Simulation, Imaging, and

Mechanics of Asphalt

Pavements

Asphalt concrete is a complex material that consists of aggregates, asphalt, and air

voids. Aggregates are held together by asphalt and form a skeleton to support the

weight of vehicles. The stiffer the asphalt, the tighter the aggregates are held together and the heavier the load a pavement is able to handle. Certain aggregate

structures have stronger skeletons than others and thus can support heavier loads.

In an effort to build more durable pavements, State highway agencies use different

mix-design procedures to find the strongest aggregate skeletons. Current mix procedures are empirical and are based on trial-and-error methods. They are timeconsuming and may sometimes lead to costly pavement failures. The Asphalt Team

at the Turner-Fairbank Highway Research Center (TFHRC) is currently working to

develop the scientific basis underlying the mix-design process and to establish criteria to predict pavement performance. These efforts involve the simulation, imaging, and mechanics of asphalt pavement (SIMAP).

Through a multidisciplinary approach, the SIMAP program is pursuing the following objectives:

To develop three-dimensional (3-D) imaging techniques to capture the internal

structure of asphalt concrete specimens.

To predict the response of asphalt concrete by applying mechanics to

3-D images.

To analyze and quantify the aggregate structure to determine aggregate orientation, segregation, and aggregate-aggregate contacts.

To develop computer models to simulate the internal structure of asphalt concrete and to predict its behavior based on volumetric and mechanics principles.

To develop a systematic process that incorporates the technology acquired in

this project into a useful tool for asphalt-mix design.

SIMAP

Research and Development

Turner-Fairbank Highway Research

Center

6300 Georgetown Pike

McLean, Virginia 22101-2296

Bridge Coating

Non-Destructive

Evaluation

Recycling

Asphalt Concrete

Portland Cement

Concrete

Highway Operations

RoSAN

SIMAP

Visit us at www.tfhrc.gov
Why?

The methods that are currently being used for the description of aggregate structures are based on consensus

properties. In fact, most of the information available on

gradation, angularity, texture, and volumetric properties

of asphalt pavement are solely based on previous field

experiences. By developing techniques to image and to

quantify the structure of aggregates in asphalt concrete,

and by applying mechanics that account for the aggregate structure, it will be possible to quantitatively relate

the raw material properties to the performance of pavements in a scientific way. This will develop the foundations to building more durable pavements.

How?

Our basic approach to studying asphalt is to develop parameters that describe the aggregate structure and relate

to pavement performance. This approach will enable us

to develop a deeper understanding of the structure, develop and optimize the parameters, and optimize pavement performance.

X-ray tomography (CAT scanning) is being used to visualize aggregate structures in 3-D. We are currently developing the capabilities of 3-D image analysis to utilize this

cutting-edge technology to its fullest potential. Through

two-dimensional (2-D) image analysis, we have successfully quantified aggregate orientation and aggregate segregation and have gained useful information about the internal aggregate structure. We are also using computer

simulations to find the relationships that exist between gradations and the internal structure of aggregates and to determine which gradations should be used to obtain a predesigned aggregate structure.

The SIMAP program treats asphalt pavement as cemented

granular material and uses granular mechanics as a means

to describe the stress-strain behavior of pavements. Photoelastic imaging and mechanics modeling are being used

to obtain qualitative and quantitative descriptions of aggregate structures. Unlike previous efforts, these techniques will incorporate the effect of asphalt/mastic on

pavement performance.

What If?

Proposed Method

Apply mechanical or

mechanical-empirical

models

Select mix that meets

requirements

Measure properties

(SST, IDT, etc.)

Predict pavement

performance

Mix

likely to

perform

well?

Asphalt,

aggregate

selection

Computer Modeling

& Simulation

YES

NO

How to Accomplish?

3-D Imaging and Simulation

Mix

likely to

perform

well?

Asphalt,

aggregate

selection

Computer Modeling

& Simulation

YES

NO

Research Topics

Aggregate structureperformance

relationships?

Optimum aggregate

structure?

Load bearing abilities?

Effect of asphalt/

mastic?

Approach

Aggregate structure

Optimum aggregate

structure

Load bearing 

abilities

Effect of asphalt/

mastic

Methods

3-D imaging (x-ray

tomography)

Computer simulation

and optimization of

aggregate packing

Granular mechanics

Contact mechanics

Where We Are

Superpave Method

Asphalt, aggregate

selection

Compact specimens

gyratory

Apply mechanical or

mechanical-empirical

models

Select mix that meets

requirements

Measure properties

(SST, IDT, etc.)

Predict pavement

performance
Contact Mechanics

Granular Mechanics

Asphalt Concrete Imaging and Analysis Aggregate Packing Simulation

A 3-D image of a 150-mm diameter core of asphalt concrete

(from the WesTrack project) was obtained by x-ray tomography

(a). This image can be treated as a virtual specimen that can be

cut (as in (b)), analyzed to study voids (c), aggregate structure (d),

and other features. Efforts are underway to quantify these features and to study the mechanical behavior of this virtual specimen. Simultaneously, 2-D image analysis is also being applied to

study aggregate alignment, aggregate segregation, voids content and distribution, and to perform gradation analysis (e). XRay micro-tomography is also being utilized to study asphalt

concrete in a finer scale such as in (f), in which the uneven distribution of asphalt in a typical pavement is shown.

(a)

(b)

(c)

(d)

(e)

(f)

32 mm

Study influence of gradation, shape, etc., on

aggregate structure.

Optimize percent voids, aggregate aggregate

contacts, etc.

Treats asphalt concrete as a collection of

aggregates cemented with mastic.

Considers each aggregate as a discrete

element.

The system’s response is equal to the sum of

the responses of individual elements.

Photo-elastic disks having a typical gradation were

viewed through polarized light. The bright lines

show the stress paths within the system. While

under 50 percent particles transmitted load, only

15 percent carried substantial load.

Stress transfer through an aggregateaggregate contact depends on the relative

stiffness of mastic and aggregate.

Optimize mastic properties for performance.
Benefits

So far, it has been possible to measure aggregate segregation and orientation and to

compare different compaction methods. Not only are we being able to characterize

an asphalt core thoroughly, but we have also started correlating pavement performance to internal aggregate structures.

The SIMAP program now has the tools necessary to verify some concepts used in

asphalt technology such as asphalt film thickness and voids in mineral aggregates.

We have also started using 3-D imaging in forensic work to determine causes for

pavement failure. In the future, we will work towards the optimization of mixture

components and the development of a mixture-design process.

For more information contact:

Naga Shashidhar (703) 285-2070

Robert Davies (703) 285-2793

Tom Harman (703) 285-2772

3-D Imaging

and Image

Analysis

Aggregate

Packing

Simulations

Granular

Mechanics

Contact

Mechanics

Asphalt Technology – Road to the Future

July 1998

FHWA-RD-98-128