This page puts forth some basic guidance for use when designing HMA pavements and is meant to be used as a checklist. When designing a pavement, the guidance listed here should at least be considered. The guidance contained on this page is not absolute, however design and construction in conflict with this guidance should (1) only be undertaken for a specific reason and (2) only carried out once the associated risks are understood.
- Pay particular attention to subgrade condition and preparation. Pavement performance is highly dependent on subgrade condition and preparation. Poor subgrade conditions must be recognized and properly addressed by the design engineer and a geotechnical consultant. Most geotechnical reports contain a clause or phrase relating to the unsuitability of a saturated subgrade.
- Paving over a poor subgrade. If paving over a subgrade that meets this Guide’s classification of “poor” pay particular attention to optimum moisture content. If subgrade moisture content is higher than this it can quickly degrade into an extremely adverse subgrade that is near impossible to pave over.
- Consider the construction time of year. During wet fall/winter/spring periods (especially west of the Cascade mountains) the subgrade may remain saturated or near saturated for long periods of time. It is difficult or almost impossible to properly compact and prepare a saturated subgrade. Where possible, pavement construction should be scheduled in dryer, warmer months. At a minimum, pavement construction in wetter months should allow for more subgrade preparation time and cost as well as a thicker pavement section. Also, consider adjusting the pavement design if the pavement construction window must be moved into a time of inclement weather.
- The minimum grade to ensure good surface drainage is about 1.5 – 2 percent. Slopes less than this are difficult to construct and may not prevent pooling of water during wet weather.
- If possible, do not specify new mix designs in paving specifications. Specify a currently approved WSDOT design or equivalent.
- Adhere to agency/owner specifications. Prior to finalizing the design of a roadway that will be dedicated as a public street, check the minimum design standards required by the City or County in its standard design specification manual.
- Understand the risks of pavement underdesign. A structurally underdesigned pavement will almost always look good in the near term. In the long term, underdesigned pavements are prone to early failure and will amount to a greater total pavement expense.
- Subgrade and drainage problems cannot be accommodated by thicker pavement sections. If fundamental subgrade problems or drainage issues are suspected, thicker pavement structures may only be of marginal benefit. As always, a better solution is to address the actual problem.
- Understand the impact of construction loads on a pavement structure. Often, construction loads are the heaviest loads a pavement structure will encounter. If these loads are not accounted for in design, a pavement may become overstressed and begin to fail before its associated construction project is complete.
- Consider alternative pavement structural designs. The minimum designs recommended in this Guide are just a starting point. Other alternative designs may be more cost effective.
- Consider a pavement design amenable to a staged construction. In many cases it is advantageous to build a pavement in stages. Early on, a layer of ATB (in accordance with site paving thicknesses) can be placed to allow construction vehicles to access the site without tracking mud or damaging the subgrade. Later on, near project completion, the ATB layer can be repaired, if needed, and a final surface course can be placed.
- Full depth asphalt pavements should not be used over frost susceptible subgrades. A full depth asphalt pavement leaves no room for ice expansion and may fail prematurely when placed over a frost susceptible subgrade.
- Smoothness. If smoothness is specified on an overlay of an existing rough road, there must be some method of leveling allowed (either a leveling course or milling). Leveling is necessary to remove the existing bumps/roughness and create a reasonably smooth surface on which to pave. The paver’s self-leveling screed can remove small grade imperfections but because HMA differentially compacts it should not be relied upon to remove large grade problems in a single lift.
- Generally, pavement thicknesses of 3.5 inches or more may be paved in two lifts.
- The nominal maximum aggregate size used can affect traffic flow during rehabilitation of existing roadways. In many urban areas off-peak construction is used to minimize traffic impacts. However, for a road to be released to traffic during peak hours, either the lane drop-off (elevation difference between adjacent lanes) must be kept below a specified minimum value (typically less than 1.5 inches with proper signage) or all lanes must be brought to the same elevation. Bringing all lanes to the same elevation at the end of each paving day may require changing traffic control and moving paving equipment, which can increase construction costs and decrease safety. Therefore it is often better to satisfy the lane drop-off requirement. However, with larger aggregate mixes the minimum lift thickness may exceed the maximum lane drop-off allowed. As a result, a smaller nominal maximum aggregate size may allow paving one lane, then releasing the road to traffic, then paving the other lane. However, never sacrifice pavement performance just to meet this requirement.
- The required HMA density to achieve satisfactory pavement performance is dependent upon the pavement function and HMA depth below the pavement surface. Generally, State agency surface course minimum density specifications range between 91 and 94 percent TMD. These specifications are generally intended for high traffic, high load roadways. Surface courses intended for lower traffic/loads may not need such high compaction standards. However, HMA with more than about 8 percent air voids (92 percent TMD) tends to be permeable and may allow water infiltration into the lower pavement layers. Furthermore, lower compaction requirements for HMA used in base layers, such as ATB, may be appropriate because (1) it can be protected from water infiltration by a relatively impermeable surface course and (2) it is further from the pavement surface and thus carries a lesser load per unit area.