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7 CHAPTER 2 State of the Practice This chapter summarizes the state of the practice with regard Individuals from 18 highway agencies, 15 industry groups, to concrete pavement surface texturing, as gleaned from the lit- and 13 international and related sources were interviewed. erature reviews and interviews with experienced and knowl- Interviewees included representatives of texture, friction, and edgeable individuals. The summary of current practices deals noise measuring equipment manufacturers/vendors; noise with the following items: testing facilities; friction and profile testing calibration cen- ters; paving contractor agencies; construction materials and · Surface properties most relevant to the selection of a texture equipment manufacturers, and tire manufacturers. type. The information obtained from the state and industry · Methods used to measure or test the relevant surface interviews was synthesized and is provided in Appendix B. properties. Key aspects of this synthesis are included in the state-of-the- · Types of textures available for use. practice summary provided in this chapter. · Properties typically exhibited or possessed by individual texture types. State-of-the-Practice Summary Literature Review Pavement Surface Properties A literature search focused on information pertaining to Pavement surface texture is made up of the deviations of the concrete pavement texture, friction, noise, and other related pavement surface from a true planar surface. These deviations surface characteristics was conducted. This search involved occur at three distinct levels of scale, each of which is defined domestic and international sources available from public agen- by the wavelength () and peak-to-peak amplitude (A) of its cies, industry, academic institutions, and other organizations. components. The three levels of texture, as established by Pertinent documents were reviewed (a synthesis of this the Permanent International Association of Road Congresses information is provided in Appendix A which is available (PIARC) (1987), are as follows: online). Key aspects of the synthesis are included in the state-of-the-practice summary presented in this chapter. · Micro-texture ( < 0.02 in. [0.5 mm], A = 0.04 to 20 mils [1 to 500 m])--Surface roughness quality at the sub- visible/microscopic level. It is a function of the surface State and Industry Interviews properties of the aggregate particles within the asphalt or The literature search effort was supplemented with inter- concrete paving material. views with several state highway agency (SHA) and industry · Macro-texture (0.02 in. < 2 in. [0.5 mm < 50 mm], representatives, and experts in the area of pavement surface A = 0.005 to 0.8 in. [0.1 to 20 mm])--Surface roughness characteristics. The interviews sought (1) information on quality defined by the mixture properties (shape, size, and SHA policies, practices, experiences (including past studies), gradation of aggregate) of an asphalt paving material and the and perspectives on pavement frictional properties, texture, method of finishing/texturing (dragging, tining, grooving; and noise; and (2) insights and information from other pub- depth, width, spacing and direction of channels/grooves) lic or private institutions engaged in these issues. Information used on a concrete paving material. was sought about in-service pavements suitable for inclusion · Mega-texture (2 in. < 20 in. [50 mm < 500 mm], in the field evaluations. A = 0.005 to 2 in. [0.1 to 50 mm])--This type of texture is
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8 the texture which has wavelengths in the same order of · Friction size as the pavementtire interface. It is largely defined Locked-wheel Friction Tester (ASTM E 274) by the distress, defects, or "waviness" on the pavement Dynamic Friction Tester (DF Tester) (ASTM E 1911) surface. British Pendulum Tester (BPT) (ASTM E 303) · Noise Pavement surface texture influences many different Controlled pass-by (CPB) method (NF S 31 119-2) pavementtire interactions. Figure 2-1 shows the ranges of [ISO 5725) texture wavelengths affecting various vehicleroad inter- Statistical pass-by (SPB) method (ISO 11819-1) actions, including friction, interior and exterior noise, splash Close-proximity (CPX) method (ISO/DIS 11819-2) and spray, rolling resistance, and tire wear. As can be seen, Coast-by (CB) method (ISO/DIS 13325 and Directive micro-texture contributes significantly to surface friction 2001/43/EC) on dry roads at all speeds and on wet roads at slower speeds, Trailer coast-by (TCB) method (ISO/DIS 13325) while macro-texture significantly influences surface friction Acceleration pass-by (APB) method (ISO 362) on wet road surfaces with vehicles moving at higher speeds. Sound intensity (SI)/On-Board Sound Intensity (OBSI) Highway noise is affected by the macro-texture and mega- method (General Motors [GM] standard and AASHTO texture of a roadway, while splash/spray is affected primarily Provisional Standard TP076-08) by macro-texture. Interior vehicle method (Society of Automotive Engi- neers [SAE] J 1477) Methods of Measuring Pavement Brief descriptions and assessments of these methods are pro- Surface Properties vided in this chapter; more details are provided in Appendix A. Several types of equipment and procedures have been devel- oped and used over the years to measure pavement surface Texture Measurement properties. Current standardized or widely accepted testing methods for measuring texture, friction, and noise include: The SPM method, the OF Meter, and the CT Meter are texture measuring equipment requiring lane closures. Also, a · Texture recently developed line laser system named RoboTex (Robotic Sand Patch Method (SPM) (ASTM E 965) Texture), which gives three-dimensional texture readings, Outflow Meter (OF Meter) (ASTM E 2380) requires lane closure. Circular Texture Meter (CT Meter) (ASTM E 2157) The SPM (ASTM E 965) is a volumetric-based spot test High-speed Laser Profiler (ASTM E 1845) method that assesses pavement surface macro-texture through Texture Wavelength 0.00001 0.0001 0.001 0.01 0.1 1 10 100 ft 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1 m Micro-texture Macro-texture Mega-texture Roughness/Unevenness Friction Ext. Noise Int. Noise Splash/Spray Rolling Resistance Tire Wear Tire/Vehicle Note: Darker shading indicates more favorable effect of texture over this range. Figure 2-1. Texture wavelength influence on pavementtire interactions (adapted from Henry, 2000 and Sandberg and Ejsmont, 2002).
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9 the spreading of a known volume of glass beads in a circle onto or smooth (ASTM E 524) test tire. This method, used for rou- a cleaned surface and the measurement of the diameter of the tine network surveys and/or project-level testing, uses a friction resulting circle. The volume divided by the area of the circle is index called the Friction Number (FN) to quantify the level of reported as the mean texture depth (MTD). available friction under wetted conditions. The speed at The OF Meter (ASTM E 2380) is a volumetric test method which the test is performed (typically 40 mi/hr [64 km/hr]) that measures the water drainage rate through surface texture and the type of test tire used (ribbed or smooth) further delin- and interior voids. It relates the hydroplaning potential of a eate the friction parameter (i.e., FN40R or FN40S represent surface to the escape time of water beneath a moving tire. The friction values obtained at 40 mi/hr [64 km/hr] with ribbed equipment consists of a cylinder with a rubber ring on the or smooth tires, respectively). bottom and an open top. Sensors measure the time required Friction measurement using a ribbed test tire does not ade- for a known volume of water to pass under the seal or into the quately assess road macro-texture, because tire grooves allow pavement. The measurement parameter, outflow time (OFT), for removal of water at the pavementtire interface, eliminat- defines the macro-texture; high OFTs indicate smooth macro- ing the need for good road macro-texture (Henry, 2000). texture and low OFTs rough macro-texture. Recent studies (PIARC, 1995) suggest the addition of lasers to The CT Meter (ASTM E 2157) is a non-contact laser device measure macro-texture, and most new testers are now being that measures the surface profile along an 11.25-in. (286-mm) ordered with texture lasers. This allows for measurements at diameter circular path of the pavement surface at intervals speeds other than the standard 40 mi/hr (64 km/hr), with a of 0.034 in. (0.868 mm). The texture meter device rotates at way to adjust the measurement to 40 mi/hr (64 km/hr). Thus, 20 ft/min (6 m/min) and generates profile traces of the pave- measurements can be done at higher speeds on interstates and ment surface, which are transmitted and stored on a portable lower speeds in towns and at intersections, and then adjusted computer. Two different macro-texture indices can be com- to a common speed of 40 mi/hr (64 km/hr). puted from these profiles--mean profile depth (MPD) and the The DF Tester (ASTM E1911) allows measuring friction root mean square deviation of the profile (RMS). The MPD, (expressed as DFT) as a function of speed over the range of 0 to which is a two-dimensional estimate of the three-dimensional 56 mi/hr (0 to 90 km/hr) (Flintsch et al., 2003). The DFT fric- MTD (ASTM 2157), represents the average of the highest pro- tion parameter is accompanied by the speed at which the test is file peaks occurring within eight individual segments constitut- performed; hence, the typical speed of 12.5 mi/hr (20 km/hr) is ing the circle of measurement. The RMS is a statistical value, designated as DFT12.5 or, more commonly, DFT(20). DFT(20) which offers a measure of how much the actual data (measured has been found to correlate well with BPN and is generally used profile) deviates from a best-fit (modeled profile) of the data as the reporting friction value (Henry, 2000). (Abe et al., 2000). High-speed methods for characterizing pavement surface Noise Evaluation texture typically are based on non-contact surface profiling techniques. An example of a non-contact profiler for use in As described by Bernhard and Wayson (2005), noise is characterizing pavement surface texture is the Road Surface defined as unwanted sound and is typically expressed in terms Analyzer (ROSANV), developed by the FHWA. ROSANV is of sound pressure level (SPL). The formula for SPL, which uses a portable, vehicle-mounted, automated system for measur- a logarithmic scale and is reported in decibels (dB), is as follows: ing pavement texture at highway speeds along a linear path SPL = 10 × log10 ( p 2 p ref 2 ) Eq. 2-1 (FHWA, 2008). ROSANV incorporates a laser sensor mounted on the vehicle's front bumper and the device can be operated where at speeds of up to 70 mi/hr (113 km/hr). The system calcu- p = Sound pressure of concern, Pa lates both MPD and estimated mean texture depth (EMTD), pref = Standard reference pressure which is an estimate of MTD derived from MPD using a trans- = 20 × 10-6 Pa formation equation. Automated profile measurement systems such as ROSANV provide a large quantity of texture data and SPL adjusted to the sensitivity of human hearing (i.e., atten- enhance safety by eliminating the traffic control required for uation of low [5,000 Hz] frequencies) manually performed volumetric methods. is referred to as A-weighted sound (Bernhard and Wayson, 2005). The unit of measure is the A-weighted decibel or dB(A). Friction Testing The primary method for detailed evaluation of highway noise in the United States (and most of Europe) is the SPB The most common method for measuring pavement fric- method, which measures the maximum sound level (Lmax) for tion in the United States is the ASTM E 274 using locked-wheel a mix of vehicles. The measurement is taken from the side of testing equipment supplied with either a ribbed (ASTM E 501) the road at a specified distance from the center of the travel lane
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10 (typically, 50 ft [15 m] in the United States and 25 ft [7.5 m] in high frequency noise, but amplifies the low frequency noise Europe) and at a specified height above the travel surface (5 ft (Rasmussen et al., 2007a). [1.5 m] in the United States and 4 ft [1.2 m] in Europe). The SPB method provides noise values that are representative of Texturing Methods for Concrete Pavements a wide range of vehicles; however, it is somewhat costly and time-consuming and results in considerable variability with The following methods are used in the United States and different vehicles using different roads. other countries for texturing new concrete pavements or A similar method, the CPB method, offers the ability to com- retexturing existing concrete pavements: pare roadside noise (Lmax) of different road sections directly using specific vehicle properties and speeds. Although a little · Plastic brushing/brooming less time-consuming than SPB, this method only provides · Transverse and longitudinal dragging the ability to compare the roadside noise properties from the · Transverse and longitudinal tining vehicle(s) used in the evaluation; CPB may not well represent · Transverse and longitudinal grooving the overall roadside noise experienced by the neighboring com- · Longitudinal diamond grinding munity. CPB was used in a study completed in 1999 (Kuemmel · Exposed Aggregate Concrete (EAC) surfacing et al., 2000), because it provided direct comparison of roadside · Porous concrete noise of road surfaces. · Shot abrading The two most common methods of measuring near-field pavementtire noise (i.e., noise at or very near the source) are In addition, in lieu of retexturing, other options have been the CPX and SI methods. The CPX method, which uses sound used for enhancing the surface characteristics of concrete pave- pressure microphones to measure average dB(A) at 0.3 to 1.6 ft ments, such as thin (1.5 in. [38 mm]) asphalt overlays, ultra- (0.1 to 0.5 m) from a reference tire in an enclosed, sound- thin (0.375 to 0.75 in. [9.5 to 19.0 mm]) bonded wearing absorbing trailer, is relatively inexpensive, fast, and can be used courses (i.e., NovaChip® proprietary treatment), and ultra-thin to continuously document the noise characteristics (including (0.12 to 0.25 in. [3.0 to 6.0 mm]) epoxied laminates (i.e., variability) of long portions of highway. It has been used in Italgrip® System proprietary treatment). Europe for many years, and a modified CPX noise trailer was FHWA Technical Advisory T5040.36 (Surface Texture for used in recent years to evaluate noise on pavement sections Asphalt and Concrete Pavements) (2005) contains recommen- in several states (Scofield, 2003; Hanson and James, 2004; dations for the applications of many of these textures. A sum- Hanson, 2002). Correlations between sound pressure CPX mary of the properties and performance characteristics of the values and roadside CPB levels have been noted as inconsistent above textures and their relative desirable rankings is provided (Chalupnik, 1996). below. Descriptions of the strengths and weaknesses of each The SI method was originally developed by GM and has method are given in Appendix A. been used in the United States since the 1990s for conducting pavementtire noise evaluations. It uses microphones mounted next to the tire of the test vehicle and measures the rate of Texture Properties and Performance Characteristics energy flow through a unit area, which when integrated over the area provides sound pressure. Because these microphone Each of the identified methods has properties and perfor- pairs are directional, they are not significantly affected by adja- mance characteristics that make them more or less desirable cent tire and wind noise. NCHRP Report 630 (Donavan and for different paving applications. Table 2-1 summarizes the Lodico, 2008) contains the SI test procedure that provided ranges of initial texture, friction, and noise properties reported a basis for the AASHTO Provisional Standard TP076 for mea- for each method in the United States. Some examples of the surement of tirepavement noise using the OBSI method texture depth produced as a result of different tine dimensions (AASHTO TP076, 2008). are provided in Table 2-2, based on measurements made on Interior vehicle noise measurement entails the continuous various in-service pavement sections (Kuemmel et al., 2000). measurement of noise inside the test vehicle as it travels along Table 2-3 summarizes the strengths, weaknesses, and typical a road at a specified speed. The measurement location is at a costs for each method based on the information available in point 2.25 ft (0.7 m) above the front passenger seat. The col- the literature (Wittwer, 2004; Chandler et al., 2003; Billiard, lected noise data for a given run are used to compute the equiv- 2004; Beeldens et al., 2004; Exline, 2004; APTech, 2001). alent sound pressure level (Leq), which is obtained by adding up all the sound energy during the measurement period and then Tentative Benefit Rankings dividing it by the measurement time (Rasmussen et al., 2007a). Interior vehicle noise is generally a much lower frequency than Selecting the appropriate methods for PCC texturing in dif- exterior noise, because the vehicle not only attenuates the ferent applications requires a balance of maintaining adequate
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11 Table 2-1. Texture, friction, and noise ranges. Texture Range Friction Range Noise Range Method MTD, mm MPD, mm FN40R FN40S CPX, dB(A) CPB Lmax, dB(A) Transverse tine (0.75 in.) 0.53 to 1.1 0.50 to 0.52 41.0 to 56.0 30.6 to 34.4 100.4 to 104.8 83.0 to 84.0 Transverse tine (0.5 in.) 0.35 to 1.00 54.0 to 71.0 37.6 to 62.0 81.9 to 83.0 Transverse tine (variable) 1.14 0.42 to 1.02 50.0 to 69.5 81.0 to 87.3 Transverse groove 1.07 48.0 to 58.0 84.1 to 84.6 Transverse drag 0.76 22.0 to 46.0 Longitudinal tine 1.22 36.0 to 76.6 96.6 to 103.5 79.0 to 85.0 Longitudinal groove 1.14 48.0 to 55.0 99.4 to 103.8 80.9 Longitudinal grind 0.30 to 1.20 35.0 to 51.0 29.9 to 46.8 95.5 to 102.5 81.2 Longitudinal burlap drag 101.4 to 101.5 Longitudinal turf drag 0.53 to 1.00 23.0 to 55.6 20.0 to 38.0 97.4 to 98.6 83.7 Longitudinal plastic brush 48.0 to 52.0 23.0 to 24.0 101.8 to 102.2 EAC 0.9 to 1.1 35.0 to 42.0 Shot abraded PCC 1.2 to 2.0 34.3 to 46.2 84.3 Porous PCC Ultra-thin epoxied laminate 1.4 79.8 Ultra-thin bonded wearing course 0.97 to 1.98 26.0 to 27.0 95.0 to 99.0 1 in. = 25.4 mm short- and long-term wet-weather friction levels, minimiz- is unlikely that one surface texturing method will always be the ing pavementtire noise, maintaining road durability, and best choice in any highway agency (FHWA, 1996a). minimizing construction and maintenance costs. The infor- mation gathered and analyzed provided a sufficient basis for Highway Agency Texturing developing tentative rankings according to these categories. Policies and Practices The texture method benefit rankings shown in Table 2-4 were determined based on a subjective assessment of the available The highway agencies interviewed in this study reported information. various policies and practices regarding texturing of new con- Each paving project includes specific demands for levels of crete pavements. The texturing methods for high-speed (>40 friction, noise, cost, and constructability. Low-speed rural or to 45 mi/hr [64 to 72 km/hr]) pavements are summarized in industrial projects in a dry climate with no curves and inter- Table 2-5. Although responses were provided by only 16 states, sections will demand less noise reduction and less friction than the general indication is that transverse tining using various an urban, high-speed throughway that includes several curves patterns and dimensions is currently the most common and intersections and bisects a residential community. Cost form of texturing; only a few agencies use longitudinal tin- restrictions for the latter may also be less stringent. Aggregate ing. Several European agencies use one- or two-layer EAC costs may also affect the texturing option chosen. Therefore, surfaces for new concrete construction. (Additional infor- the individual category rankings will need to be considered in mation on highway agency texturing policies and practices is selecting the optimum texturing methods for each project. It provided in Appendix B which is available online.) Table 2-2. Texture depths observed for different groove spacings and depths. Design Groove Avg. Groove Avg. Texture No. Sections Texture Type Spacing, in. Depth, mm Depth (MPD) Tested/Evaluated Transverse Tine, 0.5 1.4 0.54 5 Uniform Spacing 0.75 1.7 0.51 2 1.00 1.9 0.46 5 Transverse Tine, 0.75 2.1 0.64 5 Variable Spacing 1.00 2.2 0.54 2 1.50 1.9 0.38 2 Longitudinal Tine 0.75 2.2 0.82 3 1.00 0.62 2 1 in. = 25.4 mm
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12 Table 2-3. Constructability, design, and cost comparison for various surface textures. Method Strengths Weaknesses Initial Cost1, $/yd2 Longitudinal burlap Automated, simple construction Moderate initial friction and early friction 0.10 to 0.15 drag Good noise properties loss Longitudinal turf Lower noise, high friction Long-term friction not well defined 0.10 to 0.15 drag Simple construction and early cure Aggregate and mortar strength are critical application Longitudinal plastic Automated or manual application May not maintain texture, friction, and 0.10 to 0.15 brush/broom Good noise properties safety properties Transverse drag Small positive surface water drainage Slow and expensive operation N/A flow Transverse tine Durable high friction Very high noise and tonal whine 0.10 to 0.15 (0.75 in.) Automated or manual construction Variable depending on weather and operator No positive surface drainage when longitudinal slope is less than cross-slope Transverse tine Durable high friction High noise and some tonal whine 0.10 to 0.15 (0.5 in.) Automated or manual construction Variable depending on weather and operator No positive surface drainage when longitudinal slope is less than cross-slope Transverse tine Durable high friction, automated or High noise 0.10 to 0.15 (variable) manual Variable depending on weather and No tonal whine if properly operator designed/constructed No positive surface drainage when longitudinal slope is less than cross-slope Transverse tine Durable high friction, automated or High noise 0.10 to 0.15 (unless (skewed variable) manual Additional effort required to construct joints avoided) No tonal whine if properly No positive surface drainage when designed/constructed longitudinal slope is less than cross-slope Longitudinal tine High friction, lower noise and no tonal Some annoyance or perceived handling 0.10 to 0.15 whine problems may be experienced by Automated construction required motorcyclists or drivers of light vehicles, however safety not impacted No positive surface drainage channels Longitudinal groove Provides retrofit macro-texture to old Some annoyance or perceived handling 1.25 to 3.00 roads problems may be experienced by Minimal traffic interruption or worker motorcyclists or drivers of light vehicles, exposure however safety not impacted No positive surface drainage channels Longitudinal grind High friction, low noise, low worker Friction decreases rapidly on polish 1.00 to 5.45 exposure susceptible coarse aggregate with heavy Increased smoothness traffic. Transverse groove Provides retrofit macro-texture to old Slow and expensive operation 4.00 to 8.20 roads Minimal traffic interruption or worker exposure EAC Good noise and friction properties Special equipment and methods are 2.50 to 5.00 Long-term noise and friction stable required Contractor experience is critical to performance Shotblasted PCC Provides retrofit macro-texture to old Limited improvement in noise properties 1.50 to 2.00 roads Minimal traffic interruption or worker exposure Porous PCC Very good noise, high friction, low Mostly experimental designs 10.00 to 11.35 splash/spray Noise reduction reduces with void filling Thin HMA Overlay2 Very good noise properties Vertical clearance decreased 2.50 to 4.50 (1.0 to 1.5 in.) Generally good friction Splash/spray an issue, particularly for finer mixes Ultra-thin epoxied Good friction Extremely expensive 16.50 to 20.00 laminate No clearance issues Ultra-thin bonded Good noise, high friction, low Vertical clearance slightly decreased 2.50 to 5.00 wearing course splash/spray Fast application, improved smoothness 1 For concrete textures, unit costs represent only the cost of the texturing activity (or in the case of porous PCC, the added cost of producing and placing a porous mixture). For the three asphalt textures, the unit costs are representative of the specific material and its placement. 2 Assumes existing pavement is in generally good condition and needs minimal pre-overlay repairs. 1 in. = 25.4 mm 1 yd2 = 0.84 m2
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13 Table 2-4. Tentative texture method benefit rankings. Method Friction Exterior Noise Cost Constructability Transverse tine 1 8 1 2 (0.75-in spacing) Transverse tine 1 6 1 2 (0.5-in. spacing) Transverse tine 1 7 1 2 (variable spacing) Transverse groove 1 7 4 3 Transverse drag 2 6 2 Longitudinal tine 1 4 1 1 Longitudinal groove 1 5 3 3 Longitudinal grind 1 3 3 3 Longitudinal burlap drag 4 3 1 1 Longitudinal turf drag 2 3 1 1 Longitudinal plastic brush 3 3 1 1 EAC 2 3 3 4 Shotblasted PCC 1 7 2 3 Porous PCC 1 1 5 4 Ultra-thin epoxied laminate 1 2 6 3 Ultra-thin bonded wearing course 2 2 3 3 1 = Best/highest ranking Table 2-5. Highway agency texturing practices for new concrete pavements. Highway Agency Texturing Method Optional Texturing Methods Alabama Tran Tine (13 to 25 mm variable) w/ Burlap Drag California Long Tine (19 mm) w/ Burlap Drag Burlap Drag (mountains), Long Groove Colorado Long Tine (19 mm) Florida Tran Tine (13 to 25 mm variable) w/ Burlap Drag Illinois Tran Tine (19 mm) w/ Long Turf Drag Tran Tine (17 to 54 mm variable) w/ Long Turf Drag Indiana Tran Tine (variable) w/ Long Turf Drag or Burlap Drag Iowa Tran Tine (19 mm) w/ Long Turf Drag or Burlap Drag Long Tine (19 mm), Tran Tine (9.5 to 41 mm) States Kansas Long Tine (19 mm) w/ Burlap Drag or Long Turf Drag Michigan Tran Tine (13 mm slightly variable) Minnesota Long Turf Drag ( 1 mm MTD) Missouri Any method ( 0.7 mm MTD) Tran Tine (13 mm), Long Tine (13 mm), Long Grind North Carolina Tran Tine (13 to 19 mm variable) w/ Burlap Drag North Dakota Tran Tine (13 to 71 mm variable) w/ Long Turf Drag Pennsylvania Tran Tine (15 to 54 mm variable) Texas Tran Tine (25 mm) w/ Long Turf Drag Wisconsin Tran Tine (15 to 54 mm variable) w/ Long Turf Drag Austria EAC (2 layer) Belgium EAC (1 layer) Long Tine, Long Grind Germany Burlap Drag (MTD 0.4 to 0.6 mm) Japan Long Groove Countries Netherlands EAC (1 layer) Spain Long Tine-Sinusoid (25 to 30 mm) (MTD 0.6 to 0.9 mm) Sweden EAC Long Grind United Kingdom EAC 1 in. = 25.4 mm