EARLY AGE CRACKING IN HMAC Short Course Gisel Carrasco, P.E., CST-M&P

Transcription

1 EARLY AGE CRACKING IN HMAC 2014 Short Course Gisel Carrasco, P.E., CST-M&P

2 Common Myths About Cracking I can prevent it. Cracking is not that big of a deal. If I use recycled materials, I can t have a crack resistant mix. Don t worry, maintenance will take care of it. 2

3 Let s Talk about Cracks All Pavements Crack! Environment Moisture changes Temperature changes Materials Asphalt binder oxidation Aggregate breakage Traffic Loading Stresses from axle loads It will happen, the question is when 3

4 Let s Talk about Cracks 4

5 Cracking the Cracked Road Code dd = A K n dd N f = 0 h dd A( K) n Where: c = the crack length; N = the number of loading cycles; A and n = fracture properties of the asphalt mixture; ΔK = the SIF amplitude (depends on the stress level, the geometry of the pavement structure, the fracture mode, and crack length). 5

6 Cracking the Cracked Road Code We can t stop it But, we get paid the big bucks to delay it. Specifications Pavement Design Guide Best Practices 6

7 Table of Contents 1 Pavement Life Expectation Pavement Design Mix Design Construction Specifications Summary 29 7

8 Pavement Life Expectations Reflective cracking Rule of thumb: one inch per year Reported ages at which surface cracking starts France: from 3 to 5 years Japan: from 1 to 5 years United Kingdom: 10 years Florida: 5 to 10 years 8

9 Pavement Life Expectations Districts were surveyed for project Expected pavement life Mix type usage Type-C Type-D Type-F PFC-PG Service Life, Years 0% < 220% % % % 100% NOR Type-D Type-C PFC-PG SMA-D PFC-AR a) Mix Type Use in Past 5 Years CAM SP-C SP-D Type-F CMHB-F SMAR-F TBPFC CMHB-C SMA-C SMA-F UTBHMWC PFC-AR SP-C SP-D CMHB-C CMHB-F SMA-C SMA-D SMA-F SMAR-F CAM TBPFC 9

10 What Can Cause Early Age Cracking? Improper pavement design Excessive traffic loading Improper material selection Bad construction practices 10

11 Pavement Design Evaluate the existing pavement Pavement design Determine thickness Consider all the variables Select the right mix for the right job Address reflective cracking by selecting a mix type with a higher asphalt content Data! Data! Data! He cried impatiently. I can t make bricks without clay. -Sherlock Homes Do your homework! - Your teacher 11

12 Pavement Design - Durability Specify mixes that have a higher binder content 1000 Overlay Test 800 Number of Cycles %-3.9% 4.0%-4.9% 5.0%-5.9% 6.0%-6.9% 7.0%-7.9% >8.0% 12

13 Pavement Design - Durability Specify a finer gradation for your mix type 700 Overlay Test Number of Cycles Median Average 0 Type A Type B Type C Type D Type F 13

14 Superpave Usage By District AMARILLO CHILDRESS LUBBOCK WICHITA FALLS PARIS ABILENE FORT WORTH DALLAS ATLANTA EL PASO ODESSA SAN ANGELO BROWNWOOD WACO TYLER LUFKIN BRYAN AUSTIN BEAUMONT SAN ANTONIO YOAKUM HOUSTON LAREDO CORPUS CHRISTI PHARR 14

15 FY 2013 to FY 2014 FY 2013 FY 2014 Seal Coat (SY) 112,957, ,911,803 Mix Tonnage (tons) 7,581,968 8,086,538 Dense Graded (%) Superpave (%) SMA (%) PFC (%) TOM (Item 347) (%) Bonded (Item 348) (%) RAP (%) RAS (%) WMA (%)

16 Mix Design 16

17 Mix Design Specify more durable aggregates when feasible SAC B vs. SAC A Cost Availability Road class Mix type Absorption Effective asphalt content 17

18 Mix Design Specify softer binders or binders with a wider PG spread 18

19 Recycled Materials Best Practices Control the moisture and variability of your recycled materials RAS Control the moisture Grind fine RAP Fractionate 19

20 Changes in Specifications to Address Early Age Cracking Recycled Materials Recycled materials are allowed unless otherwise shown on the plans. Is not recommended to use the maximum allowable percentages on surface mixtures. Verify the asphalt content of the stockpiles during production. Ensure the ratio of the recycled asphalt binder to total binder does not exceed the limit during design and production. 20

21 The Dark Side of Recycled Materials More Recycled Materials Less Virgin Binder Stiff Mixes Premature Cracking Shorter Pavement Life 21

22 Changes in Specifications to Address Early Age Cracking Substitute Binder Use softer binder when using recycled materials Verify the substitute binder meets Item 300 For an intermediate HMA layer Originally Specified Binder PG Substitute binder PG PG PG PG Maximum ratio of recycled to total binder (%) 20% 20% 35% 35% 22

23 Mix Design - Durability Raise the target lab-density or lower n-design Item 341, Dense-Graded Mixtures Target Lab Molded density Increase density up to 97.5% Ndesign Decrease Ndesign down to 35 gyrations VMA Production VMA was increased 23

24 Hamburg and Tensile Strength Very low rut depths and high tensile strengths can indicate a relatively stiff mix

25 Changes in Specifications to Address Early Age Cracking Hamburg When the rut depth at the required minimum number of passes is less than 3 mm, the Engineer may require the Contractor to increase the target laboratory-molded density or lower the Ndesign level Pass Engineer s Discretion Fail Rut Depth (mm) Fail 25

26 Construction - Best Practices Don t over-stiffen or over-oxidize the mix WMA & rejuvenators can be use to reduce mix stiffness HMA WMA 26

27 Construction Include adequate surface preparation in the plans Ensure an adequate bond to the existing pavement Crack Seal Underseal Patching Joint Repair Level-Up Profile Milling 27

28 Construction Ensure the mix is uniform and consistent 28

29 Changes in Specifications to Address Early Age Cracking JMF adjustments Ignition oven correction factors Meet mixture requirements Maximum allowable amount of RAP Recycled Binder to Total Binder IDT and Hamburg Aging ratio 29

30 Proper Joint Construction Best Practices Control segregation at the outside edges of the mat Steer a straight line Compact the unconfined edge Maintain the correct overlap Place the proper depth for roll-down Do not lute the joint Compact the joint for density 30

31 Summary Recycled Materials Mix Type WMA Soft Binder Adequate Asphalt Content Crack Resistant Mix Expected Pavement Life 31

32 Summary Pavement Design Layer thickness HMA mix selection Traffic Materials Quality and uniformity of materials Mix design Performance tests Construction Quality Control/ Quality Assurance Testing JMF Changes 32

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