s64_evaluation of friction characteristics of wearing course mixtures in louisiana_ltc2013

7/29/2019 S64_Evaluation of Friction Characteristics of Wearing Course Mixtures in Louisiana_LTC2013

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Zhong Wu, Ph.D., P.E.

2013 Louisiana Transportation ConferenceFeb 20, 2013

Evaluation of Friction

Characteristics of Wearing CourseMixtures in Louisiana


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Background

Pavement Surface Friction is a critical issue to highway safety

and pavement performance.

Each year traffic accidents cause nearly 2.5 million injuries and over

41,000 fatalities.

approximately 25% of all crashes and 13.5% of fatal crashesoccurred under wet pavement conditions

Surface friction is generated as the tire rolls or slides over the

pavement surface.


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Background (contd..)

The friction coefficient can be affected by

Vehicle and driver characteristics

e.g.,speed, braking system, tire condition, driver controls

Pavement surface characteristics

Material properties, e.g. aggregates, binder, gradations, etc.

Surface Textures (Micro-/Macro-texture, Mega-

texture/unevenness)

Other conditions (e.g. temperature, wet or dry, clean or dirty)

Microtexture and Macrotexture are extremely important

pavement in the development of surface friction


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Primary Effects: Micro-texture/Macro-texture

Micro-texture (significant at low speeds)

Depends on surface coarse aggregate polishing resistance

To measure: polishing stone value, Dynamic friction tester, etc.

Macro-texture (better for high speeds)

Helps to reduce the potential for separation of tire from surface due to

hydroplaning

To measure: surface profiler, Circular Track Meter, etc.


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Measurement of Friction in Field


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Measurement of Friction in Laboratory

--- Based solely upon Polish stone value (PSV) of coarseaggregates Micro-texture

- British Pendulum Tester

(BPT)

- Wessex AcceleratedPolishing Device

-Result: Polish Stone Value

(PSV)

-- BPT number on

polished aggregate surface

--Aggregates micro-

texture property


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Current DOTD Specification

Friction Rating Allowable Usage

I (PSV>37) All mixtures

II (35 PSV 37) All mixtures

III (30 PSV 34) All mixtures, except travel lane wearing courses with plan ADT

greater than 70001

IV (20 PSV 29) All mixtures, except travel lane wearing courses2

1 When plan current average daily traffic (ADT) is greater than 7000, blending of Friction Rating III aggregates and

Friction Rating I and/or II aggregates will be allowed for travel lane wearing courses at the following

percentages. At least 30 percent by weight (mass) of the total aggregates shall have a Friction Rating of I, or at

least 50 percent by weight (mass) of the total aggregate shall have a Friction Rating of II. The frictional

aggregates used to obtain the required percentages shall not have more than 10 percent passing the No. 8 (2.36

mm) sieve.2 When the average daily traffic (ADT) is less than 2500, blending of Friction Rating IV aggregates with Friction Rating

I and/or II aggregates will be allowed for travel lane wearing courses at the following percentages. At least 50

percent by weight (mass) of the total aggregate in the mixture shall have a Friction Rating of I or II. The

frictional aggregates used to obtain the required percentages shall not have more than 10 percent passing the

No. 8 (2.36 mm) sieve.


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Objectives

Evaluate the surface frictional characteristics of

typical DOTD wearing course mixtures based on

both field and laboratory analyses;

Develop a pavement surface friction guideline that

can consider both micro- and macro- textures for a

mix design.


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Methodology (Testing Plan)

Laboratory Polishing/Friction-Resistant Tests

Accelerated polishing procedure

Dynamic Friction Tester (DFT)

Circular Track Meter (CTM)

Field Surface Friction Tests

Lock-Wheel Skid Test

Laser Profiler

DFT/CTM


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Accelerated Polishing Device

Three-Wheel accelerated polishing device at the National Center ofAsphalt Technology (NCAT)

Simulate the traffic-polishing effects on surface friction of slabs

Normal load - 105 lb., pressure of pneumatic tires - 50 psi

Preparation of Testing Slab

(20x20x3)


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ASTM E 1911

DFT Includes three rubber sliders

mounted on a disk at a diameter of 13.75 in. (35-cm).

measures surface coefficients of friction at 20, 40, 60, & 80 km/hr DFT@ 20km/hr is an indicator of Micro-texture

DFT


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Used to measure surface macro-texture

ASTM E 2157

Results

Mean Profile Depth (MPD)


=11.2

Mean Profile Depth (MPD)


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Laboratory Experimental Design

Twelve typical asphalt wearing course mixtures with differentcombinations of aggregate blends and mix types wereconsidered in this study:

4 asphalt mix types:

Open-grade friction course (OGFC) Stone matrix asphalt (SMA)

19-mm Superpave Level-II, and

12.5-mm Superpave Level-II

2 aggregate types:

Sandstone (SS) - AB13 --- (PSV=38, FR-I) Limestone (LS) - AA50 --- (PSV = 30, FR-I II)

3 different aggregate blends:

100% SSHigh PSV 100% LSLow PSV

30% SS +70% LS

Moderate PSV


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12 Wearing Course hot mix asphalt (HMA)

mixtures


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Measurements on Polished Slabs

Totally 36 slabs prepared (12 mixtures x 3

replicates)

Each slab was polished up to 100,000 polishingcycles

At the cycle periods of 0, 2, 5, 10, 30, 50, and

100 thousand cycles, the surface textureproperties were measured using


Circular Tracker meter (CTM)


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Discussion of Results


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DFT Results


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DFT Results (contd..)


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DFT Results (contd..)


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CTM Results (MPD for Macro-Texture)


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F(60): a friction index combined both micro-

and macro- textures

International Friction Index (IFI)

Developed at the World Road Association-PIARC in France

Used to standardize the dependency of friction measurement by

different devices and tire sliding speeds.

Reported asIFI(F(60), Sp),

F(60) - Friction number at a slip speed of 60 km/hr,

Sp - Speed number


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F(60) Results

10

20

30

40

0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000

F60

CYCLE100% LIMESTONE, OGFC 100% Sandstone, OGFC 70/30 LS+SS, OGFC

100% LIMESTONE, 19 mm superpave 100% Sandstone, 19 mm superpave 70/30 LS+SS, 19 mm superpave

100% LIMESTONE, 12.5 m superpave 100% Sandstone, 12.5 mm superpave 70/30 LS+SS, 12.5 mm superpave

100% LIMESTONE, SMA 100% Sandstone, SMA 70/30 LS+SS, SMA

Sandstone OGFC

Sandstone SMA

Limestone Superpave


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Effect of Micro- and Macro- Texture on Surface

Friction at different speeds


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Relationship of F(60) vs. DF20 & MPD

5

25

45

65

85

15 20 25 30 35 40 45

DF20

F(60)

19.5 mm Superpave, MPD: 0.37-0.50

SMA, MPD: 0.53-0.69

OGFC, MPD: 0.82-1.29

Limestone

12.5 mm Superpave, MPD: 0.37-0.53

Sandstone

LS (70%) + SS (30%)

F(60)

DF20


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10

20

30

40

0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000

F60

CYCLE100% LIMESTONE, OGFC 100% Sandstone, OGFC 70/30 LS+SS, OGFC

100% LIMESTONE, 19 mm superpave 100% Sandstone, 19 mm superpave 70/30 LS+SS, 19 mm superpave

100% LIMESTONE, 12.5 m superpave 100% Sandstone, 12.5 mm superpave 70/30 LS+SS, 12.5 mm superpave

100% LIMESTONE, SMA 100% Sandstone, SMA 70/30 LS+SS, SMA

Relationship between F(60) vs. DFT/CTM

Sandstone OGFC

Sandstone SMA

Limestone Superpave

F(60)= (2.18+13.5MPD+0.38DF20)e(-1.73E-06N)

(R2 = 88%)(Eq. 1)


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Relationship between Terminal F(60) and PSV

F(60) = 0.067(PSV)23.84PSV +74.46 for Superpave 19mm

F(60) = 0.106(PSV)26.19PSV + 108.75 for Superpave 12.5mm

F(60) = -0.121(PSV)2 + 9.417PSV153.52 for SMA

F(60) = -0.066(PSV)2 + 5.99PSV101.65 for OGFC

(Eq. 2)


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Relationship between F(60) and SN

F(60) = 0.649 SN(50R) + 0.0572

SN data obtained from a 19-mmSuperpave mixture

(Eq. 3)

where

SN(50R)

skid number at 50 mile/hr with a ribbed tire.


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Procedure for Mix Design Considering Surface Friction

Design

SN

Design

F(60)des

Select

Mix Type

Determine

DF20 &

MPD

Select

Aggregate

type by PSV

Required

PSV

Compute

F(60)

F(60)

F(60)desEnd

StartEq. 3

Eq. 1

Eq. 2

or

Yes

No

F(60) = 0.649 SN(50R) + 0.0572

F(60) = 0.067(PSV)23.84PSV +74.46 for Superpave 19mm

F(60) = 0.106(PSV)2

6.19PSV + 108.75 for Superpave 12.5mmF(60) = -0.121(PSV)2 + 9.417PSV153.52 for SMA

F(60) = -0.066(PSV)2 + 5.99PSV101.65 for OGFC

F(60)= (2.18+13.5MPD+0.38DF20)e(-1.73E-06N)


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On-Going Field Test Results

Currently, we have tested 20 field projects, among themincluded:

12 Superpave

4 OGFC

3 SMA 1 warm mix

Each selected project was 1000-ft long, following tests wereconducted

LWST rib/smooth (40 or 50 mph)

Laser profile

DFT

CTM


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On-Going Field Test Results (contd..)

Based on the limited data, the following

harmonization correlations were determined:

SN(R) = f (DFT20, MPD)

SN(S) = f (DFT20, MPD) T.I = N1.76/exp(15.14)

Our ultimate goal is to determine DFT/MPD

requirements for a lab-prepared slab

By specifying the levels of DFT and MPD, a certain level

of field SN will be achieved


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Flowchart for determining DFT/MPD-based

Friction Requirements

Determine required DFT/MPD

Input SN(R) or SN(S)

Superpave SMA OGFC Other Types

MPD

Range

MPD

RangeMPD

Range

MPD

Range

DFTmin DFTmax DFTmin DFTmax

DFTmin DFTmaxDFTmin DFTmax

SN(R) = f (DFT20, MPD)

SN(S) = f (DFT20, MPD)

T.I. = N1.76/exp(15.14)

Lab FN deterioration

curves


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Conclusions

DFT/CTM tests are sensitive to asphalt mixture typeand aggregate type

Polishing-resistant aggregates have higher DFT number

In terms of macro-texture (MPD):

OGFC > SMA > Superpave

Laboratory mix design can consider both micro- andmacro- textures to produce a cost-effective, frictionresistant mixes.

The procedure can also assist in the use of lower skid-resistant, locally-available aggregates in a wearing coursemix design,


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Questions?

s64_evaluation of friction characteristics of wearing course mixtures in louisiana_ltc2013

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