January 2011

Measuring and Analyzing Pavement Texture Concrete Pavement Surface Characteristics Program

Authors Introduction deleterious eff ect shown in red. Th e focus of this technical brief is on techniques to Pavement texture is defi ned by the Robert Otto Rasmussen measure and analyze texture, particularly Vice President, The Transtec Group, Inc. irregularities on a pavement surface those that infl uence tire-pavement noise [email protected] that deviate from an ideal, perfectly 512-451-6233 and friction. fl at surface. As shown in Figure 1, the Richard Sohaney World Road Association (PIARC) has In modern concrete pavement Project Manager, The Transtec Group, Inc. established standard categories of texture, construction, texture is deliberately Paul Wiegand classifi ed by wavelength. Th ese categories imparted in a reasonably-controlled Transportation Research Engineer, include microtexture (wavelengths up manner. Th e nominal geometry of the Institute for Transportation at Iowa State to 0.5 mm), macrotexture (0.5 to 50 texture is typically prescribed, either University mm), megatexture (50 to 500 mm), and directly or indirectly. For example, Dale Harrington roughness (wavelengths larger than 500 as illustrated in Figure 2, the spacing Senior Engineer, Snyder and Associates mm). and depth of the directional striations produced by tining are often described Texture infl uences many aspects of road in a specifi cation. Diamond-ground safety and comfort, including friction, textures will be defi ned in large part by smoothness, splash and spray, and rolling the geometry of the grinding head (blade resistance. Figure 1 also illustrates the widths and spacers). Even the texture of ranges of texture that infl uence these a drag surface will be defi ned to some pavement surface characteristics, with a degree by the specifi c mechanical means positive infl uence shown in green and used to impart these surfaces.

National Concrete Pavement Technology Center 2711 South Loop Drive, Suite 4700 1 Ames, IA 50010-8664 Texture 0.1 mil 1mil 10 mil /8 in. 1 in. 1 ft. 10 ft. 100 ft. Wavelength 1 µm 10 µm 100 µm 1mm 10 mm 100 mm 1 m 10 m 100 m www.cptechcenter.org PIARC Microtexture Macrotexture Megatexture Roughness Director Category Tom Cackler 515-294-5798 Rolling Resistance [email protected] Ride Quality Managing Editor Sabrina Shields-Cook Pavement Wet Weather Friction Surface 515-294-7124 Characteristic Dry Weather Friction [email protected] Influence Splash and Spray

Tire Wear Vehicle Wear

In-Vehicle Noise Key: Tire-Pavement Noise Good Bad

Figure 1. World Road Association (PIARC) texture defi nitions and their infl uence on pavement surface characteristics Measuring and Analyzing Pavement Texture

While it would be ideal for the With texture aff ecting so many Measuring Texture as-constructed texture to match the diff erent surface characteristics, it Laser-based profi lometry is the most as-designed (nominal) geometry, this is an ideal parameter to evaluate as commonly used technique to collect is not usually the case. Numerous part of construction quality control. pavement texture data. Various standards factors aff ect the fi nal texture, including Furthermore, because texture is simply have been developed to describe the variability in materials and construction geometry, it is a fundamental aspect of requisite equipment in such a way that technique. Th is is illustrated in Figure 3, a pavement surface to measure. Th is is relevant to the type of texture to be which shows variability of pavement technical brief discusses pavement texture measured. Of these, ISO 13473 is the texture on two areas of a project that measurement and, specifi cally, the most commonly cited. While other were constructed within minutes of each methods used in the Concrete Pavement texture measurement methods have been other. In addition to initial variability, Surface Characteristics Program advanced, they often possess technical texture changes over time due to both (CPSCP). (It also describes the methods or practical limitations. Laser-based traffi c and the environment. and metrics used to quantify texture.) texture profi lometry uses technology that is similar to road profi lers that evaluate pavement smoothness. However, when measuring texture, a greater degree

Tining D (0.125 in.) of precision and accuracy is needed with respect to small features that are normally fi ltered out when evaluating S (0.75 in.) Drag W (0.125 in.) road roughness. Most profi lers in use today measure Artificial Turf texture using a single-point laser, which 5/8” PE blades 7200 blades/sf results in a two-dimensional (2D) texture 70 oz/sy Diamond Grinding profi le with distance along the pavement surface as one dimension and the texture elevation as the second. Concrete Spacer Width Blade Width pavement texture is complex, though: it (0.110 in.) (0.125 in.) is anisotropic, meaning that the texture varies depending on the direction Figure 2. Typical texture specifi cation of the measurement (longitudinal or transverse). Th erefore, measuring a 2D profi le fails to completely describe characteristics of the texture that are important to many surface characteristics. In addition, measuring with a single-point laser can introduce artifacts in the measurement that distort the profi le and, therefore, introduce error in predicting the surface characteristic of interest. To overcome the limitation of using a spot sensor, a line laser can be used. Traditionally, line lasers are found in machine vision applications. In recent years, vendors of these sensors have worked with the highway industry to develop variants of the sensors that are Figure 3. Material and construction variability affect the as-constructed texture suitable for texture profi le applications.

2 Measuring and Analyzing Pavement Texture

Figure 4 illustrates the measurement principle of a line laser. Measuring with a line laser results in a three-dimensional (3D) texture profi le with distance along the pavement surface as one dimension, distance along the laser line as the second dimension, and texture elevation as the third dimension.

RoboTex A line laser sensor alone cannot measure pavement texture properly. A platform with a means to mobilize the sensor is required. In 2005, a prototype of an innovative system Figure 4. Measurement principle of the LMI Technologies RoLine line laser height sensor for texture measurement was developed: a robotic-based texture measurement system (RoboTex). As shown in Figure 5, the system is built on a robotic chassis for remote control of the measurement platform speed and travel direction. RoboTex includes customized software that simultaneously collects and stores data from a number of sensors. As illustrated in Figure 5, the components include: • Line laser sensor – for height data • Accelerometer – to establish an inertial reference elevation • Wheel encoder – to determine the precise position of the robot • Global positioning system (GPS) – to establish a global position of the robot for reference • Time – to determine speed and for global reference • Digital imaging system – for a visual record of the surface Figure 5. RoboTex (above) and schematic of RoboTex components (below) Th is concept device provides a better tradeoff between resolution and effi ciency in texture data collection. RoboTex is capable of sampling 100 or more texture elevation points across a 100 mm wide laser line at 1000 Hz as it travels down the road under its own power at about 0.5 m/s. Th is yields a pavement texture measurement with a spatial resolution of about 0.4 mm² and a height resolution of 0.01 mm. More importantly, the result is a 3D texture profi le along a 100 mm wide swath of pavement surface, as illustrated in Figure 6. Figure 6. Sample RoboTex data from concrete pavement textures

3 Measuring and Analyzing Pavement Texture

Geometric Texture 2.5 Metrics 2 While illustrations of the measurements, as 1.5 shown in Figure 6, are useful, they do not 1 quantify texture and a multitude of ways to 0.5 characterize texture exist. Th e selection of a proper metric begins with an understanding 0 of the fundamental mechanisms that describe Profile (mm) -0.5 the surface characteristics of interest. Ideally, -1 a texture metric describes the texture in a way -1.5 that relates its interaction with a tire. -2 Quite often, texture depth is used as a -2.5 texture metric. However, texture depth can 0 102030405060708090100 be calculated in numerous ways. To illustrate Distance (mm) this, refer to a short (100 mm) sample of Figure 7. Sample texture profi le tined concrete pavement texture, as illustrated in Figure 7. 2.5 A straightforward way to calculate texture depth is to calculate an average (rectifi ed) 2 deviation of the texture from the mean 1.5 Ra = 0.42 mm depth. Th is is illustrated on the top of Figure 1 8, where the sum of the shaded areas both above and below the mean profi le elevation 0.5 is divided by the length of the profi le (100 0 mm). Th e result is in an average texture depth Profile (mm) -0.5 (R ) of 0.42 mm. Alternatively, the texture a -1 profi le shown in Figure 7 can be squared, as illustrated on the bottom of Figure 8. Taking -1.5 this area, dividing by the length of the profi le, -2 and then taking the square root, results in -2.5 the root-mean-square (RMS) texture depth 0 102030405060708090100 Distance (mm) (Rq). In this example, the RMS is 0.63 mm. 5 Mathematically, these calculations are made as follows using measured texture data: 4.5 4 Rq = 0.63 mm

3.5 N N 2 ∑ zn ∑ zn 3 n=1 n=1 Ra = and Rq = 2.5 N N

Profile² (mm²) 2 where: 1.5

N = number of measured samples of 1 texture profi le 0.5

zn = texture profi le elevation data 0 (starting with n=1, and ending with 0 102030405060708090100 n=N) after detrending Distance (mm) One disadvantage of most texture-depth Figure 8. Calculation of average texture height, Ra (top), and RMS texture height, Rq (bottom)

4 Measuring and Analyzing Pavement Texture metrics is the lack of sensitivity to As shown in Figure 9, calculating MPD Functional Texture directionality of the texture. For example, involves dividing the 100 mm texture Metrics the same Ra and Rq values are calculated profi le into two 50 mm segment halves. for the texture profi le in Figure 7, even A peak profi le point in each segment While geometric metrics, such as if the profi le is “fl ipped over” (upside half is determined, then averaged, and, depth and skewness, are intuitive down). In the fl ipped-over case, the tined then the mean profi le elevation (in this and straightforward to calculate, they grooves become sharp spikes and, clearly, case, zero) is subtracted. Th e result in do not describe the interaction of a tire would interact with this profi le this example is 0.69 mm. Note that a tire and the pavement texture by very diff erently than the right-side-up the currently-proposed revision to the themselves. Functional texture metrics profi le. To help overcome this limitation, ISO 13473-1 specifi cation refers to the are more useful in this regard. ISO a directionality metric, termed texture calculation within a 100 mm sample as specifi cation 13565-2 describes a process skewness (Rsk), is calculated as follows: a mean segment depth (MSD), so, the of transforming a texture profi le into a MPD is then an average of numerous profi le-bearing ratio curve. As illustrated N in Figure 10, this begins with the 3 (minimum of fi ve) MSD values. ∑ zn recognition of three distinct “regions” of n=1 Rsk = 3 texture. Rq N

2.5 For textures like that shown in Figure 7, the skew is negative (in this example, 2

-1.7). If the profi le is fl ipped over, the 1.5 MPD (MSD) = 0.69 mm texture has a positive skew. 1 0.83 mm Historically, pavement texture depth 0.55 mm has been measured using the volumetric 0.5 (“sand”) patch technique and reported 0 as a mean texture depth (MTD). Th e Profile (mm) -0.5 test involves carefully spreading a known volume of small glass beads into a -1 circle on the pavement surface and the -1.5 MTD is simply the volume of beads divided by the area of the circle. As -2 laser-based profi lers began to be used, -2.5 the mean profi le depth (MPD) metric 0 102030405060708090100 was developed as a means to estimate the Distance (mm) MTD. Figure 9. Calculation of mean profi le (mean segment) depth

2.5 3 3

2 2 2 1.5 Peaks – First Region of Contact / Wear Peaks – First Region of Contact / Wear 1 Core – Working Region 1 1 0.5 Rpk 0 0 0 Rk Profile (mm) Profile (mm) -0.5 Profile (mm) Core – Working Region -1 -1 -1 Rvk

-1.5 -2 -2 -2 Valleys – Water/Air Flow Valleys – Water/Air Flow -2.5 Amplitude -3 -3 0 102030405060708090100 Density 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Distance (mm) Function Profile Bearing Length Ratio (%) Profile Bearing Length Ratio (%)

Figure 10. Calculation of profi le-bearing ratio from a texture profi le

5 Measuring and Analyzing Pavement Texture

Th e peaks are the fi rst to be contacted by Bridging Filter achieved. At this point of equilibrium, a tire, and the most rapid to wear. Th e a single fi ltered elevation is calculated Ideally, the texture metrics that are second region is the core, where most of and stored as the bridged elevation calculated should correlate well to the the texture typically lies, and therefore and the process is repeated again after pavement surface characteristics of describes the longevity of the texture. incrementally moving the fi lter forward. interest. However, this is not always the Th e lowest regions are termed valleys, Mathematically, the fi ltering process case. Tire-pavement noise, in particular, and are where air and water that are not can be further simplifi ed by sorting is a diffi cult response to predict given necessarily in contact with the tire can the texture profi le elevations. Th is is its complexity. One technique that has possibly reside. Calculation of the heights illustrated in Figure 13. successfully improved this correlation of these three regions fi rst involves is the application of a bridging (or converting the texture profi le to a profi le- envelopment) fi lter. Th e fi lter is applied Conclusions bearing length ratio (a type of cumulative to texture profi les prior to calculating key distribution of texture elevations). From Texture is a vitally important pavement texture metrics, most notably spectral this, various chord and area calculations property because it relates to most of analysis. derive the parameters of reduced peak what defi nes functional performance vis-à-vis pavement surface characteristics. height (Rpk), core roughness depth (Rk), Th e specifi cs of the bridging fi lter are It has been shown that measuring the and reduced valley depth (Rvk). governed by the response of interest. For example, in the case of tire- texture of concrete pavements in an pavement noise, the fi lter geometry accurate and relevant manner requires Spectral Texture 3D data, which, in the CPSCP, was Metrics is on the same scale as that of the tire tread. As illustrated in Figure 12, the collected using an innovative device Spectral analyses of pavement textures fi lter simulates how a tire tread would called RoboTex. Th e data from RoboTex are used to identify the presence or impregnate the texture. Th e elevation was used to calculate numerous texture signifi cance of various repeating features. of the fi ltered profi le is calculated as metrics, including those described herein. To illustrate this, Figure 11 shows a balance of the downward force on As a result, the as-constructed texture a spectral analysis of the same tined the tire and the upward reaction due could be related to surface characteristics, pavement shown in Figure 7. (More to the elasticity of the tire. Th is occurs such as tire-pavement noise and friction, on the technique used for this type of when an equivalent uniform tire and better practices could be developed analysis is included in ISO 13473-4.) rubber displacement (e.g., 1 mm) is for the design and construction of concrete pavement surfaces. From this plot, a characteristic peak at 25 mm is visible, which corresponds to the tine spacing. Other less-obvious 40 characteristics of the texture can Dominant Texture Spacing sometimes be derived from spectral 35 analyses, particularly when the texture wavelength is plotted in narrow bands (as 30 opposed to third-octave, illustrated here). 25 Furthermore, there is often a correlation between some pavement surface 20 characteristics and the texture content within select spatial frequency bands 15 (or texture wavelengths). Also note that, in Figure 11, a transformation of the Texture Level (dB ref 1 µm RMS) 10 texture elevation was made to a texture level using a decibel scale and a reference 5

RMS texture of 1 μm. (Don’t confuse 0 this with the decibel scale that is used for 1.6 2 2.5 3.15 4 5 6.3 8 10 12.5 16 20 25 31.5 40 noise measurement and reporting.) Texture Wavelength (mm) Figure 11. Spectral analysis of texture profi le

6 Measuring and Analyzing Pavement Texture

“Tire”

Actual Displaced Tire Volume = Pavement Voids Uniform 1mm Displaced “Tire” Volume

Typically 25 to 100 mm

Figure 12. Principles of tire tread bridging fi lter

D = 1 mm Equal Highest Elevation = P(1) Volumes

Uncorrected Bridged

Elevation = PB

Lowest Elevation = P(N) Elevation (mm)

1 N/4 N/2 3N/4 N Texture Profile Cumulative Distribution (point #)

Figure 13. Calculation of bridged elevations using tire tread bridging fi lter

7 Measuring and Analyzing Pavement Texture

For More Information • ISO, “Characterization of pavement Administration (FHWA), the American texture by use of surface profi les – Concrete Pavement Association (ACPA), • ASME, “Surface Texture (Surface Part 4: Spectral analysis of surface and the International Grooving and Roughness, Waviness, and Lay),” An profi les,” ISO/TS 13473-4: 2008. Grinding Association (IGGA). American National Standard, ASME B46.1-2009. • ISO, “Geometrical Product Th e mission of the program was to help Specifi cations (GPS) – Surface optimize concrete pavement surface • ASTM, “Standard Test Method for texture: Profi le method; Surfaces characteristics—more specifi cally, it was Calculating Pavement Macrotexture having stratifi ed functional properties to fi nd innovative solutions to make Mean Profi le Depth,” Standard E – Part 2: Height characterization concrete pavements quieter without 1845-96. using the linear material ratio curve,” compromising safety, durability, or cost • ASTM, “Standard Test Method for ISO 13565-2: 1996. eff ectiveness. Measuring Pavement Macrotexture • Robert O. Rasmussen, et al., “Th e Th e current program is now operating Depth using a Volumetric Little Book of Quieter Pavements,” under Pooled Fund TPF-5(139) with Technique,” Standard E 965-01. Report FHWA-IF-08-004, USDOT the additional support of state DOTs, • ISO, “Characterization of pavement Federal Highway Administration, including California, Iowa, Minnesota, texture by use of surface profi les 2007. New York, Texas, Washington, and – Part 1: Determination of Mean Wisconsin. • Steven Karamihas, “Critical Profi ler Profi le Depth,” ISO/TC 43/SC 1/ N Accuracy Requirements,” University Recent focus is on identifying specifi c 1xx, ISO/WD 13473-1, 2009. of Michigan Research Report guidance to properly design and • ISO, “Characterization of pavement UMTRI-2005-24, 2005. construct quieter concrete pavements. texture by use of surface profi les Innovative concrete pavement surfaces – Part 2: Terminology and basic About the Concrete are also being evaluated to assess their requirements related to pavement Pavement Surface potential as viable solutions. texture profi le analysis,” ISO 13473- For more information, contact Paul 2: 2002. Characteristics Wiegand of the National CP Tech • ISO, “Characterization of pavement Program Center at 515-294-7082 or pwiegand@ texture by use of surface profi les – In December 2004, a coalition was iastate.edu. Part 3: Specifi cation and classifi cation formed between the National Concrete of profi lometers,” ISO 13473-3: Pavement Technology Center (National 2002. CP Tech Center), the Federal Highway

About the National Concrete Pavement Technology Center The mission of the National Concrete Pavement Technology Center is to unite key transportation stakeholders around the central goal of advancing concrete pavement technology through research, tech transfer, and technology implementation. The sponsors of this research are not responsible for the accuracy of the information presented herein. The conclusions expressed in this publication are not necessarily those of the sponsors. Iowa State University does not discriminate on the basis of race, color, age, religion, national origin, sexual orientation, gender identity, sex, marital status, disability, or status as a U.S. veteran. Inquiries can be directed to the Director of Equal Opportunity and Diversity, (515) 294-7612.