HIGH FRICTION ROADS

Surface Testing

HIGH FRICTION:

What equipment and test procedures are used?

Aggregates

The aggregates used for HFS are generally tested for gradation, abrasion, and polishing. Below are examples of typical test requirements that have been used by various agencies. Please note that these requirements should not be considered standard practice for HFS, as the requirements vary from agency to agency.

Test	Procedure	Required Values*
Aggregate Gradation	Sieve Analysis ASTM C 136	Florida DOT: 95% min. passing No. 6 sieve, 5% max passing No. 16 sieve
Aggregate Abrasion	L.A. Abrasion Test ASTM C 131	Florida DOT: 10% max. South Carolina DOT: 20% max.
Accelerated Polishing Test	ASTM E 660 European Standard: EN 1097-8	South Carolina DOT: 75 mm max. (ASTM E660) UK: PSV >70

*Note: Florida DOT and South Carolina DOT specifications are developmental specifications and should not be considered standard specifications for HFS.

Binder

Binders may be tested for several properties, including tensile strength, pot life, viscosity, and tensile elongation, among others. Below are examples of test requirements that have been used by various agencies. Please note that these requirements should not be considered standard practice for HFS, as the requirements vary from agency to agency.

Property	Test Prodecure	Requirement*
Ultimate Tensile Strength	ASTM D 638	Florida DOT: Minimum 2,800 psi
Compressive Strength	ASTM D 695	Minimum @ 3 hrs – 1,000@ 75 °F Minimum @ 24 hrs – 5,000 psi@ 75 °F Florida DOT: 1,600 psi min.
Elongation	ASTM D 638	30-70% @ 7 days
Shore Hardness	ASTM D 638	Florida DOT: 70 min.
Abrasion Resistance	ASTM D 4060	SCDOT: 500 cycles, CS17
Gel Time	ASTM D 2471	Florida DOT: 10 minutes min.
Water Absorption	ASTM D 570	Florida DOT: Less than 0.25%
Viscosity	ASTM D2393 Florida DOT: ISO 2555	7-25 poises Florida DOT: 3,000 Mpa min.
Bleed Test	Florida DOT: Swab test visual 7 days @ room temp.	Florida DOT: Little trace
Cure Rate	Florida DOT: Thin film @ 75 °F	Florida DOT: 3 hours max.
Peak Exothermic Temperature	Florida DOT: ASTM D 2471	Florida DOT: 150 °F min.
Pot Life	ASTM C 881	15-45 min. @ 75 °F
Adhesion	Virginia Test Method 92 ASTM D 4541	Minimum @ 24 hrs – 250 psi @ 75 °F

*Note: Florida DOT and South Carolina DOT specifications are developmental specifications and should not be considered standard specifications for HFS.

Skid Resistance

Skid resistance, as defined by ASTM, is the ability of the pavement surface to prevent the loss of tire traction. Pavement skid resistance is generally quantified using either locked-wheel or fixed slip skid measurement devices which utilize standard tires for the measurements. Other stationary devices are also available for spot measurements of skid resistance. Skid Number (SN) and Skid Resistance Value (SRV) are the most common metrics for quantifying skid resistance. However, recent efforts to harmonize skid resistance values from different devices have resulted in the development of the International Friction Index (IFI), which provide a means to compare skid resistance valued between devices. Below are some of the types of equipment commonly used for measuring skid resistance:

Portable Skid Resistance Tester (a.k.a. British Pendulum Tester) – portable device for spot measurements of SRV.
Sideway-Force Coefficient Routine Investigation Machine (SCRIM®) – used to determine the Sideway Force Coefficient (SFC).
Griptester – continuous measurement device used to measure a GripNumber.
Locked-Wheel Trailer – device which uses a full-size test wheel with a standard (smooth or ribbed) tire to measure friction resistance (also produces a SN)
Dynamic Friction Tester (DFT) – portable device for spot measurements that calculates surface frictional properties at various speeds.

According to the HAPAS Guidelines Document for the Assessment and Certification of High-Friction Surfacing for Highways from the British Board of Agrément, a high friction surface is described as “having a minimum skid resistance value (SRV) of 65 measured using the portable Skid-Resistance Pendulum Tester”.

Test	Specification	Calculated Value
Pendulum Tester	ASTM E 303	SRV, BPN
Sideway-Force Coefficient Routine Investigation Machine (SCRIM®)	In accordance with TRL Report 176: Appendix E and Road Research Laboratory Road Note 27	SFC
Griptester	ASTM E 1844 (test tire specification)	GripNumber
Locked-Wheel Trailer	ASTM E 274	SN/FN
Dynamic Friction Tester	ASTM E 1911	Dynamic COF

Texture

HFS systems can also be tested for texture depth and/or profile depth, primarily for quantifying macrotexture, using the following tests and equipment. Specifications for texture depth vary from agency to agency depending on the application. In general, a minimum mean texture depth (MTD) of 1-1.5 mm is required.

Test	Specification	Type of Measurement	Value Reported
Circular Texture Meter (CTM)	ASTM E 2157	Laser	Mean Profile Depth
Sand Patch Method	ASTM E 965	Volumetric	Mean Texture Depth
Robotex	ISO 13473	Laser	Mean Profile Depth

Tire Pavement Noise

While a reduction in tire-pavement noise is not the primary reason for installing HFS, the macrotexture properties of HFS can help to reduce tire-pavement noise on certain pavement surfaces.

The following table describes types of noise testing commonly used to measure tire-pavement noise.

Test	Specification	Short Description
Statistical Pass-by Method (SPB)	ISO 11819-1	Type of wayside testing, where a microphone is located on the side of the roadway in a fixed location. Measures hundreds of individial vehicles.
Close-Proximity (CPX)	Draft ISO 11819-2	Fixed microphones are enclosed in a sound-proof trailer and are located near the test tire.
On-Board Sound Intensity	(OBSI) TP 76-08	Fixed microphone located near the tire, measures sound pressure levels as test vehicle is in motion.

Performance Measures for High Friction Surfacing

In addition to monitoring skid resistance, texture depth, and tire-pavement noise testing over time, monitoring the overall performance of the HFS system over time provides valuable information on the durability of the material.

While HFS product performance tests are still maturing in the U.S., the British Board of Agrement HAPAS process includes a number of simulative and conditioning tests for HFS products. Some of these tests are performed on the finished product in-situ, and others are performed in the laboratory on core samples removed from the finished product.

In addition to skid resistance and texture depth testing, on-site performance tests include:

Erosion Index – Examination of the HFS surface used to quantify the degree of erosion of the material.
Overall System Loss – Visual evaluation for overall system loss, including loss of aggregate or wearing away or delamination of the system from the underlying pavement surface.
Cracking – Used to identify cracking (greater than 0.5 mm in width) in the HFS system.

Additional laboratory “torture” tests of HFS systems are also conducted to evaluate the durability of the products. These tests include:

Scuffing – Used to determine the resistance to wear by scuffing a sample of the HFS at an elevated temperature. This is an accelerated loading test using a pneumatic tire to simulate actual traffic loading.
Wear – Used to determine the resistance to wear under repeated turing wheels at a low temperature. This accelerated loading test simulates wear of the HFS under traffic turning movements.
Tensile Adhesion – Used to evaluate the bond between the HFS system and the underlying pavement surface.
Heat Aging – Used to determine what effect heat aging has on the performance of the HFS system – used in conjunction with the Scuffing Test.
Freeze-Thaw Conditioning – Used to determine what effect exposure to freeze-thaw cycles has on the performance of the HFS system.
Diesel Susceptibility – Used to determine the extent to which exposure to diesel fuel has a deleterious effect on the HFS system performance.
Thermal Movement – Used to determine the thermal expansion coefficient of the HFS system.