Evaluation of Friction Characteristics of Wearing Course Mixtures in Louisiana Zhong Wu, Ph.D., P.E.

Transcription

1 Evaluation of Friction Characteristics of Wearing Course Mixtures in Louisiana Zhong Wu, Ph.D., P.E. Feb 20, Louisiana Transportation Conference

2 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.

3 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, Megatexture/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

4 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.

5 Measurement of Friction in Field

6 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 microtexture property

7 Current DOTD Specification Friction Rating I (PSV>37) II (35 PSV 37) III (30 PSV 34) All mixtures All mixtures Allowable Usage All mixtures, except travel lane wearing courses with plan ADT greater than IV (20 PSV 29) All mixtures, except travel lane wearing courses 2 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.

8 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.

9 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

10 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 lb., pressure of pneumatic tires - 50 psi Preparation of Testing Slab (20 x20 x3 )

11 Dynamic Friction Tester (DFT) ASTM E 1911 DFT Includes three rubber sliders mounted on a disk at a diameter of 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

12 Circular Track Meter (CTM) Used to measure surface macro-texture ASTM E 2157 Results Mean Profile Depth (MPD) Ф =11.2 Circular Track Meter (CTM) Mean Profile Depth (MPD)

13 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) - AB (PSV=38, FR-I) Limestone (LS) - AA (PSV = 30, FR-III) 3 different aggregate blends: 100% SS High PSV 100% LS Low PSV 30% SS +70% LS Moderate PSV

14 12 Wearing Course hot mix asphalt (HMA) mixtures

15 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)

16 Discussion of Results

17 DFT Results

18 DFT Results (contd..)

19 DFT Results (contd..)

20 CTM Results (MPD for Macro-Texture)

21 F(60): a friction index combined both microand 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

22 F60 F(60) Results 40 Sandstone OGFC Sandstone SMA Limestone Superpave 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

23 Effect of Micro- and Macro- Texture on Surface Friction at different speeds

24 Relationship of F(60) vs. DF 20 & MPD DF20 DF mm Superpave, MPD: mm Superpave, MPD: SMA, MPD: OGFC, MPD: Sandstone LS (70%) + SS (30%) Limestone F(60)

25 F60 Relationship between F(60) vs. DFT/CTM 40 Sandstone OGFC Sandstone SMA CYCLE 100% LIMESTONE, OGFC 100% Sandstone, OGFC 70/30 LS+SS, OGFC F(60)= 100% LIMESTONE, ( MPD+0.38 DF 19 mm superpave 100% Sandstone, 19 mm superpave 70/30 LS+SS, 19 mm superpave 20 ) e (-1.73E-06 N) 100% LIMESTONE, 12.5 m superpave 100% Sandstone, 12.5 mm superpave 70/30 LS+SS, 12.5 mm superpave (R 2 = 88%) 100% LIMESTONE, SMA 100% Sandstone, SMA 70/30 LS+SS, SMA Limestone Superpave (Eq. 1)

26 Relationship between Terminal F(60) and PSV F(60) = 0.067(PSV) PSV F(60) = 0.106(PSV) PSV F(60) = (PSV) PSV F(60) = (PSV) PSV for Superpave 19mm for Superpave 12.5mm for SMA for OGFC (Eq. 2)

27 Relationship between F(60) and SN SN data obtained from a 19-mm Superpave mixture F(60) = SN(50R) (Eq. 3) where SN(50R) skid number at 50 mile/hr with a ribbed tire.

28 Procedure for Mix Design Considering Surface Friction Start Design SN Eq. 3 Design F(60)des F(60) = SN(50R) Select Mix Type Eq. 2 F(60) = 0.067(PSV) PSV F(60) = 0.106(PSV) PSV F(60) = (PSV) PSV F(60) = (PSV) PSV for Superpave 19mm for Superpave 12.5mm for SMA for OGFC Required PSV No or Select Aggregate type by PSV End Yes F(60) F(60)des Compute F(60) Eq. 1 Determine DF20 & MPD F(60)= ( MPD+0.38 DF 20 ) e (-1.73E-06 N)

29 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

30 On-Going Field Test Results (contd..) Based on the limited data, the following harmonization correlations were determined: SN(R) = f (DFT 20, MPD) SN(S) = f (DFT 20, MPD) T.I = N 1.76 /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

31 Flowchart for determining DFT/MPD-based Friction Requirements Input SN(R) or SN(S) SN(R) = f (DFT20, MPD) SN(S) = f (DFT20, MPD) Determine required DFT/MPD T.I. = N 1.76 /exp(15.14) Lab FN deterioration curves Superpave SMA OGFC Other Types MPD Range MPD Range MPD Range MPD Range DFT min DFT max DFT min DFT max DFT min DFT max DFT min DFT max

32 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 skidresistant, locally-available aggregates in a wearing course mix design,

33 Questions?