Pavement Design Considerations
Formerly Federal-aid Policy Guide Non-Regulatory Supplement NS 23 CFR, Part 626,
April 8, 1999, Transmittal 25
See Order 1321.1C FHWA Directives Management
*entire document is new material
- General Pavement Design Considerations (23 CFR 626) Title 23 CFR 626 establishes the following requirement: "Pavements shall be designed to accommodate current and predicted traffic needs in a safe, durable, and cost-effective manner." The regulations do not specify the procedures to be followed to meet this requirement. Instead, each State Highway Agency (SHA) is expected to use a design procedure that is appropriate for its conditions. The SHA may use the design procedures outlined in the "AASHTO Guide for Design of Pavement Structures," or it may use other pavement design procedures that, based on past performance or research, are expected to produce satisfactory pavement designs.
- FHWA Evaluation of Pavement Design Procedures
- Consistent with FHWA's operational philosophy on process review/product evaluation (PR/PE) attached to Executive Director Carlson's November 12, 1991, memorandum, the FHWA field offices can conduct periodic reviews of the SHA's pavement design process. As part of the review, FHWA field offices will sample a sufficient number of projects to determine that the pavement design process is being followed and the process provides reasonable engineering results. If the reviews show that the SHAs have and are following an acceptable pavement design process, routine pavement design reviews of individual projects will not be required.
- The FHWA encourages the development of mechanistic pavement design procedures. To promote consistency in application of mechanistic-related design procedures, the Office of Pavement Technology will participate with the Resource Centers and Division Offices in reviewing and discussing these procedures with the State during their development.
- Pavement Design Factors. Highway agencies should pay particular attention to the following items in designing pavements.
- Traffic. Pavement designers should work closely with the SHA component responsible for traffic volume, classification, and truck weight data required for pavement design.
- Accurate cumulative load (normally expressed as 18 kip equivalent single axle loads or ESALs) estimates are extremely important to pavement structural design. Load estimates should be based on representative current vehicle classification and truck weight data and anticipated growth in heavy truck volumes and weights. Representative current traffic data should be obtained using statistically valid procedures for obtaining count, classification, and weight data, based on the concepts described in the FHWA "Traffic Monitoring Guide" and the "AASHTO Guidelines for Traffic Data Programs."
- Accurate vehicle classification data on the number and types of trucks is essential to estimate cumulative loads during the design period and should be given special emphasis. Weight information should be obtained using weigh-in-motion (WIM) equipment, for this data is more representative than data obtained using static enforcement scales, which are plagued with avoidance problems. States should continue to automate their monitoring program through installation of strategically placed automatic vehicle classification and WIM systems as soon as possible to improve the current base traffic data used to forecast future truck volumes and loads. It is anticipated that individual axle load information will be needed for future mechanistic-based design procedures.
- The SHA's forecasts of future loadings should, as a minimum, be based on two truck classes: trucks up to 4-axle combination and trucks with 5-axles or more. Changes in load factors should also be monitored and forecasted. The forecasting procedures should consider past trends and future economic activity in the area. A traffic data collection and forecasting program that identifies the most important truck types and the changes in numbers and weights of these truck types during the design period should provide realistic load estimates.
- Foundation. Providing a uniform, stiff, moisture and frost resistant foundation is the most important aspect of pavement structural design. Special attention needs to be given to subgrade uniformity and stiffness and the inclusion of subbase layers for pavements on the NHS. When the subgrade consists of fine grain clay or silt materials, stabilization of the upper 300 to 600 mm should be considered. In addition, the SHAs are encouraged to include a 200 to 600 mm thick granular subbase layer in NHS pavement foundations. In areas where frost penetration occurs, the subbase layer should be non-frost susceptible. Base courses should either be free draining or resistant to moisture related damage.
- Both the 1986 and 1993 versions of the "AASHTO Guide For Design of Pavement Structures" require the use of the Resilient Modulus (MR) (a measure of the elastic property of soils) in lieu of soil support value as the basic materials value to characterize roadbed soils for flexible pavements. The AASHTO guide strongly recommends that SHAs acquire the necessary equipment to measure (MR). SHAs who use (MR) values converted from CBR and R-value should conduct correlation studies using a range of soil types, saturation levels, and densities to determine realistic input values. The FHWA LTPP TECHBRIEF - Improved Guidance for Users of the 1993 AASHTO Flexible Pavement Design Procedures (FHWA-RD-97-091), dated August 1997, summarizes improved guidance for users of the 1993 AASHTO flexible pavement design procedures.
- For rigid pavements, the use of a k-value is required. LTPP TECHBRIEF - Phase 1: Validation of Guidelines for k-Value Selection and Concrete Performance Prediction (FHWA-RD-96-198), dated January 1997, and LTPP TECHBRIEF - Data Analysis, Validation of Guidelines for k-Value Selection and Concrete Pavement Performance Prediction (FHWA-RD-97-035), dated March 1998, provide updated guidance on selecting appropriate values for this factor. AASHTO has approved a modification to the 1986 and 1993 rigid pavement design equations that are discussed in these referenced publications and the 1998 Supplement to the AASHTO Guide for the Design of Pavement Structures. FHWA has developed a LTPP Website at:
http://www.tfhrc.gov where the LTPP TECHBRIEF can be viewed.
- Drainage is an important factor in pavement design, and should be considered on all projects. However, inadequate subsurface drainage continues to be a significant cause of pavement distress, particularly in portland cement concrete pavements. During the last 10 years, significant strides have been made in the development of positive drainage systems for new and reconstructed pavements. There have also been major developments in products and materials that can be used for retrofit longitudinal edgedrains.
- The developments in permeable base technology and longitudinal edgedrains make positive pavement drainage possible and affordable. Accordingly, pavement design procedures need to consider the effects of moisture on the performance of the pavement. Where the drainage analysis or past performance indicates the potential for reduced service life due to saturated structural layers or pumping, the design needs to include positive measures to minimize that potential. NHI Course 13126, Pavement Subsurface Drainage, is being developed to provide updated guidance and will be available in 1999.
- Shoulder Structure
- Research results have shown that widening the right pavement lane and placing the edge stripe 0.5 m from the outside pavement edge significantly improves both asphalt and concrete pavement performance by providing edge support.
- The SHAs are encouraged to use paved shoulders where conditions warrant. Shoulders should be structurally capable of withstanding wheel loadings from encroaching truck traffic. On urban freeways or expressways, strong consideration should be given to constructing the shoulder to the same structural section as the mainline pavement. This will allow the shoulder to be used as a temporary detour lane during future rehabilitation or reconstruction.
- On new and reconstructed pavement projects, the SHAs are encouraged to investigate the advantage of specifying that the shoulder be constructed of the same materials as the mainline, particularly on high-volume roadways. Constructing shoulders of the same materials as the mainline facilitates construction, reduces maintenance costs, improves mainline pavement performance, and provides additional flexibility for future rehabilitation.
- SHAs are encouraged to investigate the advantage of specifying rumble strips on the road shoulder as a safety improvement. FHWA has developed a Rumble Strip Website at:
http://safety.fhwa.dot.gov/roadway_dept/pavement/#rumble and a CD-ROM of technical information on rumble strips. The CD-ROM includes information on different types of rumble strips, placement and specifications, weather concerns, bicycle safety, benefit/cost ratio calculations, and frequently asked questions.
- Engineering Economic Analysis. The design of both new and rehabilitated pavements should include an engineering and economic evaluation of alternative strategies and materials. The project specific analysis should be evaluated in light of the needs of the entire system. The "1993 AASHTO Guide for Design of Pavement Structures" (Appendix B) and the "FHWA Pavement Rehabilitation Manual," provide guidance on engineering considerations. The engineering evaluation should include consideration of the use of recycled materials and/or pavement recycling techniques, where feasible. Economic considerations include an economic analysis based on Life Cycle Costs (LCC). The FHWA Final Policy Statement on LCC analysis published in the September 18, 1996, Federal Register provides guidance on LCC Analysis. The FHWA Memorandum "National Highway System Designation Act - Life Cycle Cost Analysis Requirements" (April 19, 1996), provides supporting information and guidance to assist in implementing Life-Cycle Cost Analysis (LCCA) requirements in the National Highway System (NHS) Designation Act of 1995. The FHWA Office of Pavement Technology's "Interim Technical Bulletin: Life Cycle Cost Analysis in Pavement Design FHWA-SA-98-079, September 1998" and FHWA's "Demonstration Project 115: Probabilistic Life Cycle Cost Analysis in Pavement Design" provide technical guidance and training on good practice.
- Pavements are long-term public investments and all the costs (both agency and user) that occur throughout their lives should be considered. LCCA identifies the long-term economic efficiency of competing pavement designs. However, the resulting numbers themselves are less important than the logical analysis framework fostered by LCCA in which the consequences of competing alternatives are evaluated. When performing LCCA for pavement design, the variability of input parameters needs to be considered. The results of LCCA should be evaluated to determine whether differences in costs between competing alternatives are statistically significant. This evaluation is particularly important when the LCC analysis reflects relatively small economic differences between alternatives.
The FHWA's policy on alternate bids, which would include bids for alternate pavement types, is addressed in 23 CFR 635.411(b). This section requires the use of alternate bid items "When ... more than one... product... will fulfill the requirements... and these... products are judged...equally acceptable on the basis of engineering analysis and the anticipated prices... are estimated to be approximately the same."
- The FHWA does not encourage the use of alternate bids to determine the mainline pavement type, primarily due to the difficulties in developing truly equivalent pavement designs.
- In those rare instances where the use of alternate bids is considered, the SHA's engineering and economic analysis of the pavement type selection process should clearly demonstrate that there is no clear cut choice between two or more alternatives having equivalent designs. Equivalent design implies that each alternative will be designed to perform equally, and provide the same level of service, over the same performance period and have similar life-cycle costs.
This section is superseded by Use of Alternate Bidding for Pavement Type Selection (T5040.39) on 12/20/2012
- Rehabilitation Pavement Design. It is essential that rehabilitation projects be properly engineered to achieve the best return possible for the money expended. When an existing pavement structure is sound, and the cost to restore serviceability is minor when compared to the cost of a new pavement structure or major rehabilitation, an engineering and economic analysis of alternative actions may not be necessary. In general, for all major rehabilitation projects, each of the following steps should be followed to properly analyze and design the project.
- Project Evaluation
- Obtain the necessary information to evaluate the performance and establish the condition of the in-place pavement with regard to traffic loading, environmental conditions, material strength, and quality. Historical pavement condition data, obtained from the Pavement Management System (PMS), can provide good initial information.
- Identify the types of pavement distresses and the factors causing the distresses before developing appropriate rehabilitation alternatives. The tools necessary to analyze pavement failures, such as coring, boring, trenching, and deflection measurements, are well known, and need to be employed more often.
- Evaluate the array of feasible alternatives in terms of how well they address the causes of the deterioration, repair the existing distress, and prevent the premature reoccurrence of the distress.
- Project Analysis
- Perform an engineering and economic analysis of candidate strategies. The engineering analysis should consider the traffic loads, climate, materials, construction practices, and expected performance. The economic analysis should be based on life cycle costs and consider service life, initial cost, maintenance costs, user costs, and future rehabilitation requirements, including maintenance of traffic.
- Select the rehabilitation alternative that best satisfies the needs of a particular project considering economics, budget constraints, traffic service, climate, and engineering judgment.
- Project Design
- Conduct sufficient testing, both destructive and non-destructive, to verify the assumptions made during the alternative evaluation phase. The SHAs should consider a new distress survey if the original condition survey was sample based or if the survey is not current in terms of the time the project is scheduled to go to contract.
- Consider and address all factors causing the distress in addition to the surface indicators in the final design. Such factors as structural capacity, subgrade support, surface and subsurface drainage characteristics need to be considered and provided for in the final design.
- Once a rehabilitation alternative is selected, design the project using appropriate engineering techniques. A number of publications are available to guide the selection of these engineering techniques. The FHWA's "Pavement Rehabilitation Manual," and the NHI training course "Techniques for Pavement Rehabilitation" provide excellent guidelines. There are also a number of excellent guides available from the asphalt and concrete industries.
- Project Implementation
- Document the intent of the design in the project plans and specifications to provide both the contractor and the construction engineering personnel a clear and concise project proposal. In addition, maintain adequate communication between the design, construction, and maintenance engineers. This will reinforce the intent of the design and provide feedback on project constructability, maintainability, and performance to aid timely evaluation of the selected rehabilitation alternative.
- The performance information should also be included as a part of the SHA's PMS. The lack of good as-constructed data on pavement rehabilitation and preventive maintenance techniques is one of the weaker points in the pavement management process. Increased emphasis should be placed on developing basic as-constructed, performance monitoring, and maintenance cost data on rehabilitation techniques where this data is not presently available.
- The SHAs should provide skid resistant surfaces on all projects, regardless of funding source.
- The SHAs should analyze pavement performance histories and existing skid data to ensure that the materials, mix designs, and construction techniques used are capable of providing a satisfactory skid resistant surface over the expected performance period of the pavement. This should include periodic analysis of wet weather crash rates on all standard surfacing types used. Each SHA's skid crash reduction program should include a systematic process to identify, analyze, and correct hazardous skid locations. The SHA's should use the same construction procedures and quality standards used in constructing new pavements in pavement maintenance operations. "Surface Finishing of Portland Cement Concrete Pavements - Final Report FHWA-SA-96-068, Tire Pavement Noise and Safety Performance, May 1996," transmitted by Messrs. Toole and Eller's memorandum dated November 12, 1996, summarizes FHWA's existing guidelines on surface related characteristics, including safety.
- Plans and specifications for proposed pavement rehabilitation, reconstruction, and maintenance projects should include items to minimize disruption and ensure adequate protection of the motorists and workers within the construction work zone, in accordance with the provisions of 23 CFR 630, Subpart J and 23 CFR 635, Subpart A.