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

Pavement Design Criteria  Traffic & loading (axle loads, # of load applications, contact area of load, vehicle speed)  Climate (temperature, precipitation, rel. 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

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

Mechanistic Design Pavement Stresses Due to Tire Load

Distribution of Stresses & Temperatures

Pavement Distresses Rutting Alligator Cracking

Pavement Distresses Longitudinal Cracking Transverse Cracking

AASHTO 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 and Nov (1,114,000 axle loads) 4 74 flexible & 49 rigid pavement sections

AASHTO 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)

AASHTO 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. Poor; 1-2:Poor; 2-3:Fair; 3-4:Good; 4-5: V. Good 4 PSR’s used to develop design equations to predict Present Serviceability Index (PSI)

AASHTO Flexible Pavement Design -Design Nomograph

AASHTO 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 )

AASHTO Design Factors - Analysis Period Highway ConditionAnalysis Period (Years) High Traffic - Urban Highway30-50 High Traffic - Rural Highway20-50 Low Traffic - Urban Highway15-25 Low Traffic - Rural Highway10-20

AASHTO Design Factors -Reliability & Standard Deviation Design Reliability Functional ClassificationRecommended Reliability UrbanRural Interstate or other freeways Principal arterials Collectors Local roads Standard Deviation CategoryStandard Deviation Performance prediction0.35 Other factors0.10

AASHTO Design Factors -Traffic Loading 4 Expressed in Equivalent 18-kip Single Axle Loads, or ESAL’s. 4 Axle load equivalency factor was introduced to convert different axle loads to Equivalent 18,000 lb (18-kip) Single Axle Loads. Load Equivalency Factors (LEF’s) Passenger Car18-Wheel Truck Laden Weight (lbs)4,00080,000 Axle Load (lbs)2,00018,000 Tire Pressure (psi)

Vehicle Types Also refer to Figure 3.1

Vehicle Weight & Size Limits Weight limits for whole vehicle, single axles & tandem axles Size limits for length, width & height

AASHTO Design Factors -Example calculation of ESAL’s Flexible pavement with p t =2.5 and SN=4.0 Axle LoadAxle TypeLEF# of AxlesESAL’s (kips) Passenger Vehicle (Two 2-kip single axles) 2 Single Total ESAL per vehicle Tractor Trailer (one 10-kip single, Two 34-kip tandem) 10Single Tandem Total ESAL per vehicle Tractor Trailer causes 5755 times more damage than one passenger car

AASHTO Design Factors -Example calculation of ESAL’s Cumulative ESAL's in the design lane=D D x D L x W 18 D D = direction distribution factor (usually 50 percent) D L = lane distribution factor (percent traffic in design lane) W 18 = Cumulative ESAL for all lanes Example: Calculate the cumulative ESAL for a pavement with the following characteristics. ADT = 5000 Design life = 20 years Traffic growth rate = 3 percent Total number of lanes in each direction = 2 Directional distribution factor = 55 percent Lane distribution factor = 75 percent

AASHTO Design Factors -Effective Roadbed Resilient Modulus (ERRM) 4 Resilient modulus (M R )=Stress / Recoverable Strain

AASHTO Flexible Pavement Design Factors (continued....) 4 Design Serviceability Loss –Initial Serviceability Index = p 0 –Terminal Serviceability= p t (2.5-major, 2.0-other) –Serviceability loss (  PSI) = p 0 - p t –  PSI may be due to both traffic & climatic factors –Design nomograph uses  PSI due to traffic only PSI Age of Pavement p0p0 Terminal Serviceability Level ptpt

AASHTO Flexible Pavement Design....) 4 Design Output: Pavament Structural Number a i =layer structural coefficient D i =layer thickness in inches m i =layer drainage coefficient D1D1 D2D2 D3D3 Surface Layer Base Layer Subbase Layer Subgrade Layer

AASHTO Flexible Pavement Design -Layer Thickness Determination Example: The required structural number (SN) of an asphalt pavement to protect the roadbed soil (subgrade) is 3.5. The required structural number to protect the subbase material is 2.84 and the required structural number to protect the base is 2.0. The pavement is constructed using the following materials. Sand subbase layer structural coefficient = 0.11 Gravel base layer structural coefficient = 0.14 Asphalt concrete surface layer structural coefficient = 0.40 Calculate the required layer thickness in inches. Assume a drainage coefficient of 1.0 for all layers.

Determination of Layer Thicknesses