TECHBRIEF

The Long-Term Pavement Performance (LTPP) program is a 20-year

study of inservice pavements across

North America. Its goal is to extend

the life of highway pavements

through various designs of new and

rehabilitated pavement structures,

using different materials and under

different loads, environments, subgrade soil, and maintenance practices.

LTPP was established under the

Strategic Highway Research Program,

and is now managed by the Federal

Highway Administration.

O2@@@@@6K? ?O2@@@@@@@@@@@6K @@@@@@@@@@@@@@@@6K V@@@?@@@@@@@@@@@@@6X ?W2@@@@@@@XI'@@@@@@@@@@)X? W&@@@@@@@@)XV'@@@@@@@@@@)X ?W&@@@@@@@@@@1?V'@@@@@@@@@@1 W&@@@@@@@@@@@@eN@@@@@@@@@@@L? 7@@@@@@@@@@@@@e?@@@@@@@@@@@1? @@@@@@@@@@@@@@e?3@@@@@@@@@@@L ?J@@@@@@@@@@@@@@e?N@@@@@@@@@@@1 ?7@@@@@@@@@@@@@@f@@@@@@@@@@@@ ?@@@@@@@@@@@@@@5e?J@@@@@@@@@@@@ ?@@@@@@@@@@@@@(Ye?7@@@@@@@@@@@@ ?@@@@@@@@@@@@0Y?e?3@@@@@@@@@@@@ ?@@@@@@@@@@(M?f?N@@@@@@@@@@@@L? ?@@@@@@@@@0Yh@@@@@@@@@@@@1? ?@@@@@@@(Mhe3@@@@@@@@@@@@? ?@@@@@@@H?heV4@@@@@@@@@@5? ?@@@@@@5e?O2@@6K?fI4@@@@@@@@H? ?@@@@@@H?W2@@@@@@6K?g?W@@@@ ?3@@@@@?W&@@@@@@@@@6K?fO&@@@@ ?N@@@@@?7@@@@@@@@@@@@6K?O2@@@@@5 3@@@@?@@@@@@@@@@@@@@@@@@@@@@@H N@@@@?@@@@@@@@@@@@@@@@@@@@@@5? ?3@@@?@@@@@@@@@@@@@@@@@@@@@(Y? ?V'@@@@@@@@@@@@@@@@@@@@@@@(Y V'@@@@@@@@@@@@@@@@@@@@@(Y? ?V'@@@@@@@@@@@@@@@@@@@(Y V4@@@@@@@@@@@@@@@@@(Y? ?I4@@@@@@@@@@@@@0Y ?I4@@@@@@@@0M? ?I40M?

?@@?e@@eW2@@@?e?@@@@6X? ?W2@?@@@@@@? ?@@? ?@@?e@@e7@@@@?e?@@@@@)X ?O26K? ?@6XheW&@5?@@@@@@? ?O@Kg?O26X@H? ?@@?e@@e@@X?f?@@?I'@1?W2@@6?2@@@6K??O2@@@@@@@@@@@@@@@6?2@6X?W2@6KO2@@@6X@@)X?eO2@6KO&@@He?@@?@@6?2@@@@@@@@@6KO2@@?@?2@6X??O2@6KO2@@@@@@6?2@@@@@@@@@?W2@@6X?@@@@6X ?@@?e@@e@@)Kf?@@??N@@W&@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@)?&@@@@@@@@@@@@@@)??@@@@@@@@@5?e?@@@@@@@@@@@@@@@@@@@@@@@?@@@@@)?2@@@@@@@@@@@@@@@@@@@@@@@@@W&@@@@)X@@@@@1 ?@@?e@@e@@@@6Xe?@@?e@@@@@??@@@@?I'@@@@(M?@@@@(M?@@?@@?@@(Y@@@@@??@@@@??@@@@?e?@@?I'@@@@H?e?@@@@@@@(M?@@@@@V'@@@@e?@@?I'@@@@?I'@@@@(M?@@@@(?'@@?@@?@@@(MI'@@@@e@@ ?@@?e@@e?I4@@1e?@@?e@@@@@@@@@@@??N@@@@H??@@@@He@@?@5?@@H?@@@@@@@@@@@??@@@@?e?@@??N@@@@f?@@@@?@@H??@@@@@?N@@@@@6X@@??N@@@@eN@@@@He@@@@H?N@@?@@?@@@e?N@@@@e@@ ?@@Le@@f?W@@e?@@?e@@@@@@@@@@@L?J@@@@L?J@@@@?e@@?@H?@@e@@@@@@@@@@@??@@@@?e?@@L?J@@@@f?@@@@?@@L??@@@@@e@@?@@@@@@??J@@@@eJ@@@@?e@@@@eJ@@?@@?@@@1??J@@@@e@@ ?3@)K?@@?O26?&@@L??@@W2@@@@@@?@@@@@)?&@@@@)?&@@@@?e@@?@e@@e@@@@@?@@@@@??@@@@?e?@@)?&@@@@f?@@@@?3@)K?@@@@@e@@@@?@@@@?O&@@@@?O&@@@@?e@@@@?O&@@?@@?@@@@?O&@@@@e@@ ?V'@@@@@@@@@@@@@@??@@@@@0MI'@@@@@@@@@@(?4@@@@@@@@?e@@?@e3@e@@V'@@@@@@@??@@@@?e?3@@@@@@@@f?@@@@?V'@@@@@@@5e@@@@@@@@@@@@(?'@@@@@@@@?e@@@@@@@@@?@@?@V'@@@@(Y@@e@@ V4@@0MeI4@0M?e?@@@0Me?V4@0M?@@@@0Y??I4@@@@@@?e@@?@eV'e@@?V4@0MI4@??@0?@?e?V4@@0MI4@f?@0?@??V4@@@0?(Ye@@@@@0Y@@@@0Y?V4@@0MI4@?e@MI4@@@@@?@@?@?V4@@0Y?@@e@@ ?@@? ?@ ?@@? ?@

?@@@@@@?he?@6X W2@?eW26Xe@@@@@@g?@@@ ?@@@@@@?he?@@1 7@@?e7@@1e@@@@@@g?@@@ ?@@@6Xg@@@?he@@@?g@@@?f?@6Xhf?@@@?@@? ?@@@@@@=O2@6X??O26X@@@?O26K?e?O@KO26Ke@@@?e@@@@e@@@@@@?O26K??@@@@6K?hfO@K?hf?@@@@1g@@@?he@@@?g@@@?f?@@)K?he?@@@?@@? ?@@@eV@@@@@)?2@@@@@@@@@@@@@@@@@@@@@@@@@@@@?e@@@@e@@@@@@@@@@@@@@@@@@@@@@6?2@6?2@@@@@@@@@@@6?2@@?eJ@@@@@L?@@@@@@@@@@@@@@@@6X@@@@@ ?@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@?@@?e@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@5?e7@@@@@)X@@@@@@@@@@@@@@@@@@@@@ ?@@@@@@@@@@@@@@@@V'@@@@@@@@@@@@(Y@@@V'@@?@@?e@@@@@@@@@@@@@@@?@@@@@@?@@@@@@@@@@@@@@@@V'@@@@@@@@@H??J@@@?@@@@@@(Y@@@@@@?@@@?@@@@@ ?@@@e?@@@@@@@@@@?V@@@@@@@@@@@@H?@@@?V@@?@@?e@@@@e@@@@@@@@@?@@@@@@?@@@?@@@@@@@@@@@@?V@@@?@@@@5e?7@@@@@@@@@@Y?@@@@@@?@@@?@@@@@@@@ ?@@@e?@@@@@@?@@@@@@@@@@@@@@?@@??@@@@@@@?@@?e@@@@e@@@@@@@@@@@@@@@@?@@@?@@@@@@@@?@@@@@@@@?3@@@He?@@@@@@@@@@@@@@@@@@@?@@@?@@@@@@@@ ?@@@e?3@@@@@@@@@@@@@V'@@@@5?@@??3@@@@@@?@@?e@@@@e@@@@@@@@@@@@@@@@?@@@?3@@@?@@@?3@@@@@@@e@@@?e?@@@?I'@@@@@@@@@@@@@?@@@?@@@@@@@@@ ?@@@e?V4@@@0MI4@@@@@?V4@@0Y?@@??V4@@@@@?@@?e@@@@e@@@@@@X@@@@@@@@@?@@@?V4@@?@@@?V4@@@@@@?7@@5?e?@@@eV4@@@@@@@@@@@@?@@@?@@@@@@@@@ ?B@@@@@@(M ?@@@H? @@@@@0Y? ?@@@

@@@@?@@@@@@@@@@X?@@@@@@@@@@(?'@@?@@@?@@@@@(Y@@@@@@?@@@ @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@1 @@@6X@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@W2@@@@6?2@@@@6X

@?@@@@@@@@@@1?@@@@@@V'@@?e@@?@@@?@@@@@Y?@@@@@@?@@@ @?@@@@@@@@@@@?@@@@@@?N@@)?&@@?@@@?@@@@@@@@@@@@@?@@@ ?@@@@@@@@@@@?@@@@@@??3@@@@@@?@@@?@@@@@@@@@@@@@?@@@ ?@@@@@@@@@@@?@0?4@@??V4@@@@@?@@@?@@0?4@@@0?4@@?@@@

ggggggg

Research and Development

Turner-Fairbank Highway

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Advanced Methods for Using

FWD Deflection-Time Data to

Predict Pavement Performance

FHWA Contact: Cheryl Richter, HNR-30, (703) 285-2183

Introduction

An important property of materials that defines the viscoelastic and

inelastic characteristics of materials is the dissipated work or dissipated

energy of the material. Dissipated energy is simply defined as the area

included in the loaded and unloaded portion of the stress-strain curve

(referred to as the hysteresis loop). Dissipated energy has been used in

the asphalt concrete fatigue area for many years by some agencies.

Similarly, the falling-weight deflectometer (FWD) load deflection-time

data can be used to measure the dissipated work during the loading and

unloading of the pavement structure from the FWD impact load. This

dissipated work should be related to the occurrence of selected surface

distresses, especially for asphalt concrete-surfaced pavements.

The deflection-time history data collected within the Long-Term

Pavement Performance (LTPP) program represent an invaluable data

source and critical data element that has yet to be thoroughly investigated and used to its full potential in pavement diagnostic studies. As

such, a limited study was undertaken under contract number DTFH61-

95-C-00029 to determine if there is any relationship between the dissipated work as measured with the FWD and levels of pavement distress.

This study also shows some of the different parameters that can be used

from the deflection-time data and the benefit of using these data for

pavement diagnostic studies and pavement classifications.

Background

Wave propagation nondestructive testing (NDT) methods were largely

initiated by the U.S. Air Force for nondestructive pavement evaluations

in the late 1960’s. However, a procedure was not formally adopted by

the Air Force for routine pavement evaluations until 1978.(1) During the

late 1970’s, transient wave propagation behavior became better understood, and more reliable instrumentation for measuring the pavement

response was available. For these types of tests, deflection-time histories of motion from an applied dynamic load are recorded by several

receivers or sensors placed on the pavement surface. By computing the

surface-wave travel time between adjacent receivers, as produced by

different frequencies, a dispersion curve is obtained relating phase

velocities to frequencies (or wavelengths). This type of testing includes

the FWD, which has the most widespread use because of its ability to

impose high-amplitude dynamic loads.
Load-Deflection Response

Data

The load pulse produced during

deflection testing with an FWD

generally occurs over a time of

about 15 to 35 ms. Both the loading time and load-pulse shape can

have an effect on the measured

peak deflection basins, especially

for viscoelastic materials. This is

graphically illustrated in figures 

1 and 2, and clearly shows differing creep effects from the applied

load.

Another parameter received

from these data is the recovery

time for the induced deflections

(i.e., the time required to recover

all of the deflection). The time to

recover all of the peak deflection

generally varies from about 25 ms

to more than 60 ms. In fact, for

some of the sites, all of the deflection still had not been recovered at

60 ms.

Using the deflection-time plots,

the pavement can be categorized

into two basic types of response:

elastic and viscoelastic. The elastic and viscoelastic properties of

the pavement structure can be

illustrated by reviewing the deflection-time data measured with the

FWD. Figures 2 and 3 show these

different types of pavement

response characteristics. Figure 2

shows the response for a pavement section that is basically elastic, while figure 3 shows pavements that are considered viscoelastic.

A pavement that behaves elastically will recover most or all of the

induced deflection immediately

after the load pulse reaches zero,

as shown in figure 2. General

Pavement Study (GPS) test section 481056 (figure 2) is a thin

asphalt concrete-surfaced pavement (less than 51 mm (2 in) in

Double-peak FWD load pulse and deflections measured by

each sensor at GPS site 011001.

FIGURE 1

Typical FWD deflection-time data from testing performed on

an asphalt concrete pavement with elastic behavior, GPS site

481056.

FIGURE 2

Typical FWD deflection-time data from testing performed

during the month of January on an asphalt concrete

pavement with some viscoelastic behavior, GPS site 481060.

FIGURE 3
thickness). Asphalt concrete mixtures are viscoelastic materials,

but the surface of this test section

is so thin that the viscoelastic

properties are insignificant in relationship to the total measured

deflection.

A highly viscoelastic pavement

will take time to recover the

induced deflection after the load

pulse reaches zero, as shown in

figures 1 and 3. As shown, the

maximum load and peak deflections are not coincident, and it

takes nearly 20 ms past the end of

the load pulse for the pavement to

recover the deflection. GPS test

section 481060 (figure 3) is a relatively thick asphalt concrete-surfaced pavement (193 mm (7.6 in)).

The time difference between peak

load and peak deflection is very

pronounced.

Dissipated Work

Dissipated work, as measured by

the FWD, was calculated for several LTPP-GPS sites during similar

time periods (summer months).

Figures 4 and 5 show examples of

the hysteresis loop used to calculate dissipated work for different

types of pavements that vary from

very thin to very thick, and from

soft to stiff. Based on a review of

selected sites, the hysteresis loop

and dissipated work do vary

extensively by structure and pavement type.

Dissipated work was also evaluated on a seasonal basis using

some of the LTPP Seasonal

Monitoring Program (SMP) sites.

For the most part, dissipated work

was found to be independent of

season or month for those sites

where the properties are more

uniform throughout the year (i.e.,

no frost penetration into the subgrade and no spring thaw occurring in the base and underlying

subgrade). It is expected that the

dissipated work will be significantly different between seasons for

those sites where frost penetration and spring thaw occur.

Pavement Performance

Comparisons

Dissipated work should be related

to the rate of pavement deterioration and/or damage. This

becomes an extremely important

parameter in evaluating pavement

structures to determine remaining

life and rehabilitation requirements. It is hypothesized that the

dissipated work calculated from

the FWD load deflection-time data

is proportional to, if not directly

related to, pavement damage in

terms of fatigue cracking and

other types of distress, excluding

permanent deformation (rutting)

that is confined to the asphalt concrete surface layer.

Various sites were selected with

varying International Roughness

Index (IRI) values, distress magnitudes, and traffic levels to deterHysteresis loop as measured by the FWD during the month of

July on an asphalt concrete pavement, GPS site 481060.

FIGURE 5

Hysteresis loop as measured by the FWD during the month of

July on an asphalt concrete pavement, GPS site 481122.

FIGURE 4
that can be used to evaluate the

performance behavior of pavement structures. More importantly, dissipated work can be measured directly with the FWD.

Summary

In conclusion, the differences

between the elastic and viscoelastic responses of a pavement structure, as measured by the FWD,

may begin to explain some of the

differences normally observed

and reported between the laboratory and backcalculated moduli of

a pavement material and/or subgrade soil. These observed differences should be studied in depth

in future data analysis studies

regarding the LTPP data base,

especially when developing

mechanistic-empirical pavement

performance models.

The authors strongly recommend that agencies begin to use

these deflection data sets to their

full potential, especially with the

nationally increased awareness of

using mechanistic-empirical

design procedures. These data

should represent a key parameter

in the development of these new

design procedures that are being

planned by the American

Association of State Highway and

Transportation Officials by the

year 2002.

References

1. Nielsen, J.P. and G.T. Baird,

Evaluation of an Impulse Testing

Technique for Nondestructive Testing

of Pavements, Report No. CEEDO-TR77-46, Civil and Environmental

Engineering Development Office,

Tyndall Air Force Base, Florida,

September 1977.

JULY 1997 FHWA-RD-97-093

Researcher: This study was performed by Brent Rauhut Engineering, Inc., Contract No. DTFH61-95-C-00029. 

Distribution: This TechBrief is being distributed according to a standard distribution. 

Availability: The publication related to this TechBrief is Analyses Relating to Pavement Material Characterizations

and Their Effects on Pavement Performance (FHWA-RD-97-085). It will be available in late 1997. Copies will be available from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161. A limited number of copies will be available from the R&T Report Center, HRD-11, FHWA, 9701 Philadelphia Court, Unit Q,

Lanham, MD 20706, telephone: (301) 577-0818, fax: (301) 577-1421.

Key Words: Falling-weight deflectometer, FWD, deflection, time history, dissipated work.

Notice: This TechBrief is disseminated under the sponsorship of the Department of Transportation in the interest

of information exchange. The TechBrief provides a synopsis of the study’s final publication. The TechBrief does not

establish policies or regulations, nor does it imply FHWA endorsement of the conclusions or recommendations. The

U.S. Government assumes no liability for the contents or their use.

mine whether there is a relationship between dissipated work and

pavement performance or the rate

of pavement deterioration. These

data are shown in figure 6 and

indicate that the greater the dissipated work, the more pavement

distress (both in magnitude and

severity) and the greater the number of different types of distresses

that were observed at these sites.

Thus, dissipated work appears to

be a material or pavement

response parameter (or property)

Comparison of dissipated work to pavement condition for

different traffic levels for selected GPS sites in the LTPP

southern region.

FIGURE 6