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Good 4 PSR’s used to develop design equations to predict Present Serviceability Index (PSI)ĪASHTO Flexible Pavement Design -Design NomographĪASHTO Flexible Pavement Design Factors 4 Design life 4Analysis period 4Performance Period 4Period by which pavement reaches terminal PSI 4Performance Period  Analysis Period 4 Reliability (Z R ) & standard deviation (S 0 ) 4 Traffic loading 4Estimated cumulative traffic in 18-kip Equivalent Single Axle Loads or ESAL’s (W 18 ) 4 Effective roadbed resilient modulus (M R )ĪASHTO Design Factors - Analysis Period Highway ConditionAnalysis Period (Years) High Traffic - Urban Highwa圓0-50 High Traffic - Rural Highway20-50 Low Traffic - Urban Highway15-25 Low Traffic - Rural Highway10-20ĪASHTO Design Factors -Reliability & Standard Deviation Design Reliability Functional ClassificationRecommended Reliability UrbanRural Interstate or other freeways85-99.980-99.9 Principal arterials80-9975-95 Collectors80-9575-95 Local roads50-8050-80 Standard Deviation CategoryStandard Deviation Performance prediction0.35 Other factors0.10ĪASHTO Design Factors -Traffic Loading 4 Expressed in Equivalent 18-kip Single Axle Loads, or ESAL’s.

1960 (1,114,000 axle loads) 4 74 flexible & 49 rigid pavement sectionsĪASHTO Road Test Major Findings for Flexible Pavements 4 Performance of base material (best to worst) asphalt treated, cement treated, untreated crushed stone, untreated gravel 4 Rutting and cracking were the major distresses 4 90% of rutting occurred in pavement layers: 32% in surface, 14% in base, 45% in subbase 4 More surface cracking during colder weather 4 Pavement deflections greater during spring than during summer (due to higher moisture levels)ĪASHTO Road Test 4 Based on Pavement Serviceability concept –Serviceability was defined as the ability of a pavement to serve its intended function under existing conditions 4 During the test, road users rated pavements –Pavement Serviceability Rating (PSR) –Based on the pavement roughness –0-1:V. Pavement Distresses Longitudinal Cracking Transverse CrackingĪASHTO Road Test 4 AASHTO: American Association of State Highway and Transportation Officials 4 Objective: To relate axle load repetitions, pavement design configurations & distresses 4 Test sections constructed in 1956 on I-80 in Ottawa, Illinois (80 miles southwest of Chicago) 4 Cost $ 27 million 4 Test pavements subjected to test traffic between Oct. Pavement Distresses Rutting Alligator Cracking Mechanistic Design Pavement Stresses Due to Tire Load

Types of Pavement Design Approaches 4 Empirical methods based on –Strength test (CBR) –Classification (soil classification) 4 Limiting shear failure methods 4 Limiting deflection methods 4 Regression methods based on Road Tests –AASHTO road test 4 Mechanistic-Empirical methods –Mechanistic model predicts stresses, strains, deflections –Regression models to predict pavement life humidity)  Materials  Failure criteria –Distress types, functional failure level of distress  Reliability needed for the pavement  Pavement management system –Life cycle cost (costs of construction & maintenance) –Pavement design life Pavement Design Criteria  Traffic & loading (axle loads, # of load applications, contact area of load, vehicle speed)  Climate (temperature, precipitation, rel. Pavement Design  A pavement consists of a number of layers of different materials 4 Pavement Design Methods –AASHTO Method –The Asphalt Institute Method for Flexible Pavements –Portland Cement Association (PCA) Method for Concrete Pavements –Texas DOT Method –Other methods Presentation on theme: "Pavement Design  A pavement consists of a number of layers of different materials 4 Pavement Design Methods –AASHTO Method –The Asphalt Institute Method."- Presentation transcript: