SuperPave Design Method

This method evaluates the components of the asphalt mixture individually (mineral
aggregate and asphalt). and their interaction when they are mixed.

The SUPERPAVE system delivers:

 New specifications for asphalt.

 New specifications for aggregates.
 New methods for designing asphalt mixtures.
 New behavior prediction models.

Method specifications

The SUPERPAVE method is made up of three levels. Because the analysis and design
of a mixture in the SUPERPAVE system is complex, the extent of use of this
methodology (according to SHRP researchers) depends on the level of traffic and the
function of the mixture in the pavement. Table 4 specifies the different levels
considered for the analysis and design of hot asphalt mixes using the SUPERPAVE
methodology.

First Level: requires a volumetric design and involves:

 Selection of the type of asphalt.

 Selection of aggregate properties.
 Preparation of test specimens.
 Selection of asphalt content.

This activity is based on the estimation of the volumetric properties of the mixture: void
content of the mixture (VA), voids in the mineral aggregate (VMA) and voids filled
with asphalt (VFA).
Intermediate Level: part of a volumetric analysis. The established tests are:

 Shear test (SST, SUPERPAVE shear test).

 Indirect tension test (IDT, indirect tensile test).

Advanced Level: Includes all the steps carried out in the previous levels, but additional
IDT and SST tests are carried out, at a wide range of temperatures. A complete mixture
analysis uses confined SST specimens and offers a higher and more reliable level of
prediction of mixture behavior.

Design Algorithm

Study and Selection of Materials

Asphalts: SUPERPAVE specifications focus on simulating, through laboratory tests,

the 3 critical states to which asphalt is exposed during its useful life.

 First state: original asphalt, a state that occurs during transportation, storage
and handling of the asphalt binder.
 Second state: aging produced after the manufacture and placement of the
asphalt mixture.
 Third state: aging of the asphalt mixture when it has remained in service for a
long period.

The measurement of physical properties is carried out by using 4 test equipment, the
purpose of which is detailed in the following table.
Asphalt Grade Selection

The nomenclature that defines the degree of performance of asphalt is PG XX-YY,

where:

 PG: Performance Grade.

 XX: Maximum Temperature (maximum temperature at which the asphalt must
maintain its properties during service).
 YY: Minimum Temperature (minimum temperature at which the asphalt must
maintain its properties during service).

The asphalts defined in the SUPERPAVE method are shown below.

Air Temperatures (TXXair and TYYair): The maximum and minimum air
temperature that must be considered in the design will depend on the required reliability
(z) and the standard deviation of the data (σ). That is to say:

 TXXair = TAverage + z* σ
 TYYair = TAverage - z* σ

A reliability of 50% considers an average summer and winter. On the contrary, higher
reliabilities assume hotter summers and colder winters.

Pavement Temperatures (TXX and TYY): They are calculated from air temperatures
and a coefficient (Lat) given by the geographical location of the area to be paved
(latitude).

 TXX = (TXXaire – 0.00618*Lat² + 0.2289*Lat + 42.2)*(0.9545) – 17.78

 TYY = TYYaire (In Canada: TYY = 0.859*TYYaire + 1.7)

Effect of Traffic: Loading Speed and Accumulated Traffic: The DSR test simulates
the loading of a vehicle at 90 kilometers per hour. At lower speeds, as the loading time
is longer, the stiffness of the mixture decreases. Therefore, it is recommended to
increase (depending on the loading speed) the required grade XX by 1 or 2 levels.

Aggregates

The SHRP program did not develop new tests for the aggregates, however, additional
tests were adopted and the specifications were reformulated, with the objective of
adjusting them within the SUPERPAVE system. This is how two types of aggregate
properties were defined: consensus properties and origin properties.

Consensus Properties : They are associated with the quality of the aggregate to
produce a resistant and durable mixture. The consensus properties (aggregate
characteristics that can be altered in crushing and selection plants) are:
  Angularity of coarse aggregate.
 Angularity of the fine aggregate.
 Flat and elongated particles.
 Clay content (sand equivalent).
 Combined granulometry.

Source Properties: These are those properties associated with the quality of the source
of the aggregate. The properties of origin, which depend on the place where the
aggregates are obtained, are:

 Tenacity or hardness.
 Durability.
 Deleterious materials.

Aggregate Grading: To specify grading, Superpave has modified the Marshall grading
approach. It uses the exponent 0.45 on the granulometry chart to define the allowed
particle size (Fuller graph), using a unique graphic technique to judge the distribution of
cumulative particle sizes of a mixture of aggregates.

The ordinates of the letter are the percentages that pass; The abscissa, on an arithmetic
scale, represents the sieve openings in mm, raised to the 0.45 power. In the example the
4.75mm mesh is graphed as 2.02

Normally these types of graphs do not show a common arithmetic scale, instead, the
scale is a function of mesh size.
An important range of this chart is the maximum density granulometry; corresponds to a
straight line extended from the abscissa of maximum aggregate size and ordinate 100%,
to the origin (0%, 0 mm).

Illustration: Limits for Superpave granulometries

The values of the parameters: Control points and restricted zone, are referenced to
five designations that the Superpave methodology establishes, in which they propose
the most used nominal maximum sizes and the criteria corresponding to the mentioned
parameters.
Asphalt Mixtures

Two key features of the SUPERPAVE design method are:

 Conditioning of the mixture.

  Compaction carried out in the laboratory.

The asphalt mixtures that are used to manufacture briquettes are conditioned for 2 hours
in an oven at the compaction temperature (the mixing and compaction temperatures are
determined the same as in the traditional design method, depending on the viscosity of
the asphalt). .

Compaction in the laboratory is carried out using the Gyratory Compactor or SGC
(SUPERPAVE Gyratory Compactor). This equipment rotates with an inclination angle
of 1.25 degrees and applies a confining pressure of 600 KPa to the mixture.

The selection of the optimal asphalt content depends on two factors, which are the
criteria established for compaction and the number of turns applied. The selection of the
number of turns is a function of the average temperature of the place and the number of
equivalent axles (ESALs) established in the design. The following table shows the
different ranges of values established to select the number of turns.

Testing of asphalt mixtures

 Volumetric requirements of the mixture: The corresponding requirements

are: air voids; mineral aggregate voids and asphalt-filled voids. Air void content
is an important property used as a basis in the selection of asphalt binder
content. Superpave defines mineral aggregate voids (VAM) as the sum of the
 volume of air voids and effective asphalt in a compacted sample. It represents
the voids between the particles of the aggregate. The specified minimum VAM
values for the 4% design air void percentage are a function of the nominal
maximum aggregate size.

The acceptable range of design VFA for 4% air voids is a function of traffic level.

 Powder Proportion: Another mixing requirement is the powder proportion; It

is calculated as the relationship between the percentage by weight of the
aggregate finer than the 0.075 mm sieve and the effective asphalt content in
percentage of the total weight in the mixture, minus the percentage of absorbed
asphalt.
 Moisture susceptibility: The moisture susceptibility test to evaluate a Hot Mix
Asphalt (HMA) to detachment is Standard T 283, "Resistance of compacted
bituminous mixtures to moisture-induced damage."

Testing Stage

Different test mixtures must be prepared (using different mixtures of aggregates) to

which, after compaction, the volumetric parameters are determined (percentage of
asphalt Pb, percentage of voids Va, voids of the mineral aggregate VMA and voids
filled with VFA asphalt).
That is why new volumetric parameters are estimated, using those initially calculated,
for the case in which the void content (VA) was 4%. The formulas to make the
estimates are the following:

 ESTIMATED Pb = INITIAL Pb – 0.4*(4 – INITIAL Va)

 %ESTIMATEDVAM = %INITIALVMAIN + C*(4 – INITIALVAM)
 %EstimatedVFA = 100* [(%ESTIMATEDVFA – 4) / (%ESTIMATEDVFA)]
 C = 0.1 if VaINITIAL < 4%
 C = 0.2 if VaINITIAL > 4%

Definitive Design

Once the design aggregate structure has been selected, the following specimens must be
prepared:

 2 with PbESTIMATED.
 2 with PbESTIMATED + 0.5%
 2 with ESTIMATED Pb - 0.5%
 2 with PbESTIMATED + 1.0%
 2 loose specimens with PbESTIMATED (to determine maximum density).

The specimens are prepared and tested in the same manner as in the case of aggregate
structure selection. With the results of the properties of the mixture depending on the
asphalt content (the graphs have been prepared), we proceed as follows:

 Determine Pb with which 4% air voids (VA) are obtained.

 Determine the properties of the mixture at the selected asphalt content.
 Compare mixture properties to SUPERPAVE design criteria (AASHTO MP2-
95).
 The sensitivity of the mixture to humidity was evaluated, analyzing the loss of
adhesion between the asphalt and the aggregate through the ASSHTO T283 test.