User:Hoskir/Revision Request 4223

REVISION REQUEST 4223

New Test Method
106.3.2.100 TM-100, Procedure to Calculate the Slope Ratio (SR) and Stripping Inflection Point (SIP) using the Hamburg Wheel-Track (HWT) Test

1. Run the HWT test in accordance with AASHTO T 324 at 50 +/- 1 º C for 20,000 passes (regardless of the mix type and contract binder grade).

Record rut depth at each cycle or at regular cycle intervals during the test.

2. Plot the average rut depth versus number of cycles up to the 20,000 passes.

Create a graph using Hamburg Test Data Analysis form that is available at MoDOT’s “Forms for Contractor Use” page on MoDOT’s website.

• X-axis: number of wheel passes or cycles.
• Y-axis: rut depth in millimeters.
• Report the average rut depth at 20,000 passes for all asphalt mixtures:

Binder Contract Grade	Minimum Wheel Passes	Maximum Rut Depth (mm)
58-28H / 64-22	20,000	20,000 4 mm
64-22H / 70-22
64-22V / 76-22

• If the average rut depth at 20,000 passes is 4 mm or less, the mixture passes the HWT stripping test and no further analysis is needed. If the rut depth exceeds 4mm, continue with Steps 3 through 5.

3. Identify the two linear portions of the curve.

Determine the two best-fit straight-line sections using a linear regression equation from excel spreadsheet or other graphing tools:

• First portion: steady-state rutting region before stripping begins.
• Second portion: post-stripping region where rutting increases more rapidly.

4. Calculate the slope of each portion.

Use linear regression for each section:

• Slope 1 = slope of the first portion – Creep Slope (M1) from linear regression equation – y = M₁x+ b₁.
• Slope 2 = slope of the second portion – Striping Slope (M2) from linear regression equation – y₂ = M₂x + b₂.

5. Calculate the slope ratio.

Slope Ratio = M₂ ÷ M₁

Where:

• M₁ = slope of the steady-state rutting portion.
• M₂ = slope of the stripping portion.

• If slope ration (M₂ / M₁) is less than 2.0 from the two linear equations from the creep slope and stripping slope for 20,000 passes using HWT test, the mixture passes the HWT stripping test and no further analysis is needed. If the slope ration (M₂ / M₁) is 2.0 or greater, continue with Step 6.

6. Calculate the stripping inflection point.

Find the intersection of the two best-fit lines using linear regression equations determined from Steps 3 through 5.

If the first line is:
y = M₁x + b1

and the second line is:
y = M₂x + b2

then:
SIP = (b2 - b1) ÷ (M₁ - M₂)

Where:

• b1 = intercept of the first portion.
• b2 = intercept of the second portion.
• M₁ = slope of the first portion.
• M₂ = slope of the second portion.

If the stripping inflection point (SIP) is greater than 15,000 wheel passes, then the mixture passes the HWT stripping test and no further analysis is needed.

7. Interpret the Combined HWT Stripping Results.

• A higher SIP generally indicates better resistance to moisture damage.
• A lower slope ratio usually indicates less acceleration in rutting after stripping begins.
• If no second slope is evident, the mix may not have shown stripping during the test.
• If the SIP is 15,000 wheel passes or less, did not pass the 4 mm rut depth in Step 2, and did not pass the slope ratio (SR) of less than 2.0 in Steps 3 through 5, the mixture is unacceptable.

EXAMPLE HWT Report:

Rut depth at 20,000 Passes = 16.3 mm

Slope Ratio = M2/M1 = -0.0014893 / -0.00015636 = 9.52

SIP = = (b2 - b1) ÷ (M₁ - M₂) =

b2 = ((-0.0014893*-17849))+(-13.5956) = 12.9869157
b1 = ((0.00015636*-3067))+(-1.8183) = -1.33874388

SIP = ((12.9869157 – (-1.33874388)) / ((-0.00015636) – (-0.0014893)) = 10,747

With rutting greater than 4 mm, Slope Ratio greater than 2.0, and SIP less than 15,000, this mixture did not pass the HWT stripping criteria and is rutting susceptible.

8. Report the results.

Include the following in the final report:

• Contract ID, Project Number, Mix Identification.
• Confirm HWT Equipment meets AASHTO T 324.
• Confirm Test temperature = 50ºC.
• Confirm Number of cycles completed = 20,000 Passes.
• Report Total Average Rut Depth at 20,000 Passes.
• Establish Rut depth curves.
• Provide equations for Creep Slope Best Fit Line and Stripping Slope Best Fit Line using Linear regression methods in excel or other programs.
• Report the Slope Ratio.
• Report the Stripping Inflection Point.
• Report the parameters that the asphalt mixture meets using the table below. When one parameter passes, the other parameters may be omitted.

Parameter	Stripping Criteria	Pass or Fail?
Total Rut Depth	Less Than or Equal to 4 mm
Slope Ratio (SR)	Less Than 2.0
Stripping Inflection Point (SIP)	Greater Than 15,000 Passes

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