By Brian J Majeska
In the last blog I focused on the importance of continuing adult learning and how it relates to the success of product commercialization. Nowhere is this more evident to me than in the asphalt industry as our team develops a number of products with clients. I am continually reminded of the need to stay current on the science of asphalt. Up-to-date knowledge is crucial in order to maintain relevance in the market and to achieve commercialization success. This blog is going to cover the following:
- Crude Oil and Petroleum Refining
- The Composition and Chemistry of Asphalt Binders
- Physical Properties of Asphalt Binders & Key Tests
- Simplified Specification Table
Introduction: Defining Asphalt and Its Use
Gaining understanding of the science of an asphalt binder, a material that is used in most road pavements in the world, is a journey. The route for this journey is straightforward, but each segment in this journey can be complex.
Most people who take trips want to first understand the destination of the trip and also, their starting point of the journey. Once the start and finish are determined, the primary rest stops along the route make the journey manageable.
With this journey in mind, let’s declare the destination understanding the science of asphalt binders. An asphalt binder’s role in the road is to glue the rocks that form a pavement together in a manner that does not promote rutting in the wheel paths of the pavement, and does not crack when the pavement gets cold, aged, or when heavy trucks drive on the road. This is what every pavement engineer is hoping to achieve.
The asphalt starts its journey during the discovery of crude petroleum oil. The heavy constituent of the crude oil is the material that becomes the binder for the road. Each source and type of crude oil has different characteristics that will have a significant impact on the performance in the road that is being paved.
As with any journey there can be as many rest stops as the explorer wants to take, but on this trip three major ones are necessary. The first stop on the trip is when the crude oil goes into the refining process. The modern refining process plays a significant role in the binder’s final performance in the road. The decision to produce other products in the refinery also has an impact on the liquid asphalt’s performance. The primary stream of a refining process is unleaded gasoline; a side stream of the refining process is asphalt. The highest economic value for the refinery is in maximizing the production of unleaded gasoline.
The second rest stop in our journey is understanding of the basic chemistry of a liquid asphalt. The chemical composition of the binder impacts its flexibility, its adhesion to rocks, and its ability to be modified with other materials like polymers.
The final rest stop is understanding the physical properties of the asphalt binder. This allows the engineer to select the appropriate binder for the particular pavement.
Now that the trip, destination, and route is understood, let’s begin with a description of liquid asphalt.
Asphalt is a viscoelastic material. It is defined by the American Society of Testing Materials (ASTM) as “a dark brown to black cementitious material in which the predominant constituents are bitumens which occur in nature or are obtained in the petroleum processing.”
As a cement, asphalt is especially valuable to the engineer for the following reasons:
- It acts as an oil at high temperatures (above 80℃) to assist lubrication of the asphalt mixtures, which are comprised of aggregate, liquid asphalt, and other additives or polymers.
- It has excellent properties for adhesion to aggregate.
- It acts as a waterproofer to the aggregate in the pavement.
- It is a durable material.
Asphalt is also resistant to reaction with most acids, alkalis, and salts. Liquid asphalt’s physical characteristics are those of a semi-solid material at ambient temperatures, but it can be readily liquefied in three ways:
- Heating to the binder (above 80℃) – the principal means of use
- Emulsification – the science of dispersing fine asphalt droplets into water through the use of emulsifiers and a high share mixer (typically a colloid mill)
- Solvent blending – blending a viscosity cutting solvent such as diesel into the liquid asphalt.
Liquid asphalt is a complex material. The best way to understand it is to learn how it is manufactured about its chemical and physical properties.
Section I: Crude Oil and Petroleum Refining
To begin to understand refining, you need to have a basic knowledge of the various sources of crude oil. There are three primary crude sources: light sweet crude, medium sour crude, and heavy sour crude. (see table 1.1)
Sophisticated refineries are capable of refining heavy sour crude by processing the heavy oils through catalytic cracking and other processes, resulting in gasoline and distillate. Distillate includes jet fuel, diesel, and heating oil. Heavy oil includes residual fuel, flux, and asphalt.
A simplified schematic of the refining process is shown in diagram 1.1.
Within the refining industry, sometimes the phrase, “unleaded gasoline factory” is used to describe the modern integrated refining process. The refinery is run using a Linear Program (LP), which optimizes the refinery in terms of the number, blends, and types of crude sources. It then balances the economic value of the finished product. In the US, most refineries’ economics are linked to three products: gasoline, jet fuel, and diesel.
Many veterans of the asphalt industry will say the quality of the liquid asphalt or asphalt cement (AC) is equally determined by the crude oil source and the refinery configuration.
Modern refiners are facing many challenges, including a diversity of crude sources and a higher rate of crude source changes. This means that refineries and asphalt terminals are needed to blend several refined streams of materials to meet the needs of the construction industry. As asphalt binder specifications change, flexibility in blending at the asphalt terminal will become increasingly important.
Section II: The Composition and Chemistry of Asphalt
Composition of Asphalt
Asphalt is derived from crude oil, as described in the above. Approximately 90 to 95 percent of asphalt is composed of carbon and hydrogen; thus, it is classified as a hydrocarbon. The breakdown of asphalt is represented in table 2.1:
A means of characterizing asphalt constituents was developed by Fritz Rostler in the early 1960s. It continues to be one of the standard methods for describing asphalt cement (AC) components. This method separates the AC by using sulfuric acid to separate some constituents, while also using a solvent (n-pentane) to separate out the insolubles, resulting in the following two components:
Asphaltenes: a hard insoluble material that drives stiffness and a number of other properties in AC.
Maltenes: paraffins, acidiffins, and nitrogen bases which have impact on cracking characteristics, compaction, and other AC properties.
The molecular structure of asphalt is complex and varies a great deal from source to source. There are three types of molecules:
- Aliphatic or paraffinic – three-dimensional, chainlike molecules which can be waxy or oily
- Aromatic – flat, stable rings of carbon that can be readily stacked
- Cyclic or naphthenics – three-dimensional saturated rings of carbon
The chemical bonds are relatively weak and are easily broken through shear forces or heat. The bonds are quickly reformed upon cooling of the AC. The components can also be described as:
- Polar – molecules that form a network and give the asphalt its elastic properties
- Non-polar – these molecules form the body of material around the network and contribute to the viscous properties of asphalt (Stokes Law)
The weak interaction between the polar and non-polar molecules at higher temperature allows asphalt to act as a Newtonian fluid. The viscosity change is proportional to the amount of temperature change. The balance of polar to non-polar appears to have a significant impact on hot mix asphalt performance. ACs with high amounts of high non-polar molecular weight molecules tend to have poor cracking performance on the road.
Section III: Physical Properties of Asphalt Binders & Key Tests
Asphalt binders and roads in general appear to be simple, but are in fact complex systems that are affected by several variables:
- soil conditions (soil strength, expansive soils),
- climate (temperature fluctuations, moisture), and
- traffic (loading and unloading of weight associated with cars and trucks driving over each square inch of pavement).
In materials testing and specifically in asphalt testing, a number of concepts are used to describe material properties. These concepts are associated with load, stress, strain, and a number of other properties that are simple when discussed one at a time, but often get confusing when discussed in combination.
The most often cited material properties are described below
Load (P): A mass or weight support by something; the forces to which a given pavement is subjected. For example, the weight associated with the traffic placed on a pavement.
Stress (σ): The load divided by the area upon which it is distributed. A simple example of the significance of stress is when a woman walks across a soggy soccer field in high heels. The stress associated with the heel versus the sole of the shoe are dramatically different (Diagram 3.1). Because the load is concentrated over a small area, the heel digs into the field. When the woman walks more on the sole of the shoe, the stress is significantly reduced because the load is distributed over a greater area, and the shoe does not dig into the field.
Tensile Stress (σ): The load divided by the cross-section of the material. For example, if a force is applied to a rod. Diagram 3.2 shows the forces being applied to the rod. The stress equation divides this force by the cross-sectional area of the rod.
Strength: When stress is applied to something, it creates failure in the material. For example, in Diagram 3.2, when enough load is applied to the rod, it breaks the rod into two pieces.
Strain (ε): This is how much the rod stretches when a load is applied to it. (Diagram 3.3). Strain is sometimes called or considered deformation of a material.
Shear (τ): This when forces are pushing a material in one direction and another part of a material in the opposite direction (Diagram 3.4).
Elastic Modulus (E): This is stress (σ) divided by strain (ε), which is called Young’s modulus. This concept becomes important for elastic materials when a material deforms upon loading and then recovers to its initial shape.
Viscosity ( η): This is the flow behavior of a material. For example, maple syrup has a different flow behavior than that of orange juice. Viscosity is the stress (σ) divided by the strain rate (ἐ).
Creep: This is the tendency of a solid material to move slowly or deform permanently under the influence of load. The impact of the load is magnified as materials are heated and typically increases near the material’s melt point.
Viscoelastic Material: This is the typical term that is used to describe an asphalt binder. It combines the following characteristics.
- Elastic Material: When a load is applied to a material and it deforms immediately, and upon the removal of the load the material returns to its original physical shape.
- Viscous Materials: when a load is applied to a material and the material immediately deforms and continues to deform while the force is being applied to the material. Upon the removal of the load, the material will not return to its original shape.
The asphalt binder is a viscoelastic material, which acts as a viscous material at high temperatures and reverts to an elastic material at cooler (ambient) temperatures.
Key Tests for Asphalt Binders
Over the years, the asphalt industry has developed a number of methods for testing asphalt cement. In the late 1990s the US embarked in the development of a number of performance tests during the Strategic Highway Research Program (SHRP) era. The SHRP program facilitated research in several transportation areas. One of the asphalt research categories was Superpave, which led to the development of new test methods and new laboratory procedures for binder, mixtures, and designs. This resulted in a new binder classification method called Performance Grade or PG. To decode the designations of binder classification an example is listed below:
This investment by the Federal government along with the states, universities, associations, and private industry was transformative in gaining understanding of asphalt binders and pavement. There has been a renewed interest in developing new specifications in an effort to produce the most cost effective AC for road agencies.
We are going to review several of the existing and emerging testing methods to gain further insight into asphalt binder properties. Specifically, we will look at the following categories:
- Binder Aging
- Binder Tests
The two key binder aging tests are probably a bit of a misnomer. The methods discussed below are tests, but also can be a means for sample conditioning methods that allow the AC to then be tested for various rheological properties.
Rolling Thin Film Oven Test (RTFOT)
This test measures two parameters of aging: oxidative related aging and evaporative related hardening. The RTFO attempts to replicate the aging associated with the high temperatures incurred during the handling and manufacture of Hot Mix Asphalt (HMA) at 75 minutes at 163℃ with periodic injection of hot air. The oxidation and stability of the binder is evaluated in two ways. The Dynamic Sheet Rheometer (DSR) is used to determine the extent of hardening and the impact of fatigue cracking, and the binder mass loss should not be ≦ 1.0%. Click this link to see the test equipment: RTFO Equipment.
Pressure Aging Vessel
Hot mix pavement becomes more brittle over time as it is exposed to environmental factors. This test attempts to age the binder in a manner that is equivalent to long-term road life and a performance of seven to ten years. The sample conditioning places samples of RTFO aged asphalt binder into a number of small stainless steel pans and ages the samples for twenty hours in a pressurized vessel (2.10 MPa or 305 psi) in a heated environment of 90℃, 100℃, or 110℃, depending on the climate conditions. Click this link to see the test equipment: PAV Equipment.
Dynamic Shear Rheomoter (DSR)
Asphalt binder behavior is impacted by many things; two that have a significant impact are loading time and temperature. The use of DSRs allows for both time and temperature to be evaluated in a consistent manner. Although the equipment is complex and expensive, the concept of the test is straightforward. Diagram 3.5 shows the concept of the operation.
The oscillating plate applies a series of stress and strains to the asphalt, which is sandwiched between the two plates. This creates the stress-strain curve for each cycle that is listed below. The test is run at 10 radians per second which is about 1.59 Hz (cycles per second) with a shear strain (strain amplitude) of approximately 10 to 12% for original (unaged) binders and for RTFO (Rolling Thin Film Oven) aged binders. The DSR is run in the controlled stress mode; this means the DSR maintains a maximum specimen stress or fixed level of torque.
The DSR is used to characterize viscous and elastic behavior by measuring:
- G* (Called G-star), which is the complex shear modulus of the AC. This is the total resistance of a material to deformation when exposed to repeated pulses of shear stress. It is an evaluation of two elements: elastic (recoverable) and viscous (non-recoverable) binder behavior.
- δ (delta), which is the phase angle of the AC. It is an indicator of relative amounts of elastic and viscous behavior.
AC is significantly impacted by temperature and frequency of loading. At high temperatures, the binder acts like a viscous liquid with no capacity for recovery. At very low temperatures, the asphalt cement will behave like an elastic solid, which means it will recover or rebound completely. See diagram 3.6.
Understanding the data output of the DSR for asphalt binders is presented in diagram 3.7.
The following are taken from the DSR to create the Superpave specifications for permanent deformation and fatigue cracking. The parameters are defined below:
- Permanent Deformation is governed by limiting G*/sin δ at the test temperatures to values greater than 1.0 kPa for original binder (unaged) and 2.20 kPa after RTFO aging
- Fatigue cracking is governed by limiting G* sin δ of PAV (Pressure Aging Vessel) to values less than 5,000kPa at test temperatures
Bending Beam Rheometer (BBR)
The purpose of the BBR to accurately evaluate asphalt binder properties at very low temperatures. The second test equipment is needed because the AC is too stiff at low temperatures to be reliably measured in the DSR. Superpave uses both the DSR and the BBR to gain a more complete understanding of fracture-related properties within binders. The BBR is used to measure two binder parameters:
- Stiffness (S) – The amount a binder deflects or creeps under constant load at a constant low temperature when the asphalt appears to be an elastic solid
- m-value – this is the rate of change in creep stiffness over time
The test is run on binders that have gone through both the short-term aging of the RTFO and the long-term aging that is conditioned in the PAV. A schematic of the equipment is shown in Diagram 3.8.
In the BBR, a beam of asphalt is placed on two load frame supports and then a downward force is applied to the specimen that has been prepared and tested at low temperatures. Loads are applied pneumatically with a transducer that monitors deflection throughout the test.
The method uses beam theory to calculate the stiffness of an asphalt beam sample under a creep load. The test applies a constant force for four minutes and measures the deflection at the center of the beam. The test method allows for the calculation of stiffness (S) and creep rate (m) which can be determined (creep can also be defined as deformation in cold temperature). Click this link to see the test equipment: BBR Test.
To achieve the target Superpave temperature, the following specifications must be met:
- Creep Stiffness (S) is not to exceed 300 MPa
- Rate of change of stiffness over time (m-value) must be greater than or equal to 0.300 at sixty-seconds
Multiple Stress Creep Recovery (AASHTO MP19)
(MSCR Test ~ pronounced massacre in the paving industry)
This test is a recent development to the Superpave Performance Graded (PG) Asphalt Binder specification. The original SHRP PG specifications were developed primarily with unmodified binders, and therefore did not capture the benefits of polymer modification in terms of rut resistance or elasticity. This new test is a high temperature binder specification that indicates the rutting performance of the asphalt binder and is blind to modification.
Additionally there is a provision for elastic recovery of binders in MSCR. The various elastic recovery type tests were used to determine the degree of polymer modification in an asphalt binder. Examples of these tests include forced ductility and elastic recovery.
This method allows states to potentially eliminate the process of grade bumping within PG binder selection. Grade bumping is defined in AASTHO (American Association of State Highway Officials) M 323; this specification outlines when a higher PG grade should be used due to increased traffic on a highway. For example, the climate may require PG 64-22 asphalt binder for a road, but if there is higher truck traffic, the binder selected should be PG 70-22 to minimize rutting of the road. The binder formulation should also include a polymer. To ensure a polymer, many states include some form of elastic recovery specification.
The rationale for the MSCR test is to develop a method that utilizes the DSR to determine rut resistance of a binder and its ability to recover from deformation, which is important for crack resistance in a binder. To characterize these properties, two additional parameters are tested in the DSR:
- MSCR Jnr : This measure determines the rut resistance of the binder. This is determined by dividing non-recoverable shear strain by applied shear stress. “J” is the variable for compliance, and “nr” stands for non-recoverable. We will go into greater detail on the specifics of this aspect of the procedure in a later section.
- MSCR % Recovery: This measure captures the performance of polymers in the binder.
The MSCR test uses creep and recovery to evaluate an AC’s potential for permanent deformation. The test uses the DSR to apply a one-second creep load in a binder sample. After the one-second load is removed, the sample is allowed to recover for nine seconds.
FHWA describes the test as follows: “The test is started with the application of a low stress (0.1 kPa) for 10 creep/recovery cycles then the stress is increased to 3.2 kPa and repeated for an additional 10 cycles. The material response in the MSCR test is significantly different than the response in the existing PG tests. In the PG system, the high temperature parameter, G*/sin δ, is measured by applying an oscillating load to the binder at very low strain. Due to the low strain level, the PG high temperature parameter doesn’t accurately represent the ability of polymer modified binders to resist rutting.”
Diagram 3.9 shows an example of the impact of rut resistance which is denoted as Jnr, non-recoverable creep compliance. Recall, creep is when a material permanently deforms due to a load being applied. The binder partially recovers from the shear strain, which is indicated in the dotted red region of the first graph. The bottom portion of the first graph indicates the non-recoverable shear strain of the asphalt binder.
The diagram also shows the impact of several cycles of loading and unloading an asphalt specimen. Think of what happens to an asphalt binder in a road when traffic goes over that square inch of the pavement.
The MSCR test evaluates higher levels of stress and strain being applied to the binder, better representing what occurs in an actual pavement. The higher levels of stress and strain in the test captures the stiffening effects of the polymer and also the elastic effects.
Diagram 3.10 is a graphical representation of binder performance from an elastic recovery perspective. The orange portion of the graph represents poor elastic properties for a binder, and the portion above the curve indicates good elastic recovery properties.
Table 3.1 highlights the minimum percent recovery needed for various Jnrs.
AASHTO MP19 has brought some new nomenclature to Performance Graded specifications. When selecting a binder, the traditional climatic temperature considerations of the PG method is applied, and an caveat for traffic loading is added to the grade name. The traffic designations are:
- “S” – Standard Jnr ≦5 kPa-1
- “H” – Heavy Jnr ≦0 kPa-1
- “V” – Very Heavy Jnr ≦0 kPa-1
- “E” – Extra Heavy Jnr ≦5 kPa–1
An example of the use of the MSCR test for a binder selection is the following:
The state of Kentucky traditionally uses a PG 64-22 for paving. They grade bump to a PG 76-22 with a traditional elastic recovery of greater than 75% when the road to be paved has high and heavy traffic loadings (e.g., an interstate with numerous semi trucks). In the MSCR test, the following criteria would be used:
- Original DSR: G*/Sin δ ≧00 kPa @ 64℃
- RTFO MSCR: Jnr ≦0 kPa-1 @ 64℃
This gives the DOT a method to get a stiffer asphalt binder without “pretending” that pavement temperatures in Kentucky have suddenly become warmer.
In the Northeastern states, the changes in polymer modified asphalt binders are shown in Table 3.2.
Section IV: Simplified Specification Table
We began this discussion with the journey of an asphalt binder. A great deal of science and engineering goes into the crude oil sourcing, the refinery process, the chemistry of asphalt, and the physical properties of this binder for roads.
Ultimately, the objective is basic: Develop cost-effective products for binding rocks together that will experience constant loading and unloading of traffic and also undergo pavement temperatures that cycle through seasonal extremes. The demand on an asphalt binder, that is typically less than five percent of the pavement mix, is significant. In future blogs we will discuss mixture designs and pavement designs. These additional topics integrate the understanding of the role of the binder into the total pavement requirements. A thorough understanding of mixture design, pavement design and asphalt binder results in building the most cost effective roads for society.
NOTES related to the references for this document
This paper could not have been written without the excellent material generated by Federal Highways, the Asphalt Institute, and a number of other sources and friends. Listed below, I have attempted to capture the key sources that were used for each section. My apologies for not using a formal footnoting process – I hope to get better at this process in the future, but would be remiss if I did not site the sources.
1. Asphalt Institute. Superpave Series (SP-1).
2. Asphalt Institute. Superpave Series (SP-1).
3. Asphalt Institute. Superpave Series (SP-1).
4. “Load,” Merriam-Webster website: http://www.merriam-webster.com/dictionary/load
5. ASTM D 2872 – EN 12607 Rolling Thin Film Oven Test Equipment, com, https://www.youtube.com/watch?v=3SwRYEg1FCQ
6. Roadtec Pavementinteractive website: http://www.pavementinteractive.org/article/pressure-aging-vessel/
7. FHWA & Asphalt Institute. The Multiple Stress Creep Recovery (MSCR) Procedure. http://www.fhwa.dot.gov/pavement/materials/pubs/hif11038/tb00.cfm and Asphalt Institute AASHTO Subcommittee presentation, Michael Anderson and John Bukowski.
8. Greg Harder, Asphalt Institute.
9. Asphalt Institute. Asphalt Binder Testing Manual (MS-25).
10. Michael Anderson, August 31, 2011. Understanding the MSCR Test and Its Use in the PG Asphalt Binder Specification. Asphalt Institute. http://www.asphaltinstitute.org/wp-content/uploads/public/asphalt_academy/webinars/pdfs/MSCR_Webinar_Aug2011.pdf
Thanks to a number of people for their contribution to this blog: Joe Lorenc, Gayle King, Dennis Muncy, Richard Steger, Jason Bausano, Chris Williams, Eric Cochran, Phil Blankenship, Julie Tenenbaum, and most importantly, my bride and children – Diane, John, & Grace Majeska