The modern use of asphalt for road and street construction began in the late 1800s, and grew rapidly with the emerging automobile industry.  Since that time, asphalt technology has made giant strides such that today the equipment and techniques used to build asphalt pavement structures are highly sophisticated.

One rule that has remained constant throughout asphalt's long history in construction is that an asphalt pavement is only as good as the materials and workmanship that go into it.  No amount of sophisticated equipment can make up for use of poor quality materials, poor sampling and testing techniques or poor construction practices.

In 1987, the Strategic Highway Research Program (SHRP) began developing a new system for specifying asphalt materials.  The final product of the SHRP asphalt research program is a new system referred to as “Superpave” which stands for Superior Performing Asphalt Pavements. Superpave is an improved system for specifying component materials, designing and analyzing asphalt mixtures, and predicting pavement performance.  The system includes test equipment, test methods, aggregate and binder specifications and mix criteria based on traffic loading, environmental considerations and other factors. This section of the manual is a discussion of materials used in quality asphalt construction - what they are, how they behave, and how to tell whether or not particular materials are suitable for a paving project.  It is basic information that technicians must have in order to make sound decisions and ensure a quality product is produced.

2.2 GENERAL DESCRIPTION OF ASPHALT PAVING MATERIALS / PAVEMENTS

 Asphalt pavements are composed of three basic components - 1) asphalt binder, 2) aggregates, and 3) air voids.  Aggregates are generally classified into two groups - coarse and fine, and normally constitute 90 to 96 percent by weight of the total mixture.  Asphalt binders are classified by various grading systems and normally constitutes 4 to 10 percent of the total mixture.  Probably the most important but often overlooked component of an asphalt mix is air voids.  In this section of this manual, only asphalt binder, aggregates and other additives are discussed.  Air voids and the role it has in asphalt mixtures and pavement performance will be discussed in later sections of this manual.

 There are many different types of asphalts and many different types of aggregates.  Consequently, it is possible to make different kinds of asphalt pavements.  Among the most common types of asphalt pavements are:

 *  Asphalt concrete (dense-graded hot mix asphalt);
 *  Open-graded asphalt friction course;
 *  Ultra-Thin Bonded Wearing Course;
 *  Asphalt Surface Treatments;
 *  Emulsified asphalt mixes (cold mixes);
 *  Permeable Asphalt Drainage Course
 *  Others, SMA, In-Place Recycled Mixes (both hot and cold)

This manual primarily addresses asphalt concrete (sometimes referred to as “hot mix asphalt” or simply “HMA”).  HMA is a paving material that consists of asphalt binder and mineral aggregate with appropriate air voids.  The asphalt binder, either an asphalt cement or a modified asphalt cement, acts as a binding agent to glue aggregate particles into a dense mass and to waterproof the mixture.  When bound together, the mineral aggregate acts as a stone framework to impart strength and toughness to the system.  The performance of the mixture is affected both by the properties of the individual components and the combined reaction in the system.

2.3 TECHNICIAN RESPONSIBILITIES

The Contractor’s Quality Control technicians and the DOT’s Quality Assurance technicians are responsible for the way asphalt and aggregate materials are handled, stored, sampled, mixed, hauled, placed, and compacted.  They have responsibilities to check such things as material sources, grades, types, temperatures, and moisture contents.  Both must also be fully capable of reviewing  and interpreting mix design data, laboratory test results and specifications, when necessary, as well as being able to perform sampling and testing.

The technician will be unable to perform his job without a working knowledge of the materials from which an asphalt concrete pavement is made, particularly material characteristics and their role in pavement performance.  He must also understand how improper handling of materials can adversely affect their properties and ultimately, their behavior in the finished pavement.  Having such information will give him the confidence to make proper day-to-day decisions and will eliminate the role of guesswork in the job, ensuring that good quality control is maintained.

Materials inspection and control demands accurate and thorough documentation.  Facts, figures, dates, names, locations and conditions are important elements in daily record-keeping.  Experience has taught us over the years that a scrap of information that seems unimportant when recorded can later turn out to be the very piece of information needed to analyze a serious problem.

2.4 ASPHALT MATERIALS

2.4.1 Refining Crude Petroleum

Asphalt is a constituent of petroleum (crude oil).  Most crude petroleum contains some asphalt, and sometimes crude oil may be almost entirely asphalt.  There are some crude oils, however, that contain no asphalt.

Crude petroleum from oil wells is separated into its constituents or fractions in a refinery.  Principally, this separation is accomplished by distillation. After separation, the constituents are further refined or processed into products meeting specific requirements.  Thus it is that asphalt, paraffin, gasoline, lubricating oil, and other highly useful products are the output of an oil refinery, depending on the nature of the crude oil being processed (See Fig. 2-1).

Because asphalt is the base or heavy constituent of crude oil, it does not evaporate or boil off when crude oil is distilled.  Accordingly, asphalt is obtained as a residue or residual product, and is valuable and essential for a great variety of engineering and architectural uses.  Practically all asphalt used in the United States is produced by modern oil refineries and is called petroleum asphalt.

Asphalt is also sometimes referred to as a bituminous material because it contains bitumen, which is a hydrocarbon material soluble in carbon disulfide (CS₂).  Tar obtained from the destructive distillation of soft coal also contains bitumen.  Consequently, both petroleum asphalt and coal tar are jointly referred to as asphaltic materials.  However, petroleum asphalt should not be confused with coal tar because their properties are significantly different.  Petroleum asphalt is composed almost entirely of bitumen, whereas in coal tar the bitumen content is relatively low.  In view of these differences, it is imperative that coal-tar products and petroleum asphalts be considered and treated as entirely separate entities.

Petroleum asphalt for use in pavements is usually called paving asphalt, or asphalt binder to distinguish it from asphalt made for non-paving uses, such as roofing and industrial purposes.

2.4.2 Classification and Grading of Paving Asphalts

Paving asphalts are classified into three general types:

(1) Asphalt Binders,
(2) Cutback Asphalts and
(3) Emulsified Asphalts.

Included in the Standard Specifications and Special Provisions are the specific requirements for the various grades and types of asphalt materials.  Table 2-1, 2-2, and 2-3 included in this manual summarizes the various grades and typical applications of asphaltic materials used in pavement construction by the NCDOT.  However, the technician should always review the project special provisions to determine if there are any additional grades or specific requirements which must be utilized.

Until recently asphalt binders used in North Carolina were graded based on asphalt viscosity.  In the viscosity system, the poise was the standard unit of measurement for absolute viscosity.  AC-20 was the grade in standard hot mix asphalt pavements in North Carolina.  Softer grades were sometimes used in recycled mixes to achieve a combined viscosity of the old asphalt and new asphalt binder which was equivalent to an AC-20.

In 1997 NCDOT began using the SUPERPAVE asphalt “”binder” specifications. These specifications were a major result of the SHRP research program which began in 1987.  The SUPERPAVE “binder” specifications are based on tests which measure physical properties of the asphalt “binder” that can be related directly to field performance by engineering principles.  The tests are conducted at temperatures encountered by in-service pavements. These “binder” specifications have now been adopted by AASHTO and are referenced under AASHTO M 320.

The new system for specifying asphalt binders is unique in that it is a performance based specification.  It specifies binders on the basis of the climate and in-place pavement temperatures at which the binder is expected to serve.  Physical property requirements remain the same, but the temperature at which the binder must attain the properties changes.  For example, the high temperature, unaged binder stiffness (G*/sin ?) is required to be at least 1.00 kPa.  But this requirement must be achieved at higher temperatures if the binder is expected to serve in a hot climate.

Performance graded (PG) binders are designated with grades such as PG 64-22.  The first number, 64, is often called the “high temperature grade.”  This means that the binder would possess adequate physical properties to perform satisfactorily at least up to 64ºC (147ºF).  This would be the high pavement temperature corresponding to the climate in which the binder is actually expected to satisfactorily serve.  Likewise, the second number, -22,  is often called the “low temperature grade” and means that the binder would possess adequate physical properties in pavements to perform satisfactorily at least down to -22ºC (-8ºF).

Additional consideration in selecting the grade to be used is given to the time of loading (vehicle speed on open highway, city streets, intersections, etc.), the magnitude of loads (heavy trucks), and at what level the material is within the pavement structure. Table 2-1 shows the current binder grades in Superpave Specifications (AASHTO M 320). Under SUPERPAVE specifications, the binder grade used in standard hot mix asphalt pavements in North Carolina is Performance Grade 64-22 (PG 64-22). Other grades are required under certain conditions, such as heavy traffic and in recycled mixes.

2.4.3 Asphalt Binder

Asphalt binder at normal atmospheric (ambient) temperatures is a black, sticky, semi-solid, highly viscous material.  Because asphalt binder is sticky, it adheres to aggregate particles and can be used to cement or bind the aggregate in an asphalt concrete mixture.  Asphalt binder is an excellent waterproofing material and is unaffected by most acids, alkalies, and salts.  It is called a thermoplastic material because it softens as it is heated and hardens as it is cooled.  This unique combination of characteristics and properties is a fundamental reason why asphalt is an important paving material.

Paving asphalt binder must be made fluid (liquified) for handling and use during construction operations, such as pumping through pipes, transporting in tanks, spraying through nozzles and mixing with aggregate. During the heating process the asphalt binder temperature must not exceed the manufacturers recommended temperature. If the asphalt binder is overheated, a process known as oxidation will occur. Oxidation causes the asphalt to become more brittle, leading to the term oxidative, or age, hardening.  Oxidation occurs more rapidly at higher temperatures.  A considerable amount of hardening occurs during HMA production, when the asphalt binder is heated to facilitate mixing and compaction.  When pavement construction operations are finished, the asphalt binder cools and reverts to its normal semi-solid condition and functions as the cementing and waterproofing agent that makes the pavement stable and durable.
Asphalt binder can be made temporarily fluid (liquified) for construction operations in two ways:

1   By liquefying with indirect heat: After construction operations, the hot liquid asphalt binder cools and returns from a fluid to a normal, semi-solid condition.

2   By emulsifying the asphalt with water: While asphalt and water ordinarily do not mix, they can be made to mix by churning asphalt in a colloid mill with water and a small amount of emulsifying agent.  The resulting product, called emulsified asphalt, is a fluid and can be handled and sprayed at relatively low temperatures.  After application, the water and asphalt separate.  The asphalt particles coalesce into a continous film that bonds the aggregate particles as the water evaporates.  When the water and asphalt separate, it is said that the emulsion breaks or sets and the asphalt residue remains.

2.4.4 Emulsified Asphalts

 As previously mentioned, heating is one way to liquefy asphalt; however, there are other ways to liquefy asphalt for construction applications.  One method is to emulsify the asphalt  in water.  Asphalt liquefied by this method is known as emulsified asphalt.  With emulsified asphalt, the basic idea is that the water will escape by absorption and evaporation, leaving the asphalt binder to do its job.

 The petroleum asphalt manufacturing flow chart (Pg. 2-4 Printed version of  2008 HMA/QMS Manual) has been extended to show the flow for making liquid asphalt materials.  It is important to note that asphalt binder is the base material that is liquified by emulsifying.

An asphalt emulsion consists of three basic ingredients: asphalt, water, and an emulsifying agent.  On some occasions the emulsifying agent may contain a stabilizer.  In the emulsification process, the asphalt binder is mechanically separated into minute globules and dispersed in water treated with a small amount of emulsifying agent.  The machine used in this process is a colloid mill.  The asphalt globules are extremely small, mostly in the colloidal size range (0.001 – 0.005 in.).

The object is to make a dispersion of the asphalt binder in water, stable enough for pumping, prolonged storage, and mixing.  Furthermore, the emulsion should break down quickly after contact with aggregate in a mixer, or after spraying on the roadbed.  Upon curing, the residual asphalt retains all of the adhesive, durability, and water-resistant properties of the asphalt binder from which it was produced.

By proper selection of an emulsifying agent and other manufacturing controls, emulsified asphalts can be produced in several types and grades.  By choice of emulsifying agent, the emulsified asphalt can be anionic (asphalt globules electro-negatively charged) or cationic (asphalt globules are electro-positively charged) or nonionic (asphalt globules are neutrally charged).  In practice, the first two types are ordinarily used in roadway construction and maintenance activities.  The letter “C” in front of the emulsion type denotes cationic.  The absence of the “C” denotes anionic or nonionic.  For example, RS-1 is anionic or nonionic and CRS-1 is cationic.

Because particles having a like electrostatic charge repel each other, the asphalt globules are kept apart until the emulsion is deposited on the surface of the aggregate particles.  At this point, the asphalt globules coalesce (join together) through neutralization of the electrostatic charges or water evaporation.  Coalescence of the asphalt globules occurs in rapid and medium-setting grades.  When this coalescence takes place, it is referred to as "breaking" or "setting".

Emulsions are further classified on the basis of how quickly the asphalt will coalesce; i.e., revert to asphalt binder.  The terms RS, MS, and SS have been adopted to simplify and standardize this classification.  They are relative terms only and mean rapid-setting (RS), medium-setting (MS), and slow-setting (SS).  The tendency to coalesce is closely related to the mixing of an emulsion.  An RS emulsion has little or no ability to mix with an aggregate, an MS emulsion is expected to mix with coarse but not fine aggregate, and an SS emulsion is designed to mix with fine aggregate.

Additional grades of high-float medium-setting anionic emulsions, designated HFMS, have been added to standard ASTM specifications.  These grades are used primarily in cold and hot plant mixes, coarse aggregate seal coats, road mixes, and tack coats.  High float emulsions have a specific quality that permits a thicker film coating without danger of runoff.

A quick-set type of emulsion (QS) has been developed for slurry seals.  Its use is rapidly increasing as the unique quick-setting property solves one of the major problems associated with the use of slurry seals.

2.4.5 Characteristics of Asphalt Binders

(A) Asphalt’s properties are temperature susceptible – asphalt is stiffer at colder temperatures.
That is why almost every asphalt binder and mixture test must be accompanied by a specified test temperature.  Without specifying a test temperature, the test result cannot be effectively interpreted.  For the same reason, asphalt binder behavior is also dependent on time of loading - asphalt is stiffer under a shorter loading time.  The dependence of asphalt binder behavior on temperature and load duration means that these two factors can be used interchangeably.  That is, a slow loading rate can be simulated by high temperatures and fast loading rate can be simulated by low temperatures.

(B) Asphalt binder is a viscoelastic material because it simultaneously displays both viscous and elastic characteristics. At high temperatures (e.g., > 100°C), asphalt binder acts almost entirely as a viscous fluid, displaying the consistency of a lubricant such as motor oil.  At very low temperatures (e.g., <0°C), asphalt binder behaves mostly like an elastic solid, rebounding to its original shape when loaded and unloaded.  At the intermediate temperatures found in most pavement systems, asphalt binder has characteristics of both a viscous fluid and an elastic solid.

(C) Because asphalt is organic, it reacts with oxygen from the environment. Oxidation changes the structure and composition of the asphalt molecules.  Oxidation causes the asphalt to become more brittle, leading to the term oxidative, or age, hardening.  Oxidation occurs more rapidly at higher temperatures.  A considerable amount of hardening occurs during HMA production, when the asphalt binder is heated to facilitate mixing and compaction.  That is also why oxidation is more of a concern when the asphalt binder is used in a hot, desert climate.

The characteristics of asphalt binder under varying temperatures, rates of loading, and stages of aging determine its ability to perform as a binder in the pavement system.  The tests and specifications used to measure and control these characteristics in the Superpave system are discussed in Performance Graded Asphalt Binder Specification and Testing, Superpave Series No. 1 (SP-1), The Asphalt Institute.

2.4.6. Testing Properties of Asphalts

A key feature in the Superpave system is that physical properties are measured on binders that have been laboratory aged to simulate their aged condition in a real pavement.  Some binder physical property measurements are performed on unaged binder.  Physical properties are also measured on binders that have been aged in the rolling thin film oven (RTFO) to simulate oxidative hardening that occurs during hot mixing and placing.  A pressure aging vessel (PAV) is used to laboratory age binder to simulate the severe aging that occurs after the binder has served many years in a pavement.

 Binder physical properties are measured using four devices:

• dynamic shear rheometer  - controls stiffness at high temperatures
• rotational viscometer  - assures binder can be pumped at 135° C
• bending beam rheometer  - controls stiffness at low temperatures, and
• direct tension tester  - controls resistance to low temperature cracking

2.4.7 Specific Gravity of Asphalt Binder

Specific gravity is the ratio of the weight of any volume of a material to the weight of an equal volume of water, both at a specified temperature.  As an example, an aggregate with a specific gravity of 2.653 weighs 2.653 times as much as water.  Asphalt binder has a specific gravity of approximately 1.030 at 60° F (15.6° C).

The specific gravity of an asphalt binder is not normally indicated in the job specifications.  Nonetheless, knowing the specific gravity of the asphalt binder being used is important for two reasons. Asphalt binder expands when heated and contracts when cooled.  This means that the volume of a given amount of asphalt binder will be greater at higher temperatures than at lower ones.  Specific gravity measurements provide a means for making temperature-volume corrections, which are discussed later.

Specific gravity is usually determined by the pycnometer method (AASHTO T 228).  Because specific gravity varies with the expansion and contraction of asphalt binder at different temperatures, results are normally expressed in terms of Sp. Gr. (Specific Gravity) at a given temperature for both the material and the water used in the test.  (Example: Sp. Gr. 1.023 at 60°/60°F (15.6°/15.6° C) means that the specific gravity of the asphalt binder tested is 1.023 when both the asphalt binder and the water are at 60°F (15.6° C).

2.4.8 Asphalt Additives

(A) Silicone:  Silicone is used in asphalt because of its foam suppressing capabilities and also because it helps prevent the tearing and pulling of an asphalt mix behind the paving machine.  Section 620-2 of the Standard Specifications requires that silicone is to be added to asphalt binder used in all surface course mixtures, including open-graded asphalt friction courses.  The silicone is added at the rate of 1 ounce per 2000-2500 gallons (4 ml per    1000-1250 liters) of asphalt binder and may be added either at the asphalt plant or at the supplier's terminal when so noted on the delivery ticket. The silicone should be adequately circulated throughout the asphalt binder storage tank prior to use. The brand used must have been previously approved by the Department.  A listing of approved sources of silicone may be obtained through the M&T Lab in Raleigh, N.C.

(B) Anti-Strip Additive: Heat stable liquid chemical or hydrated lime anti-strip additives are required to be incorporated into asphalt mixes in an effort to prevent the separation of the asphalt from the aggregate particles (stripping).  Chemical anti-strip additives are blended with the asphalt binder prior to introduction of the binder into the mix. Hydrated lime is blended with the aggregate prior to the aggregate entering the drier.

All mixes including recycled mixes require either chemical or  lime anti-strip additive or a combination of both.  The technician should always refer to the JMF to determine the type, rate required and the brand specified.  The Contractor may use a different brand or grade, provided the proper TSR testing has been performed with satisfactory results prior to its use.  If a different rate is required, a new JMF must be obtained by the Contractor.

2.4.9 Asphalt Binder Storage

The asphalt binder storage capacity at the plant must be sufficient to allow uniform plant operation.  Where more than one grade of asphalt binder is required for a project, at least one tank will be needed for each grade or the tank must be completely emptied before a different grade is added.  Different grades must not be mixed.

Asphalt contents of storage tanks must be capable of being measured so that the amount of materials remaining in the tank can be determined at any time.  This is necessary in order to determine the amount of an additive to be added, when required.  They also must be heated to keep the asphalt fluid enough to move through the delivery and return lines; however, the maximum storage temperature should not exceed the supplier’s recommendation.  Heating is done either electrically or by circulating steam or hot oil through coils in the tank.  Regardless of the heating method used, an open flame must never come in direct contact with the tank or contents.  Where circulating hot oil is used, the oil level in the reservoir of the heating unit should be checked frequently.  A drop in the level could indicate leakage of the hot oil into the tank, leakage which results in contamination of the asphalt.  All transfer lines, pumps and weigh buckets also must have heating coils or jackets so that the asphalt will remain fluid enough to pump.  One or more thermometers must be placed in the asphalt feed line to ensure control of the asphalt temperature, as it is being introduced into the mixer or drum.  The asphalt tanks must be equipped with a circulation system capable of uniformly dispersing and mixing additives throughout the total quantity of asphalt binder in the tank.

Adequate pumps must be furnished so that asphalt binder can be unloaded from tankers and still continue to operate the plant.  A sampling valve or a spigot must be installed in the circulating system or tank to allow sampling of the asphalt.  When sampling from the circulating system, exercise extreme care, as pressure in the lines may cause the hot asphalt to splatter.

Safety:   Asphalt foaming can be a safety hazard, and specifications usually require that asphalt not foam at temperatures up to 350°F (175°C).   In addition, asphalt binder, if heated to a high enough temperature, will flash in the presence of a spark or open flame.  The temperature at which this occurs is well above the temperatures normally used in paving operations. However, to be sure there is an adequate margin of safety, the flash point of the asphalt should be known. The Minimum Flash Point temperature specified for all performance graded asphalts is 446°F (230°C).

2.4.10 Delivery and Acceptance of Asphalt Materials

Obtain Performance graded asphalt binder (PGAB) only from sources participating in the Department’s Quality Control/Quality Assurance (QC/QA) program.  The PGAB QC/QA program is designed to give Producers or Suppliers more responsibility for controlling the quality of material they produce and to utilize the QC information they provide in the acceptance process by the Department.  It requires Producers or suppliers to perform QC sampling, testing, and recordkeeping on materials they ship for use by the Department.  Also, it requires the Department to perform QA sampling, testing, and recordkeeping to confirm the performance of the Producer’s quality control plan set forth in the QC/QA program.

NCDOT specifications (Article 1020-1) require that asphalt materials used in asphalt pavement construction be tested and certified as meeting all applicable specification requirements (AASHTO M 320).  This certification for acceptance purposes is furnished with each delivered load of material, subject to certain conditions outlined in the specifications.

This Article of the specifications also requires that all asphalt transport tankers, rail, and truck tankers must have a sampling valve in accordance with Asphalt Institute Publication MS-18, Sampling Asphalt Products for Specification Compliance and AASHTO T 40 or a comparable device acceptable to the Engineer.  A picture of a typical sampling device is shown in the Figure below. (Pg. 2-10 Printed version of  2008 HMA/QMS Manual)

The sample must be taken from the sampling device on the transport tanker. Sample containers must  be new and are available from the M & T Laboratory.  Glass containers should not be used.  The sample container should not be washed, rinsed out, or wiped off with oily cloths.  The top of the container must fit securely.  In obtaining a sample from the sampling valve, approximately 1 gallon (4 liters) of the asphalt material should be drawn from the valve and discarded for sampling purposes.  The container should then be filled from the valve and the lid securely fastened to the container.  Samples shall not be transferred from one container to another.  The sample should then be forwarded to the Materials and Tests Unit with the appropriate sample identification cards.

This Article also outlines the information that is to be shown on load delivery tickets for all asphalt  materials.  Also included is an example statement of certification forms which must be included on the delivery ticket.  The Contractor must furnish a ticket from the supplier which includes a statement of certification of the grade and amount of asphalt material, including a statement relative to the brand, grade, and quanity or rate of anti-strip additive added to the material. In addition, a separate statement of certification that the tanker was clean and free of contaminating material is required from the transporter on the ticket.  Each certification shall be signed by an authorized representative of the supplier or transporter.  These certifications may be either stamped, written, or printed on the delivery ticket, or may be attached to the delivery ticket.  Failure to include or sign the certifications by either the supplier or transporter will be cause to withhold use of the material until a sample can be taken and tested, except where an alternative testing and invoicing procedure has been preapproved by the Engineer.

It will not be necessary to fill out Materials Received Reports (MRRs) for liquid asphalt (asphalt binder, emulsions).  All liquid asphalt materials will be accepted by certification in accordance with Section 1020-1 of the Standard Specifications and the following procedures.

When a shipment of asphalt binder is received at an asphalt plant, the Contractor’s plant personnel will send a copy of the bill of lading to the QA Supervisor.  The QA Supervisor will attach the bill of lading to the appropriate QC-1 report from that plant and maintain same in a separate file for that plant.  When a shipment of emulsified or  asphalt is recieved at the asphalt plant or on a project, a copy of the bill of lading will be sent to the QA Supervisor who will attach it to the appropriate QC-1 report from that plant and maintain same in a separate file for that plant.

M&T Unit representatives will take verification samples from the asphalt terminals which will be logged in and tested at the M&T central facility with results entered into a Liquid Asphalt Database.

If a sample fails but the failure is considered by the Chemical Testing Engineer to be immaterial, the terminal will be notified of the test results and allowed to continue shipping, provided corrective action is taken.  Samples will continue to be taken at the normal frequency.

If a sample fails and the failure is considered by the Chemical Testing Engineer to be significant, the terminal will be notified of the results and they will be instructed to discontinue shipments and take corrective action.  M&T will resample and retest the material at the terminal.  Any materials from this batch in a Contractor’s storage tank will be evaluated for acceptability.

In the case of a significant material failure, the Chemical Testing Engineer will send a failure notification form to all QA Supervisors.  The QA Supervisors will review the bills of lading in their files to determine if they have recieved any material from that batch.  If so, they will notify the appropriate Resident Engineers.   They will then review the appropriate QC records for any possible related test deviations.  The failure notification form will include an investigation section to be filled out by the QA Supervisor.  They should include information concerning test deviations and any actions they took concerning or involving the Resident Engineers on this form and attach it to the appropriate bill of lading and QC-1 report in their file and send a copy to the Chemical Testing Engineer.

Resident Engineers will not be receiving direct notification of failures from the Chemical Testing Engineer because there is no way he can determine who should receive the notifications.  By sending these notifications to the QA Supervisors, a relatively small number of forms can be sent out and the appropriate Resident Engineers will be notified by the QA Supervisors.

All actions taken by the Chemical Testing Engineer, QA Supervisors and Resident Engineers will be noted in the database summary.

2.4.11 Temperature-Volume Relationships of Asphalts

As with all liquids and most solids, asphalt expands when heated and contracts when cooled.  These changes in volume must be taken into consideration because, regardless of the temperature at which asphalt is shipped and stored, the basis for buying and selling asphalt materials, for making asphalt plant settings and mix design calculations is the asphalt's volume and specific gravity at 60°F (15.6°C).  For example, when 5,000 gallons (19,000 liters) of asphalt at 300°F (149°C) is loaded into a tanker at a terminal for sale and delivery to an asphalt plant, the volume at 60°F (15.6°C) may need to be calculated and recorded for various purposes.

The calculation involved is rather simple.  It requires that  two pieces of information be known:

*  The temperature of the asphalt when used.
*  The asphalt specific gravity or Group No. @ 60°F (15.6°C).

The asphalt temperature and specific gravity are used to locate the proper correction factor on one of the following tables.  These tables have been in use for at least three decades and are the only data currently available for temperature corrections above 300°F (149°C).  (See Table 2-4)

When the Technician knows the asphalt temperature and the necessary correction factor, the following formula is used to calculate the asphalt volume at 60°F (15.6°C):

The following example illustrates how the calculation is made.

A truck has just delivered 5,000 gallons (19,000 liters) of asphalt at a temperature of 300°F (149°C).  The Specific Gravity (Sp.Gr.) of the asphalt is 0.970.  What would the asphalt's volume be at 60°F (15.6°C)?

Because the asphalt Specific Gravity is above 0.966, the tables for Group O (TABLE 2-4) are used to find the correction factor.  For 300°F (149°C), the correction factor listed is 0.9187.

The volume of the particular asphalt at 60°F (15°C) is therefore, 4,594 gallons (17,455 liters).

V₆₀ = Volume at 60°F (15.6°C)
V_t = Volume at given temperature
CF = Correction Factor from TABLE

2.5 MINERAL AGGREGATES

2.5.1 Introduction

The amount of mineral aggregate in asphalt paving mixtures is generally 90 to 96 percent by weight and 75 to 85 percent by volume.  Mineral aggregate is primarily responsible for the load supporting capacity of pavement.  Asphalt pavement performance is also heavily influenced by aggregate characteristics and properties.

Mineral aggregate has been defined as any hard, inert mineral material used for mixing in graduated particles or fragments.  It includes sand, gravel, crushed stone, slag, rock dust or powder.  Aggregates may also include recycled materials, such as reclaimed asphalt pavement (RAP) or reclaimed asphalt shingle material (RAS).

2.5.2. Sources of Aggregates

Aggregates for asphalt paving are generally classified according to their source or means of preparation.  They include natural aggregate (pit or bank-run aggregates), processed aggregates (from quarries), synthetic or artificial aggregates (manufactured), and recycled aggregates.

(A) Natural Aggregates:  Gravel and sand are natural aggregates and are typically pit or bank-run (river deposits) material.  Exposed rocks are eroded and degraded by many processes of nature, both physical and chemical.  The products of the degradation processes are usually moved by wind, water, or moving ice, and deposited as a soil material in various land forms.

Gravels are widely distributed but deposits are rarely found without some proportion of sand and possibly silt.  Sandy soil mixtures frequently have some clay and silt.  Beach sands, some of which are now far inland, are a uniform sized material, but river sand often contains large amounts of gravel, silt, and clay.  Both gravels and natural sands usually consist of smooth, rounded particles due to the tumbling action of water.  Gravels are usually washed to remove undesirable materials and then screened to proper size before use.  Local sands are normally used without any special preparation other than the way they are removed from the pit; however, screening may be required to remove undesirable materials, such as clay balls and debris.

(B) Processed Aggregate:  Processed aggregate includes both quarried aggregate and natural gravel or stone that has been crushed and screened to desired sizes.  Natural gravel is usually crushed to make it more suitable for use in asphalt paving mixtures and to meet specification requirements for fractured faces.  The quality may be improved by crushing, which changes the surface texture of the particles, changes the rounded particle shapes to angular shapes, and improves the distribution and range of particle sizes.

Crushed stone results from crushing fragments of bedrock or large stones, with all the aggregate particles having fractured faces.  In the manufacture of crushed stone, solid ledges of bedrock are broken up in a quarry by blasting and further reduced in size by rock crushers.  The crushed product is then screened to produce desired sizes of aggregate.  Screenings are a by-product of the stone crushing, washing and screening operation and are desirable for use in asphalt mixtures, provided they meet specification requirements.

(C) Synthetic or Artificial Aggregates:  Aggregates resulting from the modification of materials, which may involve both physical and chemical changes, are sometimes called synthetic or artificial aggregates.  They may take the form of the by-product that is developed in the refining of ore, or those specially produced or processed from raw materials for ultimate use as aggregate.
 Blast-furnace slag is the most commonly used artificial aggregate.  It is the by-product of the smelting or iron in blast furnaces.  It is nonmetallic and floats on molten iron.  It is drawn off at intervals and reduced in size either by quenching in water or by crushing if after it has air-cooled.

Manufactured aggregates are relatively new in asphalt paving.  Typically, they are lightweight and have unusual resistance to wear.  Their use is often preferred in bridge-deck paving and in surface layers of asphalt pavements where a high degree of skid resistance is required. These aggregates are manufactured by a firing process and are usually made from clay, shale, slate, processed diatomaceous earth, volcanic gasses, expanded slag, and other like materials.  They are produced and marketed under a variety of trade names.  Flyash and bottom-ash are similar synthetic aggregates.  These are produced as by-products from the burning of coal in power plants and other coal burning processes.  A listing of approved sources of aggregates may be obtained through the M&T Lab in Raleigh, N.C.

(D) Recycled Aggregates:  These are salvaged aggregates obtained from the reclaiming of existing pavements (both asphalt and concrete), from waste shingle manufacturing material, or from other sources.  Normally, recycled aggregate from pavements are obtained by milling an existing pavement or by breaking up the pavement and then processing the material through a crusher. Waste shingle material is obtained by processing  manufacturing waste by grinding and screening to acceptable sizes.

2.5.3 Evaluating the Quality of Aggregates

2.5.3(a)  Aggregate Gradation

To specify aggregate gradation, Superpave uses the 0.45 power gradation chart with control limits and a restricted zone to specify the mix gradation limits and to develop a design aggregate structure.   A Superpave design aggregate structure must pass between the control points while avoiding the restricted zone.  The maximum density gradation is drawn from the 100 percent passing the maximum aggregate size through the origin.  Maximum aggregate size is defined as one size larger than the nominal maximum aggregate size.  Nominal maximum size is defined as one size larger that the first sieve size to retain more than 10 percent.  The design aggregate structure approach ensures that the aggregate will develop a strong, stone skeleton to enhance resistance to permanent deformation while achieving sufficient void space (VMA) for mixture durability.  Standard sizes of coarse and fine aggregate are shown in Table 1005-1 of the Standard Specifications (See Table 1005-1 at the end of this Section).

Selecting an aggregate material for use in an asphalt pavement depends upon the availability, cost, and quality of the material, as well as the type of construction that is intended.  Mineral aggregates play a key role in HMA performance.  Two types of aggregate properties are specified in the Superpave system: source properties and consensus properties.

2.5.3(b)  Source Properties

Source properties are those which are often used to qualify local sources of aggregate.  These tests must be completed prior to allowing the use of any particular aggregate in a HMA mix.  These properties are determined on the individual components rather than the aggregate blend.  The source properties are:

• toughness,
• soundness, and
• deleterious materials

(1) Toughness:  Toughness is the percent loss of materials from an aggregate blend during the Los Angeles Abrasion test.  The procedure is stated in AASHTO T 96, “Resistance to Abrasion of Small Size Coarse Aggregate by Use of the Los Angeles Machine.”  This test estimates the resistance of coarse aggregate to abrasion and mechanical degradation during handling, construction, and in-service performance.  The test is performed by subjecting the coarse aggregate, usually larger than 2.36 mm, to impact and grinding by steel spheres.  The test result is percent loss, which is the weight percentage of coarse material lost during the test as a result of the mechanical degradation.  Maximum loss values typically range from approximately 35 to 55 percent.  According to current  NCDOT specifications, the maximum abrasion loss allowed on an aggregate for use in Superpave mixes is 55%.

(2) Soundness:  Soundness is the percent loss of materials from an aggregate blend during the sodium sulfate soundness test.  The procedure is stated in AASHTO T 104, “Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate.”  This test estimates the resistance of aggregate to weathering while in-service.  It can be performed on both coarse and fine aggregate.  This test is performed by alternately exposing an aggregate sample to repeated immersions in saturated solutions of sodium sulfate each followed by oven drying.  One immersion and drying is considered one soundness cycle.  During the drying phase, salts precipitate in the permeable void space of the aggregate.  Upon re-immersion the salt re-hydrates and exerts internal expansive forces that simulate the expansive forces of freezing water.  The test result is total percent loss over various sieve intervals for a required number of cycles.  Average loss values range from approximately 10 to 20 percent for five cycles.  According to current NCDOT specifications, the maximum loss allowed for use in Superpave mixes is 15% during a 5-cycle period using sodium sulfate.

(3) Deleterious Materials: Deleterious materials are defined as the weight percentage of contaminants such as shale, wood, mica, and coal in the blended aggregate.  This property is measured by AASHTO T 112, “Clay Lumps and Friable Particles in Aggregates.”  It can be performed on coarse and fine aggregate.  The test is performed by wet sieving aggregate size fractions over prescribed sieves.  The weight percentage of material lost as a result of wet sieving is reported as the percent of clay lumps and friable particles.  A wide range of maximum permissible percentage of clay lumps and friable particles is evident.  Values range from as little as 0.2 % to as high as 10 %, depending on the exact composition of the contaminant.  Current NCDOT specifications allow a maximum of 0.3% in Superpave mixes.

2.5.3(c)  Consensus Properties

 Once the aggregate sources have been selected and the source properties approved for use in Superpave mixes, an aggregate blend will be determined.  The aggregate blend will consist of the percentages needed to meet the Job Mix Formula.  Once the blend is determined, the following consensus properties will be analyzed to determine if the blend conforms to NCDOT requirements.

Consensus properties are those properties which are critical in achieving high performance HMA.  These properties must be met at various levels depending on traffic level and level (position) within the pavement.  High traffic levels and surface mixtures (i.e., shallow pavement position) require more strict values for consensus properties.  These properties are determined on the aggregate blend rather than individual components.  They are:

• Coarse Aggregate Angularity (Fractured Faces)
• Fine Aggregate Angularity
• Flat and Elongated Particles, and
• Clay Content (Sand Equivalent).

(1) Coarse Aggregate Angularity (Fractured Faces): This property ensures a high degree of aggregate internal friction and rutting resistance.  It is defined as the percent by weight of aggregates larger than 4.75 mm with one or more fractured faces.

Superpave specifies that coarse aggregate angularity be determined in accordance with ASTM D 5821.  This involves manually counting particles to determine fractured faces.  A fractured face is defined as any fractured surface that occupies more than 25 percent of the area of the outline of the aggregate particle visible in that orientation. The Superpave Specifications include  the minimum requirements for coarse aggregate angularity for each mix type.

(2) Fine Aggregate Angularity:  This property ensures a high degree of fine aggregate internal friction and rutting resistance.  It is defined as the percent air voids present in loosely compacted aggregates smaller than 2.36 mm.  Higher void contents mean more fractured faces.

The test procedure used by Superpave to measure this property is AASHTO TP 33 (Method A).  In the test, a sample of fine aggregate is poured into a small calibrated cylinder by flowing through a standard funnel.  By determining the weight of fine aggregate (W) in the filled cylinder of known volume (V), void content can be calculated as the difference between the cylinder volume and fine aggregate volume collected in the cylinder.  The fine aggregate bulk specific gravity (Gsb) is used to compute fine aggregate volume. The Superpave Specifications  include  the minimum requirements for fine aggregate angularity (uncompacted void content) for each mix type.

(3) Flat and Elongated Particles: This characteristic is the percentage by weight of coarse aggregates that have a maximum to minimum dimension of greater than five.  Elongated particles are undesirable because they have a tendency to break during construction and under traffic.  The test procedure used is ASTM D 4791 (Section 8.4), “Flat and Elongated Particles in Coarse Aggregate” and it is performed on coarse aggregate larger than 4.75 mm sieve.

The procedure uses a proportional caliper device to measure the dimensional ratio of a representative sample of aggregate particles.  The aggregate particle is first placed with its largest dimension between the swinging arm and fixed post at position A.  The swinging arm then remains stationary while the aggregate is placed between the swinging arm and fixed post at position B.  If the aggregate passes through this gap, then it is counted as a flat or elongated particle.  Two values are measured: percentage of flat particles and percentage of elongated particles. The Superpave Specifications state that the maximum allowed Flat and Elongated Particles is 10% by weight at a 5:1 ratio for all mix types except S 4.75A, SF 9.5A, and S 9.5B
.

(4) Clay Content (Sand Equivalent):  Clay content is the percentage of clay material contained in the aggregate fraction that is finer than a 4.75 mm sieve.  It is measured by AASHTO T 176, “Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalent Test.”

In this test, a sample of fine aggregate is placed in a graduated cylinder with a flocculating solution and agitated to loosen clayey material into suspension above the granular aggregate.  After a period that allows sedimentation, the cylinder height of suspended clay and sedimented sand is measured.  The sand equivalent value is computed as a ratio of the sand to clay height readings expressed as a percentage. The Superpave Specifications include the minimum sand equivalent requirements for each mix type.