KSCE Journal of Civil Engineering (0000) 00(0):0-0 Copyright ⓒ2014 Korean Society of Civil Engineers DOI 10.1007/s12205-014-0516-0 Highway Engineering pISSN 1226-7988, eISSN 1976-3808 www.springer.com/12205 TECHNICAL NOTE Effect of Aggregate Shape on the Surface Properties of Flexible Pavement Burak Sengoz*, Amir Onsori**, and Ali Topal*** Received October 22, 2012/Accepted September 2, 2013/Published Online March 15, 2014 ·································································································································································································································· Abstract One of the most important properties of flexible pavements in terms of tire-pavement interface is surface texture. The texture of a pavement surface and its ability to resist polishing effect of traffic is of prime importance in providing skidding resistance. Pavement surface texture greatly contributes to tire-pavement skid resistance which has a direct effect on traffic operation and safety particularly at high speeds. Doubtless, there exists a close relationship between pavement surface texture and aggregate angularity within the wearing course. This paper is aimed to determine the effect of aggregate shape on the surface properties of Hot Mix Asphalt (HMA). Two different mineralogical types of aggregate (basalt and limestone) have been crushed with impact, jaw and roll type of crushers. Various types of aggregate with different shapes have been mixed with 50/70 penetration grade bitumen to form dense graded mixtures. Test methods related with the evaluation of shape and texture characteristics have been utilized to characterize the geometrical properties of aggregates. The texture and friction properties of asphalt slabs have been evaluated by means of sand patch test, 3D laser scanner and dynamic friction tester respectively. The results indicated that a relationship exists between the shape characteristics of aggregate and the surface properties of HMA. Keywords: aggregate shape, mineralogy, crusher, friction, surface texture ·································································································································································································································· 1. Introduction Each year unacceptably large number of fatalities and injuries resulting from accidents on highways make roadway safety one of the most important international issue. With continuous growth in the amount of highway traffic and capacity, traffic crashes increase annually over the whole world. Along this increase, a great demand and focus on the need for safer roads and highways become prior in road projects. In addition to security factors such as highway geometry, operating speed driver dynamics, pavement surface properties are also critical factor in highway safety. A proper design to provide adequate pavement surface properties and practically monitoring these properties has been high priority in recent projects worldwide. Many factors influence pavement surface properties such as: age of the surface course, seasonal variations and rainfall, traffic intensity, road geometry and aggregate mineralogy and shape properties (Chelliah et al., 2003). Among these categories, shape properties of aggregates gained attention in the last decade. Investigation of a potential relationship between pavement surface characteristics, such as friction and texture, and shape characteristics of aggregate will help better understand and mitigate the problem. Pavement texture is defined as the deviations of the pavement surface from a true planar surface (PIARC, 1987). Macrotexture is characterized by wavelengths between 0.5 to 50 mm and peak to peak amplitudes usually between 0.1 to 20 mm. Microtexture is characterized by wavelengths shorter than 0.5 mm and with peak to peak amplitudes usually between 0.001 and 0.5 mm (PIARC, 1995; ISO, 2002). Texture wavelength is defined as the minimum distance between periodically repeated parts of the curve in its direction along the surface plane (ISO, 2002). Both microtexture and macrotexture influence the skid resistance of a pavement. It is widely recognized that pavement surface texture influences many different pavement-tire interactions. Fig. 1 illustrates the ranges of texture wavelengths affecting various vehicle-road interactions, including friction, interior and exterior noise, splash and spray, rolling resistance, and tire wear. Pavement friction is the force that resists the relative motion between a vehicle tire and a pavement surface. This resistive force is generated as the tire rolls or slides over the pavement surface (Rado, 2006). Pavement friction plays a vital role in keeping vehicles on the road, as it gives drivers the ability to control/maneuver their vehicles in a safe manner, in both longitudinal and lateral directions. The surface properties of Hot Mix Asphalt (HMA) are also affected substantially by the shape characteristics of aggregates. The successful quantification of aggregate geometric irregularities *Associate Professor, Dept. of Civil Engineering, Dokuz Eylul University, 35160 Buca, Izmir, Turkey (Corresponding Author, E-mail: burak.sengoz@deu.edu.tr) **MSc. Researcher, Graduate School of Natural and Applied Sciences, Dokuz Eylul University, 35160 Buca, Izmir, Turkey (E-mail: a.onsori@gmail.com) ***Associate Professor, Dept. of Civil Engineering, Dokuz Eylul University, 35160 Buca, Izmir, Turkey (E-mail: ali.topal@deu.edu.tr) −1− Burak Sengoz, Amir Onsori, and Ali Topal the Type 1 wearing course of Turkish specifications. Various types of aggregate with different shapes have been mixed with 50/70 penetration grade bitumen to form a dense graded mixture. The shape characteristics of the aggregate have been determined by ASTM C1252, modified ASTM C1252, EN 933-6, ASTM D4791 and BS 812. The primary indices used to characterize texture are Mean Texture Depth (MTD) determined by sand patch test and Mean Profile Depth (MPD) determined by 3D laser scanner. The friction measurements have been performed by means of dynamic friction tester. The hypothesis behind this experimental design is that it is possible to improve the frictional performance of the pavement surface by the selection of aggregate shape characteristics and mineralogical types of aggregates. Fig. 1. Texture Wavelength Influence on Pavement Tire Interactions is essential for understanding their effects on pavement surface properties and for selecting aggregates to produce pavements of adequate quality. Thus, the quantification of the shape is important as high-quality pavements are needed to meet increases in traffic volume and load. A number of studies related skid resistance with surface texture. Masad et al. studied the influence of texture on tire and pavement surface friction. Skid numbers were governed by six texture parameters. The three macrotexture parameters and three microtexture parameters were expressed in terms of texture size, spacing or distribution, and shape (Masad et al., 2009). Do et al. (2007) defined three parameters for characterizing a surface texture; size, interspace (or density) and shape. Ergun et al. (2005) developed a friction-coefficient prediction model which is based on texture profiles measured by using an image capturing technique. Wu and King (2011) showed how differences on texture of pavement surfaces influence peak brake coefficients of a standard test tire. Gardiner et al. (2001) demonstrated how skid resistance tests taken from different pavement surfaces are different based on their texture. This research aims to characterize the surface properties of HMA slabs by means of texture and friction measurements. Two different mineralogical types of aggregate (basalt and limestone) were crushed with three different types of crushers (impact crusher, jaw crusher and roll crusher) and screened with ASTM E11 sieves. Grading of aggregate was chosen in conformity with 2. Experimental 2.1 Materials The base bitumen with a 50/70 penetration grade was procured from Aliaga/Izmir Oil Terminal of the Turkish Petroleum Refinery Corporation. In order to characterize the properties of the base bitumen, conventional test methods such as: penetration test, softening point test, ductility were performed. These tests were conducted in conformity with the relevant test methods that are presented in Table 1. The aggregate samples were prepared by limestone and basalt type aggregate crushed with impact, jaw, and roll type crushers. The general properties of aggregates are presented in Table 2. Grading of aggregate was chosen in conformity with the Type 1 wearing course of Turkish Specifications. Table 3 presents the aggregate gradation board. 2.2 Preparation of HMA Slabs The aggregate type for producing HMA slabs are divided into three main groups. Limestone and basalt aggregates (as coarse, fine and filler fraction) constitute the first two group whereas mix aggregate (basalt aggregate as coarse fraction and limestone aggregate as fine fraction) constitute the third group. It should be noted that both limestone and basalt aggregate was crushed with different types of crushers. Table 4 presents the detailed description of the slabs produced. Related to the preparation of each slab, Marshall stability and flow tests were carried out at various bitumen contents based on ASTM D1559. The bitumen content corresponding to 4% air Table 1. Properties of Base Bitumen Test Penetration (250°C; 0.1 mm) Softening point (°C) Viscosity at (135°C)-Pa.s TFOT (163°C; 5h) Change of mass (%) Retained penetration (%) Specific gravity Specification ASTM D5 EN 1426 ASTM D36 EN 1427 ASTM D4402 ASTM D1754 EN 12607-1 ASTM D5 EN 1426 ASTM D70 −2− Results 55 49.1 412.5 137.5 0.04 51 1.030 Specification limits 50-70 46-54 0.5 (max) 50 KSCE Journal of Civil Engineering Effect of Aggregate Shape on the Surface Properties of Flexible Pavement Table 2. Properties of Aggregate Samples Specific gravity (coarse aggregate) Bulk SSD Apparent Specific gravity (fine aggregate) Bulk SSD Apparent Specific gravity (filler) Los Angeles abrasion (%) Sodium sulphate soundness (%) Specification Limestone ASTM C 127 ASTM C 128 ASTM C 131 ASTM C 88 Table 3. Aggregate Gradation Board Sieve Size/No. Specification 3/4'' 1/2'' 3/8'' No. 4 No. 10 No. 40 No. 80 No. 200 ASTM C 136 Gradation (%) and results 100 92 80.5 47.3 33.0 13.5 9.0 5.3 Specification limits 100 83-100 70-90 40-55 25-38 10-20 6-15 4-10 Table 4. Optimum Bitumen Content for Each of Specimens Slab identification LIP LJP LRP BIP BJP MIP MJP MRP Aggregate type Limestone Limestone Limestone Basalt Basalt Mix Mix Mix Crusher type Impact Jaw Roll Impact Jaw Impact Jaw Roll Optimum bitumen content 4.65 4.60 4.60 4.70 4.75 4.70 4.65 4.70 void in total mixture were taken as optimum bitumen content. Table 4 illustrates the optimum bitumen content related to each slab. Following the determination of optimum bitumen content, approximately 50 mm thick asphalt slabs were produced (650 × 650) using a kneading slab laboratory compactor. Basalt 2.686 2.701 2.727 2.666 2.810 2.706 2.687 2.703 2.732 2.725 22.600 1.470 2.652 2.770 2.688 2.731 14.200 2.600 Specification limits Max 30 Max 10-20 of the fine aggregates (ASTM, 1998). This method estimates the angularity, sphericity and surface texture of the aggregate having a given grading. There are three methods for running this test: Methods A, B and C. The mass of the sample for all methods is fixed at 190 g. Method A specifies a standard gradation ranging from No. 8 (2.36 mm) sieve to No. 100 (0.15 mm). Method B specifies that the test be run on the three individual size fractions; No. 8-16 (2.36-1.18 mm), No. 16-30 (1.18-0.6 mm) and No. 3050# (0.6-0.3 mm). Method C specifies that the test is run on the as received gradation. Since the aggregate samples were produced by mean of laboratory type crusher, method C is not included in the scope of the study. Modified ASTM C1252 “Uncompacted Void Content of coarse Aggregate” (AASHTO TP 56) was also used to determine the angularity, sphericity and surface texture of the coarse aggregate. The mass needed to perform the test is 5000 g. Method A specifies a standard gradation ranging from 19 mm sieve to 4.75 mm. Method B specifies that the test be run on the three individual size fractions; 3/4''-1/2'' (19 mm-12.5 mm), 1/2''-3/8'' (12.5 mm-9.5 mm) and 3/ 8''-No4 (9.5 mm-4.75 mm). The EN 933-6 ‘‘Geometrical properties of aggregates assessments of surface characteristics, flow coefficient of aggregates’’ test method was used to determine the flow coefficient of aggregates (European standard tests, 2001). The test performed using No.200-No.10 (0.08-2 mm) and No.200-No.5 (0.08-4 mm) sized sand samples. In addition to the above tests, the flat and elongated particles (ASTM D4791) and flakiness index of coarse aggregate (BS 812) characteristics were also determined on the aggregate samples. 2.3 Test Methods 2.3.1 Test Methods Related to Aggregate Shape Characteristics ASTM C1252, modified ASTM C1252, EN 933-6, ASTM D4791 and BS 812 were performed to evaluate the shape characteristics of the limestone and basalt types of aggregates. The ASTM C1252 ‘‘Uncompacted void content of fine aggregate’’ was used to determine the uncompacted unit weights Vol. 00, No. 0 / 000 0000 2.3.2 Test Methods Related to Surface Texture and Friction of HMA There are many methods developed to measure texture and friction properties of a pavement so far. Methods and the associated tests used in this study are mostly based on ASTM standards. These methods are accordingly, the Sand patch test method (ASTM, 1998) to measure Mean Texture Depth (MTD), a recent and more reliable Laser Scanner (ASTM E 1845-01, −3− Burak Sengoz, Amir Onsori, and Ali Topal 2003) to obtain Mean Profile Depth (MPD) (ASTM, 2003), Dynamic Friction Tester (ASTM E 1911-98, 1999) used to measure the friction coefficient of a surface at a regular speed (0 -80 Km/h) (ASTM, 1999). 2.3.2.2 3D Laser Scanning In this study, the Model Maker 3D laser scanner (class 2M) including enhanced sensors was utilized to inspect full range of colors and depths on the selected asphalt pavement surfaces. The laser equipment was mounted on a portable vehicle attached to a computer as presented in Fig. 2. The Model Maker D with true digital camera technology includes several groundbreaking innovations such as second generation Enhanced Sensor Performance (ESP2). This device provides a better tradeoff between resolution and efficiency in texture data collection. As shown in Fig. 2, the device measures texture by means of laser light. Laser intensity output is controlled by the processing unit to maintain a constant level of light on the detector. The possible angle of incidence will depend on the measured material and on the surface geometry. The sensor consists of a light source and a detector integrated with optics and electronics. It is insensitive to ambient light. When the light source projects a beam to hit a pavement surface, a scattered reflection will occur. This light spot on the surface is viewed by a camera mounted inside the sensor. Depending on the distance between the laser head and the measured spot, the image of the light spot will be reflected to focus on a certain position on the detector. As the resolution depends on the range to the object, in the field studies the scanners enhanced sensor was established to provide an optimum resolution of 15 µm in the lateral direction and an optimum resolution of 10 µm in the vertical direction. For the 3D laser scanner utilized, the accuracy is expressed in terms of standard deviation of the ten measurements made on the same test surface. The standard deviation related to the calibration surface was found as 0.04 mm. The Model Maker D is capable of sampling 1000 texture elevation points across a 100 mm wide laser line at 150 Hz as it scans the road surface at about 0.1 m/sec. More importantly, the result is a 3D texture profile along a 100 mm wide swath of pavement surface. The laser scanner adapts its laser power to suit the surface characteristics of pavement through enhanced scanning performance. During scanning process, laser device automatically tracks changes based on the surface conditions (both color and reflectivity of the bitumen as well as some minerals) parallel to the direction of the moving traffic and adapts laser power and Fig. 2. 3D LASER Scanning Test Device Fig. 3. Scanning Process 2.3.2.1 Sand Patch Test Method The test procedure used for the study follows the procedures included in ASTM E965. It uses a volumetric approach of measuring pavement macrotexture defined as mean texture depth. The principle is fairly obvious that the greater the texture, the more the sand will be taken up by it and the smaller the circle that can be achieved from the standard quantity of sand. In this study a known volume of glass spheres (24.6 mL) was spread evenly over the pavement surface to form a circle, thus lling the surface voids with glass beads. The diameter of the circle was measured on four axes and the value averaged. This value was used to calculate the Mean Texture Depth (MTD) in mm (Eq. 1). 4V MTD = --------------------2 Π.D avg (1) Where; Davg = Average diameter of sand patch in, mm V = Exact volume of glass spheres, mL −4− KSCE Journal of Civil Engineering Effect of Aggregate Shape on the Surface Properties of Flexible Pavement Fig. 4. The profile and a Cross Section Example of an Asphalt Pavement Surface Fig. 5. Standard Method used for Calculating MPD sensor settings. Fig. 3 depicts the scanning process by the 3D laser scanner. The surfaces scanned with the Model Maker D were also captured by 12.1 Mp CCD camera as illustrated in Fig. 4. The Model Maker D laser scanner are also supplied with a data acquisition software (Kube®) which is integrated and specifically designed for capturing and processing the laser stripe data. Following the acquisition procedure of the set of surface (Fig. 4a) and cross-section (Fig. 4b) of pavement samples with Kube® software, it is necessary to characterize them with appropriate indicators such as mean profile depth (MPD). Based on descriptions given in ASTM E 1845 standard, before computing the MPD, the surface profile was filtered by applying a low pass filter in order to remove wavelengths 2.5 mm followed by suppressing the profile slope by subtracting a regression line from the profile. The MPD was computed from a sample baseline divided into two equal half as presented in Fig. 5. The peak level in each half was determined and the average of the two peaks was termed as MPD. 2.3.2.3 Dynamic Friction Tester The Dynamic Friction Tester (DFT), as shown in Fig. 6, is a portable device for measuring surface friction. The test procedures are covered in ASTM E-1911. The fundamental principle is the Coulomb’s friction law. This device consists of a horizontal Vol. 00, No. 0 / 000 0000 Fig. 6. Dynamic Friction Tester (DFT) spinning disk fitted with three spring-mounted rubber sliders. During testing, the disk is lowered so that the three sliders are in contact with the test surface under a constant force perpendicular to the test surface. The disk is driven by a motor and rotates at a tangential speed varying from 0 to 80 km/h which is determined from the rotary speed of the disk. Water is delivered to the test surface by a water supply unit. The horizontal force required to overcome the friction is measured by a transducer. The test result is reported as the coefficient of friction and is plotted against the speed. DFT measurement corresponding to 20 km/h is taken as the friction value (PIARC, 1995). 3. Results and Discussion 3.1 Aggregate Characteristics Figure 7 and Fig. 8 present the uncompacted void (U) content (%) related to fine aggregate and coarse aggregate respectively. In figures the first letter L and B are the abbreviations of limestone and basalt aggregates respectively. The second letter presents the type of crusher. As seen in Fig. 7, method B yields higher U content (which is an indicator of higher angularity and aggregate surface texture) compared to −5− Burak Sengoz, Amir Onsori, and Ali Topal Fig. 7. Uncompacted Void Contents for Fine Aggregate (ASTM C1252) Fig. 10. Flat and Elongated Particle Results (ASTM D4791) Fig. 11. Flakiness Index Values Fig. 8. Uncompacted Void Contents for Coarse Aggregate (Modified ASTM C1252) Fig. 9. Flow Rates of Fine Aggregate Aamples by EN 933-6 method A. Since method B specifies the test be run on the individual size fractions, the internal friction between the aggregate particles is higher. As depicted in Fig. 7; basalt type aggregate has higher U content compared to limestone aggregate for all type of crushers. This is due to the mineralogical properties of the basalt aggregate (Topal, 2008). Besides, based on the same mineralogical type of aggregate, among the crushers utilized the sample crushed with impact crusher yields the highest U content; whereas the sample crushed with roll crusher yields the lowest U content. Also, LR sample failed to satisfy the minimum requirement of U content. Also similar conclusions can be made for Fig. 8. The flow coefficients of fine aggregates passing both No.5-200 and No.10-200 are given in Fig. 9. As seen in Fig. 9 the flow coefficients related to No.10-200 yield higher values (which are an indicator of higher angularity) compared to No.5-200 which demonstrates the effect of gradation on the flow coefficients of the samples. As expected regardless of the crusher type, basalt aggregates depicts higher flow rate than limestone aggregates. As presented in Fig. 9, for all type of aggregate, impact crushers yield highest flow rates. Unlike the results obtained from ASTM C1252, aggregates crushed with jaw crushers yield lowest flow rate. Flow rates for the samples crushed with roll crushers (R) are higher than the flow rates related to the samples crushed with jaw crusher (J). This is because the roll crushers produce more flat and elongated particles than the other type of crushers. Irregular aggregate particles lead blockages passing through the orifice of the funnel and also the more irregular particles the more air space under them. Similar conclusions can also be made based on Fig. 10 and 11. As seen in Fig. 10 and 11 aggregates crushed with Roll crushers have the highest flat and elongated particle values. Higher flat and elongated particle value is not desirable since the aggregate can be easily broken down during mixing with bitumen and in compaction process. 3.2. Surface Characteristics The calculated MTD values, MPD values analyzed with MATLAB program, and DFT20 values obtained from DFT performed on asphalt slabs involving only limestone, basalt aggregate as well as combination of basalt and limestone aggregate (mix type) can be seen in Fig. 12 and 13. −6− Fig. 12 MPD and MTD Values KSCE Journal of Civil Engineering Effect of Aggregate Shape on the Surface Properties of Flexible Pavement types of crushers. As a consequence, it is possible to consider that, the sharp and angular particles produce deep texture and profile values as compared to more rounded particles. 5. Dynamic friction tester is used to estimate the frictional properties of flexible pavements. The sample exhibiting higher texture and profile depth values also demonstrates higher DFT20 values. Regardless of the crusher type, basalt type aggregates display higher friction values compared to limestone aggregates. Based on the findings from DFT it can also be concluded that the aggregates crushed with impact crusher yield the higher friction values. Fig. 13. Measured Friction Coefficients at 20 km/h As presented in Fig. 12 and Fig. 13, regardless of the crusher type, the asphalt slab involving only basalt aggregate demonstrates the highest surface texture (MTD and MPD) and friction values. As expected the lowest texture and friction values are obtained from the slab involving only limestone aggregate. As depicted in figs, based on the slab involving the same type of aggregates, the slab compacted with aggregates crushed with impact crusher yields the highest texture and friction values. Besides, the LRP slab, exhibited the lowest surface characteristics among the other asphalt slabs. 4. Conclusions One of the most important properties of the pavement is surface texture which contributes to tire-pavement skid resistance. Pavement surface texture mainly depends on the aggregate shape and gradation. Based on the results obtained, the following conclusions can be with drawn. 1. In the light of findings from laboratory investigations, EN 933-6 and ASTM C1252 clearly designates the type of aggregate (whether basalt and limestone) and the type of crusher (whether the aggregate crushed with impact, jaw or roll crusher). 2. Basalt type aggregate exhibits higher angularity values compared to limestone aggregate which is crushed with same type of crusher. This is due to the mineral grain size and the abrasion resistance of the basalt aggregate. 3. Regardless of the aggregate type, a clear distinction between the crushers indicate that the aggregate crushed with impact crusher exhibits the highest angularity value whereas roll crusher displays the lowest angularity value. It is therefore necessary to use an impact crusher in order to achieve cubical particle shape. 4. Sand patch and 3D laser scanner are used to evaluate the texture properties of the flexible pavement types involving different aggregate types produced with different crushers. The slab prepared with basalt type aggregate yields higher MTD and MPD values compared to slab prepared with limestone aggregate. Besides, regardless of the aggregate type, the slab prepared with aggregate crushed by using impact crusher display the highest MTD and MPD values since impact crushers produce aggregates that exhibit higher angularity and more cubical particles compared to the other Vol. 00, No. 0 / 000 0000 References ASTM (1999). Standard test method for measuring pavement surface frictional properties using the dynamic friction tester, ASTM E 1911-98, Volume 04.03, ASTM, West Conshohocken, Pennsylvania. ASTM C1252 (1998). Standard test method for uncompacted void content of fine aggregate (as influenced by particle shape, surface texture, and gradaing), American Society for Testing and Materials, West Conshohoken, Philadelphia, PA. ASTM E 965-96 (1998). Standard test method for measuring pavement macro-texture depth using a volumetric technique, Vol. 04.03, West Conshohocken, Pennsylvania. ASTM E 1845-01 (2003). Standard practice for calculating pavement macro-texture mean profile depth, Vol. 04.03, West Conshohocken, Pennsylvania. Chelliah, T., Stephanos, P., Shah, M.G., and Smith, T. (2003). “Developing a design policy to improve pavement surface characteristics.” Paper presented at 82nd Annual Meeting of the Transportation Research Board, Washington, D.C.,USA. Do, M., Zahouani, H., and Vargilor, R. (2007). “Angular parameter for characterizing road surface microtexture.” Transportation Research Record, Vol. 1723, pp. 66-72. EN 933-6 (2001). European standard tests for geometrical properties of aggregates, Part 6, Assessments of Surface Characteristics, Flow Coefficient Of Aggregates. Ergun, M., Iyinam, S., and Iyinam, A. F. (2005). “Prediction of road surface friction coefficient using only macro and microtexture measurements.” Journal of Transportation Engineering, Vol. 131, pp. 311-319, DOI: 10.1061/(ASCE) 0733-947X(2005)131:4. Gardiner, M.S., Studdard, C., and Wagner, C. (2001). “Influence of HMA macrotexture on skid resistance.” Auburn University Civil Engineering Department. Henry, J. J. (2000). Evaluation of pavement friction characteristics, NCHRP Synthesis 291, National Cooperative Highway Research Program (NCHRP), Washington, D.C. ISO (2002). Characterization of pavement texture by use of surface profiles-Part 2, Terminology and Basic Requirements Related to Pavement Texture Profile Analysis, International Organization of Standardization. Masad, E., Rezaei, A., Chowdhury, A., and Harris, P. (2009). 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Development of new digital image analysis methods for determination of the geometrical properties of aggregates, PhD Thesis (in Turkish), Institute of Natural and Applied Science, Dokuz Eylul University, Izmir/Turkey. Wu, Z. and King, W. (2011). Development of surface friction guidelines for LADOTD, Report by Lousiana Department of Transportation, Report No: FHWA/LA.11/485. −8− KSCE Journal of Civil Engineering