How the PG Specification Addresses Pavement Performance

Safety

In the PG specification, safety is accounted for by requiring a minimum

flash point temperature using the Cleveland Open Cup Flash Point test.

This test procedure (AASHTO T 48) indicates the temperature at which an

asphalt binder will instantaneously flash in the presence of an open flame.

The flash point is well below the fire point, or the temperature at which

the asphalt binder will actually burn.

The minimum temperature required for all PG asphalt binders is 230°C

(approximately 450°F). This test is performed on the unaged binder.

Pumping and handling

To ensure that asphalt binders, especially modified asphalts, can be

pumped and handled at the hot-mixing facility, the PG specification

contains

a maximum viscosity requirement on the unaged binder as

determined

by AASHTO T 316. This value is 3.0 Pascal-seconds (Pa-s) and

it must be achieved at 135°C for all grades. The specifying agency may

waive this requirement if the supplier warrants that the binder can be

handled and pumped at the necessary temperatures.

While the limiting viscosity of 3.0 Pa-s is not a problem for most PG asphalt

binders, it may be exceeded for some highly modified asphalt binders. Users

requesting an asphalt binder having a useful temperature interval, UTI,

(temperature range from high-temperature grade to low-temperature grade)

of greater than 100 degrees should expect the increased likelihood that the

supplied asphalt binder will exceed the rotational viscosity limit of 3.0 Pa-s

(3000 centipoise) at 135°C. Exceeding the limit in these instances should not

be considered a material property failure since this is simply a result of the

high modification level needed to achieve the required temperature spread.

The user agency, hot mix asphalt contractor, and asphalt binder supplier

should work together to ensure that the required grade is appropriate for the

project and that the supplied asphalt binder can be properly handled by the

hot mix asphalt contractor to produce a mixture having the desired in-place

properties.

Permanent deformation

As discussed in the section describing the DSR, the total response of an

asphalt binder to load consists of elastic (recoverable) and viscous (nonrecoverable)

components. Pavement rutting, or permanent deformation, is

the accumulation of nonrecoverable deformation in the asphalt mixture in

response to repeated load applications at high temperatures (Figure 4.73).

To address the response of the asphalt binder to these repeated loads, the

PG specification defines and places requirements on a rutting factor, G*/

sin  (read as “G star over sine delta.”), which represents a measure of the

high-temperature stiffness of the asphalt binder. This factor is determined

by dividing the complex shear modulus (G*) by the sine of the phase angle

(), as determined using the dynamic shear rheometer (DSR) test procedure

(AASHTO T 315). To minimize the contribution of the asphalt binder to rutting,

the value of G*/sin  must be a minimum of 1.00 kiloPascal for the original

asphalt binder and 2.20 kiloPascal for the short-term-aged asphalt binder

(after using the RTFO procedure) at the appropriate grade temperature.

A minimum RTFO G*/sin  value ensures that the asphalt binder will have

sufficient stiffness after construction to minimize the contribution of the asphalt

binder to permanent deformation in the asphalt mixture. High values of G* and low values of  are considered desirable attributes from the standpoint

of rutting resistance. Thus, the PG specification promotes the use of stiff,

elastic binders (unaged and RTFO-aged) to address permanent deformation.

Excessive aging

As with previous grading systems, a mass-loss requirement is specified

to help prevent the use of an asphalt binder that would age excessively

from volatilization during hot mixing and construction. The mass-loss

requirement is calculated using the RTFO procedure (AASHTO T 240).

The mass loss for any PG asphalt binder should not exceed 1.00 percent.

Long-term stiffness

Like permanent deformation, G* and  are also used in the PG specification

to characterize asphalt binder stiffness after long-term aging, when

subjected to cumulative loading at moderate pavement temperatures. Excessive

stiffness at intermediate temperatures could be a contributing factor in

cracking (durability) of an asphalt pavement (Figure 4.74). Because cracking

generally occurs at lower to moderate pavement temperatures after the

pavement has been in service for some time, the specification addresses

the response of the asphalt binder after aging by both the RTFO and PAV.

The DSR is again used to generate G* and . However, instead of dividing

the two parameters, the two are multiplied to produce a factor that

may be related to the long term oxidation and aging properties of an

asphalt binder at intermediate temperatures. The factor, G*sin , (read as

“G star sine delta”), is the product of the complex shear modulus, G*, and

the sine of the phase angle, . The PG specification requires that all PG

asphalt binders have a maximum value of 5000 kiloPascal for G*sin  at

the appropriate grade temperature.

The maximum value of 5000 kiloPascal was considered to be an acceptable

specification limit when the PG specification was first adopted,

based on the observed fatigue cracking performance of asphalt pavement

sections used in the Zaca-Wigmore test sections. Low values of G*sin 

indicate low energy dissipation. Thus, a maximum allowable value was

considered for the specification. Subsequent research has not validated

that the G*sin  parameter is related to fatigue performance. Rather, the

parameter is considered to provide an indication of stiffness at intermediate

temperatures as a result of long-term aging.

Thermal cracking

When temperature decreases, asphalt pavements contract, causing

thermal stress to develop in the pavement. As the temperature drops,

the asphalt binder contracts to a much greater degree than the aggregate

in an asphalt pavement. When these stresses exceed the tensile strength

of the asphalt mixture, a low-temperature crack develops (Figures 4.75

and 4.76). Asphalt becomes more brittle with time, which is why thermal

cracking occurs at low pavement temperatures after the pavement has

been in service for a time. Because of this, the specification addresses

the response of the asphalt binder after aging in both the RTFO and PAV.

In the PG specification, the main way of examining the propensity of

an asphalt binder to develop thermal stresses at a specified low temperature

is to use data generated from the BBR. If the stiffness is too high, the

asphalt binder will behave in a brittle manner, indicating that cracking is

more likely to occur. To minimize the contribution of the asphalt binder

to thermal cracking, the creep stiffness (S) after 60 seconds of loading

at the appropriate temperature must not exceed 300 MPa. In addition

to stiffness, the rate at which the asphalt binder stiffness changes with

time at low temperatures is regulated through the m-value. In the PG

specification, a higher m-value is an indication that the asphalt binder

may not increase in stiffness as rapidly when the temperature decreases

and contraction occurs, leading to smaller tensile stresses in the asphalt

binder and less chance for low temperature cracking. A minimum m-value

of 0.300 after 60 seconds of loading at the appropriate temperature is

required by the PG specification.

Past studies have also indicated that if a binder can stretch at least

1.0 percent of its original length during this thermal contraction period,

cracks are less likely to occur. As a result, the direct tension test (DTT)

is included in the PG specification as an alternative requirement to using

creep stiffness and m-value. In the specification, the direct tension test

requirement only applies to asphalt binders that have a creep stiffness

greater than 300 MPa but less than 600 MPa, with an m-value of 0.300

or greater. If the creep stiffness at the specified temperature is 300 MPa

or less, then the direct tension test is not required. If the direct tension

test is used, the failure strain must be a minimum of 1.0 percent at the

appropriate

temperature.

Although stiffness can also be used to estimate failure or strength properties,

for some asphalt binders, especially modified asphalts, the relationship

between stiffness and strength properties is not well known.

This is why some researchers believe that the alternate procedure for

determining a critical low cracking temperature specified in Table 2 of the

AASHTO M 320 specification represents a more rigorous approach than

the use of BBR stiffness and m-value alone (as is commonly used in the

PG specification).