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7/25/2019 Lec 16 Highway Engineering - Flexible Pavemen Design
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Lecture 16 195
Highway Eng. Design of Flexible Pavements 1415
Dr. Firas Asad
In this lecture;
A-Types of Pavements
B-Design of HMA Pavements
C-AASHTO 1993 Method.
1-Loading
2- Materials & Soil
3- Enviroment
Structural Design of Flexible Pavements
Information listed in this lecture is mainly taken from Traffic and Highway
Engineering (Garber, 2009), Asphalt Pavements (Lavin, 2003),Pavement Analysis and
Design(Huang, 2004),http://www.pavementinteractive.org(Accessed on 2015) and
Highways (OFlaherty, 2007).
A- Types of Pavements
Generally, hard surfaced pavements are typically categorized into flexible and rigid
pavements:
Flexible pavements. Those which are surfaced with bituminous (orasphalt)
materials. These types of pavements are called "flexible" since the total pavement
structure "bends" or "deflects" due totraffic loads.A flexible pavement structure is
generally composed of several layers of materials which can accommodate this
"flexing". Flexible pavements comprise about 94 percent of U.S. paved roads.
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Rigid pavements. Those which are surfaced with portland cement concrete (PCC).
These types of pavements are called "rigid" because they are much stiffer than
flexible pavements due to PCC's high stiffness. Rigid pavements comprise 6 percent
of U.S. paved roads.
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B-
Structural Design of HMA Pavements
As shown above, the flexible pavement structure is typically composed of several
layers of material each of which receives the loads from the above layer, spreads
them out, then passes them on to the layer below. Thus, the further down in the
pavement structure a particular layer is, the less load (in terms of force per area) it
must carry (see Figure in P. 195).
B-1 Basic Structural Elements
Material layers are usually arranged within a pavement structure in order of
descending load bearing capacity with the highest load bearing capacity material
(and most expensive) on the top and the lowest load bearing capacity material (and
least expensive) on the bottom. A typical flexible pavement structure (Figure 2)
consists of:
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Surface Course. The layer in contact withtraffic loads. It provides characteristics
such asfriction,smoothness, noise control,rut resistance anddrainage.In addition,
it prevents entrance of surface water into the underlyingbase,sub
base andsubgrade . This top structural layer of material is sometimes subdivided
into two layers: the wearing course (top) and binder course (bottom). Surfacecourses are most often constructed from hot-mix asphalt HMA.
Base Course. The layer immediately beneath the surface course. It provides
additional load distribution and contributes to drainage. Base courses are usually
constructed out of crushedaggregate or HMA (stabilised).
Subbase Course. The layer between the base course and subgrade. It functions
primarily as structural support but it can also minimize the intrusion offines from
the subgrade into the pavement structure and improve drainage. The subbase
generally consists of lower quality materials than the base course but better than
the subgrade soils. A subbase course is not always needed or used. Subbase courses
are generally constructed out of crushed aggregate or suitable fill.
http://www.stmuench.com/modules/06_design_factors/06_loads.htmhttp://www.stmuench.com/modules/10_pavement_evaluation/10_categories.htm#skid_resistancehttp://www.stmuench.com/modules/03_general_guidance/03_pavement_distress.htm#ruttinghttp://www.stmuench.com/modules/06_design_factors/06_drainage.htm#surfacehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#basehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#subbasehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#subbasehttp://www.stmuench.com/modules/06_design_factors/06_subgrade.htmhttp://www.stmuench.com/modules/05_materials/05_aggregate.htmhttp://www.stmuench.com/modules/05_materials/05_aggregate.htm#fine_aggregatehttp://www.stmuench.com/modules/05_materials/05_aggregate.htm#fine_aggregatehttp://www.stmuench.com/modules/05_materials/05_aggregate.htmhttp://www.stmuench.com/modules/06_design_factors/06_subgrade.htmhttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#subbasehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#subbasehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#basehttp://www.stmuench.com/modules/06_design_factors/06_drainage.htm#surfacehttp://www.stmuench.com/modules/03_general_guidance/03_pavement_distress.htm#ruttinghttp://www.stmuench.com/modules/10_pavement_evaluation/10_categories.htm#skid_resistancehttp://www.stmuench.com/modules/06_design_factors/06_loads.htm7/25/2019 Lec 16 Highway Engineering - Flexible Pavemen Design
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B-1 Methods of Design
The goal of structural design is to determine the number, material composition and
thickness of the different layers within apavement structure required to
accommodate a given loading regime
. This includes thesurface course as well as
any underlyingbase orsubbase layers.
Calculations are chiefly concerned with traffic loading stresses. The principal
methods of structural design in use today are (from simplest to most
complex)design catalogs,empirical andmechanistic-empirical.
Design Catalogs
The simplest approach to HMA pavement structural design involves selecting a
predetermined design from a catalog. Typically, design catalogs contain a listing of
common loading, environmental and service regimes and the corresponding
recommended pavement structures. State and local agencies often include them in
their design manuals.
Empirical Design
Many pavement structural design procedures use an empirical approach. This
means that the relationships between design inputs (e.g., loads, materials,layer
configurations andenvironment) and pavement failure were determined using
experience, experimentation or a combination of both.
Although the scientific basis for these relationships is not firmly established, they
can be used with confidence as long as the limitations with such an approach are
recognized. Specifically, it is not wise to use an empirically derived relationship to
describe phenomena that occur outside the range of the original data used to
develop the relationship. Examples of these methods is 1993 AASHTO method.
http://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htmhttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#surfacehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#basehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#subbasehttp://www.stmuench.com/modules/06_design_factors/06_loads.htmhttp://www.stmuench.com/modules/08_structural_design/08_methods.htm#design_catalogshttp://www.stmuench.com/modules/08_structural_design/08_methods.htm#empirical_designhttp://www.stmuench.com/modules/08_structural_design/08_methods.htm#mechanistic_empiricalhttp://www.stmuench.com/modules/06_design_factors/06_loads.htmhttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htmhttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htmhttp://www.stmuench.com/modules/06_design_factors/06_environment.htmhttp://www.stmuench.com/modules/06_design_factors/06_environment.htmhttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htmhttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htmhttp://www.stmuench.com/modules/06_design_factors/06_loads.htmhttp://www.stmuench.com/modules/08_structural_design/08_methods.htm#mechanistic_empiricalhttp://www.stmuench.com/modules/08_structural_design/08_methods.htm#empirical_designhttp://www.stmuench.com/modules/08_structural_design/08_methods.htm#design_catalogshttp://www.stmuench.com/modules/06_design_factors/06_loads.htmhttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#subbasehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#basehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm#surfacehttp://www.stmuench.com/modules/08_structural_design/08_pavement_structure.htm7/25/2019 Lec 16 Highway Engineering - Flexible Pavemen Design
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Mechanistic-Empirical Design
The most advanced pavement structural design uses a mechanistic-empirical
approach. Unlike anempirical approach, a mechanistic approach seeks to explain
phenomena only by reference to physical causes. In pavement design, the
phenomena are thestresses, strains anddeflections within a pavement structure,
and the physical causes are the loads and material properties of the pavement
structure. The relationship between these phenomena and their physical causes is
typically described using various mathematical models. AASHTO has a full procedure
and software for conducting mechanistic-empirical pavement design.
http://www.stmuench.com/modules/08_structural_design/08_methods.htm#empirical_designhttp://www.stmuench.com/modules/08_structural_design/08_pavement_response.htm#stresshttp://www.stmuench.com/modules/08_structural_design/08_pavement_response.htm#deflectionhttp://www.stmuench.com/modules/08_structural_design/08_pavement_response.htm#deflectionhttp://www.stmuench.com/modules/08_structural_design/08_pavement_response.htm#stresshttp://www.stmuench.com/modules/08_structural_design/08_methods.htm#empirical_design7/25/2019 Lec 16 Highway Engineering - Flexible Pavemen Design
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C- 1993 AASHTO Empirical Design Method for Flexible Pavements
The 1993 AASHTO Guide for the Design of Pavement Structures is the basis for the
AASHTO method of flexible pavement design.
Design Considerations
The factors considered in the AASHTO procedure for the design of flexible pavement
as presented in the 1993 guide are:
Pavement performance
Traffic
Roadbed soils (subgrade material)
Materials of construction
Environment
Drainage
Reliability
Pavement Performance. The primary factors considered under pavement
performance are the structural and functional performance of the pavement.
Structural performance is related to the physical condition of the pavement with
respect to factors that have a negative impact on the capability of the pavement to
carry the traffic load. These factors include cracking, faulting, raveling, and so forth.
Functional performance is an indication of how effectively the pavement serves the
user. The main factor considered under functional performance is riding comfort.
To quantify pavement performance, a concept known as the serviceability
performance was developed. Under this concept, a procedure was developed to
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determine the present serviceability index (PSI) of the pavement, based on its
roughness and distress. The scale of PSI ranges from 0 to 5, where 0 is the lowest PSI
and 5 is the highest.
Two serviceability indices are used in the design procedure: the initial serviceability
index (pi), which is the serviceability index immediately after the construction of the
pavement; and the terminal serviceability index (pt), which is the minimum
acceptable value before resurfacing or reconstruction is necessary. Recommended
values for the terminal serviceability index are 2.5 or 3.0 for major highways and 2.0
for highways with a lower classification.
PSI = Po - Pt
Traffic Load.In the AASHTO design method, the traffic load is determined in terms
of the number of repetitions of an 18,000-lb (80 kilonewtons (kN)) single-axle load
applied to the pavement on two sets of dual tires. This is usually referred to as the
equivalent single-axle load (ESAL).
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The equivalence factors used in this case are based on the terminal serviceability
index to be used in the design and the structural number (SN) (see definition of SN
in Page ). The Tables (1a & 1b) below give traffic equivalence factors for pt of 2.5 for
single and tandem axles respectively.
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To determine the ESAL, the number of different types of vehicles such as cars,
buses, single-unit trucks, and multiple-unit trucks expected to use the facility during
its lifetime must be known. These can then be converted to equivalent 18,000-lb
loads using the equivalency factors given in the two Tables above.
The total ESAL applied on the highway during its design period can be determined
only after the design period and traffic growth factors are known. The design period
is the number of years the pavement will effectively continue to carry the traffic
load without requiring an overlay. Flexible highway pavements are usually designed
for a 20-year period.
Since traffic volume does not remain constant over the design period of the
pavement, it is essential that the rate of traffic growth be determined and applied
when calculating the total ESAL. Annual growth rates can be obtained from regional
planning agencies or from state highway departments. These usually are based on
traffic volume counts over several years. The overall growth rate in the United
States is between 3 and 10 percent per year. The growth factors (Grn) for different
growth rates and design periods can be obtained from Equation below:
where
r = i / 100 and is not zero. If annual growth is zero, growth factor = design period.
i = growth rate.
n = design life, yrs.
The Table below shows calculated growth factors (Grn) for different growth rates (r)
and design periods (n) which can be used to determine the total ESAL over the
design period.
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The portion of the total ESAL acting on the design lane (fd) is used in the
determination of pavement thickness. Either lane of a two-lane highway can be
considered as the design lane whereas for multilane highways, the outside lane is
considered. The identification of the design lane is important because in some cases
more trucks will travel in one direction than in the other or trucks may travel heavily
loaded in one direction and empty in the other direction. Thus, it is necessary to
determine the relevant proportion of trucks on the design lane. A general equation
for the accumulated ESAL for each category of axle load is obtained as
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Materials of Construction. The materials used for construction can be classified
under three general groups: those used for subbase construction (a3), those used
for base construction (a2), and those used for surface construction (a1). The quality
of the material used is determined in terms of the layer coefficient, a3, a2 and a1,
which are used to convert the actual thickness of the subbase, base and surface to
an equivalent SN respectively. Figures below used to find layer coefficients.
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Reliability:
The AASHTO guide incorporates in the design a reliability factor R% to account for
uncertainties in traffic prediction and pavement performance. R% indicates the
probability that the pavement designed will not reach the terminal serviceability
level before the end of the design period.
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Drainage requirements:
Criteria on ability of various drainage methods to remove moisture from the
pavement are depend on engineer and the drainage quality which depend on the
time that water removed from pavement granular materials within.
Drainage quality effect represent in pavement thickness by sample of m. taken from
AASHTO recommendation that depend on the selected quality of drainage and
percent of time pavement structure is exposed to moisture level approaching to
saturated during a year.
Thickness Requirements:
Using the input parameters described in the preceding sections, the total pavement
thickness requirement is obtained from the monograph in terms of structural
number SN. SN is an index number equal to the weighted sum of pavement layer
thicknesses, as follows:
SN= a1D1+ a2D2m2+ a3D3m3
Where: a1, a2, and a3are numbers known as layer coefficients can be find from layermodules(Mr) by using AASHTO specific charts or default formula;
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D1, D2, and D3are layer thicknesses; and
m2and m3are layer drainage coefficients used for granular layers.
The values of D1, D2and D3have to meet certain minimum practical thicknesses asshown inTable 2.
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R-value : soil resistance value
Chart below for base courses.
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Structural No. (SN):
An index number derived from an analysis of traffic, roadbed, soil conditions,
and reliability. That may be converted to thickness of several of flexible pavement
layers:
SN1 a1 D1 therefore D1minSN1/ a1
SN2 a1 D1 + a2 D2m2 therefore D2min(SN2- a1 D1)/a2m2
SN3a1 D1 + a2 D2m2 + a3 D3m3 therefore D3min(SN3- a1 D1- a2 D2m2) /a3m3
In other words: SN3= a1 D1 + a2 D2m2 + a3 D3m3
In general, a1taken as 0.44 for plant asphalt mix high stability, a 2taken as 0.14 for
crushed stone base course, a3taken as 0.11 for sandy gravel subbase course.
AASHTO asphalt pavement design procedure
In the AASHTO design procedure for asphalt pavements, the basic design equation
(or design chart) and the structural number SN are the key focus of the procedure.
The following steps summarize the procedure:
1 Determine the required reliability R% and overall standard deviation So for the
pavement.
2 Determine the total accumulated ESALs (w18) for the design life of the pavement
and annual growth rate.
3 Determine the subgrade soil resilient modulus, MR.
4 Determine the design serviceability loss, PSI.
5 Using the four values selected above and the AASHTO design nomograph (chart),
determine the required structural number (SN) for the asphalt pavement.
6 The selected structural number and SN equation and its required values are then
computed to determine the thickness of each layer.
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Example:
Design the pavement for an expressway consisting of an asphalt concrete
surface, a crushed-stone base,and a granular subbase using the 1993 AASHTO
design chart. The cumulative ESAL in thedesign lane for a design period of 15 years is
(7*106). The area has good quality drainage with 10% of thetime the moisture level
is approaching saturation. The effective roadbed soil resilient modulus is 7 ksi, the
subbase has a CBR value of 80, the resilient modulus of the base is 40 Lb, and the
resilient modulus ofasphalt concrete is 4.5 * 105psi. Assume a reliability level = 95%,
So= 0.45, Po = 4.6 and Pt = 3.0.
Solution
Step 1: Reliability (R) = 95% and overall standard deviation (So)= 0:45 (Given)
Step 2: Step 3: W18= 7 * 106(Given)
Step 3: Effective road-bed soil resilient modulus = 7 ksi (Given); Resilient modulus of
subbase = 20 ksi (Figure); Resilient modulus of base = 40 ksi (Given) and Resilient
modulus of asphalt concrete surface = 450 ksi (Given)
Step 4: PSI = PoPt = 4.6 - 3.0 = 1.6
Step 5: SN3 = 5.2 ( design chart; subgrade MR of 7 ksi)
SN2= 3.5 (design chart; subbase MR of 20 ksi)
SN1= 2.7 (design chart; base MR of 40 ksi)
Step 6: a3= 0.14 (Figure); a2= 0.17 (Figure); a1= 0.44 (Figure) ;
Drainage coefficients = m2= m3= 1.1 (Table)
SN Equation ---- > SN1 = a1 D1------ > 2.7 = 0.44 D1
D1=6.1 in. (Round to 6.5 in.)
SN Equation ---- > SN2 = a1 D1 + a2 D2m2 ----- > 3.5 = 0.44 * 6.5 + 0.17 *D2 * 1.1
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D2=3.4 in. (Use a minimum value of 6 in.) (Table)
SN Equation --- > SN3= a1 D1 + a2 D2m2 + a3 D3m3 ---- > 5.2 = 0.44 * 6.5 + 0.17 * 6 *
1.1+ 0.14 * D3 * 1.1
D3=7.9 in. (Round to 8 in.)
Hence. For design use ----- > D1=6.5 in.; D2=6 in. and D3=8 in.
===========================================
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