A Balanced Mix Design! -...

12/14/2016

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“Where Excellence and Transportation Meet”

Illinois Center for TransportationUniversity of Illinois at Urbana Champaign

I-FIT (Illinois Flexibility Index Test) Protocol – Contractor Perspectives

Imad L. Al-Qadi, PhD, PE, Dist.M.ASCE

Founder Professor of Engineering

ICT/ATREL Director

UIUC



Not too Brittle that can cause cracking:

Not too Flexible that can cause rutting:

Balanced Mixes!

A Balanced Mix Design!

12/14/2016

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Superpave™

-40°C -20°C 20°C 40°C

Low Temperature

CrackingFatigue/Block/Other

Forms of Cracking

Permanent

Deformation

Volumetrics Rut TestCracking Test

Solution: Testing Scheme



I-FIT Protocol• Developed in ICT R27-

128 study (2015)

• Published as AASHTO TP 124

• It is now being experimented by many states and researchers all over the US

• Currently, 5 manufacturers are making devices for I-FIT

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Fracture Energy Ambiguities



Development of Flexibility Index

Flexibility Index (FI) = 𝑮𝑭 ×𝟏

𝒂𝒃𝒔(𝒎)

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I-FIT Variability

• Goal: Identify a difference in the performance of:

Mix ABR Binder Constant

Properties

Expected

Behavior

L4 0% 64-22 AC (%): 6

VMA (%):

15.3

Increment

in ABR

Decreases

Toughness

L7 20% 58-28

L9 30% 58-28

L10 60% 52-34



Low Temperature SCB (-12ºC):

0

0.01

0.02

200 400 600 800 1000

Pro

babili

ty D

ensity

Fracture Energy (J/m2)

SCB (-12ºC, 1 mm/min, CMOD Control)ABR

0% vs 20%,

Overlap: 43%

ABR

20% vs 30%,

Overlap: 11%

ABR

30% vs 60%,

Overlap: 58%

(0%) μ = 742 σ = 57

(20%)μ = 795 σ = 22

(30%) μ = 677 σ = 55

(60) μ = 635 σ = 130

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Low Temperature SCB:

• High Repeatability:

• ~ 9% COV

• Low

Discrimination:

• 40% Overlap

30 %

40%

Normal Distribution Overlap:



0

0.002

0.004

0.006

0.008

0.01

700 1200 1700 2200

Pro

ba

bili

ty D

en

sity

Fracture Energy (J/m2)

SCB (25ºC, 50 mm/min)Intermediate Temperature SCB:

ABR

0% vs 20%,

Overlap: 3%

ABR

20% vs 30%,

Overlap: 37%

ABR

30% vs 60%,

Overlap: 21%

(20%) μ = 1622 σ = 51

(0%) μ = 1895σ = 73

(60%) μ = 1280 σ = 164

(30%)μ = 1703 σ = 174

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30 %

40%

Intermediate Temperature SCB:

• Intermediate

Repeatability:

• ~ 10% COV

• Intermediate

Discrimination:

• ~ 25%

Overlap




Flexibility Index SCB (25 ºC):

0

0.3

0.6

0.9

1.2

1.5

0 5 10 15 20

Pro

ba

bili

ty D

istr

ibu

tio

n

Flexibility Index

L4

L7

L9

L10

SCB (25ºC, 50 mm/min)

Overlaps:

< 3%

Increase in ABR = Reduction in FI

60% 30% 20% 0%

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30 %

40%

Flexibility Index SCB (25 ºC):

• Reduced

Repeatability:

• ~ 15% COV

• High

Discrimination:

• ~ 3% Overlap




Balanced Mix Approach

Fle

xib

ility

In

de

x (

FI)

Rut Depth (mm) @ X Passes

Ru

t D

ep

th T

hre

sh

old

FI Threshold

Fail*

FailFail

Pass

High PerformanceMixes

12.5 mmSTIFF & BRITTLE

STIFF & FLEXIBLE SOFT & FLEXIBLE

UNACCEPTABLE

Ru

t D

ep

th T

hre

sh

old

FI Threshold

Fail*

FailFail

Pass

High PerformanceMixes

12.5 mmSTIFF & BRITTLE

STIFF & FLEXIBLE SOFT & FLEXIBLE

UNACCEPTABLE

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0

2

4

6

8

10

12

14

16

18

20

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25

Flex

ibili

ty In

dex

(FI

)

Hamburg Wheel Rut (mm)

64-22

64-28

64-22 and Plus 0.4% AC

64-22 with 3% Additive

Note: 70-22 Rut Depth is at 15,000 passesAll others are at 7,500 passes

Starting with borderline mix (PG64-22)

Softer grade: FI Rut

More binder: FI Rut

Additive:FI Rut

Optimizing Performance: Binder Strategies



Accelerated Loading Facility• Simulates truck traffic with controlled loading and

pavement temperatures at FHWA Turner Fairbank

Research Laboratory.

• Up to 35,000 cycles can be applied per week.

• Wheel load can be varied from 33 kN (7,500 lb)

representing a light truck to 84 kN (19,000 lb) simulating a

heavy axle.

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I-FIT Prediction of Fatigue (ALF)

R² = 0.8322

0

4

8

12

16

10 100 1,000

Fle

xib

ility

Index

ALF Cycles ( x 1000)



Effect of Construction Year

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Tests on Different MachinesMachine A• Spring Rollers

• Hydraulic

Machine B• Spring Rollers

• Screw driven

Machine C• Bearing rollers

• Hydraulic

Machine D• Spring Rollers

• Hydraulic



Closing Remarks

• I-FIT protocol is progressing in the right direction for DOTs and industry

• AASHTO provisional standard and the test have received nationwide attention

• Precise and cost-effective equipment are in the market

• Better understanding of our mixes when combined with Hamburg (better mix = better product)

A Balanced Mix Design! -...

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