PAVEMENT DESIGN CONSIDERATIONS Gisel Carrasco, P.E CST, Materials and Pavements Flexible Pavements Branch Footer Text
Introduction 2
What is a Pavement Design? Analytical process to calculate the required pavement layer thicknesses based on: the expected performance of the pavement, the properties of materials intended to be used, the projected traffic loads, and environmental conditions. 3
The Big Picture Climate Desired Pavement Performance Pavement Design Analysis Material Properties Required Pavement Structure Traffic 4
FPS-21 & TxME Texas Flexible Pavement Design System (FPS-21) Required for design flexible pavements High volume highways (>10,000 ADT, 5 M EASALs) Uses a system approach centered upon deflection measurements from the FWD Traffic loading in terms of cumulative 18-kip ESALs TxME Mechanistic-Empirical pavement design program based on AASHTO s MEPDG Developed by TTI through a TxDOT s research project 5
Top 10 Pavement Design Dos and Don ts Run test for stabilized subgrade/bases Characterize the existing pavement Use postprocessing checks Avoid high flex base modulus for virgin material Avoid single lift HMA directly of flex base BEST PRACTICES Evaluate ground truthfulness of traffic Address existing weak locations Characterize the existing subgrade Remove outlier backcalculated layer moduli Insure proper bond between layers 6
Characterize the existing pavement prior to developing the design 7
Do your homework RECORDS PMIS Plans Maintenance Soils Maps SITE INVESTIGATION Drainage Cross slope/crown ROW issues Distress survey NON- DESTRUCTIVE TESTING (NDT) Ride Skid FWD GPR DCP SAMPLING HMA cores Soil borings Trench 8
Non-Destructive Testing (NDT) 9
Falling Weight Deflectometer (FWD) Measures pavement deflections Used for load zoning, design, forensic studies, and projects for determining structural adequacy. Calculates modulus of subgrade and other existing pavement layers Determine limitations of existing materials Spot weak areas Segmentation option allows diving project up into likestrength segments 10
Ground Penetrating Radar (GPR) Provides a continuous profile of layer thicknesses and dialectric variations to a maximum depth of about 24 in. beneath the pavement surface Used to locate areas with high moisture pavement layers low density areas areas where further testing is necessary stripped HMA layer weak areas 11
Crystal Core 16 Crystal Core C17 FM 1187, EB, FM 1902 to BF 1187 Left WP Low density interface at 8-9 depth. Dielectric ~ 20; depth 5-6 Soaked PMS Dielectric ~ 30; depth 5-6 Uniform HMA mass Uniform HMA mass Dielectric ~ 20; depth 5-6 Soaked PMS Right WP
USE RELIABLE DATA 13
Traffic & Climate Evaluate ground truthfulness of TPP traffic characterization Traffic based on ESALS ADT (Beginning/End) 18 kip ESALs 20 years Asses actual % trucks in the traffic stream from project location thru a 48- hr classification count Make sure TPP knows about any recent traffic generators in the area Avoid outdated pavement design traffic data Climate Environmental influences including seasonal changes in material stiffness are not directly considered by FPS-21 14
TxME TRAFFIC Load Spectra Percent Trucks Axle Configuration Vehicle Class Distribution and Growth Axle Per Truck CLIMATE Climatic data from weather stations Interpolated climatic data for a given location 15
LAYER THICKNESS SELECTION 16
Layer Thickness Selection Based on Pavement design program Cost District experience (SOP) Material availability HMA layers Avoid single lift HMA applications directly on flexible base Avoid 2-4 HMA surface on weak sub-structure 17
Zone of high tensile strain typical for 2-4 HMA surface on weaker substructure. Note limited fatigue life.
Result 19
Better fatigue life using thinner, more compliant (lower modulus, higher AC content) mixture or multiple surface seals.
Thin Overlay Options Thin Mix or ST Base Subgrade
DESIGN CHECKS Texas Triaxial Check (Shear strength) Mechanistic Check (Rutting and Cracking) 22
Design Checks Use post-check performance results in determining the adequacy of FPS design. Modified Triaxial ME fatigue ME cracking TxME 23
TxME Relationship to FPS21 FPS21: Overall design options TxME: Mechanistic Check Specific layer material properties Specific climatic conditions Specific traffic (load spectrum) Specific variability: Inputs
TxME Performance Criteria Rutting Thermal Cracking Fatigue Cracking Variability of Input Parameters 25
Final Thoughts TxME workshops will be provided throughout the state Load spectra traffic data research project to start in the fall Updated pavement manual 27
Questions 28