Introduction to Pavement Design Concepts. Pavement Types of Pavement Principal of Pavement Design Failure Criteria Aspects of Pavement Design Relative Damage Concept Pavement Thickness Design approaches Empirical Method Mechanistic-Empirical Method. PAVEMENT.
The pavement is the structure which separates the tyres of vehicles from the underlying foundation material. The later is generally the soil but it may be structural concrete or a steel bridge deck.
Flexible Pavements are constructed from bituminous or unbound material and the stress is transmitted to the sub-grade through the lateral distribution of the applied load with depth.
Aggregate Base Course
Natural Soil (Subgrade)
Aggregate Subbase Course
Pavement design is the process of developing the most economical combination of pavement layers (in relation to both thickness and type of materials) to suit the soil foundation and the traffic to be carried during the design life.
The concept of design life has to be introduced to ensure that a new road will carry the volume of traffic associated with that life without deteriorating to the point where reconstruction or major structural repair is necessary
For roads in Britain the currently recommended design is 20 years for flexible pavements.
A road should be designed and constructed to provide a riding quality acceptable for both private cars and commercial vehicles and must perform the functions i.e. functional and structural, during the design life.
If the rut depth increases beyond 10mm or the beginning of cracking occurs in the wheel paths, this is considered to be a critical stage and if the depth reaches 20mm or more or severe cracking occurs in the wheel paths then the pavement is considered to have failed, and requires a substantial overlay or reconstruction in accordance with LR 833.
Failure Mechanism (Fatigue and Rut)
Elastic Modulus ’E1’
Poison’s Ratio ‘ v1’
Bituminous bound Material
Maximum Tensile Strain at Bituminous Layer
Elastic Modulus ’E2’
Poison’s Ratio ‘ v2’
Maximum Compressive on the top of the sub-grade
Elastic Modulus ’E3’
Poison’s Ratio ‘ v3’
The following relationship can be used to calculate permissible tensile and compressive strains by limiting strain criterion for 85% probability of survival to a design life of N repetition of 80 kN axles and an equivalent pavement temperature of 20C;
log N = -9.38 - 4.16 logr (Fatigue, bottom of bituminous layer)
log N = - 7.21 - 3.95 logz (Deformation, top of the sub-grade)
r = is the permissible tensile strain at the bottom of the bituminous layer
z = is the permissible Compressive strain at the top of the sub-grade.
Can sustain Traffic Load
ASPECTS OF DESIGN
Pavement Thickness Design is the determination of required thickness of various pavement layers to protect a given soil condition for a given wheel load.
Given Wheel Load
Asphalt Concrete Thickness?
Base Course Thickness?
Subbase Course Thickness?
Given In Situ Soil Conditions
Roadbed Soil (Subgrade)
Asphalt Concrete Thickness ?
Base Course Thickness ?
Sub-base Course Thickness ?
SELECTED DESIGN LIFE
DESIRED STRUCTURAL AND FUNCTIONAL PERFORMANCE
STRUCTURAL PERFORMANCE CURVE
PREDICTED DESIGN TRAFFIC
Pavement Thickness Design Are Developed
To Account For The Entire
Spectrum Of Traffic Loads
Failure = 10,000 Repetitions
Failure = 100,000 Repetitions
Failure = 1,000,000 Repetitions
Failure = 10,000,000 Repetitions
Failure = Repetitions ?
18000 - Ibs
Damage per Pass = 1
16.4 Tons Axle
8.2 Tons Axle
Consider two single axles A and B where:
4.1 Tons Axle
8.2 Tons Axle
Consider two single axles A and B where:
Pavement Thickness Design is the determination of thickness of various pavement layers (various paving materials) for a given soil condition and the predicted design traffic in terms of equivalent standard axle load that will provide the desired structural and functional performance over the selected pavement design life.
PAVEMENT THICKNESS DESIGN APPROACHES
A given set of paving materials and soils, geographic location and climatic conditions
Pavement performance, traffic loads & pavement thickness
These methods or models are generally used to determine the required pavement thickness, the umber of load applications required to cause failure, or the occurrence of distress due to pavement material properties, sub-grade type, climate, and traffic conditions.
One advantage in using empirical models is that they tend to be simple and easy to use. Unfortunately they are usually only accurate for the exact conditions for which they have been developed. They may be invalid outside of the range of variables used in the development of the method
Proposed FA 1 Route 80
Layout of the AASHO Road Test.
WEIGHT IN TONS
AXLE WEIGHTS & DISTRIBUTIONS USED ON VARIOUS LOOPS OF THE ASSHO ROAD TEST
Asphalt Concrete = ?
Base = ?
Subbase = ?
AASHTO being an EMPIRICAL procedure is applicable to the AASHO Road TEST conditions under which it was developed.
The mechanistic –empirical method of design is based on the mechanics of materials that relates an input, such as a wheel load, to an out put or pavement response, such as stress or strain. The response values are used to predict distress based on laboratory test and field performance data. Dependence on observed performance is necessary because theory alone has not proven sufficient to design pavements realistically
Kerkhoven and Dormon (1953) first suggested the use of vertical compressive strain on the surface of sub-grade as a failure criterion to reduce permanent deformation, while Saal and Pell(1960) recommended the use of horizontal tensile strain at the bottom of asphalt layer to minimize fatigue cracking. The use of above concepts for pavement design was first presented in the United States by Dormon and Metcalf (1965)
By limiting the elastic strains on the sub-grade, the elastic strains in other components above the sub-grade will also be controlled; hence, the magnitude of permanent deformation on the pavement surface will be controlled as well. These two criteria have since been adopted by Shell Petroleum International (Claussen et al., 1977) and the Asphalt Institute (Shook et al., 1982) in their mechanistic-empirical methods of design, the ability to predict the types of distress, and the feasibility to extrapolate from limited field and laboratory data.
Researchers assumes that mechanistic - empirical design procedures will model a pavement more accurately than empirical equations. The primary benefits that could result from the successful application of mechanistic empirical procedures include:
One of the biggest drawbacks to the use of mechanistic design methods is that these methods require more comprehensive and sophisticated data than typical empirical design techniques. The modulus of resilience, creep compliance, dynamic modulus, Poisson's ratio, etc., have replaced arbitrary terms for sub-grade and material strength used in earlier empirical techniques.
Inadequately Designed Pavements Will Fail Prematurely Inspite
Of Best Quality Control & Construction Practices
PAKISTAN Vs USA