Innovations for Predicting Pavement Performance - Measurements and Models

Transcription

1 Geoffrey M. Rowe PAVEMENT PERFORMANCE PREDICTION SYMPOSIUM Innovations for Predicting Pavement Performance - Measurements and Models Organized by Western Research Institute Hilton Garden Inn & University of Wyoming Conference Center July 17, 2014, Laramie, Wyoming

2 Linear visco-elastic parameters Why these? What we measure? Methods for deducing parameters 2

3 Thermal Fatigue Top down surface cracking Block cracking Durability cracking 3

4 Linear visco elastic Measurements at small strain areas BBR, DSR measurements Can make master curves in linear region Note linear behavior is not full story for binders particularly modified but an important start/consideration 4

5 Generally we consider crack initiation and propagation Both aspects are dependent upon loading time and temperature Can use binder stiffness as descriptor for this! 5

6 Hukelom, AAPT, vol 35, p 358, Observations on the rheology and fracture of bitumens and asphalt mixtures Tensile Strength, kg/cm Tensile strength of binder is a function of binder stiffness (S b ) and can be presented as master curve; S b considers both time of loading and temperature. This location equates to G* 15MPa ,000 10,000 Stiffness Modulus of Bitumen, kg/cm 2 6

7 Extended testing to mixtures with same result Done for 8-mix types 7

8 Similar results for polymers Example: Styrene-butadiene rubber Tensile strain Data is shifted to a reduced strain rate that captures both time and temperature 8

9 Styrene-butadiene rubber Tensile strength Data is shifted to a reduced strain rate that captures both time and temperature 9

10 Binder stiffness important to describe strength, strain and properties at break Properties are both a function of loading rate and temperature! 10

11 Fatigue and fracture will exhibit a brittle to ductile transition! 11

12 Failure Stress, MPa A B C 5 Test Temperatures σ f ε f D E Failure strain, percent -12-

13 Number of cycles at failure, "True" fatigue Nf G* (MPa) Instability flow Stiffness modulus (MPa) Temperature ( C) Anderson, Marasteanu, Planche, Martin and Gauthier - Evaluation of Fatigue Criteria for Asphalt Binders TRB 2001

14 Range in stiffness where fatigue cracking and instability flow dominate Binder Fatigue cracking Instability flow Unmodified 28 to 55 MPa 5 to 18 MPa SB crosslinked 15 to 45 MPa 5 to 10 MPa EVA modified 13 to 45 MPa 5 to 9 MPa

15 Stiffness describe range were true fatigue versus visco-flow is expected 15

16 R-value G c T VET δ Relaxation spectra noted as being related to cracking in roads in France All above parameters related to relaxation spectra 16

17 Simple- easy to use model that describes time-temperature behavior of asphalt binders Development of BBR-DSR test equipment that provide measurements needed for model and to describe the combined effects of time and temperature on material response * G c * 17

18 Important to understand shape of stiffness response relationship Stiffness and relaxation is correlated to cracking 18

19 Require that we determine stiffness characteristics accurately for range that effects cracking Model stiffness master curve with BBR, DSR and CAM fit with Kaelble Possible from standard data that is collected 19

20 BBR data (a) raw data, 3 BBR isotherms and 2 DSR data points b: master curve of G* and δ DSR data 20

21 Free shifting keep analysis for temperature dependency and time dependency as two steps CAM model fit works well in limited range 1e5 to 1e9 Pa with Kaelble modification to WLF Density inclusion in shifting is important Data acceptable if rms % error 2.25%!! Combination of BBR data seems reasonable using interconversion via spectra fit, Hopkins and Hamming, etc. R value and cross-over frequency should only be expresses reliability when data in range Use free shifting to produce master curve Don t fit for time and temperature properties at same time Can develop master curve from standard PAV data set collected with M320 analysis using BBR and DSR data Minimum data set is two BBR isotherms and two intermediate DSR results associated with fatigue criteria 21

22 log a T = C 1 C 2 T Td + T T Tr Td + T T R= log 2/(β) δ (T,ω) = 90 / [1 + (ω r / ω c ) β ] d C 2 r d From these we can calculate all linear parameters as needed! Each parameter has a specific frequency of loading. Fatigue cracking: G* sinδ 5 MPa T varies, 10 rads 15 o C, rads 15 o C, rads T varies, rads

23 Models facilitate extrapolation! 23

24 What would a limit be? 24

25 1. Limits may change depending upon location, climate and type of cracking! 2. Other functions could be considered that describe stiffness and relaxation properties. 25

26 Rate of loading effects range of results that will be obtained in temperature domain if properties are dependent upon stiffness as shown earlier. Rate of any fracture test is key to understanding behavior. 26

27 800 DTT Test Vialit Test Cohesion 1.6 DTT - Energy (J) Brittle Ductile Brittle Ductile Vialit - Energy (J/cm2) Temperature (C) Really a stiffness effect needed to explain these brittle to ductile transitions. This shift is just related to loading time/rate! Width is related to rate! 0 27

28 Range is dependent upon assessed rate of loading of test being used. 28

29 If using E(t) previously observed ductile to brittle range covered from about 1MPa to 1000 MPa Note G* E(t)/3 300 kpa to 300 MPa 3e5 to 3e8 Pa Earlier noted that CAM model fit works well in limited range 1e5 to 1e9 Pa This range covers our the above range that we would describe the fracture behavior 29

30 E(t) = 1MPa to 1000 MPa Normalization of ultimate of fracture properties via linear visco-elastic modulus. 30

31 Stiffness G*, E(t) vital to use as descriptor for ultimate properties Data collection only use G* >1e5Pa in model fits This is enough for describing brittle to ductile fracture, etc. Linear VE properties of binder are being shown to relate to cracking of mixes thermal, fatigue, durability, etc. Dependent upon shape and position of master curve Stiffness and relaxation properties. 31

32 32

33 Questions? Comments! 33