Lecture 6 Characterization of Asphaltic Materials Part 1-General Concepts

• Asphalt concrete is basically a mixture of asphalt binder and aggregates, hot-mixed in an asphalt plant and then hot-laid to form the surface course of a flexible pavement.

• The mix proportion is approximately 95% aggregate by weight, 75% aggregate by volume and ideally 3%-5% air voids.

• The properties of asphalt concrete depend on:

• Quality of its components (i.e., asphalt binder and aggregates).

• Mix proportions (void content, binder content, aggregate geometry and etc.).

• Construction process.

• Asphalt concrete must provide a

stable, safe, and durable road surface.

Introduction

• Stability of the asphalt concrete depends on the strength and flexibility of the mixture which is influenced by mix properties and field compaction.

• The strength must be sufficient to carry the load without developing excessive plastic deformation. The structure must remain intact to provide good ride quality.

• The main contributor to strength is friction between particles as well as adhesive and cohesive bonds between aggregates and mastic.

• A dense-graded mixture, composed of particles with angular rough faces, with a relatively thin asphalt film between them is best for high strength mixes.

• Flexibility is also an important phenomena in pavement engineering as the pavement is expected to rebound (recovery of elastic strains) during unloading process without cracking or brittle fracture. This is imporant for fatigue performance under repeated traffic loads.

• Therefore it is imperative to consider both strength and flexibility to account for the stability of flexible pavements.

Asphalt Concrete: Stability

• Its important to design the asphalt mixes so that they provide proper frictional properties between the tire and the pavement surface.

• Safety is achieved by making the surface course skid resistant and able to allow quick drainage of water from the surface.

• Hydroplaning happens when a layer of water builds between wheels and the pavement surface which leads to loss of traction.

Asphalt Concrete: Safety

• Durability of the asphalt concrete is critical to ensure that it maintains the stability and skid resistance properties during service life.

• As asphalt ages, the HMA layer becomes less flexible and more prone to loss of service with time and repeated traffic loads.

• Pavements fail (i.e., loss of serviceability) due to:

• Changes in the aggregates (loss of surface macro-texture, aggregate breakage due to excessive loads or environmental conditions, and etc.).

• Permanent deformation or rutting (improper compaction, bad mix design (high air voids or tender mixes which leads to vertical lateral movement of materials).

• Cracking, either due to fatigue, or low temperatures (can be controlled with proper mix design).

• Bleeding of asphalt (excess binder content in the mix which compromises the surface frictional properties).

Asphalt Concrete: Durability

Refinery Operations

• During the distillation process, lighter

molecules vaporize and asphalt binder

remains.

• The consistency of the residual is highly

variable and depends on the source and the distillation process.

Chemical Composition of Asphalt Binder

- Large organic molecules of varying size and polarity Carbon 80 - 87% Nitrogen 0 - 1% Hydrogen 9 - 11% Sulfur 0.5 - 7% Oxygen 2 - 8% Heavy metals 0 - 0.5% Heavy metals play an important role as they contribute to polarity.

- Molecular structure very complex Asphaltenes - largest and most polar Resins - intermediate, also polar Oils - smallest, paraffin - like, non - polar

- Colloidal model Asphaltenes surrounded by resins Oils continuous medium

Asphaltenes Resins Oils

Asphalt Concrete Mixtures

• Mixtures of aggregate and asphalt binder.

• About 95% aggregate by weight

• About 75% aggregate by volume

• Ideally, 3-5% air voids

Challenges for Characterization of “Asphaltic Materials”

20 year old pavement

Variation of layer Moduli with Time

Mechanical Behavior of Asphalt Mixes

• Thermoplastic Material

• Material properties change with temperature.

• Rheological Material

• Material properties change with time or frequency of loading.

Temperature Regimes where Distress Predominates

-25 7550250

Approximate Temperature, C

Co

nsis

ten

cy

Low-temperature thermal

Shrinkage cracking

Intermediate-temperaturetraffic-associated fatigue

High-temperaturerutting

Plexiglas

Salt Water

Taffy

Molasses

Binder Characterization

The main purpose of binder grading systems is to classify binders based on their rheological

and mechanical properties, assuming that these properties relate to field performance.

• The asphalt grading systems are:

• Penetration Grading (ASTM D 946) • Based on a penetration of a needle in 0.1 mm in five seconds.

• Five Grades: Pen 40-50, Pen 60-70, Pen 85-100, Pen 120-150, and Pen 200-300.

• Viscosity Grading (ASTM D 3381) • Based on absolute viscosity at 140 0F (60 0C).

• Five Grades: AC-2.5, AC-5, AC-10, AC-20, and AC-40. (the number following AC indicates the absolute viscosity in hundreds of poise. (e.g. AC-5: binder with absolute viscosity of 500 poise).

• Viscosity of Aged Residue Grading (ASTM D 3381) • Based on absolute viscosity of RTFO aged binder at 140 0F (60 0C).

• Five Grades: AR-1000-AR-2000, AR-4000, AR-8000, and AR-16000

• Superpave Performance Grading (most commonly used). • Based on temperature range and climatic conditions of a specific geologic location.

Binder Grades (Classification)

Asphalt Penetration Test (Consistency Test)

• The penetration test started out using a No. 2 sewing machine needle mounted on a shaft for a total mass of 100 g. This needle was allowed to sink into a container of asphalt binder at room temperature (25 oC) for 5 seconds.

• The consistency (stiffness) of a given asphalt binder was reported as the depth in tenths of a millimeter (dmm) that the needle penetrated the asphalt binder.

• Based on penetration test asphalts are categorized

into five grades:

Pen 40 - 50

Pen 60 - 70

Pen 85 - 100

Pen 120 - 150

Pen 200 - 300

Temperature25C (77F)

High

Medium

Low

• Temperature susceptibility (i.e., the rate of change in material properties with a change in temperature) can be estimated by determining the penetration at two (or more) temperatures.

• The most commonly used temperatures are 4 oC and 25 oC with a 100 g load for 5 seconds.

• This slide highlights one of the major problems with the penetration grading system. For example, three sources of asphalt binder can have the same penetration at 25 oC but decidedly different properties above and below this temperature.

• This helps explain the differences in observed

pavement performances even though the same penetration grade of asphalt binder is specified.

Penetration Graded Binders- Disadvantage

Comparison of Grading Systems

4050

6070

85100

120150

200300

Penetration Grades

AC 40

AC 20

AC 10

AC 5

AC 2.5

100

50

10

5

Vis

cosi

ty, 6

0C (

140F

)

AR 16000

AR 8000

AR 4000

AR 2000

AR 1000

PG 64 - 22

Performance

Grade

Average 7-day max. pavement

temperature

Min. pavement

temperature

Superpave Performance Grade (PG) Asphalt Binder Specification

The PG Binder designation is based on expected extremes of hot and cold pavement temperatures.

Performance Graded (PG) Binder Specification

MaximumTemperature

(ºC)Minimum Temperature ( ºC)

PG 46 -34 -40 -46

PG 52 -10 -16 -22 -28 -34 -40 -46

PG 58 -16 -22 -28 -34 -40

PG 64 -10 -16 -22 -28 -34 -40

PG 70 -10 -16 -22 -28 -34 -40

PG 76 -10 -16 -22 -28 -34

PG 82 -10 -16 -22 -28 -34

As an example, a PG 64--28 is acceptable for use in a

climatic region where the maximum temperature is

64°C and the minimum temperature is -28°C.

(0C)

Observed Air Temperatures Topeka, KS

36

40

-23-31

0 10 20 30 40 50 60-10-20-30-40

average winter

> standard

deviation of 4°C

very cold winter

56

60

-23-31

0 10 20 30 40 50 60-10-20-30-40 70

Calculated Pavement Temperatures Topeka, KS

For the low temperature use the air temperature.

PG Asphalt Binder Grades Topeka, KS

0 10 20 30 40 50 60-10-20-30-40 70

PG 64-34 (98% minimum reliability)

PG 58-28 (50 % minimum reliability)

PG asphalt binder grades - six degree

increments

Typical Asphalt Binder Tests

Flash Point

Temperature at which a material will ignite with an open flame. Important for safety during the mix preparation.

Viscosity Rotational viscometer measures the viscosity at a standard temperature (e.g. 135 0C)

Complex Shear Modulus

Dynamic Shear Rheometer (DSR) to determine viscoelastic material properties (dynamic shear modulus and phase angle).

Flexural Creep Bending Beam Rheometer (BBR) as an indicator of creep stiffness properties of the binder.

Asphalt Aging

Rolling Thin Film Oven Test ( RTFOT) as an Indicator of the aging effect of short term high temperatures when producing the asphalt mix.

Tensile Strength

ASTM D 4402 or AASHTO T 316

Rotational Viscometer

4

)/1/1( 2

0

2 RRT i

Torque (T) required to maintain a rotational

speed () at constant temperature (e.g. 135 0C).

. Rotational viscosity, :

Ri = Spindle Radius

Ro = Chamber Radius

Binder Tests-Viscosity

Asphalt Binder Viscosity and Temperature

Mixing and Compaction Temperatures

.1

.2

.3

.5

1

10

5

100 110 120 130 140 150 160 170 180 190 200

Temperature, C

Vis

co

sity

, P

a.

s

Compaction Range

Mixing Range

Information from viscosity test can be used to estimate appropriate mixing and compaction temperatures.

AASTHO T 315

Dynamic Shear Rheometer (DSR) to measure dynamic properties (|G*|, d):

3

max /2 rT

Parallel plate rheometer

applies shear stress

T= Maximum applied torque

h= Specimen height

q= Deflection angle

r = Plate radius

Binder Tests- Dynamic Shear Modulus

hr /max q

hr /max q

Dynamic Shear Rheometer (DSR)

Area for

Liquid Bath

Motor

Parallel Plates

with Sample

Time A

A

B

C

Test operates at 10 rad/sec or 1.59 Hz 360o = 2 p radians per circle 1 rad = 57.3o

C

A

B

Fixed Plate

Oscillating Plate

One Cycle

Dynamic Shear Modulus and Phase Angle

• In a perfectly elastic material, there is no lag between the applied stresses and measured strains, therefore the stresses and strains are in phase and the phase angle is zero (d=00).

• The stresses applied on a fully viscous material is out of phase with strains therefore (d=900).

• Asphalts are viscoelastic materials therefore (00<d<900).

• ASTM D 6648 or AASTHO T 313

• Bending Beam Rheometer (BBR) Used to measure creep caused by a load applied in the middle of the beam.

Binder Tests- Flexural Creep

Sample Results of BBR and DSR Tests on the Same Binder

AASTHO T 134

Direct Tension Test (DTT)

Direct tension load applied to maintain a constant displacement rate at a standard low temperature.

Result: tensile strain and stress at maximum load.

Binder Tests- Tensile Strength

• ASTM D 6521

Pressure Aging Vessel (PAV) test to simulate long term aging: asphalt binder subjected to oxygen at high pressure and high temperature.

• ASTM D 2872 or AASTHO T 240

Rolling Thin-Film Oven (RTFO) test to simulate short term aging (aging during mixing and construction).

Binder Tests- Asphalt Aging

Bottles with asphalt placed in a rotating rack at a high temperature (mixing temperature) to simulate short term aging of asphalt.

Types of Asphalt Mixes

Asphalt concrete mixtures can be classified into following two types based on whether hot-mixed, hot laid or cold-mixed, cold-laid:

Hot-mixed, Hot-laid Asphalt (HMA) Concrete Mixture.

Cold-mixed, Cold-laid Asphalt Concrete Mixture.

Asphalt concrete mixtures can be classified into following two types based on whether in-situ-mixed or plant-mixed:

Road-mixed Or In Place-mixed Asphalt Concrete Mixture.

Plant-mixed Asphalt Concrete Mixture.

HMA concrete mixtures can be classified into following three types based on type of aggregate grading used:

Dense-graded HMA Concrete Mixture.

Stone Matrix Asphalt (Sma) Concrete Mixture.

Open-graded Hma Concrete Mixture.

Asphalt concrete mixtures can be classified into following three types based on the type of additives used:

Rubber-modified Asphalt Concrete Mixture.

Polymer-modified Asphalt Concrete Mixture.

Sulfur-modified Asphalt Concrete Mixture.

Types of Asphalt Concrete

• HMA is produced and laid in the following steps:

• Both aggregate and asphalt are heated prior to mixing to drive off moisture from the particles and make the asphalt sufficiently fluid (maximum temperatures for heating asphalt and emulsified asphalt are 176.6 °F and 82.2 °F, respectively)

• After heating, all the raw materials are mixed in the plant, and the hot mixture is transported to the paving site and spread on a loosely compacted layer to a uniform, even surface with the help of a paving machine.

• While the mixture is hot it is compacted by heavy, motor-driven rollers to produce a smooth, well-compacted paving course.

• Since the aggregates are thoroughly dried prior to mixing, stripping of asphalt (i.e., disintegration from the pavement) is expected to be minimal or nonexistent for hot-mixed, hot-laid asphalt pavements

Hot Mix Asphalt (HMA) Production Process

Warm Mix Asphalt (WMA)

Warm asphalt mixes are separated from half-warm asphalt mixtures by the resulting mix temperature. If the resulting temperature of the mix at the plant is less than 100 °C (212 °F), the mix is considered a half-warm mix. If the temperature of the mix at the plant is above, 100 °C (212 °F), the mixture is considered a warm mix. There is still a wide range of production temperatures within warm mix asphalt, from mixes that are 20 °C to 30 °C below HMA to temperatures slightly above 100 °C (212 °F).

Similar to hot-mixed asphalt concrete, cold-mixed asphalt concrete is also a mixture of asphalt, fine aggregate or both fine and coarse aggregates, and mineral filler (optional).

Cold-mixed asphalt concrete is produced and laid at normal temperature, however, some heating of both the aggregates and asphalt may be required during winter season.

Drying of aggregates is not necessary except when the particles have surface moisture.

Commercial additives are needed in this type of asphalt concrete to improve bonding.

Cold Mix Asphalt

• A bituminous surface or base course produced by mixing aggregates and asphalt at the jobsite is called road-mixed or mixed-in place asphalt concrete.

Road-Mixed and

Plant-Mixed Asphalt Mixes

• A mixture of aggregates and emulsified or cutback asphalt prepared at a central mixing plant and spread and compacted at the jobsite at near ambient temperature is called plant-mixed, cold-laid asphalt concrete.

A dense graded HMA mix is produced using well-graded aggregates, and intended for general use.

When properly designed and constructed, a dense-graded HMA concrete is relatively impermeable.

Dense-graded HMA concrete mixes are generally referred to by their nominal maximum aggregate size.

Fine-graded mixes have more fine and sand sized particles than coarse-graded mixes.

Dense Graded HMA

Stone matrix asphalt (SMA) is a gap-graded HMA that is designed to maximize deformation (rutting) resistance and durability by using a structural basis of stone-on-stone contact.

Because the aggregates are all in contact, rut resistance relies on particles interlock rather than binder adhesive and cohesive bonds.

Since aggregates do not deform as much as asphalt binder under load, this stone-to-stone contact is believed to reduce rutting.

SMA is generally more expensive than a typical dense-graded HMA (about 20 - 25 percent) because it requires more durable aggregates, higher asphalt content and, typically, a modified asphalt binder.

In the right situations it should be cost-effective because of its increased rut resistance and improved durability.

Stone Matrix Asphalt (SMA)

An open-graded HMA mixture is designed to be water permeable. (dense-graded mixes usually are not permeable).

Open-graded mixes use only crushed stone and a small percentage of manufactured sands.

There are three types of open-graded mixes typically used in the U.S.:

Open-Graded Friction Course (OGFC): Typically 15 percent air voids, no minimum air voids specified, lower aggregate standards than Porous European mixes (PEM).

Porous European mixes (PEM): Typically 18 - 22 percent air voids, specified minimum air voids, higher aggregate standards than OGFC and requires the use of asphalt binder modifiers.

Asphalt Treated Permeable Bases (ATPB): Less stringent specifications than OGFC or PEM since it is used only under dense-graded HMA, SMA or PCC for drainage.

Open-Graded HMA

Asphalt rubber also called ‘crumb rubber’, which is a recycled product from old tires, is added ranging from 1% to 5% (by weight of asphalt) as an additive in the production of HMA for improving the flexibility and therefore fatigue performance of pavement systems.

Rubber addition increases the viscosity and the softening point of the asphalt.

Polymers (such as ethyl vinyl acetate, silicone, and epoxies) are added to asphalt as additive to produce polymer-modified asphalt concrete

Polymer addition increases dispersion, ductility, and adhesiveness of asphalt. It’s often used to reduce the temperature sensitivity of the stiffness properties of the mixes.

Sulfur is added to asphalt concrete to provide higher stiffness at elevated temperatures.

Modified Asphalt Mixes

Asphalt Mixture

Weight-Volume Relationships

Asphalt Concrete Mixtures

• Mixtures of aggregate and asphalt binder.

• About 95% aggregate by weight

• About 75% aggregate by volume

• Ideally, 3-5% air voids

During the asphalt mix preparation, some of the asphalt is absorbed in the pores of the aggregate particles. The portion of asphalt absorbed by aggregate particles is called “absorbed asphalt”.

The net amount of asphalt available to coat and bind aggregates together is called “effective asphalt”.

It is important to account for the absorbed asphalt when using porous aggregates during mix design.

Effective Asphalt Content

Asphalt Concrete Mixture-Phase Diagram

The mass/volume relationships of a compacted asphalt mixture are illustrated in the following figure:

M Total mass (= MG + MB)

MG Mass of aggregate

MB Mass of asphalt (binder) (= MBE + MBA)

MBE Mass of effective asphalt, the asphalt binder between particles

MBA Mass of absorbed asphalt, absorbed into the pores of the aggregate

particles

V Total volume of the compacted mix

VG Volume of aggregate, the bulk volume including the aggregate

pores

VBE Volume of effective asphalt

VBA Volume of absorbed asphalt

VB Volume of asphalt (= VBE + VBA)

VA Volume of air between the coated aggregate particles in the mix

VGE Effective volume of aggregate (= VG – VBA)

VMM Volume of voidless mix (maximum mix volume)

Mass/volume relationships for an asphalt concrete mixture:

Density () = M/V

Asphalt content (PB) PB = MB / M

Effective asphalt content (PBE) PBE = MBE / M

Asphalt absorption (PBA) PBA = MBA / MG

Air voids (AV) AV = VA / V

Voids in mineral aggregate (VMA) VMA = (VBE + VA)/V

Voids filled with asphalt (VFA) VFA = VBE / (VBE + VA)

Asphalt Concrete Mixture-Example