Pressure Aging Vessel (PAV): Long-Term Binder Aging Test

A pressure aging vessel, or PAV, is how a lab fast-forwards asphalt binder through years of sun and air in less than a day. Binder that has already been through the rolling thin film oven is spread thin in steel pans, sealed in a hot chamber and pressurized with air for 20 hours. What comes out is stiffer and more brittle, much like binder in a pavement that has been in service for several years. That aged residue is what the PG system uses to judge fatigue and low-temperature cracking. The governing practice is AASHTO R 28, with ASTM D6521 as the matching standard.

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Quick facts

Standards: AASHTO R 28, ASTM D6521.
Pressure: 2.1 MPa (about 305 psi) of clean, dry air.
Time: 20 hours.
Temperature: 90, 100 or 110 degrees C, picked for the climate.
Simulates: aging equal to roughly 5 to 10 years on the road, per FHWA's lab manual.
Origin: adopted from the Strategic Highway Research Program (SHRP) in the early 1990s.

Why binder needs two aging steps

Binder ages in two phases. The first is short and violent: it is heated and mixed with hot stone at the plant, hauled and laid. The rolling thin film oven recreates that. The second is slow: years of oxygen and heat in the finished mat. That long-term hardening is what makes old pavements crack, so the PG system tests cracking resistance on binder that has been through both steps. The PAV is step two.

Equipment

Pressure vessel: a heated stainless chamber with a rack that holds at least 10 pans, with temperature logged throughout the run.
Pans: shallow stainless steel pans; the standard charge is 50 g of binder per pan, giving a thin film.
Air supply: regulated bottled air to pressurize the vessel.
Vacuum degassing oven: to pull trapped air bubbles out of the aged residue.
Balance and calibration gear: the temperature and pressure sensors are checked on a regular schedule.

How a PAV run works

Start with RTFO residue. Pour the short-term aged binder into the pans while it is fluid.
Preheat the vessel to the selected aging temperature, then load the rack of pans.
Pressurize. Once the vessel is back near temperature, bring it up to 2.1 MPa with air.
Age for 20 hours, holding temperature and pressure within tight limits.
Release slowly. Bleed the pressure off gradually so the binder does not foam.
Degas and store. Scrape and combine the residue, vacuum-degas it if required, and store it for the next tests.

Expect about a day from pouring to finished residue. The PAV itself produces no pass or fail result; it only conditions the material.

Choosing the aging temperature

PAV temperature	Typical use
90 degrees C	Binders for cooler climates (lower high-temperature grades)
100 degrees C	The common choice for most moderate-climate grades
110 degrees C	Hot desert climates

The researchers who developed the method picked these temperatures for practical reasons: an early version at lower heat took days and did not age the binder enough. The grade specification tells the lab which one to use.

Tests that follow the PAV

Dynamic shear rheometer at an intermediate temperature, to check fatigue cracking resistance.
Bending beam rheometer at low temperature, which sets the second number of a PG grade. Our explainer on the low-temperature number shows how.
Direct tension, when a spec calls for it; this one requires degassed residue.

Many labs also run extra measurements on PAV residue, such as Delta Tc, and some use a longer 40-hour PAV when they want a harsher aging check, for example on mixes with a lot of recycled binder.

What it means for a paving customer

If your quote says PG 64-22 or PG 58-28, part of that label was earned in a pressure aging vessel. You do not need to test it yourself. The decisions that are yours are thickness, base and drainage: use the driveway asphalt calculator, the asphalt base calculator and the crushed stone calculator.

Questions people ask

What does a pressure aging vessel do?

It artificially ages asphalt binder so labs can test what the binder will be like after years on the road. Thin films of binder sit in a heated, pressurized chamber for 20 hours, and the hardened residue is then tested for cracking resistance.

What are the PAV conditions?

AASHTO R 28 uses 2.1 MPa of air pressure, about 305 psi, for 20 hours, at an aging temperature of 90, 100 or 110 degrees C. The temperature is chosen to suit the climate the binder is graded for.

How many years of aging does PAV represent?

FHWA's lab manual describes PAV as simulating roughly 5 to 10 years of service aging. It is an approximation; real aging depends on climate, air voids and the mix.

Is the binder aged in the RTFO first?

Yes. The normal sequence is rolling thin film oven aging to mimic the plant and paver, then PAV aging of that residue to mimic service life. Testing on PAV residue usually comes only after both steps.

Which tests use PAV residue?

In the PG system: the dynamic shear rheometer at intermediate temperature for fatigue, the bending beam rheometer at low temperature, and the direct tension test when it is used. Degassing after PAV is required before direct tension and optional for the other two.

What is a 40-hour PAV?

Some agencies and researchers double the aging time to 40 hours to represent more severe or longer aging, especially when judging mixes with recycled binder. It is an add-on to the standard 20-hour practice, not a replacement.

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