Section 5: Pavement Type SelectionAnchor: #i1009166
Selecting a pavement type is an important decision. Like other aspects of pavement design, the 1993 American Association of State Highway and Transportation Officials (AASHTO) Guide states, “The selection of pavement type is not an exact science but one in which the highway engineer must make a judgment on many varying factors. . ." Appendix B of the 1993 AASHTO Guide provides a list of principal and secondary factors to consider in the selection process.
The use of alternate bidding has proven to stimulate competition between pavement industries. When this type of bidding is used, the contractor with the lowest bid may ultimately decide the pavement type.
Alternate bidding on Federal-aid National Highway System (NHS) projects is allowed only with prior written approval from the Texas Division Office of FHWA. Failure to obtain approval prior to contract letting will make the projects ineligible for Federal-aid funding.
Alternate bidding on Non-NHS projects does not require FHWA approval. However, the pavement design reports must be coordinated and submitted at an early stage of the PS&E development to TxDOT’s Construction Division.
The recommended screening criteria for considering an alternate bid contract include:
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- project cost > $5M Anchor: #IODKHYCI
- ADT > 10,000 and Anchor: #NMBAKDSJ
- limited constructability constraints such as limitation in width, traffic control, drainage, construction sequencing, etc. that might dictate the selection of a specific payment type.
Chart 1 provides the criteria of project selection for alternate pavement design.
Projects with greater than 30 percent trucks or 100,000 average daily traffic (ADT), or any combination of percent trucks and ADT that falls above the curve in chart 1, will require a Life Cycle Cost Analysis (LCCA) and a design life of 30 years for both rigid and flexible pavements. These projects are presented in the orange portion of the chart. The flexible pavement will require a perpetual design without staged construction. The LCCA will be used to determine the bid adjustment factors for these projects.
Projects with up to 30 percent trucks and ADT greater than 10,000 but less than 100,000, which are represented by the yellow region in chart 1, will be referred to as "Thickness Design." Both the rigid and flexible alternates will have a 20-year design life. The rigid pavement is further defined by a minimum slab thickness of 6 inches and the flexible alternate will be limited to one structural overlay, maximum 2 inches. LCCA is not required but may be used for these projects at the district discretion. When LCCA is used, the cost of the overlay will included to determine the bid adjustment factors for these projects.
For projects with less than 10,000 ADT, alternate pavement design is not required. These projects are represented by the green portion of the chart.
Figure 2-8. Criteria for alternate pavement design.Anchor: #i1076972
Several exceptions may justify utilization of district preference in lieu of incorporating an alternate pavement design in the construction plans. These exceptions are:
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- Projects with a total pavement cost less than $5 million. Anchor: #MVTLVWRM
- Projects where it is necessary to match adjacent pavement types (i.e intersections or projects of minimal length) requires specific documentation in the pavement design file; and, Anchor: #RVKQXEEP
- Projects where constructability constraints such as limitations in width, traffic control, drainage, construction sequencing, etc, dictate a specific pavement type, requires specific documentation in the pavement design file
The decision factors for pavement design type should be included in the pavement design report.Anchor: #i1009261
Life-cycle Cost Analysis (LCCA)
LCCA is an engineering economic analysis that allows engineers to quantify the differential costs of alternative investment options for a given project. LCCA can be used to compare alternate pavement types (flexible versus rigid) on new construction projects and rehabilitation projects. LCCA considers all agency expenditures and user costs throughout the life of the facility, not just the initial investment, and allows for cost comparison of options with varying design lives to be compared on an equivalent basis.
More than a simple cost comparison, LCCA offers methods to determine and demonstrate the economic merits of the selected alternative in an analytical and fact-based manner. LCCA helps engineers answer questions like:
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- Which design alternative results in the lowest total cost to the agency over the life of the project? Anchor: #EWHTHNHI
- To what level of detail have the alternatives been investigated? Anchor: #LKRXANXR
- What are the user-cost impacts of alternative strategies?
LCCA’s structured methodology provides the information and documentation necessary for successful open dialogue. Because of this, LCCA is a valuable analysis to support pavement type selection decisions.
The Federal Highway Administration (FHWA) has developed general guidance for running a detailed life-cycle cost analysis. This website is a good starting point for information on running a life-cycle cost comparison of alternatives.
TxDOT research project 0-1734 (Center for Transportation Research [CTR]) developed a computer program, TxPTS (Texas Pavement Type Selection). More information may be found at TxDOT Online Training. The program evaluates candidate flexible and rigid pavement strategies for pavement rehabilitation, reconstruction and new construction projects. The economic evaluations are based on life cycle cost analysis. Also, CTR developed a life-cycle cost analysis program for rigid pavements under project 0-1739.
LCCA is only one of many processes for selecting a pavement type. The reliability of output from any LCCA is a function of the reliability of the input data. It may be beneficial to evaluate user costs separately from agency costs when looking at different structural options.