Tests that predict pavement performance show the benefits of PMAs made with Elvaloy® RET

Keeping asphalt pavements from failing prematurely is an increasingly sophisticated material science. Testing labs routinely perform sophisticated testing of the dynamic viscoelastic properties of asphalt binders, including SHRP-specified Rolling Thin Film Oven (RTFO) and dynamic rheometry tests. The goal of the testing is to predict and prevent the four main ways that asphalt pavements fail:

• Cold cracking
• Fatigue
• Rutting
• Stripping

These failure mechanisms, and the ways that modified asphalts improve performance, are summarized below.

Cold-cracking

Cold-cracking is a well-known problem in areas subject to extreme thermal cycling. For every mile of roadway, asphalt pavement shrinks by about 1 ft. for each 10-degree drop in temperature. The result is typically seen as cracks running perpendicular to the road centerline. Softer asphalts can help combat the cold cracking problem, but these tend to rut. Mixtures of hard and soft asphalts often are employed, and modifiers can added to improve the viscoeleastic properties even further.

One of the principal reasons for considering a polymer-modified asphalt (PMA) is to enable the use of paving material that can resist cold cracking and yet still remain resistant to rutting.

Fatigue resistance

There's a limit on how many times pavement can flex before it fatigues and breaks. Fortunately, a well designed flexible pavement -- and especially a polymer-modified system -- can incur a huge number of flexes, thus yielding a long service life even under today's heavy traffic loads. But it's often useful to double-check this, using actual mixes being used on a project.

In the lab, one can simulate the traffic stresses on pavement by fixturing a sample beam of test material and subjecting the beam to cyclical loading comparable to that of a passing wheel, and seeing how many load cycles can be withstood before the beam breaks.

Test result: Elvaloy® RET outlives the rest

Fatigue life tests have been conducted on beams of representative dense grade (4% voids) asphalt mixes including unmodified, EVA-modified, and SBS-modified grades, and hard and soft grades modified with Elvaloy® RET. The beams were subjected to various amounts of deflection and flexed until the beam broke.

At a strain of 400 microinches (about what a truck will exert on a well-designed road), a beam containing unmodified asphalt failed at about 10,000 cycles.

EVA-modified and SBS-modified grades, which performed almost identically, failed at 100,000 cycles.

The beams made with hard and soft asphalts modified with Elvaloy® RET failed at 1,000,000 and at 10,000,000 cycles, respectively, as shown in the test results.

Load-related stress failure can also been seen in data generated at the University of Maryland by Mat Witczak, whose paper is presented elsewhere at this Web site. This data results from the imposition of a load on dense graded samples produced with a Conoco asphalt containing 0, 1.5 and 2.0% Elvaloy® RET. The testing was performed at 100 degrees F at three different loads.

One can see that at the highest load (30 psi ) that the samples failed compressive shear in about 200 cycles when the asphalt was not modified with Elvaloy® RET. If, however, the same sample was modified with 2% Elvaloy® RET, no sign of compressive shear failure was noted even after 10,000 cycles, at which point the test was stopped.

There's a photo (14 KBytes) showing test cylinders after the compressive shear tests.

Rutting resistance

One of the more interesting developments in recent years is the use of tests that simulate the dynamics of heavy traffic loads by passing laboratory-scale wheels repeatedly over a pavement surface. So-called "French wheel" and "Hamburg wheel" tests -- presumably named after places of origin -- are increasingly being used by Departments of Transportation (DOTs) to improve predictability of pavement rutting resistance.

One state's DOT recently shared with DuPont scientists results of Hamburg wheel test results, comparing two PG 64-28 grade asphalts. One grade was modified by air blowing; the other by adding Elvaloy® RET. Both passed SHRP binder tests for their grade rating; however, the grade modified by adding Elvaloy® RET clearly resisted rutting much better. The plot looked like this.

In this test, rut depth of 20 mm occurred with the air-blown product after 10,000 cycles. In contrast, after twice as many cycles, ruts were only half as deep (10 mm) in the product made with Elvaloy® RET.

Stripping resistance

The modified Lottman test (AASHTO 283-89) was run on a collection of samples to determine the effect of Elvaloy® RET on resistance to stripping. The results indicate that Elvaloy® RET works as an antistrip agent and at the same time improves the indirect tensile strength of the mixes.

The moisture damage tests were run at the University of Maryland using Conoco asphalt from Billings, Montana, at different levels of modification using Elvaloy® RET.

The aggregate used was limestone, which shows much better resistance to stripping than the Watsonville granite used in similar tests. The results show a 2 to 3 times improvement in tensile properties compared to the wet, non-modified sample.

Using Watsonville granite, split wet-dry tensile strengths were determined using the modified Lottman test. The data shows that the tensile strength of the Elvaloy® RET modified system was much higher in the dry samples. In addition, the Elvaloy® RET modified samples, when exposed to moisture, retained much more of their tensile strength compared with the SBS modified sample.

The wet-dry resilient modulus was also measured using the modified Lottman test. Again, we see the advantage of Elvaloy® RET versus SBS; i.e., much higher resilient modulus on dry samples, and much better retention of the resilience after exposure to moisture.

To test moisture damage with and without Elvaloy®, laboratory freeze-thaw tests were conducted by DuPont. A photograph demonstrates the results.