Section 9: Pavement Design Reports

9.1 Projects Requiring Pavement Design and Pavement Design Reports

A pavement design and a pavement design report are required for the following projects that are over 500 ft. long:
  • new location projects (flexible and rigid)
  • reconstruction projects (flexible and rigid)
  • rehabilitation (3R) projects (flexible and rigid)
  • unbonded concrete overlays of existing rigid pavements.
Tie-ins, such as bridge approaches, do not require pavement designs when following department or district proven standards.
A new design is not always necessary. Previously approved designs can be used if through an analysis, considering traffic, environmental, and subgrade conditions, the pavement design analysis yields the same thickness. However, adjustments to designed thicknesses and specific conditions, even within a project, should be considered in the design process for budgetary control purposes.
HMA overlays, approximately 2 in. thick and less, are considered pavement preservation; therefore, a pavement design report is not required where adequate structural capacity is documented. Considering the significant investment thin overlays represent, these treatments should be taken into account in an overall pavement preservation program. An analysis should be performed that substantiates the appropriateness of this maintenance strategy.
The pavement design for special cases will typically be based on engineering judgment, historical performance, district policy, and other guidelines (e.g., this manual, industry guidelines, and research findings). A design report may be required for documentation purposes.
The following list provides examples of special cases that do not require a full design report but do require documentation of the criteria and rationale for the strategy selected for projects greater than 500 ft. long:
  • approaches on a bridge replacement
  • detours
  • pavement widening including shoulders
  • HMA overlays of rigid pavements. The TxACOL (Texas Asphalt Concrete Overlay software) developed through research
    project 0-5123
    should be considered when designing these overlays. This process evaluates suitability of proposed overlay HMA mixtures and thicknesses for reflective cracking and rutting performance. Another approach for designing an HMA overlay to an existing rigid pavement is the AASHTO Overlay procedure (automated in DARWin® 3.1). However, this process is highly subjective.
  • bonded concrete overlays on rigid pavements (consult with MNT – Pavement Asset Management)
  • thin whitetopping of flexible pavements (consult with MNT – Pavement Asset Management).
For design categories not covered above, contact the district pavement engineer for guidance about recommended design procedures and documentation requirements.

9.2 Pavement Design Report and Other Documentation

A pavement design report is a formal engineering document that presents all analyses, data, policies, and other considerations used to design the structural aspects of a pavement. The pavement design report shall include the following when applicable:
  • Cover sheet showing highway designation, district, county, project CSJ, geographical limits, and signatures of persons involved in the preparation and approval.
  • Narrative discussing the overall objective, site particulars (location, facility type, soil conditions and subgrade Texas Triaxial Classification [TTC], drainage considerations), multi-year PMIS (PA) data analysis/pavement condition surveys for 3-R projects, conclusions, and recommended pavement structure. The narrative should include a discussion of the factors that significantly affect pavement performance and a summary of laboratory tests conducted on any materials extracted from the existing structure.
  • If the pavement structure selected is different from the structure recommended by the design procedure, a discussion of the selection process must be included in the report.
  • Location map. Maps should be detailed enough to distinguish urban or rural project locations and the presence of water features such as lakes, streams, etc.
  • Soils map of the project area with a brief description of each type of soil located within the project area. The USDA NRCS website at
    http://websoilsurvey.nrcs.usda.gov/app/websoilsurvey.aspx
    is an excellent resource for generating maps and soil summaries. Provide information pertaining to shrink/swell potential and plasticity.
  • The study of the presence of sulfate bearing compounds, organic content, and any mitigation technique selected.
  • Determination of PVR mitigation requirements, if any. Obtain and provide approval to use PVR mitigation techniques when roadway characteristics do not meet policy criteria (see, Chapter 3, Section 2, Geotechnical Investigation for Pavement Structures
  • Existing and proposed typical sections. For the proposed structure, clearly define the various pavement layers, thickness, and materials with specification item. For the existing structure, sections should be as detailed as possible. Proposed or existing positive drainage systems or use of geosynthetics should be indicated on the typical sections.
  • The project specific factors used for selecting the pavement type.
  • TPP Traffic Data and any adjustments to the traffic data.
  • Identification of the base grade chosen, whether shown on the typical section or in the report text.
  • Form 2088, Surface Aggregate Selection Form as part of the flexible pavement design only. Information from this form will determine the appropriate Surface Aggregate Classification (SAC) of the aggregate used for the final hot-mix asphalt (HMA) riding surface.
  • Results of NDT to characterize the existing structural condition (including the MODULUS backcalculation summary).
  • Design input values and output:
    • FPS 21 summary, modified Texas Triaxial check, mechanistic checks, stress analysis, etc., for flexible pavement.
    • AASHTO (DARWin® 3.1) design summary for CPCD rigid pavements.
    • TxCRCP-ME Design summary for CRCP rigid pavements.
    • Alternate pavement design, if appropriate, using past successful practices/district SOP.
  • Conclusion. The pavement design report will conclude with a recommended pavement design based on the data, analyses, and procedures included in the report. The information included in the report should be a synthesis of all work performed to arrive at the recommended pavement structure.
  • Appendices:
    • Surface Aggregate Selection Form 2088 (Wet Surface Crash Reduction program, flexible pavements only).
    • Additional appendices (results of borings, material lab tests, raw PMIS (PA) data, life-cycle cost analysis, drainage analysis, design exceptional approvals, etc.), as needed.
For other reporting requirements, contact the DPE for guidance.

9.3 Completing the Pavement Design Report

The pavement design report may be prepared by anyone with knowledge of the specific project under development and familiar with the analysis tools used. The first licensed engineer in the chain of responsibility will review and sign the report. After completion of the pavement design report content, finalize the report by using the following procedure:
Table 2-5: Completing Pavement Design Report
Step
Action
1
  • The district pavement engineer (DPE) reviews the technical content of the draft report and appropriateness of the design for conditions cited within the report.
  • Submit the draft report to MNT – Pavement Asset Management for review and comment.
2
  • Upon DPE approval of the report, the DPE signs and provides their respective professional engineer (PE) license number on the report cover. NOTE: If the district does not have an assigned DPE, the report must be forwarded to the director of MNT – Pavement Asset Management for review and endorsement.
  • The District Engineer (DE) provides approval of final pavement design. If the report is approved, the approval is indicated by the DE signature and date on the report cover sheet. In metropolitan districts, the DE may delegate final pavement design approval to the District Director of Construction, Maintenance Operations, or Transportation Planning and Development for projects less than $20 million.
3
  • Add the statement “This document is released for the purpose of interim review and is not intended for bidding, construction, or permitting purposes.” after the engineer’s license number on the approved pavement design report, in accordance with paragraph 137.33[e] of the Texas Engineering Practice Act.

9.4 Pavement Design Report Review and Archive

MNT – Pavement Asset Management archives pavement designs for the state for the purposes of forensics and knowledge-based pavement performance. Submit a scanned copy of completed pavement design reports to
pavementdesign@txdot.gov
.
Archived reports can be viewed on the TxDOT intranet (site not available to internet users) at the “
Plans Online”
page.