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

2013 Louisiana Transportation Conference Feb 20, 2013

Evaluation of Friction Characteristics of Wearing Course

Mixtures in Louisiana

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 crashes

occurred under wet pavement conditions

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

pavement surface.

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

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.

Measurement of Friction in Field

Measurement of Friction in Laboratory

--- Based solely upon Polish stone value (PSV) of coarse

aggregates – Micro-texture

- British Pendulum Tester

(BPT)

- Wessex Accelerated

Polishing Device

-Result: Polish Stone Value

(PSV)

-- BPT number on

polished aggregate surface

--Aggregate’s micro-

texture property

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.

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.

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

Accelerated Polishing Device

• Three-Wheel accelerated polishing device at the National Center of

Asphalt 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

(20”x20”x3”)

Dynamic Friction Tester (DFT)

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

Circular Track Meter (CTM)

Used to measure surface macro-texture

ASTM E 2157

Results – Mean Profile Depth (MPD)

Circular Track Meter (CTM)

Ф =11.2”

Mean Profile Depth (MPD)

Laboratory Experimental Design

Twelve typical asphalt wearing course mixtures with different combinations of aggregate blends and mix types were considered 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-III)

3 different aggregate blends:

100% SS High PSV

100% LS Low PSV

30% SS +70% LS Moderate PSV

12 Wearing Course hot mix asphalt (HMA)

mixtures

Measurements on Polished Slabs

Totally 36 slabs prepared (12 mixtures x 3

replicates)

Each slab was polished up to 100,000 polishing

cycles

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

100 thousand cycles, the surface texture

properties were measured using

Dynamic Friction Tester (DFT)

Circular Tracker meter (CTM)

Discussion of Results

DFT Results

DFT Results (contd..)

DFT Results (contd..)

CTM Results (MPD for Macro-Texture)

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 as IFI(F(60), Sp),

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

Sp - Speed number

F(60) Results

10

20

30

40

0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000

F60

CYCLE 100% 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

Effect of Micro- and Macro- Texture on Surface

Friction at different speeds

Relationship of F(60) vs. DF20 & MPD

5

25

45

65

85

15 20 25 30 35 40 45

DF

20

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

10

20

30

40

0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000

F60

CYCLE 100% 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+13.5×MPD+0.38×DF20)×e(-1.73E-06×N)

(R2 = 88%)

(Eq. 1)

Relationship between Terminal F(60) and PSV

F(60) = 0.067(PSV)2 – 3.84PSV +74.46 for Superpave 19mm

F(60) = 0.106(PSV)2 – 6.19PSV + 108.75 for Superpave 12.5mm

F(60) = -0.121(PSV)2 + 9.417PSV – 153.52 for SMA

F(60) = -0.066(PSV)2 + 5.99PSV – 101.65 for OGFC

(Eq. 2)

Relationship between F(60) and SN

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

• SN data obtained from a 19-mm Superpave mixture

(Eq. 3)

where

SN(50R) – skid number at 50 mile/hr with a ribbed tire.

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)des

End

Start Eq. 3

Eq. 1

Eq. 2

or

Yes

No

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

F(60) = 0.067(PSV)2 – 3.84PSV +74.46 for Superpave 19mm

F(60) = 0.106(PSV)2 – 6.19PSV + 108.75 for Superpave 12.5mm

F(60) = -0.121(PSV)2 + 9.417PSV – 153.52 for SMA

F(60) = -0.066(PSV)2 + 5.99PSV – 101.65 for OGFC

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

On-Going Field Test Results

Currently, we have tested 20 field projects, among them included:

12 Superpave

4 OGFC

3 SMA

1 warm mix

Each selected project was 1000-ft long, following tests were conducted

LWST rib/smooth (40 or 50 mph)

Laser profile

DFT

CTM

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

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

Range MPD

Range

MPD

Range

DFTmin DFTmax DFTmin DFTmax

DFTmin DFTmax DFTmin DFTmax

SN(R) = f (DFT20, MPD)

SN(S) = f (DFT20, MPD)

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

Lab FN deterioration

curves

Conclusions

DFT/CTM tests are sensitive to asphalt mixture type and 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- and macro- textures to produce a cost-effective, friction resistant mixes.

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

Questions?