Guide for Pavement Friction (2009)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

87 CHAPTER 6. PAVEMENT FRICTION DESIGN INTRODUCTION Although pavement friction design is a relatively small component of the overall pavement design process, it is particularly critical because of the safety issue. Its importance and complexity have increased over the years due to increased demands for safer roads and the desire for greater highway user comfort, which sometimes contradicts friction. Friction design requires a thorough understanding of the factors that influence friction and knowledge of the materials and construction techniques (including equipment) that ultimately dictate initial and long-term friction. It also requires an understanding of the economic and engineering tradeoffs associated with different materials and techniques, such as the costs/benefits of utilizing one friction strategy over another and how each strategy impacts structural design and other functional aspects (e.g., noise, splash/spray). Designing pavement surfaces so that they have adequate friction—whether as part of a new pavement structure or a rehabilitation activity—involves identifying materials and construction activities that produce an appropriate combination of micro-texture and macro-texture. The micro-texture is a function of the type of aggregate used in the surface mix, while the macro-texture is generally dictated by the gradation/size of the aggregate in the mix or the type of texturing applied to the surface of the in-place mix. This chapter discusses in detail the issues of micro-texture and macro-texture, and how they form the basis for designing pavement surfaces for friction. Both the network policy aspects and the project-level engineering aspects of friction design are discussed, along with the economics and other pavement–tire interaction issues that often must be addressed. FRICTION DESIGN POLICIES Friction design policies represent a highway agency’s overall framework and procedural manner for ensuring that all pavement projects fully and properly account for friction needs. As evidenced by the survey carried out in this study (see table C-7 in appendix C), SHA policies largely focus on the selection and use of (a) aggregates for micro-texture and (b) paving mixtures and surface texturing techniques for macro-texture. Discussions about each of these aspects are provided below. Consideration of Aggregate in Friction Design As noted earlier, micro-texture plays a key role in the development of pavement–tire frictional forces and is primarily governed by the properties of the aggregate used in the surface. While asphalt binder and cement paste can affect micro-texture—particularly just after a surface mix is placed—it is aggregate that makes up the bulk of asphalt and concrete mixtures, and thus serves as the primary contact medium with the vehicle tires.

88 Aggregate generally is viewed as two distinct fractions—coarse aggregate and fine aggregate. Coarse aggregate pieces are greater than the No. 4 sieve (0.19 in [4.75 mm]), with most pieces between 0.375 and 1.5 in (9.5 and 38 mm). Fine aggregate, on the other hand, is the collection of natural or crushed/manufactured particles less than 0.19 in (4.75 mm), but greater than the No. 200 sieve (0.003 in [75 µm]). Aggregate testing and characterization must be targeted to the fraction(s) of aggregate in a mix that will control the frictional performance. In general, coarse aggregate controls the frictional properties of asphalt mixtures, while fine aggregate controls the frictional properties of concrete mixes. Exceptions include fine-graded asphalt mixes, where fine aggregates are in greater abundance, and concrete mixes in which coarse aggregates are either intentionally exposed at the time of construction (exposed aggregate concrete, porous concrete) or will become exposed in the future (diamond grinding/grooving, surface abrading). Research by Dahir and Henry (1978), Kandhal and Parker (1998), and Folliard and Smith (2003), among others, indicates that the following aggregate properties have a significant influence on pavement friction performance: • Hardness. • Mineralogy (i.e., mineral composition and structure). • Shape. • Texture. • Angularity. • Abrasion Resistance. • Polish Resistance. • Soundness. Aggregate hardness and mineralogy largely dictate the wear characteristics (i.e., durability, polish) of the aggregate. Aggregates that exhibit the highest levels of long-term friction are typically composed of hard, strongly bonded, interlocking mineral crystals (coarse grains) embedded in a matrix of softer minerals (Henry, 2000). The differences in grain size and hardness provide a constantly renewed abrasive surface because of differential wear rates and the breaking off of the harder grains from the softer matrix of softer minerals. Aggregates made up of hard minerals alone typically resist wear and other forms of degradation, yet may polish easily when subjected to traffic. Aggregates made up of moderately soft minerals alone resist polishing, but wear quickly when subjected to traffic. Thus, while a wear-resistant aggregate is desired in the mixture, some wearing of the pavement surface must occur to ensure good levels of skid resistance (Davis, 2001). As summarized in table 22, aggregate angularity, shape, and texture are important parameters for defining both micro-texture and macro-texture. Fine aggregates that exhibit angular edges and cubical or irregular shapes generally provide higher levels of micro-texture, whereas those with rounded edges or elongated shapes generally produce

89 Table 22. Effect of aggregate angularity, shape, and texture properties on pavement friction. Effect of Aggregate Property on Pavement Friction Aggregate Fraction Aggregate Property Asphalt Surface Concrete Surface Angularity and shape No effect. Defines pavement micro-texture, which highly impacts friction. Fine Texture No effect. Little to no effect. Angularity and shape Defines pavement macro-texture, which significantly impacts friction via hydroplaning potential. If exposed, helps define pavement macro-texture, which impacts friction via hydroplaning potential. Coarse Texture Defines pavement micro-texture, which highly impacts friction. If exposed, helps define pavement micro-texture, which impacts friction. lower micro-texture. For coarse aggregates, sharp and angular particles interlock and produce a deep macro-texture as compared to more rounded, smooth particles. Moreover, in asphalt mixes, platy (i.e., flat and elongated) aggregate particles tend to orient themselves horizontally, resulting in lower macro-texture depth. The abrasion resistance of aggregates is an indicator of their resistance to mechanical degradation. The use of abrasion-resistant aggregates is important to avoid the breakdown of fine and/or coarse aggregates. During handling, stockpiling, mixing, and construction, the breakdown of fine and/or coarse aggregates can significantly alter the mix gradation, thereby affecting the porosity of open-graded friction course (OGFC) asphalt mixes and porous concrete mixes. For concrete mixes, it can result in the loss of strength due to the production of excess fines in the concrete mix. In asphalt mixes, the increase in fines can alter the volumetric properties and result in insufficient binder or may contribute to rutting and shoving. After construction, the breakdown of fine and/or coarse aggregates due to traffic shear forces can result in a loss of macro-texture. Polish-resistant aggregates are those that retain their micro-texture under the grinding and shearing effects of repeated traffic loadings. For asphalt surface mixes, it is the hardness and mineralogy of the coarse aggregate particles that determine the degree of polishing that takes place. For concrete mixes, because the surface is composed primarily of mortar and is initially devoid of coarse aggregates, the polishing resistance of fine aggregates is the most critical parameter (Folliard and Smith, 2003). The coarse aggregate becomes an influencing factor only if it is made or becomes exposed. Soundness refers to an aggregate’s ability to resist degradation caused by climatic/ environmental effects (i.e., wetting and drying, freezing and thawing). Similar to abrasion resistance, sound and durable aggregate properties are important for avoiding the breakdown of fine and/or coarse aggregates, particularly when used in harsh climates. Aggregate Tests Many laboratory material tests were noted in the literature as pertinent in defining aggregate frictional properties. Many of these same tests were reported in the state friction

90 survey (see listing in appendix C) as being used to ensure the use of aggregates with good frictional properties. Several of the tests cited may be conducted for reasons other than friction performance. For example, for concrete pavements, mineralogical tests are very important in assessing the potential development of alkali-aggregate reactivity, D-cracking, and spalling. For asphalt pavements, coarse and fine aggregate particle shape and texture are good indicators of permanent deformation and fatigue cracking potential. Based on recent thorough evaluations of aggregate tests related to performance (Kandhal and Parker, 1998; Folliard and Smith, 2003) and the proactive work of various states— Maryland, Michigan, Ohio, New York, and Texas, to name a few—the following tests are considered most relevant in characterizing frictional properties and potential performance. • Scratch Hardness (Mohs). • Petrographic Analysis (ASTM C 295). • Uncompacted Voids (UV) for Fine Aggregate (AASHTO T 304 or ASTM C 1252). • UV for Coarse Aggregate (AASHTO T 326). • Fractured-Face Particles (ASTM D 5821). • Micro-Deval for Fine Aggregates (Canadian Standards Association [CSA] A23.2- 23A). • Micro-Deval for Coarse Aggregates (AASHTO TP 58 or ASTM D 6928). • LA Abrasion (AASHTO T 96 or ASTM C 131 for small-sized coarse aggregates; ASTM C 535 for large-sized coarse aggregates). • Acid Insoluble Residue (AIR) (ASTM D 3042). • Polished Stone Value (PSV) (AASHTO T 278 and T 279 or ASTM E 303 and D 3319). • Magnesium Sulfate Soundness (AASHTO T 104 or ASTM C 88). Table 23 provides a brief description of these tests and shows their recommended applications. Further discussion about the selection of tests is provided below. It is important to note that no individual test provides a full and accurate prediction of friction performance. Selecting and using multiple tests will increase the reliability, but even then there is no total guarantee of friction performance in the field. Thus, it is essential that testing be used in conjunction with field performance history to identify acceptable aggregate types. Aggregate Composition/Structure and Mineral Hardness While a visual inspection (using the descriptive nomenclature in ASTM C 294) of the aggregate can provide a basic understanding of mineral composition and structure, more detailed information can be obtained through advanced testing using petrographic analysis (ASTM C 295). Among other things, petrographic analysis provides important information on the types and relative amounts of constituent minerals comprising an aggregate. Although the Mohs hardness test can be performed on the individual mineral components, an experienced petrographer will know the approximate hardness values of each component. Thus, a range of hardness can be established, as can the proportion of hard versus soft minerals.

91 Table 23. Test methods for characterizing aggregate frictional properties. Aggregate Property Aggregate Type Test Name Test Protocol Test Description Applications Fine Scratch Hardness test Mohs Rough measure of the resistance of a mineral’s surface to scratching. Expressed using a 1-to-10 scale (1 being very soft, 10 being very hard), Mohs hardness is determined by observing whether its surface is scratched by minerals of a known or defined hardness. • New concrete surfacings. Hardness Coarse Scratch Hardness test Mohs Same as above. • New asphalt surfacings and asphalt mixes used for friction restoration. • New concrete surfacings (conventional and innovative).a Descriptive Nomenclature for Constituents of Concrete Aggregates ASTM C 294 Provides brief descriptions of commonly occurring natural or artificial aggregates from which mineral aggregates are derived. The descriptions provide a basis for understanding the potential effects on pavement friction of using different aggregate materials. Fine Petrographic Analysis ASTM C 295 Used to assess aggregate (1) constituent minerals and structure, (2) surface texture, and (3) mineralogy, and to develop a petrographic database for aggregate sources to serve as a basis for linking aggregate sources to pavement field performance (Folliard and Smith, 2003). • New concrete surfacings. Descriptive Nomenclature for Constituents of Concrete Aggregates ASTM C 294 Same as above. Mineralogy (i.e., Aggregate Composition & Structure) Coarse Petrographic Analysis ASTM C 295 Same as above • New asphalt surfacings and asphalt mixes used for friction restoration. • New concrete surfacings (conventional and innovative).a a For conventional PCC surfaces, where coarse aggregates are expected to be exposed, and innovative surfaces, such as porous concrete and exposed aggregate concrete.

92 Table 23. Test methods for characterizing aggregate frictional properties (continued). Aggregate Property Aggregate Type Test Name Test Protocol Test Description Applications Fine Uncompacted Voids (UV) test for fine aggregates AASHTO T 304 (or ASTM C 1252) Fine aggregate of prescribed gradation is allowed to flow through orifice of a funnel and fill a 6.1-in3 (100- cm3) cylinder. Excess material is struck off and cylinder with aggregate is weighed. Uncompacted void content is computed using this weight and the bulk dry specific gravity of the aggregate (Kandhal et al., 1997). Higher uncompacted void contents are generally the result of more fractured faces and rougher textures, which are desirable for pavement friction. • New concrete surfacings. Uncompacted Voids (UV) test for coarse aggregates AASHTO T 326 b Coarse aggregate angularity, shape, and texture can be determined using principles similar to those described above for fine aggregates. Again, higher uncompacted void contents are generally the result of more fractured faces and rougher textures, which are desirable for pavement friction. • New asphalt surfacings and asphalt mixes used for friction restoration. • New concrete surfacings (conventional and innovative).a Angularity, Shape, & Texture Coarse Fractured-Face Particles test ASTM D 5821 Determines the amount (percent) of fractured-faced (an angular, rough, or broken surface of an aggregate particle) aggregate particles, by visual inspection. The fractured face of each aggregate particle must meet a minimum cross-sectional area. • New asphalt surfacings and asphalt mixes used for friction restoration. • New concrete surfacings (conventional and innovative).a Fine Micro-Deval test for fine aggregates Canadian Standards Association (CSA) A23.2-23A A fine aggregate sample is subjected to wet attrition by placing it in a steel jar with 0.375-in (9.5-mm) diameter steel balls and water. The jar is rotated at 100 rpm for 15 minutes, after which aggregate damage is assessed by mass loss using a No. 200 (75 μm) sieve. Higher percentages of loss indicate greater potential for aggregate breakdown (Folliard and Smith, 2003). • New concrete surfacings (conventional). LA Abrasion test AASHTO T 96 (or ASTM C 131 [for small-sized coarse aggregates] ASTM C 535 [for large-sized coarse aggregates]) A dry aggregate sample is placed in a steel drum with six to twelve 420-gram steel balls, and the drum is rotated for 500 to 1,000 revolutions. Degradation by impact of the aggregate sample is determined by the percentage passing the No. 12 (1.7-mm) sieve. • New asphalt surfacings and asphalt mixes used for friction restoration. • New concrete surfacings (conventional and innovative)a Abrasion/Wear Resistance Coarse Micro-Deval test for coarse aggregates AASHTO TP 58 (or ASTM D 6928) A coarse aggregate sample is subjected to wet attrition by placing it in a steel jar with 0.375-in (9.5-mm) diameter steel balls and water. The jar is rotated at 100 rpm for 2 hours, after which aggregate damage is assessed by mass loss using a No. 16 (1.18-mm) sieve. • New asphalt surfacings and asphalt mixes used for friction restoration. • New concrete surfacings (conventional and innovative)a a For conventional PCC surfaces, where coarse aggregates are expected to be exposed, and innovative surfaces, such as porous concrete and exposed aggregate concrete. b Formerly AASHTO TP 56.

93 Table 23. Test methods for characterizing aggregate frictional properties (continued). Aggregate Property Aggregate Type Test Name Test Protocol Test Description Applications Fine Acid Insoluble Residue (AIR) test ASTM D 3042 Estimates the percent by weight of insoluble, hard, non- carbonate residue in carbonate aggregates (e.g., limestone, dolomite), using hydrochloric acid solution to react the carbonates. Higher acid insoluble residue (AIR) values indicate larger percentages of siliceous minerals, which are considered more polish resistant than carbonate materials (Kandhal et al., 1997). • New concrete surfacings. Polished Stone Value (PSV) test AASHTO T 278 & T 279 (or ASTM E 303 & D 3319) Aggregate coupons (aggregates embedded in epoxy resin) are fabricated, subjected to accelerated polishing (using British polish wheel) for a specified time (usually 9 hrs), and then tested for frictional resistance (expressed as British Pendulum Number [BPN]) using the British Pendulum Tester. The BPN value associated with accelerated polishing is defined as the polished stone value (PSV), which is a quantitative representation of the aggregate’s terminal frictional characteristics. Higher values of PSV indicate greater resistance to polish. • New asphalt surfacings and asphalt mixes used for friction restoration. • New concrete surfacings (conventional and innovative)a Polish Resistance Coarse Acid Insoluble Residue (AIR) test ASTM D 3042 Same as above. • New asphalt surfacings and asphalt mixes used for friction restoration. • New concrete surfacings (conventional and innovative)a Fine • New concrete surfacings. Soundness Coarse Magnesium Sulfate Soundness test AASHTO T 104 (or ASTM C 88) An aggregate sample is immersed in a solution of magnesium sulfate for a period of 16 to 18 hours at a temperature of 70°F (21°C). The sample is then removed, drained for 15 minutes, and oven-dried to a constant weight (5 cycles of immersion and drying is typical). During the immersion process, the salt solution penetrates the permeable pore spaces of the aggregate. Oven drying dehydrates the sulfate salt precipitated in the pores. The internal expansive force of the re-hydration upon re-immersion simulates the expansion of water upon freezing. Upon completion of the final cycle, the sample is sieved over various sieves and the maximum weighted average loss is reported as the sulfate soundness loss. Higher percentages of loss indicate less sound or durable aggregate (Khandal et al., 1997). • New asphalt surfacings and asphalt mixes used for friction restoration. • New concrete surfacings (conventional and innovative)a a For conventional PCC surfaces, where coarse aggregates are expected to be exposed, and innovative surfaces, such as porous concrete and exposed aggregate concrete.

94 Aggregate Angularity, Shape, and Texture The uncompacted voids (UV) test (AASHTO T 304 [ASTM C 1252]) is the most commonly used test for assessing fine aggregate angularity, sphericity, and texture (Folliard and Smith, 2003). As indicated by Meininger (1994), this test does not require performing detailed petrographic evaluations of shape and texture. Three feasible options for assessing coarse aggregates are the fractured-face particles test (ASTM D 5821), the UV test (AASHTO T 326 [formerly AASHTO TP 56]), and the flat/elongated particles test. Highway agencies use both the fractured-face and flat/elongated particles tests extensively, primarily for controlling rutting in asphalt mixes. Because there are concerns with the subjectivity of the former test, NCHRP Project 4-19 recommended the UV test as a replacement for it (Prowell et al., 2005). However, given that the UV test has yet to be adopted by any state, the option of either test is recommended. Furthermore, because it is believed that the fractured particles test conveys a better sense of the micro-texture characteristics of an aggregate as compared to the flat/elongated test, it is recommended over the flat/elongated test. Abrasion/Wear Resistance While the Micro-Deval test (AASHTO TP 58 [ASTM D 6928]) for coarse aggregates has been reported to be a better indicator of the potential for aggregate breakdown (Folliard and Smith, 2003; Kandhal and Parker, 1998), the LA Abrasion test is commonly used with good success. Both tests are recommended. Polish Resistance There are no direct tests for assessing fine aggregate polish characteristics. The acid insoluble residue (AIR) test (ASTM D 3042), which indicates the amount of softer polishing carbonate material in an aggregate, is widely used and accepted, and has been reported to best relate to friction in concrete pavements. It is therefore recommended for fine aggregate. For coarse aggregates, both the AIR test and the polished stone value (PSV) test (AASHTO T 278 & T 279 [ASTM E 303 & D 3319]) have been used with good success. Both tests are recommended. Feasible alternatives to the AIR and PSV tests exist—some old, some new; some standard, some non-standard. The Tennessee Terminal Textural Condition Method (T3CM), for instance, developed in the mid-1990s, utilizes the LA abrasion device and a modified version of the UV test apparatus to assess the texture retention characteristics of an aggregate (Crouch et al., 1996). Aggregates with good micro-texture and micro-texture retention characteristics exhibit higher UV contents and smaller reductions in loss when subjected to LA abrasion aging revolutions. The Tennessee DOT has used the T3CM test with fairly good success for several years and an improved version of the test (termed MDV9) that utilizes the Micro-Deval abrasion apparatus has been developed and evaluated (Crouch and Dunn, 2005). Other test alternatives include Circular Track Polishing tests. Like the PSV test, these tests consist of polishing an aggregate sample using an accelerated polishing device and then evaluating the micro-texture of the aggregate using a friction testing device. Three

95 particular tests examined in this study include the North Carolina State University (NCSU) wear and polishing test (represented by ASTM standard E 660, Standard Practice for Accelerated Polishing of Aggregates or Pavement Surfaces Using a Small-Wheel, Circular Track Polishing Machine), the Michigan aggregate wear test, and the NCAT polishing test. The polishing devices used in these tests are bigger than the accelerated polishing machine (APM) used in the PSV test, with polishing tracks ranging from about 12 in (305 mm) (NCAT device) to 7 ft (2.1 m) (Michigan device) in diameter and test tires ranging from 8 in (203 mm) in diameter (NCAT device) to full-scale smooth friction test tires (Michigan device). Although the size, configuration, and operation of these devices appear to simulate real-world conditions better than the APM, the equipment and operational costs tend to be greater than the APM. At issue also is the availability and usage of the various polishing-type tests. McDaniel and Coree (2003) reported that, although ASTM E 660 was re-approved in 2002, it is not being used by the university or the North Carolina DOT. Furthermore, as per the survey results of this study, no states reported using the ASTM E 660 test. Though the Michigan DOT has reported good success with the Michigan aggregate wear test (which uses a laboratory version of the ASTM towed friction tester), it is the only agency that uses it. The NCAT test is primarily designed for mixture samples instead of aggregate samples. Nominal 20-in (508 mm) square slabs are polished and tested with the DFT and CTM, resulting in both micro-texture and macro-texture assessments of the prepared mix. This test is more appropriate for use as a mix design and/or QC/QA test. Soundness The test method considered to best characterize aggregate soundness is the sulfate soundness test (AASHTO T 104 [ASTM C 88]). This widely used test was developed to simulate, without the need for refrigeration equipment, the effects of freeze-thaw water action on aggregate particles (Khandal and Parker, 1998). Two options for sulfate solution are given in this test—sodium sulfate and magnesium sulfate. The preferred option is the latter, as it has been reported to produce less variation in mass loss (Folliard and Smith, 2003) and provide a better indication of good versus poor aggregates (Kandhal and Parker, 1998). Aggregate Test Criteria Just as no single test can distinguish good friction performance from bad, no single test value can be used as a standard for the same purpose. The factors that influence friction performance do so in an interactive manner and on a continuous scale, making it difficult to pinpoint specific discrimination values. Nevertheless, research and current practices shed light on what can be considered as basic guidance in establishing friction performance-related test criteria. Table 24 provides

96 Table 24. Typical range of test values for aggregate properties. Aggregate Property Aggregate Fraction Test Type Typical Property Range for Good Friction Performancea Supporting Documentation Fine Mohs Scratch Hardness ≥ 6 Hardness Coarse Mohs Scratch Hardness Hard minerals: ≥ 6 Soft minerals: 3 to 5 Differential hardness (hard minus soft): 2 to 3 Fine Visual Examination (Constituents of Concrete Aggregates) and Petrographic Analysis Hard siliceous mineral aggregate Aggregate Composition & Structure Coarse Visual Examination (Constituents of Concrete Aggregates) and Petrographic Analysis Percent of Hard Fraction Natural Aggregate: 50 to 70 Artificial Aggregate: 20 to 40 Hard Grain or Crystal Size 150 to 300 µm, average 200 µm Hard Grain or Crystal Shape Angular Tips • Dahir and Henry (1978) provided Mohs hardness values for a variety of minerals, as follows: Diamond: 10 Feldspar: 6 - 6.5 Corundum: 9 Pyroxene Group: 5 - 7 Topaz: 8 Amphibole Group: 4 - 6.5 Sillimanite: 7.5 Apatite: 5 Cordierite: 7 - 7.5 Zeolites: 3.5 - 5.5 Quartz: 7 Flourite: 4 Garnet Group: 6.5 - 7.5 Dolomite: 3.5 - 4 Olivine Group: 6.5 - 7 Calcite: 3 Epidote Group: 6 - 7 Gypsum: 2 Chalcedony: 6 • As noted by Dahir and Henry (1978), arbitrarily, but rather widely, Mohs Hardness of 5 has been used as dividing number between minerals termed as soft and those termed as hard. • Dahir and Henry (1978) reported that aggregates made up of hard minerals (Mohs hardness ≥ 6) alone typically resist wear and other forms of degradation, yet may polish easily when subjected to traffic. Aggregates made up of moderately soft minerals (Mohs hardness of 3 to 6) alone resist polishing, but wear quickly when subjected to traffic. • The ideal coarse aggregate should consist of 50 to 70 percent coarse-grained and hard minerals embedded in a matrix of 30 to 50 percent softer minerals (Dahir and Henry, 1978). Coarse aggregates that contain larger and more angular mineral grains or crystals exhibit higher levels of micro-texture and have a higher frictional resistance.

97 Table 24. Typical range of test values for aggregate properties (continued). Aggregate Property Aggregate Fraction Test Type Typical Property Range for Good Friction Performancea Supporting Documentation Fine Uncompacted Voids content, % ≥ 45 Uncompacted Voids content, % ≥ 45 • Guideline value of 45% minimum is based solely on addressing permanent deformation concerns (as noted by Prowell et al. [2005], several studies have supported the 45% minimum criteria). No research was available to indicate what minimum value should be used from a friction performance standpoint. Angularity, Shape, & Texture Coarse Fractured-Face Particles Agg. Particle Size: 0.12 to 0.5 in (3 to 13 mm) Agg. Particle Shape: Conical, Angular At least 90% by weight of the combined aggregates retained on No. 4 (4.75 mm) sieve should have two or more mechanically fractured faces. • Guideline value of 90% minimum is based on addressing rutting potential (as suggested by Prowell et al. [2005], a reasonable minimum target for high traffic pavements is 95% with two or more crushed faces). No research was available to indicate what minimum criteria should be used from a friction performance standpoint. Fine Micro-Deval, % Loss ≤ 17 to 20 Micro-Deval, % Loss ≤ 17 to 20 • Research performed under NCHRP Project 4-19 (Kandhal and Parker, 1998) resulted in a recommendation of 18% as the maximum allowable percentage loss. Ontario has longstanding requirement of 17% for aggregate used in surface courses (Kandhal and Parker, 1998). Abrasion/Wear Resistance Coarse LA Abrasion, % Loss ≤ 35 to 45 • As reported by Prowell et al. (2005), the LA abrasion test is used extensively by state agencies, with specification values ranging from 30 to 55% maximum and the most frequently cited specification value being 40% maximum • Wu et al. (1998) reported that majority of states have a maximum allowable loss of 40 or 45%, and noted that criteria are more restrictive for surface courses than base courses. • FHWA (2005) recommended range of 35 to 45% as maximum loss using the LA Abrasion test.

98 Table 24. Typical range of test values for aggregate properties (continued). Aggregate Property Aggregate Fraction Test Type Typical Property Range for Good Friction Performancea Supporting Documentation Fine Acid Insoluble Residue (AIR), % ≥ 50 to 70 AIR, % ≥ 50 to 70 • Dahir and Henry (1978) recommended 50 to 70 percent minimum for heavily traveled pavements. • According to Liang (2003), Kentucky DOT specifies 50% minimum for class A aggregate sources. • Liang and Chyi (2000) reported that New York DOT requires minimum of 15% for ADT greater than 3,000 veh/day. Polish Resistance Coarse Polished Stone Value (PSV) ≥ 30 to 35 • Texas DOT criteria for PSV for different traffic levels (Liang and Chyi, 2000): ¾ ADT < 750: No requirements ¾ ADT between 750 and 2,000: 28 min. ¾ ADT between 2,000 and 5,000: 30 min. ¾ ADT > 5,000: 32 min. • Louisiana DOT governs use of asphalt mixtures based on four levels of PSV (Liang, 2003): ¾ Friction Rating I: PSV > 37 ¾ Friction Rating II: PSV between 35 and 37 ¾ Friction Rating III: PSV between 24 and 30 ¾ Friction Rating IV: PSV between 20 and 29 • New Jersey DOT rated surface coarse aggregates with minimum PSV between 25 and 30 as marginal and with minimum PSV greater than 30 as good (Liang, 2003). To ensure year-round SN40S greater than 35, they recommended minimum PSV of 33 (Liang, 2003). • Tennessee DOT categorizes aggregates based on silica dioxide content, calcium carbonate content, AIR, and PSV. Minimum PSV values for each category are as follows (Liang, 2003): ¾ Type I: 33 ¾ Type II: 30 ¾ Type III: 25 • Utah DOT specifies minimum PSV of 38 for aggregates to be used in surface courses (Liang, 2003). • Senior and Rogers (1991) recommended minimum PSV of 50 for high-volume roadways in Ontario. Fine Magnesium Sulfate Soundness (5 cycles), % Loss ≤ 10 to 20 Soundness Coarse Magnesium Sulfate Soundness (5 cycles), % Loss ≤ 10 to 20 • While Kandhal et al. (1997) reported a fairly wide range (10 to 30) in the maximum percentage loss specified by some states, subsequent research performed under NCHRP Project 4-19 (Kandhal and Parker, 1998) resulted in a recommendation of 18% as the maximum value. • FHWA (2005) recommended a range of 15 to 20% as the maximum loss using the magnesium sulfate test.

99 guideline values in the form of acceptable ranges for the tests recommended in the previous section. It also presents and discusses the source information used to support the guideline criteria. The information presented pertains to typical virgin aggregates and may not apply to lightweight, heavyweight, or recycled aggregates. Surface Mix Types and Texturing Techniques Pavement surface drainage is in part a function of the surface macro-texture, which is defined largely by the aggregate gradation characteristics and finish quality of the surface mix. Surfaces with greater amounts of macro-texture provide greater resistance to sliding via hysteresis, and they help facilitate drainage, thereby reducing the potential for hydroplaning. Several different surface mix types and finishing/texturing techniques are available for use in constructing new pavements and overlays, or for restoring friction on existing pavements. Tables 25 and 26 describe the commonly used mix types and texturing techniques, respectively, and they present the typical macro-texture levels achieved. Pavement–tire considerations, such as noise, splash/spray, and hydroplaning, and general considerations, such as constructability, cost, and structural performance, are not discussed here, but they are an integral part of any policies developed for these mixes and texturing techniques. Design Policy for Friction and Texture The way aggregates and/or surface mixtures/textures are specified and selected for pavement projects, varies widely throughout the U.S. While the survey conducted in this study provides some indication of current practices, an earlier survey by Jayawickrama et al. (1996) provided an insightful characterization of friction design practices that most likely hasn’t changed. The approaches are categorized as follows: • Category I—No Specific Guidelines to Address Skid Resistance. Experience indicates that no prior classification of aggregates is necessary and, as such, no special procedure is followed to ascertain that the frictional characteristics of the aggregate used are satisfactory. The primary reason cited for such a policy is the availability of good quality aggregates. • Category II—Skid Resistance is Accounted for Through Mix Design. States in this category also don’t use any procedure to evaluate aggregate frictional properties. Instead, they base their friction policies on proper mix design. Again, experience shows that these states have no major problems related to pavement friction. • Category III—No Specific Guidelines to Address Skid Resistance. States in this category consider friction of surface courses in the design of new pavements. Sufficient friction is obtained by controlling the quality of aggregate used in the construction of the pavement surface courses. Quality of the aggregates is controlled through experience by specifying the type and allowable percentages of a particular type of aggregate.

100 Table 25. Asphalt pavement surface mix types and texturing techniques. Application Mix/ Texture Type Description Macro-texture Deptha Dense Fine-Graded HMA Dense-graded HMA is a dense, continuously graded mixture of coarse and fine aggregates, mineral filler, and asphalt cement (5 to 6 percent). It is produced in a hot-mix plant, delivered, spread, and compacted on site. Dense-graded HMA can be modified with polymers or crumb rubberb, and may include recycled materials. Nominal maximum sizes for surfacing applications can range from 0.38 in (9.5 mm) to 0.75 in (19.0 mm). Fine HMA mixes contain gradations that pass above the maximum density line (MDL) at the No. 8 (2.36-mm) sieve (WSDOT, 2005). Typically ranges from 0.015 to 0.025 in (0.4 to 0.6 mm) Dense Coarse-Graded HMA Coarse HMA mixes have gradations that pass below the MDL at the No. 8 sieve (2.36-mm) (WSDOT, 2005). Typically ranges from 0.025 to 0.05 in (0.6 to 1.2 mm) Gap-Graded HMA or Stone Matrix Asphalt (SMA)b SMA is a gap-graded mixture of course aggregate (typically, 0.4 to 0.6 in [10 to 15 mm]), filler, fibers and polymer-modified asphalt (typically, between 6 and 9 percent) produced in a hot-mix plant. Its primary advantage is resistance to deformation, but its relatively coarse surface yields good frictional characteristics. Typically exceeds 0.04 in (1.0 mm). New AC or AC Overlay Open-Graded HMA or Open-Graded Friction Course (OGFC)b OGFC is an open-graded mixture of mostly coarse aggregate, mineral filler, and asphalt cement (3 to 6 percent). It is produced in a hot-mix plant, contains a high percentage of air voids (17-22 percent) in the mix, and is spread and compacted on site. Friction, texture, and drainage properties can be controlled by the aggregate gradation, size, angularity, and type. Open-graded HMA can be modified with polymers, fibers, and/or crumb rubberc. Typically ranges from 0.06 to 0.14 in (1.5 to 3.0 mm) a Based in part on Hanson and Prowell, 2004; Meegoda et al., 2002; FHWA, 1996; FHWA, 2005; Richardson, 1999. b Fine- and coarse-graded SMAs and OGFCs are being developed and increasingly used. c Crumb rubber asphalt is a blend of 5 to 10 percent asphalt cement, reclaimed tire rubber, and additives in which the rubber component is 15 to 20 percent by weight of the total blend. The rubber must react in the hot asphalt cement sufficiently to cause swelling of the rubber particles.

101 Table 25. Asphalt pavement surface mix types and texturing techniques (continued). Application Mix/ Texture Type Description Macro-texture Deptha Chip Seal Thin surface treatment containing single-sized, high-quality, angular aggregates (0.38 to 0.63 in [9.5 to 15 mm]), spread over and rolled into a liquid asphalt or asphalt emulsion binder. Aggregates are sometimes pre-coated with asphalt emulsion prior to spreading. Completed surface is somewhat coarse, yielding good frictional characteristics. Typically exceeds 0.04 in (1 mm). Slurry Seal Slurry mixtures of fine aggregate, mineral filler, and asphalt emulsion. They are similar to micro-surfacing, without interlocking aggregates. Polymers are not always used in the emulsion. Their surface is typically gritty. Typically range from 0.01 to 0.025 in (0.3 to 0.6 mm). Micro-Surfacing (polymer-modified slurry seal) A slurry mixture containing high-quality crushed, dense-graded aggregate, mineral filler, and polymer-modified asphalt emulsion. It is placed over a tack coat and is capable of being spread in variable thickness layers for rut-filling, correction courses, and wearing course applications. Typically range from 0.02 to 0.04 in (0.5 to 1 mm). HMA Overlay See HMA surface mixes above. Ultra-Thin Polymer- Modified Asphalt (e.g., NovaChip) Thin gap-graded asphalt surfaces placed using specialized equipment immediately over a thick polymer-modified asphalt emulsion membrane. Following slight compaction the surface provides a semi-porous texture. Typically exceeds 0.04 in (1 mm). Friction Restoration of Existing AC Pavement Epoxied Synthetic Treatment (e.g., Italgrip) A very thin surface treatment consisting of a two-part polymer resin placed on an existing pavement and covered with a man-made aggregate of re-worked steel slag (0.12 to 0.16 in [3 to 4 mm]). The surface is designed to substantially improve the frictional characteristics of pavements. Typically exceeds 0.06 in (1.5 mm). Retexturing of Existing AC Pavement Micro-Milling Milling equipment, consisting of a self-propelled machine with carbide teeth mounted on a rotating drum, typically removes 0.75 to 1.25 in (19 to 32 mm) from the asphalt surface. Spacing of cuts is approximately 0.2 in (5 mm) versus 0.62-in (6-mm) cut of conventional cold-milling machines. Resulting surface has a fine, smooth pattern that gives smoother ride. Typically exceeds 0.04 in (1 mm) a Based in part on FHWA, 1996; FHWA, 2005; Hanson and Prowell, 2004; Mockensturm, 2002; Wade et al., 2001; McNerney et al., 2000; HITEC, 2003; Gransberg and James, 2005; Yaron and Nesichi, 2005.

102 Table 26. Concrete pavement surface mix types and texturing techniques. Application Mix/ Texture Type Description Macro-texture Deptha Broom Drag (longitudinal or transverse) A long-bristled broom is mechanically or manually dragged over the concrete surface in either the longitudinal or transverse direction. Texture properties are controlled by adjusting the broom angle, bristle properties (length, strength, density), and delay behind the paver. Uniform striations approximately 0.06 to 0.12 in (1.5 to 3.0 mm) deep are produced by this method. Typically ranges from 0.008 to 0.016 in (0.2 to 0.4 mm). Artificial Turf Drag (longitudinal) An inverted section of artificial turf is dragged longitudinally over a concrete surface following placement. Texture properties are controlled by raising/lowering the support boom, adding weight to the turf, and delaying application to allow surface hardening. This method produces uniform 0.06 to 0.12 in (1.5 to 3.0 mm) deep surface striations. Typically ranges from 0.008 to 0.016 in (0.2 to 0.4 mm), but a deep texture (min depth of 0.04 in [1.0 mm]) has been specifiedb. Burlap Drag (longitudinal) One or two layers of moistened coarse burlap sheeting are dragged over the concrete surface following placement. Texture properties are controlled by raising/lowering the support boom and adjusting the delay following concrete placement. This method produces uniform 0.06 to 0.12 in (1.5 to 3.0 mm) deep striations in the surface. Typically ranges from 0.008 to 0.016 in (0.2 to 0.4 mm). Longitudinal Tine A mechanical assembly drags a wire comb of tines (~ 5 in [127 mm] long and 10 ft [3 m] wide) behind the paver (and usually following a burlap or turf drag). Texture properties are controlled by the tine angle, tine length, tine spacing, and delay for surface curing. Grooves from 0.12 to 0.25 in (3 to 6 mm) deep and 0.12 in (3 mm) wide are produced by this method, typically spaced at 0.75 in (19 mm). Typically ranges from 0.015 to 0.04 in (0.4 to 1.0 mm). New PCC or PCC Overlay Transverse Tine Accomplished using methods similar to longitudinal tining, however, the mechanical assembly drags the wire comb perpendicular to the paving direction. Variations include skewing the tines 9 to 14° from perpendicular and using random or uniform tine spacing from 0.5 to 1.5 in (12 to 38 mm). Typically ranges from 0.015 to 0.04 in (0.4 to 1.0 mm). a Based in part on Hoerner et al., 2003; Hoerner and Smith, 2002; FHWA, 1996; FHWA, 2005. b Minnesota Department of Transportation.

103 Table 26. Concrete pavement surface mix types and texturing techniques. Application Mix/ Texture Type Description Macro-texture Deptha Diamond Grinding (longitudinal) A self-propelled grinding machine with a grinding head of gang-mounted diamond sawing blades removes 0.12 to 0.75 in (3 to 19 mm) of cured concrete surface, leaving a corduroy- type surface. Blades are typically 0.08 to 0.16 in (2 to 4 mm) wide and spaced 0.18 to 0.25 in (4.5 to 6 mm) apart, leaving 0.08 to 0.16 in (2 to 4 mm) high ridges. This method is most commonly used to restore surface characteristics of existing pavements, however, in recent years, it has been used to enhance the surface qualities of new PCC pavements or PCC overlays. Typically ranges from 0.03 to 0.05 in (0.7 to 1.2 mm). Porous PCC Gap-graded, small-diameter aggregate are combined with cement, polymers, and water to form a drainable surface layer (typically 8 in [200 mm] thick). That surface layer is bonded to the underlying wet or dry dense concrete layer. Texture properties are controlled by aggregate sizes and gradations. Air voids range from 15 to 25 percent. Typically exceeds 0.04 in (1 mm). New PCC or PCC Overlay Exposed Aggregate PCC A set retarder is applied to the wet concrete surface and the surface is protected for curing. After 12 to 24 hours, the unset mortar is removed to a depth of 0.04 to 0.08 in (1 to 2 mm) using a power broom. The large diameter aggregate is exposed by this process leaving a uniform surface. Typically exceeds 0.035 in (0.9 mm). Friction Restoration of Existing PCC Pavementb HMA Overlay See HMA surface mixes above. Diamond Grinding (longitudinal) See diamond grinding above. Longitudinal Diamond Grooving A self-propelled grooving machine saws longitudinal grooves in the road surface about 0.12 to 0.25 in (3 to 6 mm) deep and spaced 0.5 to 1.5 (13 to 38 mm) apart. This method adds macro-texture for drainage but relies on the original surface for micro-texture. Typically ranges from 0.035 to 0.055 in (0.9 to 1.4 mm). Transverse Diamond Grooving Completed in a manner similar to longitudinal diamond grooving, except the grooves are sawn transverse to the travel direction. This method also adds macro-texture and positive drainage for surface water. It relies on the original surface for micro-texture. Typically ranges from 0.035 to 0.055 in (0.9 to 1.4 mm). Retexturing of Existing PCC Pavement Shot Abrading An automated machine hurls recycled round steel abrasive material at the pavement surface, abrading the surface and/or removing the mortar and sand particles surrounding the coarse aggregate to a depth of up to 0.25 in (6 mm). Texture properties are controlled by adjusting the steel abrasive material velocity and approach angle and by modifying the forward equipment speed. Typically ranges from 0.025 to 0.05 in (0.6 to 1.2 mm). a Based in part on Hoerner et al., 2003; Hoerner and Smith, 2002; FHWA, 1996; FHWA, 2005; HITEC, 2003; Rao et al., 1999. b Other treatments, such as micro-surfacing, ultra-thin polymer-modified asphalt, epoxy-bonded laminates, and thin-bonded PCC overlays, have been used but often have structural performance and/or cost issues.

104 • Category IV—Evaluate Aggregate Frictional Properties Using Laboratory Test Procedures. States in this category use laboratory tests, such as AIR, PSV, fractured particles, and soundness, to determine the acceptability of an aggregate or aggregate source for a particular job. • Category V—Incorporates Field Performance in Aggregate Qualification—With shortcomings in the correlation between laboratory test results and actual field performance, some states incorporate a two-pronged design approach consisting of laboratory testing and historical field performance. Thus, while some states are fortunate to have good quality aggregates or are less in need of special mixes or textures for macro-texture, others are compelled, at some level, to more fully evaluate and specify their aggregates and mixes/textures. Presented in the sections below are some of the friction design practices reported in the literature and through the surveys and interviews with selected SHAs. The practices described represent examples of how traffic and other site conditions can be utilized in specifying aggregates and mixes/textures. Illinois DOT The Illinois DOT selects and designs pavement surfaces in accordance with the following criteria (Rowden, 2004): • PCC Pavements: Final finishing on highways with posted speed limits in excess of 40 mi/hr (65 km/hr) receives a Type A final finish (transverse tining with 0.75-in [19- mm] spacing, 0.1- to 0.125-in [2.5- to 3.1-mm] width, and 0.125- to 0.19-in [3.1- to 4.8-mm] depth. Final finishing on highways with posted speed limits not exceeding 40 mph (65 km/hr) receives a Type A or Type B (artificial turf drag) final finish. • HMAC Pavements: New surface courses must have friction qualities equivalent to or greater than those provided by the following guidelines. Traffic levels from the expected year of construction are used to determine the mixture. ¾ Mixture C is used as the Class I surface course on roads and streets having an ADT of 5,000 veh/day or less. ¾ Mixture D is used as the Class I surface course on two-lane roads and streets having an ADT greater than 5,000 veh/day, on four-lane highways having an ADT between 5,001 and 25,000 veh/day, and on six-lane (or greater) highways having an ADT of 60,000 veh/day or less. ¾ Mixture E is used as the Class I surface course on four-lane highways having an ADT between 25,001 and 100,000 veh/day or on six-lane (or greater) highways having an ADT between 60,001 and 100,000 veh/day. ¾ Mixture F is used as the Class I surface course on any facility having an ADT greater than 100,000 veh/day. The HMAC specification describes the allowable coarse aggregates and proportions for use in each mixture type. For instance, aggregates for mixture C may consist of crushed gravel, crushed stone, crushed sandstone, crushed slag, crushed steel slag, or gravel (in certain instances). Aggregates for the highest mixture type (F), on the other hand, may only

105 consist of crushed gravel, crushed stone (except limestone), or adequately blended crushed sandstone. Although the Department employs some friction-related lab tests, such as sodium sulfate soundness and LA Abrasion, they place much greater emphasis on testing friction in the field and linking the results to the respective aggregates/aggregate sources. Louisiana DOT Although the Louisiana DOT does not utilize a friction demand identification process, the Department does classify aggregates for asphalt mixtures according to four different friction ratings, that are based on PSV test results (Rasoulian, 2004). These friction rating categories are distinguished by layer and application, as illustrated below. • Friction Rating 1 (all mixtures): PSV > 37. • Friction Rating 2 (all mixtures): PSV = 35 to 37. • Friction Rating 3 (all mixtures, except wearing courses with ADT > 7,000 veh/day): PSV = 30 to 34. • Friction Rating 4 (all mixtures except wearing courses): PSV = 20 to 29. Maryland SHA Maryland ensures HMAC friction by recommending minimum levels of coarse aggregate PSV in the HMAC mixture. The actual PSV required to ensure adequate levels of pavement surface friction is dependent on friction demanded by a specific site (i.e., site category) and expected traffic level, as depicted in table 27 (Flintsch et al., 2002). According to the Maryland procedure, the use of limestone, marble, or serpent aggregates in the surface mixture is avoided regardless of their PSV value. Table 27. Recommended levels of aggregate PSV for various site and friction requirement categories (Flintsch et al., 2002). PSV of Coarse Aggregates Traffic (Heavy Commercial Vehicles per Lane per Day) Site/Demand Category 250 1,000 1,750 2,500 3,250 4,000 Design FN 1—Approach railroad crossing, traffic lights, pedestrian crossing, roundabouts, stop and give way controlled intersections. 7 7 8 8 9 9 55 2—Curves with radius <820 ft (250 m), downhill gradients >10 percent, and 164-ft (50-m) long freeway/highway on/off ramp. 6 7 7 8 8 9 50 3—Approach to intersections, downhill gradients 5 to 10%. 6 6 7 7 8 8 45 4—Undivided highways without any other geometrical constraints which influences frictional demand. 5 6 6 7 7 8 40 5—Divided highways without any other geometrical constraints which influences frictional demand. 5 5 6 6 7 7 35

106 Michigan DOT Michigan determines the polishing potential of HMAC coarse aggregates for design of high- friction pavements through laboratory testing (wear track testing or petrographic analysis) (Skerritt, 2004). The wear-track testing program consists of a large-scale indoor polishing track and a tire-mounted friction tester. Aggregate test specimens are subjected to 4 million wheel passes on the wear track, during which surface friction is measured. The normalized value of friction at the end of the test is used to calculate an Aggregate Wear Index (AWI), which is a measure of the polishing potential of the aggregate source tested. Aggregates are specified for use as follows, based on anticipated traffic (Liang, 2003): • ADT < 100 veh/day/lane: no AWI requirement. • ADT ≥ 100 and < 500 veh/day/lane: AWI ≥ 220. • ADT ≥ 500 veh/day/lane: AWI ≥ 260. Pennsylvania DOT All coarse aggregate sources approved by the Pennsylvania DOT are assigned a Skid Resistance Level (SRL) rating that is used to decide (for asphalt wearing courses only) what aggregate sources may be used in which wearing courses. The five levels of SRL ratings are defined as low (L), medium (M), good (G), high (H), and excellent (E). Based on the SRL, aggregates are specified for pavements with different ADT values as follows (Liang, 2003): • ADT > 20,000 veh/day: E • 5,000 < ADT < 20,000 veh/day: E, H, E/M blend, or E/G blend. • 3,000 < ADT < 5,000 veh/day: E, H, G, H/M blend, or E/L blend • 1,000 < ADT < 3,000 veh/day: E, H, G, M, H/L blend, G/L blend, or E/L blend • ADT < 1,000 veh/day: Any After the results of the above tests are available, they are evaluated and the SRL rating is assigned to the new aggregate source, based on the petrography of the aggregate, and how closely it matches that of older, petrographically similar aggregate sources whose skid performance is known from previous skid studies. Texas DOT Designing for friction in Texas begins with the identification of friction demand. The Texas DOT uses various factors for assessing overall friction demand, including rainfall, traffic, speed, trucks, grade, curves, intersections, cross slope, surface design life, and the macro- texture of the proposed surface (Stampley, 2004). For asphalt pavements, the Department has developed an aggregate rating system that classifies coarse aggregate source materials into four categories (A, B, C, and D) to match their demand classifications. These ratings are updated semi-annually based on aggregate properties from approved resources (Texas DOT, 2004). Source aggregates are rated according to PSV, LA Abrasion, and magnesium sulfate soundness for HMAC and surface treatment applications. Suggestions for blending are also provided (Texas DOT, 2004).

107 Framework for Comprehensive Friction and Texture Policy State highway agencies (SHAs) are encouraged to develop or update policies concerning the friction design of new and restored pavements. Such policies should clearly define the aggregate friction testing protocol (i.e., test types and criteria) and surface mix/texturing techniques that are applicable for the friction demand categories established in the PFM program. As conceptually illustrated in figure 37, friction design categories should be established that link combinations of rated aggregate sources and agency mix types/texturing techniques with PFM sections having different levels of friction demand (defined by investigatory/intervention level). Each category should include a design friction level that takes into consideration expected friction loss over time due to aggregate polishing and/or macro-texture erosion. As a minimum, friction design categories should be established according to highway design speed and traffic (or design loadings in terms of equivalent single axle loads [ESALs]), since these factors largely determine micro-texture and macro-texture needs. Other factors that could be used in establishing categories include roadway facility type (i.e., functional or highway class, access type), facility setting (rural, urban), climate (e.g., wet, dry), number of lanes, and truck percentages. Figure 37. Example illustration of matching aggregate sources and mix types/texturing techniques to meet friction demand. Aggregate Sources Friction Design Category Agency Aggregate Testing Protocol Agency Mix Types/ Texturing Techniques Source A (high polish) Source B (moderate polish) Source C (low polish) Test Criteria 1 > 5 2 < 25 3 > 50 . . . . . . . . Mix/Texture Type X (low macro-t) Y (moderate macro-t) Z (high macro-t) Category I Low demand α ≤ Design Friction < β Category II Moderate demand β ≤ Design Friction < χ) Category III High demand Design Friction > χ C–Y C–Z B–Z C–X B–Y A–Z B–X A–X A–Y Agg. Source–Texture Options

108 Although several factors can be used in establishing friction design categories, the number of categories should be limited to between three and five. When developing aggregate source–texture options for a given design category, economics should be considered from the standpoint that, if the local sources contain only low-polish aggregate, it may be justifiable to use such aggregate for low friction demand situations. In addition, agencies should be mindful of any existing classification schemes set forth in their wet-weather crash reduction programs, materials and/or construction specifications, or other pavement-related policies and systems, as they may reflect the desired friction priorities. Once the design categories have been set, aggregate test protocols and mix/texture type options can be developed for each category, along with design friction levels. The test protocol should list the specific tests to be performed and the criteria/parameters to be used. The criteria should be based on established links between historical friction performance and laboratory test data. PROJECT-LEVEL FRICTION DESIGN Project-level friction design entails selecting aggregates and mix types/texturing techniques that satisfy both initial and long-term friction requirements. Although safety over the established pavement design life is the paramount concern, the design process should target a surface that most economically satisfies the following criteria: • Adequate levels of micro-texture over the life of the pavement, as produced by sharp, gritty aggregate with low polish and high wear resistance characteristics. • Adequate levels of macro-texture over the life of the pavement for efficient displacement of water on the pavement surface. • Low levels of splash/spray, noise generation, glare, tire wear, and rolling resistance. A five-step process for designing surfaces for new asphalt or concrete pavement, as well as restoration treatments of existing asphalt or concrete pavement, is as follows: 1. Determine design friction level. 2. Select aggregates. 3. Establish surface mix types and/or texturing techniques. 4. Develop construction specifications. 5. Formulate design strategies. These design steps are described in detail in the sections below. Step 1—Determining Design Friction Level For each new construction or restoration project, a design friction level (expressed as F(60) if IFI is used or as FN) must be selected to satisfy agency policy requirements. The selected design level must ensure that adequate amounts of micro-texture and macro-texture are available throughout the design period.

109 The selected design level should take into consideration the design levels of individual PFM sections. Either one overall level can be established for the project corresponding to the PFM section with the highest demand, or multiple levels can be used. In the latter case, care must be taken such that the multiple levels do not result in an excessive number of mix types and/or surface textures to be used along the project. Once an agency sets the goal for friction for a particular project, the process of selecting aggregates and mix types/texturing techniques that satisfy the design friction level can begin. An initial list of aggregate source–texture options can be derived from the feasible combinations identified previously for each design category (e.g., B-X, A-X, and A-Y for design category I in figure 37). These, and other potential combinations, can be evaluated more thoroughly for adequacy using the IFI model, as described below in step 3. Step 2—Selecting Aggregates The most important factor in achieving long-lasting friction is aggregate selection. Aggregates should have the physical, chemical, and mechanical properties needed to satisfy both the initial and long-term friction requirements of a pavement project. Aggregates must comply with an agency’s testing requirements. Aggregate samples should be tested early in a project to determine their suitability and compliance with specifications. Frequently, two or more aggregate sources must be combined in appropriate percentages to meet project gradation requirements. Aggregates not meeting the specified test parameters should be rejected (prior to any mix design effort) and either new materials should be considered and tested or a suitable blend of high- and low-polish susceptible aggregates should be identified. As discussed earlier, micro-texture in asphalt surface mixes is provided by the coarse aggregate surface texture. Coarse aggregates that exhibit “rough sandpaper” surface textures provide higher levels of micro-texture than those with smooth “fine sandpaper” textures. Micro-texture in concrete surfaces is generally provided by the fine aggregates in the cement mortar/paste (for concrete mixes with exposed aggregates, the surface properties of the coarse aggregate will dictate micro-texture). Fine aggregates that exhibit angular edges and cubical or irregular shapes generally provide higher levels of micro-texture, whereas those with rounded edges or elongated shapes generally produce lower micro- texture. Aggregates comprised of a matrix of both hard and soft minerals offer a continuously renewable micro-texture that helps ensure friction durability. Ascertaining the long-term micro-texture of the selected aggregate is a crucial part of the design process. It generally entails either retrieving historical PSV test data (if available) for the aggregate or aggregate source in question or conducting formal PSV testing of the aggregate.

110 Step 3—Establishing Surface Mix Types and/or Texturing Techniques Framework for Achieving Design Friction Level As discussed earlier, potential combinations of aggregate source and mix type/texturing technique can be evaluated in detail using the IFI model (equations 9 through 11). Using DFT(20) as a surrogate for micro-texture and the CTM to get MPD, FR(S) in equation 10 can be set to DFT(20) at S equal to 20 km/hr. Furthermore, substituting equation 9 into equation 10, one gets the following: Eq. 26 Inserting equation 26 into equation 11, adding in the A, B, and C calibration constants (0.081, 0.732, and 0, respectively) for DFT(20) as given in ASTM E 1960, and re-arranging to solve for DFT(20), the following equation is obtained: Eq. 27 Figure 38 is a plot of the above equation. As an example application, consider a project where it is desired that a locked-wheel smooth-tire friction test give a friction number of 40 at a speed limit of 60 km/hr. Then F(60) is 40 and equation 27 becomes as follows: Eq. 28 To achieve the design friction level of 40, the pairs of DFT(20) and MPD given in table 28 are needed. The first pair includes a rather high DFT(20) and the last two pairs include high MPD values. Therefore, the second and third pairs containing MPD values of 0.813 and 1.524 mm would need to be selected to give the F(60) or FN needed. If the polishing characteristics have been measured or are already known, higher levels of micro-texture and/or macro-texture should be selected to meet the required levels at the end of the design life. For example, if the polished DFT(20) (i.e., PSV) and the MPD are satisfactory, then the initial DFT(20) from the test would need to be specified. If the polished DFT(20) is too low and thus requires a MPD that is too high to meet, then a higher DFT(20) or different aggregate is needed to get the required polished DFT(20) at the end of the design life. This method is then a guide for evaluating the levels of micro-texture (DFT(20)) and macro- texture (MPD) needed to achieve the design friction level established for a project. It can be used directly in identifying a suitable combination(s) of aggregate and mix type/texturing technique for a project or it can serve as a framework for agencies interested in developing their own customized procedure. It should also be noted that a similar process utilizing the combination of BPN (micro-texture) and MTD (macro-texture) could be established and used. ) 7.892.14 6020( )20()60( MPDeDFTFR ×+ − ×= ( )[ ] )7.892.14 2060(732.0/0081.0)60()20( MPDeMPDFDFT ×+ −××−−= ( )[ ] )7.892.14 40(732.0/081.040)20( MPDeDFT ×+×−=

111 Figure 38. Example of determining DFT(20) and MPD needed to achieve design friction level. Table 28. Pairs of MPD and DFT(20) needed to achieve design friction level of 40. MPD, mm 0.457 0.813 1.524 2.921 4.343 DFT(20) 112.5 86.3 71.1 63.0 60.2 Detailed Consideration of Macro-texture in Friction Design During the mix design stage of an asphalt project, there may become the need to “fine-tune” the gradation of a mix to satisfy the friction design requirement. A method for doing this was developed by Sullivan (2005). This method, illustrated in figure 39, uses PSV and MPD to compute IFI (as given in ASTM E 1960) and subsequently determine the design vehicle stopping distance. Figure 40 shows an example vehicle response chart for a selected speed of 50 mi/hr (80 km/hr). The Sullivan method uses an equation for computing the MPD based on key asphalt mix characteristics (maximum aggregate size, gradation, binder content). While historical data on asphalt surface mix textures can be used in this process, the MPD equation (derived using comprehensive mix design and surface texture data from the NCAT test track) gives the mix designer greater flexibility in establishing a mix design that will meet friction requirements. 0 20 40 60 80 100 120 140 160 20 25 30 35 40 45 50 55 F(60) or FN D FT (2 0) 0.457 0.813 1.524 2.921 4.343 MPD, mm

112 Figure 39. Asphalt pavement friction design methodology (Sullivan, 2005). Figure 40. Vehicle response as function of PSV and MTD (Sullivan, 2005). Polished Aggregate Friction Value D es ig n V eh ic le S to pp in g D is ta nc e, m MPD = 0.2 mm 0.4 mm 0.65 mm 1.2 mm 2.0 mm 350 300 250 200 150 100 50 0 10 20 30 40 50 60 70 80 90 100

113 Although a similar process for conventional concrete mixes could be developed, it is not as important, since the macro-texture is designed separately from the micro-texture. However, agencies are encouraged to quantify the macro-texture (MPD or MTD) of both newly applied and in-service surface texturings (e.g., tined, grooved, or ground surfaces with different groove dimensions, spacings, and orientations), so as to ensure the right supplement for the chosen fine aggregate, Asphalt Mix Design Macro-texture in asphalt surface mixes (and exposed concrete surfaces) is primarily governed by the size and gradation of the aggregate used. Generally speaking, the larger the aggregates in the mix, the greater the macro-texture produced. Also influencing macro- texture are mix volumetric properties, such as voids in the mineral aggregate (VMA), voids in the total mix (VTM), and the percentage of aggregate passing the 0.38-in (9.5-mm) through No. 10 (2.36-mm) sieve sizes. Mix type selection and design are important for identifying a mix with sufficient macro- texture (MPD) that, when combined with the aggregate PSV, satisfies the friction design requirements (Step 1). Further discussion about asphalt mix design, and in particular aggregate size/gradation and volumetric properties, is provided in the sections below. Aggregate Size Aggregate size may be qualified in terms of either maximum size (MS) or nominal maximum size (NMS). The MS of an aggregate is defined as the smallest sieve that all of a particular aggregate must pass through. The NMS of an aggregate is defined as the smallest sieve size through which the major portion of the aggregate must pass. The NMS sieve may retain 5 to 15 percent of the aggregate depending on the size number. Superpave defines NMS as one sieve size larger than the first sieve to retain more than 10 percent of the material (Roberts et al., 1996). Aggregate Gradation The gradations of commonly used asphalt surface mixes can be categorized and described as follows: • Dense- or well-graded—Refers to a gradation that is near maximum density. The most common HMA mix designs tend to use either dense fine-graded or dense coarse-graded aggregate. • Gap-graded—Refers to a gradation that contains only a small percentage of aggregate particles in the mid-size range. The curve is flat in the mid-size range. Gap-graded surface mixes include SMA and proprietary mixes such as NovaChip. • Open-graded—Refers to a gradation that contains only a small percentage of aggregate particles in the small range. This results in more air voids because there are not enough small particles to fill in the voids between the larger particles. The curve is flat and near-zero in the small-size range. Open-graded surface mixes include OGFC.

114 • Uniformly graded—Refers to a gradation that contains most of the particles in a very narrow size range. In essence, all the particles are the same size. The curve is steep and only occupies the narrow size range specified. Common uniformly graded surface mixes include most slurry seals, micro-surfacing, and chip seals. Figure 41 illustrates these four gradations. Note that dense fine-graded HMA mixes contain gradations that pass above the maximum density line (MDL) at the No. 8 (2.36- mm) sieve, whereas the gradations for dense coarse-graded HMA pass below the MDL at the No. 8 (2.36-mm) sieve. Figure 41. Typical asphalt mix aggregate gradations (WAPA, 2004). Percentage of Aggregate Material Passing the 0.38-in (9.5-mm) through No. 10 (2.36-mm) Sieve Sizes The percentage of material passing the 0.375-in (9.5-mm) through No. 10 (2.36-mm) sieve sizes affects the asphalt mix macro-texture. Evidence suggests that increasing the amount of material passing these sieve sizes reduces the asphalt mix macro-texture. Generally, the amount of aggregate passing these sieve sizes depends on the asphalt mix type (i.e., dense graded, open graded, and so on). To increase asphalt mix macro-texture, the lower bound values of agency recommendations for percentage of aggregate material passing these sieve sizes should be used. Sieve Size P er ce nt P as si ng 0.075 mm 0.15 mm 0.30 mm 0.60 mm 1.18 mm 2.35 mm 4.75 mm 9.5 mm 12.5 mm 19.0 mm 0 20 40 60 80 100 No. 200 No. 100 No. 50 No. 30 No. 16 No. 8 No. 4 3/8-in 1/2-in 3/4-in Dense Gradation Open Gradation Uniform Gradation Gap Gradation 0.45 Power Curve (line of maximum density for ¾-in maximum aggregate size)

115 Voids in the Mineral Aggregate (VMA) Increasing the VMA increases macro-texture and porosity. Excessively high VMA can, however, adversely affect asphalt mix durability. Hence, increasing this asphalt mix property must be done with caution. Where a high VMA is required to meet macro-texture requirements or to ensure that the asphalt mix is open or porous, additives (i.e., polymers) can be added to the mix to increase durability. Typically, a minimum VMA value ranging from 13 to 15 percent is specified for a dense aggregate mix. This value can be increased to enhance the asphalt mix macro-texture requirements by altering the packing characteristics of aggregate particles in the mix. In particular, lowering the minus No. 200 content in a mixture to the lower end of the specification or reducing the amount of aggregate particles between two successive sieves (i.e., gap grading) will increase VMA. Estimating Texture Depth Using Mix Design Parameters Several studies have been conducted attempting to model texture depth as a function of aggregate gradation/size characteristics and mix volumetric properties. Presented below are three particular models reported in the literature which could be considered for use in assessing macro-texture of laboratory-designed asphalt mixes. • NCHRP Report 441 (Stroup-Gardiner and Brown, 2000)—The model below predicts estimated mean texture depth (ETD) based on aggregate size and gradation characteristics. As also shown, the sieve sizes associated with 10, 30, and 60 percent passing are used to compute the coefficients of uniformity and curvature (CC and CU, respectively). EMTD = 0.0198×MS – 0.004984×P200 + 0.1038×CC + 0.004861×CU Eq. 29 where: EMTD = Estimated mean texture depth (computed using ROSANV laser texture measurement). MS = Maximum size of the aggregate, mm. P200 = Percentage passing No. 200 (4.75-mm) sieve. CC = 6010 2 30 DD D × CU = 10 60 D D D10 = Sieve size associated with 10 percent passing, mm. D30 = Sieve size associated with 30 percent passing, mm. D60 = Sieve size associated with 60 percent passing, mm. • Virginia Smart Road (Davis, 2001)—MPD at the Virginia Smart Road, as measured using a laser profiler, was analyzed according to mixture properties of the pavement to determine which properties had the largest effect on MPD. The equation resulting from the regression analysis is provided below. The regression coefficient for the equation was 0.9724, indicating an excellent fit.

116 MPD = –3.596 + 0.1796×NMS + 0.0913×P200 – 0.0294×VTM + 0.1503×VMA Eq. 30 where: MPD = Mean profile depth. NMS = Nominal maximum size of the aggregate. P200 = Percentage passing No. 200 (4.75-mm) sieve. VTM = Total voids in the mixture. VMA = Voids in the mineral aggregate. • NCAT-Derived Model (Sullivan, 2005)—The results of an evaluation of the effect of mix gradation and binder content on in-service surface texture measurements from 17 NCAT test mixes found that texture depth can be accurately estimated using binder content and the gradation’s weighted mean distance from the MDL. The developed model is presented below. The correlation between predicted and measured texture depth (in the form of MPD) was excellent, with an R2 of 0.96. Eq. 31 where: Ω = Weighted distance from maximum density line. SivS = Sieve size. MaxAgg = Maximum aggregate size in mix. %Pass = Percent of mix passing the sieve size. MPD = 0.025×Ω2 + 0.037×Ω – 0.0265×Pb + 0.052 Eq. 32 where: Pb = Percent binder by weight. Macro-Texture Durability For asphalt mix types, high permeability, high air voids, and thin asphalt coatings on aggregate particles are the primary causes of excessive aging of the asphalt binder. This aging contributes to lack of durability and loss of long-term pavement friction (Kandhal, Foo, and Mallick, 1998). Thus, mix proportioning must optimize the asphalt mix properties. In the special case of open-graded asphalt mixes, maintaining the long-term durability and pavement friction while ensuring a high porosity/permeability is required. Additives and polymers can be used to prevent moisture damage and excessive aging of the asphalt binder. Specific recommendations for ensuring durable asphalt mixes are presented below. • Dense, Uniform, and Gap-Graded Mixes—As a consequence of their low void content and thick binder films, these mixes have proven to be durable and resistant to age hardening. Some pavement friction-related considerations are as follows: ¾ Make a careful choice of aggregate size, shape, and grading to produce a dense asphalt surface that will meet micro-texture and macro-texture requirements. Ω = ∑ (SivS/MaxAgg)0.45 × 100 – %Pass × SivS

117 ¾ Limit the voids in the asphalt mix to ensure adequate durability. However, if the void content is too low, deformation can occur resulting in a loss of macro- texture. ¾ Ensure a thick film of asphalt binder around the coarse aggregate to prevent thin asphalt binder films and excessive aging. However, the binder content must not be excessive to cause bleeding. ¾ SMA mixes may require a stiff asphalt binder to ensure durability. This can be achieved by using the harder asphalt binder grades or by adding polymers to the binder. • Open-Graded Mixes—Maintaining high levels of permeability/porosity is important for maintaining the drainage characteristics of open-graded asphalt mixes. This is achieved by using open-graded aggregates held together by asphalt binder to form a matrix with interconnecting voids through which water can pass. Unfortunately the interconnected voids allow excellent access to air; so aging and embrittlement of the asphalt binder may be exacerbated. To ensure both permeability and durability in the long term, the following is recommended for design: ¾ Enhance asphalt mix durability by using softer grade binders and as high a binder content as possible. The binder content must be optimized through testing to ensure adequate permeability. ¾ Avoid lean asphalt mixes, as these types of mixes are mostly not durable. ¾ Avoid rich asphalt mixes, as these types of mixes are likely to flush/bleed, resulting in patches of binder on the road surface causing low pavement friction and an impermeable surface (poor drainage). ¾ The design binder content (optimized through testing) represents the maximum quantity of binder that can be incorporated into the porous asphalt mix without introducing excessive binder drainage causing segregation during mixing, transportation, and placement. ¾ Excessive binder content and/or excessive mixing temperature causes binder drainage and mixture segregation during transportation from the mixing plant, leading to inconsistency of the finished surface, with areas either rich or lean in binder content. ¾ Temperature controls and maximum target binder contents must be incorporated into the design specification to reduce the occurrence of defective surfaces. ¾ If it is necessary to improve bonding characteristics and durability, polymer- modified binders should be used. Noise Considerations The two biggest keys to producing low noise asphalt pavements are surface texture and porosity (Newcomb and Scofield, 2004). A relatively flat surface with voids in it (i.e., negative texture) has better acoustical performance than one that has protrusions above the surface (i.e., positive texture). For pavement–tire noise reduction, smaller maximum aggregate size and negative texture are better (Newcomb and Scofield, 2004). Larger sized surface texture tends to produce greater noise, which is why coarse chip and coarse-graded dense HMA surface mixes can be

118 noisier than those having a smaller maximum aggregate size. Mixes containing a 0.18- or 0.25-in (5- or 6-mm) maximum aggregate size produce the quietest pavements, compared to reference dense-graded mixes containing a 0.55- or 0.62-in (14- or 16-mm) maximum aggregate size. Porosity in the surface is a means to achieve even further pavement–tire noise reduction (Newcomb and Scofield, 2004). OGFC combined with a smaller aggregate size is very effective in reducing noise from traffic. Two-layer OGFCs (coarser underlying porous layer and finer porous surface layer) help maintain safety and reduce noise. In closing, while macro-texture should be kept as low as practical to reduce noise—in the 0.4- to 2.0-in (10- to 50-mm) range—it should not be done at the price of good surface friction (Wayson, 1998). Concrete Mix Design and Texturing Selection Concrete surface macro-texture is determined by the type of texturing applied to the surface of the concrete (whether freshly placed or hardened). As with asphalt surface mixes, designers must identify a texturing application that produces a macro-texture (MTD) that, when combined with the aggregate PSV, satisfies the friction design requirements (Step 1). Extensive recommendations for applying the finishing methods listed in table 26 have been presented in several references, including FHWA Technical Advisory T 5040.36 (FHWA, 2005). Macro-Texture Durability The strength/abrasion properties of the cement mortar/paste largely determine the wearing characteristics of new concrete surfaces. Increasing the cement content (or decreasing the water-cement ratio) and implementing sound construction practices maximizes cement paste/mortar strength and, thus, abrasion resistance. Additionally, the use of air-entrained cement paste/mortar where freezing and thawing is encountered, can relieve pressure in the paste during freezing, thereby reducing the potential for the paste to crack. Noise Considerations Tine or groove depth, width, spacing, and orientation are all major factors affecting pavement-tire noise (Hoerner et al., 2003). Transverse tinings with uniformly spaced tines 0.5 in (13 mm) or greater have been found to produce an objectionable tonal quality (tire whine). Randomly varying the transverse tine spacing can reduce the tonal quality problems. Tire noise increases with tine width; research shows mixed data regarding the impact of tine depth on noise. Skewing of transverse tining has been found to reduce pavement–tire noise (Hoerner et al., 2003). Longitudinal tining, shallow turf drags, and abrading do not exhibit same prominent objectionable tonal spikes observed with uniform transverse tining (Hoerner et al., 2003). Recommended transverse tining types, with respect to noise, are as follows (Hoerner and Smith, 2002):

119 • Repeated random, with spacing of 0.4 to 3.0 in (10 to 76 mm), depth of 0.125 in to 0.25 in (3 to 6 mm), width of 0.125 in (3 mm), and skew of 1:6. • Repeated random, with spacing of 0.4 to 2.0 in (10 to 51 mm), depth of 0.125 in to 0.25 in (3 to 6 mm), width of 0.125 in (3 mm), and skew of 1:6. Recommended longitudinal tining type, with respect to noise, are as follows (Hoerner and Smith, 2002): • Uniform, with spacing of 0.75 in (19 mm), depth of 0.125 in to 0.25 in (3 to 6 mm), and width of 0.125 in (3 mm). Finally, based on Wisconsin’s results and Virginia’s experience (FHWA, 1996), using transverse and longitudinal tining together (i.e., cross-hatching) produces consistently higher total noise. Step 4—Development of Construction Specifications All agencies have standard specifications for construction of pavement surfaces that provide guidance on requirements for aggregates, mixes, handling, placement, compaction, curing, and protection of new surfaces. For some agencies, these specifications do not specifically address friction properties of the wearing surface. To ensure quality friction on new or rehabilitated pavement surfaces, requirements for aggregate properties and test methods presented in this section may included in project specifications as needed. Special Provisions Each project has unique requirements because of the design and construction constraints and special demands. Items such as aggregate blending, noise mitigation, and QA should be clarified in the special provisions of the construction documents and specifications. Blending Frequently, aggregates from two or more sources must be blended to meet the specification limits. Several studies (Mullen et al., 1974; Underwood, 1971; Liang, 2003) have reported that the blended aggregate properties tend to be the same as the weighted average of the properties of the individual aggregates. Thus, the goal of blending aggregate is to set the percentages of each aggregate used such that the final blend has properties that lies within the specification limits of the tests to be performed. Quality Assurance Among other things, a QA program often stipulates the frequency of testing aggregate sources. It is strongly suggested that an aggregate source be tested extensively whenever substantially new aggregate deposits are to be used for pavement surfacing. The extent and frequency can be reduced as the agency becomes more familiar with the aggregate source and there is a history of performance for aggregates from the given source (Folliard and Smith, 2003).

120 Construction Issues Construction deficiencies and poor construction practices can contribute to inadequate friction. Construction issues involve control of aggregate and mix quality during production, handling, stockpiling, mixing, placing, and finishing. Friction restoration treatments in particular, such as chip seals, slurry seals, micro-surfacing, and proprietary surfaces, are susceptible to providing less than expected friction, if poor construction practices are employed. Step 5—Formulation of Design Strategies Both monetary and non-monetary factors are considered in selecting preferred pavement design strategy the various feasible alternatives. The main inputs required are (a) estimates of costs, (b) estimates of benefits (if the benefit cost option is selected, not that benefit cost analysis is required only if there is a significant difference in benefits between alternatives, and (c) non-monetary factors. Important cost elements related to the inclusion of surface friction in the design strategy are: • Agency costs. ¾ Additional design and engineering costs. ¾ Aggregate materials with required frictional properties. ¾ Additives, including polymers, to improve surface properties and performance. ¾ Frequency/duration of restoration activities. ƒ Design strategies involving frequent M&R are typically more costly overall because of the effects of highway user delay costs, traffic control, and so on. ƒ Timing of M&R can significantly escalate costs if M&R to restore surface friction does not coincide with M&R to restore structural capacity. • User costs ¾ Travel delays (time/delay) for friction restoration impact life cycle cost. ¾ Friction can adversely influence pavement–tire factors such as tire wear, rolling resistance, and fuel consumption. ¾ Safety associated factors that impact crash costs. ƒ Frequency of crashes. ƒ Value of crashes. Benefits from ensuring adequate levels of friction throughout the pavement life are quantified through: • Improved highway safety (i.e., reduction in crash costs). ¾ Value of lives saved. ¾ Value of injuries avoided (medical, loss income, psychological damage). ¾ Savings in pain and suffering of crash victims and their families due to a reduction in crashes. ¾ Reductions in property damage due to reduction in crashes.

121 Non-monetary factors can be included in the decision matrix and addressed through (a) agency policies and criteria on these factors and (b) appropriate weights to these factors to reflect the importance assigned to them by the agency. The non-monetary design considerations include (AASHTO, 1993): • Service life. • Duration of construction. • Traffic control problems. • Reliability, constructability, and maintainability of design. Non-monetary considerations associated with pavement friction include: • Pavement–tire noise. • Splash and spray. • Fuel consumption/rolling resistance. • Tire wear. • Reflectance and glare.

This page intentionally left blank.