Hamburg Wheel Tracking Test: Procedure and Pass Criteria

The Hamburg wheel tracking test is a lab test that runs a loaded steel wheel back and forth over compacted asphalt samples sitting in hot water, thousands of times, while a sensor tracks how deep a rut forms. It answers two questions at once: will this mix push into ruts under heavy trucks in summer heat, and will water strip the binder off the stone? Highway agencies such as Texas, Missouri and Iowa use it when they approve mix designs. Spotted “HWTT,” “Hamburg” or “SIP” on a mix design sheet? Below we cover the machine, the procedure, how to read the results and the pass limits agencies use.

The short answer

What it does: an 8-inch steel wheel carrying about 158 pounds rolls over two asphalt samples submerged in water, usually held at 122 °F (50 °C), for up to 20,000 passes.
What it measures: rut depth in millimeters against the number of wheel passes. The shape of that curve shows both rutting and moisture damage (stripping).
Typical pass limits: they vary by agency. Texas DOT, for example, allows no more than 12.5 mm of rutting at 10,000, 15,000 or 20,000 passes depending on the binder grade, while Missouri DOT looks for 4 mm or less at 20,000 passes, or a passing slope ratio or stripping inflection point.
Standard methods: AASHTO T 324 is the national method; agencies such as Texas (Tex-242-F) and Iowa (IM 319) publish their own versions or changes.

Where the test came from

According to a Texas Transportation Institute research report for TxDOT, the City of Hamburg, Germany, built the first version of the device in the 1970s, borrowing the idea from a British machine that used a rubber tire. A Hamburg company, Helmut-Wind Inc., later settled the test method and wrote pass/fail requirements for rutting and stripping. The original German test called for 9,450 wheel passes at 40 or 50 °C. When researchers ran samples longer, to about 19,200 passes, some mixes fell apart from moisture damage soon after the 10,000 mark, which is part of why longer runs became the norm. The same report notes that TxDOT adopted the test and set its own rut-depth limits.

The machine and the samples

Texas DOT’s current Tex-242-F procedure (effective July 2026) spells out the hardware in detail:

Wheel: steel, 8 inches (203 mm) across and 1.85 inches (47 mm) wide, carrying a 158 ± 5 lb (705 N) load.
Speed: 52 ± 2 passes per minute, moving back and forth in a smooth, sinusoidal motion and peaking near 1 foot per second at the middle of the samples.
Water bath: able to hold any temperature from 77 to 158 °F (25 to 70 °C) within ± 4 °F, with circulating water so the tank stays even.
Rut sensor: an LVDT that reads to 0.01 mm over at least 20 mm of travel, taking readings at 11 points along the wheel path at least every 100 passes, without stopping the wheel.

Lab-made samples are two gyratory-compacted cylinders 150 mm (5.9 in.) across and 62 mm (2.4 in.) tall. TxDOT wants them compacted to 93 ± 0.5 percent density, meaning about 7 percent air voids, close to what a new road has after rolling. Each cylinder is sawn along one edge so the two can sit snugly side by side in plastic molds, forming one continuous track for the wheel. Road cores can be tested too: TxDOT asks for 145 ± 5 mm diameter cores trimmed or capped to the same 62 mm height, with no density target. For the math behind density and voids, see air voids in asphalt.

Running the test, step by step

Prepare and rest the samples. TxDOT has lab-molded specimens sit at room temperature for at least 24 hours and tests them within three days of molding.
Mount them. The two cut samples go into molds in a stainless tray bolted into the empty bath, pushed tight against each other so there is no gap where they meet.
Heat and soak. The bath fills to the test temperature, most often 122 °F (50 °C). Once it gets there, TxDOT soaks the samples another 30 minutes. Iowa DOT uses 40 °C for its softer binder grades and 50 °C for the rest, with a 45-minute soak.
Roll. The wheel lowers onto the samples and the machine runs until it hits the set number of passes, commonly 20,000, or a maximum rut depth (Iowa sets 20 mm). Reports from FHWA and TTI put a full 20,000-pass run at roughly 6.5 hours.
Record and inspect. The software logs rut depth through the run. TxDOT records it at every 5,000 passes, notes the pass count at failure and then looks at the samples. Bare, clean stone, lots of fines in the bath or muddy water are signs the mix is prone to stripping.

How to read the results

The output is a curve of rut depth against wheel passes. It usually has up to three parts:

Early settling: in the first part of the run the wheel packs the sample a little tighter, so the rut can deepen faster at the start before it settles into a steady trend.
Creep slope: a long, steady stretch where the rut grows at a slow, nearly constant rate as the mix flows under load.
Stripping slope: if water starts breaking the bond between binder and stone, the curve bends and the rut grows much faster.

The stripping inflection point (SIP) is the pass count where straight lines fitted to the creep and stripping parts cross. A higher SIP means the mix held out longer before water damage took over. Missouri DOT’s method also uses the slope ratio, the stripping slope divided by the creep slope; a ratio under 2.0 means there is no real stripping bend, so the mix passes on that count. Iowa DOT applies the same 2.0 cutoff and treats the mix as passing when the ratio is lower. A mix can also have no visible second slope at all, which simply means it showed no stripping in the test.

Two different problems show up in the same test. A deep rut with a steady slope points to a soft or poorly built aggregate skeleton (rutting). A sharp late bend points to moisture damage, which our guide to asphalt stripping covers in detail.

Typical pass/fail criteria

There is no single national limit. Each agency writes its own, and the results only mean something when compared with the spec on your project. A few published examples:

Texas DOT (dense-graded mixes): rut depth must stay under 12.5 mm at 50 °C for at least 10,000 passes with PG 64 or lower binder, 15,000 passes with PG 70 and 20,000 passes with PG 76 or higher, according to NCAT’s Texas case study and the TTI report. Stiffer binder grades have to last longer.
Missouri DOT: tests every mix at 50 °C for 20,000 passes. It passes if the average rut is 4 mm or less, or the slope ratio is under 2.0, or the SIP is above 15,000 passes. It fails only if it misses all three.
FHWA research (2000): a Turner-Fairbank study described the device’s customary pass/fail line as 10 mm of rutting at 20,000 passes.

The binder grade matters because a higher first number in a PG grade means a binder that stays stiffer when the road is hot. See what PG 76-22 means and what PG 64-22 means on a quote.

What happens when a mix fails

A failing Hamburg result sends the mix design back for changes before it can be used. Common fixes include a different stone blend or gradation, a stiffer binder grade or an anti-strip additive. Iowa DOT’s procedure, for example, has the contractor test at least three dosages of an anti-strip additive to find a rate that brings the SIP up to the state’s minimum. The approved recipe then is written into the job mix formula that the plant follows.

Limits of the test

It is empirical. The FHWA study found the results highly variable for some mixes and advised against ranking mixes whose results were statistically alike.
The stone can crush. In the same study the wheel crushed a limestone aggregate, so all three of those mixes failed quickly whatever binder they had. That said more about the test than about how the road would perform.
One temperature. The usual 50 °C was set in Europe for a climate the FHWA report compares to a Superpave high-temperature grade of 58. It says nothing about cold-weather cracking.
It sits alongside other tests. Hamburg is usually one piece of a mix design, next to the volumetric checks in Superpave or the Marshall stability test. Our overview of asphalt testing shows how the lab and field tests fit together.

Why it matters for a paving job and your tonnage

On a home driveway or a small lot you will rarely see a Hamburg report, but the mix you buy may come from a design that had to pass one, especially if the plant also supplies state work. It is a fair question to ask a supplier whether their surface mix is an agency-approved design. If ruts are already showing up in your own pavement, our guide to fixing ruts in an asphalt driveway walks through the repair.

The test does not change how many tons you need, but the same lab work fixes the mix’s compacted density, which drives tonnage. A common planning number is roughly 110 lb for each square yard at 1 inch thick; ask for the design density if you want a tighter number, then run your area and depth through the asphalt tonnage calculator.

Questions people ask

What does the Hamburg wheel tracking test measure?

It measures how deep a rut a loaded steel wheel makes in compacted asphalt samples kept under hot water, tracked against the number of wheel passes. The curve shows both rutting resistance and moisture damage, also called stripping.

What temperature is the Hamburg test run at?

Most often 122 degrees F (50 degrees C). Texas DOT allows up to 158 degrees F (70 degrees C) when the plans call for it, and Iowa DOT uses 40 degrees C for some softer binder grades.

How many passes are in a Hamburg test?

Commonly up to 20,000 passes, or until the rut reaches a set maximum depth. A full 20,000-pass run takes roughly 6.5 hours, according to FHWA and Texas Transportation Institute reports.

What is the stripping inflection point?

It is the number of wheel passes where lines fitted to the slow, steady creep part of the curve and the steeper stripping part cross. A higher stripping inflection point means the mix resisted water damage longer.

What is a passing Hamburg result?

It depends on the agency. Texas DOT allows at most 12.5 mm of rutting at 10,000, 15,000 or 20,000 passes depending on binder grade. Missouri DOT passes a mix with 4 mm or less at 20,000 passes, a slope ratio under 2.0 or a stripping inflection point above 15,000 passes.

What standard covers the Hamburg wheel tracking test?

AASHTO T 324 is the national method. Some states publish their own versions or changes, such as Texas DOT Tex-242-F and Iowa DOT IM 319.

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