As stated in Section 2 of this manual, an asphalt mixture is composed of three (3) basic components: 1) asphalt binder, 2) aggregates and 3) air voids.  Mineral filler and other additives are used when required.  The asphalt material, which can be asphalt binder, modified asphalt binder, emulsified liquid asphalt or some other form of asphaltic material, acts as a binding agent to glue the aggregate particles into a cohesive mass. Asphalt Concrete, sometimes referred to as hot mix asphalt (HMA) is a paving material that consist primarily of asphalt binder and mineral aggregate and is mixed in a hot mix plant or by some other procedure.  When bound by the asphalt binder, the mineral aggregate acts as a stone framework to impart strength and toughness to the system.  Because it is relatively impervious to water, the asphalt binder also functions to waterproof the mixture.  Because HMA contains both asphalt binder and mineral aggregate, the volumetric properties and subsequently the behavior of the mixture is affected by the properties of the individual components and how they react with each other in the system.  In order to determine if the behavior and performance of the mixture under traffic will be satisfactory, a mix design must be performed to determine the proper combination of the individual materials prior to beginning mix production.

In North Carolina, HMA mixtures have historically been designed using the Marshall Method of Mix Design. This is an empirical laboratory procedure, meaning that field experience is required  to determine if the laboratory analysis correlates with pavement performance. While this design method has served well over the years, it is not always reliable. In addition, traffic volumes and loading have increased tremendously, tire pressures have increased, and legal load limits are frequently raised without regard to damage caused to pavements. In 1987, the Strategic Highway Research Program (SHRP) began developing a new system for specifying asphalt materials. The new design system is called Superpave, which is an acronym for Superior Performing Asphalt Pavements. The three major components of the Superpave mix design system are (1) asphalt binder specifications, (2) mineral aggregate specifications, and (3) a HMA design and analysis system, including computer software that integrates the system components.

The Superpave design system represents an improved system for specifying asphalt binders and mineral aggregate and then combining the components to meet certain criteria based on expected performance. Although the name may imply that Superpave is a type of asphalt concrete used only on heavily loaded roads, this is not the case. It will be used to design all mixes, including mixes to be used on low volume secondary roads. This system is a new and improved mix design procedure which employs many of the same materials and mix properties that were used in conjunction with the Marshall Method procedure.  The terminology has changed in some cases, but address the same issues. The major difference with Superpave is that the mix is appropriately designed for the environmental conditions and anticipated traffic to which the pavement will be exposed. The mix design criteria for the various mix types and levels within each type take these factors into consideration. The most dramatic change in the mix design and field testing procedures is the use of the Superpave gyratory compactor (SGC). This compactor kneads the mix and better simulates field compaction and traffic use. This results in better prediction of actual field performance of a mixture.

4.2   PURPOSES OF MIX DESIGNS

While there are many types of asphalt mixtures used in highway construction, there are three basic types: surface mixes, intermediate mixes and base mixes.  The Specifications for Superpave asphalt plant mixes cover each type in more detail than we are here, but it is well to note that each type has a specific purpose and location within an asphalt pavement structure.

As noted above, there are certain properties and performance characteristics that are desirable in an asphalt mix.  The relative proportions of aggregate, asphalt binder, and air voids significantly affect the physical properties of the mix and ultimately, how it will perform as a finished pavement.  While it would be very easy to "mix some asphalt with some rock", this very likely would result in a poor quality mix.  Designing asphalt mixes, as with other engineering materials designs, is largely a matter of selecting and proportioning materials to obtain the desired qualities and properties in the finished construction.  The overall objective is to determine an economical blend and gradation of aggregates (within the specification limits) and a corresponding asphalt content that yields a mix having:

(a) Sufficient asphalt to ensure a durable pavement by thoroughly coating the aggregate particles and waterproofing and bonding them together under suitable compaction.

(b) Sufficient mix resistance to permanent deformation to satisfy the service requirement and demands of traffic without distortion or displacement.

(c) Sufficient voids in the total compacted mix to provide for a slight additional amount of compaction under traffic loading without bleeding and rutting, yet be low enough to keep out excessive air and moisture.

(d) Sufficient workability to permit efficient placement and proper compaction operations without segregation.

4.3   PERFORMANCE CHARACTERISTICS CONSIDERED IN MIX DESIGN

Hot-mix asphalt pavements function properly when they are designed, produced and placed in such a way as to give them certain desirable performance characteristics.  These characteristics contribute to the quality of hot-mix pavements.  These include permanent deformation (rutting) resistance, durability, flexibility, fatigue resistance, skid resistance, impermeability, workability and economics.

Ensuring that a paving mixture has each of these properties is a major goal of the mix-design procedure.  Therefore, the technician should be aware of what each of the properties is, how it is evaluated, and what it means in terms of pavement performance.  These properties are discussed below.

4.3.1  Permanent Deformation (Rut Resistance) - The ability of an asphalt mix to resist permanent deformation from imposed loads.  Unstable mixes are marked by channeling (ruts) and corrugations (washboarding) in the pavement.  Rut resistance  is dependent upon both internal friction and cohesion.

Internal friction is dependent on particle shape, surface texture, gradation of aggregate, density of mix, and quantity of asphalt.  It is a combination of the frictional and interlocking resistance of the aggregate in the mix.  Frictional resistance increases with the surface roughness of the aggregate particles.  It also increases with the area of particle contact.  Interlocking resistance is dependent upon particle size and shape.

The figure above demonstrates that with more angular (cubical) particle shape and more contact between particles greater resistance to rutting and permanant deformation is achieved.

For any given aggregate, the rut resistance increases with the density of the confined particles, which is achieved by dense gradations and adequate compaction.  Excessive asphalt in the mix tends to lubricate the aggregate particles and lower the internal friction of the stone framework.

Cohesion is that binding force that is inherent in the asphalt mixes.  The asphalt serves to maintain contact pressures developed between aggregate particles.  Cohesion varies directly with the rate of loading, loaded area, and viscosity of the asphalt.  It varies inversely with the temperature.  Cohesion increases with increasing asphalt content up to a maximum point and then decreases.

4.3.2 Durability - The ability of an asphalt mix to resist disintegration by weathering and traffic.  Included under weathering are changes in the characteristics of asphalt such as oxidation, volatilization and changes in the pavement and aggregate due to the action of water, including stripping, freezing and thawing.

Durability is generally enhanced by high asphalt contents, dense aggregate gradations, and well-compacted, impervious mixes.  One argument for an increased amount of asphalt is the resultant thicker asphalt film coating around the aggregate particles.  Thicker films are more resistant to age-hardening.  That is, a longer period of time is required to reduce a thicker film of asphalt to the same degree of brittleness as a thin film.  Another favorable argument is that the increased amount of asphalt reduces the pore sizes of the interconnected voids or seals them off in the mix, making it more difficult for air and water to enter the interior of the mix and cause damage.

To resist the action of water, the same requirements (dense-graded aggregates, high asphalt contents, and adequate compaction) apply.  It is desirable to use aggregates that retain an asphalt coating in the presence of water.  If the mixture is dense, displacement of asphalt from aggregate by water generally does not occur.

Sufficient asphalt must be incorporated in the mix to provide bonding properties adequate to resist the tractive or abrasive forces of traffic.  Insufficient asphalt may result in aggregate being dislodged from the surface.  This is known as raveling.  Abrasion may also take place if the asphalt has become brittle.  Overheating of asphalt in the hot-mixing process is a cause of brittleness, which leads to pavement disintegration.

A mix having a high asphalt content with voids completely filled with asphalt would provide the ultimate in durability.  However, this would be undesirable from the standpoint of rut resistance.  When placed in the roadway, the mix would rut and displace under traffic.  Bleeding or flushing of asphalt to the surface would also take place, thereby reducing skid resistance.

Maximum rut resistance is not reached in an aggregate mass until the amount of asphalt coating the particles has reached some critical value.  Additional asphalt then tends to act as a lubricant rather than a binder, reducing the rut resistance of the mix, even though the durability may be increased.  It is therefore necessary to compromise by keeping the asphalt content as high as possible while maintaining adequate rut resistance.

4.3.3 Flexibility - The ability of an asphalt mix to conform to gradual settlements and movements of the base and subgrade.  Differential settlements in the fill embankment occasionally occur.  Thus, it is impossible to develop uniform density in the subgrade during construction because sections or portions of the pavement tend to compress and settle under traffic.  Therefore, the asphalt pavement must have the ability to conform to localized and differential settlement without cracking.  Generally, flexibility of the asphalt mix is enhanced by high asphalt content and relatively open-graded aggregates.

4.3.4 Fatigue (Cracking) Resistance - The ability of asphalt pavement to withstand repeated flexing or slight bending of the pavement structure caused by the passage of wheel loads.  Tests have shown that the quantity of asphalt is extremely important when considering the fatigue resistance of a pavement.  As a rule, the higher the asphalt content, the greater the fatigue resistance.  Tests indicate that low air-void content asphalt mixes have more fatigue resistance than higher air-void content mixes.  Well-graded aggregates that permit higher asphalt content without causing flushing or bleeding in compacted pavement should be incorporated in the mix.

4.3.5 Skid Resistance - The ability of an asphalt surface, particularly when wet, to provide resistance to slipping or skidding of vehicles.  The factors for obtaining high skid resistance are generally the same as those for obtaining high stability. Proper asphalt contents and aggregates with a rough surface texture are the greatest contributors.  However, not only must the aggregate have a rough surface texture, it must also resist polishing.  Aggregates containing non-polishing minerals with different wear or abrasion characteristics provide continuous renewal of the pavement's texture, maintaining a skid-resistant surface.  Examples of non-polishing aggregates are granites, crushed gravel, silica sands and slag.  An example of a polishing type aggregate is limestone.

Mixes so rich in asphalt as to fill the voids in the compacted pavement will probably cause asphalt to flush to the surface.  This is usually called bleeding.  Free asphalt on the pavement surface can cause slippery conditions.

4.3.6 Impermeability - The ability an asphalt pavement to provide resistance to the passage of air and water into or through the pavement.  While the void content may be an indication of the susceptibility of a compacted mix to the passage of air and water; of more significance is the interconnection of the voids and their access to the surface.  Imperviousness to air and water is extremely important from the standpoint of lasting durability in asphalt mixes.

4.3.7 Low Temperature / Shrinkage Cracking - The ability of an asphalt pavement to resist low temperature/shrinkage cracking. Low temperature/shrinkage cracking is caused by adverse environmental conditions rather than applied traffic loads. It is characterized by surprisingly consistently spaced transverse cracks (perpendicular to the direction of traffic). It is caused by a build-up of tensile stresses as the pavement shrinks due to extremely cold weather or due to shrinkage caused by oxidation (aging) of the pavement. Hard asphalt binders or binders which have hardened (oxidized) due to high void content in the as constructed mix are more prone to low temperature cracking.

4.3.8 Workability - The ease with which an asphalt mix may be placed and compacted.  With careful attention to proper design and with the use of machine spreading, workability is not a problem.  At times, the properties of the aggregates that promote high stability make asphalt mixes containing these aggregates difficult to spread or compact.  Since workability problems are discovered most frequently during the paving operation, mix design adjustments should be made quickly to allow the job to proceed as efficiently as possible.

4.3.9 Economics - The cost of the in-place pavement must be considered.  Mix components, production and placement costs, haul distances, safety considerations, quality, expected pavement performance and other factors need to be evaluated when selecting the final mix design.

4.4 THE SUPERPAVE MIX DESIGN SYSTEM

The Superpave mix design system is based on volumetric proportioning of the asphalt and aggregate materials and laboratory compaction of trial mixes using the Superpave Gyratory Compactor (SGC).  The SGC’s primary function is to fabricate test specimens by simulating compaction and the effects of traffic on an asphalt pavement.  The specimens fabricated with the gyratory compactor are used to determine the volumetric properties (air voids, voids in the mineral aggregate, and voids filled with asphalt) of Superpave mixes.  Those properties, measured in the laboratory, are an indicator of how good the mix will perform in the field. The gyratory compactor is also well suited for quality control/quality assurance, inasmuch as it can be set up at the plant site to verify that the delivered asphalt mix meets the job mix volumetric specifications.

By kneading mixes to simulate construction compaction and traffic loads, the gyratory compactor provides specimens that are much more representative of actual in-service pavements.  The level or amount of compaction is dependent on the environmental conditions and traffic levels expected at the job site.

The basic mixture design procedures in Superpave consist of an evaluation of the following characteristics once the type and amount of traffic and the environmental conditions under which the pavement will be expected to perform have been determined:

1.     aggregate properties and gradation requirements,
2.     asphalt grade selection and requirements,
3.     mixture volumetric properties and requirements,
4.     dust proportion (dust to effective binder ratio),
5.     moisture susceptibility, and
6.     permanent deformation (rutting resistance)

The Superpave mix design system will eventually include performance based tests and performance prediction models that will predict how well a mix will actually perform under traffic. These procedures are intended to provide additional information on asphalt mixes that will be utilized in pavements with very high traffic volumes and loads.

Two new, sophisticated pieces of laboratory equipment - the Superpave shear tester and the indirect tensile tester - are used to measure specific engineering properties of the laboratory-compacted asphalt mix.  The test results are then entered into software models that predict how many equivalent single-axle loads (ESAL's) the pavement will carry, or how much time will elapse, before a certain level of rutting, fatigue cracking, or low-temperature cracking develops.

The test procedures for the Superpave shear tester and the indirect tensile tester are currently being refined to ensure that the procedures are sound and that the results are repeatable.  The performance prediction models are also undergoing evaluation and validation and will be refined as necessary.  Further discussion of these performance analysis procedures is beyond the scope of this manual.

4.4.1 Aggregate Properties and Gradation Requirements – Aggregate physical properties for Superpave mixes are specified on the basis of both “consensus” (blend) properties and “source” (individual) properties. These criteria are discussed in more detail in Section 2 of this manual.

To specify gradation, Superpave uses a 0.45-power gradation chart with control points on various sieves to define a permissible gradation of the designated mix type.  Control points function as master ranges through which gradations must pass.  Control points are placed at the nominal maximum size sieve, an intermediate size sieve (2.36 mm), and the smallest sieve (0.075 mm).  The control points vary, depending on the nominal maximum size of the mix.

This chart uses a unique graphing technique to judge the cumulative particle size distribution of an aggregates blend.  The vertical axis of the chart is the percent passing.  The horizontal axis is an arithmetic scale of sieve sizes in millimeters, raised to the 0.45 power (See Example 0.45-power chart included in the example mix design later in this section of the manual).

An important feature of the 0.45-power chart is the maximum density gradation.  This gradation plots as a straight line from the maximum aggregate size through the origin.  Superpave uses a standard set of ASTM sieves and the following definitions with respect to aggregate size.

• Maximum Size:  One sieve size larger than the nominal maximum size.

• Nominal Maximum Size: One sieve size larger than the first sieve to retain more than 10 percent.

The maximum density gradation represents a gradation in which the aggregate particles fit together in their most dense possible arrangement.  In general, this is a gradation to avoid because there will most likely be inadequate void space within the aggregate structure to allow adding adequate asphalt binder in order to develop sufficiently thick asphalt films for a durable mixture and still maintain the desired air void content.

The term used to describe the cumulative frequency distribution of aggregate particle sizes is the design aggregate structure.  The design aggregate structure gradation should lie between the control points and meet the Superpave aggregate gradation requirements detailed in Table 1 (Fig. 4-2) of this Manual.

4.4.2   Asphalt Binder Grade Selection and Requirements - In general, Superpave mix design guidelines specify that the binder grade to be used be initially selected based on the climate, both high and low temperatures, in which a pavement will be performing. The guidelines then recommend the high temperature grade be adjusted (upward) based on other factors, such as the amount and type of traffic loading, operating speed of the traffic, and position of the pavement layer within the pavement structure. These guidelines have been taken into consideration when specifying the binder grades to be used in the various mixes. See Fig. 4-3 of this Manual for the PG binder grade required for the various mix types specified by NCDOT.

4.4.3 Mixture Volumetric Properties and Requirements - A major factor that must be taken into account when considering asphalt mixture behavior is the volumetric properties of the mixture.  Mixture volumetric requirements consist of air voids (VTM), voids in the mineral aggregate (VMA), voids filled with asphalt (VFA) and effective asphalt content (Pbe). These volumetric properties for NCDOT mixes are illustrated in Figure 4.1.

Air void content (VTM) is an extremely important property because it is used as the basis for selecting the asphalt binder content.  In Superpave, the design air void content is usually 4.0 %; however, the mix designer should always check the specifications.

Superpave defines voids in the mineral aggregate (VMA) as the sum of the volume of air voids and effective (i.e., unabsorbed) binder in a compacted sample.  It represents the void space between the aggregate particles.  Specified minimum values for VMA at the design air void content of 4.0 % are a function of nominal maximum aggregate size. Fig. 4-3 of this Manual shows Superpave VMA requirements.

Voids filled with asphalt (VFA) is defined as the percentage of the VMA containing asphalt binder.  Consequently, VFA is the volume of effective asphalt binder expressed as a percentage of the VMA.  The acceptable range of design VFA at 4.0 % air voids is a function of traffic level as shown in Figure 4-3.

Effective asphalt content (P_be) is defined as the total asphalt content of a paving mixture minus the portion of asphalt absorbed into the aggregate particles (See Figure 4-1).

Obtaining the correct air void content is critical in both mix design and the in-service performance of a pavement.  As discussed in Section 2, asphalt binder expands and contracts with variations in temperature.  In hot weather, air voids in the mix provide room for the expanding asphalt binder.  If there are not enough voids within the mix to allow for the expansion, the asphalt binder expands to fill all existing voids, and then begins pushing the aggregate particles apart, reducing aggregate interlock and contact friction. This causes the pavement to become unstable, more susceptible to pushing, shoving, and rutting.  The binder eventually may bleed or flush to the surface.  This significantly reduces the skid resistance of the pavement.

Imperviousness to air and water is extremely important for the mix to be and remain durable. If the air void content is too high, the air voids may interconnect and allow water and air to penetrate into the mix. Water penetration may cause the asphalt binder to strip from the aggregate.  Exposing asphalt binder to both water and air will cause it to oxidize more rapidly, causing it to become hard and brittle, and therefore resulting in early fatigue failure.

4.4.4 Dust to Effective Binder Ratio - Another mixture requirement is the dust to binder ratio.  This is computed as the ratio of the percentage by weight of aggregate finer than the 0.075 mm sieve (by washing) to the effective asphalt content expressed as a percent by weight of total mix.  Effective binder content is the total binder used in the mixture less the percentage of absorbed binder.  Dust / Binder Ratio is used during the mixture design phase as a design criteria.  Specifications require the dust / binder ratio be in the range of 0.6 to 1.4, inclusive, for all Superpave mixtures.

4.4.5 Moisture Susceptibility -  Moisture Susceptibility, also know as stripping, is the separation of the asphalt film from the aggregate through the action of water and may make an aggregate material unsuitable for use in asphalt paving mixes.  Such material is referred to as hydrophilic (water loving).  Siliceous aggregates such as quartzite and some granites are examples of aggregates that may require evaluation of stripping potential.  Aggregates that exhibit a high degree of resistance to asphalt film stripping in the presence of water are usually most suitable in asphalt paving mixes.  Such aggregates are referred to as hydrophobic (water hating) aggregates.  Limestone, dolomite, and traprock are usually highly resistant to asphalt film stripping.  Why hydrophobic or hydrophilic aggregates behave as they do is not completely understood.  The explanation is not as important as the ability to detect the properties and avoid use of aggregates conductive to asphalt stripping.

The moisture susceptibility test used to evaluate HMA for stripping is AASHTO T 283, “Resistance of Compacted Bituminous Mixtures to Moisture Induced Damage.”  This test serves two purposes.  First, it identifies whether a combination of asphalt binder and aggregate is moisture susceptible.  Second, it measures the effectiveness of anti-stripping additives.

In the test, two subsets of test specimens are produced.  Specimens are compacted to achieve an air void content target value of 7.0 +/- 0.5 % with the exception of the S 4.75A mix type, which is 13.0 +/- 0.5 %.  Test specimens should be sorted so that each subset has approximately the same air void content.  One subset is moisture conditioned by vacuum saturation to a constant degree of saturation in the range from 65 to 80 percent.  This is followed by an optional freeze cycle. (NCDOT does not currently require the freeze cycle).  After conditioning both subsets are tested for indirect tensile strength.  The test result reported is the ratio of tensile strength of the conditioned subset to that of the unconditioned subset.  This ratio is called the “tensile strength ratio” or TSR.  NCDOT Superpave Specifications require a minimum TSR of 85 percent, except for base and S 4.75 mm mixes, the requirement is 80%.

4.4.6 Permanent Deformation (Rut Resistance) - One of the major objectives of Superpave Mix Design System is to provide pavements which would be highly resistant to permanent deformation (rut resistance).  As stated earlier, rut resistance is the ability of an asphalt mix to resist permanent deformation from imposed loads.  This is especially important for surface mixes since this is where the wheel loads are concentrated and the potential for rutting is greatest.  The aggregate and binder specifications are established such that a rut resistant mix should be obtained;  however, once a mix has been designed based on the specified criteria, the mix should be physically tested to evaluate the anticipated performance under traffic.  To accomplish this objective the Department will perform rut resistance evaluation on surface mix specimens prepared by the Contractor as a part of the mix design process.

In addition to the required mix design submittal forms, the Contractor will prepare and deliver six (6) Superpave Gyratory Compactor specimens to the Department’s Central Asphalt Laboratory for the following surface mix types: SF 9.5A, S 9.5B, S 9.5C, S 9.5D, S 12.5C and S 12.5D.  The Contractor will prepare these specimens using lab produced mix in accordance with AASHTO T 312 (Modified).  These specimens shall be compacted to a height of 75mm and to a void content (VTM) of 4.0% +/- 0.5%.  These specimens will be tested for rutting susceptibility using the Asphalt Pavement Analyzer in the Materials and Tests Central facility.  The maximum rut depth allowed for the various surface mixes is specified in Table 610-2 of the Specifications (See Figure 4-3).

MIXTURE VOLUMETRIC PROPERTIES AND RELATIONSHIPS

Illustration of VMA in a Compacted Mix Specimen
(Note: For simplification the volume of absorbed asphalt is not shown)

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Diagram Illustrating the Air Voids and Voids in Mineral Aggregate (VMA)
Figure 4.1

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The Contractor is required to design the asphalt mix and to obtain an approved Job Mix Formula (JMF) issued by the Department. A mix design and proposed JMF targets for each required mix type and combination of aggregates must be submitted both in writing and in electronic format to the NCDOT Asphalt Mix Design Engineer for review and approval at least 10 days prior to start of asphalt mix production.

The mix design must be prepared in an approved mix design laboratory by a certified Superpave mix design technician.  The design laboratory must be approved by the Mix Design Engineer prior to submission of the mix design.  The mix design shall be prepared in accordance with AASHTO R 35, “Standard Practice for Designing Superpave HMA” as modified by the Department, recommended procedures in the Asphalt Institute publication "Superpave Series No. 2 (SP-2, 3rd edition) Mix Design Manual” and the latest edition of Department mix design computer programs, policies, procedures, and forms.  The request for the MD/JMF approval will be submitted to the Mix Design Engineer on Form QMS-1 (See Page 4-19) with attached design data, proposed JMF target values, and forms as noted. In addition, the Contractor is required to submit the design data in electronic form using the Department’s mix design program.

The information and data that are required on the mix design are described in detail in Article 610-3 of the Standard Specifications or Project Special Provisions.  When the mix design is submitted, include the original recording charts detailing the TSR results to Asphalt Design Engineer in accordance with Section 7.16 of this Manual.  In addition, when requested by the Mix Design Engineer, the Contractor must submit representative samples of each mix component, including RAP, RAS, mineral filler, asphalt binder, chemical anti-strip additive and hydrated lime to the Department’s mix design laboratory.

In addition, the Contractor will prepare and deliver six (6) Superpave Gyratory Compactor specimens to the Department’s Central Asphalt Laboratory for the following surface mix types: SF 9.5A, S 9.5B, S 9.5C, S 9.5D, S 12.5C and S 12.5D.

4.6   THE JOB MIX FORMULA

 NCDOT Specifications require that all asphalt plant mixes, either virgin or recycled, be proportioned and graded such that they meet the requirements of a job mix formula approved and issued by the Department.  This job mix formula will be based on a mix design performed by the Contractor and approved by the Materials and Tests Asphalt Design Lab. Once the Asphalt Mix Design Engineer has evaluated and/or confirmed the data, the mix design data and request form will then be forwarded to the Pavement Construction Engineer. The mix design and job mix formula target values must be within the design criteria for the particular type of asphalt mixture specified. The source and grades of materials, blend proportions of each of the various aggregates used, specific gravity information, and other applicable data and notes will  be given on the formula.  Specific details on “Master” job mix formula procedures are discussed below.

Once the JMF has been approved and production is ready to begin, the component materials must be combined in such proportions that the completed mixture meets the specification requirements for the particular mix type specified. During production  the materials are heated and blended together in a hot mix asphalt plant such that the mixture is uniformly mixed and coated with asphalt binder.  The mixture is then transported to the roadway where it is spread, finished and compacted to the required grades, thickness and typical section required by the plans and contract.

The job mix formula (JMF) gradation target values will be established within the design criteria specified for the particular type of asphalt mixture to be produced. The JMF  asphalt binder content will be established at the percentage  which will produce voids in total mix (VTM) at the midpoint of the specification design range for VTM, unless otherwise approved.  The formula for each mixture will establish the following: blend percentage of each aggregate fraction, the percentage of reclaimed aggregate, if applicable, a single percentage of combined aggregate passing each required sieve size, the total percentage (by weight of total mixture) and grade of asphalt binder required by the specifications for that mix type (Table 610-2) unless otherwise approved by the Engineer, the percentage and grade of asphalt binder actually to be added to the mixture (for recycled mixtures), the percentage of chemical anti-strip additive to be added to the asphalt binder or percentage of hydrated lime to be added to the aggregate, the temperature at which the mixture is to be discharged from the plant, the required field density, and other volumetric properties.

The mixing temperature at the asphalt plant will be established on the job mix formula. The mixing temperatures will be different depending on which grade of asphalt binder is being used. The mixing temperature is based on the grade of asphalt binder required by the specifications for a specific mix type (Table 610-2),  unless otherwise approved by the Engineer. The normal mixing temperatures for Superpave mixes are as follows unless otherwise requested by the Contractor and approved by the Engineer:

At the end of this section (in the printed version of the 2007 QMS Manual) are examples of the currently approved computer generated mix design forms and supporting mix design data forms for the Contractor's use in preparing and submitting Mix Design/JMF request. The Contractor is required to use and therefore, must obtain from the Department,  at no charge, the Mix Design computer spreadsheet program that will perform the calculations and  generate the completed forms once the appropriate data has been entered. To obtain a copy of this spreadsheet, contact either the Asphalt Mix Design Engineer, M&T Unit, Blue Ridge Rd., Raleigh, NC, phone (919) 329-4060, or the Pavement Construction Section (Room 255), NCDOT, 1 South Wilmington Street, Raleigh, NC, phone (919) 733-3579, or the spreadsheet program can also be downloaded from the Division of Highways/Pavement Section website.

(a) For Type S 4.75A, a minimum of 50% of the aggregate components shall be manufactured material from the crushing of stone.
(b) For Type SF 9.5A, the percent passing the 2.36mm sieve shall be a minimum of 60% and a maximum of 70%.
(c) For the final surface layer of the specified mix type, utilize a mix design with an aggregate blend gradation above the maximum density line on the 2.36 mm and larger sieves.

(b) When Recycled Mixes are used, select the binder grade to be added in accordance with Subarticle 610-3(A).

(c) Volumetric Properties based on specimens compacted to Ndes as modified by the Department.

(d) AASHTO T 283 Modified (No Freeze-Thaw cycle required).  TSR for Type S 4.75A, Type B 25.0 and Type B 37.5 mixes is 80% minimum.

(e) Mix Design Criteria for Type S 4.75A may be modified subject to the approval of the Engineer

Once a mix design for a specified mix type has been approved, and if the Pavement Construction Engineer is in concurrence with the design and proposed target values, the JMF data will be entered into the NCDOT HiCams computer system. The Contractor will then be furnished 2 copies of the approved "Master" JMF with attached copies of the mix design data. This "Master" JMF will be for a specific plant and will serve for all projects on which that given JMF for the specified mix type is to be used. The Contractor will then place one copy of this MD/JMF assembly on file at the asphalt plant QC field laboratory for use by all QMS personnel.  It is suggested that a bulletin board, preferably with a glass enclosure or a durable notebook with transparent plastic sheeting be used for this purpose.  In situations where the JMF is to be used for DOT work and no lab is present, the JMF should be placed on file in the plant control room.

This is the JMF that both the Contractor QC and the DOT QA personnel will be using for producing and testing the mixture, respectively. One should keep in mind that this JMF will possibly be used for a significant period of time and must be kept in a safeguarded manner.  This posted copy will be readily available to all QC/QA personnel and will also serve for all projects until voided or revisions are authorized by the Pavement Construction Engineer or his representative.

When the Contractor is ready to begin producing mixture, he will advise the QA Supervisor which JMF he intends to produce. Inasmuch as there will very likely be several valid JMF's for a given mix type at each plant using different material sources and combinations, the Contractor must use caution to assure that the appropriate materials as required by the formula are being used. In addition, he must assure that the latest version of the formula is being used and the correct JMF number is being recorded on weight tickets. The QA Supervisor will compare his test results with this JMF for compliance with specifications.

As a JMF is revised in the field for whatever reasons, the Pavement Construction Engineer will send to the Contractor an updated copy showing the revisions and the effective date.  The Contractor must make certain that these updated copies are posted in the field lab as quickly as possible and that the voided copies are removed.  (There may be situations where verbal approval is given by the Pavement Construction Section prior to the actual posting of the JMF data).  While it would be desirable to have the valid JMF posted at the plant at all times, it is realized that delays due to mailing will occur.  Verbal approval can be given in these situations but everyone must strive to keep this to a minimum.

Master Job Mix Formulas for the standard mix types covered by the specifications will not be issued directly by the Pavement Construction Engineer for a specific project unless some special circumstance exists.  The JMF data will be entered into the NCDOT HiCAMs computer system and the Resident Engineer will obtain his Project/County specific file copy of the applicable JMF through a computer terminal.  Detailed instructions for this procedure are available through the Pavement Construction Section.

Included in this Manual are examples of both virgin mix JMF's and recycled mix JMF's.  Note that the owner's name, plant location, and plant certification number shown on the JMF are the same as shown on the plant certification certificate.  No project number or county is indicated on the "master" copy; however, this will be indicated on the copy the Resident Engineer obtains through the DOT Computer for his files.  On JMF's the asphalt binder and tack coat suppliers will normally indicate "SPECS.”, meaning that any specification material may be used.  JMF's will indicate a specific anti-strip additive supplier, brand, and rate and must be used unless otherwise approved by the Engineer.

4.8 COMPOSITION OF RECYCLED MIXTURES (JOB MIX FORMULA)

When the Contractor elects to use a recycled mixture on a project, he must submit to the Department's Materials and Tests Unit his proposed mix design and JMF target values in accordance with Article 610-3 of the Standard Specifications (as modified) and this Manual.

The reclaimed asphalt materials (RAP or RAS) shall be tested for the following properties: (1) asphalt content, (2) aggregate gradation, (3) aggregate effective specific gravity, and (4) asphalt viscosity and performance grade (PG) of the RAP asphalt, if more than 25% RAP is proposed.

The gradation of the reclaimed aggregates is analyzed to determine the gradation of the virgin aggregates required. Using the gradation of the aggregate from the RAP material and the new aggregates, the approved design lab will design a combined gradation meeting the specifications. The asphalt content of the RAP material is used to determine the amount of asphalt binder to be added in the recycled mixture.  The performance grade parameters of the asphalt in the RAP material (if more than 25%) will determine the required grade of the additional asphalt binder in the recycled mixture.  The new asphalt binder added to the recycled mix serves two purposes.  It increases the total asphalt content to meet the requirements of the mix and it blends with the aged asphalt in the reclaimed portion of the mix to yield an asphalt meeting the desired specifications.

Reclaimed asphalt pavement may constitute up to 50% of the total material used in recycled mixtures, except mix Types S 9.5D and S 12.5D.  Types S 9.5D and S 12.5D are limited to a maximum of 20% RAP and must be produced using asphalt binder grade PG 76-22.

For all other recycled mix types, when the percentage of RAP is 20% or less of the completed mix, use the same virgin binder PG grade as specified in Table 610-2 for the required mix type. When the percentage of RAP is greater than 20% but not more than 25% of the total mixture, use a virgin binder PG grade which is one grade (both high and low temperature grade) below the grade required in Table 610-2.  For Example: Table 610-2 requires that a PG 70-22 binder be used for an S 9.5C mix, but due to the RAP exceeding 20%, the binder grade to be used would be a PG 64-28 binder.  When the percentage of RAP is greater than 25% of the total mixture, the Engineer will establish and approve the asphalt binder grade.  Should a change in the source of RAP be made, a new mix design and/or job mix formula may be required in accordance with the Specifications.  Any questions concerning this procedure should be directed to the Asphalt Design Engineer at the M&T Lab.

Reclaimed asphalt shingle (RAS) material may constitute up to six percent (6%) by weight of total mixture.  When both RAP and RAS are used, do not use a combined percentage of RAS and RAP greater than 20% by weight of total mixture, unless otherwise approved.  When the percent of binder contributed from RAS or a combination of RAS and RAP exceeds 20% of the total binder in the completed mix, the virgin binder PG grade must be one grade below (both high and low temperature grade) the binder grade specified in Table 610-2 for the mix type.

Samples of the completed recycled mixture may be taken by the Department on a random basis to determine the PG grading on the recovered asphalt binder in accordance with AASHTO M 320. If the grading is determined to be a value other than specified by Table 610-2 for the required mix type, the Engineer may require the Contractor to adjust the grade and/or percentage of additional asphalt binder, and/or the blend of reclaimed material to bring the grade to the specified value.

Once the total asphalt demand has been determined, the amount of new asphalt binder to be added in the recycled mixture is then calculated.  This quantity equals the calculated asphalt demand minus the percentage of asphalt in the reclaimed asphalt pavement.  Trial mix designs are then made using the Superpave mix design procedures to determine the estimated design asphalt content.  The same design criteria are used for recycled mixes as are used with virgin mixes of the same type.

The Job Mix Formula will establish the percentage of reclaimed aggregate, the percentage of each additional aggregate required, a single percentage of combined aggregate passing each sieve size, the total percentage of asphalt binder in the mixture, a single percentage of additional asphalt material to be added, the percentage of chemical anti-strip additive to be added to the additional asphalt material or percentage of hydrated lime to be added to the aggregate, a single temperature at which the mixture is to be discharged from the plant, the required field density, and other volumetric properties. In addition, the Job Mix Formula will establish the blend ratio and percent binder in the RAP.

Should a change in the source of virgin aggregate be made, a new job mix formula will be required before the new mixture is produced.  Should a change in the source or properties of the RAP be made, a new mix design and/or JMF may be required based on the requirements of Article 1012-1 of the Standard Specifications.  See Section 8.3 of this manual.

Samples of the completed recycled asphalt mixture may be taken by the Department on a random basis to determine the performance grading on the recovered asphalt binder in accordance with AASHTO M 320.  If the viscosity is determined to be out of this specified range, the Engineer may require the Contractor to adjust the additional asphalt material formulation and/or blend of reclaimed material to bring the viscosity within the allowable range.

4.9 PROJECT FILE JOB MIX FORMULA PROCEDURES

Job Mix Formulas (JMF) are maintained in the Highway Construction and Materials System (HiCAMS), including revised and voided JMF.  HiCAMS automatically pulls information from the JMF to calculate the quantity of asphalt binder to be paid based upon the quantity of plant mix material placed and JMF in effect at the time the work is performed.  Since copies of those JMF can be obtained at any time, the Resident Engineer is not required to maintain paper copies of the JMF within the project Files.

When a given JMF is revised, the void date will be entered on the voided formula by the Pavement Construction Engineer's office and this date will appear on all copies obtained through the computer after that date.  The new or revised JMF will show the new number assigned and the effective date.  This new JMF will be entered into the computer system and the cycle repeated as noted in the "Master" JMF procedures.  Again, it is critical that the QC technician has the correct JMF number and shows same on his daily reports.  If the JMF is revised, the technician at the plant will be advised of the new JMF number at that time and will note the revised number and date on the copy posted at the plant.  This revised JMF will be used until the Contractor receives and posts the new JMF at the plant.

The instructions for obtaining a computer listing of all Job Mix Formulas issued to a specific asphalt plant are also available through the Departments computer system.  It should be noted that this listing shows all Job Mix Formulas issued to a plant including any "voided" formulas.  Therefore, everyone must be careful to assure that the Contractor is using the most current JMF and not a voided formula.

Note: Job Mix Formulas are now accessable via the NCDOT HiCams computer system.

Note:  NCDOT Mix Composition is by Weight of Total Mixture (See Pg. 42-43 TAI SP-2 3rd Edition)

Example:  6.0% binder is by weight of total mixture.