Hamburg Wheel Tracking (HWT) Test
Gary L. Fitts, P.E. Sr. Field Engineer Asphalt Institute
Topics
Background of HWT Data trends (Texas) Specifications
Special thanks to Dale Rand and TxDOT
Background
Developed in Germany, used since the mid-1970s
Spurbildungsgerat, auf Deutsch
A loaded steel wheel tracks over the samples in a heated water bath, the deformation is observed vs. the number of loading passes Has mostly been used in North America as an mixture evaluation tool
Hamburg Wheel Tracking Test
TxDOT uses specimens molded in the SGC or 6 cores cut from the pavement
10 cores can also be used
~50 cycles per minute, < 7 hrs for 20,000 passes
Courtesy, Koch Pavement Solutions
Specimen Configuration, TEX-242-F
Lab-compacted specimens molded to 93% Gmm 1% Circular specimens sawed as shown in the figure
Top View of Test Specimen Configuration for the Hamburg Wheel-tracking Device.
Courtesy, TxDOT
HWT
Precision Machine & Welding
HWT Specimens, After Testing
HWT Indices
Courtesy, Koch Pavement Solutions
Colorado DOT Test Criteria: CPL 5112
Test temperature based on PG high temperature grade PG 5240C PG 5845C PG 6450C PG 7055C Data reported include: Number of passes Maximum impression Test temperature Sample(s) air voids Creep slope Stripping slope Stripping inflection point
TxDOT 2004 Specifications
No longer use the Modified Lottman (TEX-531C, AASHTO T283)
TxDOT still uses the boil test (TEX-520-C)
All HMA items, except Items 292 (Plant mix, asphalt-treated base) and 342 (Permeable Friction Course) include HWT criteria Only the maximum rut depth (0.5 inches) is specified, the number of passes depends on the PG grade
Not using the other Hamburg parameters for mix design acceptance
HWT-TxDOT Materials & Tests Laboratory, Cedar Park
TxDOT has seven HWTs Four in their central laboratory (Cedar Park)
Three in MAT labs around the state Three approved commercial labs
Testing performed at 50C, for prescribed number of passes or until failure (1/2 inch deformation)
TxDOT HWT Requirements Mixture Design & Production
Hamburg Wheel Test Requirements High-Temperature Binder Grade Test Method Minimum # of Passes @ 0.5" Rut Depth, Tested @122F 4
PG 64 or lower PG 70 PG 76 or higher
Tex-242-F
10,000 15,000 20,000
4. May be decreased or waived when shown on the plans. Test not required for RBL. Note: TxDOT also includes a maximum indirect tensile strength requirement to avoid brittle mixtures
HWT Testing Requirements
Mixture design (for approval) Trial batch (Lot 1) Minimum on 1 HWT during the project Engineer may test as desired
Usually not done unless there are other indications of potential problems
During production, failing test results result in suspension of operations and may require removal and replacement of material
Passing Result
Dense-graded mixture PG 76-22 14% RAP
Contrasting results
Failed @10,200 passes PG 76-22, Limestone
Passed, 2.8 mm @ 20,000 passes Same aggregate, different binder source
General Trends
HWT does a better job of identifying mixtures that are susceptible to premature failure than other laboratory tests (Lottman, Hveem stabilometer, Creep, etc) Stiffer asphalt binders (higher high temperature grades) do better Adding liquid antistrip or lime usually improves the HWT results. Improvement with lime is usually more dramatic than with liquid. Harder aggregates do better (igneous -vs- limestone) Stone on stone mixes do better than dense mixes There are no absolutes!Do not assume! Measure! (D. Rand)
Trends in the data
There are exceptions!!!
Caution!!!
HWT favors stiff asphalt binders and mixtures
Good for thick pavements, bad for thin, deflecting pavement structures
HWT does not identify mixtures that are susceptible to cracking
Other uses
Forensic tool
In combination with distress survey, other sampling and testing, NDT Not enough data (yet) to identify a test result limit that suggests failure
Pavement evaluation tool
Test samples from existing pavement before deciding on rehabilitation or resurfacing strategy
Particularly where there is evidence of moisture trapped within the pavement, raveling, or a history of moisture damage with existing materials
Moisture Damage
Note lateral deformation Ruts > 1 in. deep Moisture damage in underlying material This can be avoided!!!
In summary
HWT is used by TxDOT in lieu of stability and Lottman tests for design and project control Other agencies use HWT as a forensic tool HWT or other loaded wheel tests can and should be used for verifying designs and for evaluating existing materials
Thanks!
