lec 16 highway engineering - flexible pavemen design

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.
http://www.pavementinteractive.org/http://www.pavementinteractive.org/http://www.pavementinteractive.org/http://www.stmuench.com/modules/05_materials/05_asphalt.htmhttp://www.stmuench.com/modules/06_design_factors/06_loads.htmhttp://www.stmuench.com/modules/06_design_factors/06_loads.htmhttp://www.stmuench.com/modules/05_materials/05_asphalt.htmhttp://www.pavementinteractive.org/


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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.htm


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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.htm


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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_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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lec 16 highway engineering - flexible pavemen design

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