S43_Why the Multiple Stress Creep Recovery (MSCR) Test Should Be Implemented_LTC2013

7/29/2019 S43_Why the Multiple Stress Creep Recovery (MSCR) Test Should Be Implemented_LTC2013

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Multiple Stress Creep Recovery (MSCR)

Why it Should Be ImplementedDanny Gierhart, P.E.

Regional Engineer Asphalt Institute


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Whats insufficient with the Superpave binder classification system

Why polymerized binders are different

What the PG Plus test Elastic Recovery tells usand the time savings gained by eliminating it

What research supports MSCR testing doing a

better job of predicting rutting susceptibility Making the MSCR jargon clear

Implementation of MSCR

Overview


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The use of polymer modifiedbinders has grown tremendouslyover the past several years

However, the most widely usedbinder specification in the U.S.,

AASHTO M 320, was based on

a study of neat (unmodified)binders, and may not properlycharacterize polymer modifiedbinders


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When would a polymer-modified asphalttypically be used?

Adjustment to High-Temp Grade

Traffic Load RateESALs (M) Standing Slow Standard

< 0.3 - - -0.3 - < 3 2 1 -

3 - < 10 2 1 -10 - < 30 2 1 -

30 2 1 1

AASHTO M 323 - Table 1


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PG Grading Alone Does Not AlwaysPredict Performance

Study of the two mixes with the sameaggregate structure, but different binders.

PG 63-22 modified, no rutting PG 67-22 unmodified, 15mm rut


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Current spec, G* and are measured in thelinear visco-elastic range.

For neat asphalts, flow is linear and notsensitive to the stress level of the test.

For polymer-modified binders, the responseis not necessarily linear, and can besensitive to the stress level of the test. Thepolymer chains can be rearrangedsubstantially as the stress increases.

Why doesnt M 320 properly characterizepolymer-modified binders?


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States with a PG Plus Specification

PG Plus Spec

No PG Plus Spec

*

*

*


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The Elastic Recovery Test

Step 1 - Equilibrate specimensto testing temperature, typically 25 C (77 F)* Some states: 10 C (50 F)

Step 2 Stretch at controlledrate of 5 cm/min (2 in/min) totypically 20 cm (8 in)* Some states: 10 cm

* Some states run on original binder,some on RTFO-aged binder Some states use parallel-sided molds, some v-sided molds


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Step 3 After holding stretchedspecimens in place for 5 min(typically) , cut each in the center * Some states: cut immediately

Step 4 (Photo takenimmediately after cutting) Letsample stand undisturbed for 1hour (typically)



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Step 5 Retract ductilometer

and measure distance until endstouch.

% Recovery = Total cm recovered

20 cm

Polymer-modified Samples

Neat (Unmodified) Sample


States minimum requirementsrange from 40% to 75%


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ER Information and Test Time

The Elastic Recovery Test is an excellent tool toestablish the presence of polymer modification.

It takes about 4 hours to prepare and test

samples for this information. However, it is a poor tool to evaluate the rutting

performance of polymer-modified binders. The MSCR test can use the same sample

already being run in the DSR to give moreinformation in a few extra minutes.


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ALF Study - 7 Asphalt Binders

AZCRM----70-22

1

PG70-22Control

2

AirBlown

3

SBS

4

TXTBCR

5

TP

6

PG70-22+Fibers

7

PG70-22

8

SBS64-40

9

AirBlown

10

SBS

11

TP

12


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ALF Loading

The pavement was heated to a constant64C.

The FHWA ALF uses an 18,000 lbs wheelload with no wheel wander.

The speed is 12 MPH. This loading condition is more severe than on

an actual highway.


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y = -7.4519x + 10.956R 2 = 0.1261

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

rutting inches

G*/sind64C

Relationship between G*/ sin and ALFrutting

Existing SHRP specification has poor relationshipto rutting for modified systems.

r2 = 0.1261

G

* / s i n d

@ 6

4 C

ALF Rutting, inches


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Relationship between J nr and ALF rutting

y = 4.7357x - 1.1666R 2 = 0.8167

0

0.5

1

1.5

2

2.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

ALF Rutting in

Jnr

MSCR can adjust forfield conditions and hasexcellent relations toperformance.

ALF Rutting, inches

J n r @ 3

. 2 k P a - 1

r2 = 0.8167


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Mississippi I-55 6-year rutting study

y = 0.2907x + 0.1297R2 = 0.7499

00.5

1

1.52

2.53

3.54

0 2 4 6 8 10 12

6 yr Jnr 3.2 kPabinder mod true grade rut mm 70C

Ultrapave SBR 70-27 4.5 1.7Styrelf SB 77-29 2 0.44

GTR 80 75-29 1.5 1.21Sealoflex SBS 82-27 3 0.19

Multigrade 72-24 5 2.13Cryo Rubber 75-28 7 1.62

Control 70-24 11 3.5

Rutting mm

J n r

@ 3

. 2 k P a - 1


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The Multiple Stress Creep Recovery test,commonly referred to as the MSCR (pronouncedmassacre ) test, is an asphalt binder test which:

allows for a performance-related binder specthat is blind to modification type

can relate polymer- modified binders potential

rutting performance to in-service pavements allows for a much more economic use of polymers to improve performance

takes less lab time to perform

MSCR Overview Talking Points


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Multiple Stress Creep Recovery

The test method is detailed in AASHTO TP 70

The test uses the same Dynamic Shear Rheometer (DSR) as required in the original M 320

Only minor software changes are need to run theMSCR test

The test uses the creep and recovery method tomeasure the percent recovery and non-recoverablecreep compliance (J nr )


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Multi Stress Creep and Recovery

Sample prep is exactly the same as theexisting DSR on RTFO-aged material.


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Multiple Stress Creep Recovery

Definitions :Creep and recovery a standard test protocol whereby aspecimen is subjected to a constant load for a fixed timeperiod and then allowed to relax (recover) at a zero load

for a fixed time periodPercent Recovery A measure of how much the samplereturns to its previous shape after being repeatedlystretched and then relaxed

Non-Recoverable Creep Compliance (J nr ) a measureof the amount of residual strain left in the specimen after repeated creep and recovery, relative to the amount of stress applied


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C r e e p

S t r e s s

time

time

C r e e p

S t r a

i n

0.1 kPa

3.2 kPa

1 10

Multi Stress Creep and Recovery

Test usingthe DSRapplying a 1sec creepstressfollowed by 9

sec recovery.


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Stress Sensitivity Parameter

J nr, diff =(J nr, 3.2kPa - J nr, 0.1kPa )

J nr, 0.1kPa 75%

For polymer-modified binders, the strain response isnot linear and sensitive to the stress level of the test.The polymer chains can be rearranged substantiallyas the stress increases. This parameter is a check on the phenomenon.

x 100


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0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

time s

Strain%

r = recovered strain

What is % Recovered Strain?

u = un-recovered strain

p = Peak strain

% recovery= ( r / p) x 100

Stress is released at this point,and the binder begins recovery

toward its initial shape


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Determination of J nr

0

5

10

15

20

25

30

0 20 40 60 80 100

Time s

Normalized Strain

u = Avg. un-recovered

a strain

Jnr = u /

= applied stress during creep kPa

Jnr = non-recoverable compliance

b dd d


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y = 29.371x -0.2633

0

10

20

30

40

50

60

70

80

90

100

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1

Jnr kPa

% recovery

MSCR % recovery can be added tovalidate polymer modification

% Recovery above the line meansasphalt binder is modified with anacceptable elastomeric polymer

Below line - not modified with an elastomeric polymer

AASHTO TP 70, Appendix X2


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New PG Grading System (MSCR)

Environmental grade plus traffic leveldesignation; i.e. PG 64-22E Four traffic levels

S = Standard : < 10 million ESALs andstandard traffic loading H = Heavy: 10 30 million ESALs or

slow moving traffic loading

V = Very Heavy: > 30 million ESALs or standing traffic loading

E = Extr. Heavy: > 30 million ESALs andstanding traffic loading


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Implementation

Please recognize that the refineries that serve your state may also serve bordering states.

This may be a good reason for an unusual

implementation process - working with other statesto implement regionally

Note that every current Performance Grade may

not equate to a distinct MSCR grade - for example,the polymer loading in both a PG 70-22 andPG 76-22 may be high enough that both grade to aPG 64 -22 E


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Full vs. Partial Implementation

Some agencies may be reluctant to implementMSCR fully, since the names by which they refer to binder types will necessarily change.

PG 64 -22 H instead of PG 70 -22, for a possibleexample

If this is the case, a partial MSCR implementation

could be done as outlined in AIs Guidance on theUse of the MSCR Test with the AASHTO M 320Specification.

* Guidance document on AIs web site in Engineering section


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Full vs. Partial Implementation

Importantly, AI recommends that if the MSCR test isimplemented to evaluate the delayed elastic response of binders, then other PG Plus tests with a similar purpose -such as Elastic Recovery, Force Ductility, and Toughnessand Tenacity tests - should be eliminated .If you are conducting side-by-side testing for a while as aprecaution, keep in mind that elastic recovery andtoughness and tenacity tests give much more simplifiedresults with a much higher degree of error than the MSCR,so agencies should not expect a strong correlation betweenthem and MSCR results.


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S43_Why the Multiple Stress Creep Recovery (MSCR) Test Should Be Implemented_LTC2013