Fatigue assessment of conventional and highly modified asphalt materials with ASTM and AASHTO specifications

Phillip Blankenship and Geoffrey M. Rowe

BATT and Abatech

Objectives and Acknowledgments

• To review and comment on the development of fatigue specifications as they apply to the implementation of ASTM and ASSHTO standards for 4-point bending beam fatigue tests as conducted in the USA

• Involves reviews of data collected from various sources over the past 30-years +

• Acknowledgments are due to many who contributed– UCB, University of Nottingham

• Carl Monismith, Steve Brown– Task Group Members

• Satish Ramaia, John D'Angelo John Casola, Gerald Reinke Gayle King, K. Nam, David Anderson, Bob Kluttz, Mark Bouldin, Geoff Rowe

• Tom Bennert, Louay Mohammad, Richard Steger, Richard Willis, Phil Blankenship, Geoff Rowe

– Others – in ASTM and AASHTO who contributed to standards development

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• Fatigue testing started late 1950s/early 1960s– Pioneers – Prof. Carl Monismith, Prof. Peter Pell– Bending beams or push pull of different

configurations– Strain criteria recognized (1960s)– Used in pavement design methods (1970s)

• Examples – Asphalt Institute, Shell Pavement Design Manual (SPDM)

• Developments in South Africa by Freeme and others• Inclusion of stiffness

The early days ….

C.R. Freeme

P.S. Pell

C.L. Monismith

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• 1980s – further development of bending beam in Europe –particularly in Netherlands– Pronk, Hopman, etc.

• 1980s to early 1990– SHRP study

• Definition of AASHTO standards around Four-Point Bending Beam• Rowe’s ideas of failure definition from Trapezoidal work

• Late 1990s/early 2000s– Rowe’s ideas adopted by Koch Materials as analysis method– Rich May lead concept for development of ASTM standard based on new

failure definition – defined failure with PmBs in more robust manner– Now 2 methods in USA!!!!

Lets look at the two USA methods - early 2000s

Ideas that influence USA development

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• Shape of curve assumed to follow S=AebN

– Regression analysis to fit curve –originally based on 10 to 20 data points

– Failure defined at point when stiffness has dropped to 50% based on equation

– Data collection on logarithmic basis• Remember in late 1980/early 1990

– the standard data storage in an asphalt lab was a 1.44MB floppy disk!– Limited data size was important!

ASSHTO T321 method 1990s to 2014

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Typical Fatigue Data – Bending Beam – SHRP A003a

• This is an example of a typical test – towards end of SHRP study – a little more data than the earlier slide– Stiffness scale has

been normalized to % of initial stiffness

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• Data normally plotted on log-log scale• Test normally discontinued at 50% stiffness reduction• Exponential equation fitted

Typical Fatigue Data (SHRP A003a)

e A = Stiffness bN

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Typical Fatigue Data (SHRP A003a)

• Data and two lines using S = AebN fitted equation– Method 1 – the ASSHTO

standard as defined– Method 2 – most often

applied by those using the test

• Neither method is really a good fit to the data

% In

itial

Stif

fnes

s

8

Stiffness reduction with load cycles

• I might convince you that this is a fatigue curve for an asphalt mix – stiffness % reduction versus load cycles!– We can often be deceived by

not understanding the data for what it is!

– Need to be very careful when fitting data with curves and saying that it’s a good fit because we get a “great” r2 value !!!

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Stiffness reduction

• Both methods confirm that exponential equation is not a good description of the stiffness reduction in the test method

• Large errors can be introduce in the definition of number of load cycles to failure

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Stiffness reduction

• Stiffness reduction is best considered on linear –linear scale and not log-log scale– Real events can be best

examined from data expressed in this manner

– Question – how do we define the failure condition – point at first cracks appearing?

% In

itial

Stif

fnes

s

11

• If we consider the fatigue stages– Need to define point

between micro-crack formation and crack formation

– Often around 50% stiffness reduction

• But not always!

Typical Fatigue Test Result

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Definition of Failure

• Energy ratio concept• Stiffness x Number of Load

Cycles• Applicability to controlled

strain and stress data– A “simplified” approach– Assumption is that phase

angle change is second order compared to change in stiffness

) () ( n = RatioEnergy

iii

ooo

δεσπδεσπ

sinsin

i*

E

n R =εControlled strain

i*

E n R =σControlled stress

Hopman, Kunst, Pronk - 1989

Hopman, P.C., Kunst P.A.J.C and Pronk A.C., "A Renewed Interpretation Method for Fatigue Measurement, Verification of Miner's Rule," 4th Eurobitume Symposium, Volume 1, Madrid, 4-6 October 1989, pp 557-561.

Rowe, 1993, 1996 & Rowe and Bouldin, 2000

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Energy Ratio Concept (Hopman et al.)

• The shape on this curve is from a controlled displacement test (strain)– Shape remains same if

simplified ratio plotted of full “energy ratio”

– However, the early part of this curve is not a straight line as suggested but rather a gradual curve

– Make the definition of N1 subjective!

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• Hopman’s approach involved fitting linear line to first portion of curve

• Method can be improved by applying controlled stress format to both test types since peak is produced at point data deviates from linear damage -formation of cracks– Well defined peak – consistent

with N1 possition in previous slide

Implementation of Method

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• Based on this approach ASTM method was developed

• When published this followed a different concept to define failure based on S.n– Data collection linearized

using a scheme that captures both an early and late failure

– Definition of peak using alternate methods allowed (cubic spline, polynomial equation, curve fitting, etc.)

• Example show is from four point beam – collected from a controlled displacement (strain) test

ASTM D7460 – early 2000s

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• How do we deal with additional data being generated– Labs – doing their own

thing!– Not strict conformance– Regression methods allows

different options– Different to ATSM

• Task group – under Mixture ETG asked to unify methods

Definition of the problem – mid 2000s – ASSHTO vs. ASTM

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Work of the task group

• Comparison between material performance was more pronounced with modified materials

• Modified materials have significantly greater drop in stiffness before cracking in specimen occurs

Why?• Materials have lower

stiffness• Heating, thixotropic

effects are larger

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• The product of n x stiffness for controlled load (stress) loading in a trapezoidal fatigue test– SHRP Project A003a – work at

University of Nottingham– Same concept applied to any

test with repeated loading and definition of an change of behavior capture of the stiffness or compliance response

– Initially proposed in 1993 with 4-point bending beam analysis –adopted in 2014!

Controlled Stress Fatigue Test S.n vs. Load Cycles

N x

Stif

fnes

s

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• Need to capture data on a linear basis and not a log basis in order to define failure peak– Data collection scheme in

linearized in each decade – more data collected earlier in test to cope with an “early failure”

– Ten points per decade– In Nottingham at end of

SHRP project – testing scheme was changed to collect data – full hysteresis loop at every 5% stiffness reduction

A note on data collection

Specimen 6b

0.0E+00

2.0E+05

4.0E+05

6.0E+05

8.0E+05

1.0E+06

1.2E+06

1.4E+06

0 5000 10000 15000 20000

Number of Load Cycles

E* (p

si)0.0E+00

1.0E+09

2.0E+09

3.0E+09

4.0E+09

5.0E+09

6.0E+09

7.0E+09

n.E*

(psi)E*

n.E*

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• The peak in the stiffness x cycles curve coincides with other key parameters– Indicative of localization

of cracking– Better and clearer

location than phase angle or dissipated energy ratio concepts

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Concept of ASTM failure

• Using the old AASHTO definition we noticed that the analysis was much more sensitive to the modification level– BDWC (Bridge Deck Water

Proofing Course) with very high percentage of polymer

– PG64-22 is conventional asphalt binder

Effect of data inclusion

0

20

40

60

80

100

120

140

160

180

20253035404550

% Sini (defined by in AASHTO method) that test data was included analysis of Nf

Nf c

hang

e, %

9.5mm 64-22 650

9.5mm 64-22 500

BDWC-1500

BDWC-1000

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Fit type

This example is a cubic spline fit. The method is generally insensitive to data included – except ….

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• If data is included which goes significantly beyond a 15-20% value of the original stiffness the Stiffness x Cycles curve can go through a minimum and then start to increase. The data past this point needs to be removed from the analysis.

• ASTM – recommends fitting simple 6-order polynomial

Data trimming

This type of data inclusion can cause a difference typically of 10%.

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• The ASTM and AASTHO methods now use a (N vs. S.n) to define the peak– Ad Pronk and co-workers have suggested that this maybe better

represented by N* vs. S.n• Where N* is N raised to an exponent (a value of 0.5) has been suggested • This effectively is …. N vs. (S.n)2

– Alternate approach – but again we need to evaluate multiple data sets to see if this offers any improvement

• Extends failure cycles from N vs. S.n approach – so gives longer fatigue life

– Key takeaway don’t use 50% stiffness reduction

What about Pronk’s method (2019)

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1. The old AASHTO method currently did not define a comparable failure condition for modified materials

2. Conventional materials will produce similar fatigue curves regardless of standard being used at this time (ASTM or ASSHTO)

3. Failure for modified materials should be based on definition as currently in ASTM method and the AASHTO method should be changed to be consistent with this

4. New materials with high modification levels will then have a performance level more correctly defined

5. Excess data should be trimmed beyond the first significant peak - need to establish rule – suggest max 10% drop in ratio from peak value

6. Changing the AASHTO definition of life will not have a significant impact on lives calculated in previous evaluations – i.e. – legacy relationships will be valid

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Summary and action of the ETG work in 2012/2013

1. In 2014 the new AASHTO version of T321 changed the definition of failure from 50% stiffness reduction to S.n peak

Failure is generally between 35% and 65% stiffness reduction but can be as low as 20% of initial stiffness for heavily modified materials

2. A task group was formed to rewrite the ASTM standard with the intent of standardizing the waveform to haversine loading and replaced this with sine loading

The specimen moves in a visco-elastic manner making the maintenance of a haversine load impossibleTesting showed that results from tests considered to be started using haversine gave the same results when peak-to-peak strain magnitude considered

3. ASTM D8238 was published in 2018 – now the two standards will give essentially the same result although the ASTM is normalizing the S.n curve by dividing the number by the initial stiffness. This makes no difference to result since it is diving the numbers used to define the peak by a constant!

Actions – 2013 to present

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Thanks for listening!

• Comments?• Questions?