{ CE 414Pavement Analysis

and design Spring 2013

Facebook Group:www.facebook.com/ce414

Pavement Types

Pavements

Flexible Rigid Composite

Flexible Pavement

Surface CourseBase Course

Subbase Course

Subgrade

Rigid Pavement

Rigid PavementConcret Pavement Contraction Design (CPCD)

Rigid PavementJointed Reinforced Concrete Pavement (JRCP)

Rigid PavementContinuously Reinforced Concrete Pavement

Rigid Pavement

Post-tensioned Concrete Pavements

Post-tensioned concrete pavements remain in the experimental stage and their design is primarily based on experience and engineering judgment. Post-tensioned concrete has been used more frequently for airport pavements than for highway pavements because the difference in thickness results in greater savings for airport pavements than for highway pavements.

Composite Pavement

A composite pavement is composed of both hot mix asphalt (HMA) and hydraulic cement concrete. Typically, composite pavements are asphalt overlays on top of concrete pavements. The HMA overlay may have been placed as the final stage of initial construction, or as part of a rehabilitation or safety treatment. Composite pavement behavior under traffic loading is essentially the same as rigid pavement.

Rigid and Flexible Pavement Characteristics

The primary structural difference between a rigid and flexible pavement is the manner in which each type of pavement distributes traffic loads over the subgrade. A rigid pavement has a very high stiffness and distributes

loads over a relatively wide area of subgrade – a major portion of the structural capacity is contributed by the slab itself.

The load carrying capacity of a true flexible pavement is derived from the load-distributing characteristics of a layered system

Pavement Type Selection

• Selecting a pavement type is an important decision. Like other aspects of pavement design,

• The selection of pavement type is not an exact science but one in which the highway engineer must make a judgment on many varying factors. .

Project Selection

Life-cycle Cost Analysis (LCCA)

• LCCA is an engineering economic analysis that allows engineers to quantify the differential costs of alternative investment options for a given project.

• LCCA can be used to compare alternate pavement types (flexible versus rigid) on new construction projects and rehabilitation projects.

Life-cycle Cost Analysis (LCCA)

• LCCA considers all agency expenditures and user costs throughout the life of the facility, not just the initial investment, and allows for cost comparison of options with varying design lives to be compared on an equivalent basis.

• LCCA offers methods to determine and demonstrate the economic merits of the selected alternative in an analytical and fact-based manner.

Life-cycle Cost Analysis (LCCA)

LCCA helps engineers answer questions like:

1. Which design alternative results in the lowest total cost to the agency over the life of the project?

2. To what level of detail have the alternatives been investigated?

3. What are the user-cost impacts of alternative strategies?

Questions ?

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Design FactorsCE 414 Lecture 2

Ser

vice

abil

ity

(PS

I)

p0 - pt

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Design Factors

1. Traffic and Loading2. Environment3. Materials4. Failure Criteria5. Reliability6. Pavement Management Systems (PMS)

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1. Traffic and Loading Axle loads, load repetitions, tire-contact areas , vehicle speeds

Passenger

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Axle Loads

o Single axle with single tires

o Single axle with dual tires

o Tandem axles with dual tires

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Number of repetitions for each axle load (SPECTRA) Dividing axle loads into groups (PCA, AI& AASHTO) Equivalent axle (80 KN)

Number of Repetitions

Contact Area

• Axle load• Tire load • Tire pressure• Contact pressure • Contact area

Generally contact pressure is assumed to be equal to tire pressure

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Contact Area “ Ac”

+(0.4L)(0.6L) = 0.5227L

L

0.6L0.3L

Actual Area

0.8712 L

0.6 LArea = 0.5227 L2

Equivalent Area

𝑳=√ 𝑨𝒄

𝟎 .𝟓𝟐𝟐𝟕

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Example

Draw the most realistic contact area for an 18-Kip (80 KN) single-axle load with a tire pressure of 80 psi (552 kPa).

What are the other configurations of contact area that have been used for pavement design?

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Solution

P=18-Kip (80 kN) single-axle load p= 80 psi (552 kPa).

Single axle load is applied on 4 tires , Tire load =18,000/4=4500 lb or 80/4 =20 kN The contact area of each tire is Ac =4500/80 = 56.25 in2 (3.6X104 mm2)

Width of tire is 0.6L = 0.6X10.37 =6.22 in. (158

mm).

= 10.37 in. (263 mm)

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Solution

263 mm

158 mm79 mm

Actual Area

229.13mm

158 mm

Area = 3.6x104

mm2

Equivalent Area

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(a) Most realistic contact area (rectangle + 2 cemicircles)(b) Rectangular contact area for use in finite element analysis(c) Two circles of radius 4.23 in. (107 mm), used by AI(d) Single circle with contact radius of 5.98 in. (152 mm), FHWA

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Vehicle speed

Visco-elastic theory

Speed is directly related to the duration of loading

Elastic theory

Resilient modulus of paving material is selected for vehicle speed (the greater the speed is the larger the modulus)

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Environment

• Temperature• Precipitation Both affecting the moduli of the various layers

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Material

General Properties (for both flexible and rigid pavements) :

1. Linear elastic – E & μ 2. Elastic modulus varied with time - Resilient

modulus selected with load durations 3. Non-linear elastic – constitutive equation should

be provided

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Material

Flexible pavements:1. For HMA-creep compliance (1/E) and

temperature- time shift factor2. Fatigue properties of asphalt mix3. Permanent deformation for each layer4. Asphalt stiffness

Rigid pavements:1. Modulus of subgrade reaction2. Coefficient of thermal expansion3. Modulus of rupture and fatigue properties4. Dowel properties (diam. & spacing)

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Failure criteria

• mechanistic-empirical methods, distress criteria• AASHTO method, PSI Flexible Pavements:

1. Fatigue cracking2. Rutting3. Thermal cracking

Rigid pavements:1. Fatigue cracking2. Pumping or erosion3. Other criteria

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5. Reliability

In view of the fact that the predicted distress at the end of a design period varies a great deal, depending on the variability of predicted traffic and the quality control on materials and construction, it is more reasonable to use a probabilistic approach based on the reliability concept .

If PSI is used as a failure criterion, the reliability of the design, or the probability that the PSI is greater than the terminal serviceability index , can be determined by assuming the PSI at the end of a design period to be a normal distribution with a mean and a standard deviation . Conversely, given the required reliability and terminal serviceability index, the acceptable PSI at the end of the design period can be computed.

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6. Pavement Management SystemPMS

Pavement design is a part of the total pavement management process, which includes: planning, design, construction, maintenance, evaluation, rehabilitation .

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6. Pavement Management SystemPMS

With the use of a computer, a pavement management system (PMS) can be developed to assist decision makers in finding optimum strategies for providing, evaluating, and maintaining pavements in a serviceable condition over a given period of time.

Pavement management can be divided into two generalized levels: network project .

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6. Pavement Management SystemPMS

1. At the network level, the pavement management system provides information on the development of an overall program of new construction , maintenance, or rehabilitation that will optimize the use of available resources .

2. At the project level, consideration is given to alternative design, construction, maintenance, or rehabilitation activities for a particular project within the overall program.