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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: July/August 1998|
Issue No: Vol. 62 No. 1
Date: July/August 1998
This article is adapted from a portion of Developing Long-Lasting, Lower-Maintenance Highway Pavement: Research Needs, published in 1997 by the Transportation Research Board, a unit of the National Research Council in Washington, D.C.
The Research and Technology Coordinating Committee (RTCC), a special committee convened by the Transportation Research Board (TRB) and funded by the Federal Highway Administration (FHWA), provides continuing guidance to FHWA on highway research opportunities and priorities.
In the course of committee reviews of FHWA's annual research and technology program plans, Laurence J. Adams of the National Academy of Engineering (NAE), a former committee member with extensive experience managing National Aeronautics and Space Administration research programs, often asked why the highway industry did not aim its research programs at more ambitious goals. One such goal would be developing the technology to build pavements that last 50 years. Such pavements, he believed, ultimately would be more cost-effective than current pavement. When pavement industry representatives indicated to him that longer-lasting pavements could be built but that higher initial costs limit their potential application, he urged the committee to look at the issue in more detail.
As a result of these and other discussions, RTCC initiated an investigation of the feasibility of building highway pavements to last longer and require less maintenance. A task force was formed under the direction of Dale Stein (NAE), president emeritus of Michigan Technological University. It focused on three tasks: (1) to determine if it is technologically feasible to construct pavement that will last as long as 50 years in service, (2) to identify the research needed to make long-lived, lower maintenance pavement a reality, and (3) to describe the benefits as well as the constraints associated with pavement that lasts longer and requires less maintenance than current pavement.
Highway pavement design standards are based on extensive past and continuing research supported by federal and state agencies. New cost-saving design tools and products were developed in the recently completed, five-year, $150 million Strategic Highway Research Program (SHRP). The ongoing Long Term Pavement Performance Program (LTPP) studies hold promise for yielding more information about the physical relationships that influence pavement performance. Such efforts are aimed at improving the basic pavement design and construction practices adopted for the Interstate Highway System. Experience indicates that most pavements of the interstate construction era have been highly successful. Designed to last for 20 years, many continue to serve well at traffic levels much higher than originally projected. Nevertheless, as SHRP research has pointed out, improvements are still possible. The committee sought to determine whether it is possible, on the basis of current knowledge, to raise pavement design and construction standards to 50 years of service.
The issues examined in this report are important because major sections of the Interstate Highway System, particularly urban routes with high traffic volumes, are in need of rehabilitation and reconstruction. As state and local highway officials prepare for these projects, some of which will be very large and expensive, attention is being focused on the increasing magnitude of project-related costs that have little to do with pavement. These include delays in passenger and freight transportation, accidents involving road users and highway construction personnel in work zones, and traffic in work zones. In addition, the time to complete a project is greatly affected by the need to accommodate existing traffic. Many of these costs could be reduced or postponed if pavement lasted longer.
The topic of this study recently took on added urgency. Within the past two years, as several committee members noted, an increasing number of unexpected pavement failures have occurred in several states; some of these failures remain unexplained, further illustrating gaps in knowledge about pavement design and pavement performance. In addition to the issue of pavement reconstruction, limitations are imposed by existing road design features, surrounding development, drainage requirements, vertical clearances, and other factors that affect how projects can be carried out and how long they take.
Past and current research activities address ways to improve on pavement design principles and practices aimed at yielding pavements that last 20 years. Some states have decided that lower life-cycle and user costs over the life of a pavement justify building long-lasting, lower maintenance pavements on heavily traveled sections of road. As highway use continues to increase and the need for rehabilitation and reconstruction grows, more states will address the feasibility of building long-lasting, lower maintenance pavements. The issues identified in this report and the findings and recommendations presented should provide state highway officials and researchers with useful information for their decision-making.
Overview of Pavement Design and Performance
Because highways are an essential part of passenger and freight transportation, there is always considerable public interest about them, especially if potholes are not repaired quickly or if maintenance or repairs cause lengthy delays.
Now that the interstate highway program is virtually complete, state and local highway agencies are faced with its success - it is used more and more each day. As much of the system reaches its design life, the need for rehabilitation and reconstruction of some of the most heavily traveled sections is growing. As noted before, several states have begun pavement reconstruction projects that feature long-lasting, lower maintenance pavement. These states have concluded that despite higher initial costs, such pavements will cost the state and road users less than current pavements over the life of the pavements.
Contemporary pavement design is based on procedures recommended by the American Association of State Highway and Transportation Officials (AASHTO) and included in what is known as the AASHTO Design Guide. Pavements typically are designed for a 20-year period of performance although some states use 30- to 40-year periods; 20 years was adopted early in the interstate program as the standard design life for federal-aid projects. It was considered a reasonable length of service in view of the system's extent, proposed budget, and available information about pavement design and future traffic growth.
Highway pavements are mixtures of aggregate and portland cement or asphalt with minor secondary constituents. The pavement supports vehicle loads and transfers them to the soil below through layers of surface, base course, and subgrade. Pavement design involves determining the most economical combination of pavement layers (taking into account both thickness and type of materials) appropriate for the soil foundation and the traffic to be carried, while addressing such variables as environmental conditions, soil drainage, and pavement aging and weathering. The pavement performance period is the actual length of time that the pavement is in service before major rehabilitation is needed. Pavement performance depends on many factors, including:
Design life is the basis for the combination of design features chosen by the pavement engineer. Obviously, there are uncertainties associated with all the variables involved. In designing new and reconstructed pavements, designers can provide thicker pavements for increased traffic loadings because a given percentage of increase in the expected loads can be accommodated by a much smaller percent increase in the pavement thickness and cost. This implies that thicker pavements will last longer - all other things being equal.
Other high-performance design features can yield additional benefits. Such features include shorter slabs (for PCC pavements), full-depth paved shoulders, positive subsurface drainage, and improved materials specifications. But because thicker pavement and high-performance design features cost more than conventional pavement, a highway agency is faced with difficult choices. Building long-lasting, lower maintenance pavement involves tradeoffs between higher initial costs and lower life-cycle costs.
Despite the limited experience with long-lasting, lower maintenance pavement, there is some cost information. Engineers at the New York State Department of Transportation (NYSDOT) estimate that for the 60 projects initiated in New York using its 50-year pavement design, the ratio of estimated cost per mile for the new design to actual cost per mile for the traditional design is 1.24 for 12-inch (30-centimeter) concrete pavement and 1.4 for 10-in (25-cm) asphalt pavement. NYSDOT engineers believe that such costs could decrease as contractors gain more experience with the changes in construction techniques and are able to amortize any additional equipment costs resulting from the new designs over a larger number of projects.
Life-cycle costs include the full range of construction and maintenance costs during the life of the pavement as well as costs associated with the maintenance of traffic in work zones. Other costs include those incurred by passenger and freight transportation on the section being reconstructed. These are delay costs due to construction activities, lane blockages, and lower speeds, and costs associated with the higher incidence of accidents involving road users and highway construction personnel in work zones. Although life-cycle costs are a strong argument for using long-lasting, lower maintenance pavement, higher initial costs can meet with resistance from the highway construction industry and the public.
The committee recognizes that many factors affect how different groups react to changes associated with such pavement. For example, small construction and paving firms, which make up the majority of the highway construction industry, might resist such pavement if it requires them to purchase new equipment, facilities, and so forth.
Findings and Recommendations
The committee report included findings and recommendations in the three primary areas addressed by the study - feasibility, need for research, and benefits and constraints.
Finding: Pavement that lasts longer and requires less maintenance than current standard pavement is technologically feasible and is being pursued in several states.
Several states, including Georgia, New York, and Wisconsin, have undertaken pavement rehabilitation programs by constructing what they call "premium" pavement at some heavily trafficked locations. To achieve longer lasting pavements, these states are using a combination of high-performance design components, including thicker pavement sections, improved design details such as shorter slabs, full-depth paved shoulders, and positive subsurface drainage, and improved materials specifications. Some questions remain, as would be expected when climate, geography, traffic, and other variables must be accounted for in pavement design and when there is considerable uncertainty about many critical factors that affect the construction, maintenance, use, and performance of pavement.
Although long-lasting, lower-maintenance pavements have higher initial costs, under certain conditions they could have lower life-cycle costs than standard pavement. If a pavement located on a heavily trafficked highway performs for 50 years without requiring major rehabilitation, it could cost a highway agency less than standard pavement and would be a more efficient public investment.
Highway agencies, however, are faced with fixed budgets and pressure to spread their budgets over as many miles of highways as possible. Many would be reluctant to embark on highway construction and rehabilitation programs that involve fewer highway miles even though it might be a more efficient use of public funds. The committee believes that this hurdle can be addressed by pavement research that is aimed at providing answers to specific questions about design, materials, construction techniques, and maintenance procedures that apply to long-lasting, lower maintenance pavement. Improvements in these areas could reduce the initial costs of such pavement and make it more attractive to highway agencies.
The committee found other hurdles to adopting long-lasting, lower maintenance pavement. For example, forecasting future traffic and vehicle loadings has proven difficult in the past, and attempting to forecast farther into the future will require better forecasting methods than are currently available. Development and implementation of effective pavement management svstems and pavement maintenance programs are essential to the success of long-lasting, lower maintenance pavement.
Also, life-cycle cost methodologies for long-lasting, lower maintenance pavement are not yet well refined, and further development of such methodologies could provide decision-makers with a more substantial basis for arguing the merits of life-cycle cost accounting. The committee believes that decision-makers and road users would be willing to pay for road improvements if the arguments for them were convincing; well-developed life-cycle cost analyses could help support such arguments.
Pavement-Related Research Needs
Addressing Key Unresolved Questions
Finding: Well-focused research is needed to address remaining unresolved questions about long-lasting, lower maintenance pavement.
Although SHRP provided much valuable information on essential pavement issues, particularly with regard to pavement materials, pavement experts identified several key topics that require additional research if long-lasting, lower maintenance pavement is to be implemented successfully. Research is needed to improve design procedures, ensure materials reliability, improve the performance of local materials, and better integrate construction and maintenance. Research is needed on the bonding mechanisms of pavement materials and on their long-term durability, on appropriate quality-control techniques in paving construction, and on the requirements for pavement management systems for long-lasting, lower maintenance pavement.
Recommendation: FHWA, the state highway agencies, and the National Cooperative Highway Research Program (NCHRP) should coordinate their research efforts to aim at improving design procedures, ensuring materials reliability, improving the performance of local materials, and integrating construction and maintenance programs with design expectations in support of long-lasting, lower maintenance pavement.
Using Data From the Long Term Pavement Performance Program (LTPP)
Finding: Current LTPP activities could yield significant technical information to assist in developing long-lasting, lower maintenance pavement.
The ongoing LTPP is aimed at discovering - through a series of long-term field experiments on highways in service - the physical relationships governing the long-term performance of highway pavements. LTPP is a systematic approach to: (1) providing data to determine pavement distress and performance resulting from loading, environment, material properties and variability, construction quality, and maintenance levels; and (2) determining the effects of specific pavement design features on pavement performance. It is designed to collect data on pavements throughout the United States and Canada for 20 years. It would be useful to develop short-term laboratory tests that could predict long-term pavement performance.
Recommendation: FHWA and the state highway agencies should support current LTPP program activities through the program's 20-year period of data collection and should use the data collected on pavement performance to develop: (1) a better understanding of the physical relationships governing long-term pavement performance, and (2) much needed short-term laboratory tests for predicting long-term pavement performance.
Improving Pavement Management and Pavement Maintenance
Finding: Cost-effective implementation of long-lasting, lower maintenance pavement requires improved pavement management systems and a continuing commitment to pavement maintenance.
Design, materials, and construction quality issues notwithstanding, successful long-lasting, lower maintenance pavement installations will rely on pavement management systems aimed at preventing distress rather than repairing pavement failures. Distress-specific pavement management programs also need accurate construction data, maintenance records, traffic data, and performance observations for the life of the pavement. Also important is timely recognition of unusual changes in service conditions that could contribute to an accelerated rate of distress accumulation.
In addition, researchers need to make use of the information on pavement distress generated in the LTPP program to develop a better understanding of the design-materials-performance relationship, enabling better prediction of pavement performance for specific conditions.
Recommendation: FHWA should continue to support the development and implementation of pavement management systems and should examine the potential for incentives that ensure that state highway agencies implement effective pavement management programs.
Improving Traffic Information, Prediction, and Monitoring for Pavement Design
Finding: A key component of pavement design - predicting traffic and loads - is extremely difficult and highly problematic as the design life becomes longer.
Pavement performance depends on traffic loads over time; so, traffic prediction - especially information about the number, types, axle configurations, and weights of vehicles expected to use the pavement - is critical to pavement design. The AASHTO Design Guide requires that anticipated traffic over the life of the pavement be expressed as a single uniform factor - the equivalent single-axle load (ESAL). The calculations from the AASHTO Design Guide call for a range of information, including predictions of traffic and traffic-growth rates for up to 13 vehicle categories. Simplified assumptions suitable for local circumstances and data availability can be used to reduce the amount of information needed. Thus, the procedure relies heavily on the ability of the highway agency to make good predictions or good assumptions in the design process. Moreover, uncertainties in predicting traffic volumes and vehicle characteristics can make it difficult to select specific locations within a state that would benefit most from long-lasting, lower maintenance pavement. However, research results suggest that traffic estimates can be improved significantly if site-specific data are used in traffic prediction. According to the Texas Transportation Institute's 1991 Research Report No. 1235-1, Traffic Load Forecasting for Pavement Design, traffic-load forecasting accuracy could be improved by more than 30 percent from current levels by conducting 24-hour manual vehicle classification sessions at specific pavement project sites and by more than 85 percent by conducting week-long, weigh-in-motion sessions at specific pavement project sites.
Recommendation: Research is needed to develop better traffic-prediction methods and low-cost improvements for traffic monitoring to support the design and implementation of long-lasting, lower maintenance pavement.
Benefits and Constraints
The benefits of long-lasting, lower maintenance pavement include fewer delays to motorists due to road closures for maintenance, fewer complaints about road closures and construction incidents, less accident exposure for both motorists and construction workers in and near construction zones, and more consistent speeds. If maintenance backlogs can be overcome, then stability of construction and maintenance will also be an asset. There could be a public relations value in using new road construction technology to achieve long-lasting, lower maintenance pavement.
A major disadvantage of long-lasting, lower maintenance pavement is its higher initial construction cost. This could result in a reduction in the total surfacing/maintenance program as measured by miles of construction. The actual reduction could be small depending on how much long-lasting, lower maintenance pavement is constructed and the portion of the project cost attributed to the use of such pavement. Pavement costs are usually a small portion of overall construction costs, particularly in heavily trafficked urban areas. Other disadvantages include the possibility of higher user fees or taxes, lack of political commitment for long-term projects with virtually no immediate political return, opposition from construction contractors and the materials supply industry, and negative public reaction to any unexpected failure of the pavement before its expected service life regardless of the cause (e.g., changes in predicted traffic, loads, and weather).
Developing and Using Life-Cycle Cost Accounting Methods
Finding: State highway agencies have limited incentives for building long-lasting, lower maintenance pavements. However, for heavily trafficked highways, life-cycle cost accounting indicates that such pavement yields considerable benefits due to reduced maintenance and highway user costs.
The strongest argument for rehabilitating heavily trafficked highways with long-lasting, lower maintenance pavement involves life-cycle cost accounting. The higher cost of construction is outweighed by reductions in maintenance costs and user delay costs. State highway agencies are under pressure to improve pavement performance while also reducing the amount of time that the driving public is delayed by highway maintenance and rehabilitation activities. Moreover, it appears that maintenance and rehabilitation needs are increasing faster than the corresponding budgets.
However, decision-makers must also weigh the certainty of the current liability (higher cost) against the uncertainty of future benefits (reduced maintenance and user delay costs). This is particularly difficult because any change in which benefits are diffuse (and uncertain and hard to capture) and in which costs are concentrated (and current and certain) will run into opposition. Nevertheless, based on the experience of several states, life-cycle cost accounting can be a useful tool.
Recommendation: FHWA should continue to support efforts to develop life-cycle cost accounting methods for pavement construction and rehabilitation decision-making and should encourage state highway agencies to use these methods to support long-lasting, lower maintenance pavement.
Using Road User Fees for Highway Improvements
Finding: Highway users generally support fees used to improve highways, so long-lasting, lower maintenance pavement can be a reality if the costs and benefits are articulated clearly and the users are assured that user fees are improving the highways they use.
Road users are responsive to better pavement performance, especially if it translates into fewer disruptions or road closings due to pavement repair and maintenance. It might be possible to increase highway user fees so that long-lasting, lower maintenance pavement can be a reality. Issues such as equity in the distribution of gas tax revenues across the various highway classes and across states would also have to be examined.
Recommendation: If long-lasting, lower maintenance pavements are to be successfully implemented, FHWA, state highway agencies, and the entire highway industry must find a way to clearly articulate the costs and benefits and to convince road users that their user fees are improving the highways they use.
Coordinating the Various Aspects of the Highway System
Finding: Because each aspect of the nation's highway system - including pavement design, pavement procurement and construction, pavement maintenance, and vehicle design and operation - is optimized independently, the overall system is considerably suboptimized.
The committee found that individual parts of the nation's highway transportation system are largely independent of one another, leading to a considerable sub-optimization of the overall system that leaves room for improvement. For example, trucks are built with suspensions and wheel configurations that protect cargo, provide comfort for the driver, and improve fuel efficiency with little regard to the impact on pavement performance. Highway agencies frequently make pavement engineering decisions without addressing the vehicle-pavement interface. In addition, highway construction generally is performed on the basis of a low-bid process in which a low initial price is the predominant factor in evaluating bids and awarding the contract.
A recent Transportation Research Board report points out that "among the advantages of low first price as a central criterion in evaluating bids is that it is a simple quantitative measure that assures contractors of fair and consistent bid assessments and promotes administrative efficiency by providing public officials with a straightforward and impartial evaluation metric. A disadvantage of this practice, however, is that it generates simple price competition with little emphasis on product timeliness, quality, and durability." The result is that there is little incentive for contractor innovation in product quality. Finally, highway financing is based on cost-per-mile of highway built rather than on durability or some other measure that accounts for the value of long-lasting, lower maintenance pavement.
As a result, although longer lasting pavements can be achieved by improvements in design, materials, construction, and maintenance, additional improvement could be achieved if all aspects of the highway system could be coordinated more completely in design and operation.
Recommendation: Research is needed to determine how pavement design, pavement procurement and construction, pavement maintenance, and vehicle design and operation can be coordinated more systematically to yield pavements that last longer and perform better.
Providing Incentives for Long-Lasting, Lower Maintenance Pavement
Finding: Incentives aimed at reducing project construction times could also be structured to yield longer lasting pavements.
Several recent highway construction projects have successfully featured financial incentives aimed at reducing construction times; the incentives were based on savings of time and cost to road users. Reducing construction times involves a variety of tactics, including improved project activity scheduling, the application of a range of best practices, and the use of innovative construction technologies, including automated systems.
Consideration should be given to how incentives for reducing construction times could be combined with construction techniques needed to build long-lasting, lower maintenance pavement to yield more long-term benefits.
Recommendation: FHWA should work with state highway agencies to determine how procurement innovations aimed at reducing construction times could be broadened to include incentives that favor innovative time-saving construction techniques and also yield longer lasting pavements.
Ensuring the Availability of Qualified Pavement Designers
Finding: Because there is a shrinking cadre of experienced pavement design engineers, efforts to develop long-lasting, lower maintenance pavement must include ways to ensure that qualified pavement engineers are available to design such pavements.
Discussions with pavement experts indicated that although the AASHTO Design Guide is a useful tool, insufficient attention is being paid to how the basic design alternatives can be improved through judicious use of the guide's procedures in combination with an understanding of the underlying engineering mechanics and design principles. Compared to the standard curriculum 20 years ago, undergraduate civil engineering programs now require fewer courses in which fundamentals of mechanics and materials are provided. Similarly, fewer opportunities exist for elective courses for undergraduate specialization in these or virtually any other aspect of civil engineering. Therefore, specialization occurs at the graduate level or in post-baccalaureate training courses concentrating on aspects of a particular specialty (e.g., pavement design).
In addition, the cadre of knowledgeable pavement design engineers within state highway agencies is slowly shrinking as a result of attrition, downsizing, and retirement. Because young engineers are not attracted to pavement design, few seek such positions. As a result, staffing for designing and implementing higher performing pavements that require considerable knowledge of engineering mechanics and materials principles will be difficult unless steps are taken by the states to provide educational incentives and other opportunities.
Recommendation: If state highway agencies are to design and implement higher performing pavements, they must provide educational incentives and other opportunities for young engineers interested in pavement technology so that staff with the requisite knowledge of engineering mechanics and materials principles will be available.
This study was performed under the overall supervision of:
The RTCC members are: