HAPI Design Checklist
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.
- Ensure you specify not only the mix number but also whether it is a "State" or "County" mix. Prior to 1994, HDOT and the Counties used different mix designations with the State using roman numerals II, III, IV and V and the Counties using arabic numerals 2, 3, 4 and 5. These mixes are not always the same. For instance, a State mix III is very similar to a County mix 3, but a State Mix V is very similar to a County mix 4. If given a choice, specify mixes using State (HDOT) designations.
- State Mix V is the best choice for low traffic roads, parking lots, paths and recreational facilities. A State Mix V can provide a smoother surface texture than a State Mix III or IV while maintaining adequate strength for medium to low load applications. State Mix V is more pedestrian friendly due to its smooth, tight surface texture. It is also less likely to segregate, easier for handwork and easier to use in small quantities.
- Superpave Use. Limit Superpave use to arterials, highways and heavy industrial use. Superpave mix can be stiffer and more difficult to place than traditional HDOT mixes. Where heavy traffic is not a concern, a better option may be a State Mix IV or State Mix V.
- SMA use. SMA is most appropriate for high traffic, high load arterials, highways and industrial facilities. It is not cost effective to use SMA in low traffic, low load situations such as driveways, residential streets, collectors or non-vehicular pavements.
- OGFC use. OGFC has been used several times in the distant past in Hawai'i with mixed results at best. However, more recent OGFC mix designs and improved understanding of OGFC mixes make current OGFC designs a viable surface mix. In 2012, Chain of Craters Road in Volcanoes National Park was surfaced with an OGFC.
- Investigate the subgrade condition before specifying a pavement structural design. Pavement structural design is dependant upon subgrade support. Some knowledge of subgrade support is critical to developing an adequate pavement structural design. Pavement design catalogs make assumptions about subgrade support (as the HAPI design catalog does) and these assumptions must be reviewed carefully to ensure they apply to the project at hand.
- Investigate the subgrade over the entire length of the project. It is not uncommon to see the subgrade strength/stiffness vary greatly over the length of a paving project. A pavement structure designed for one level of subgrade support may be wholly inadequate for the subgrade support offered in another area of the project.
- 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", consider the following:
- 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 using a geotextile fabric over a poor subgrade. A geotextile fabric can help prevent fines infiltration into the aggregate base layer. When using an untreated permeable base course, the use of a geotextile fabric is especially warranted.
- Consider a subbase layer of large sized aggregate (4 to 6 inches maximum size). Placing large aggregate directly on a poor subgrade can help give the subgrade some initial stiffness and structure to support a base layer. Without this added subbase layer, the first base layer may sink partially into the subgrade if it is particularly bad.
- Consider stabilizing the subgrade. Stabilization with concrete, lime, asphalt or other material can increase subgrade support enough to allow using designs intended for "fair" subgrade.
- 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 HDOT design or equivalent. For instance "State mix V or equivalent". If a new mix design is required, specify a Marshall mix design that substantiates an already existing HDOT mix design. There are no private laboratories in the Hawai'i that can perform a Hveem mix design and few that can perform a Superpave mix design.
- Different Counties have different preferences for mixes and specifications. Check with the local County before specifying mixes.
- Consider the entire pavement life-cycle when designing
a pavement. Although a pavement with a thin HMA section
(less than about 5 inches) is less expensive to initially
construct, it will likely be more expensive
over the life of a particular road because:
- Thin HMA pavements (less than about 5 inches) fail from the bottom up. When cracking becomes visible on the surface the entire pavement structure is likely to be severely damaged leaving no opportunity to repair just the surface layer. Therefore, the thin HMA layer usually has to be entirely removed and replaced. This also involves repairing and regrading the underlying aggregate base layer.
- HMA layers less than 4 inches cannot be effectively milled. A realistic minimum milling depth is about 2 inches. For a 4 inch pavement, this results in a depth after milling of about 2 inches. If the remaining pavement is thinner than 2 inches it is likely to (1) break as a result of the impact and energy of the milling process or (2) break from the subsequent weight of construction equipment. These broken sections can quickly reflect through any subsequent HMA overlay.
- 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 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 should not be accommodated just 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. This can be a particular problem when developing a large area. Main access roads that are built early on and turned over to the County may continue to carry construction traffic for years to come. If they are not structurally designed to support this heavy traffic they are likely to fail prematurely.
- 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 ACB (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 ACB layer can be repaired, if needed, and a final surface course can be placed.
- When designing shoulders, it is often better to keep base layer thickness consistent with roadway base thicknesses. It is difficult for the paving contractor to place a thinner base depth for shoulder areas only. The gain in construction and final pavement quality obtained by using a shoulder base layer thickness consistent with roadway base layer thickness will likely outweigh any savings obtained by using a thinner base layer to save material costs.
- 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.
- Overlay smoothness is directly related to the smoothness of the underlying pavement. Therefore, the smoother the underlying pavement, the smoother the overlay. The best way to think of this is that overlays can improve smoothness as a percentage of existing pavement smoothness. While an IRI of 90 inches/lane-mile may be achievable on an overlay of a reasonably smooth existing pavement, it may not be realistic on an overlay of an excessively rough existing pavement.
- Generally, pavement thicknesses of 3 inches or more may be paved in two lifts. For thicknesses less than 3 inches, 2-lift construction can give poor results because both lifts will have difficulty meeting the lift thickness vs. nominal maximum aggregate size thumb rule.
- 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 2 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, HDOT surface course specifications require a density between range between 91 and 96 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 ACB, 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.
- Ensure that field compaction specifications are consistent with laboratory mix design parameters. The air voids specified in the field after compaction should be less than or equal to the air voids used in mix design. In other words, a HMA mixture should be field compacted to no more than what it was designed for in the laboratory.
- Shoulder paving may require different quality metrics from mainline paving. Often, the mainline is repaved to a substantial depth (or even fully reconstructed) while the shoulder is kept in place and only improved with a relatively thin overlay. In such cases the thinner shoulder section can be more difficult to compact (because it is a relatively thin lift) and may warrant different compaction specifications than the mainline.
- Consult a HAPI contractor before designing a paved swale (see Figures 1 and 2). The geometry of most paved swales makes them quite difficult to pave and compact. It is prudent to consult an experienced paving contractor to determine what can reasonably be constructed. Roadway specifications for smoothness and density are intended for use on flat, wide pavement surfaces, which is substantially different from the relatively large cross slopes and narrow width of a typical paved swale.
|Figures 1 and 2: Paved swale on the Big Island.|