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July/August 2002
Road Map to the Future
by Theodore R. Ferragut, Dale Harrington, and Marcia BrinkEnvisioning what we want concrete pavements to look like in the next 20, 50, or 100 years and developing a long-term research plan to make the vision a reality will require the industry to examine itself closely and perhaps even redefine itself.
How can we narrow the gap between the design life expectancy and the actual life of concrete pavements? Why is concrete consistently perceived as the more expensive alternative?
Can concrete compete as a reliable, cost-effective, short-term paving alternative?
Can concrete be a 100-year pavement?
Can we construct concrete pavements that meet lighter operational demands and tough budget restrictions?
Can new technologies or processes make today's marginal mix materials, such as low-grade aggregates, tomorrow's useful materials?
These and other questions are guiding a national project to develop a far-reaching road map to tomorrow's concrete pavements. Under the Transportation Equity Act for the 21st Century (TEA-21), Congress authorized the Federal Highway Administration (FHWA) to undertake a significant research program to improve the performance of concrete pavements. A Long-Term Plan for Concrete Pavement Research and Technology is being developed through the
Concrete Pavement Technology Program (CPTP). The CPTP selected an Iowa State University-led team of program planners, university researchers, engineering consultants, and practitioners from industry and public agencies to create the plan.
Why Another Research Plan?
In recent years several groups have conducted or sponsored pivotal research programs that continue to have significant impact on the concrete paving community. Consider, for example, the possible ramifications of ultra-high-performance concrete pavement, currently being evaluated at FHWA's Turner-Fairbank Highway Research Center in McLean, VA. This steel fiber-reinforced product typically displays twice the compressive strength of high-performance concretes now used in the United States.
The Transportation Research Board, American Concrete Pavement Association, American Association of State Highway Transportation Officials, and FHWA's Long-term Pavement Performance Program also have conducted research that will lay the foundation for CPTP's new long-term research plan.
Nevertheless, the CPTP plan will be unlike any other. Broad in scope and vision, it will guide investigations into innovative technologies, processes, and systems that result in superior concrete paving solutions for 21st century needs and budgets.
Desirable Pavement CharacteristicsThe long-term research plan will be the product of extensive input from stakeholder representatives from Federal, State, and local agencies; contractors, suppliers, and consultants; professional associations; and academia, particularly those conducting applied research. At regional workshops, these stakeholders are helping to envision concrete pavements of the future and identify their desirable characteristics.
The value of the long-term research plan will depend on the vision and practicality of stakeholders' input regarding desirable characteristics, and the integrity and thoroughness with which the team identifies and prioritizes needed research.
Many characteristics identified to date are easy to agree on: (1) stable, long-lived joints; (2) quiet, safe, comfortable ride; (3) low permeability; (4) initial designs that consider future rehabilitation; (5) high reliability (predictable performance); and (6) environmental friendliness.
Other pavement characteristics are generating more discussion. One nontraditional characteristic is the level(s) of service future concrete pavements could or should provide. Do we want a menu of cost-effective, reliable concrete pavements"mixes of fixes" that serve a variety of needs and design lives? Only research can tell us if such a menu is feasible. But, first, stakeholders must decide if mixes of fixes are part of their vision for the next generation of concrete pavements.
The CPTP Research Plan
The plan will:
- Focus on research that results in improved measurement and prediction of pavement performance.
- Integrate performance-based research in four focus areas: (1) pavement design, (2) mix design and materials, (3) construction and maintenance, and (4) performance.
- Serve as the concrete paving community's plan, not that of any particular agency or organization.
- Be innovative and progressive, looking beyond traditional concrete paving solutions.
The CPTP's long-term research plan will result from an iterative process between the planning team and the concrete paving community. Stakeholders are defining the characteristics they want in the next generation of concrete pavements, and the research team is comparing those characteristics with research to date to determine where research is needed.
Performance-Based Research
The planning team is convinced that desirable characteristics to improve pavement performance and more accurately predict the length of a pavement's life can become cost-effective realities only if they are developed through a performance-based process. Such a process is cyclical, based on identifying and measuring performance criteria, or performance predictors, for various elements of a paving system.
In a performance-based process, pavement designs, mixes, and construction and maintenance practices are developed and applied. Performance is measured during and after mixing and construction to determine to what extent optimums were missed and adjustments are needed. Additional research is conducted to further improve designs, mixes, and construction and maintenance practices.
This research cycle of trial, error, improvement, and early discovery and rehabilitation of performance problems results in performance-based pavements, concrete pavements that are not over- or under-built. Such a cycle requires reliable, accurate performance-based measurement and prediction tools to ensure that the best materials are used, reduce initial costs, improve pavement reliability, reduce eventual maintenance and repair costs, and provide for future rehabilitation of the pavement (through its use as a subbase, for example).
Integrated Performance Prediction Tools
The concrete community already monitors several performance criteria, and several software products currently under development will expand and improve such monitoring:
- 2002 American Association of State Highway and Transportation Officials (AASHTO) Pavement Design Guide software will incorporate variations in materials and construction factors into AASHTO design procedures.
- User-friendly software, CONCLIFE, recently developed as part of an FHWA-sponsored study, predicts concrete pavement service life by measuring soroptivity, the most relevant permeability characteristic of concrete.
- HIPERPAV software, developed and regularly updated for FHWA by the Transtec Group, Inc., helps contractors predict the potential for concrete pavement to crack during its first 72 hours. (See "Paving the Way" on p. 20.)
- COMET software, recently developed for FHWA as part of HIPERPAV II, is a Microsoft® Windows-based adaptation of a powerful technique for selecting mixture proportions based on a number of performance criteria.
- PaveSpec software, being developed for FHWA by ERES Consultants, Inc., focuses on predicting and improving long-term pavement performance with regard to specific distresses.
Tools like these are likely to affect performance significantly but more can be done, according to Suneel Vanikar, concrete materials engineer at FHWA. We need "new, distinct performance and protocol tools to measure performance. These tools need to be linked together to predict long-range performance accurately." This linkage requires measuring and integrating the performance of variables throughout an entire paving system in the roadbed, subgrade, drainage, slope, mix materials, placement practices, surface finish, and weather. It also requires integrating variable performance measurements throughout various stages in the pavement life cycle: design, mixing, construction, and maintenance.
One important criterion and predictor of concrete pavement performance the concrete's air void system provides just one example of the need to integrate measuring tools across the system and stages in the pavement life cycle. Concrete must contain a minimum percentage of air voids, adequately spaced, to accommodate freezing water during freeze-thaw cycles, or it may be susceptible to early cracking and premature failure.
The air void system is affected by practices during mixing and construction. We need high-speed tests to measure air void spacing during those stages. Combined with results of air void percentage tests already being conducted in newly poured concrete, these measurements will help workers evaluate and adjust mixing and construction processes quickly to optimize the entire air void system.
After the concrete has hardened, however, environmental and other factors in the rest of the pavement system can affect the air void system. If, for example, one stretch of subgrade fails to drain properly, moisture will interact with cement where the pavement and subgrade meet, causing a chemical reaction that, over time, fills the air voids.
Ideally, tests of performance criteria in the subgrade before and during construction would lead to the discovery and correction of potential drainage problems. However, we also need tools for quickly, economically, conveniently, and accurately measuring the air void system (and other performance criteria) as pavements age. If measurements change enough to indicate performance problems are developing, we can rehabilitate the pavement before it fails.
In another example, the ability to predict mix performance could result in enhanced materials compatibility in today's increasingly complex mix designs, where fluctuations in supplementary materials and chemicals and other variables exponentially complicate the compatibility issue. If the compatibility and performance of complex mix designs can be predicted, performance could be improved substantially, and maintenance and repair costs minimized. Integrated performance predictions can help mix designers increase the usefulness of today's marginal (and less expensive) materials, such as fly ash and other by-products.
Even if aggregates, cements, admixtures, and other variables in the mix are coordinated for a compatible, predictable 25-year mix, the concrete must sit on a predictable 25-year subgrade. Placed on inadequate subgrade, a "perfectly" designed and constructed concrete pavement will fail prematurely.
Again, integrated performance predictions are necessary to improve the performance of the pavement system as a whole.
The ability to measure and predict pavement performance accurately and reliably is integral to achieving Congress's goal for concrete research outlined in TEA-21. The long-term research plan therefore will include problem statements for designing, improving, and implementing performance-based measurement tools that integrate as many elements of the paving system as possible.
Research can determine the feasibility of a variety of nontraditional, innovative concrete paving applications, like this whitetopping-over-brick street project.
Mixes of Fixes
In addition to the potential for saving money and improving the performance of traditional heavy-duty concrete pavements, other implications of performance-based pavements are even more intriguing. The ability to develop performance-based pavements could expand their usefulness beyond traditional applications to cost-effective fixes for light service pavements, which may compose up to three-quarters of pavements identified as needing improvement. Hot-mix asphalt, the traditional fix for light-service roads and streets, is not always satisfactory for pavements with short design lives or for repair of problems like intersection rutting.
What if we did not have to rely solely on asphalt for these improvements? What if we had a menu of affordable, viable concrete solutions"mixes of fixes"that provide concrete's traditional advantages of durability and strength for the short and very long term?
Consider the possibilities:
- Stop-gap rehabilitation projects and staged improvements, later used as subbase; low-cost overlays for haul roads; short- or very short-term load transfer restorations
- High-traffic arterials (traditional long term)
- High-density urban interstates, tunnels, and approaches to large bridges (very long term)
Design Life
Of the many variables to be considered when designing pavements to provide a particular level of service, design life is pivotal. Most concrete pavements constructed in the last half century have significantly outlived their intended design lives. Rarely, one fails prematurely. Given current design, mix, and construction and maintenance practices, survival curves for a variety of theoretical design lives (8-, 15-, 30-, and 60-year designs) show that a large percentage of pavements will survive far beyond design life.
When a pavement outlasts its intended life, it may have been overbuilt, and the original design and construction costs may have been too high. On the other hand, when a pavement fails before its intended design life, the pavement owner may incur excessive repair and rehabilitation costs.
As the planning team and their stakeholder partners around the country discuss potential mixes of fixes based on design lives, they are suggesting a 90/5 percent life expectancy as the target performance. That is, at least 90 percent of pavements are expected to last the number of years for which they were designed, and no more than 5 percent of pavements should last longer than 120 percent of design life. Currently, up to 10 percent of pavements generally last twice as long as intended; 50 percent last up to half again as long as planned.
The better we can fine-tune pavement performance to achieve 90/5 life expectancy, the more costs can be saved by not over- or under-building. Performance-based pavement designs, mixes, and construction/maintenance practices will likely make such fine-tuning possible. Only research can determine whether the curves can be sharpened enough to make short- or very short-term pavements cost effective.
Research is needed to determine if potential cost savings for performance-based concrete pavements could make a variety of design lives—mixes of fixes—economically feasible.
Determining Needed Research
After identifying desirable concrete pavement characteristics, the planning team will develop a road map for getting there. As the team collects prioritized ideas about desirable characteristics, it is cataloging them in a database.
Concurrently with the catalog of prioritized characteristics, the team is inventorying completed, in-progress, and planned concrete pavement research to compile a research-to-date database. This database is categorized according to specific topics within pavement design, mixes and materials, construction/maintenance (including repair, rehabilitation, and reconstruction), and performance.
Each category contains 42 data fields, or subareas, and all research to date is linked to at least one subarea. This system allows the team to summarize the scope and focus areas of research efforts to date and identify studies that relate to any one or more search criteria. To identify existing research on both aggregates and additives, for example, the research database is queried using "aggregates" and "additives" as search parameters.
Other database operations such as statistical analyses and graphing are possible as well. These capabilities are extremely important for digesting and understanding research to date. The large number of projects and sometimes overwhelming amount of related information can be challenging to analyze for identifying meaningful trends and needs.
When the research-to-date catalog is complete, the team will compare it with the developing catalog of prioritized characteristics. Desired characteristics with no, or only partial, research matches will be considered gaps in research. The team members will prioritize the gaps based on benefit-cost analyses. For each gap, they will include problem statements in the draft long-term research plan.
Concurrently with the above steps, the planning team is identifying criteria for predicting and measuring pavement performance. The criteria also will be included in research problem statements.
Based on information collected to date, the team has begun drafting a working plan and will be sharing it with workshop participants. As additional stakeholders contribute fresh ideas, this iterative process will result in a fluid draft-in-progress.
The final plan will include a cooperative financing program, a systematic method for determining research benchmarks and milestones, and a focused, concerted approach for conducting recommended research. The team also will develop a plan for implementation, suggesting strategies to communicate the national plan to the concrete paving community.
Potential Roadblocks
The team does not expect that the plan will have the corner on innovative, nontraditional concrete paving solutions. Ultra-thin whitetopping, bonded and unbonded overlays, cost-effective and continuously improved reinforced concrete pavements constructed without transverse joints, and ultra fast-track pavements are examples of new concrete applications under evaluation.
Before highway agencies can implement mixes of fixes like these and others effectively and on a large scale, however, researchers will need to address some obstacles.
First, highway agencies will have to overcome the entrenched institutional philosophy that concrete pavements are built for heavy loads and last about 30 years. To many people in the paving community, the idea that reliable, cost-effective concrete pavements can and should be built for a variety of service requirements is a revolutionary concept.
Second, although some States have experimented with nontraditional concrete solutions, most agencies have little experience in this area.
The planning team believes that the process of developing the plan in concert with the community of stakeholders, along with implementation of a communications program being developed concurrently with the research plan, will result in stakeholder buy-in and knowledge of current and future options throughout the concrete paving community.
Looking Forward
The performance-based generation of concrete pavements is still in its infancy. But concrete pavements are about to grow up, and the next generation could alter the paving industry significantly. With concrete's load-bearing strength, long life expectancy, and durability, it will likely continue to be the preferred material for interstates and other major paving projects. Through the CPTP's long-term research plan to develop integrated, performance-based concrete solutions, the 21st century could see concrete become a viable, useful material for a wide array of pavement service needs.
Whitetopping as a pavement rehabilitation strategy is gaining acceptance.
Theodore (Ted) Ferragut is coordinating the effort to develop the Long-term Plan for Concrete Pavement Research and Technology. He is owner and president of TDC Partners, Ltd., a sole proprietorship in Alexandria, VA, which works with government agencies and private companies to move innovative technology into practice in the areas of highway pavement design, construction, maintenance, and performance. Ferragut is a registered professional engineer in Virginia and has a B.S. in civil engineering from the University of Massachusetts.
Dale Harrington is administering the long-term research planning effort. Director of the Center for Portland Cement Concrete Pavement Technology at Iowa State University, he has 30 years of public and private experience in pavement design, performance evaluation, construction, and rehabilitation. He is a registered professional engineer in Iowa and has a degree in civil engineering technology from Iowa State University.
Marcia Brink is the communications manager for Iowa State University's Center for Transportation Research and Education, administrative home of the Center for Portland Cement Concrete Pavement Technology and the U.S. Department of Transportation's University Transportation Program serving Iowa, Kansas, Missouri, and Nebraska. She is also adjunct professor of English (professional and technical communications) at Iowa State. Brink has a master's degree in English from Iowa State University.
Other Articles in this issue:
Taking Concrete to the Next Level
Fine-Tuning Innovative Technologies
Making Roads Better and Better
Texas Tests Precast for Speed and Usability
The Biggest Bang for Your Buck
New Software Promises to Put Whitetopping on the Map
Road Map to the Future
