Top 10 Best Practices For New Pavement Design
- Characterize the existing subgrade soils along the ROW. Use on-line USDA soils maps, DCP & Geotech surveys to establish limits for soil types, degree of swell potential.
- Run TxDOT's Sulfate and Organics tests on soils. Where required, run Texas Methods 146 E and 149 E to determine the presence of high levels of sulfates or organics. If levels are found to be higher than proposed limits 7000 ppm sulfate and 1% organics exercise care if lime treatment of the soil is proposed in the pavement design.
- Run 120/121 for stabilization design of subgrade/bases. For bases, use retained strength criteria (typically 80%) after a 10-day soak. Stabilized bases < 8” thick are not recommended when placed just below a thin surface. Consider micro-cracking when placed just below a bituminous surface for moderate to heavy stabilization.
- Avoid artificially high flex base modulus design inputs for virgin material. Flex base stiffness is dependent to a large degree on what it sits on. Do not exceed a 4:1 ratio of flex base to subgrade modulus for the very best quality unbound granular bases. Add a designed subbase layer to increase support for flex base and avoid working platform scenarios.
- Evaluate ground accuracy of TPP traffic characterization. Actual % of trucks in the traffic stream for project location through a 48-hour classification count. Make sure TPP knows about any recent traffic generators in the area. Portable Weigh In Motion (P-WIM) data collection is available for high profile projects.
- Use FPS post-processing checks (Modified Triaxial, ME fatigue/rutting). Use post-check performance results in determining the adequacy of FPS design. Avoid HMA surfaces 2”-4” unless the substructure support is exceptional.
- Use uniform section design when designing Super 2 or widening projects. Avoid scabbing on new and different base material when widening projects. The potential to trap moisture can cause major performance problems which are costly to mitigate. Recycling and spreading the existing materials to form a sub-base is the recommended option. The new base should be spread evenly to create a uniform section.
- Do not waive test requirements for any of the pavement layers. The poor performance of new sections can often be traced to relaxing test requirements for pavement layers. The less testing required the lower quality materials will be used. This is especially true for flexible base materials and stabilized layers.
- Ensure proper bond between layers. Effectively use MC 30 or AEP approved prime and tack. Ensure uniformity of application and prevent damage to these by construction operations. This is a fundamental performance assumption in the design theory. Layers working independently will cause the system to fail prematurely. A successful prime should penetrate at least 0.25 inches into the base material; also ensure the base is adequately cured before applying the prime – do enforce the 2% below OMC; also tout advantages of trackless tack.
- Avoid single lift HMA applications directly on a flexible base. Single lift HMA applications leave no room for error in mix design, production, or placement operations. A balanced mix design becomes critical. For this reason, underseals are strongly recommended for single lift applications.
Courtesy of Magdy Y. Mikhail, P.E., Section Director, Pavement Asset Management, TxDOT Maintenance Division