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WAPA 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.
Mix Type Selection
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Modified
Class B (Commercial Mix) is often the best choice for pavements with
anticipated medium to low loading. A modified Class B (Commercial) mix
can provide a smoother surface texture that a Class A or Class B mix while
maintaining adequate strength for medium to low load applications. Modified
Class B (Commercial) mixes are more pedestrian friendly due to the
smooth surface texture, are less likely to
segregate,
easier for handwork and easier to use in small quantities.
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Superpave Use.
Limit Superpave use to arterials, highways
and heavy industrial use.
Superpave mix is often stiffer and more difficult to place than traditional
WSDOT mixes. Where heavy
traffic is not a concern, a better option may be Class
A,
B,
G or
Modified Class
B (Commercial Mix).
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SMA and
OGFC. These
mix types are generally most appropriate for high traffic, high load
arterials and highways.
It is not cost effective to use them in low traffic, low load situations such as
driveways,
residential streets,
collectors or non-vehicular
pavements.
Subgrade
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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.
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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.
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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.
Drainage
Mix Designs
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If
possible, do not specify new mix designs in paving specifications.
Specify a currently approved WSDOT design or equivalent. For instance
"Class B or equivalent". If a new mix design is required, specify a
Marshall mix design that substantiates an already existing WSDOT mix design.
Most private labs do not have the equipment to perform
Hveem or
Superpave mix designs.
Structural Design
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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.
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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.
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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.
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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.
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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.
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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.
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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.
Construction
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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.
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Generally, pavement thicknesses of 3.5 inches or more may be paved in two lifts.
For typical Class A
and B mixes, the
first paving lift should be no less than 2 inches.
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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.
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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.
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