Module 2-4 Roughness and Surface Friction Testing

Learning Objectives  Explain the importance of conducting roughness and surface friction evaluations  Describe the different types of roughness survey methods, equipment types, and roughness indices  Describe the different types of friction measuring equipment  List the different categories of surface texture

Learning Objectives (continued)  Explain the importance of measuring surface texture as part of a surface friction evaluation  Describe the different types of texture- measuring devices  Explain the advantages of using IRI and IFI over other indices  Describe how roughness- and friction- related information influences the selection of rehabilitation activities

Why is it important to assess roughness and friction?  Both affect “functional” performance  Roughness leads to increases in:  Vehicle operating costs  User delay  Accidents  User dissatisfaction  Friction loss leads to increases in wet weather accidents

Roughness Surveys

Definition of Roughness  Deviations in pavement surface that affect ride quality  Caused by:  Built-in surface irregularities  Traffic and environment

Purpose of a Roughness Survey  Identify areas of severe roughness  Determine relative roughness between projects  Gauge benefits of various rehabilitation activities by measuring roughness before and after construction

Roughness Survey Types 1.Ride quality surveys (serviceability)  Subjective method  Simple assessment tool 2.Objective roughness surveys  Quantitative unbiased method  Various types of equipment are available

Ride Quality Surveys Serviceability  Subjective measure of user’s perception of pavement rideability  Measurement scale  Zero (very poor) to Five (very good)  Working range: 1.5 to 4.5  Trigger levels for rehabilitation depend on speed and level of traffic  Correlations with other roughness indices

Roughness Measuring Equipment  Response-Type Road Roughness Measuring Systems (RTRRMS)  Maysmeter  PCA Roadmeter  BPR Roughometer  Inertial Road Profiling Systems (IRPS)  ICC Profiler  K.J. Law Profilometer  South Dakota Profiler

Response-Type Road Roughness Measuring Systems (RTRRMS)  Measure vehicle response  Advantages  Low initial and operating costs  Ease of operation  High measuring speeds  Disadvantages  Output sensitive to vehicle characteristics  Requires frequent calibration

Response-Type Systems Maysmeter

Inertial Road Profiling Systems Equipment (IRPS)  Measure actual pavement profile  Advantages  Relatively accurate and repeatable profile measurements  Used to calibrate RTRRM systems  Disadvantages  High capital and operating expenses  Complexity of systems

Inertial Profiler Systems

International Roughness Index (IRI)  Current roughness measurement standard  Correlates to RTRRM systems  Scale: 0 to 20 m/km (0 to 1267 in/mi)  2 m/km (125 in/mi) is typical break point between rough and smooth pavements  Standard for FHWA HPMS Database

Roughness Data Variability  Seasonal and daily environmental effects  Lateral positioning  Differences in operator practice (between- operator variability)  Differences between equipment devices (between-device variability)  Manufacturing differences  Calibration problems

Surface Friction Testing

Definition of Surface Friction  Force developed at pavement-tire interface that resists sliding  Influenced by:  Surface texture  Surface drainage (cross-slope)

Purpose of a Surface Friction Survey  Assess safety concerns  Hydroplaning  Wet weather accidents  Target testing in areas of poor surface condition:  Smooth macrotexture (polishing or inadequate finishing)  Inadequate pavement cross slope

How do you adequately assess surface frictional characteristics?  Historical method—Measure friction directly (“skid number”)  Recent research indicates that “surface texture” MUST also be considered

What is surface texture?  Characteristics that contribute to surface friction  Microtexture – Roughness of individual pieces of aggregate  Macrotexture  General coarseness of pavement surface  Formed water channels (grooving)  Large impact on surface friction

Friction Survey Measurement Equipment Types  Locked-wheel testers  Side force testers (Mu Meter, SCRIM)  Fixed slip testers (Gripster)  Variable slip testers (Norsemeter ROAR) Note: These devices do NOT measure texture!

Friction Measuring Devices Locked-Wheel Trailer

Friction Measuring Devices Examples of Other Devices Mu Meter (side force tester) SCRIM Norsemeter ROAR (variable slip tester)

Surface Texture Measuring Devices  Volumetric (“Sand Patch”) method  Outflow meter  Circular track meter (CTMeter)  High-speed laser-based devices (ROSAN, most laser-based profilers)

Surface Texture Measurement Sand Patch Method

Surface Texture Measurement High-speed Laser-Based Devices ROSANv High-Speed Inertial Profilers

International Friction Index (IFI)  Incorporates simultaneous measurements of friction and macrotexture  Speed constant (S p )  Friction number (F60)  Becoming friction measurement standard  Modern high-speed measuring equipment measure IFI directly

Review  Why is it important to conduct roughness and surface friction evaluations?  What are the two types of roughness measuring equipment?  What are two types of friction measuring equipment?  What are the three categories of surface texture?

Review (continued)  Why it is important to measure surface texture as part of a surface friction evaluation?  What are two of the different types of texture-measuring devices?  What are the advantages of using IRI and IFI (over other indices)?  How do roughness and friction affect the selection of rehabilitation alternatives?

 Henry, J. J Evaluation of Pavement Friction Characteristics. Synthesis of Highway Practice No National Cooperative Highway Research Program, Transportation Research Board, Washington, DC.  Karamihas, S. M., T. D. Gillespie, R. W. Perera, and S. D. Kohn “Guidelines for Longitudinal Pavement Profile Measurement.” NCHRP Report 434. Transportation Research Board, Washington, DC.  Ksaibati, K., R. McNamara, W. Miley, and J. Armaghani “Pavement Roughness Data Collection and Utilization.” Transportation Research Record Transportation Research Board, Washington, DC. Key References

 Sayers, M. W. and S. M. Karamihas The Little Book of Profiling—Basic Information about Measuring and Interpreting Road Profiles. University of Michigan Transportation Research Institute, Ann Arbor, MI. Web address:  Smith, K. L., K. D. Smith, L. D. Evans, T. E. Hoerner, and M. I. Darter “Smoothness Specifications for Pavements.” Web Document #1, Final Report NCHRP Project Transportation Research Board, Washington, DC. Key References (continued)

 ASTM E 274, Skid Resistance of Pavements Using a Full- Scale Tire  ASTM E 965, Measuring Pavement Macrotexture Depth Using a Volumetric Technique  ASTM E 1845, Calculating Pavement Macrotexture Profile Depth  ASTM E 1859, Friction Coefficient Measurements Between Tire and Pavement Using a Variable Slip Technique  ASTM E 1926, Calculating International Roughness Index of a Pavement Surface  ASTM E 1960, Calculating International Friction Index of a Pavement Surface Applicable ASTM Standards