needs further refinements. Advances in devices that use electromagnetic waves to determine pavement thickness, condition, surface texture, and drainage (ground penetrating radar and LiDAR for example) are needed.
Rapid quality control and quality assurance test methods are needed. The industry has largely been unsuccessful in developing meaningful, automated, rapid test methods that can be utilized for equipment production process control and construction quality control. Hand-held instruments that evaluate and classify RAP and provide recommendations for the type and quantity of rejuvenator would be particularly useful.
Improvements in plants and equipment are constantly being undertaken by industry. Asphalt mixture plant production automation, intelligent compaction, automated machine guidance, GPS-based profile milling and laydown of pavement layers, e-ticketing and automated data collection from plant and field records for quality assurance purposes are examples of a few advancements that are underway.
Automated process control and quality control testing equipment that will sample and test materials rapidly will improve the quality of construction. Automated aggregate sampling and gradation testing equipment is an example of needed equipment that has been developed but has been difficult to deploy in the industry. More comprehensive testing and evaluation of in-place materials through the deployment of current and future technology, such as dielectric profiling systems, will provide engineers with more thorough information. Additional developmental efforts are needed in this and other equipment.
Accelerated construction techniques need to be developed. The goal of accelerated construction is to reduce construction project time by as much as 70 percent. This is a large effort involving a workforce skilled in several disciplines including contracting methods, traffic management, work zone safety, materials, construction operations, economics and construction equipment. Challenging long-held beliefs of acceptable maximum lift thickness, for example, can speed the delivery of construction projects while potentially improving their performance.
The construction industries are facing increasing challenges in manning their crews. Data
from 2019 as reported by NAPA indicates that 91 percent of construction companies were looking for workers. Eighty to 94 percent of those companies also reported that workers were hard to find.
To attract workers to our industry, negative perceptions of the industry need to be rebutted. Pavement construction is generally seen as a dirty and dangerous work environment. Targeted messages about the skills needed, industry opportunities, pay equity and diversity of our workforce have been shown to improve perceptions of our great industry by double digits. Concerted efforts with educators are extremely valuable in improving perceptions and attracting the next generation to construction.
Efforts to recruit future engineers and scientists to the asphalt world need to be encouraged. Organizations such as the Asphalt Institute Foundation and its education programs can pay untold dividends in developing these needed resources.
Small improvements in the performance of asphalt pavements will result in substantial dollar savings. The greatest advancements in the understanding of asphalt and technologies to capture that understanding have occurred through large-scale research efforts. Although small individual projects mostly performed at universities provide value, large, reasonably focused projects are needed to make significant contributions. Relating measured test parameters to performance is key to improving the quality and life of pavements. Large projects involving accelerated field testing and long-term pavement performance monitoring projects that are integrated with the research will be required. The SHRP Long-Term Pavement Performance (LTPP) project is an example of this type of effort in the United States.
Local government needs should be recognized in research. Local governments also need to provide financial support. Research and development efforts led by industry have produced significant improvements in pavements. Typically, industry-led technology research and implementation efforts are deployed without a reasonably complete understanding of the benefits and drawbacks. Public-supported research is often needed to refine the product or process.
Public-supported research is often slow to reach the market or enter the implementation stage. Understanding how to best move the research to market is important to the future of the paving industry. FHWA’s “Everyday Counts” efforts have been one way to speed the path of technology from development to implementation.
Universities should play a significant role in future research, development and deployment efforts. The evaluation systems for faculty at universities need to be altered to reward professors for their contributions to society, not just for securing the latest high technology, nationally competitive research project.
With fewer dollars and a limited public agency workforce available to construct, reconstruct, rehabilitate, and maintain the highways, the design and building of the pavements must be smarter, time in the work zones must be shorter, and the pavements constructed must last longer. The process must be ‘‘smarter, faster and provide a longer life pavement.’’ A mantra for accelerated construction of ‘‘get-in, stay-in, and get-out and stay-out’’ has application to this industry.
Dr. Jon A. Epps retired as a distinguished professor from Texas A&M University.
Johnson is an Asphalt Institute Senior Regional Engineer based in Montana.
Epps and Johnson co-authored a companion article about the key advancements of the asphalt industry over the previous 50 years. It can be found on asphaltmagazine.com in the Summer 2021 edition.