Chapter 2: Materials

1.0 Material Types

This chapter covers important aspects of the materials requirements for the preventive maintenance treatments described in subsequent chapters.  It includes aspects of the materials’ functions and how they should be stored, handled, and combined to achieve desired outcomes and meet required specifications.

Materials are important to the efficiency and effectiveness of maintenance treatments.  Most materials used in maintenance treatments are covered in an agency’s specifications. This section discusses the primary materials, their composition, manufacturing, storage and handling techniques, and special application requirements.  In some cases, the materials themselves are derived from a mixture of raw materials.  This section will also address some of those issues.

The two main materials comprising maintenance treatments are binder and aggregate.  Typical binders could include:

• Standard Paving Asphalt
• Performance Grade (PG)
• Viscosity Grade (AC)
• Aged Residue Grade (AR)
• Penetration Grade (Pen)
• Asphalt Emulsion
• Polymer Modified Asphalts, including performance-based asphalts (PBA grades)
• Asphalt Rubber and Modified Binder (MB) grades

Aggregates are typically drawn from a range of geological types and vary geographically.  The general requirements, gradings and physical properties are covered in an agency’s specifications.

1.1 Standard Paving Asphalt

1.1.1 What is Standard Paving Asphalt?

Conventional paving asphalt is a complex hydrocarbon mixture derived from the refining of crude oil.  The crude type and the processing method have a significant effect on the final physical properties (1).

1.1.2 How is Standard Paving Asphalt Manufactured?

• Steam Distillation:  Steam distillation begins with the desalting and de-waxing of the crude oil.  Once completed, the crude is heated to approximately 300°C (572°F) (1).  A furnace is then used to heat the crude to 400°C (752°F), and the heated crude is continuously delivered to the flash zone of the atmospheric tower.  The material is separated (by its boiling point) with the most volatile components rising to the top and the less volatile escaping on the sides of the tower.  The residue in the tower is stripped using steam to remove volatiles.  In some very heavy crude oils, this residue may be suitable asphalt.

• Straight Run/Blending:  Vacuum tower residue may be suitable as paving asphalt or it may require blending with other feedstocks, fluxes from the vacuum tower, or from other parts of the process such as solvent de-asphalting.

• Solvent Refining:  Solvent refining takes advantage of the varying solubility of different asphalt fractions.  A short chain hydrocarbon (usually propane) is injected into the asphalt-rich fraction and precipitates asphalt fractions out, as they are not soluble in the hydrocarbon.  The intent is to remove aromatic oil fractions from the asphalt for other uses, such as extender oils and solvents.  Propane precipitated asphalt (PPA) is an aromatic rich fraction that may be used as asphalt alone, although it often exhibits tenderness. (Tenderness is a behavior that occurs during compaction or within a few weeks if plastic deformation is exhibited.)  It can also be blended with other straight-run fractions.

• Air Blowing:  Air blowing has been used to harden asphalt to create higher viscosity grades.  This process may be done continuously or in a batch process.  However, this has often led to asphalts with poor aging resistance.  Some processes have been developed whereby light blowing is employed to modify feedstocks of specific composition to create multi-grade asphalts of low thermal susceptibility.

The properties of the asphalt produced will depend on its chemical composition, the crude type, and the processing method used.  Specification is the key to producing asphalts that will perform well in the field.

1.1.3 Specification of Conventional Asphalts

Until Superpave introduced the PG grading system, asphalt cements (AC) were either penetration or viscosity graded.  Most states used the penetration system until the mid-1970s. 

One problem with the AC graded systems is that it is based on empirical tests.  Empirical specifications rely solely on practical experience and observations without regard for pavement performance theory.  Therefore, the specification is based on the results from a given situation.  Once the conditions change, the results may no longer be the same.  The penetration test is a measure of asphalt stiffness, but the stiffness requirements were gained through experience.  If conditions change, the stiffness requirements may no longer be accurate.  The accuracy will not be known until results are obtained under the new conditions.

Another drawback of the AC graded system is that long-term asphalt aging is not taken into consideration.  The tests are performed on unaged or ”tank” asphalt and on artificially short term aged asphalt to simulate construction aging.  No tests are performed to simulate in-service aging, which occurs when the asphalt undergoes oxidation, i.e. reacts with the oxygen in the atmosphere.

A variation of the AC viscosity grading system is the aged residue viscosity grading system, or AR viscosity grading system.  The AR viscosity grading system characterizes asphalt using aged residue from the rolling thin film oven, RTFO.  Like the AC viscosity grading system, the consistency of the RTFO-aged asphalt is characterized based on viscosity at 60 °C (140°F).  Additional specifications include a minimum penetration and minimum viscosity at 25 °C (77°F) and 135 °C (275°F), respectively.  The AR viscosity grading system has been used primarily by western states.

Asphalt is specified by:

• Consistency (or viscosity)
• Aging characteristics
• Purity
• Safety

Special knowledge is required to assess and evaluate asphalt properties.  While this chapter discusses basic asphalt properties, the intent is to provide only an appreciation of the subject and the complexities involved with asphalt grading. Table 1 provides the specification for various AR grades, whereas the following paragraphs provide additional details regarding this specification:

• Viscosity:  Viscosity describes the fluidity of asphalt at a given temperature under a given rate of shear.  The viscosity will vary depending on the conditions at the time of the test due to asphalt’s viscoelastic properties.  At temperatures of around 60°C (140°F), unmodified asphalt behavior becomes less shear-dependent than at lower temperatures.  For this reason, many specifications require that dynamic viscosity be measured at 60°C (140°F).  Kinematic viscosity is often used for specifications at higher temperatures 135°C (275°F).

Another consistency test used is penetration.  Penetration involves the insertion of a standard needle into an asphalt sample under a standard weight (shear force) over a standard time.  The amount that the needle penetrates at 25°C (77°F) loaded with a 100g (3.5 oz) weight over 5 seconds is normally used.  Lower or higher temperatures can be used to provide an indication of the temperature susceptibility of asphalt. 

The relationship between penetration grade and viscosity grade asphalt is given in Figure 1.  For example, a penetration grade 40-50 has viscosity characteristics similar to an AC-40 or AR 16000.  In AR graded asphalts, the aging process increases the viscosity relative to un-aged material.  Different asphalts age at different rates depending on the crude source and processing method.  The initial un-aged properties will vary in applications not involving hot mix plants, i.e., emulsions.

Figure 1: Relationships Between Asphalt Grades by Viscosity (3)

Note: Figure 1 above shows the relationship between grades of asphalt in general viscosity terms. In no way should it be interpreted to determine actual viscosity values.

Other consistency tests include the Ring and Ball Softening Point Test and the Ductility Test. The Ring and Ball Softening Point Test determines the temperature at which the weight of a ball bearing pushes through a circular sample of material. This is a measure of the temperature at which the penetration is approximately 800. Ductility is the elongation of a sample at a set temperature and set strain rate. Ductility indicates the colloidal stability or internal cohesion of the asphalt.

• Aging Characteristics: Aging in asphalt is associated with oxidation and hardening of the asphalt. During processing and while in service in the pavement, asphalt will lose volatile materials. The loss of this volatile material can lead to hardening. Aging is one cause of asphalt failure as the material becomes brittle, shrinks, and cracks. Specification of short-term aging characteristics is usually determined after exposure to air in a thin film oven or rolling thin film oven. Aging during service life is measured using a pressure aging vessel test (AASHTO T-49). In the AR specification, the properties of the asphalt are measured after these aging tests are performed.

• Purity and Safety: Purity tests are based on the solubility of the asphalt in a solvent such as Trichloroethane, the most commonly used. Safety tests are based on the flammability or flash point of the fumes emitted by asphalt during heating. The flash point for asphalt binder material can be determined using the Cleveland Open Cup test.

Aged Residue (AR) asphalts are used in all conventional mixes. The higher the designation number, the higher the in-service viscosity. For example, AR 4000 has a higher in-service viscosity than AR 2000. The higher the designation number, the harder and stiffer the asphalt. Stiffer asphalt produces higher stability in mixes and resistance to deformation. Conversely, stiffer asphalt may result in lower resistance to cracking.

The AC system’s tests do not cover the temperature extremes that a pavement endures. Binders that produce similar results at the temperature used for penetration and viscosity testing may have very different results at other temperatures experienced by the pavement.  For example, the three binders shown in Figure 2 below (1, 2, 3) all meet the same AC grade specification; therefore, each may erroneously be expected to have the same characteristics during construction and the same performance during hot and cold weather conditions.

The chemistry of asphalts at the molecular and intermolecular levels may help explain the variation of performance characteristics.  There are over tens of thousand of different molecular variations that may be present in asphalts and having to measure the properties would be an impractical task.

Figure 2: Different Temperature Response of Same AC Grades

1.1.4 Changing Times

Penetration and viscosity tests were developed in an era of significantly lower pavement loading.  In the past, truck weights were around 72,000 pounds with tires at 75 psi.  Today, truck weights exceed 80,000 pounds with 125 psi radial tires.

That 10% increase in truck weights yields a 40% increase in the stresses applied to the pavement, not to mention the increase in the number of trucks on the road.  With such changes in road conditions, past experience is no longer sufficient to establish asphalt grading.

1.1.5 Performance Driven Binders

As part of the Superpave research, a new asphalt binder specification was devised.  Grading based on viscosity and penetration has been replaced with a performance graded (PG) system.  No longer are the tests empirical.  The PG specification uses tests to measure physical properties that can be directly related to field performance by engineering principles.

PG binders are designed to resist pavement rutting, fatigue, and low temperature cracking for the range of temperatures under which they are expected to perform. PG 58-28 is a commonly specified PG binder in many states.  In this case, the number 58 refers to the average seven-day high pavement temperature (Degrees Celsius, 58°C = 136°F) to which the pavement is expected to be subjected.   The number -28 refers to the minimum air temperature (Degrees Celsius, -28°C = -18°F) under which the pavement will be required to perform.  

By matching the PG binder to the expected pavement temperature and loading conditions, the following occurs:

• Rutting is reduced by ensuring the binder remains stiff when exposed to high temperatures and heavy traffic loading conditions.  To account for heavy traffic loading conditions, a stiffer binder should be specified by “bumping” the high temperature grade.  For example, a PG 64-22 normally specified for expected pavement temperatures would be “bumped” to a PG 70-22 to account for heavy traffic loading conditions.
• Fatigue cracking is reduced by ensuring the binder remains elastic enough to expand and contract under cyclic traffic and thermal loading throughout the temperature range.
• Low-temperature cracking is reduced by ensuring the binder remains flexible enough when exposed to low temperatures.  However, for single course overlays, the lower temperature requirement may be modified since cracks, including thermal cracks, in the existing pavement will reflect through to the surface even if the appropriate low temperature is specified.  For example, a PG 64-28 is required, but when placing a single course overlay, a PG 64-22 may be cost effective since reflective cracks will occur with either binder.

PG binders are tested under conditions that are similar to the three critical stages of a binder’s life.  The binder is tested for the first stage of transport, storage, and handling.  A rolling thin film oven is used to process the binder for the second stage, mix production and construction, by exposing binder films to heat and air that approximate exposure during mixing and laydown conditions. 

For the third stage, long-term aging, the binder is aged in a pressure aging vessel.  The pressure aging vessel exposes samples to heat and pressure to simulate years of in-service aging of a pavement.  Characteristic PG binders are shown in Table 2 and PG test methods are shown in Table 3.

Table 2: Performance Graded Binders

Table 3: PG Test Methods

1.1.6 Modified PG Binders

Some PG binders may require modifiers, such as polymers, to meet low and high temperature requirements.  Although modifiers may affect many properties, the majority of modifiers attempt to decrease the temperature dependency and oxidation hardening of asphalt mixtures.  A rule of thumb to determine whether a given grade will typically require some type of modifier is based on the working temperature range.  For example, a PG 64-22 has a working range of 86 °C  [64+22 = 86].

Most crude oils have a working range of approximately 90°C, whereas higher quality crude oils can achieve a maximum temperature differential of approximately 92°C.  Many agencies recommend polymer-modification for any PG grade with a temperature differential of 92°C or greater.

Unmodified PG binders should cost about the same as comparable AC grades, but modified binders can increase the cost of an HMA.  As such, modified binders should not be prescribed without careful consideration of the benefits and costs associated with their use.

1.2 Asphalt Emulsions

1.2.1 What is an Emulsion?

Normally, a suspension made up of small drops of one liquid with another liquid will not mix.  If a suspension is properly mixed, either by mechanical agitation or by chemical processes, an emulsion forms. Unstable emulsions will separate with time or temperature.  Stable emulsions will not separate.

Because an emulsion is a dispersion of one immiscible phase in another, an asphalt emulsion is asphalt dispersed in water.  This is not a solution, because the two phases (oil in water) are susceptible to separation.  So, like a good salad dressing, the oil is stabilized with an emulsifier to keep it dispersed.  Figure 3 shows an emulsion in schematic and an emulsion micrograph.

The process of returning from this dispersed form to the asphalt form is called “breaking” (4).  The process by which the asphalt expels water and dries to an integral film or layer on the aggregate is called “curing” (4).  The mechanisms associated with breaking and curing are covered in this chapter, Section 1.2.4.

Emulsions allow the formation of an asphalt binder with low enough viscosity for easy application.  The dispersion in water gives the asphalt many of the properties of water such as low viscosity, lower temperature requirements for both application and storage, and less sensitivity to application on damp surfaces.

1.2.2    How are Emulsions Made?

Asphalt is semi-solid at ordinary temperatures 10°C to 60°C (50°C to 140°F).  To make an emulsion, the asphalt must be sheared into small droplets and coated/reacted with a chemical stabilizer or emulsifier.  Figure 4 shows the cross section of a typical colloid mill, the device that is used to shear the asphalt.  It should be noted that there are other methods of shearing the asphalt to produce an emulsion.  These include homogenizers, pressure reducers (venturi effect) and pumps.  The colloid mill is the most commonly used to produce asphalt emulsions.  The chemical emulsifier solution, also known as the soap solution, is combined with the asphalt and introduced into a gap between a high-speed rotor and a stator (or other rotor rotating at a lower speed).  The resulting shear breaks the asphalt particles down to the required size.  The geometry of the sheared particles affects the particle size distribution, which in turn affects the properties of the emulsion (7).

In an emulsion plant, several operations must be carried out.  Figure 5 shows the key elements of an emulsion plant.

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Figure 4: Colloid Mill Cross Section (4)

Figure 5: Emulsion Plant Operations Schematic (5)

The asphalt must be stored correctly and at the right temperature.  For normal operation (non- polymer binder emulsification), the storage temperature will range from 135 to 140°C (275 to 284°F).  For polymer-modified binders, the storage temperature will range from 160 to 170°C (320 to 338°F).  If higher temperatures are required, the colloid mill must be operated under back pressure, about 2 to 3 bar (29 to 43 psi) and a heat exchanger on the mill outlet is required to ensure that the emulsion is cooled to below boiling temperatures before the back pressure is reduced to atmospheric pressure.  If the backpressure and heat exchange operations are not properly carried out, the emulsion will boil and be destroyed.

The soap solution (emulsifier solution) preparation is required because the emulsifiers usually need to react with a base or acid to create the surface-active or emulsifying form (salt).  This may be done in a continuous fashion or in a batch fashion.  Because the reactions are between an acid and an alkaline emulsifier (cationic systems), or an alkaline chemical and an acid emulsifier (anionic systems), the pH of the soap solution and the pH of the resulting emulsion are key factors in the quality of the emulsion.

In many cases, additives for emulsion stability or modification are introduced.  The most common method of modifying an emulsion is by adding rubber latex, synthetic or natural.  The latex is either introduced via the soap solution or directly injected into the mill via the soap line.  This method has the advantage that no heat exchanger or pressure operation is required.

1.2.3 What Are Emulsifiers and What Types of Emulsions Are Used?

Emulsifiers in their neutralized state may have a negative charge (anionic), a positive charge (cationic), or no charge (nonionic) (4).  The exact chemistry and type of emulsifier determines the application of the finished emulsion.  Other factors that determine physical and application characteristics include pH of the emulsion, the binder content, the particle size and distribution, and the compatibility with the aggregate sources.

Anionic emulsifiers are based on fatty acids.  These are reacted with a base such as caustic soda to produce an acid salt.  This acid salt is the active emulsifier (see Figure 6).  The emulsifier has a long fatty chain that is soluble in the asphalt and a polar head that provides a surface charge (see Figure 7).  Repulsion created by these charges allows stabilization of the emulsion.

Figure 6: Chemical Structure of an Anionic Emulsifier (6)

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Figure 7: Anionic Emulsified Asphalt Particle (6)

Cationic emulsifiers are based on amines of various types.  The exact type used will depend on the application.  Some typical types include quaternary ammonium compounds (slow set), fatty diamines (rapid set), amidoamines (quick set), and immidazolines (microsurfacing).  These are reacted with an acid such as hydrochloric acid to produce a salt, which is an active emulsifier.  The emulsifier has a long fatty chain that is soluble in the asphalt and a polar head that provides a surface charge.  Repulsion created by these charges allows stabilization of the emulsion as may be seen in Figures 8 and 9.

• Nonionic emulsifiers are amphoteric; that is, at low pH they have a positive charge and can be cationic, at high pH levels they can have a negative charge and can be anionic emulsifiers.

The emulsifier type and concentration determines the emulsion’s performance.  The emulsifier type may control the break period.  More rapid break times equal higher charge and lower concentrations of emulsifier.  Slow break times equal lower charge, and higher concentrations of emulsifier.

Figure 8: Chemical Structure of an Cationic Emulsifier Particle (6)

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Figure 9: Cationic Emulsified Particle (6)

1.2.4 Anionic or Cationic:  Which to Use?

At present, either cationic or anionic emulsions are used in roadway applications.  However, the use of non-ionic emulsions may become more common as emulsion technology advances.  The choice between anionic and cationic is based on the application requirements and the characteristics of the aggregate to be used in the mix.  Generally, anionic emulsions of the slow set variety are more compatible with soils and easier to dilute with water.  Thus, they are normally chosen for soil stabilization and fog seals.  Anionic emulsions break by flocculation and coalescence.  In this process, as water evaporates from the emulsion and the particles come into close contact, they stick together, as illustrated in Figure 10.  These particles then “floc” or coalesce into larger particles.  This process continues until the particles begin to form films.  No specific reaction occurs with siliceous aggregates, but with calcareous aggregates a reaction does occur (8).  Thus, anionic emulsions are suitable for use with calcareous aggregates such as limestone.

Figure 11 illustrates material compatibility in general terms along with the associated breaking process.  Cationic emulsions may be formulated for all application types and aggregates, also illustrated in Figure 12.  These emulsions are most useful for rapid setting chip seals, slurry emulsions and microsurfacing emulsions.  This is due to a cationic emulsion’s specific reaction with all compatible aggregates that creates a stronger adhesive bond.  For the same reason, cationic emulsions are also less susceptible to cooler conditions and dampness than anionic emulsions.

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Figure 11: Material Compatibility and Reactivity of Emulsions (8)

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Figure 12: Cationic Emulsion Physio-Chemical Reaction with Aggregate (6)

The curing process (illustrated in Figure 13) is the same for both types of emulsion, except the reaction mechanism for cationic emulsion pushes water away from the aggregate surface.  Thus, cationic emulsions tend to cure faster.

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Figure 13: Emulsion Break and Cure Stages (6)

The decreased curing time for cationic emulsions has implications in the application and handling of these emulsions.  These implications will be discussed in the sections on specifications and storage and handling.

1.2.5 Specifications and Testing

1. Commonly used emulsion types are briefly described below:

Caltrans Specifications: Caltrans uses several common emulsion types.  These are described in detail in Standard Specifications Section 94 (2), and are briefly described below:

Anionic Emulsions:  Rapid Set (RS), Medium Set (MS) and Slow Set (SS).  There are subcategories that describe the base asphalt (“h” equals hard or 80/100-penetration grade or if there is no ‘h” designation, it refers to the use of a softer grade- 120-150 pen grade).  Numbers describe the binder content of the emulsion (1 for lower and 2 for the higher level).  In anionic emulsions, these binder content designations are different for different grades; RS-1 is typically 55% minimum binder content, and RS-2 contains 65% minimum.  Medium sets are 55 and 65% respectively and SS grades are only designated as SS-1 or SS-1h and are 57% minimum binder content.

Cationic Emulsions:  Rapid set (CRS), Medium set (CMS) and Slow set (CSS).  There are subcategories that describe the base asphalt (“h” equals hard or 80/100-penetration grade.  If there is no ‘h” designation, it refers to the use of a softer grade).  Numbers describe the binder content of the emulsion (1 for lower and 2 for the higher level).  In cationic emulsions, these binder content designations are different for different grades; CRS-1 is 60% minimum binder content, and CRS-2 65% minimum.  Medium sets are 55 and 65% respectively and SS grades are only designated as CSS-1 or CSS-1h and are 57% minimum binder content.

• Polymer Modified Emulsions: These may be anionic or cationic.  They are all rapid set and have the letter P at the start of the designation.  Many highway agencies designate the letter P, either as a prefix or suffix to the designation.  For example PMCRS-2h CRS-2hP is a polymer modified cationic rapid set emulsion with the hard binder.  All the emulsion binder contents for this class of emulsions are 65% minimum.  These emulsion types are further discussed in Chapter 5 “Chip Seals.”

Quickset Slurry Emulsions:  These may be anionic or cationic (QS or CQS) and have minimum binder contents of 57%.  In general, the polymer modified (latex) versions of these emulsions have the letter “L” preceding the designation (e.g., LMCQS-1h).  Such emulsions may be made with the hard binder or the softer binder.  This is further discussed in Chapter 7 on Slurry Surfacings.

2. What the Specifications Mean:  The test methods listed in the specifications (2) are designed to provide an indication of the stability, physical characteristics, and performance of the emulsion.  This section presents a general overview of tests contained in the specifications.

• Binder content is measured by distillation or evaporation.  This is important to know because application rates are based on residual binder.
• Viscosity indicates the application properties, i.e., whether the emulsion can be pumped and sprayed, and whether it will remain where it is applied without running off.  The viscosity of an emulsion is a function of the binder content within the emulsion, as illustrated in Figure 14.  This figure indicates that as the binder content of the emulsion increases, so does its viscosity.  Emulsions with higher viscosities are more difficult to pump and spray at a given temperature than are emulsions with lower viscosities.
• Settlement and storage stability are determined by the same test, but performed over different periods of time.  They determine if an emulsion can be stored without “breaking” in the storage container.  If settlement occurs during the test (as shown in Figure 15), and is not re-dispersed, this is an indication that the emulsion may flocculate and coalesce (“break”) during storage.
• Demulsability is the measure of an emulsion’s resistance to breaking and gives an idea of whether the emulsion is rapid or slow setting.
• Coating test refers to mixing characteristics with soil or aggregate.
• Cement-mixing test is a stability test that is relevant for mixing emulsions with soils or aggregates.
• Sieve test provides an indication of foreign matter in the emulsion that might cause problems such as clogging nozzles during spraying or clogging in-line sieves during pumping operations.  It is also an indication of stability.  Figure 16 illustrates the Sieve Test.

Tests on residual binder are carried out to check the base asphalt and the polymer.  Penetration and ductility are conducted on the residue of both conventional and polymer modified emulsions.  Torsional recovery and infrared testing are used to examine polymer content.  Torsional recovery is carried out using the equipment shown in Figure 17.  The recovery from a torsional load is measured and related to polymer content.

Figure 14: Relative Viscosity vs. Binder Content (9)

Figure 16: Sieve Test (Note: Normally only 1 sieve is used in the AASHTO t-59) (8)

Figure 17: Torsional Recovery Test

1.3 Cutbacks

1.3.1 What Are Cutbacks?

A cutback is a solution of asphalt in a hydrocarbon solvent (e.g., kerosene, diesel, or naphtha).  Solvents are used to reduce the asphalt’s viscosity so that the cutback can be pumped and sprayed at lower temperatures [40 to 145ºC (104 to 293°F)] than that required for conventional asphalt.  The solvent performs no other function in road applications.  The solvent selected depends on the grade of the cutback, which in turn is based on the expected setting rate.

During the energy crisis of the 1970s and in response to environmental concerns on volatile emissions (i.e., evaporation of the solvent during the application and curing processes), use of cutbacks has generally been discontinued.  Cutbacks are principally used as prime coats over aggregate base materials before placing an asphalt-wearing course in new construction.

1.3.2 Manufacturing

Cutbacks are easily manufactured because they are solutions of asphalt and solvent.  They can be made on site by circulation in a tank.  In refinery applications, inline blending or emulsion colloid mills have been used to manufacture cutbacks. 

1.3.3 Specifications and Testing

The Caltrans cutback specifications are found in the Standard Specifications Section 93, and they are referred to as “Liquid Asphalts” (2).

If the solvent used to make the cutback asphalt is highly volatile, it will evaporate quickly.  The less volatile the solvent, the slower it evaporates.  On this basis, cutback asphalt is divided into three types:

• Slow curing (SC), containing a heavy oil solvent (SC-70, SC-250, SC-800, and SC-3000).  The number refers to kinematic viscosity of the cutback.
• Medium Cure (MC), made with a kerosene type solvent and having the same viscosity designations as SC grades.  Asphalt cement and a light diluent of intermediate volatility, generally in the kerosene boiling point range (MC-38, MC-70, MC-250, MC-800, MC-3000).
• Rapid Cure (RC), made with asphalt cement and a light diluent of high volatility, generally in the gasoline or naphtha boiling point range (RC-70, RC-250, RC-800, RC-3000).

The advantage of cutbacks is that various types and grades of asphalt cutbacks can be manufactured in the field with standard equipment.  Thinner cutbacks can be produced from the more viscous grades.  Field manufacture of SC with diesel oil and MC with kerosene is more practicable than field manufacture of RC.  Fire is a serious concern because of the rapid evaporation of gasoline from RC.

The degree of fluidity in each case depends principally on the proportion of solvent to asphalt cement.  It may also be affected to a minor degree by the hardness of the base asphalt used to formulate the cutback.  The degree of fluidity results in several grades of cutback asphalt.  Some are fluid at ordinary temperatures while others are more viscous, requiring some heating to make them fluid enough for construction purposes.

The main specifications relate to safety as measured by flash point and water content, viscosity and boiling range of the solvent, application and cure rate, residue percentage for residual application rate, and residue tests to ensure the correct base asphalt grade had been used.  The main effect of increasing the viscosity is to increase the application temperature requirements.  This is covered in the Standard Specifications Section 93.

1.4 Polymer Modified Binders (PMBs) & Performance Based Asphalts (PBAs)

1.4.1 What  Are Polymers and Polymer Modified Binders?

Polymer modified binders can bring real benefits to highway performance in terms of cost savings and better and longer lasting roads.  The term "polymer" does not automatically mean a synthetic material.  There are a large number of naturally occurring polymers which can be organic or mineral substances.  For example, rubber and sulphur are naturally occurring polymers. Rubberized asphalt for wearing courses and base courses has been used with some success for over 40 years.  Today, polymers are often manufactured in a chemical process to combine particular molecules in a way that would not occur naturally.  These are known as synthetic polymers.

Polymers are large molecules that enhance the properties of virgin asphalt.  Depending on the basic polymer units or monomers used, a wide range of properties can be achieved.  It is possible to categorize polymers in a number of ways, but for engineering purposes they are conveniently described as having glassy (stiffness) or rubbery (elastomeric) properties.  Often this is termed plastomeric or elastomeric. Plastomers will deform but will not return to their original dimensions when the load is released.  Elastomers will deform and return to their original dimensions when the load is released.  However, this is very dependent on conditions such as temperature, rate of loading and strain level (10).  As the demands of a modern road system have, in some areas, exceeded the capacity of conventional bituminous materials, polymer additives are a means by which pavement performance may be enhanced.

The use of a polymer has no value if it does not substantially improve the life cycle cost of the material in which it is used or solve a specific problem.  The selection of polymer modification should almost always be based on improved performance related to cost.  Although there are a substantial number of polymers in use today for a variety of products, only a relative few are commonly used in asphalt mixtures (11).  Examples of polymers commonly used in asphalt mixtures include:

• Styrene Butadiene Copolymer (radial and linear) (SBS)
• Polyethylene (PE)
• Styrene Butadiene Rubber (SBR)
• Polybutadiene (PB)
• Ethylene Vinyl Acetate (EVA)
• Ethylene Methyl Acrylate (EMA)
• Atactic Polypropylene (PP)
• Epoxies and Urethanes
• Tire Rubber (Crumb)

1.4.2    How Polymer Modified Binders Are Manufactured

There are many types of manufacturing configurations to make polymer-modified asphalts.  Manufacturing may be done at high or low shear, on site, or in a factory.  The main stages of manufacturing require the following procedures:

• Metering of polymer, asphalt, and additives
• Wetting of the polymer by the asphalt/additive mix
• Dispersion of the polymer
• Allowance for any interaction (reaction) of the polymer with the asphalt
• Storage and transportation

Most of these are mechanical issues and are achieved by relatively simple techniques.  Figure 18 illustrates a typical manufacturing (blending) plant.

The most important steps in the manufacturing process are dispersion and reaction.  This is what determines the structure (i.e., morphology) of the final binder and hence its properties.  These steps also determine the level of polymer required to achieve the desired results.  Compatible systems usually have superior rheological, aging, and stability properties to those of incompatible systems at the same polymer level (10).  The micrographs in Figure 19 show the structure (morphology) of SBS at 3% dispersion in (a) a compatible, and (b) an incompatible polymer system.  The scale for both micrographs is the same.

Figure 18: Typical Polymer Blending Plant (10)

a) Compatible System	b) Incompatible System
Figure 19: Micrographs of Polymer Systems (10)

1.4.3 What are Performance Based Asphalts (PBAs)?

Some PBAs are polymer modified, notably PBA 6a, PBA 6b, and PBA 7.  The PBA specification defines the performance characteristics of the binder.  It also incorporates many of the standard consistency tests as well as some items that were a precursor to the Strategic Highway Research Program (SHRP) rheological parameters, such as shear susceptibility of delta and viscosity.

The different grades are suited for different climatic applications.  PBA 1, the base conventional material, is similar to AR 4000.  PBA 6a and PBA 6b are polymer modified asphalts and provide better high and low temperature properties than AR grades and are used in areas with hot summers and cold winters.  This is achieved through the use of SBS copolymers.  The main difference is the lower temperature cracking resistance, PBA 6b being superior to PBA 6a by 5 to 6°C.  Both products resist rutting at very high pavement temperatures.  PBA 6a is also useful in open graded mixes where the application temperatures are at the lower end of the requirements or work is being done at night.  This is possible due to PBA 6a’s good compaction characteristics in such mixes.  PBA 7 is more heavily modified than the other materials and is suitable for high desert areas.  This material has better aging resistance and may be used for this purpose in milder climates.

1.5 Asphalt Rubber

1.5.1 What is Asphalt Rubber?

Rubber is a naturally occurring polymer.  When added to asphalt, it will increase elasticity and the softening point and decrease brittleness.  The benefit that rubber imparts to asphalt binders includes resistance to cracking and rutting.  Asphalt rubber is the largest single market for ground rubber, consuming an estimated 12 million tires annually. California and Arizona use the most asphalt rubber in highway construction, with Florida the next largest user.

Scrap rubber, crumb rubber, and reclaimed rubber all describe recycled rubber.  The largest recycled rubber source is automobile and truck tires, referred to as Chemically Modified Crumb Rubber Asphalt (CMCRA) crumb rubber modified (CRM).  This rubber is not a pure polymer but a blend.  Most car tires in the USA are made of mainly Styrene Butadiene Rubber (SBR) or polyisoprene and carbon black.  Tires are not uniformly formulated or compounded; other polymers are included in some blends.  Truck tires generally contain a higher percentage of natural rubber than car tires – up to 30% of the combined polymer content.

The variations in the (CMCRA) CRM may affect the properties.  However, in asphalt rubber binders, the particle size of the added (CMCRA) CRM is relatively large and the chemical properties are less important than in a polymer system.  Asphalt rubber binder is typically made in the field; that is, near to the job site for chip seal applications or at the hot plant site for hot mix applications.

The asphalt rubber specification is a recipe specification and is detailed in the SSPs (12).  Most asphalt rubber specifications blend asphalt and rubber to increase elasticity and flexibility. The anti-oxidants and carbon black in the rubber significantly increase the longevity of the binder.  Two rubber types are specified: one is tire rubber and the other is a high natural rubber recycled material.  The required rubber properties are controlled by the SSP requirements (23).  Mixing temperatures should be kept between 190 and 226°C (375 and 440°F).  Grading is important in determining the rate of digestion and the binder’s final properties.  These materials are mixed into asphalt that has been modified with extender oil (high aromatic hydrocarbon) at about 2% (12).

Compared to other binders, asphalt rubber binder improves fatigue life, resistance to rutting, and provides stone retention and crack alleviation in chip seals (13).

1.5.2 How is Asphalt Rubber Manufactured?

To produce asphalt rubber binder, the neat asphalt is heated to approximately 190°C to 226°C (375°F to 440°F), at which time the tire rubber is added via a hopper system into a pre-wet tank.  The asphalt contacts and wets the (CMCRA) CRM particles.  This mix is then transferred into a reaction vessel where it reacts with the lighter fractions in the asphalt, mostly aromatic and naphthenic oils (extender oils) that swell the outer areas of the particles.  Extender oils are used in the manufacture of tires as softeners.  These extender oils offer low volatility, good oxidation resistance, and excellent solvency power necessary to blend the tire rubber with asphalt.

During the reaction, the asphalt and the rubber particle interact to form a gel-coated particle (13), similar to the process of swelling that occurs in polymer asphalt systems (14).  This reaction is shown schematically in Figure 20.

How well this model reflects the actual situation and the relative effect of particle sizing is not clear but, based on polymer and asphalt chemistry, it seems adequate.  It also explains why a significant change in properties occurs over time, since this type of system is not thermodynamically stable.  Further, the large increase in viscosity over its early life is due to the continuation of this solvation process (15, 16).

The influence of extender oils can be shown by examining micrographs (see Figure 21) of asphalt rubber digested with and without extender oil (relative sizing is important; all micrographs are to the same scale and the largest particles are 100μm).

Figure 20: Asphalt Rubber "Reaction" (13)

a) Without Extender Oil	b) With Extender Oil
Figure 21: Micrographs: Asphalt Rubber Extender Oil Effects (10)

1.6 Aggregates

1.6.1 What Are Aggregates?

Aggregates are the primary building material for pavements.  The aggregate’s role is to form the matrix of strength in a mix; aggregate properties are critical to the success of a mix (3).  Local sources are generally used, but some other materials such as expanded clay (light weight aggregate) or slag may be used if they meet the required specification.  Major aggregate types that may be encountered in California include (3):

• Igneous rocks:  Volcanic rocks such as granite and basalt that form from molten rock. 
• Sedimentary rocks:  Rocks such as limestone, sandstone, and chert that form from layers of material that are then compressed.
• Gravel:  Usually in rivers or waterways, formed from the breakdown of any natural rock, such as river gravel.
• Sands:  Formed from the deterioration of any natural rock, they often contain clay or silt and require washing.
• Slag:  A generally hard but absorbent by-product of metallurgical processing of tin, steel, or copper.

There are two major categories of aggregate properties of interest in pavement applications:  chemical and physical. 

Chemical Properties:  Chemical properties of aggregates identify the changes an aggregate may go through due to chemical action.  Some aggregates contain substances that are soluble in water, and are subject to oxidation, hydration or carbonation.  The main chemical property that affects asphalt applications, however, is the affinity the aggregate has for the asphalt.  Asphalt must wet the surface of the aggregate and adhere to it.  Failure to do so may produce the phenomena of stripping and disintegration failure of the hot mix, or loss of stone in other treatment types such as slurry or chip seal.  There are no reliable indicators for determining stripping potential based on chemistry of the aggregate alone.  Most tests are based on testing the mixture (AASHTO T283).

Physical Properties:  The most important aggregate properties are listed below:

• Grading or Particle Size Distribution:  Grading requirements are discussed in the chapters that deal with individual treatments.  Grading determines the mixture characteristics with respect to its physical properties.  In HMA, for example, this includes fatigue resistance and load bearing.  In open-graded asphalt concrete mixtures, grading will determine porosity, while in chip seals and slurry surfacing it will determine seal durability.  The individual grading requirements are further discussed in the later chapters on treatments.
• Cleanliness or Presence of Deleterious Materials:  Dirty aggregates may hamper adhesion in chip seal and HMA and cause cohesion problems in slurry surfacing.  Lumps of clay may disintegrate under freeze thaw conditions or cause pockmarks in a HMA pavement.  Specific testing and requirements are discussed in the chapters concerned with treatments. 
• Hardness or Abrasion Resistance:  Aggregates transmit wheel loads to the subgrade.  They must be resistant to crushing and wear to maintain this function.  They must also resist crushing and degradation during stockpiling.  A polished or worn aggregate will reduce skid resistance.  The LA abrasion test is used to measure hardness and abrasion resistance.
• Durability or Soundness:  Aggregates must be resistant to breakdown due to the cyclic action of wetting and drying and freeze and thaw cycles. 
• Particle Shape and Surface Texture:  Aggregate particles for use in most treatments should be cubical rather than flat or elongated.  This creates more interlock and internal friction in generating higher deformation resistance.  In chip seals, it creates greater seal texture depth and skid resistance.  The surface texture and shape determine its workability in mixes and may affect compaction.  A rough fractured particle has a higher surface area and forms tougher adhesive bonds. 
• Absorption Characteristics:  Aggregates may absorb asphalt; reducing the effective volumetric percentage of the binder mixtures or the effective application rate in chip seals.  These changes can result in raveling of the pavement. 

Special aggregate requirements for specific treatments are considered in the relevant chapters.

1.6.2 How are Aggregates Manufactured?

Aggregates are manufactured in quarry operations first by blasting, if necessary, and then using a series of crushers and screens to create the desired stone sizes.  Several methods of crushing may be used, including jaw crushers (usually the primary crusher), impact crushers (these produce cubical aggregates and are generally used later in the process), attrition mills, hammer mills, and gyratory cone crushers.  The right combination must be chosen to meet required specifications.