SEAUPG 2002 CONFERENCE

Transcription

1 SEAUPG CONFERENCE Top-Down Cracking: Causes and Potential Solutions Dr. Rey Roque, P.E. University of Florida Introduction Surface-initiated longitudinal wheel path cracking: Common mode of failure Recently identified Mechanisms now better understood Occurs primarily under critical conditions Existing design methods are inadequate Understanding leads to more resistant pavements Background In the Past, Cracking Assumed to Always Start at Bottom of AC Layer (Structure has a Strong Effect) CRACKING IN WHEEL PATHS Have evaluated effects of factors on response at this location CORE EXTRACTED FROM FIELD Objectives To Summarize Findings Related to Top-Down Cracking - Mechanisms for Initiation and Propagation - Key Factors Dominating the Mechanisms - Implications for Mitigation and Design (improved guidelines for mixture and pavement design) 1

2 SEAUPG CONFERENCE Pavement Surface Stresses Measured Tire-Pavement Interface Stresses (Smithers Scientific Services, Inc.) Tire-Pavement Interface Stresses - Including lateral stresses Thermal Stresses Pavement Structure Effects Layer Stiffness and Thickness Bed Motion 1 Transducers Bed Tire σ x δ x Tire Rolling Direction σ z δ z σ y δ y Transducer Detail Transverse Contact Stresses Transverse Surface Stress Distribution Tension Compression..... Radial (R.5) Truck Tire Transverse Stress, σ xx Transverse Stress, xx (MPa) Tension Compression Tread Specified Load/Pressure Transverse Distance, X (mm) Thermal Stress Thermal Stress (MPa) Time (hr) mm Depth 5 mm Depth 1 mm Depth Aged AC-3 Findings for Crack Initiation Primary Contributors to Surface-Initiated Longitudinal Wheel Path Cracking - Transverse contact stresses induced by radial truck tires - Thermal stresses - Age-Hardening near surface

3 SEAUPG CONFERENCE Propagation of Top-Down Cracks Surface Contact Stresses (Tire or Thermal) Cannot Explain Propagation of Top-Down Cracks - Continued work to identify mechanisms and factors controlling propagation RANGE OF FACTORS Load Spectra Positioning with respect to crack Realistic tire contact stresses (vertical and lateral) Cracks and Discontinuities Crack depth Asphalt Pavement Thickness Interstate highway pavements thick/stiff Surface and Base Layer Stiffness Stiffness Gradients Induced in Asphalt Layer Daily temperature and environmental fluctuations STRUCTURAL PARAMETERS Crack Length.5,.5,.75, 1., 1.5 Complex Vertical Load Positions: Wide Rib, 7, 15,, 5, 3, AC E 1 =,1ksi 1 Base E =,.5ksi Subgrade 9 E 3 = 1.5ksi MODELING SYSTEM STEP 1 Boundary Condition Applied to Sides: Rollers Vertical and Transverse Tire Contact Load AC Layer Predict Vertical and Base Layer Horizontal Reaction Forces Subgrade Layer Rollers Applied to Bottom Side Pins Applied to Bottom Corners MODELING SYSTEM STEP USE OF FRACTURE MECHANICS Model geometry of crack & increase mesh refinement Boundary Condition Applied on Sides: Rollers Crack Vertical and Transverse Tire Contact Load AC Layer Predicted Vertical and Horizontal Reaction Forces Applied Along Bottom Nodes Spring Constant k Applied Mode I Stress Intensity Factor, K I lim σ xx (πr) 1/ r Mode II Stress Intensity Factor, K II lim τ yx (πr) 1/ r Fracture Energy Release Rate, J = (K ( ) 1 υ I + K II ) 1-ν J = K I + KII * E Stresses Predicted in Process Zone (Contours) Ahead of ECrack Provides local Tip Tire Loading description of crack tip Crack Depth region Predicted Stresses Under Crack Tip 3

4 SEAUPG CONFERENCE PARAMETRIC STUDY - STRUCTURE Mode I and II Fracture K I, K II (psi-(in).5 ) Tension >> Shear failure at crack tip Higher stiffness ratio increases tension at crack tip Crack Length, a (in) E 1 :E = E 1 :E = E 1 :E = 7 E 1 :E = 1 K1 - E1:E = :ksi K1 - E1:E = :ksi K1 - E1:E = 1:ksi K1 - E1:E = 1:ksi K - E1:E = :ksi K - E1:E = :ksi K - E1:E = 1:ksi K - E1:E = 1:ksi Load Position/Crack Length CRACK CRACK Tensile Zone (+) Undeformed Pavement Deformed Pavement LOAD LOAD Transverse Stress Distribution Along Surface of Pavement (-) Compressive Zone (+) (-) PARAMETRIC STUDY - LOADING Mode I Fracture, K I (psi-(in).5 ) Load position most critical factor for tension at crack tip 5 Crack Located Under Wide Rib 5 Crack Located " Away From Center of Load Crack Located 5" Away From Center of Load Crack Located 3" Away From Center of Load E 1:E = : ksi STIFFNESS GRADIENTS Temperature differentials, age-hardening, sudden rains induce sub-layers of variable stiffness within asphalt concrete Case 1: Uniform temperature distribution (i.e., no stiffness gradient) mean pavement temperature computed at 1/3-depth when temperature conditions = warm Case : Sharpest temperature gradient near surface - temperatures at 7 PM represent this condition Case 3: Highest temperature differential between surface & bottom of asphalt concrete layer - temperatures at 5 AM represent this condition Case : Rapid cooling near the surface represent case of sudden rain showers Crack Length (in) STIFFNESS GRADIENTS Major increase in tension key factor in propagation of cracks Mode I Fracture, K 1 (psi-(in).5 ) Base Stiffness E =ksi in AC, Case 1 - Uniform Stiffness E1 = ksi in AC, Case - Sharp Gradient in AC, Case 3 - Highest Temperature Differential in AC, Case - Sudden Rain STAGES OF CRACK GROWTH Short Cracks, a =.5,.5 a Wide Rib Load -3 d AC Intermediate Cracks, a =.75,1.,1.5 a d AC Load 7 Load -3 Crack Length, a (in)

5 SEAUPG CONFERENCE MPLICATION OF LOAD SPECTRA Load Wander & Magnitude Instrumental in Mechanism - Critical position not always directly in wheel path on top of crack - Depends on crack length, pavement structure, type of stiffness gradient - Need to determine how many loads induce crack tip Key Findings Top-Down Cracks Develop Even in Thick Pavements (Previous Field Studies) Mechanisms Cannot be Captured Without Considering - Realistic Contact Stresses -Temperature Gradients -Load Wander - Presence of Cracks (Fracture Mechanics) Most Recent Field Test s 5 pairs of poor and good performing sections throughout Florida Location: Performance: SR 1-C SR 1-U US 19-1U TPK 1U US 19-C TPK C 5

6 / 1/ 3/ TPK 1U TPK C SEAUPG CONFERENCE Sampling Coring WP BWP Total SR 1-C 7 3 NW 39-1C SR 1-U US 19-1U US 19-C SR 375-1U 1 1 NW 39-U NW 39-1C SR 375-C 1 1 TPK-1U 1 1 TPK-C 1 NW 39-1C NW 39-U Total 7 NW 39-U Sample selection: closest to average BSG Crack depth: Crack Depth (in) Rating < 1/ 1 1/ / 1 1/ > 3 Performance Rating SR 1 - C SR 1 - U US 19-1U 1 US 19 - C SR 375-1U 1 SR C TPK 1U 1 TPK C NW 39-1C NW 39 - U 1 Effective Asphalt Conten Volumetric Properties Air Void Content % Air Void Content SR 1-U SR 1-C US 19-1U US 19-C SR 375-1U SR 375-C TPK 1U TPK C NW 39-U NW 39-1C Effective Asphalt Content WP BWP. SR 1-U SR1-C US 19-1U US 19-C SR 375-1U SR 375-C TP K 1U TP K C NW 39-U NW 39-1C Aggregate Gradation Maximum Density Line SR 1-C SR 1-U 1 9 M aximum Density Line SR 375-1U SR 375-C SR SR / 1/ Sieve sizes Sieve Sizes Maximum Density Line US 19-1U US 19-C 1 9 Maximum Density Line TPK 1U TPK C 7 US 19 TPK / 1/ 3/ / 1/ 3/ / 1/ 3/ 1 Sieve sizes Sieve sizes Sieve sizes

7 SR 1-U SR 1-C US 19-1U US 19-C SR 375-1U S R 375-C TP K 1U TPK C NW 39-U NW 39-1C SEAUPG CONFERENCE 1 9 NW 39 Maximum Density Line NW 39-1C NW 39-U Theoretical Film Thickness Theoretical Film Thickness (micro-meters) SR 1-U SR1-C US 19-1U US 19-C SR 375-1U SR 375-C TP K 1U TP K C NW 39-U NW 39-1C 7 5, 3 35, / 1/ 3/ 1 Sieve sizes Binder Viscosity (poises) 3, 5,, 15, 1, Binder Viscosity 5, SR 1-U SR 1-C US 19-1U US 19-C SR 375-1U SR 375- TP K 1U TP K C NW 39-U NW 39-1C C Mixture Testing Resilient Modulus Resilient Modulus (GPa) Tensile Strength Indirect Tensile Strength (MPa) SR 1-U SR 1-C US 19-1U US 19-C SR 375-1U SR 375-C TPK 1U TPK C NW 39-U NW 39-1C.7 Creep Compliance at 1 seconds (1/GPa) SR 1-U SR 1-C US 19-1U US 19-C SR 375-1U SR 375-C TPK 1U TPK C NW 39-U NW 39-1C Creep Compliance m - Value SR 1-U SR 1-C US 19-1U US 19-C SR 375-1U SR 375-C TPK 1U TPK C NW 39-U NW 39-1C m-value 1 1 Failure Strain Failure Strain (microstrain) DCSE. SR 1 -U SR 1 - C US 19-1U US 19 - C SR 375-1U SR 375- C TPK 1U TPK C NW 39 -U NW 3 9-1C DCSE (KJ/m 3 ) Fracture Energy (KJ/m 3 ) Fracture Energy. SR 1 -U SR 1 - C US 19-1U US 19 - C SR 375-1U SR 375- C TPK 1U TPK C NW 39 -U NW 3 9-1C.5. SR 1 -U SR 1 - C US 19-1U US 19 - C SR 375-1U SR 375- C TPK 1U TPK C NW 39 -U NW 3 9-1C 7

8 SEAUPG CONFERENCE FWD Analysis Geometry Friction Course AC Base Sub-base SR 1-C SR 1-U US 19-1U US 19-C SR 375-1U SR 375-C TPK-1U TPK-C NW 39-1C NW 39-U Pavement Structure AC Base Sub-base Sub-grade US 19-1U US 19-C 1, 37 TPK-1U TPK-C NW 39-1C 1,5 71 NW 39-U 1, E1/E US 19-1U 11 US 19-C 7 TPK-1U TPK-C NW 39-1C 5 NW 39-U 3 Crack Growth Model Loading stresses Design load: 9 lbs Loading Stresses (psi) Initiation DCSE Nf 1, 1, 1,,, ,, 1 US 19-1U US 19-C TPK-1U TPK-C NW 39-U NW 39-1C SR 1-U SR 1-C US 19-1U US 19-C SR 375-1U SR 375- TP K 1U TP K C NW 39-U NW 39-1C C , Thermal Stresses (psi) 1 Thermal Stresses Cooling rate: 1ºC/hr FE Nf 1, 1,,,,, US 19-1U US 19-C TPK-1U TPK-C NW 39-U NW 39-1C SR 1-U SR 1-C US 19-1U US 19-C SR 375-1U SR 375- TP K 1U TP K C NW 39-U NW 39-1C C Propagation Nf to propagation 1, 1,,,,, Individual Analysis of the s SR 1: high air voids, high m-value, low FE possible effect of friction course US 19: stiffness overwhelmed FE, low AC content and film thickness SR 1-U SR 1-C US 19-1U US 19-C SR 375-1U SR 375-C TPK 1U TPK C NW 39-U NW 39-1C SR 375: low FE, high m-value, high air voids

9 SEAUPG CONFERENCE TPK: low base stiffness, high m-value, low film thickness NW 39: high E 1 /E ratio, low compliance, low tensile strength, low FE, possible effect of friction course Summary of Findings Structure has effects on surface-initiated cracking Load + Thermal stresses plays significant role High MR + Low Compliance / Load and thermal σ UF Cracking model properly explained performance differences No single property will assure adequate mixture performance. Need of conditions and model Key Findings (Continued) Design Approach Using Averaged Pavement and Load Conditions Inadequate - Critical Conditions Must Be Identified Mitigation Should be Addressed Primarily Through - More Crack-Resistant Surface Materials - Use of Less Damaging Tires Implications for Design Focus on Improving Surface Materials Dialogue With Tire Designers Pavement Design/Management - Realistic contact stresses - Load spectra (magnitude and position) - Temperature gradients - Cracks/discontinuities (fracture mechanics) Potential Solutions Improved Mixture Design - Maximize Fracture Resistance of Mixtures - Improved Gradation/Volumetrics - Appropriate Mixture Design Parameters (e.g. Fracture Energy) -Modifiers Specialized Thin Surface Layers - Highly Modified, Low Stiffness/Stress Relief, High Strain Tolerance Conclusions Stiffness and Compliance should be limited Thermal stresses should be considered in structural design More attention should be paid to the effect and design of friction courses It is critical to identify appropriate design conditions to evaluate performance using the parameters from the laboratory UF cracking model adequately represents cracking mechanisms 9

10 SEAUPG CONFERENCE Recommendations Continue evaluation of field performance to better understand failure mechanisms Create database of mixture, traffic, environment and pavement structural characteristics Concentrate more on friction course and structure Testing at multiple temperatures for thermal analysis 1