3PavementTypes of PavementPrincipal of Pavement DesignFailure CriteriaAspects of Pavement DesignRelative Damage ConceptPavement Thickness Design approachesEmpirical MethodMechanistic-Empirical Method
4PAVEMENTThe 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.
5TYPES OF PAVEMENTFlexible PavementsRigid Pavements
6FLEXIBLE PAVEMENTSFlexible 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.
7Natural Soil (Subgrade) Aggregate Subbase CourseAggregate Base CourseAsphalt Concrete
8Typical Load Distribution in Flexible Pavement Wheel LoadBituminous LayerSub-grade
9Typical Stress Distribution in Flexible Pavement. Vertical stressFoundation stress
10RIGID PAVEMENTSIn rigid pavements the stress is transmitted to the sub-grade through beam/slab effect. Rigid pavements contains sufficient beam strength to be able to bridge over localized sub-grade failures and areas of inadequate support.Thus in contrast with flexible pavements the depressions which occur beneath the rigid pavement are not reflected in their running surfaces.
12PRINCIPLES OF PAVEMENT DESIGN The tensile and compressive stresses induced in a pavement by heavy wheel loads decrease with increasing depth. This permits the use, particularly in flexible pavements, of a gradation of materials, relatively strong and expensive materials being used for the surfacing and less strong and cheaper ones for base and sub-base.The pavement as a whole limit the stresses in the sub-grade to an acceptable level, and the upper layers must in a similar manner protect the layers below.
13PRINCIPLES OF PAVEMENT DESIGN 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.
14DESIGN LIFEThe 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
15Philosophy of Pavements Pavements are alive structuresThey are subjected to moving traffic loads that are repetitive in natureEach traffic load repetition causes a certain amount of damage to the pavement structure that gradually accumulates over time and eventually leads to the pavement failure.Thus, pavements are designed to perform for a certain life span before reaching an unacceptable degree of deterioration.In other words, pavements are designed to fail. Hence, they have a certain design life.
16DESIGN LIFEFor roads in Britain the currently recommended design is 20 years for flexible pavements.
17PERFORMANCE AND FAILURE CRITERIA 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.
18PERFORMANCE AND FAILURE CRITERIA 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.
20Typical Strains in Three Layered System Elastic Modulus ’E1’Poison’s Ratio ‘ v1’Thickness ‘H1’Bituminous bound MaterialErMaximum Tensile Strain at Bituminous LayerElastic Modulus ’E2’Poison’s Ratio ‘ v2’Thickness ’H2’Granular base/Sub-baseEzMaximum Compressive on the top of the sub-gradeElastic Modulus ’E3’Poison’s Ratio ‘ v3’Sub-grade
21The 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 = logr (Fatigue, bottom of bituminous layer)log N = logz (Deformation, top of the sub-grade)r = is the permissible tensile strain at the bottom of the bituminous layerz = is the permissible Compressive strain at the top of the sub-grade.
24RUDIMENTARY DEFINITION PAVEMENT THICKNESS DESIGNRUDIMENTARY DEFINITIONPavement 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 Load150 Psi3 PsiGiven In Situ Soil ConditionsAsphalt Concrete Thickness?Base Course Thickness?Subbase Course Thickness?
25PAVEMENT DESIGN PROCESS Climate/EnvironmentLoad MagnitudeVolumeTrafficMaterial PropertiesAsphalt ConcreteRoadbed Soil (Subgrade)BaseSubase
34RELATIVE DAMAGE CONCEPT EquivalentStandard ESALAxle LoadIbs(8.2 tons)Damage per Pass = 1Axle loads bigger than 8.2 tons cause damage greater than one per passAxle loads smaller than 8.2 tons cause damage less than one per passLoad Equivalency Factor (L.E.F) = (? Tons/8.2 tons)4
35Consider two single axles A and B where: EXAMPLEConsider two single axles A and B where:A-Axle = 16.4 tonsDamage caused per pass by A -Axle = (16.4/8.2)4 = 16This means that A-Axle causes same amount of damage per pass as caused by 16 passes of standard 8.2 tons axle i.e,16.4 Tons Axle=8.2 Tons Axle
36Consider two single axles A and B where: EXAMPLEConsider two single axles A and B where:B-Axle = 4.1 tonsDamage caused per pass by B-Axle = (4.1/8.2)4 =This means that B-Axle causes only times damage per pass as caused by 1 pass of standard 8.2 tons axle.In other works, 16 passes (1/0.625) of B-Axle cause same amount of damage as caused by 1 pass of standard 8.2 tons axle i.e.,=4.1 Tons Axle8.2 Tons Axle
38PAVEMENT THICKNESS DESIGN Comprehensive Definition 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.
40Pavement performance, traffic loads & pavement thickness EMPIRICAL PROCEDURESThese procedures are derived from experience (observed field performance) of in-service pavements and or “Test Sections”betweenPavement performance, traffic loads & pavement thicknessforA given set of paving materials and soils, geographic location and climatic conditionsThese procedures define the interactionThese procedures are only accurate for the exact conditions for which they were developed and may be invalid outside the range of variables used in their development.EXAMPLEAASHTO Procedure (USA)Road Note Procedure (UK)
41EMPIRICAL PROCEDURESThese 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.
42EMPIRICAL PROCEDURESOne 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
43AASHTO PROCEDUREEmpirical Procedure developed through statistical analysis of the observed performance of AASHTO Road Test Sections.AASHTO Road Test was conducted from 1958 to near Ottawa, Illinois, USA.234 “Test Sections” (160 feet long), each incorporating a different combination of thicknesses of Asphalt Concrete, Base Course and Subbase Course were constructed and trafficked to investigate the effect of pavement layer thickness on pavement performance.
44Layout of the AASHO Road Test. 178UticaUtica Road2371US6NorthOttawaLoop 4Loop 5Loop 6Loop 3Frontage RoadMaintenance BuildingAASHO Adm’n12Proposed FA 1 Route 80Army BarracksPre-stressed /Reinforced ConcreteTypical LoopXTest TangentRigidFlexibleSteel I-Beam
46AXLE WEIGHTS & DISTRIBUTIONS USED ON VARIOUS LOOPS OF THE ASSHO ROAD TEST LOOP LANELOAD1FRONT LOAD2WEIGHT IN TONS0.9FRONT AXLELOAD AXLEGROSS WEIGHT220.127.116.113655.512.710.918.104.22.168.114.622.214.171.124.240.513.631.421.849.1
47AASHO ROAD TEST RIDE QUALITY Initial Terminal Subbase = ? ESALs “Test Sections” were subjected to million applications of load.Performance measurements (roughness, rutting, cracking etc.) were taken at regular intervals and were used to develop statistical performance prediction models that eventually became the basis for the current AASHTO Design procedure.AASHTO performance model/procedure determines for a given soil condition, the thickness of Asphalt Concrete, Base Course and Subbase Course needed to sustain the predicted amount of traffic (in terms of 8.2 tons ESALs) before deteriorating to some selected level of ride quality.ESALsTerminalInitialRIDE QUALITYAsphalt Concrete = ?Base = ?Subbase = ?Soil
48LIMITATIONS OF THE AASHTO EMPIRICAL PROCEDURE AASHTO being an EMPIRICAL procedure is applicable to the AASHO Road TEST conditions under which it was developed.
49MECHANISTIC-EMPIRICAL PROCEDURES These procedures, as the name implies, have two parts:=> A mechanistic part in which a structural model (theory) is used to calculate stresses, strains and deflections induced by traffic and environmental loading.=> An empirical part in which distress models are used to predict the future performance of the pavement structure.The distress models are typically developed from the laboratory data and calibrated with the field data.EXAMPLESAsphalt Institute Procedure (USA)SHRP Procedure (USA)
50Mechanistic- Empirical Methods 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
51Mechanistic- Empirical Methods 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)
52Mechanistic- Empirical Methods 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.
53Mechanistic - Empirical Design Approach 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:
54Benefits of Mechanistic - Empirical Design Approach The ability to predict the occurrence of specific types of distress.Stress dependency of both the subgrade and base course.The time and temperature dependency of the asphaltic layers.
55Benefits of Mechanistic - Empirical Design Approach Estimates of the consequences of new loading conditions can be evaluated. For example, the damaging effects of increased loads, high tire pressures, and multiple axles, can be modeled by using mechanistic processes.Better utilization of available materials can be accomplished by simulating the effects of varying the thickness and location of layers of stabilized local materials.Seasonal effects can be included in performance estimates.
56Benefits of Mechanistic - Empirical Design Approach One of the most significant benefits of these methods is the ability to structurally analyze and extrapolate the predicted performance of virtually any flexible pavement design from limited amounts of field or laboratory data prior to full scale construction applications. This offers the potential to save time and money by initially eliminating from consideration those concepts that have been analyzed and are judged to have little merit.
57Draw Back of Mechanistic - Empirical Design Procedures 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.
58However, the potential benefits are believed to far outweigh the drawbacks. In summary, mechanistic-empirical design procedures offer the best opportunity to improved pavement design technology for the next several decades.
59SOURCES OF PREMATURE PAVEMENT FAILURE Thickness DesignConstruction Practices&Quality ControlMaterial DesignThickness DesignMaterial DesignConstruction Practices&Quality ControlMaterial DesignThickness DesignConstruction Practices&Quality ControlInadequately Designed Pavements Will Fail Prematurely InspiteOf Best Quality Control & Construction Practices
60Causes of Premature Failure in Pakistan Causes of premature failure of pavements in PakistanRutting due to high variations in ambient temperatureUncontrolled heavy axle loadsLimitations of pavement design procedures to meet local environmental conditionsPROBLEM STATEMENT
61COMPARISON OF TRUCK DAMAGE PAKISTAN Vs USA17131928142039154101621511176121822