heological instruments and rheologicalmeasurements find applications in any in-d u s t ry where the flow characteristics of a

material determine its processibility, performance,and/or consumer acceptance. Characterizing andunderstanding the rheological properties of asphaltbinders in both the molten and solid state is funda-mental in formulating the chemistry and predictingthe end-use performance of these materials.

Rheology is the branch of science dealing with theflow and deformation of materials. The materials un-der investigation can range from low-viscosity fluids,semisolids, and polymer-modified asphalt to solid-state rigid samples. In the case of asphalt, rheologicalbehavior is controlled directly by molecular structure,c ry s t a l l i n i t y, association crosslinking, and filler typeand amount. Whether involved in manufacturingthe asphalt, producing the machinery used in asphaltproduction, or processing the asphalt into a finalproduct, rheological measurements are of practicalimportance. These quantitative, objective measure-ments bridge the gap between molecular structure,p r o c e s s i b i l i t y, and ultimate performance properties.

Asphalt binders’ behavior depends on tempera-ture and time of loading. Unlike steel, which is con-sidered a completely elastic material that can storeenergy indefinitely, asphalt binders cannot sustaina load without showing time-dependent deforma-tion known as creep. Additionally, some of the in-put energy is dissipated in the material and resultsin a permanent set. This is called viscoelastic behav-i o r. Samples exhibiting viscoelastic behavior arewell suited for rheological characterization.

At high temperatures or long time of loading, as-phalt binders act like high-viscosity flowable fluids.Viscosity is defined as the resistance to flow. As theviscosity decreases, a sample will flow more readilyunder the same conditions of temperature and load.At low temperatures or short loading times, asphaltbinders act like elastic solids. Elastic solids approachthe response of steel, where all the input energy isrecoverable.

A d d i t i o n a l l y, there remains another unique char-acteristic of asphalt binders: environmental and/orchemical aging. Since asphalts are organic materials,they are susceptible to oxidation, chemical associa-tion, and physical hardening. All these phenomenacause changes in the asphalt binder with time. Rhe-ology is a useful tool in quantifying these effects.

Traditional tests used to characterize asphaltbinders were either penetration or simple one-pointviscosity tests using a capillary device held at con-

A p p l i c a t i o n N o t e

16 / JULY 1999

Dynamic shear rheometers pave the way for quality asphalt binders

Peter K.W. Herh, Steven M. Colo, Kaj Hedman, and Bruce K.S. Rudolph

stant temperature. Both of these measurementshave shortcomings and provide only limited in-sight into the overall flow behavior and ultimateend-use performance of an asphalt binder. Oftenthese traditional tests provide misleading results inregard to the effect of additives, modifiers, and en-vironmental conditions.

In 1987, the Strategic Highway Research Pro-gram (SHRP) began developing new tests for mea-suring the properties of asphalt. The new tests mea-sure physical properties that can be directly relatedto field performance by rigorous engineering princi-ples. Since asphalt binder behavior depends onboth time and temperature, the ideal test instru-ment would evaluate both of these variables. TheVISCOTECH dynamic shear rheometer (DSR) (Figure1), a research-grade DSR, and SPECTECH DSR (F i g -u re 2), a specification-grade DSR (both from AT SRheoSystems, Bordentown, NJ), are capable of per-forming both of these tests. Both units feature dryheating/cooling of the test specimen. In a dry sys-tem, the test sample is not immersed in a fluid me-dia to control the sample temperature. The dry sys-tem has many advantages such as response time,ease of use, ergonomics of sample edge trimming,and temperature range and stability.

Temperature sensitivity of asphalt binders

Much has been written about the temperaturesensitivity of asphalt binders. As a result, specifica-tion grading requires temperature control to ±0.1 ° C .

F i g u re 3 shows the response of a typical original as-phalt binder at 64 °C as a function of time run withthe VISCOTECH DSR. The data show the tempera-ture stability and constant complex shear modulusG′ over 4000 sec.

Specification grading requires that the asphaltbinder be evaluated at a variety of temperatures from5 to 85 °C. Therefore, the time required for the DSRto change and then equilibrate the asphalt sample atthe test temperature is paramount. F i g u re 4 shows anoriginal asphalt binder response when heated from64 to 70 °C and F i g u re 5 shows the same samplecooled from 64 to 58 °C. The important parameter isthe time for the sample, not the instrument tempera-ture controller, to reach thermal equilibrium.

The time to attain a steady response is greater incooling mode than in heating mode. From the cool-

R

Figure 1 VISCOTECH DSR.

Figure 2 SPECTECH DSR.

Figure 3 VISCOTECH DSR stability at 64 °C on an original asphalt binder. Figure 4 VISCOTECH DSR heating transient response 64–70 °C.

ing mode results, the sample has attained steady-state response after 5 min. In a comparable wet sys-tem, this time can exceed 30 min.

To determine the relative temperature sensitivity ofan asphalt binder, a sample of DSR verification fluidN2700000SP was also run from 64 to 58 °C. The resultsare shown in F i g u re 6. The asphalt binder shows an ap-prox. factor of 2 greater temperature dependence thanthe DSR verification fluid in this range.

Polymer-modified asphalt binder

Polymers are often added to asphalt binders to im-prove the performance behavior with particular em-phasis on obtaining desired viscoelastic properties.As an example, a polymer-modified asphalt demon-strates increased ring and ball softening point, re-duced penetration, increased ductility, and increasedtoughness and tenacity. These effects become morepronounced with increasing polymer loading. Al-though extensive efforts have been directed at im-proving the quality and performance of asphaltpaving materials, there appears to be no consensusamong the various authorities concerning the leastexpensive and most effective methods to reduce oreliminate cracking, potholes, distortion, and otherforms of pavement distress. Since the first sign ofpavement failure is usually the formation of tiny,hairline cracks, it is particularly important to pro-duce road surfaces that are tough and crack resistant,preferably through the use of additives that impart

18 / JULY 1999

frequencies, the elastic modulus approaches a limit-ing value, which is the glassy state modulus. At lowfrequencies, the slope of a log–log plot of the elasticmodulus versus frequency approaches 2, which sig-nifies that viscous flow has been reached, and thatthe asphalt is behaving as a Newtonian fluid. Usingthe characteristic parameters of the master curv e ,along with the mathematical relationships fortime–temperature dependence, it is possible to con-struct a unified mathematical model for the rheo-logical behavior of asphalt binders.

Tests were performed on two asphalt binders inthe temperature range from –10 to 50 °C in order todetermine the most significant viscoelastic function.At temperatures from –10 to 35 °C, 8-mm-diam par-allel plates were used in performing the dynamictesting. Shear moduli (G′, G′ ′) and complex viscosity(η*) are plotted versus frequency (rad/sec) in F i g u re7a and b and F i g u re 8a and b, r e s p e c t i v e l y. The com-plex viscosity (η*) of sample 2 increases considerablywith decreasing frequencies. On the other hand,sample 1 exhibits Newtonian behavior in the regionof lower frequencies. Unmodified paving asphaltusually exhibits Newtonian flow behavior at tem-peratures in excess of 50 °C.

F i g u res 9 and 1 0 are master curves using theWilliams, Landel, and Ferry (WLF) shifting proce-dure in the temperature range of –10 to 50 °C forsamples 1 and 2, respectively. In this case, the refer-ence temperature selected was 25 °C. Sample 2, con-taining 5% styrene butadiene rubber (SBR), has a

Figure 7 Shear modulus versus frequency results for samples 1 and 2 at selected temperatures.

a b

Figure 6 Temperature dependence of an original asphalt binder versus a calibration stan -dard.Figure 5 VISCOTECH DSR cooling transient response 64–58 °C.

greater strength and extendability, especially at lowtemperatures when shrinkage forces and embrittle-ment are likely to promote failure.

In the past few years, much experience has beenacquired in petrochemical research with regard tothe rheology of polymers. At the same time, the tech-nologies applied to the research of rheological char-acteristics of asphalt binders have been developed.This new research shows that asphalt binders can becharacterized more rigorously with viscoelastic data.

Time–temperature superposition

One of the primary analytical techniques used inanalyzing the dynamic viscoelastic data for asphaltinvolves the construction of master curves for dy-namic complex moduli, complex viscosity, andphase angle. In constructing such master curv e s ,use is made of the time–temperature superpositionprinciple. In constructing a master curve usingtime–temperature superposition, dynamic data arefirst collected over a range of temperatures and fre-quencies. A standard reference temperature mustthen be selected for the master curve. The data at allother temperatures are then shifted with respect totime until the curves merge into a single, smoothfunction. The time–temperature superposition prin-ciple also applies to materials undergoing a transi-tion such as the glass transition.

In a master curve, one or more of the viscoelasticfunctions is plotted against frequency. At very high

APPLICATION NOTE continued

crossover point of G′ and G′ ′, very close to sample 1.H o w e v e r, in the terminal zone, sample 1 approachesNewtonian behavior with a phase angle near 90°, in-dicating a constant viscosity at long times, whilesample 2 levels off at 65°, indicating an increasingviscosity at long times. The addition of a smallamount of SBR results in a dramatic effect on therheological properties and ultimate end-use perfor-mance characteristics of this asphalt binder in thelong time, i.e., low rates of deformation and applica-tion areas such as rutting.

Creep tests

Creep tests give extremely important practical in-formation and, at the same time, useful data to oneinterested in the theory of the mechanical propertiesof asphalt binders. Since asphalt pavements are de-signed to be flexible, they must quickly return to theiroriginal configuration after loading. Rutting, push-ing, and shoving are just a few of the failure mecha-nisms associated with inelastic or permanent defor-mation. Repeated loading without complete binderr e c o v e ry is also a cause of fatigue cracking. Althoughthe quality and gradation of the aggregate are impor-tant parts of the asphalt mix performance, the creepresponse of the binder is also a contributing factor. Ascreep is a time-dependent function, it is necessary tomonitor recovery per unit time or to stipulate a timei n t e rval for an expected recovery. An important mea-sure of polymer performance is its ability to recoverafter deformation. Since asphalt pavements are de-signed to be flexible, they must quickly return to theiroriginal configuration after loading. F i g u re 11 i l l u s-trates a creep/recovery test at a constant stress of 1000Pa applied over a time of 500 sec and recovery time of1000 sec at 25 °C for samples 1 and 2. The results indi-cate that sample 2 creeps less and recovers moreslowly than sample 1. As mentioned above, thesedata indicate that sample 2 is a stiffer binder with su-perior long-time creep properties.

Rheometer setup

The VISCOTECH DSR and SPECTECH DSR aresupplied standard will all measuring systems andsoftware to perform asphalt binder testing accordingto AASHTO TP5-97 specification. The base-modelVISCOTECH rheometer is a modular product with awide range of measuring systems and accessories.Additional measuring systems for VISCOTECH areconcentric cylinders, cone/plate, parallel plate, dou-ble concentric cylinders, closed/pressure cells, anddynamic mechanical analysis (DMA). Special mea-suring systems for low volume, high shear rates, andhigh sensitivity are also available. The measuring ge-ometries can be made in stainless steel, titanium,polycarbonate, or any user-defined material. The in-

20 / JULY 1999

strument includes normal force capability for repro-ducible sample loading history, thermal expansionmeasurements, and can be configured with a pat-ented Differential Pressure Quantitative NormalF o rce Sensor for quantitative normal stress measure-ments. The diffusion air bearing has a low inertiawith high axial and radial mechanical stiffness.

The SPECTECH DSR was designed to be applica-tions specific for testing of asphalt binders accord-ing to AASHTO TP5. The instrument features dis-posable plates and user-independent, automaticsample trimming.

Electronic unit

The VISCOTECH DSR instrument electronics arecontained within the mechanical unit, and the in-strument is built around a dedicated, high-speed 32-bit CPU. This consolidation enhances performanceand versatility due to having electrical connectionson the motherboard bus rather than through cablesto a separate electronics cabinet. In addition, valu-able bench space is kept to a minimum. The motorcontrol is based on digital rather than analog drivet e c h n o l o g y. The unit includes a built-in diagnosticsystem and quick diagnostic service port for serv i c eengineers. Also included is a modem port for re-mote-control operation and fault diagnostics for ser-vice. The electronics power supply is designed to op-erate on a line voltage of 180–260 V or 90–140 V andan operating frequency of 47–63 Hz.

Software package

The standard software package is a true multitask-ing Microsoft (Redmond, WA) Wi n d o w s ™ - b a s e drheological software that has many advantages tothe user. The computer is not dedicated to simplyrunning the instrument and is available for otheruse when making measurements. It can be used forprinting previous results, writing a report, or per-forming measurements with another instrument.The software runs under all Windows platforms.

The DSR software package offers measuring pro-grams designed to be user friendly with few subdia-logue levels and a general recognizable design. Prepro-grammed test parameters and pass/fail test resultlimits are included. Additional experiment methodsand analyses are available in the following test modes:v i s c o m e t ry, yield stress, constant rate, oscillation, os-cillation stress sweep, oscillation strain control,c r e e p / r e c o v e ry, compare, analyze, time–temperaturesuperposition, and spectrum transformation.

The software includes possibilities to link user-designed methods, including instrument setup andzero gapping using the project software. The dia-logue windows have many storable, editable func-tions for unique testing requirements, and can be

reset to default values using default buttons. An ex-ample is the Oscillation Frequency Step measuringprogram, where stresses, delay times, integrationperiods, and sample sizes can be set individually forall frequencies. Another example is the zoomingfunction, which is presented in both Vi s c o m e t ryStress Step and Oscillation Frequency Step, allowingany number of steps and increments to be selected.The instrument also performs controlled strain, andconstant shear rate measurements, and includes au-tomatic gap adjustments and compensation usingthe normal force sensor. This system enhances mea-surement reproducibility since the sample loadingh i s t o ry is reproduced identically each time, using aconstant loading force.

The VISCOTECH DSR is operated with a separatepower supply unit that should be left on continu-

Figure 8 Complex viscosity versus frequency results for samples 1 and 2 at selected temperatures.

a b

Figure 9 Master curve at 25 °C, sample 1.

Figure 10 Master curve at 25 °C, sample 2.

APPLICATION NOTE continued

ously. This reduces startup times and makes it possible for the instrument proces-sor to maintain values as gap and other user-defined settings.

The VISCOTECH DSR standard temperature control cell is a resistive heatingand adiabatic cooling system. Other temperature cells are available including cir-culating fluid and cryogenic cooling, covering the range –180 to 500 °C. All mea-suring geometries are supported, i.e., cone/plate/parallel plate, concentric cylin-d e r, and solid in torsion. A patented sealed cell for operating at elevatedpressures with full oscillatory capabilities is available.

AMERICAN LABORATORY / 21

Figure 11 Creep/recovery results for samples 1 and 2.

M r. Herh is Applications Manager, and Mr. Colo is President, ATS RheoSystems, 5 2G e o rgetown Rd., Bordentown, NJ 08505, U.S.A.; tel.: 609-298-2522; fax: 609-298-2795; e-mail: atsrheosystems@cwixmail.com; home page: www. a t s rh e o s y s t e m s . c o m /~ats. Dr. Hedman is with RHEOLOGICA Instruments AB, Lund, Sweden; tel.: 46 46127760; and Mr. Rudolph is Vi c e - P resident, Sales, ATS RheoSystems, To ronto, Ontario,Canada; tel.: 416-422-5474.

continued on p. 23