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Introduction
This design catalog is intended to give general
structural design and mix type selection guidance for some typical Washington
State HMA pavements. It is organized by general purpose, with each use
type being addressed on a different page. These guidelines are intended
for use in lieu of other guidance or specification. They are not
intended to supercede official guidelines or specifications.
Recommended structural designs use the
basic assumptions and classifications discussed below. The limitations
and applicability of these assumptions and classifications must be understood
before using the recommended structural designs contained in this section.
Traffic is classified in a broad sense only.
There are many different traffic classification schemes available and traffic can vary widely in both make-up and
volume. This design catalog uses the
"vehicle manufacturer truck
classification", which broadly divides vehicles into the three classes listed in
Table 1.
Table 1: Design
Catalog Vehicle Classification
|
Vehicle Category |
Gross Vehicle Weight Range |
Assumed ESALs per Vehicle |
Representative Vehicles |
|
Cars and
Light Trucks |
0 - 6,000 lbs. |
0.0007 |
cars, SUVs,
pickup trucks, ambulances, delivery vehicles |
|
Medium Trucks and Buses |
14,001 - 16,000 lbs. |
0.25 |
city cargo van, beverage delivery truck, wrecker, school bus |
|
Heavy Trucks and Buses |
33,000 lbs. and over |
1.00 |
semi tractor trailer, concrete mixer, dump truck, fire truck, city bus |
|
Note: this classification is different from
the FHWA vehicle classification system |
Subgrade soil conditions
vary widely across the State. This design catalog section classifies soils into three broad
categories based on
strength and stiffness (CBR,
MR) as listed in Table
2. These categories are broad and should be used with caution. A subgrade should be categorized based on
its strength or stiffness and not its soil type alone. For example, it is
not enough to know that a subgrade is primarily classified as an SW in the
Unified
Soil Classification System (USC), something must also be known about its
strength or stiffness. Furthermore, although a subgrade may be reasonably
stiff or strong, it may also be frost susceptible, which must be accounted for
in pavement structural design.
A Note on Extremely
Adverse Subgrades
The "Poor" classification shown in Table 2 is meant to describe subgrades with
low strength and stiffness values and/or high
fines content. Extremely
adverse subgrade conditions such as water saturated soil, standing water and organic
peat provide extremely poor support and do not meet the "poor"
classification in Table 2. Paving over
extremely adverse subgrades should be avoided if possible. If paving is
necessary, the pavement designer and a geotechnical engineer should collaborate
to develop a feasible pavement design.
Table 2: Design Catalog Subgrade
Classification
| Classification |
CBR |
MR (psi) |
Typical Description
(by USC) |
Frost Potential |
| Good |
≥ 10 |
20,000 |
Gravels, crushed stone
and sandy soils. GW, GP, GM, SW, SP, SM soils are often in
this category. |
Less than 6 - 7 percent
passing
the No. 635 sieve (0.02 mm)
Essentially Not Frost Susceptible |
| Fair |
5 - 9 |
10,000 |
Clayey gravel and clayey
sand, fine silt soils. GM, GC, SM, SC soils are often in this
category. |
6 - 20 percent passing
the No. 635 sieve (0.02 mm)
Potentially Frost Susceptible |
| Poor |
3 - 5 |
5,000 |
Fine silty sands, clays,
silts, organic soils. CL, CH, ML, MH, CM, OL, OH soils are
often in this category. |
Over 15 percent passing
the No. 635 sieve (0.02 mm)
Highly Frost Susceptible |
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All recommended HMA mix designations correspond to
standard WSDOT mixes.
In some cases a
Modified Class B (Commercial Mix) may be substituted for the recommended
Class A,
Class B or
Class G mix.
-
"Aggregate base" refers to crushed aggregate. The WSDOT crushed
aggregate definitions of "crushed surfacing top course" (CSTC) and "crushed
surfacing base
course" (CSBC) both meet the intended definition of "aggregate base".
-
The minimum recommended
crushed aggregate base thickness is 4 inches. Aggregate
layers less than 4 inches thick are possible, however (1) they do not add much
strength to the overall pavement structure, (2) they provide little protection
from frost heave, (3) fines from the underlying subgrade may contaminate a substantial
portion of the layer and inhibit drainage, (4) they are difficult to compact,
(5) they are frequently subject to quantity overruns and
(6) it is difficult to construct and maintain smoothness in aggregate layers
thinner than 4 inches.
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The minimum recommended HMA layer thickness is 2 inches. Pavement
layers thinner than about 2-3 times the
nominal maximum aggregate size may be difficult
to compact, tear under the screed, and rollers may crush the larger particles
during compaction. Because of its smaller
nominal maximum
aggregate size, Class
G mixes can be laid in thinner lifts (about 1 inch minimum thickness ).
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For
many of the recommended structural designs presented here traffic is not the
controlling factor. For lower volume roads, parking lots and recreational
facilities considerations such as
HMA aging,
constructability and subgrade
conditions are primary concerns.
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ATB can be substituted for crushed aggregate.
Structurally, ATB is
about three times as strong as an untreated granular base (such as crushed
surfacing base or top course). The
ATB page lists important
substitution considerations.
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Class E can be used
in place of ATB. Class E is a large aggregate
heavy duty mix. It has tighter asphalt content and gradation controls
than ATB and can be used in place of ATB in all conditions.
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Class B can be used in place of
ATB. Class B is generally a better quality mix (based
on gradation and aggregate angularity requirements) and can be used in place of ATB in
most conditions.
-
Class A can be used
in place of Class B. Class A is identical to Class B except that it
requires more of the coarse aggregate to have at least one fractured face.
It can be used in place of Class B mix in all conditions.
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0.5 inch Superpave can be substituted for Class A or B.
See Superpave Use
for more guidance.
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Sometimes,
Modified Class B
(Commercial Mix) can be substituted for
Class A or
Class B in medium to
low traffic/load situations. Situations where this substitution is
advisable vary,
consult your local
WAPA member for further guidance.
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In some situations,
ATB or
Class E can be substituted for
Class A or Class B
within the middle or bottom of the pavement structure. However, the
appropriateness of this substitution depends on many other considerations (e.g.,
maintaining a thick enough surface course, subsurface drainage, etc.).
Therefore,
this substitution should only be
undertaken after
consulting your local
WAPA member for further guidance.
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If
a bus route is anticipated on a particular low traffic pavement such as a
residential street or collector it may often control the structural design.
Therefore, pavements with bus routes or anticipated bus routes should be
designed using an approved design procedure. Otherwise, the heavy bus
traffic may cause premature pavement failure.
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All recommended designs in this section were checked using the 1993 AASHTO empirical structural
design equation using the following values: Reliability = 75 %, So = 0.50,
ΔPSI = 2.0, design life = 20 years.
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