|FEDERAL-AID POLICY GUIDE
April 8, 1999, Transmittal 25
|23 CFR 500B|
Title 23 CFR 500.205(d) 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. Rather each State Highway Agency (SHA) is expected to use a design procedure which is appropriate for their conditions. The SHA may use the design procedures outlined in the AASHTO Guide for Design of Pavement Structures or they may use other pavement design procedures that, based on past performance or research, are expected to produce satisfactory pavement designs.
(1) 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 will 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.
(2) 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 Division offices in reviewing and discussing these procedures with the State during their development.
Highway agencies should pay particular attention to the following items in designing pavements.
(1) Traffic. Pavement designers should work closely with the SHA component responsible for the development of the Traffic Monitoring System for Highways (TMS/H) required under 23 CFR 500.801. The TMS/H should reflect the accuracy of traffic volume, classification, and truck weight data required for pavement design.
(a) Accurate cumulative load (normally expressed as 18 kip equivalent single axle loads or ESALs) estimates are extremely important to structural pavement 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."
(b) Accurate vehicle classification data on the number and types of trucks is essential to estimating cumulative loads during the design period and should be given special emphasis. Weight information should be obtained using weigh-in-motion (WIM) equipment since 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.
(c) 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.
(2) Roadbed Soils. 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. For rigid pavements, theuse of a k-value is required. NCHRP Report 372, Support Under Portland Cement Concrete Pavements, provides improved guidance on selecting appropriate values for this factor. Proper roadbed soil support is needed for longer pavement service lives and more cost-effective pavement design.
(a) Drainage is one of the more important factors in pavement design, yet 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 which can be used for retrofit longitudinal edgedrains.
(b) 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.
(4) Shoulder Structure
(a) Recent studies demonstrate that full structural shoulders improve both mainline pavement and shoulder performance. Research results haveshown that widening the right pavement lane and placing the edge stripe 0.5 m from the outside pavement edge significantly improves pavement performance.
(b) 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.
(c) 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.
(5) Engineering and 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. Appendix B of the 1993 "AASHTO Guide for Design of Pavement Structures," and the "FHWA Pavement Rehabilitation Manual," provide guidance on engineering considerations. The Engineering evaluation should include consideration of the use of recycled materials or pavement recycling techniques where feasible. Economic considerations include an economic analysis based on Life Cycle Costs (LCC). The FHWA interim policy statement on LCC analysis published in the July 11, 1994 Federal Register provides guidance on LCC Analysis.
(a) 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.
(b) 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 theanticipated prices... are estimated to be approximately the same.
(1) 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.
(2) 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 over the same performance period and have similar life-cycle costs.
(l) Project Evaluation
(a) 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.
(b) 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.
(c) 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.
(2) Project Analysis
(a) 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.
(b) Select the rehabilitation alternative which best satisfies the needs of a particular project considering economics, budget constraints, traffic service, climate, and engineering judgment.
(3) Project Design
(a) 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 originalcondition survey was sample based or if the survey is not current in terms of the time the project is scheduled to go to contract.
(b) 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.
(c) 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 training course "Techniques for Pavement Rehabilitation" provide excellent guidelines. There are also a number of excellent guides available from the asphalt and concrete industries.
(4) Project Implementation
(a) 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 and construction engineers. This will reinforce the intent of the design and provide feedback on project constructability and performance to aid timely evaluation of the selected rehabilitation alternative.
(b) The performance information should also be included as a part of the SHA's PMS. The lack of good performance data on pavement rehabilitation techniques is one of the weaker points in the rehabilitation process. Increased emphasis should be placed on developing basic performance and maintenance cost data on rehabilitation techniques where performance data is not presently available.