U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
|This report is an archived publication and may contain dated technical, contact, and link information|
Publication Number: FHWA-HRT-05-052
Date: September 2005
Performance-based concrete pavement systems use sophisticated and objective QC/QA systems at every step of a pavement project to ensure the desired performance. Designing for performance is the first critical step. In subsequent phases of paving projects, performance is predicted during mix design to ensure, for example, materials compatibility, then measured regularly during and after mixing and construction to determine to what extent optimums were missed and adjustments are needed.
Such systems require reliable, accurate performance-based prediction and measurement tools. They require tools for responding quickly to needed adjustments. They should be based on more and better history data. They should eliminate, to a significant degree, variabilities and inaccuracies due to human factors.
Chapters 4 through 7 discuss the performance-related considerations related to design, mixture materials and design, construction, and pavement management and business systems that are addressed in the various tracks of the CP Road Map.
Design is the plan and basis for pavement dimensions, joint and reinforcement details, materials selection, mix design, construction, maintenance, preservation, and rehabilitation.
If pavement design is deficient, the pavement owner will incur extra costs and motorists will experience more lane closures over many years to come, no matter how good the subsequent maintenance and upkeep activities are.
Designers, owner agencies, and roadway users need better tools and innovative ideas for developing improved designs that are more reliable (have a better chance of serving their design lives without premature distress) and economical (provide good benefit-cost ratios for initial construction, preservation, and rehabilitation), as well as functional, constructible, and maintainable. That is, designs should be based on performance.
The CP Road Map will provide tools for performance-based design.
Concrete pavement designs will be performance-based; that is, reliable, economical, constructible, and maintainable throughout their design life while meeting or exceeding the multiple needs of the traveling public, taxpayers, and owning highway agencies.
The overall design goal will be met through several specific objectives:
These design objectives can be accomplished only if adequate mathematical and computer models exist to make it possible to relate all aspects of design-such as site conditions, design features, and economic analyses-to the functional needs of highway users to successfully develop performance-based, predictive designs. The CP Road Map provides resources to develop these prediction models.
The following paragraphs discuss several design issues addressed throughout the research tracks.
These design objectives can be accomplished only by creating adequate models that relate all aspects of design to the needs of highway users. For example, mathematical models already exist that relate site conditions (e.g., natural subgrade, traffic, climate, and existing pavement) to functional requirements (e.g., ride, friction, noise, and fatigue cracking). Of course, other factors like foundation (e.g., base, subbase, and subdrainage) and design features (e.g., shoulders, slab dimensions, and coefficient of expansion of concrete) also affect functional requirements. Mathematical models, therefore, should be available for those relationships.
Ever since the first structural response equations were published in 1926 by H.M. Westergaard, interest in engineering-based design of concrete pavements has been intense, demonstrated most recently in the Mechanistic-Empirical Pavement Design Guide.(13) Although significant progress has been made over the past 75 years in improving mechanistic modeling of pavement structural behavior and deterioration, many fundamental problems still need to be resolved. These problems include, but are not limited to:
Better understanding of concrete pavement structural responses under a wide range of common structural and climatic conditions will lead to improved designs that will provide more reliable and economical solutions with lower risks of premature failure.
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 past half century have significantly outlived their intended (designed) years of service and traffic loadings. Rarely does a concrete pavement fail prematurely when both design age and design traffic are considered. This indicates that concrete pavements have been designed to a very high level of reliability. Therefore, estimated first costs and future preservation and rehabilitation costs used in LCCAs likely have been higher than necessary because pavements are lasting longer.
Clearly, in today's pavement type selection environment, it is not desirable to design concrete pavements to a higher-than-needed level of reliability. Rather, design reliability methods should be improved to be more accurate than the current approach, which uses a multiplier on equivalent single-axle loads (ESAL). This is a very great challenge, because so many design, construction, material, traffic loading, and climatic factors affect reliability.
Existing procedures (e.g., AASHTO, PCA) have many limitations and deficiencies for designing concrete pavements economically at a desired level of design reliability. While this is true for nearly all situations, it is particularly true for the following conditions:
Many concrete pavements have been designed on both rural and urban low-volume roadways. Often, these are placed directly on the natural subgrade after some type of preparation. Designing a concrete pavement for low-volume roads is in many ways more challenging than designing high-volume pavements because of the often greater desire for long-lasting pavements (e.g., pavements for an upscale residential area should last a long time). Currently, no performance data exist for these pavements to help verify or improve design. Such data are greatly needed in all major climatic areas because climate has an even greater effect on performance with the slab resting on the subgrade or on a thin granular or treated soil layer.
One of the major limitations of concrete pavement design is the ability to consider materials, construction, traffic loading, climate, preservation treatments, rehabilitation, and performance requirements simultaneously to produce reliable and economical designs and strategies. This limitation has led to many problems in concrete pavement performance. A design is judged successful only if it performs well after many years of traffic loadings. Increased design capabilities are linked to understanding and knowledge of the construction process as well as material properties. For example, greater joint spacing (e.g., more than 6.1 meters (20 feet)) may work well if construction processes and material selections are compatible with the design. It also may result in excessive slab cracking if large temperature gradients are built in during construction and/or aggregates with a high coefficient of thermal expansion are used in the mix. Only through a major research effort can such an integrated and systematic approach to design be achieved.
Current design procedures, including the widely used AASHTO procedure, are almost completely empirical, so they carry many deficiencies and limitations. The major national effort to develop an M-E-based design procedure under NCHRP 1-37A has been completed and is under consideration and implementation by sponsoring agencies. This design methodology represents a paradigm shift forward in concrete pavement design. It is expected to significantly improve concrete pavement design because it is based on fundamental engineering principles, uses a finite-element model for structural responses, predicts key distress with mechanistic-based models and an incremental damage computation approach, and is calibrated using national performance data.
Nonetheless, successful implementation in many highway agencies will require significant effort and continual improvement or upgrading over the years. One key item might be called a high-speed computer analysis opportunity. This incremental damage approach (in which the increments are brought down to hourly analyses of damage) could address even more aspects of pavement design, such as early opening. These improvements will be identified as time goes on and agencies use the procedure and sponsor research to fund the improvements. Many improvements will be needed in the next 5 to 10 years to provide for and meet all of the design visions in this long-range research plan.
Designing preservation and rehabilitation treatments has always been very difficult, as the design procedures are almost completely empirical. The Mechanistic-Empirical Pavement Design Guide will provide mechanistically based procedures to more fully consider the existing pavement and subgrade structure, making it possible to use in-place materials from the pavement structure to minimize life cycle costs and lane closures.(13) Implementing this procedure will require a major effort, and many improvements will be needed to introduce it into highway agencies’ daily use. In addition, many expansions will be needed to achieve this vision completely
There is a significant lack of technical knowledge and ability to design base and subbase courses that provide key benefits for a concrete pavement. These benefits include permanent uniform support, bonding or lack of bonding ability, no erosion over time, economical construction platform for the slab, and subdrainage. Additional material tests are needed for erosion and bonding characteristics. Criteria and tests are needed to determine the adequacy of layers in an existing pavement structure to be used as the base or subbase for a reconstructed concrete pavement. Determinations of required base and subbase layer thicknesses also are needed.
A method is needed to directly, reliably, and economically consider subdrainage of the pavement structure. Do we need costly permeable bases and edge drains? Can they be maintained for many years? Can we design more reliably and more economically without this level of positive subdrainage? How? What type of recycled materials can be used as bases, subbases, and other sublayers in concrete pavement?
Ample opportunity exists to develop new and innovative design concepts for concrete pavements. For example, alternative types of pavements such as jointed ultrathin concrete overlays, thin jointed structurally reinforced concrete pavements, thin prestressed concrete pavements, and thin precast concrete pavements have all been constructed and have demonstrated some advantages. Innovations with the greatest potential should be identified and tested so that their benefits and costs are known to determine their viability for certain design situations (e.g., very heavy traffic, special design needs to determine maximum thickness in reconstruction).
When unusual underlying strata exist that may shrink or expand, concrete pavements often are not used because of the fear of slab cracking. New and innovative ways are needed to design concrete pavements to handle these difficult underlying strata conditions economically and reliably. This also would include construction on bedrock, which often occurs in tunnels and major cuts. Such a stiff foundation may require special joint spacing or other design changes.