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1 Construction and Building Materials 38 (2013) Contents lists available at SciVerse ScienceDirect Construction and Building Materials journal homepage: A study on volumetric versus surface properties of wearing courses F.G. Praticò a, R. Vaiana b, a University Mediterranea, Reggio Calabria, Italy b University of Calabria, Arcavacata Campus, Cosenza, Italy highlights " The volumetric and surface characteristics of HMA specimens were analyzed. " Gyratory compactor, roller compactor and laser profilometer were used. " A model for predicting surface texture and volumetrics was proposed. " The model is mainly a function of compaction effort and process. article info abstract Article history: Received 8 May 2012 Received in revised form 11 August 2012 Accepted 20 September 2012 Keywords: Surface characteristics Volumetric characteristics Laser profilometer Gyratory compactor Slab roller compactor The main purpose of this study was to analyze the volumetric and surface characteristics of hot mix asphalt (HMA) specimens as a function of compaction process. Specimens were produced in the laboratory by two different compaction devices, a gyratory compactor and a roller compactor. The volumetric and surface characteristics (air void content, bulk specific gravity) of these specimens, as well as the relationships among surface texture, volumetrics and compaction, were investigated. Analysis of these results may allow determinations of how material movements under compaction determine volumetrics distribution and variations and surface properties. A tentative theoretical framework for synergistically pursuing texture and volumetric targets was formulated. Outcomes of this study are expected to benefit both practitioners and researchers. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Pavement texture and volumetrics of hot mix asphalts (HMAs) play an important role in pavement management, expected pavement life, road safety, sustainability [1,2] and mechanistic performance [3]. The surface properties of sustainability (noise pollution reduction [4] and environmental impact [5,6]), efficiency (consumption reduction) and user safety are strongly influenced by the texture of the upper layer of wearing courses [7,8]. Indeed, texture, defined by ISO Standards as the deviation of a pavement surface from a true planar surface, is related to the accident ratio [9,10]. In particular, surface texture has been found to affect friction and its evolution over time [11 15]. At high wavelengths, texture is related to roughness [16,17], which affects user comfort and has an impact on the general cost of transportation. Furthermore, texture affects drainability [18] and therefore acceptance procedures [19]. Further research, however, is needed on the evolution of texture properties as a function of time Corresponding author. Tel.: ; fax: address: vaiana@unical.it (R. Vaiana). and/or compaction energy and type, both in the laboratory and on site [20]. HMA volumetric properties have been shown to depend on the relationship between G mm (maximum theoretical specific gravity) and G mb (bulk specific gravity). The specific gravity is the ratio of the weight in air of a volume of material at 25 C to the weight in air of an equal volume of water. HMA maximum specific gravity (G mm or the corresponding theoretical maximum density, termed Rice density) can be derived from the ratio of the weight of a loose sample to the weight of an equal volume of water at a standard temperature of 25 C [21]. G mm is dependent on G se (effective aggregate specific gravity), P b (asphalt binder content), and G b (specific gravity of the asphalt binder). Air voids (AVs) are determined from G mm and the bulk specific gravity (G mb ) of the compacted mixture. Both in situ and laboratory compaction processes have been shown to affect the mechanical, volumetric and surface properties of HMAs. Among the important parameters involved in and affected by compaction processes are reduction of air voids; the internal structure of samples [22 25]; transport of asphalt binders; re-orientation and segregation of aggregates [26,27]; /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.

2 F.G. Praticò, R. Vaiana / Construction and Building Materials 38 (2013) re-organization of aggregate-bitumen matrices; specimen surface, shape and morphology; and, consequently, surface texture, friction properties and other surface performance parameters, including drainability and noise emissions. Laboratory procedures are important in the determination of volumetrics. In contrast, on site procedures and methods, even if very useful from a management standpoint, do not present a comparable level of reliability and accuracy [28,29]. The same concept can be extended to the measure of surface texture [20]. Although there have been many studies investigating the volumetric and surface properties of HMAs, many issues require further research, including those related to the relationships among compaction level, texture spectrum and specific gravity. A unifying theoretical framework would be of great interest for enhancing our understanding of how to improve mix composition and compaction to optimize both surface and volumetric properties. Due to sample inhomogeneity and boundary factors, it has become more difficult to formulate a comprehensive theoretical model able to explain texture and volumetrics variations. 2. Research objectives and scope The main purpose of this research was to analyze the volumetric and surface characteristics of HMA specimens as a function of the compaction process. Based on the relevance of these characteristics over the entire boundary surface of a specimen and to determine sample inhomogeneity, both the upper and lower surfaces were investigated. HMA specimens were produced in the laboratory using two different compaction devices, a gyratory compactor (GC) and a Unical slab roller compactor (USRC). This paper is organized as follows: The above introduction describes the importance of pavement bulk and surface performance. The experimental program is described in the next section. Results are presented and discussed and several equations are proposed. Finally conclusions are drawn. 3. Experiments and results 3.1. Experimental program The experimental program was developed at the Department of Territorial Planning, University of Calabria (Italy), and is summarized in Fig. 1. Two compaction devices were used, a GC (Table 1) and a USRC (Table 2). The USRC device is a mechanical, self-propelled smooth steel roller with forwardreverse control, designed according to the standard UNI-EN The slabs were compacted at different pressures and numbers of passes until the desired height of the slab was reached. The compaction parameters chosen for both devices are summarized in Tables 1 and 2. Initially, only gyratory samples were produced, at three different levels of compaction, corresponding 10, 60, and 200 gyrations, respectively. In the second phase, the specific gravity of the gyratory specimens was derived for each level of compaction. Starting from these values, 3 cm- and 6 cm-thick slabs were produced by varying the weight of the material. A schematic illustration of the experimental program is shown in Fig. 1. Overall, 36 samples were produced (see Table 3): 12 Gyratory compacted specimens (6 for the bituminous mix herein termed MA and 6 for the bituminous mix herein termed MB, see Section 4.1); 24 Roller compacted specimens (12 for MA and 12 for MB). At the end, the actual density of each slab sample was calculated to verify whether the fixed %G mm at each level of compaction was reached. Each GC specimen was cut with a wet saw into three parts (bottom, center, top) to separate the top and bottom parts (see Fig. 2). The slabs ( h cm) were cut and divided into 9 sectors. The central zone measured 15 cm 15 cm and only the top surface of this compacted zone was analyzed (see Fig. 2). The central part of the 6 cm-thick slabs was divided into two parts (top and bottom, each 3 cm thick). Table 4 summarizes the tests performed (see also Fig. 2). In reference to texture measurements, MPD (mean profile depth), ETD (estimated texture depth), and RMS (root mean square) are indicators referring to surface texture (for microtexture wavelengths of mm), regardless of texture wavelength. In contrast, L t is the texture level for a given wavelength k. Note that the lowest wavelength that can be measured depends on the minimum step of the device and that the latter interacts with the diameter of the spot of the laser device. Therefore, it is relevant that the highest frequency that can be represented correctly by a sampled signal (f) is half the sampling frequency (f s ) and that, if the signal contains frequency components above the Nyquist frequency (i.e., above f s /2), these will be misinterpreted as lower frequencies in the spectrum of the sampled signal, a phenomenon known as aliasing. Furthermore, the required evaluation length depends on the frequency analysis to be performed. For one-thirdoctave bands, the evaluation length (l) must be at least (5 to) 15 times k max, where k max is the longest (one-third-) octave-band-center wavelength used in spectral analyses. These requirements imply that the octave-band levels, or one-third-octave band levels, determined under these evaluation lengths will be within a 95%-confidence interval of approximately ±3 db of the true band levels (ISO/TS :2008(E)). By referring to volumetrics, and hypothesizing that AV = 0, G mm will represent the specific gravity of a mixture. G mb would therefore represent the actual specific gravity (6G mm ) corresponding to the actual air void content, AV (P0). Despite G mm, the definition of which is independent of the state of matter of components (bitumen liquid; aggregates solid; air gas), G mb and AV will be affected by the actual characteristics of the bituminous mixture. Thus, for a given aggregate gradation and bitumen content, the lowest AV can be appreciably greater than 0 and the highest G mb much lower than G mm, regardless of the level of compaction. Furthermore, P b indicates the content in terms of liquid phase, while G sb provides a measure of aggregate density, with both affecting compaction performance Materials The first step was to select two different asphalt mixes and to produce gyratory compacted samples and slab specimens in the laboratory at different energy levels and at two different thicknesses. Table 5 shows aggregate gradation and the main aggregate and asphalt binder properties for both of the mixes studied. The two mixes had the same gradation (BRZ, below the restricted zone), but were composed of different types of aggregate, limestone for MA and limestone + basalt for MB. The aggregate blend used for MB contained 30% by mass of basaltic material (aggregate size > 5 mm). In contrast, only limestone aggregate was used in the MA mix, with a nominal maximum aggregate size of 9.5 mm Gyratory compacted specimens Figs. 3 7 and Table 6 summarize results and analyses performed on the gyratory compacted specimens. Eqs. (1) (5) refer to the proposed model. The volumetric properties of GC specimens are summarized in Fig. 3. In the top panels, the x-axes indicate air void content and the y-axes indicate the number of gyrations (N). In the bottom panels, the x-axes indicate the number of gyrations and the y-axes indicate %G mm. These results indicate that: Increased compaction energy (number of gyrations) resulted in increased %G mm and decreased air void content. The center of each specimen was always denser than its top and bottom. However, the cutting process may have caused appreciable changes in the top and bottom surfaces of the central part of the specimens [28]. Furthermore, X-ray computed tomography and image analysis techniques showed similar reductions in air void content [23]. A study of air void homogeneity using gammaray measurements indicated that the maximum compaction level was obtained approximately 5 cm from the surface (i.e., at the center of the specimens) [31]. The air void content was always higher at the top than at the bottom of specimens. The air void content of the MA mix was similar to that of the MB mix. Fig. 4 illustrates texture levels (L T (k), or L TOP L BOT, y-axis) as a function of wavelength (k, mm, x-axis). An increase of around 20 db was observed for transitions around wavelengths of mm, regardless of the number of gyrations and of the position at the top or bottom (see Fig. 4a d). Moreover, regardless of mix, a higher number of gyrations yielded lower levels of texture, especially in the domain of macrotexture (wavelengths in the range mm, see Fig. 4a d). Top surfaces usually had higher texture levels than bottom surfaces, for each wavelength in the macrotexture domain (Fig. 4e). In this domain, the difference L TOP L BOT ranged from 1 to 5.

3 768 F.G. Praticò, R. Vaiana / Construction and Building Materials 38 (2013) GC specimens USRC slabs %G %G %G %G %G %G Cutting into 3 parts 3 cm 6 cm Cutting into 2 parts TOP CENTER BOT TOP BOT GC specimens Texture and volumetric tests Top surface, TS Top (2.5 cm) 3 cm Top surface Center (3 cm) Bottom(2.5cm) Bottom surface, BS USRC slabs 6 cm Top surface, TS Top (3 cm) Bottom (3 cm) Bottom surface, BS Legend GC: gyratory compactor; USRC: Unical Slab Roller Compactor; %Gmm@N: level of compaction at a given number of gyrations. Top, center, bot(tom): the three parts of a cylindrical specimen compacted by a GC; USRC slabs: 3cm- or 6cm-thick slabs, compacted by the roller compactor USRC. Fig. 1. Schematic diagram of the experimental program for the two mixtures. Table 1 Compaction parameters for the gyratory compactor (GC). Gyratory compactor (GC) Parameters Values Reference Compaction temperature 165 C UNI EN Sample height >114 mm Rotation speed 29.8 rpm Hand punch diameter 150 mm Inclination 0.84 Pressure 600 kpa Finally, Fig. 4g (below, right) compares our results with those reported previously [32 34] on gyratory samples, on laboratory compacted slabs and on site, for the same nominal maximum aggregate size (open-graded mixes not included). Dotted lines refer to the minimum and maximum value for each wavelength. At wavelengths of mm, the texture levels of GC specimens had first derivatives around 8 and intercepts around 24 (natural logarithm regression curves). The variability of data was represented by the error bars, which indicate the percent uncertainty in the reported measurements. The graph shows that GC values were fairly low at lower wavelengths, but at higher wavelengths were within the minimum to maximum value reported in the literature. Previous studies have reported first derivatives around 4 6 (i.e., smaller) in the same range of wavelengths. The texture level for a wavelength of 5 mm was found to be well correlated with the noise level measured by the cpx-method (close proximity method, see also [32]). Fig. 5 shows the relationship between AV and macrotexture, in terms of MPD iso, MPD piarc, ETD, and RMS. The mean profile depth, MPD (and, as a consequence, the estimated texture depth, ETD) was derived according to both PIARC and ISO methods. The PIARC method is based on the difference between peak and average levels (average z), whereas in the ISO method the profile is divided into two parts and the average peak level is determined. An analysis of texture indicators showed that: Regardless of mix (MA or MB) and specimen vertical position (i.e., top or bottom), macrotexture increased when AV increased. GC compacted specimens usually had rougher top than bottom surfaces. Furthermore, lower first derivatives were obtained for MA mixes (without basalt).

4 F.G. Praticò, R. Vaiana / Construction and Building Materials 38 (2013) Table 2 Compaction parameters for the Unical slab roller compactor (USRC). Unical slab roller compactor (USRC) Parameters Values Reference Compaction temperature 165 C UNI EN Sample height 30 mm or 60 mm Horizontal dimensions of samples 405 mm 305 mm Passage series 1 7th Pressure MPa Number of passes 2 10 Table 3 Summary of samples produced by the GC and USRC devices in experimental survey. Mixes MA MB Devices GC USRC GC USRC Specimens 15 cm 3 cm-thick Slab 6 cm-thick slab 15 cm 3 cm-thick slab 6 cm-thick slab Levels of compaction %G %G %G Fig. 2. Gyratory compactor specimens (above) and USRC slab samples (below). For both mixes, the macrotexture depth was lower for the bottom than for the top surface. This finding was also observed during texture spectral analysis. Indeed, the difference in texture level between top and bottom was generally positive and higher in the range of macrotexture wavelengths (k > 0.5 mm), see Fig. 4e. The macrotexture depth was generally higher for MA than MB. This difference was minimal only at high air void contents (lower level of gyrations). Based on these findings, our volumetric analysis showed that the internal structure of the gyratory compacted specimens changed with the depth of the sample (see also [15,22,24]). The vertical distribution of air void content confirmed the above differences when top, center and bottom parts were considered. Regardless of the type of mix, position (top or bottom) and wavelength, a lower level of compaction (e.g., the number of gyrations) was associated with a higher texture level. Furthermore, (a) a greater number of cycles/gyrations was associated with a higher %G mm ; (b) a greater level of compaction was associated with a lower macrotexture; and (c) the method of compaction greatly affected the level of compaction. In other words, even if G mm expresses an intrinsic property of the mix (P b, G b, PA, and G se ), %G mm and its derivatives were method-dependent. In analyzing and understanding the above results, it is relevant to discuss the theoretical and practical thresholds of G mm, AV and macrotexture aggregate indicators (e.g., MPD). G mm is dependent on asphalt binder content (P b, by weight of mixture), its specific gravity (G b ), aggregate content (1 P b, by weight of mixture), and effective specific gravity (G se ). Therefore, a theoretic interpretation of G mm is independent

5 770 F.G. Praticò, R. Vaiana / Construction and Building Materials 38 (2013) Table 4 Summary of tests. Laser profilometer scanning Method/devices/standards: Three profiles, i, j, k, were measured for the top and bottom surfaces of the GC samples, each 120 degrees from the other. Five profiles (1 5) were measured on the top of the USRC slab central samples along the same direction as compaction. From profile analyses, aggregate and disaggregate texture indicators were derived (MPDiso, MPDpiarc, ETDiso, RMS, L t, see [11]; ISO ; ISO/CD TS ). Profiles were determined as (x,z) coordinates, where z represents profile depths. A laser profilometer based on conoscopic holography was used. The device has the following characteristics (ISO ): (i) Mobility: stationary, slow; (ii) Texture wavelength range: Range covered BD class mm; (iii) Pavement contact: Contactless devices; (iv) Principle of operation: laser profilometer; (v) Objective focal length: 100 mm; (vi) Maximum vertical measuring range: 35 mm; (vii) Vertical resolution for class mm: mm; (viii) Stand-off distance: 90 mm; (ix) Minimum horizontal resolution Dx (sampling interval) BD for class mm: 0.01 mm; (x) Angle coverage: 170 Maximum specific gravity of mixture, G mm Method/devices/standards: Corelok device, Weighing Station, AASHTO T209 and Corelok method Bulk specific gravity, G mb, and air void content, AV Method/devices/standards: ASTM D /AASHTO T (2008); AASHTO T (2007); ASTM D Note that G mb, G mm % G mb /G mm 100 Asphalt binder content (P b ) and aggregate bulk specific gravity (G sb ) Method/devices/standards: P b : UNI EN G sb : MoDOT TM 81-AASHTO T85/T85 Aggregate gradations and aggregate shape angularity Method/devices/standards: SC, FC, EC, UNI EN 933-3/4 Table 5 Aggregate and asphalt binder properties for MA and MB mixes. Aggregate size d > 5 mm Limestone Basalt References Shape coefficient (SC) UNI EN 933-3/4 Flakiness coefficient (FC) Elongation coefficient (EC) Binder (hard modified) MA MB References Asphalt Binder content of mixture - P b (%) 5.80% 6.06% UNI EN Binder content on aggregates-p b (%) 6.15% 6.46% Softening point ( C) 74 UNI EN 1427 Penetration (0.1 mm) 50 UNI EN 1426 Penetration index 0.44 [30] of the aggregates being composed of solid (and not liquid) particles. In contrast, G mb, AV and MPD% are dependent on the actual characteristics and internal structure, including aggregate gradation. AV is related to VMA (voids in mineral aggregate), with VMA depending on many factors [35], including aggregate gradation (dense gradations decrease VMA), aggregate shape (more rounded aggregates decrease VMA), aggregate texture (smooth or polished aggregates decrease VMA), asphalt absorption (increased asphalt absorption results in lower effective asphalt content and lower VMA, for the same level of compaction), dust content (higher dust contents increase surface area, decrease film thickness, and tend to lower VMA), baghouse fines/generation of dust (increased fines and dust increase surface area, decrease film thickness, and tend to lower VMA), plant production temperature (higher plant production temperatures decrease asphalt binder viscosity, resulting in more asphalt absorption, lower effective asphalt binder and lower VMA), temperature of HMA during paving (higher temperatures during paving create softer mixtures, reduce air voids, and lower VMA), hauling time (longer hauling times allow for increased asphalt absorption, lower effective asphalt content and lower VMA), and aggregate handling (more steps in aggregate handling increase the potential for aggregate degradation, resulting in an increase in fines, and lower VMA). If VMA is expressed as a function of asphalt binder contents (i.e., the aggregate gradation remaining constant), it has a minimum around for asphalt binder contents around 4.5 6%. Superpave requires a minimum allowable VMA of as a function of the nominal maximum aggregate size (in the range mm). These findings indicate that a theoretical range 0, 1 cannot be applied to air void content (where AV is expressed in decimal form). The same concept can be extended to macrotexture aggregate indicators. Indeed, for a given composition, and

6 F.G. Praticò, R. Vaiana / Construction and Building Materials 38 (2013) Fig. 3. Differences in AV and %G mm contents among gyratory specimens. Fig. 4. Texture spectra and compaction effort. Fig. 5. Surface texture versus AV for MA and MB mixes, Top and bottom specimens.

7 772 F.G. Praticò, R. Vaiana / Construction and Building Materials 38 (2013) regardless of the energy of compaction, the lowest AV will be mix-specific and, in general, different from 0. The upper limit of 1 %G mm will therefore be independent of both mix compaction and aggregate gradation. For each mix composition (P b, G se, and G b ) and set of boundary conditions (temperature and compaction level), it seems reasonable to assume a minimum MPD and a minimum AV different from 0, as the number of gyrations tends to become infinite. Based on the above hypotheses, we have formulated Eqs. (1) (5). Air void content AV can be expressed as: AV 100 ¼ VMA V T 100 V be ¼ VMA 100 P beg mb 100 G b ð1þ In Eq. (1), V be is the effective binder volume, i.e., the volume of bituminous binder external to the aggregate particles and not absorbed into the aggregate, while V T is the total volume, i.e., the bulk aggregate volume and the effective binder volume. VMA (voids in mineral aggregate) and G mb (mix bulk specific gravity) will be affected by the level of compaction, with P be referring to effective bitumen content and G b to bitumen specific gravity. Thus, for a given P b (and P be ), it is possible to define a minimum, mix-specific air void content, AV, herein termed AV mms, as: AV mms ¼ lim N!1 V a ¼ VMA mms P beg mb G b 0; ð2þ where VMA mms is the corresponding value of VMA. Similarly, a minimum AV will be associated with a minimum MPD, herein termed MPD mms. Eqs. (3) (5) were formulated to represent and describe the dependence of %G mm, %MPD and %AV on the number of gyrations: %G mm 100 ¼ G mb G mm ¼ a b e N s a þ b e N s %MPD mms ¼ MPD mb ¼ a þ b e N s 100 MPD mms a b e N s %AV mms ¼ AV mb ¼ a þ b e N s 100 AV mms a b e N s where N is number of gyrations; a, b, and s are regression coefficients (with a > b and s > 0); e is Napier s constant; MPD mms is the minimum mix-specific profile depth; AV mms is the minimum mix-specific air void content; and G mm is the maximum theoretical specific gravity. MDP mb and AV mb can be easily derived from Eqs. (4) and (5), respectively (see also Fig. 6), with both referring to the level of compaction pertaining to G mb. Eqs. (3) (5) were applied to our data, where the values for MA and MB were averaged. Table 6 summarizes the best-fit regression coefficients. The theoretical ranges of the variations in Table 6 were derived from the above hypotheses (see also Fig. 6). Fig. 6 illustrates how %G mm (6a and b), AV (6c and d, i.e., AV mb as in Eq. (5)) and MPD (6e and f, i.e., MPD mb as in Eq. (4)) vary as a function of the number of gyrations. Note that Fig. 6 compares experimental data and models (Eqs. (3) (5)). The quotient max/min, a measure of compaction susceptibility, ranged from 1.1 (%G mm ) to 1.5 (%MPD mms ) to 8.45 (AV mms ). From a practical standpoint, these findings indicate that small variations in MPDs correspond to appreciable variations in ð3þ ð4þ ð5þ Fig. 6. Volumetrics of GC specimens as a function of the number of gyrations.

8 F.G. Praticò, R. Vaiana / Construction and Building Materials 38 (2013) Fig. 7. Densities of compacted slabs. Table 6 Best fit coefficients for Eqs. (3) (5). Independent variable Top Bottom a b s MPD mms /AV mms Range a b s MPD mms /AV mms Range %G mm %MPDmms %AVmms Notes: Dependent variable = N; %G mm, %MPD mms, %AV mms are expressed in decimal form. AVs. Based on MPD and AV target values, the above equations can be used to predict the two corresponding optimal levels of compaction and to determine whether there is a suitable intermediate compaction level Slab roller compacted specimens Figs summarize the results for HMA slabs. Fig. 7 shows AV and %G mm as a function of compaction level. Two different types of mixes (MA and MB) and three different types of slabs (3 cm, 6 cm top, 6 cm bottom, see Figs. 1 and 8) were considered. We found that. Regardless of mix (MA or MB), the initial air void distribution (@10) was fairly even. In general the maximum difference was 1%. Again, regardless of mix (MA or MB), when compaction energy was increased the 3-cm slabs and 6-cm bottom slabs maintained differences of 1% relative to target value (AV). In contrast, the 6-cm top slabs showed an increase in the difference to 2.5%, suggesting that the distribution of the air void content across the slab was uneven. The bottom of the middle part was always more compacted because the forward-reverse system of passes pushed the material downwards. Fig. 8. Compacted slabs (after sawing). Based on a survey of the slabs, it is likely that the material was shoved sideways under the roller passes: at the end of compaction the slab was more compacted in the middle than in the lateral zones. The bottoms of the 6-cm-thick slabs were denser than their tops, regardless of the number of cycles. Fig. 9 shows AV versus macrotexture plots. Regardless of mix (MA or MB) and slab thicknesses, the macrotexture increased when AV increased (i.e., from right to left in Fig. 8). By comparing the surface textures of 3-cm and 6-cm thick slabs, we found that the MPD (AV) curves were always steeper for the 6-cm thick slabs (i.e., the first derivative was higher). These findings illustrate a condition in which thicker slabs undergo higher MPD variations for the same AV variations. Fig. 10 illustrates the texture spectra for the two mixes under consideration (MA, MB) and for the two thicknesses (3 cm and 6 cm). Different levels of compaction were considered. Higher levels of compaction yielded lower texture spectral levels, regardless of mix type (A or B), texture wavelength and slab thickness (3 or 6 cm). In more detail, the effect of compaction on texture levels increased at higher wavelengths, at lower thicknesses and for the mix MA. On average, for a given wavelength in the macrotexture range, the following relationship can be derived: L f ¼ L i k t h At a given texture wavelength, k, L f indicates the final level of texture (e.g., %G L i refers to the corresponding initial texture level (e.g., %G k and h are coefficients, and t refers to the specimen thickness (cm). These parameters depend on HMA and compaction characteristics. For the cases under consideration, h ffi 2 and k ffi 75. Finally, Fig. 10g compares our results with previous findings [32 34]. The dotted lines refer to the minimum and maximum values for each wavelength, while the solid line refers to the averages at the final compaction level (@200). At wavelengths of mm, the texture levels of slabs had first derivatives around 7 and intercepts around 23 (natural logarithm regression curves). The texture levels of the slabs were thus fairly low over the entire spectrum of wavelengths in the considered range. ð6þ

9 774 F.G. Praticò, R. Vaiana / Construction and Building Materials 38 (2013) Fig. 9. Compacted slabs: macrotexture versus AV or%g mm. Fig. 10. Compacted slabs: texture level versus compaction level. 4. Conclusions Previous studies have shown the importance of understanding how laboratory compaction techniques can determine the organization of the internal structure of samples. Indeed, compaction affects pavement management, expected life, road safety. Few studies to date have analyzed the effects of compaction methods on volumetrics and surface properties, and further research is needed on the terms of relationships among compaction level, texture spectrum and specific gravity. As a consequence, in this study the volumetric and surface characteristics of HMA specimens as a function of compaction process were examined. Two different laboratory compaction devices, a GC and a USRC were used, and the degree to which material movements under compaction determine volumetrics distribution and variations and surface properties was investigated. Several key-factors emerged by the analysis and interpretation of our data. At a given level of compaction, the thickness of the sample affects volumetrics and texture spectrum and their related major consequences, including the expected life of the pavement, safety, and pavement management. In jointly considering volumetrics and surface texture as the main issues in mix design, it is useful and logical to use regression curves in which the upper and lower limits take into account the relevant role of mixture composition (aggregate gradation, etc.). Even if the transferability of inferences from laboratory tests and methods to on site tests and methods may be limited, our theoretical approach provides a tentative framework of reference for practical applications. The results of this study can be used in the design, construction and quality control of asphalt mixtures, targeting, for example, surface macrotexture and air void content. Several issues emerged that call for further research. Nonetheless, based on the present results, it seems imperative that specific guidance for the construction of surface courses be provided to minimize the potential for differences in surface and volumetric performance. Further enhancement of the spectral analysis of micro- and macrotexture partitions on the upper and bottom surfaces is recommended. The outcomes of this study are expected to benefit both practitioners and researchers. Acknowledgments The authors wish to acknowledge Eng. Francesco De Masi (Road Material Laboratory Technician, University of Calabria), Eng. Teresa Iuele and Eng. Vincenzo Gallelli for their contributions during experimental stages. References [1] Romanoschi SA, Metcalf JB. The evaluation of the probability distribution function for the life of pavement structures. J Transport Res Board 2000;1730: [2] Gubler R, Partl MN, Canestrari F, Grilli A. Influence of water and temperature on mechanical properties of selected asphalt pavements. Mater Struct/Mater et Constr 2005;38(279): [3] Canestrari F, Santagata E. Temperature effects on the shear behaviour of tack coat emulsions used in flexible pavements. Int J Pavement Eng 2005;6(1): [4] Anfosso-Ledee F, Do MT. Geometric descriptors of road surface texture in relation to tire-road noise. Transportation Research Record: Journal of the

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