Cracking Tests for Asphalt Mixtures and How to Mixture Condition Region 4 Topic

Transcription

1 Cracking Tests for Asphalt Mixtures and How to Mixture Condition Region 4 Topic Phillip B. Blankenship Consulting Engineer Blankenship Asphalt Tech & Training

2 A Fun Time to Be in Asphalt Industry is demanding a cracking test SHRP started on cracking tests in ~1990 but never fully developed Binder spec (PAV DSR) was meant for unmodified binders and was a catch all o Not stout enough for the cracking component that we need today Cracking tests are popping up seemingly everywhere Users are now in the game and notice the cracking 2

3 What Should Have Happened Superpave required Level 1, 2, and 3 testing based on traffic (ESAL) load o Level 1 (Volumetrics + TSR) was intended only for 1 million ESALS or less o Level 2 and 3 were to be used for higher traffic loads and included rutting and cracking performance test Since we saw such good initial Superpave pavement performance in , Levels 2 and 3 complication were soon forgotten

4 The Need for Performance Testing

5 It all started in 1919 Asphalt Association (later Asphalt Institute) was formed and hired Prevost Hubbard and Frederick Field as researchers Research led to the Hubbard-Field design method using rammers (like a Marshall hammer but with 2 size hammers) in mid 1920 s AI Magazine article by Gerry Huber 2/15/2013

6 Hubbard-Field Stability Hubbard-Field Stability test at AI headquarters Hubbard-Field Stability is the first known asphalt performance test. Sample was loaded by turning the wheel Dial gage recorded the maximum load

7 Testing Then and Now By the 1940 s: o Hubbard-Field stability test o Hveem stability test o Marshall stability and flow o Recorded data by hand or charts Today o TSR, Hamburg wheel tracker, APA, Texas overlay tester, 4-point flexural fatigue, fracture energy (3-4 tests), resilient modulus, shear modulus, dynamic modulus, AMPT dynamic modulus/flow number, and more

8 Technology Today We can control test from 0.01 Hertz to 25 Hertz (25 cycles a second) Technology allows us to record data at fast rates like 100+ points a second Temperature control to the nearest 0.5 C (mix) and 0.1 C (binder) o Need of strict temperature control is something we learned during the SHRP research The problem still remains

9 Traffic & Load Growth on Rural Interstate System Change Since % 600% 500% 400% 300% 200% 100% Rural Average Daily Load Rural Average Daily Traffic FHWA Highway Statistics %

10 Why do we need a mixture cracking test if the binder (PG 64-xx) is certified to meet a low temperature? 10

11 The Basics of Performance Testing Allow us to verify our estimates Design and check for potential distresses Custom design for specific loading Think out-of-the-box with new materials and modifiers

12 Fundamental Performance Tests Flexural Beam Fatigue o Brittleness Asphalt Mixture Performance Test o Dynamic modulus (used in MEPDG for design) o Flow number (rutting) Indirect Tension Test o Low temperature cracking b creep compliance (resting load on cold samples) to see how the move Photos from Asphalt Institute

13 Performance Tests Other tests o Hamburg Wheel Tester o Asphalt Pavement Analyzer o Several energy based tests o Overlay (crack) tester

14 Cracking Test Evaluation Project

15 We Need a Test(s) that Is Sensitive properties of mix components Sensitive to mixture aging Repeatable and reproducible Easy to implement o Less trimming/coring o Small footprint o Easy to run Practical, low cost 15

16 Review of Tests 1. AMPT Push/Pull Fatigue (S-VECD) 2. Indirect Tensile Strength (IDT) 3. 4-Point Bending Beam Fatigue 4. Dissipated Creep Strain Energy (DCSE) 5. Disk-Shaped Compact Tension [DC(T)] 6. Texas Overlay Test 7. Semi-Circular Bending (SCB)-ASTM 8. Semi-Circular Bending (SCB)-AASHTO (later version called ifit) 9. Indirect Tensile Asphalt Cracking Test (IDEAL-CT) at Intermediate Temperature 16

17 AMPT Push/Pull Fatigue (S-VECD) 17

18 AMPT Push/Pull Fatigue (S-VECD) Draft AASHTO standard by Richard Kim 18 C / 23 C o Not recommended to run over 21 C Various Strains Software builds curve based on three tests

19 AMPT Push/Pull Fatigue (S-VECD) Good test for design Not intended for 24-hr aged mixtures

20 Indirect Tensile Strength (IDT) 20

21 Indirect Tensile Strength (IDT) ASTM D 6931 Related AASHTO T C and 4.0 C Rate of Movement: 12.5 and 50 mm/min

22 Indirect Tensile Strength (IDT) Simplest test, but just says that mix gets stiffer

23 Indirect Tensile Strength (IDT)

24 Indirect Tensile Strength (IDT) So what can we learn? Confirms that we need correct temperature/loading rate for cracking sensitivity. Peak load alone is not the answer but combine with time/distance FRACTURE ENERGY

25 4-Point Bending Beam Fatigue 26

26 4-Point Bending Beam Fatigue 4-point bending beam fatigue (1950 s / SHRP) AASHTO T321 & ASTM 7460 Examined 20.0 C & 15.0 C Sine & haversine waves Rate of Movement: 10Hz, various strains (strain rates) Ex: 300 ms = 0.16mm/0.1sec or 98mm/min 2 beams for average (per strain)

27 Beam Fatigue 20 C & sine

28 Beam Fatigue - 15 C & sine

29 EXAMPLE: KY Density Study Findings with 24-hr Loose Mix Conditioning M. Anderson Alireza Zeinali, Phillip B. Blankenship, Kamyar C. Mahboub Beam fatigue device has been used to better understand pavement cracking potential.

30 Dissipated Creep Strain Energy (DCSE) 32

31 Dissipated Creep Strain Energy (DCSE) Draft standard by Rey Roque Uses IDT configuratio n Creep based on load & time 10 C 3 samples for average

32 Disk-Shaped Compact Tension [DC(T)] 35

33 Disk-Shaped Compact Tension [DC(T)] ASTM D 7313 Run at +10 C from critical low temp PG C Rate of Movement: 1 mm/min 3 samples for average

34 EXAMPLE: Pavement Preservation with Chip Seal

35 Texas Overlay Test 39

36 Texas Overlay Test Tx DOT Standard Tex-248-F 25 C Rate of Movement: 0.6 mm/5 sec and returns (fatigue) or 7.2mm/min 0.1 Hz

37 Texas Overlay Test Note: High error. Data is usually trimmed average.

38 Semi-Circular Bending (SCB)- ASTM 42

39 Semi-Circular Bending (SCB)-ASTM ASTM standard by Louay Mohammad 25 C Rate of Movement: 0.5 mm/min

40 Load (kn) Semi Circular Bend (SCB) Test Fracture mechanics Temperature: 25 C Half-circular Specimen Laboratory prepared Field core 150mm diameter X 57mm thickness simply-supported and loaded at mid-point Notch controls path of crack propagation 25.4-, 31.8-, and 38.0-mm Loading type Monotonic 0.5 mm/min To failure Record Load and Vertical Deformation Compute Critical Strain Energy: Jc Peak Load U Deflection (mm) notch a1

41 Jc, Kj/m2 Semi-Circular Bend Test Results, 25 C Can have high error. Usually based on 6 samples Higher temps or lower PG yields lower energy o This is opposite of what should happen hr 24-hr Mixture Type

42 Semi-Circular Bending (SCB)- AASHTO (later version called ifit) 47

43 What About ifit? Semi-Circular Bending (SCB)-AASHTO AASHTO TP- 124 by Imad Al-Qadi 25 C Rate of Movement: 50 mm/min Focus on latest standard on Flexibility Index (FI)

44 What about ifit? From Research Report No. FHWA-ICT , Testing Protocols to Ensure Performance of High Asphalt Binder Replacement Mixes Using RAP and RAS by Al-Qadi, et.al.

45 What about ifit? Showing much promise Current work on field mixes More work to come on longer aged mixes

46 Indirect Tensile Asphalt Cracking Test (IDEAL-CT) at Intermediate Temperature Following slides from Texas A&M 51

47 Development of IDEAL-CT IDEAL-CT concept

48 Development of IDEAL-CT CT index Equation

49 IDEAL-CT sensitivity Sensitivity to RAP/RAS

50 IDEAL-CT sensitivity Sensitivity to binder type

51 IDEAL-CT sensitivity Sensitivity to binder content

52 Refer to NCHRP 9-57 for Further Info

53 How to Condition Mixtures Short-term (absorption) o 2 to 4 hours on loose mix at 135 C Stir hourly Longer time needed for > 2% water absorption 60

54 How to Condition Mixtures Long-term (aging to simulate time in field) o Currently three (3) options AASHTO R-30: 120 ± 0.5 hours at 85 ± 3 C on compacted sample UIUC/AI: 24 hours at 135 C on loose mix Proposed that aging at 135 C causes changes in the chemistry of the binder that are not realistic National Cooperative Highway Research Program (NCHRP) 09-54: Suggests 95 C as optimal temperature for aging loose mix. Time is adjusted based on climate conditions and pavement depth. One study cited 5-12 days corresponding to preliminary recommendations of NCHRP study 61

55 Why 24 Hour Loose Mix Aging Focus on aging of the top ~1-2 inches University of Illinois study on in-place mixtures o Andrew F. Braham, William G. Buttlar, Timothy R. Clyne AAPTP non-load associated cracking study o Also found that 18hr loose mix 20hr PAV

56 Conclusions We need to condition mixtures to simulate proper field conditions at 7 to 10 years o 95C aging or 24-hr loose mix 135 All tests seem to recognize the conditioned mixtures except for the IDT strength o Strength alone is not enough Need to accept tests for what they are and designed to do Adjust tests for climates

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58 Thank You Phillip B. Blankenship, PE Consultant Blankenship Asphalt Tech and Training, PLLC