This section consists of a brief history of the Superpave mix design method followed by a general outline of the actual method. This outline emphasizes general concepts and rationale over specific procedures. Typical procedures are available in the following documents:

• Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996^[1]). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.
• Asphalt Institute. (2001^[2]). Superpave Mix Design. Superpave Series No. 2 (SP-02). Asphalt Institute. Lexington, KY.
• American Association of State Highway and Transportation Officials (AASHTO). (2000^[3] and 2001^[4]). AASHTO Provisional Standards. American Association of State Highway and Transportation Officials. Washington, D.C.

Superpave History

Under the Strategic Highway Research Program (SHRP), an initiative was undertaken to improve materials selection and mixture design by developing:

1. A new mix design method that accounts for traffic loading and environmental conditions.
2. A new method of asphalt binder evaluation.
3. New methods of mixture analysis.

When SHRP was completed in 1993 it introduced these three developments and called them the Superior Performing Asphalt Pavement System (Superpave). Although the new methods of mixture performance testing have not yet been established, the mix design method is well-established.

Superpave Procedure

The Superpave mix design method consists of 7 basic steps:

1. Aggregate selection.
2. Asphalt binder selection.
3. Sample preparation (including compaction).
4. Performance Tests.
5. Density and voids calculations.
6. Optimum asphalt binder content selection.
7. Moisture susceptibility evaluation.

Aggregate Selection

Superpave specifies aggregate in two ways.  First, it places restrictions on aggregate gradation by means of broad control points.  Second, it places “consensus requirements” on coarse and fine aggregate angularity, flat and elongated particles, and clay content.  Other aggregate criteria, which the Asphalt Institute (2001^[2]) calls “source properties” (because they are considered to be source specific) such as L.A. abrasion, soundness and water absorption are used in Superpave but since they were not modified by Superpave they are not discussed here.

Gradation and Size

Aggregate gradation influences such key HMA parameters as (read about these parameters here) stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance and resistance to moisture damage (Roberts et al., 1996^[1]).  Additionally, the maximum aggregate size can be influential in compaction and lift thickness determination.

Gradation Specifications

Superpave mix design specifies aggregate gradation control points, through which aggregate gradations must pass.  These control points are very general and are a starting point for a job mix formula.

Aggregate Blending

It is rare to obtain a desired aggregate gradation from a single aggregate stockpile.  Therefore, Superpave mix designs usually draw upon several different aggregate stockpiles and blend them together in a ratio that will produce an acceptable final blended gradation.  It is quite common to find a Superpave mix design that uses 3 or 4 different aggregate stockpiles (Figure 1).

Figure 1. Screen shot from HMA View showing a typical aggregate blend from 4 stockpiles.

Figure 2. Gyratory compactor.

Figure 3. Superpave gyratory compactor sample (left) vs. Hveem/Marshall compactor sample (right).

The Superpave gyratory compactor establishes three different gyration numbers:

1. N_initial. The number of gyrations used as a measure of mixture compactability during construction. Mixes that compact too quickly (air voids at N_initial are too low) may be tender during construction and unstable when subjected to traffic. Often, this is a good indication of aggregate quality – HMA with excess natural sand will frequently fail the N_initial requirement. A mixture designed for greater than or equal to 3 million ESALs with 4 percent air voids at N_design should have at least 11 percent air voids at N_initial.
2. N_design. This is the design number of gyrations required to produce a sample with the same density as that expected in the field after the indicated amount of traffic. A mix with 4 percent air voids at N_design is desired in mix design.
3. N_max. The number of gyrations required to produce a laboratory density that should never be exceeded in the field. If the air voids at N_max are too low, then the field mixture may compact too much under traffic resulting in excessively low air voids and potential rutting. The air void content at N_max should never be below 2 percent air voids.

Typically, samples are compacted to N_design to establish the optimum asphalt binder content and then additional samples are compacted to N_max as a check. Previously, samples were compacted to N_max and then N_initial and N_design were back calculated. Table 6 lists the specified number of gyrations for N_initial, N_design and N_max while Table 7 shows the required densities as a percentage of theoretical maximum density (TMD) for N_initial, N_design and N_max. Note that traffic loading numbers are based on the anticipated traffic level on the design lane over a 20-year period regardless of actual roadway design life (AASHTO, 2001^[4]).

The standard gyratory compactor sample preparation procedure is:

• AASHTO TP4: Preparing and Determining the Density of Hot-Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor

Performance Tests

The original intent of the Superpave mix design method was to subject the various trial mix designs to a battery of performance tests akin to what the Hveem method does with the stabilometer and cohesiometer, or the Marshall method does with the stability and flow test.  Currently, these performance tests, which constitute the mixture analysis portion of Superpave, are still under development and review and have not yet been implemented.  The most likely performance test, called the Simple Performance Test (SPT) is a Confined Dynamic Modulus Test.

Density and Voids Analysis

All mix design methods use density and voids to determine basic HMA physical characteristics. Two different measures of densities are typically taken:

1. Bulk specific gravity (G_mb).
2. Theoretical maximum specific gravity (TMD, G_mm).

These densities are then used to calculate the volumetric parameters of the HMA. Measured void expressions are usually:

• Air voids (V_a), sometimes expressed as voids in the total mix (VTM)
• Voids in the mineral aggregate (VMA)
• Voids filled with asphalt (VFA)

Generally, these values must meet local or State criteria.

VMA and VFA must meet the values specified in Table 8.  Note that traffic loading numbers are based on the anticipated traffic level on the design lane over a 20-year period regardless of actual roadway design life (AASHTO, 2000b^[5]).

Selection of Optimum Asphalt Binder Content

The optimum asphalt binder content is selected as that asphalt binder content that results in 4 percent air voids at N_design.  This asphalt content then must meet several other requirements:

1. Air voids at N_initial > 11 percent (for design ESALs ≥ 3 million).  See Table 5 for specifics.
2. Air voids at N_max > 2 percent.  See Table 5 for specifics.
3. VMA above the minimum listed in Table 2.
4. VFA within the range listed in Table 2.

If requirements 1,2 or 3 are not met the mixture needs to be redesigned.  If requirement 4 is not met but close, then asphalt binder content can be slightly adjusted such that the air void content remains near 4 percent but VFA is within limits.  This is because VFA is a somewhat redundant term since it is a function of air voids and VMA (Roberts et al., 1996^[1]).  The process is illustrated in Figure 4 (numbers are chosen based on 20-year traffic loading of ≥ 3 million ESALs).

Figure 4. Selection of optimum asphalt binder content example: 4 basic steps.

Moisture Susceptibility Evaluation

Moisture susceptibility testing is the only performance testing incorporated in the Superpave mix design procedure as of early 2002.  The modified Lottman test is used for this purpose.

The typical moisture susceptibility test is:

• AASHTO T 283: Resistance of Compacted Bituminous Mixture to Moisture-Induced Damage.

Surveys

Superpave Dust to Binder Ratio Survey

Questions

AASHTO M 323 (Superpave Volumetric Mix Design) allows an agency to modify the required dust to binder ratio from 0.6-1.2 to 0.8-1.6 if the aggregate gradation passes beneath the PCS Control Point. Does your agency allow (or require) a dust to binder ratio of 0.8 – 1.6? If so, when is this allowed/required?

Results

Superpave Dust to Binder Ratio Survey by NJDOT

In general, weight and volume terms are abbreviated as, G_xy,

For example, G_mm = gravity, mixture, maximum = the maximum gravity of the mixture.  Other common abbreviations are:

HMA Constituents

In general, HMA is made up of 3 materials: aggregate, asphalt binder and air. Typically, HMA is described by volume so it is important to know how these 3 materials relate to one another volumetrically. The animation in Figure 1 shows this both in a close-up sketch of in-place HMA (left) and a volume diagram (right) that is similar to volume diagrams seen in the geotechnical arena. Further definitions of these terms can be found below.

Specific Gravities

Bulk Specific Gravity of the Compacted Asphalt Mixture (G_mb)

The ratio of the mass in air of a unit volume of a permeable material (including both permeable and impermeable voids normal to the material) at a stated temperature to the mass in air (of equal density) of an equal volume of gas-free distilled water at a stated temperature.  This value is used to determine weight per unit volume of the compacted mixture.  It is very important to measure G_mb as accurately as possible.  Since it is used to convert weight measurements to volumes, any small errors in G_mb will be reflected in significant volume errors, which may go undetected.

The standard bulk specific gravity test is:

• AASHTO T 166: Bulk Specific Gravity of Compacted Bituminous Mixtures Using Saturated Surface-Dry Specimens

Theoretical Maximum Specific Gravity of Bituminous Paving Mixtures (G_mm)

The ratio of the mass of a given volume of voidless (V_a = 0) HMA at a stated temperature (usually 25 °C) to a mass of an equal volume of gas-free distilled water at the same temperature.  It is also called Rice Specific Gravity (after James Rice who developed the test procedure).  Multiplying G_mm by the unit weight of water gives Theoretical Maximum Density (TMD).

The standard TMD test is:

• AASHTO T 209 and ASTM D 2041: Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures

Voids (expressed as percentages)

Air Voids (V_a)

The total volume of the small pockets of air between the coated aggregate particles throughout a compacted paving mixture, expressed as a percent of the bulk volume of the compacted paving mixture.  The amount of air voids in a mixture is extremely important and closely related to stability and durability.   For typical dense-graded mixes with 12.5 mm (0.5 inch) nominal maximum aggregate sizes air voids below about 3 percent result in an unstable mixture while air voids above about 8 percent result in a water-permeable mixture.

Voids in the Mineral Aggregate (VMA)

The volume of intergranular void space between the aggregate particles of a compacted paving mixture that includes the air voids and the effective asphalt content, expressed as a percent of the total volume of the specimen.  When VMA is too low, there is not enough room in the mixture to add sufficient asphalt binder to adequately coat the individual aggregate particles.  Also, mixes with a low VMA are more sensitive to small changes in asphalt binder content.  Excessive VMA will cause an unacceptably low mixture stability (Roberts et al., 1996^[1]).  Generally, a minimum VMA is specified and a maximum VMA may or may not be specified.

Voids Filled with Asphalt (VFA)

The portion of the voids in the mineral aggregate that contain asphalt binder.  This represents the volume of the effective asphalt content.  It can also be described as the percent of the volume of the VMA that is filled with asphalt cement.  VFA is inversely related to air voids: as air voids decrease, the VFA increases.

Other Definitions

Effective Asphalt Content (P_be)

The total asphalt binder content of the HMA less the portion of asphalt binder that is lost by absorption into the aggregate.

Volume of Absorbed Asphalt (V_ba)

The volume of asphalt binder in the HMA that has been absorbed into the pore structure of the aggregate.  It is the volume of the asphalt binder in the HMA that is not accounted for by the effective asphalt content.

Figure 1: California Kneading Compactor

Figure 2: California Kneading Compactor

The basic concepts of the Hveem mix design method were originally developed by Francis Hveem when he was a Resident Engineer for the California Division of Highways in the late 1920s and 1930s.

The Hveem mix design method consists of three basic steps:

1. Aggregate selection. Different agencies/owners specify different methods of aggregate acceptance. Typically, a battery of aggregate physical tests is run periodically on each particular aggregate source. Then, for each mix design, gradation and size requirements are checked. Normally, aggregate from more than one source is required to meet gradation requirements.
2. Asphalt binder selection. This depends on state design catalogs and any special use the pavement may be used for, such as airports.
3. Optimum asphalt binder content determination. In the Hveem method, this step can be broken up into 5 substeps:

• Prepare multiple initial samples, each at a different asphalt binder content. For instance, one sample each might be made at 4.5, 5.0, 5.5, 6.0, 6.5 and 7 percent asphalt by dry weight for a total of six samples.
• Compact these trial mixes in the California Kneading Compactor (see Figures 1 and 2). This compactor is specific to the Hveem mix design method.
• Test the samples for stability and cohesion using the Hveem stabilometer and cohesiometer. These tests are specific to the Hveem mix design method. Passing values of stability and cohesion depend upon the mix class being evaluated. Typically, all samples pass the cohesion test and three or four pass the stability test.
• Determine the density and other volumetric properties of the samples.
• Select the optimum asphalt binder content. The asphalt binder content corresponding to 4 percent air voids is selected as long as this binder content passes stability and cohesion requirements.

When aggregate and asphalt binder are combined to produce a homogenous substance, that substance, HMA, takes on new physical properties that are related to but not identical to the physical properties of its components.  Mechanical laboratory tests can be used to characterize the basic mixture or predict mixture properties.  HMA mix design has evolved as a laboratory procedure that uses several critical tests to make key characterizations of each trial HMA blend.  Although these characterizations are not comprehensive, they can give the mix designer a good understanding of how a particular mix will perform in the field during construction and under subsequent traffic loading.

This section covers mix design fundamentals common to all mix design methods.  First, two basic concepts (mix design as a simulation and weight-volume terms and relationships) are discussed to set a framework for subsequent discussion.  Second, the variables that mix design may manipulate are presented.  Third, the fundamental objectives of mix design are presented.  Finally, a generic mix design procedure (which Hveem, Marshall and Superpave methods all use) is presented.

Concepts

Before discussing any mix design specifics, it is important to understand a couple of basic mix design concepts:

• Mix design is a simulation
• HMA weight-volume terms and relationships

Mix Design is a Simulation

First, and foremost, mix design is a laboratory simulation.  Mix design is meant to simulate actual HMA manufacturing, construction and performance to the extent possible.  Then, from this simulation we can predict (with reasonable certainty) what type of mix design is best for the particular application in question and how it will perform.

Being a simulation, mix design has its limitations.  Specifically, there are substantial differences between laboratory and field conditions.  Certainly, a small laboratory setup consisting of several 100 – 150 mm (4 – 6 inch) samples, a compaction machine and a couple of testing devices cannot fully recreate actual manufacturing, construction and performance conditions.  For instance, mix design compaction should create the same general density (void content) to which the traffic will finally compact a mix in the field under service conditions (Roberts et al., 1996^[1]).  However, it is difficult to calibrate a number of tamper blows (laboratory compaction) to a specific construction compaction and subsequent traffic loading (field compaction).  Currently used correlations between these densities are empirical in nature and extremely rough (e.g., high, medium and low traffic categories).   However, despite limitations such as the preceding, mix design procedures can provide a cost effective and reasonably accurate simulation that is useful in making mix design decisions.

HMA Weight-Volume Terms and Relationships

Mix design, and specifically Superpave mix design, is volumetric in nature.  That is, it seeks to combine aggregate and asphalt on a volume basis (as opposed to a weight basis).  Volume measurements are usually made indirectly by determining a material’s weight and specific gravity and then calculating its volume.  Therefore, mix design involves several different void and specific gravity measurements. It is important to have a clear understanding of these terms before proceeding.

• See HMA Weight-Volume Terms and Relationships

Variables

HMA is a rather complex material upon which many different, and sometimes conflicting, performance demands are placed.  It must resist deformation and cracking, be durable over time, resist water damage, provide a good tractive surface, and yet be inexpensive, readily made and easily placed.  In order to meet these demands, the mix designer can manipulate all of three variables:

1. Aggregate.  Items such as type (source), gradation and size, toughness and abrasion resistance, durability and soundness, shape and texture as well as cleanliness can be measured, judged and altered to some degree.
2. Asphalt binder.  Items such as type, durability, rheology, purity as well as additional modifying agents can be measured, judged and altered to some degree.
3. The ratio of asphalt binder to aggregate.  Usually expressed in terms of percent asphalt binder by total weight of HMA, this ratio has a profound effect on HMA pavement performance.  Because of the wide differences in aggregate specific gravity, the proportion of asphalt binder expressed as a percentage of total weight can vary widely even though the volume of asphalt binder as a percentage of total volume remains quite constant.

Objectives

Before embarking on a mix design procedure it is important to understand what its objectives are.  This section presents the typical qualities of a well-made HMA mix.  By manipulating the variables of aggregate, asphalt binder and the ratio between the two, mix design seeks to achieve the following qualities in the final HMA product (Roberts et al., 1996^[1]):

1. Deformation resistance (stability).  HMA should not distort (rut) or deform (shove) under traffic loading.  HMA deformation is related to one or more of the following:
   • Aggregate surface and abrasion characteristics.  Rounded particles tend to slip by one another causing HMA distortion under load while angular particles interlock with one another providing a good deformation resistant structure.  Brittle particles cause mix distortion because they tend to break apart under agitation or load.  Tests for particle shape and texture as well as durability and soundness can identify problem aggregate sources.  These sources can be avoided, or at a minimum, aggregate with good surface and abrasion characteristics can be blended in to provide better overall characteristics.
   • Aggregate gradation.  Gradations with excessive fines (either naturally occurring or caused by excessive abrasion) cause distortion because the large amount of fine particles tend to push the larger particles apart and act as lubricating ball-bearings between these larger particles.  A gradation resulting in low VMA or excessive asphalt binder content can have the same effect.  Gradation specifications are used to ensure acceptable aggregate gradation.
   • Asphalt binder content.  Excess asphalt binder content tends to lubricate and push aggregate particles apart making their rearrangement under load easier.  The optimum asphalt binder content as determined by mix design should prevent this.
   • Asphalt binder viscosity at high temperatures.  In the hot summer months, asphalt binder viscosity is at its lowest and the pavement will deform more easily under load.  Specifying an asphalt binder with a minimum high temperature viscosity (as can be done in the Superpave asphalt binder selection process) ensures adequate high temperature viscosity.
2. Fatigue resistance.  HMA should not crack when subjected to repeated loads over time.  HMA fatigue cracking is related to asphalt binder content and stiffness.  Higher asphalt binder contents will result in a mix that has a greater tendency to deform elastically (or at least deform) rather than fracture under repeated load.  The optimum asphalt binder content as determined by mix design should be high enough to prevent excessive fatigue cracking.  The use of an asphalt binder with a lower stiffness will increase a mixture’s fatigue life by providing greater flexibility.  However, the potential for rutting must also be considered in the selection of an asphalt binder.  Note that fatigue resistance is also highly dependent upon the relationship between structural layer thickness and loading.  However, this section only addresses mix design issues.
3. Low temperature cracking resistance.  HMA should not crack when subjected to low ambient temperatures.  Low temperature cracking is primarily a function of the asphalt binder low temperature stiffness.  Specifying asphalt binder with adequate low temperature properties (as can be done in the Superpave asphalt binder selection process) should prevent, or at least limit, low temperature cracking.
4. Durability.  HMA should not suffer excessive aging during production and service life.  HMA durability is related to one or more of the following:
   • The asphalt binder film thickness around each aggregate particle.  If the film thickness surrounding the aggregate particles is insufficient, it is possible that the aggregate may become accessible to water through holes in the film.  If the aggregate is hydrophilic, water will displace the asphalt film and asphalt-aggregate cohesion will be lost.  This process is typically referred to as stripping.  The optimum asphalt binder content as determined by mix design should provide adequate film thickness.
   • Air voids.  Excessive air voids (on the order of 8 percent or more) increase HMA permeability and allow oxygen easier access to more asphalt binder thus accelerating oxidation and volatilization.  To address this, HMA mix design seeks to adjust items such as asphalt content and aggregate gradation to produce design air voids of about 4 percent.  Excessive air voids can be either a mix design or a construction problem and this section only addresses the mix design problem.
5. Moisture damage resistance.  HMA should not degrade substantially from moisture penetration into the mix.  Moisture damage resistance is related to one or more of the following:
   • Aggregate mineral and chemical properties.  Some aggregates attract moisture to their surfaces, which can cause stripping.  To address this, either stripping-susceptible aggregates can be avoided or an anti-stripping asphalt binder modifier can be used.
   • Air voids.  When HMA air voids exceed about 8 percent by volume, they may become interconnected and allow water to easily penetrate the HMA and cause moisture damage through pore pressure or ice expansion.  To address this, HMA mix design adjusts asphalt binder content and aggregate gradation to produce design air voids of about 4 percent.  Excessive air voids can be either a mix design or a construction problem and this section only addresses the mix design problem.
6. Skid resistance.  HMA placed as a surface course should provide sufficient friction when in contact with a vehicle’s tire.  Low skid resistance is generally related to one or more of the following:
   • Aggregate characteristics such as texture, shape, size and resistance to polish.  Smooth, rounded or polish-susceptible aggregates are less skid resistant.  Tests for particle shape and texture can identify problem aggregate sources.  These sources can be avoided, or at a minimum, aggregate with good surface and abrasion characteristics can be blended in to provide better overall characteristics.
   • Asphalt binder content.  Excessive asphalt binder can cause HMA bleeding.  Using the optimum asphalt binder content as determined by mix design should prevent bleeding.
7. Workability. HMA must be capable of being placed and compacted with reasonable effort.  Workability is generally related to one or both of the following:
   • Aggregate texture, shape and size.  Flat, elongated or angular particles tend to interlock rather than slip by one another making placement and compaction more difficult (notice that this is almost in direct contrast with the desirable aggregate properties for deformation resistance).  Although no specific mix design tests are available to quantify workability, tests for particle shape and texture can identify possible workability problems.
   • Aggregate gradation.  Gradations with excess fines (especially in the 0.60 to 0.30 mm (No. 30 to 50) size range when using natural, rounded sand) can cause a tender mix.  A gradation resulting in low VMA or excess asphalt binder content can have the same effect.  Gradation specifications are used to ensure acceptable aggregate gradation.
   • Asphalt binder content.  At laydown temperatures (above about 120 °C (250 °F)) asphalt binder works as a lubricant between aggregate particles as they are compacted.  Therefore, low asphalt binder content reduces this lubrication resulting in a less workable mix.  Note that a higher asphalt binder content is generally good for workability but generally bad for deformation resistance.
   • Asphalt binder viscosity at mixing/laydown temperatures.  If the asphalt binder viscosity is too high at mixing and laydown temperatures, the HMA becomes difficult to dump, spread and compact.  The Superpave rotational viscometer specifically tests for mixing/laydown temperature asphalt binder viscosity.

Knowing these objectives, the challenge in mix design is then to develop a relatively simple procedure with a minimal amount of tests and samples that will produce a mix with all the above HMA qualities.

Basic Procedure

HMA mix design is the process of determining what aggregate to use, what asphalt binder to use and what the optimum combination of these two ingredients ought to be.  In order to meet the demands placed by the preceding desirable HMA properties, all mix design processes involve three basic steps:

1. Aggregate selection.  No matter the specific method, the overall mix design procedure begins with evaluation and selection of aggregate and asphalt binder sources.  Different authorities specify different methods of aggregate acceptance.  Typically, a battery of aggregate physical tests is run periodically on each particular aggregate source.  Then, for each mix design, gradation and size requirements are checked.  Normally, aggregate from more than one source is required to meet gradation requirements.
2. Asphalt binder selection.  Although different authorities can and do specify different methods of asphalt binder evaluation, the Superpave asphalt binder specification has been or will be adopted by most State DOTs as the standard (NHI, 2000^[2]).
3. Optimum asphalt binder content determination.  Mix design methods are generally distinguished by the method with which they determine the optimum asphalt binder content.  This process can be subdivided as follows:
   • Make several trial mixes with different asphalt binder contents.
   • Compact these trial mixes in the laboratory.  It is important to understand that this step is at best a rough simulation of field conditions.
   • Run several laboratory tests to determine key sample characteristics.  These tests represent a starting point for defining the mixture properties but they are not comprehensive nor are they exact reproductions of actual field conditions.
   • Pick the asphalt binder content that best satisfies the mix design objectives.

Job Mix Formula

The end result of a successful mix design is a recommended mixture of aggregate and asphalt binder.  This recommended mixture, which also includes aggregate gradation and asphalt binder type is often referred to as the job mix formula (JMF) or recipe.

Summary

HMA mix design is a laboratory process used to determine the appropriate aggregate, asphalt binder and their proportions for use in HMA.  Mix design is a process to manipulate three variables: (1) aggregate, (2) asphalt binder content and (3) the ratio of aggregate to asphalt binder with the objective of obtaining an HMA that is deformation resistant, fatigue resistant, low temperature crack resistant, durable, moisture damage resistant, skid resistant and workable.  Although mix design has many limitations it has proven to be a cost-effective method to provide crucial information that can be used to formulate a high-performance HMA.

“The objective in designing concrete mixtures is to determine the most economical and practical combination of readily available materials to produce a concrete that will satisfy the performance requirements under particular conditions of use.”

PCC mix design has evolved chiefly through experience and well-documented empirical relationships.  Normally, the mix design procedure involves two basic steps:

1. Mix proportioning.  This step uses the desired PCC properties as inputs then determines the required materials and proportions based on a combination of empirical relationships and local experience.  There are many different PCC proportioning methods of varying complexity that work reasonably well.
2. Mix testing.  Trial mixes are then evaluated and characterized by subjecting them to several laboratory tests.  Although these characterizations are not comprehensive, they can give the mix designer a good understanding of how a particular mix will perform in the field during construction and under subsequent traffic loading.

This section covers mix design fundamentals common to all PCC mix design methods.  First, two basic concepts (mix design as a simulation and weight-volume terms and relationships) are discussed to set a framework for subsequent discussion.  Second, the variables that mix design may manipulate are presented.  Third, the fundamental objectives of mix design are presented.  Finally, a generic mix design procedure is presented.

Concepts

Before discussing any mix design specifics, it is important to understand a couple of basic mix design concepts:

• Mix design is a simulation
• Weight-volume terms and relationships

Mix Design is a Simulation

First, and foremost, mix design is a laboratory simulation.  Mix design is meant to simulate actual PCC manufacturing, construction and performance.  Then, from this simulation we can predict (with reasonable certainty) what type of mix design is best for the particular application in question and how it will perform.

Being a simulation, mix design has its limitations.  Specifically, there are substantial differences between laboratory and field conditions.  For instance, mix testing is generally done on small samples that are cured in carefully controlled conditions.  These values are then used to draw conclusions about how a mix will behave under field conditions.  Despite such limitations mix design procedures can provide a cost effective and reasonably accurate simulation that is useful in making mix design decisions.

Weight-Volume Terms and Relationships

The more accurate mix design methods are volumetric in nature.  That is, they seek to combine the PCC constituents on a volume basis (as opposed to a weight basis).  Volume measurements are usually made indirectly by determining a material’s weight and specific gravity and then calculating its volume.  Therefore, mix design involves several key aggregate specific gravity measurements.

Variables

PCC is a complex material formed from some very basic ingredients.  When used in pavement, this material has several desired performance characteristics – some of which are in direct conflict with one another.  PCC pavements must resist deformation, crack in a controlled manner, be durable over time, resist water damage, provide a good tractive surface, and yet be inexpensive, readily made and easily placed.  In order to meet these demands, mix design can manipulate the following variables:

1. Aggregate.  Items such as type (source), amount, gradation and size, toughness and abrasion resistance, durability and soundness, shape and texture as well as cleanliness can be measured, judged and altered to some degree.
2. Portland cement.  Items such as type, amount, fineness, soundness, hydration rate and additives can be measured, judged and altered to some degree.
3. Water.  Typically the volume and cleanliness of water are of concern.  Specifically, the volume of water in relation to the volume of portland cement, called the water-cement ratio, is of primary concern.  Usually expressed as a decimal (e.g., 0.35), the water-cement ratio has a major effect on PCC strength and durability.
4. Admixtures.  Items added to PCC other than portland cement, water and aggregate.  Admixtures can be added before, during or after mixing and are used to alter basic PCC properties such as air content, water-cement ratio, workability, set time, bonding ability, coloring and strength.

Objectives

By manipulating the mixture variables of aggregate, portland cement, water and admixtures, mix design seeks to achieve the following qualities in the final PCC product (Mindess and Young, 1981^[2]):

1. Strength.  PCC should be strong enough to support expected traffic loading.  In pavement applications, flexural strength is typically more important than compressive strength (although both are important) since the controlling PCC slab stresses are caused by bending and not compression.  In its most basic sense, strength is related to the degree to which the portland cement has hydrated.  This degree of hydration is, in turn, related to one or more of the following:
   • Water-cement ratio.   The strength of PCC is most directly related to its capillary porosity.  The capillary porosity of a properly compacted PCC is determined by its water-cement ratio (Mindess and Young, 1981^[2]).  Thus, the water-cement ratio is an easily measurable PCC property that gives a good estimate of capillary porosity and thus, strength.  The lower the water-cement ratio, the fewer capillary pores and thus, the higher the strength.  Specifications typically include a maximum water-cement ratio as a strength control measure.
   • Entrained air (air voids).  At a constant water-cement ratio, as the amount of entrained air (by volume of the total mixture) increases, the voids-cement ratio (voids = air + water) decreases.  This generally results in a strength reduction.  However, air-entrained PCC can have a lower water-cement ratio than non-air-entrained PCC and still provide adequate workability.  Thus, the strength reduction associated with a higher air content can be offset by using a lower water-cement ratio.  For moderate-strength concrete (as is used in rigid pavements) each percentile of entrained air can reduce the compressive strength by about 2 – 6 percent (PCA, 1988^[1]).
   • Cement properties.  Properties of the portland cement such as fineness and chemical composition can affect strength and the rate of strength gain.  Typically, the type of portland cement is specified in order to control its properties.
2. Controlled shrinkage cracking.  Shrinkage cracking should occur in a controlled manner.  Although construction techniques such as joints and reinforcing steel help control shrinkage cracking, some mix design elements influence the amount of PCC shrinkage.  Chiefly, the amount of moisture and the rate of its use/loss will affect shrinkage and shrinkage cracking.  Therefore, factors such as high water-cement ratios and the use of high early strength portland cement types and admixtures can result in excessive and/or uncontrolled shrinkage cracking.
3. Durability.  PCC should not suffer excessive damage due to chemical or physical attacks during its service life.  As opposed to HMA durability, which is mainly concerned with aging effects, PCC durability is mainly concerned with specific chemical and environmental conditions that can potentially degrade PCC performance.  Durability is related to:
   • Porosity (water-cement ratio).  As the porosity of PCC decreases it becomes more impermeable.  Permeability determines a PCC’s susceptibility to any number of durability problems because it controls the rate and entry of moisture that may contain aggressive chemicals and the movement of water during heating or freezing (Mindess and Young, 1981^[2]).  The water-cement ratio is the single most determining factor in a PCC’s porosity.  The higher the water-cement ratio, the higher the porosity.  In order to limit PCC porosity, many agencies specify a maximum allowable water-cement ratio.
   • Entrained Air (Air voids).  Related to porosity, entrained air is important in controlling the effects of freeze-thaw cycles.  Upon freezing, water expands by about 9 percent.  Therefore, if the small capillaries within PCC are more than 91 percent filled with water, freezing will cause hydraulic pressures that may rupture the surrounding PCC.  Additionally, freezing water will attract other unfrozen water through osmosis (PCA, 1988^[1]).  Entrained air voids act as expansion chambers for freezing and migrating water and thus, specifying a minimum entrained air content can minimize freeze-thaw damage.
   • Chemical environment.  Certain chemicals such as sulfates, acids, bases and chloride salts are especially damaging to PCC.  Mix design can mitigate their damaging effects through such things as choosing a more resistant cement type.
4. Skid resistance.  PCC placed as a surface course should provide sufficient friction when in contact with a vehicle’s tire.  In mix design, low skid resistance is generally related to aggregate characteristics such as texture, shape, size and resistance to polish.  Smooth, rounded or polish-susceptible aggregates are less skid resistant.  Tests for particle shape and texture can identify problem aggregate sources.  These sources can be avoided, or at a minimum, aggregate with good surface and abrasion characteristics can be blended in to provide better overall characteristics.
5. Workability. PCC must be capable of being placed, compacted and finished with reasonable effort.  The slump test, a relative measurement of concrete consistency, is the most common method used to quantify workability.  Workability is generally related to one or more of the following:
   • Water content.  Water works as a lubricant between the particles within PCC.  Therefore, low water content reduces this lubrication and makes for a less workable mix.  Note that a higher water content is generally good for workability but generally bad for strength and durability, and may cause segregation and bleeding.  Where necessary, workability should be improved by redesigning the mix to increase the paste content (water + portland cement) rather than by simply adding more water or fine material (Mindess and Young, 1981^[2]).
   • Aggregate proportion.  Large amounts of aggregate in relation to the cement paste will decrease workability.  Essentially, if the aggregate portion is large then the corresponding water and cement portions must be small.  Thus, the same problems and remedies for “water content” above apply.
   • Aggregate texture, shape and size.  Flat, elongated or angular particles tend to interlock rather than slip by one another making placement and compaction more difficult.   Tests for particle shape and texture can identify possible workability problems.
   • Aggregate gradation.  Gradations deficient in fines make for less workable mixes.  In general, fine aggregates act as lubricating “ball bearings” in the mix.  Gradation specifications are used to ensure acceptable aggregate gradation.
   • Aggregate porosity.  Highly porous aggregate will absorb a high amount of water leaving less available for lubrication.  Thus, mix design usually corrects for the anticipated amount of absorbed water by the aggregate.
   • Air content.  Air also works as a lubricant between aggregate particles.  Therefore, low air content reduces this lubrication and makes for a less workable mix.  A volume of air-entrained PCC requires less water than an equal volume of non-air-entrained PCC of the same slump and maximum aggregate size (PCA, 1988^[1]).
   • Cement properties.  Portland cements with higher amounts of C₃S and C₃A will hydrate quicker and lose workability faster.

Knowing these objectives, the challenge in mix design is then to develop a relatively simple procedure with a minimal amount of tests and samples that will produce a mix with all the qualities discussed above.

Basic Procedure

In order to meet the requirements established by the preceding desirable PCC properties, all mix design processes involve four basic processes:

1. Aggregate selection.  No matter the specific method, the overall mix design procedure begins with evaluation and selection of aggregate and asphalt binder sources.  Different authorities specify different methods of aggregate acceptance.  Typically, a battery of aggregate physical tests is run periodically on each particular aggregate source.  Then, for each mix design, gradation and size requirements are checked.  Normally, aggregate from more than one source is required to meet gradation requirements.
2. Portland cement selection.  Typically, a type and amount of portland cement is selected based on past experience and empirical relationships with such factors as compressive strength (at a given age), water-cement ratio and chemical susceptibility.
3. Mix proportioning.  A PCC mixture can be proportioned using experience or a generic procedure (such as ACI 211.1).
4. Testing.  Run laboratory tests on properly prepared samples to determine key mixture characteristics.  It is important to understand that these tests are not comprehensive nor are they exact reproductions of actual field conditions.

The selected PCC mixture should be the one that, based on test results, best satisfies the mix design objectives.

Summary

PCC mix design is a laboratory process used to determine appropriate proportions and types of aggregate, portland cement, water and admixtures that will produce desired PCC properties.  Typical desired properties in PCC for pavement are adequate strength, controlled shrinkage, durability, skid resistance and workability.  Although mix design has many limitations it had proven to be a cost-effective simulation that is able to provide crucial information that can be used to formulate a high-performance PCC.

This section consists of a brief history of the Marshall mix design method followed by a general outline of the actual method. This outline emphasizes general concepts and rationale over specific procedures. Detailed procedures vary from state-to-state but typical procedures are available in the following documents:

• Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996^[2]). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.
• National Asphalt Pavement Association. (1982^[3]). Development of Marshall Procedures for Designing Asphalt Paving Mixtures, Information Series 84. National Asphalt Pavement Association. Lanham, MD.
• Asphalt Institute. (1997^[4]). Mix Design Methods for Asphalt, 6th ed., MS-02. Asphalt Institute. Lexington, KY.

Marshall Method History

(from White, 1985^[1])

During World War II, the U.S. Army Corps of Engineers (USCOE) began evaluating various HMA mix design methods for use in airfield pavement design. Motivation for this search came from the ever-increasing wheel loads and tire pressures produced by larger and larger military aircraft. Early work at the U.S. Army Waterways Experiment Station (WES) in 1943 had the objective of developing:

“…a simple apparatus suitable for use with the present California Bearing Ratio (CBR) equipment to design and control asphalt paving mixtures…”

The most promising method eventually proved to be the Marshall Stability Method developed by Bruce G. Marshall at the Mississippi Highway Department in 1939. WES took the original Marshall Stability Test and added a deformation measurement (using a flow meter) that was reasoned to assist in detecting excessively high asphalt contents. This appended test was eventually recommended for adoption by the U.S. Army because:

1. It was designed to stress the entire sample rather than just a portion of it.
2. It facilitated rapid testing with minimal effort.
3. It was compact, light and portable.
4. It produced densities reasonably close to field densities.

WES continued to refine the Marshall method through the 1950s with various tests on materials, traffic loading and weather variables. Today the Marshall method, despite its shortcomings, is probably the most widely used mix design method in the world. It has probably become so widely used because (1) it was adopted and used by the U.S. military all over the world during and after WWII and (2) it is simple, compact and inexpensive.

Marshall Mix Design Procedure

The Marshall mix design method consists of 6 basic steps:

1. Aggregate selection.
2. Asphalt binder selection.
3. Sample preparation (including compaction).
4. Stability determination using the Hveem Stabilometer.
5. Density and voids calculations.
6. Optimum asphalt binder content selection.

Aggregate Selection

Although Hveem did not specifically develop an aggregate evaluation and selection procedure, one is included here because it is integral to any mix design. A typical aggregate evaluation for use with either the Hveem or Marshall mix design methods includes three basic steps (Roberts et al., 1996^[2]):

1. Determine aggregate physical properties. This consists of running various tests to determine properties such as:
   • Toughness and abrasion
   • Durability and soundness
   • Cleanliness and deleterious materials
   • Particle shape and surface texture
2. Determine other aggregate descriptive physical properties. If the aggregate is acceptable according to step #1, additional tests are run to fully characterize the aggregate. These tests determine:
   • Gradation and size
   • Specific gravity and absorption
3. Perform blending calculations to achieve the mix design aggregate gradation. Often, aggregates from more than one source or stockpile are used to obtain the final aggregate gradation used in a mix design. Trial blends of these different gradations are usually calculated until an acceptable final mix design gradation is achieved. Typical considerations for a trial blend include:
   • All gradation specifications must be met. Typical specifications will require the percent retained by weight on particular sieve sizes to be within a certain band.
   • The gradation should not be too close to the FHWA’s 0.45 power maximum density curve. If it is, then the VMA is likely to be too low. Gradation should deviate from the FHWA’s 0.45 power maximum density curve, especially on the 2.36 mm (No. 8) sieve.

Asphalt Binder Evaluation

The Marshall test does not have a common generic asphalt binder selection and evaluation procedure.  Each specifying entity uses their own method with modifications to determine the appropriate binder and, if any, modifiers.  Binder evaluation can be based on local experience, previous performance or a set procedure.  The most common procedure is the Superpave PG binder system. Once the binder is selected, several preliminary tests are run to determine the asphalt binder’s temperature-viscosity relationship.

Sample Preparation

The Marshall method, like other mix design methods, uses several trial aggregate-asphalt binder blends (typically 5 blends with 3 samples each for a total of 15 specimens), each with a different asphalt binder content.  Then, by evaluating each trial blend’s performance, an optimum asphalt binder content can be selected.  In order for this concept to work, the trial blends must contain a range of asphalt contents both above and below the optimum asphalt content.  Therefore, the first step in sample preparation is to estimate an optimum asphalt content.  Trial blend asphalt contents are then determined from this estimate.

Optimum Asphalt Binder Content Estimate

The Marshall mix design method can use any suitable method for estimating optimum asphalt content and usually relies on local procedures or experience.

Sample Asphalt Binder Contents

Based on the results of the optimum asphalt binder content estimate, samples are typically prepared at 0.5 percent by weight of mix increments, with at least two samples above the estimated asphalt binder content and two below.

Compaction with the Marshall Hammer

Each sample is then heated to the anticipated compaction temperature and compacted with a Marshall hammer, a device that applies pressure to a sample through a tamper foot (Figure 1).  Some hammers are automatic and some are hand operated.  Key parameters of the compactor are:

• Sample size = 102 mm (4-inch) diameter cylinder 64 mm (2.5 inches) in height (corrections can be made for different sample heights)
• Tamper foot = Flat and circular with a diameter of 98.4 mm (3.875 inches) corresponding to an area of 76 cm² (11.8 in²).
• Compaction pressure = Specified as a 457.2 mm (18 inches) free fall drop distance of a hammer assembly with a 4536 g (10 lb.) sliding weight.
• Number of blows = Typically 35, 50 or 75 on each side depending upon anticipated traffic loading.
• Simulation method = The tamper foot strikes the sample on the top and covers almost the entire sample top area.  After a specified number of blows, the sample is turned over and the procedure repeated.

Figure 1. Marshall drop hammers.

The standard Marshall method sample preparation procedure is contained in:

• AASHTO T 245: Resistance to Plastic Flow of Bituminous Mixtures Using the Marshall Apparatus

The Marshall Stability and Flow Test

The Marshall stability and flow test provides the performance prediction measure for the Marshall mix design method. The stability portion of the test measures the maximum load supported by the test specimen at a loading rate of 50.8 mm/minute (2 inches/minute). Basically, the load is increased until it reaches a maximum then when the load just begins to decrease, the loading is stopped and the maximum load is recorded.

During the loading, an attached dial gauge measures the specimen’s plastic flow as a result of the loading (Figure 2). The flow value is recorded in 0.25 mm (0.01 inch) increments at the same time the maximum load is recorded.

Figure 2. Marshall stability testing apparatus.

Typical Marshall design stability and flow criteria are shown in Table 1.

One standard Marshall mix design procedure is:

• AASHTO T 245: Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus

Density and Voids Analysis

All mix design methods use density and voids to determine basic HMA physical characteristics. Two different measures of densities are typically taken:

1. Bulk specific gravity (G_mb).
2. Theoretical maximum specific gravity (TMD, G_mm).

These densities are then used to calculate the volumetric parameters of the HMA. Measured void expressions are usually:

• Air voids (V_a), sometimes expressed as voids in the total mix (VTM)
• Voids in the mineral aggregate (VMA)
• Voids filled with asphalt (VFA)

Generally, these values must meet local or State criteria.

Selection of Optimum Asphalt Binder Content

The optimum asphalt binder content is finally selected based on the combined results of Marshall stability and flow, density analysis and void analysis (Figure 3).  Optimum asphalt binder content can be arrived at in the following procedure (Roberts et al., 1996^[2]):

1. Plot the following graphs:
   • Asphalt binder content vs. density.  Density will generally increase with increasing asphalt content, reach a maximum, then decrease.  Peak density usually occurs at a higher asphalt binder content than peak stability.
   • Asphalt binder content vs. Marshall stability.  This should follow one of two trends:
   • * Stability increases with increasing asphalt binder content, reaches a peak, then decreases.
   • * Stability decreases with increasing asphalt binder content and does not show a peak.  This curve is common for some recycled HMA mixtures.
   • Asphalt binder content vs. flow.
   • Asphalt binder content vs. air voids.  Percent air voids should decrease with increasing asphalt binder content.
   • Asphalt binder content vs. VMA.  Percent VMA should decrease with increasing asphalt binder content, reach a minimum, then increase.
   • Asphalt binder content vs. VFA.  Percent VFA increases with increasing asphalt binder content.
2. Determine the asphalt binder content that corresponds to the specifications median air void content (typically this is 4 percent).  This is the optimum asphalt binder content.
3. Determine properties at this optimum asphalt binder content by referring to the plots.  Compare each of these values against specification values and if all are within specification, then the preceding optimum asphalt binder content is satisfactory.  Otherwise, if any of these properties is outside the specification range the mixture should be redesigned.

Figure 1: Gyratory Compactor

Figure 2: Gyratory Compactor

The Superpave mix design method consists of three basic steps:

• Aggregate selection. Aggregate is specified in three ways. First, restrictions on aggregate gradation are specified by using gradation specifications. Second, there are physical property requirements on aggregate angularity, flat and elongated particles, and clay content. Third, aggregate criteria, which the Asphalt Institute (2001^[1]) calls “source properties” (because they are considered to be source specific), such as durability and soundness are specified.
• Asphalt binder selection. Superpave PG asphalt binders are selected based on the expected pavement temperature extremes in the area of their intended use. These extremes can be calculated using software (such as LTPPBind) or, more commonly, a standard grade of PG 64-16 is used statewide.
• Optimum asphalt binder content determination. In the Superpave method, this step can be broken up into 4 substeps:
  • Prepare several initial samples, usually two at the proposed design asphalt content, two at 0.5 percent below the design asphalt content and two at 0.5 percent above the design asphalt content.
  • Compact these trial mixes in the Superpave Gyratory Compactor (see Figures 1, 2 and 3). This compactor is specific to the Superpave mix design method.
  • Determine the density and other volumetric properties of the samples.
  • Select the optimum asphalt binder content. The asphalt binder content corresponding to 4 percent air voids.

In the Superpave mix design process there is are no accepted standard performance tests so nothing analogous to the Hveem stability and cohesion tests is used. Research into creating a standard performance test is ongoing.

Figure 1: Marshall Hammer

Figure 2: Marshall Stability

Figure 3: Marshall Samples

The basic concepts of the Marshall mix design method were originally developed by Bruce Marshall of the Mississippi Highway Department around 1939 and then refined by the U.S. Army.

The Marshall method is very popular because of its relative simplicity, economical equipment and proven record.

Typically, the Marshall mix design method consists of three basic steps:

1. Aggregate selection. Different agencies/owners specify different methods of aggregate acceptance. Private labs may or may not run periodic aggregate physical tests on a particular aggregate source. For each mix design, gradation and size requirements are checked. Often, aggregate from more than one source is required to meet gradation requirements.
2. Asphalt binder selection.
3. Optimum asphalt binder content determination. In the Marshall method, this step can be broken up into 5 substeps:

• Prepare a series of initial samples, each at a different asphalt binder content. For instance, two to three samples each might be made at 4.5, 5.0, 5.5, 6.0 and 6.5 percent asphalt by dry weight for a total of 10 to 15 samples. There should be at least two samples above and two below the estimated optimum asphalt content.
• Compact these trial mixes using the Marshall drop hammer (see Figure 1). This hammer is specific to the Marshall mix design method.
• Test the samples in the Marshall testing machine (see Figure 2) for stability and flow. This testing machine is specific to the Marshall mix design method. Passing values of stability and flow depend upon the mix class being evaluated.
• Determine the density and other volumetric properties of the samples.
• Select the optimum asphalt binder content. The asphalt binder content corresponding to 4 percent air voids is selected as long as this binder content passes stability and flow requirements.

Figure 1: Aggregate Gradation for Mix Design

Figure 2: Operating the Marshall Hammer

Figure 3: Mixing Aggregate and Asphalt Binder

Variables

HMA is a complex material upon which many different, and sometimes conflicting, performance demands are placed. It must resist deformation and cracking, be durable over time, resist water damage, provide a good tractive surface, and yet be inexpensive, readily made and easily placed. In order to meet these demands, the mix designer can manipulate all of three variables:

• Aggregate. Items such as type (source), gradation and size, toughness and abrasion resistance, durability and soundness, shape and texture as well as cleanliness can be measured, judged and altered to some degree.
• Asphalt binder. Items such as type, durability, rheology, purity as well as additional modifying agents can be measured, judged and altered to some degree.
• The ratio of asphalt binder to aggregate. Usually expressed in terms of percent asphalt binder by total weight of HMA or total weight of aggregate, this ratio has a profound effect on HMA pavement performance. Because of the wide differences in aggregate specific gravity, the proportion of asphalt binder expressed as a percentage of total HMA or aggregate weight can vary widely even though the volume of asphalt binder as a percentage of total volume remains quite constant.

Objectives

By manipulating the variables of aggregate, asphalt binder and the ratio between the two, mix design seeks to achieve the following qualities in the final HMA product (Roberts et al., 1996^[1]):

• Deformation resistance. HMA should not distort (rut) or deform (shove) under traffic loading. HMA deformation is related to aggregate surface and abrasion characteristics, aggregate gradation, asphalt binder content and asphalt binder viscosity at high temperatures.
• Fatigue resistance. HMA should not crack when subjected to repeated loads over time. HMA fatigue cracking is related to asphalt binder content and stiffness.
• Low temperature cracking resistance. HMA should not crack when subjected to low ambient temperatures. Low temperature cracking is primarily a function of the asphalt binder low temperature stiffness. Obviously, this is not a major concern in Hawai’i.
• Durability. HMA should not age excessively during production and service life. HMA durability is related to air voids as well as the asphalt binder film thickness around each aggregate particle.
• Moisture damage resistance. HMA should not degrade substantially from moisture penetration into the mix. Moisture damage resistance is related to air voids as well as aggregate mineral and chemical properties.
• Skid resistance. HMA placed as a surface course should provide sufficient friction when in contact with a vehicle’s tire. Low skid resistance is generally related to aggregate characteristics or high asphalt binder content.
• Workability. HMA must be capable of being placed and compacted with reasonable effort. Workability is generally related to aggregate texture/shape/size/gradation, asphalt binder content and asphalt binder viscosity at mixing and placement temperatures.

Basic Procedure

No matter what specific method is used, the basic mix design procedure remains the same. All mix design processes involve three basic steps:

1. Aggregate selection. Different agencies/owners specify different methods of aggregate acceptance. Typically, a battery of aggregate physical tests is run periodically on each particular aggregate source. Then, for each mix design, gradation and size requirements are specified. Normally, aggregate from more than one quarry stockpile is required to meet gradation specifications.
2. Asphalt binder selection. Different authorities can and do specify different methods of asphalt binder evaluation. In Hawai’i, most agencies/owners use the Superpave PG system. Formerly, the aged residue (AR) viscosity grading system (a type of viscosity grading system) was used.
3. Optimum asphalt binder content determination. Mix design methods are generally distinguished by the way in which they determine the optimum asphalt binder content. This process can be subdivided into:

• Make several trial mixes with different asphalt binder contents.
• Compact these trial mixes in the laboratory. This compaction is meant to be a rough simulation of actual field conditions.
• Run laboratory tests to determine key sample characteristics.
• Pick the asphalt binder content that best satisfies the mix design objectives.

Result: The Job Mix Formula (JMF)

The end result of a successful mix design is a recommended mixture of aggregate and asphalt binder. This recommended mixture, which includes aggregate gradation and asphalt binder type is often referred to as the job mix formula (JMF). The JMF may subsequently be altered based on field performance, however at a minimum the mix design provides the initial JMF. For HMA manufacturing, target values of gradation and asphalt binder content are specified based on the JMF along with allowable tolerance limits to allow for inherent material and production variability (see Table 1 and Figure 4). These target values and tolerance bands are based on the JMF and are much tighter than general HMA gradation requirements. Thus, the mix designer is allowed substantial freedom in choosing a particular gradation for the JMF and then the manufacturer is expected to adhere quite closely to this JMF gradation during production.

• Mix design fundamentals. These are the fundamental philosophies and parameters of mix design such as (1) why it is done, (2) what basic assumptions are made and (3) the specific goals of mix design.
• Mix design methods. These sections cover the various mix design procedures used. For HMA, the Hveem, Marshall and Superpave methods are covered. For PCC, the ACI Method is covered.

• Performance Tests. These are the tests performed on laboratory designed mixes (or field samples) to characterize their performance. They can consist of basic physical property measurements (such as stiffness or strength) or laboratory simulation of field conditions (such as rutting potential or chloride penetration).

This section is only meant to provide a brief overview of mix design methods as well as their assumptions, inputs and outputs. Resources that provide a detailed description and analysis of each mix design method are listed in the beginning of each section.

• Fundamentals
• Hveem Method
• Marshall Method
• Superpave Method
• References

Suggested Reading

Asphalt Institute. (1997). Mix Design Methods for Asphalt, Manual Series No. 2 (MS-02). Asphalt Institute. Lexington, KY.

Asphalt Institute. (2001). Superpave Mix Design. Superpave Series No. 2 (SP-02). Asphalt Institute. Lexington, KY.

Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.

Publications Cited

Asphalt Institute. (2001). HMA Construction. Manual Series No. 22 (MS-22). Asphalt Institute. Lexington, KY.