This section will cover the various types of modified asphalt binders and suggested new tests to characterize these asphalt binders. References: Stroup-Gardiner, M., and Newcomb, D.E. Polymer Literature Review. Minnesota Department of Transportation Report MN/RC – 95/27. Sept. 1995. Bahia. H.U., Hanson, D.I., Zeng, M., Zhai, H., Khatri, M.A., Anderson, R.M. NCHRP Report 459-Characterization of Modified Asphalt Binders in Superpave Mix Design. Transportation Research Board, National Research Council. 2001.
All plastic additives used in flexible pavement construction are either polyethylenes or modified polyethylenes. Polyethylenes are alkane chains with a high molecular weight. The regular shape of the chains take on a crystalline structure in the bulk state. The degree of crystallinity influences the properties of the polymer. A high level of structuring results in a polymer with a high tensile strength but little ability to deform before failure. Modifiers are added to disrupt the structuring and improve the flexibility of the plastomer.
Low density polyethylenes (LDPE) are generally defined as having a specific gravity of about 0.915 to 0.94. One example of LDPE is a plastic garbage bag. High density polyethylenes (HDPE) typically have a specific gravity of around 0.96. An example of HDPE is a one gallon plastic milk bottle. Only LDPE products have been used to any extent in the paving industry. Only one company, Novophalt, produced this type of modified binder. Special equipment was needed to keep the polymer mixed with the asphalt. The most commonly used plastomers is an ethylene vinyl acetate (EVA). The vinyl acetate is used to decrease the crystallinity of the ethylene structure and to help make the plastomer more compatible with the asphalt. Copolymers with about 30% vinyl acetate are classified as flexible resins that are soluble in toluene and benzene.
Elastomers are usually either natural rubbers or synthetic rubbers. In 1860, natural rubber was identified as a polymer of isoprene (C5H8). This is a straight chain structure with an ability to deform when stretched, then recover its shape with the load is removed.
The supply of natural rubber was interrupted after the bombing of Pearl Harbor. This led to the development of synthetic rubber compounds which eventually became into common usage after the end of World War II because of the slow recovery of the natural rubber industry in South America. Synthetic rubber is a styrene-butadiene copolymer (SBR). The most common example of this material is rubber gloves. The early synthetic rubber eventually evolved into a thermoplastic rubber, styrene-butadiene-styrene (SBS).
This slide gives several examples of products commonly used in HMA construction applications.
There are number of general polymer terms that are used to describe the various structures. The most basic one is the straight chain, or linear, configuration. This is the form of natural rubber. Modifiers, or branches are added to interrupt the regular structure of the straight chain configuration to disrupt the tendency of the polymers to form a tightly packed crystalline structure. Two chains can be chemically connected at the center to form a star, or radial, configuration. In the case of vulcanized SBS, sulfur crosslinks are used to periodically chemically bind SBS chains together. This forms a rubber-like network. Other terms that describe how the polymers are combined in the chains are shown on this slide. When two homogeneous polymer chains are linked together, this is called a diblock copolymer. When three chains are linked in alternating order, this is called a triblock (three chains) copolymer (only two types of polymer). A random block copolymer is a random collection of polymer chains of varying sizes.
Thermoplastic polymers use both thermal and chemical bonding to form a network. A physically crosslinked structure can be developed using a number of triblock copolymers. In the case of SBS, thermodynamics forced the styrene components to “clump” together. This forms large spheres of polystyrene domains connected by elastic threads of butadiene. The styrene domains provide locations for multiple physical crosslinks; they also act as filler. This structure is stable and has good strength over a wide range of temperatures, up to the glass transition temperature (about 100oC) of the styrene domains. Above about 150oC, the triblock straight chains are randomly distributed throughout the asphalt binder (as in during blending and mixing-Step 2). Once the temperature drops, the styrene domains will again try to come together to form crosslinks within the asphalt binders. The ability of the polymers to form a completely crosslinked system will depend upon the concentration of the polymer within the asphalt binder and the thermodynamics of the combined system.
Traditional vacuum viscosity can be determined for light to moderately polymer modified asphalt binders because the shear flow rate can be accounted for with this straight-walled tubes. Ductility testing has been used to evaluate the elastic recovery of the modified asphalt binder by measuring the “recoil” of the asphalt binder once it breaks. The softening point has been used to evaluate the polymer content and concentration. The toughness and tenacity test immerses a rounded head with an edge on it into the asphalt binder then pulls this tension head out of the asphalt binder at a rate of 50 cm/min (20in/min). A continuous curve of force versus deformation is recorded. Toughness is the area under this curve. That is, the work needed to pull the head out of the binder. Tenacity is a measure of the increasing force needed to stretch the binder to its initial peak in force. Polymers may be incompatible with some asphalt chemistries. When this happens, it is referred to as phase separation. This causes problems with construction as the polymer cannot be pumped out of the storage tanks and the desired modified properties of the asphalt binders are not achieved. Historically, the “cigar tube” test is used to determine if the polymer will separate from the asphalt binders. In this test, a metal cigar tube is filled with the modified asphalt binders then stored at and elevated temperature for a prescribed time. The tube is then removed, cooled and cut into 3 sections. The softening point can then be determined for each section. A maximum difference in softening point of 4oF is specified for either SB or SBS polymer modified asphalt (PMA) binders.
These transmission electron microscope (TEM) microphotographs show the morphology (structure) of the polymer domains in the asphalt cement. At 2% Kraton D1101 (linear SBS), the polymer is distributed in small, discrete, widely spaced domains. At 4%, the polymer is still the discontinuous (discrete) phase but the domains are much larger and closer together. It is this structure that show up in the master curve as a small rubbery plateau. This shows that the polymer does not have to be the continuous phase for it to produce a rubbery plateau in the storage modulus master curves.
At 6%, the polymer is just at the continuous phase (it is hard to see the fine connections in this photograph). At 10%, the asphalt (white regions) is obviously the discontinuous (discrete) phase. Both of these polymer networks are easily seen as rubbery plateaus in the storage modulus master curve (previous slide).
A continuous polymer network is formed at 4% when radial SBS Kraton D1184 polymer is used. This is about a 2% lower concentration than was needed with the linear SBS polymer.
This report covers a wide range of sophisticated topics and concepts. Only the more practical, easily implemented recommendations are covered in this section.
This NCHRP report evaluated the use of the PG asphalt binder tests and came up with this list of topics that were either not addressed in the original research or needed to be added to the current testing program.
Two general classifications of asphalt binders were defined: rheologically simple asphalt binders, and rheologically complex binders. Complex asphalt binders are those that have properties that do not meet the assumptions used to set the PG asphalt binder test parameters. For example, asphalt binders with large particle sizes cannot be tested with the DSR using the required gap height.
This slide presents the requirements for a asphalt binder to be classified as “simple”. This classification will apply to all neat asphalt binders and to some asphalt binders with light to moderate modification.
The particulate additive test (PAT) was developed so that the amount and size of additive particles could be measured.
A second test was developed to determine if the modified asphalt binder was stable during long term storage (2 or more days).
Several other suggestions were made for altering the low temperature testing program requirements for predicting the critical thermal cracking temperature. However, the concepts and modeling associated with these suggestions are complicated and somewhat preliminary in acceptance. Therefore, they will not be covered in this module.
– Quick early strength but with little ability to
strain without brittle failure
– Any strain tends to be permanent
• Little elastic behavior
Plastomer Product Examples
EMA or EVA
– Resist permanent deformation by an ability
to stretch and recover shape once load is
– Only contribute to tensile strength when
Elastomer Product Examples
Common Polymer Terms
Straight Chain or Linear
Polymer with Branches
in Asphalt Binders
in hot AC
in cool AC
Traditional Testing for Modified
• Vacuum viscosity
– Asphalt Institute tubes
– Elastic recovery
• Softening Point
– Increases with polymer concentration
• Toughness and Tenacity
White area = Asphalt Binder and Black area = polymer
2% Linear SBS
4% Linear SBS
White area = Asphalt and Black area = polymer
6% Linear SBS
10% Linear SBS
White area = Asphalt Binder and Black area = polymer
4% Radial SBS
NCHRP Report 459
Characterization of Modified Asphalt
Binders in Superpave Mix Design
• Storage stability of modified asphalt
• Shear-rate dependency of viscosity
• Strain dependency of rheological
• Effect of mechanical working
• Loading-rate dependency and time-temp
Modified Asphalt Binders
• Simple asphalt binders
– Do not violate assumptions of Superpave
• Complex asphalt binders
– Behavior (characteristics) violate one or
Simple Asphalt Binders
• <2% by vol of additives retained on No. 200
– PAT with toluene
• Stable for 2 or more days at mix/compact
• Reasonably linear over range of stress and
• Insensitive to cyclic damage during testing
Particulate Additive Test (PAT)