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Evaluation of functional characteristics of laboratory mix design of porous pavement materials
Construction and Building Materials 191 (2018) 281–289
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Evaluation of functional characteristics of laboratory mix design of porous pavement materials L. Chu a, B. Tang a, T.F. Fwa b,c,⇑ a
School of Highway, Chang’an University, South Erhuan Middle Section, Xi’an 710064, China Chang’an University, China c National University of Singapore, Singapore b
h i g h l i g h t s Current pavement mix design methods inadequately address functional requirements. Laboratory tests are proposed to address functional requirements during mix design. The proposed tests address drainage, skid resistance and noise related properties. Test data allow evaluation of drainage capacity and skid resistance of mix design. Test data also allow prediction of tire-pavement noise of mix design.
a r t i c l e
i n f o
Article history: Received 11 April 2018 Received in revised form 14 August 2018 Accepted 1 October 2018
Keywords: Porous pavement Porosity Permeability Skid resistance Sound absorption Tire-pavement noise
a b s t r a c t Porous pavements are designed and constructed today to provide one or more of the following functional benefits: lower traffic noise, improved wet-weather driving safety, lower stormwater peak load, and replenishment of groundwater supplies. However, depending on the mix design and material properties of a porous pavement, not all these benefits can be achieved fully by the porous pavement. The factors that affect such functional benefits include the porosity, the properties of binder and aggregates, and the binder content and aggregate gradation of the porous mixture. Currently no agencies have specified any laboratory test procedure for pavement engineers to evaluate the functional characteristics during the mix design phase. Such laboratory procedures are useful in practice for porous pavement design. This study demonstrates that a set of laboratory tests based on currently available equipment and methods, along with related functional assessment analysis, can be performed to meet the purpose. The following laboratory tests on the design mix are proposed in this study: (i) Permeability test. (ii) 3dimensional scanning of surface texture, (iii) Skid resistance test, and (iv) Sound absorption test. The test data may then be used for drainage capacity analysis, wet-weather skid resistance assessment, and tirepavement noise analysis. The proposed laboratory test procedures are illustrated using two porous pavement materials. The test results show that practically useful information on functional characteristics of a design porous mix can be obtained using the proposed procedures. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Porous pavements consist of one or more layers of high porosity to provide one or more of the following functional benefits: lower traffic-generated noise [1–3], improved wet-weather pavement skid resistance [4–6], lower peak flow load of stormwater runoff [7,8], and replenishment of groundwater supplies [9,10]. Other ⇑ Corresponding author at: School of Highway, Chang’an University, South Erhuan Middle Section, Xi’an 710064, China. E-mail address: [email protected] (T.F. Fwa). https://doi.org/10.1016/j.conbuildmat.2018.10.003 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
associated benefits include reduced splash and spray, and improved visibility of pavement markings on rainy days, as well as higher average traffic flow speeds during wet weather [10,11]. However, depending on the mix design and properties of the porous materials, not all of the above-mentioned benefits can be achieved in a given porous pavement. As will be shown later in the review of current porous mixture mix design practices, the standard laboratory mix design procedures today do not adequately evaluate the functional characteristics of a porous mixture design during the mix design phase. The present study was motivated by the fact that there already exist
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in the literature sufficient knowledge and available test methods and analysis techniques to obtain the required data that can be used to estimate the desired functional characteristics of a porous pavement constructed with a given design mix. It is demonstrated in this study that using currently available test equipment and analysis methods, data can be generated to predict the functional performance of a porous pavement in the following three aspects: its drainage capacity, its wet-weather skid resistance, and the tirepavement noise generated under traffic. The following laboratory tests on a design mix are proposed: (i) Permeability test (ii) 3-dimensional scanning of surface texture, (iii) Skid resistance test, and (iv) Sound absorption test. The test procedure, the measured data obtained from each test, and how the measured data can be used to predict the desired functional performance are explained in this paper. To illustrate the concept and working of the proposed laboratory test procedures, the results of tests performed on two porous asphalt design mixes are presented and explained. The test data produced from the tests serve to demonstrate that the proposed tests were able to differentiate different functional performances of different porous mix designs.
2. Review of current porous mixture mix design practices Various forms and designs of porous pavement have been used in North America and Europe for more than 50 years [12– 14]. Two important aspects of the mix design of a porous pavement material are that the high-porosity mixture should be (i) structural sound and durable, and (ii) able to maintain its functional benefits over the entire design life. The National Center for Asphalt Technology, a leading asphalt research center in North America, has developed a mix design procedure for OGFC (open-graded friction course) mixes which are a common form of high-porosity mixture used in North America [12]. The mix design procedure consisted of 5 steps: materials selection, formulate trial aggregate gradations, determination of optimum gradation, determination of optimum binder content, and moisture susceptibility evaluation. In the Texas PFC (porous friction course) mix design method, specimens at the selected optimum binder content were evaluated for draindown, moisture susceptibility and durability [15]. In Alabama, a minimum tensile strength ratio (TSR) was also specified in addition to draindown test [16]. More recently, an improved OGFC mix design procedure was recommended to the California Department of Transportation (Caltrans) [17]. The mix design procedure included two primary components: Volumetric design and performance testing. The first component was similar to the first 4 steps of the NCAT procedure. The performance testing in the second component comprised Cantabro, draindown, and Hamburg wheel tracking testing [16]. In Europe, the mix design of porous asphalt is governed by the European standard EN 13108-7:2016 [18]. The European standard specifies requirements for the constituent materials as well as the porous mixture. In its latest 2016 edition, the standard has included the following test requirements for the mix design: (i) Water sensitivity assessed using either indirect tensile strength ratio or compression strength ratio [19]; (ii) Resistance to abrasion by the Cantabro wear test [20]; (iii) Resistance to permanent deformation by wheel-tracking test [21]; (iv) Polishing resistance by the Wehner and Schulze method [22]; (v) Binder drainage by either the basket or beaker method [23]; (v) Permeability by a constant-head permeameter [24]. It is significant to note that, in contrast with North American practices, the latest European standard has incorporated some functional performance requirements into the mix design of por-
ous pavement. This is reflected by requirements (iv) and (v) in the preceding paragraph. These additional functional requirements represent a recognition of the need to incorporate functional requirements into porous pavement mix design. However, the tests in requirements (iv) and (v) alone are inadequate to address the main functional benefits of porous pavements. A more complete set of tests and analysis is proposed in the present study, as explained in the next section, to cover the functional requirements of porous pavements.
3. Concept and scope of study The main objective of the present study was to develop a set of laboratory procedures to test a mix design for the purpose of obtaining the material properties required for evaluating pavement functional characteristics based on mechanistic methods. Compared with predictions using empirical statistical regression models, improved and more accurate predictions of functional performance of pavement mix designs can be achieved with the proposed procedure. This is because mechanistic prediction models for skid resistance and tire-pavement noise are now available. Specifically, the proposed laboratory tests would be used for the prediction of the following three functional properties of a porous pavement: Surface runoff drainage capacity of the porous pavement layer; Wet-weather skid resistance properties of the porous pavement; and Tire-pavement noise. Fig. 1 presents a flow diagram that shows the proposed additional laboratory tests (i.e. Phase II in the figure) to be included during the mix design phase of a porous pavement mixture. With these data, analyses can be performed to estimate the expected functional performance of the mix design. To estimate the drainage capacity of a porous pavement layer, one needs to determine the permeability coefficient of the porous mix considered. This drainage property, together with pavement surface texture characteristics (both microtexture and macrotexture of the porous pavement surface), are required for assessing the wet-weather skid resistance performance of the porous pavement. Finally, by measuring the sound absorption coefficient of the porous pavement material, the expected tire-pavement noise can be predicted by means of a computer simulation model using the sound absorption coefficient and pavement surface texture data as input. The rationale and test procedures for the various laboratory tests proposed in this study are explained in the next section.
4. Proposed laboratory tests: Rationale and test procedures 4.1. Permeability test of porous mixture The current standard permeability tests for testing cohesive or cohesionless materials are not suitable for measuring the permeability coefficients of porous pavement materials. For instance, ASTM D5084-16 [25] is applicable only for materials with permeability coefficients less than 10-5 m/s, which is much lower than those of common porous pavement materials; while ASTM D2434-68(2006) [26] and AASHTO T215-14 [27] both assume laminar flow that follows Darcy’s Law as follows,
m ¼ ki where
ð1Þ
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Fig. 1. Proposed framework of enhanced mix design to include functional characteristics evaluation.
4.2. Characterization of microtexture and Macrotexture of porous mixture
v = specific discharge, m/s k = coefficient of permeability, m/s i = hydraulic gradient, m/m. Researchers have found that the assumption of laminar flow does not hold in permeability measurement of a pervious coarsegrained material, and that Darcy’s Law is not valid unless the hydraulic gradient is less than about 0.05 [28,29]. Several modifications to Darcy’s equation have been made by researchers to describe the non-laminar flow condition in pervious granular materials [29,30], and the following form were found to describe permeability test results closely [31,32]:
m ¼ ki0:7
ð2Þ
where the permeability coefficient k can be determined experimentally. Both constant-head and falling-head tests have been used to determine the permeability coefficient [31–33]. However, due to the high permeability coefficient of porous pavement materials, which are of the order of 1 to about 20 mm/s, as well as the nonlaminar characteristics of the flow, a falling-head test has been found to be a more suitable experiment to be performed in the laboratory [31]. Fig. 2 shows a laboratory falling-head test set-up for permeability coefficient measurement. It should be noted that to ensure successful permeability testing of a falling-head apparatus, it must be equipped with a high precision sensor capable of detecting changes of as small as 0.3 mm in water level, and a high speed data acquisition unit that can record 100 or more data points per second [31].
Surface texture characteristics of a porous pavement are a key factor that has major impacts on both the skid resistance development and tire-pavement noise generation mechanisms of the pavement. It is generally believed that pavement surface texture affects the development of wet-pavement skid resistance in two major ways [24,34]: (i) Generation of skid resistance through tirepavement adhesion when a tire comes water trapped at tirepavement interface due to inefficient drainage. The former is a function of the microtexture properties of the porous mixture concerned, while the latter is affected by the ability of the mixture’s macrotexture to discharge pavement surface water from the tirepavement contact area. Pavement surface microtexture and macrotexture also contribute to tire-pavement noise in different ways. While pavement surface microtexture generates tire-pavement noise by means of friction- or adhesion-related stick-slip and stick-snap mechanisms, its macrotexture creates tire-pavement noise through other mechanisms, such as air sucking, air pumping, air resonant and horn effect, and vibrations of tire walls [35]. Hence, quantitative characterization of surface texture is necessary in the evaluation of functional characteristics of a porous mixture design. Due to the different impact mechanisms of pavement microtexture and macrotexture on the functional characteristics of porous mixtures, the two forms of surface texture are often quantified and represented in different ways, as explained in the following two subsections.
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Fig. 2. Laboratory set-up for falling-head permeability testing of porous materials.
4.2.1. Characterization of microtexture The microtexture of pavement surface has been defined as the irregularity with wavelengths below 0.5 mm, and deviations of less than 0.2 mm from a true planar surface, measurable only by means of photomicrography [31,36]. However, studies on the effects of microtexture using its geometric characteristics have been very much restricted to establishing statistical relationships with pavement skid resistance. Such relationships are applicable to a limited scope of applications in terms of pavement materials and mix design [36,37]. As a result, low-speed measurements of skid resistance are often used to represent the effect of microtexture because it is the dominant factor influencing skid resistance at low speeds [36,38]. This is also the approach adopted in the present study, as described later under Section 4.3. The effect of microtexture on tire-pavement noise is considered to be minor compared with that of macrotecture, and is usually ignored in tire-pavement noise analysis [39]. 4.2.2. Characterization of Macrotexture Pavement surface macrotexture is commonly referred to as the visible irregularity having wavelengths between 0.5 and 50 mm and amplitudes from 0.1 to 20 mm, and is attributed to aggregate size, shape, angularity, spacing and gradation [31,36]. It governs the drainage of pavement surface water under a skidding tire, and is chiefly responsible for the deterioration of pavement skid resistance when vehicle speed increases. The standard methods of characterizing pavement macrotexture in practice are to determine the mean texture depth (MTD) by sand patch method or the mean profile depth (MPD) derived from the data of a laser texture scanner [11]. Both MTD and MPD can be measured in the laboratory, and have been applied to predict skid resistance of pavement mixtures based on statistical regression relationships. Eq. (3) shows a typical example of such a relationship [40]:
SN64R ¼ 26:066 þ 2:761ðMPDÞ þ 40:0ðDFT20Þ R2 ¼ 0:59
ð3Þ
where SN64R = skid number measured at 64 km/h (40 mph) in accordance with ASTM standard method E274 [41] using rib tire; and
DFT20 = friction values measured by the Dynamic Friction Tester at test speed of 20 km/h. Relationships such as Eq. (3) are simply statistical prediction models since neither MTD nor MPD could represent fully the effects of all surface texture factors on pavement surface drainage. The important factors that affect drainage, and hence skid resistance, include the length, size and tortuosity of channels formed by connected surface voids. A similar situation also exists in the case of tire-pavement noise prediction. Since the mechanisms of skid resistance development and tire-pavement noise generation are very much dependent on the number, size and distribution of surface voids, a realistic 3-dimensional representation of pavement surface texture is necessary to correctly analyze the effects of surface texture on skid resistance and tire-pavement noise. It is proposed in this study to obtain a numerical threedimensional representation of surface texture from scanning a pavement mixture surface using a laser scanner. Threedimensional laser scanners are now commonly available commercially that are able to produce texture reading precision of 0.1 mm or better for both depth and spatial measurements. Such scanners would be able to provide a sufficiently accurate representation of pavement mixture macrotexture for numerical analysis of pavement skid resistance and tire-pavement noise. 4.3. Measurement of low-speed skid resistance Low-speed skid resistance measurements provide useful information for the estimation of pavement skid resistance at normal traffic speeds and different water film thicknesses. Fig. 3 presents a schematic diagram to illustrate this point. It has been found through research studies and experimental measurements that at a low measurement speed (such as 20 km/h or lower), the skid resistance of a pavement is not affected by the magnitude of measurement speed and the depth of water film thickness [34,42]. This low-speed skid resistance value is also known as the zero-speed skid resistance, designated as SN0 in Fig. 3. For a pavement with a water film thickness w, its skid resistance value begins to deteriorate as the vehicle speed increases beyond the low speed range.
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Fig. 3. Low-speed and high-speed skid resistance as a function of vehicle speed and water film thickness.
The rate of skid resistance deterioration is a function of the macrotexture of the pavement surface. It is therefore of importance to measure SN0 and surface macrotexture of a mix design in order to evaluate its skid resistance performance. The value of low-speed skid resistance can be measured in the laboratory by means of a suitable skid resistance tester. The British pendulum tester (BPT) and the Dynamic Friction Tester (DFT) are the two most commonly used equipment currently in use for this purpose. Unfortunately, both devices are not suitable for the purpose of mechanistic evaluation of pavement surface skid resistance. The contact mode and skid resistance generation mechanism of both BPT and DFT are not the same as that between moving vehicle tires and a pavement surface [43,44]. The friction contact area in BPT testing is restricted to a narrow strip along the front edge of its rubber slider. On the other hand, DFT measures skid resistance using three spinning rubber sliders with their lower flat faces in contact with the test surface. Because of their modes of testing, both forms of tests could display unrealistic results when tested on coarse-textured surfaces, including porous pavement surfaces [38,43,44]. For the purpose of mechanistic evaluation of pavement skid resistance, a more appropriate form of low-speed skid resistance testing device is one that measures skid resistance between a rubber tire and test surface. There are several such test devices currently available, such as the Micro GripTester, T2GO Portable Friction Tester, and the Walking Friction Tester (WFT) [38]. Han et al. [38] conducted laboratory low-speed skid resistance measurements using WFT, BPT and DFT, and found that WFT produced more reliable test measurements than BPT and DFT because of its more realistic mode of friction measurement. The low-speed skid resistance measurements obtained from devices using a rubber test tire, such as WFT, are suitable for mechanistic evaluation of pavement skid resistance. 4.4. Measurement of sound absorption coefficient The total tire-pavement noise generated from a vehicle moving on a pavement is the combined effect of the tire-pavement interaction noise transmitted directly through the air and the noise reflected from the pavement. The noise generated from tirepavement interaction can be estimated by means of appropriate mechanistic models, while the reflected noise can be calculated
by first measuring the sound absorption coefficient of the porous mixture concerned. The reflected noise from a porous pavement consists of the component reflected from the pavement surface as well as the other component reflected after penetrating into the porous pavement. Laboratory sound absorption tests have been performed by researchers to determine the sound absorption coefficients of different pavement materials, including porous pavement materials [45,46]. The ASTM E1050-10 standard procedure [47] using acoustic impedance tubes may be employed for this purpose. Past researchers have found that the frequency range between 630 and 2000 Hz was of practical importance in tire-pavement noise studies [46,48]. Hence, the frequency range of 100–2500 Hz was considered adequate for laboratory sound absorption coefficient determination. In this study, following the recommendations of ASTM E1050-10 standard procedure [47], a 100 mm diameter impedance tube was used for low frequencies below 500 Hz, and an impedance tube of 29 mm diameter for higher frequency. 5. Application of measured data for functional performance assessment 5.1. Surface runoff drainage capacity assessment Most porous road pavements are designed to discharge surface runoff laterally toward roadside drainage systems. Since the main function is to avoid or minimize accumulation of rainwater on the porous pavement surface, the runoff drainage capacity of the pavement needed can be assessed based on its ability to maintain pavement surface water film thickness below a desired level under the design rainfall intensity. Several theoretically derived surface runoff models are available for performing this analysis, including the numerical simulation model proposed by Ranieri et al. [49] and the PAVDRN computer model [50]. Both models provide solutions for lateral flow of runoff toward the edge of a pavement under a steady-state rainfall condition. The former model was derived from the Boussinesq equation for steady flow while the latter model was formulated using a one-dimensional steady-state kinematic wave equation. Both require input data of permeability coefficient, pavement cross slope, porous surface thickness, and rainfall intensity. The PAVDRN model has been widely used for pavement drainage analysis due to its easy availability [51].
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The two models above calculate water-film thickness on a porous pavement surface as an output. A mix design may be selected to provide zero water-film thickness under a design rainfall intensity for a given geometric dimensions of the porous pavement concerned. Alternatively, since the water-film thickness varies from lane to lane in a multi-lane roadway [51], one may allow a small thickness of water film along the outer slow lane while maintaining zero water-film thickness on the fast lanes. In many situations in practice under heavy rainstorm, it is not possible to maintain zero water-film thickness even for the fast lane, and the choice of required permeability for mix design would have to be based on risk analysis of skidding and hydroplaning, as explained in the next section. 5.2. Wet-weather skid resistance assessment Knowing the water-film thickness, pavement surface macrotexture, permeability coefficient and low-speed skid resistance, it is possible to predict the hydroplaning speed and pavement skid resistance at different vehicle speeds. This can be achieved by performing a numerical skid resistance simulation analysis. Different numerical models based on mechanistic theories are available for determining pavement skid resistance using finite-element software, such as ADINA, ANSYS, and ABAQUS [52–55]. The simulation models analyze tire-water-pavement interaction to calculate the skid resistance generated under the influence of pavement surface macrotexture and microtexrture. The simulation analysis is able to generate (i) the hydroplaning speed of a mix design for a given water-film thickness, (ii) the skid resistance of the mix design at any vehicle speed and water-film thickness, and (iii) the braking distance required. The acceptability of the mix design is established on the following basis: (i) Checking the calculated hydroplaning speed against the design speed, (ii) Checking the skid resistance against the corresponding minimum safe skid resistance specified by the authority concerned, and (iii) Checking the braking distance against the safe braking distance [56].
types of tire. Standard test tire can be used in the simulation and sound pressure or sound intensity levels at specified locations can be computed to obtain tire-pavement noise corresponding to either the On-Board Sound Intensity (OBSI) Method [59] or the Close Proximity (CPX) Method [60]. The acceptability of the mix design concerned may thus be determined by comparing with the maximum noise level allowed.
6. Application illustration 6.1. Illustrative example Two porous asphalt mix designs PA-13 and PA-20 were prepared in this study to illustrate the application of the proposed laboratory test data. The steps outlined in the flow diagram of Fig. 1 was followed in the analysis. Table 1 lists the mix proportions and laboratory determined properties of two mix designs for the wearing course of a porous asphalt pavement. For the testing of each mix property, three replicate specimens were prepared. The permeability coefficients of the mixes were determined using the falling-head apparatus shown in Fig. 2. The low speed friction coefficients were measured using the Walking Friction Tester (see Section 4.3). Adopting a design rainfall intensity of 150 mm/h, and knowing the pavement permeability coefficient, the water-film thickness in the outermost lane of a 3-lane one-directional porous pavement was calculated using the PAVDRN model (see Section 5.1). The water-film thicknesses were found to be 2.27 mm and 1.98 mm for PA-13 and PA-20 respectively. With the low-speed friction coefficients given in Table 1, the skid resistance characteristics of the two types of porous pavement surfaces were predicted by applying the ANSYS finite element model developed by Zhang et al. [53], The computed skid resistance characteristics with speed are plotted in Fig. 4. The PA-20 mix produced higher skid resistance than PA-13 due to its better drainage properties. 80
5.3. Tire-Pavement noise assessment Mechanistic simulation models currently available are able to model the tire-pavement noise generated through the interaction of rolling tire and porous pavement. Both Zhang et al. [57] and Yang and Wang [58] respectively developed coupled finite element method-boundary element (FEM-BEM) model to study the noise generated from a tire rolling on different pavement types, including porous pavements. FEM was used to obtain tire vibration under excitations of different texture levels, which in turn was used as the boundary condition input to compute sound pressure levels around the tire using BEM. The simulation models can be applied to obtain tire-pavement noise generated on different porous mix designs under different
Skid Number (SN)
70 60 50 40 30 20 10 0
10
20
30
40
50
60
70
90
80
Vehicle Speed (km/h) Fig. 4. Predicted skid resistance performance of design mixes.
Table 1 Properties of mix designs of example problem. (a) Mix proportions Sieve Size (mm) Mix Type
20 PA-13 PA-20
Binder Content
16
13.2
9.5
– – 100 85 100 95 85 72 5.0% polymer-modified binder PG76-22
4.75
2.36
1.18
0.6
0.3
0.075
45 22
30 18
25 –
20 13
13 9
4 6
(b) Mix properties Mix Design
Nominal Top Aggregate Size (mm)
Percent Air Voids (%)
Permeability Coefficient (mm/s)
Low-Speed Friction Coeff.
Wearing course Thickness (mm)
PA-13 PA-20
12.7 19.6
13.7 21.3
4.2 16.6
0.66 0.69
50 50
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Fig. 5. Sound absorption coefficient spectra of design mixes.
For tire-pavement noise evaluation, the required data of measured sound absorption coefficients of the two mixes are shown in Fig. 5, and the texture level spectra obtained from the measured surface texture of the design mixes are plotted in Fig. 6. The sound absorption coefficients were measured using the acoustic impedance tube method according to the ASTM E1050-10 standard procedure [47]. The texture level spectra were obtained using a laser scanner with a texture measurement precision of 0.1 mm. Next, the tire-pavement noise was estimated by applying the finite element model developed by Zhang et al. [57]. The overall tirepavement noise levels calculated at three different vehicle speeds of 70, 80 and 90 km/h, respectively, based on the microphone positions of the CPX method for the design mixes PA-20 and PA-13 are listed in Table 2. There are little differences between the sound absorption properties of the two mixes, but the finer surface texture of PA-13 (as depicted in the surface texture level plot in Fig. 6) causes lower tire vibrations and hence lower overall dBA
Fig. 6. Surface texture level spectra of design mixes.
Table 2 Calculated CPX noise levels of example problem. Vehicle Speed (km/h)
dBA of Design Mix PA-13
dBA of Design Mix PA-20
70 80 90
95.7 97.9 99.6
97.6 100.1 101.9
than PA-20. These calculated results are in general agreement with field measurements recorded from a field trial conducted in a parallel study using similar porous mixes [61].
6.2. Further remark on application of proposed laboratory testing Although the preceding illustrative example deals with porous asphalt mix design, the proposed procedure is equally applicable to pervious concrete mix design. The proposed procedure for evaluating surface runoff drainage capacity, skid resistance, and tirepavement noise are equally valid for pervious concrete. The proposed laboratory tests presented above can also be applied to assess the long-term functional performance (i.e. drainage capacity, skid resistance and tire-pavement noise) of a porous design mix, including the durability of the functional properties of the design mix. The functional properties of a typical porous pavement tend to deteriorate with time under the actions of traffic loadings. The deterioration of skid resistance of a porous pavement is caused by two major actions: polishing of pavement surface materials, and clogging of pores in the porous pavement structure. The polishing effect affects the low-speed skid resistance directly, while the clogging effect affects the drainage capacity of the porous pavement, thereby causing its high-speed skid resistance to fall. The clogging effect also adversely affects the sound absorption coefficient of the porous pavement, leading to higher reflected noise. To predict the long-term deterioration and durability of drainage capacity, skid resistance and tire-pavement noise reduction properties of a design porous mix, laboratory polishing treatment and laboratory clogging can be performed. Many different laboratory polishing devices and clogging set-ups have been used by pavement engineering professionals and researchers [45,62,63]. Applicable laboratory polishing devices for porous pavement mixtures include the Wehner and Schulze method [34] and the NCAT Three-Wheel Polishing Device (TWPD) [63]. A suitable clogging set-up is one developed by Fwa et al. [33] that permits permeability monitoring during the clogging treatment process. Having performed the desired polishing and/or clogging treatments, the proposed laboratory procedure described in this study can be performed to study the deterioration trend and durability of drainage capacity, skid resistance and tire-pavement noise generation properties of the design mix concerned.
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7. Conclusion This paper has proposed a procedure comprising (i) conducting a set of tests to determine the properties of a design mix related to its drainage capacity, skid resistance and tire-pavement noise performance; and (ii) performing assessment analysis of the corresponding functional characteristics of the mix. The proposed tests are additional to the conventional standard tests in pavement mix design. Specifically, the proposed tests include the following: permeability test using either constant- or falling-head set-up, macrotexture characterization by means of laser scanning, microtexture characterization using low-speed skid resistance testing, and sound absorption test by means of acoustic impedance tubes. These tests have been used by different researchers and professionals for the respective stated purposes, and the equipment required are currently available commercially. The functional characteristics evaluation analysis to be performed using the measured data include drainage capacity evaluation, skid resistance and tirepavement noise prediction. The required techniques and software for the analysis are also within the reach of the pavement community today. Using the proposed laboratory tests and analysis would overcome the major limitation of the current mix design methods of not addressing the functional needs of porous pavements, and significantly enhance the capability of pavement engineers in providing porous mix designs that would satisfy the desired functional requirements of highway pavements. Conflict of interest None. References [1] P.G. Abbott, P.A. Morgan, B. McKell, A Review of Current Research on Road Surface Noise Reduction Techniques, Transport Research Laboratory, Wokingham, UK, 2010. [2] A. Vaitkus, T. Andriejauskas, V. Vorobjovas, A. Jagniatinskis, B. Fiks, E. Zofka, Asphalt wearing course optimization for road traffic noise reduction, Constr. Build. Mater. 152 (2017) 345–356. [3] V. Audrius, C. Donatas, V. Viktoras, A. Tadas, Traffic/road noise mitigation under modified asphalt pavements April 18-21, in: Transportation Research Procedia, Transport Research Arena TRA2016, Amsterdam, 2016, pp. 2698– 2703. [4] C. McGovern, P. Rusch, D.A. Noyce, State Practices to Reduce Wet Weather Skidding Crashes‘‘, Publication FHWA-SA-11-21, U.S. Department of Transportation, Washington D.C, Federal Highway Administration, 2011. [5] G. Dell’Acqua, M. De Luca, F. Russo, R. Lamberti, Analysis of Rain-Related Crash Risk on Freeway Pavements in Porous and Dense Asphalt, Transportation Research Board 91st Annual Meeting Compendium of Papers DVD, Transportation Research Board of the National Academies, Washington, D.C., 2012. [6] J.W. Hall, K.L. Smith, L. Titus-Glover, J.C. Wambold, T.J. Yager, Z. Rado, Guide for Pavement Friction. NCHRP Web-Only Document 108, National Cooperative Highway Research Program, Washington, DC., 2009. [7] D. Pezzaniti, S. Beecham, J. Kandasamy, Influence of clogging on the effective life of permeable pavements, Proc. ICE – Water Manage. 162 (3) (2009) 211– 220. [8] H.M. Imran, S. Akib, M.R. Karim, Permeable pavement and stormwater management systems: a review, Environ. Technol. 34 (17) (2013) 2649–2656. [9] J. Augenstern, T.B. Boving, M. Stolt, Porous Pavement Parking Lot and its impact on Subsurface Water Quality, in: Proceedings of the International Association of Hydrologists, XXXIII Annual meeting, Zacatecas, Mexico, 2004, pp. 1749– 1754. [10] R. Field, H. Masters, M. Singer, Porous pavement: research, development, and demonstration, J. Transp. Eng. 108 (3) (1982) 244–258. [11] J.W. Hall, K.L. Smith, L. Titus-Glover, J.C. Wambold, T.J. Yager, Z. Rado, Guide for Pavement Friction. NCHRP Web-Only Document 108, National Cooperative Highway Research Program, Washington DC, 2009. [12] R.B. Mallick, P.S. Kandhal, L.A. Cooley, D.E. Watson, Design, Construction, and Performance of New-Generation Open-Graded Friction Courses, NCAT Report 00–01, National Center for Asphalt Technology, Auburn, Alabama, April 2000. [13] J.T. van der Zwan, T. Goeman, J.A.J. Gruis, J.H. Swart, R.H. Oldergurger, Porous Asphalt Wearing Courses in the Netherlands: State of the Art Review, Transportation Research Record, No. 1265, Transportation Research Board, 1990, pp. 95–110.
[14] A. Ruiz, R. Alberola, F.E. Perez-Jimenez, B. Sanchez, Porous Asphalt Mixtures in Spain, Transportation Research Record, No. 1265, Transportation Research Board, 1990, pp. 87–94. [15] C.K. Estakhri, A.E. Alvarez, A.E. Martin, Guidelines on Construction and Maintenance of Porous Friction Courses in Texas Report No. FHWA/TX-08/05262-2, Texas Department of Transportation, Austin, Texas, 2008. [16] Alabama Department of Transportation, Standard Specifications for Highway Construction – Division 420: Polymer Modified Open Graded Friction Course, Montgomery, Alabama, 2002. [17] B.W. Tsai, J.T. Harvey, C.L. Monismith, Evaluation of Open-Graded Friction Course (OGFC) Mix Design: Summary Version. Summary Report, University of California Pavement Research Center, 2013. UCPRC-SR-2013-02. [18] European Standard EN 13108-7:2016, Bituminous mixtures – Material specifications – Part 7: Porous Asphalt. English version, The British Standards Institution, 2016. [19] European Standard EN 12697-12:2018, Bituminous mixtures. Test methods. Determination of the water sensitivity of bituminous specimens English version, The British Standards Institution, 2018. [20] European Standard EN 12697-17:2017, Bituminous mixtures. Test methods. Particle loss of porous asphalt specimens English version, The British Standards Institution, 2017. [21] EN 12697-22, Bituminous mixtures – Test methods for hot mix asphalt – Part 22: Wheel tracking. English version, The British Standards Institution (2003). [22] European Standard EN 12697-49:2014, Bituminous mixtures – Test methods for hot mix asphalt – Part 49: Determination of friction after polishing English version, The British Standards Institution, 2014. [23] European Standard EN 12697-18:2017, Bituminous mixtures. Test methods. Binder drainage English version, The British Standards Institution, 2017. [24] European Standard. EN 12697-19:2012, Bituminous mixtures. Test methods for hot mix asphalt. Permeability of specimen English version, The British Standards Institution, 2012. [25] ASTM D5084-16a, Standard Test Methods for Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter, ASTM International, West Conshohocken, PA, 2016. [26] ASTM D2434–68(Reapproved 2006), Standard Test Methods for Permeability of Granular Soils (Constant Head), ASTM International, West Conshohocken, PA, 2006, p. 2006. [27] AASHTO T215–14, Standard Method of Test for Permeability of Granular Soils (Constant Head), Standard by American Association of State Highway and Transportation Officials, Washington D.C., 2014. [28] R.H. Jones, H.A. Jones, Granular Drainage Layers in Pavement Foundations in Unbound aggregates, in Roads R.H. Jones, A.R. Dawson, 1989, 55-69 [29] A.E. Scheidegger, The Physics of Flow Through Porous Media, University of Toronto Press, Toronto, Canada, 1963. [30] J. Bear, Dynamics of Fluids in Porous Media, Elservier, New York, 1972. [31] T.F. Fwa, S.A. Tan, C.T. Chuai, Permeability measurement of base materials using falling-head test apparatus, Transp. Res. Record 1615 (1998) 94–99. [32] S.A. Tan, T.F. Fwa, C.T. Han, Clogging evaluation of permeable bases, J. Transp. Eng. 129 (3) (2003) 309–315. [33] T.F. Fwa, E. Lim, K.H. Tan, Comparison of permeability and clogging characteristics of porous asphalt and pervious concrete pavement materials, Transp. Res. Record 2511 (2015) 72–80, https://doi.org/10.3141/2511-09. [34] J.J. Henry, Evaluation of Pavement Friction Characteristics – A Synthesis of Highway Practice. NCHRP Synthesis 291, National Cooperative Highway Research Program, Washington, DC, 2000. [35] Sandberg, U. and J. A. Ejsmont. Tyre/Road Noise Reference Book, INFORMEX, Kisa, 466, Sweden, 2002. [36] A.G. Kokkalis, O.K. Panagouli, Fractal evaluation of pavement skid resistance variations I: Surface Wetting, Chaos, Solitons Fractals 9 (11) (1998) 1875– 1890. [37] A.B. Slimane, M. Khoudeir, J. Brochard, M.-T. Do, Characterization of road microtexture by means of image analysis, Wear 246 (2008) 464–468. [38] S. Han, M. Liu, T.F. Fwa, Walking Friction Tester for Low-Speed Skid Resistance Testing of Pavement January 7-11, Compendium 97th Annual Meeting of Transportation Research Board, Washington, DC, 2018. [39] M. Miljkovic´, M. Radenberg, C. Gottaut, Characterization of noise-reducing capacity of pavement by means of surface texture parameters, J. Mater. Civ. Eng. 26 (2) (2014.), https://doi.org/10.1061/(ASCE)MT.1943-5533.0000821. [40] M.A. Khaswneh, TheDevelopment and Verification of a New Accelerated Polishing Machine PhD Thesis, Graduate School, University of Akron, Akron, OH, 2008. [41] ASTM E274/E274M-11, Standard Test Method for Skid Resistance of Paved Surfaces Using a Full-Scale Tire, ASTM International, West Conshohocken, PA, 2011. [42] P.G. Roe, A.R. Parry, H.E. Viner, High and Low Speed Skidding Resistance: The Influence of Texture Depth. TRL Report 367, Transport Research Laboratory, Crowthorne, UK, 1998. [43] G.F. Salt, Research on skid resistance at the transport and road research laboratory (1927–1977), Transp. Res. Rec. 622 (1977) 1977. [44] Y.P.K. Lee, T.F. Fwa, Y.S. Choo, Effect of pavement surface texture on british pendulum test, J. Eastern Asia Soc. Transp. Stud. 6 (2005) 1247–1257. [45] L. Chu, T.F. Fwa, K.H. Tan, Evaluation of wearing course mix designs on sound absorption improvement of porous asphalt pavement, Constr. Build. Mater. 141 (2017) 402–409. [46] F.G. Praticò, R. Fedele, D. Vizzari, Significance and Reliability of Absorption Spectra of Quiet Pavements, Constr. Build. Mater. 140 (1) (2017) 274–281.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Evaluation of functional characteristics of laboratory mix design of porous pavement materials L. Chu a, B. Tang a, T.F. Fwa b,c,⇑ a
School of Highway, Chang’an University, South Erhuan Middle Section, Xi’an 710064, China Chang’an University, China c National University of Singapore, Singapore b
h i g h l i g h t s Current pavement mix design methods inadequately address functional requirements. Laboratory tests are proposed to address functional requirements during mix design. The proposed tests address drainage, skid resistance and noise related properties. Test data allow evaluation of drainage capacity and skid resistance of mix design. Test data also allow prediction of tire-pavement noise of mix design.
a r t i c l e
i n f o
Article history: Received 11 April 2018 Received in revised form 14 August 2018 Accepted 1 October 2018
Keywords: Porous pavement Porosity Permeability Skid resistance Sound absorption Tire-pavement noise
a b s t r a c t Porous pavements are designed and constructed today to provide one or more of the following functional benefits: lower traffic noise, improved wet-weather driving safety, lower stormwater peak load, and replenishment of groundwater supplies. However, depending on the mix design and material properties of a porous pavement, not all these benefits can be achieved fully by the porous pavement. The factors that affect such functional benefits include the porosity, the properties of binder and aggregates, and the binder content and aggregate gradation of the porous mixture. Currently no agencies have specified any laboratory test procedure for pavement engineers to evaluate the functional characteristics during the mix design phase. Such laboratory procedures are useful in practice for porous pavement design. This study demonstrates that a set of laboratory tests based on currently available equipment and methods, along with related functional assessment analysis, can be performed to meet the purpose. The following laboratory tests on the design mix are proposed in this study: (i) Permeability test. (ii) 3dimensional scanning of surface texture, (iii) Skid resistance test, and (iv) Sound absorption test. The test data may then be used for drainage capacity analysis, wet-weather skid resistance assessment, and tirepavement noise analysis. The proposed laboratory test procedures are illustrated using two porous pavement materials. The test results show that practically useful information on functional characteristics of a design porous mix can be obtained using the proposed procedures. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Porous pavements consist of one or more layers of high porosity to provide one or more of the following functional benefits: lower traffic-generated noise [1–3], improved wet-weather pavement skid resistance [4–6], lower peak flow load of stormwater runoff [7,8], and replenishment of groundwater supplies [9,10]. Other ⇑ Corresponding author at: School of Highway, Chang’an University, South Erhuan Middle Section, Xi’an 710064, China. E-mail address: [email protected] (T.F. Fwa). https://doi.org/10.1016/j.conbuildmat.2018.10.003 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
associated benefits include reduced splash and spray, and improved visibility of pavement markings on rainy days, as well as higher average traffic flow speeds during wet weather [10,11]. However, depending on the mix design and properties of the porous materials, not all of the above-mentioned benefits can be achieved in a given porous pavement. As will be shown later in the review of current porous mixture mix design practices, the standard laboratory mix design procedures today do not adequately evaluate the functional characteristics of a porous mixture design during the mix design phase. The present study was motivated by the fact that there already exist
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in the literature sufficient knowledge and available test methods and analysis techniques to obtain the required data that can be used to estimate the desired functional characteristics of a porous pavement constructed with a given design mix. It is demonstrated in this study that using currently available test equipment and analysis methods, data can be generated to predict the functional performance of a porous pavement in the following three aspects: its drainage capacity, its wet-weather skid resistance, and the tirepavement noise generated under traffic. The following laboratory tests on a design mix are proposed: (i) Permeability test (ii) 3-dimensional scanning of surface texture, (iii) Skid resistance test, and (iv) Sound absorption test. The test procedure, the measured data obtained from each test, and how the measured data can be used to predict the desired functional performance are explained in this paper. To illustrate the concept and working of the proposed laboratory test procedures, the results of tests performed on two porous asphalt design mixes are presented and explained. The test data produced from the tests serve to demonstrate that the proposed tests were able to differentiate different functional performances of different porous mix designs.
2. Review of current porous mixture mix design practices Various forms and designs of porous pavement have been used in North America and Europe for more than 50 years [12– 14]. Two important aspects of the mix design of a porous pavement material are that the high-porosity mixture should be (i) structural sound and durable, and (ii) able to maintain its functional benefits over the entire design life. The National Center for Asphalt Technology, a leading asphalt research center in North America, has developed a mix design procedure for OGFC (open-graded friction course) mixes which are a common form of high-porosity mixture used in North America [12]. The mix design procedure consisted of 5 steps: materials selection, formulate trial aggregate gradations, determination of optimum gradation, determination of optimum binder content, and moisture susceptibility evaluation. In the Texas PFC (porous friction course) mix design method, specimens at the selected optimum binder content were evaluated for draindown, moisture susceptibility and durability [15]. In Alabama, a minimum tensile strength ratio (TSR) was also specified in addition to draindown test [16]. More recently, an improved OGFC mix design procedure was recommended to the California Department of Transportation (Caltrans) [17]. The mix design procedure included two primary components: Volumetric design and performance testing. The first component was similar to the first 4 steps of the NCAT procedure. The performance testing in the second component comprised Cantabro, draindown, and Hamburg wheel tracking testing [16]. In Europe, the mix design of porous asphalt is governed by the European standard EN 13108-7:2016 [18]. The European standard specifies requirements for the constituent materials as well as the porous mixture. In its latest 2016 edition, the standard has included the following test requirements for the mix design: (i) Water sensitivity assessed using either indirect tensile strength ratio or compression strength ratio [19]; (ii) Resistance to abrasion by the Cantabro wear test [20]; (iii) Resistance to permanent deformation by wheel-tracking test [21]; (iv) Polishing resistance by the Wehner and Schulze method [22]; (v) Binder drainage by either the basket or beaker method [23]; (v) Permeability by a constant-head permeameter [24]. It is significant to note that, in contrast with North American practices, the latest European standard has incorporated some functional performance requirements into the mix design of por-
ous pavement. This is reflected by requirements (iv) and (v) in the preceding paragraph. These additional functional requirements represent a recognition of the need to incorporate functional requirements into porous pavement mix design. However, the tests in requirements (iv) and (v) alone are inadequate to address the main functional benefits of porous pavements. A more complete set of tests and analysis is proposed in the present study, as explained in the next section, to cover the functional requirements of porous pavements.
3. Concept and scope of study The main objective of the present study was to develop a set of laboratory procedures to test a mix design for the purpose of obtaining the material properties required for evaluating pavement functional characteristics based on mechanistic methods. Compared with predictions using empirical statistical regression models, improved and more accurate predictions of functional performance of pavement mix designs can be achieved with the proposed procedure. This is because mechanistic prediction models for skid resistance and tire-pavement noise are now available. Specifically, the proposed laboratory tests would be used for the prediction of the following three functional properties of a porous pavement: Surface runoff drainage capacity of the porous pavement layer; Wet-weather skid resistance properties of the porous pavement; and Tire-pavement noise. Fig. 1 presents a flow diagram that shows the proposed additional laboratory tests (i.e. Phase II in the figure) to be included during the mix design phase of a porous pavement mixture. With these data, analyses can be performed to estimate the expected functional performance of the mix design. To estimate the drainage capacity of a porous pavement layer, one needs to determine the permeability coefficient of the porous mix considered. This drainage property, together with pavement surface texture characteristics (both microtexture and macrotexture of the porous pavement surface), are required for assessing the wet-weather skid resistance performance of the porous pavement. Finally, by measuring the sound absorption coefficient of the porous pavement material, the expected tire-pavement noise can be predicted by means of a computer simulation model using the sound absorption coefficient and pavement surface texture data as input. The rationale and test procedures for the various laboratory tests proposed in this study are explained in the next section.
4. Proposed laboratory tests: Rationale and test procedures 4.1. Permeability test of porous mixture The current standard permeability tests for testing cohesive or cohesionless materials are not suitable for measuring the permeability coefficients of porous pavement materials. For instance, ASTM D5084-16 [25] is applicable only for materials with permeability coefficients less than 10-5 m/s, which is much lower than those of common porous pavement materials; while ASTM D2434-68(2006) [26] and AASHTO T215-14 [27] both assume laminar flow that follows Darcy’s Law as follows,
m ¼ ki where
ð1Þ
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Fig. 1. Proposed framework of enhanced mix design to include functional characteristics evaluation.
4.2. Characterization of microtexture and Macrotexture of porous mixture
v = specific discharge, m/s k = coefficient of permeability, m/s i = hydraulic gradient, m/m. Researchers have found that the assumption of laminar flow does not hold in permeability measurement of a pervious coarsegrained material, and that Darcy’s Law is not valid unless the hydraulic gradient is less than about 0.05 [28,29]. Several modifications to Darcy’s equation have been made by researchers to describe the non-laminar flow condition in pervious granular materials [29,30], and the following form were found to describe permeability test results closely [31,32]:
m ¼ ki0:7
ð2Þ
where the permeability coefficient k can be determined experimentally. Both constant-head and falling-head tests have been used to determine the permeability coefficient [31–33]. However, due to the high permeability coefficient of porous pavement materials, which are of the order of 1 to about 20 mm/s, as well as the nonlaminar characteristics of the flow, a falling-head test has been found to be a more suitable experiment to be performed in the laboratory [31]. Fig. 2 shows a laboratory falling-head test set-up for permeability coefficient measurement. It should be noted that to ensure successful permeability testing of a falling-head apparatus, it must be equipped with a high precision sensor capable of detecting changes of as small as 0.3 mm in water level, and a high speed data acquisition unit that can record 100 or more data points per second [31].
Surface texture characteristics of a porous pavement are a key factor that has major impacts on both the skid resistance development and tire-pavement noise generation mechanisms of the pavement. It is generally believed that pavement surface texture affects the development of wet-pavement skid resistance in two major ways [24,34]: (i) Generation of skid resistance through tirepavement adhesion when a tire comes water trapped at tirepavement interface due to inefficient drainage. The former is a function of the microtexture properties of the porous mixture concerned, while the latter is affected by the ability of the mixture’s macrotexture to discharge pavement surface water from the tirepavement contact area. Pavement surface microtexture and macrotexture also contribute to tire-pavement noise in different ways. While pavement surface microtexture generates tire-pavement noise by means of friction- or adhesion-related stick-slip and stick-snap mechanisms, its macrotexture creates tire-pavement noise through other mechanisms, such as air sucking, air pumping, air resonant and horn effect, and vibrations of tire walls [35]. Hence, quantitative characterization of surface texture is necessary in the evaluation of functional characteristics of a porous mixture design. Due to the different impact mechanisms of pavement microtexture and macrotexture on the functional characteristics of porous mixtures, the two forms of surface texture are often quantified and represented in different ways, as explained in the following two subsections.
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Fig. 2. Laboratory set-up for falling-head permeability testing of porous materials.
4.2.1. Characterization of microtexture The microtexture of pavement surface has been defined as the irregularity with wavelengths below 0.5 mm, and deviations of less than 0.2 mm from a true planar surface, measurable only by means of photomicrography [31,36]. However, studies on the effects of microtexture using its geometric characteristics have been very much restricted to establishing statistical relationships with pavement skid resistance. Such relationships are applicable to a limited scope of applications in terms of pavement materials and mix design [36,37]. As a result, low-speed measurements of skid resistance are often used to represent the effect of microtexture because it is the dominant factor influencing skid resistance at low speeds [36,38]. This is also the approach adopted in the present study, as described later under Section 4.3. The effect of microtexture on tire-pavement noise is considered to be minor compared with that of macrotecture, and is usually ignored in tire-pavement noise analysis [39]. 4.2.2. Characterization of Macrotexture Pavement surface macrotexture is commonly referred to as the visible irregularity having wavelengths between 0.5 and 50 mm and amplitudes from 0.1 to 20 mm, and is attributed to aggregate size, shape, angularity, spacing and gradation [31,36]. It governs the drainage of pavement surface water under a skidding tire, and is chiefly responsible for the deterioration of pavement skid resistance when vehicle speed increases. The standard methods of characterizing pavement macrotexture in practice are to determine the mean texture depth (MTD) by sand patch method or the mean profile depth (MPD) derived from the data of a laser texture scanner [11]. Both MTD and MPD can be measured in the laboratory, and have been applied to predict skid resistance of pavement mixtures based on statistical regression relationships. Eq. (3) shows a typical example of such a relationship [40]:
SN64R ¼ 26:066 þ 2:761ðMPDÞ þ 40:0ðDFT20Þ R2 ¼ 0:59
ð3Þ
where SN64R = skid number measured at 64 km/h (40 mph) in accordance with ASTM standard method E274 [41] using rib tire; and
DFT20 = friction values measured by the Dynamic Friction Tester at test speed of 20 km/h. Relationships such as Eq. (3) are simply statistical prediction models since neither MTD nor MPD could represent fully the effects of all surface texture factors on pavement surface drainage. The important factors that affect drainage, and hence skid resistance, include the length, size and tortuosity of channels formed by connected surface voids. A similar situation also exists in the case of tire-pavement noise prediction. Since the mechanisms of skid resistance development and tire-pavement noise generation are very much dependent on the number, size and distribution of surface voids, a realistic 3-dimensional representation of pavement surface texture is necessary to correctly analyze the effects of surface texture on skid resistance and tire-pavement noise. It is proposed in this study to obtain a numerical threedimensional representation of surface texture from scanning a pavement mixture surface using a laser scanner. Threedimensional laser scanners are now commonly available commercially that are able to produce texture reading precision of 0.1 mm or better for both depth and spatial measurements. Such scanners would be able to provide a sufficiently accurate representation of pavement mixture macrotexture for numerical analysis of pavement skid resistance and tire-pavement noise. 4.3. Measurement of low-speed skid resistance Low-speed skid resistance measurements provide useful information for the estimation of pavement skid resistance at normal traffic speeds and different water film thicknesses. Fig. 3 presents a schematic diagram to illustrate this point. It has been found through research studies and experimental measurements that at a low measurement speed (such as 20 km/h or lower), the skid resistance of a pavement is not affected by the magnitude of measurement speed and the depth of water film thickness [34,42]. This low-speed skid resistance value is also known as the zero-speed skid resistance, designated as SN0 in Fig. 3. For a pavement with a water film thickness w, its skid resistance value begins to deteriorate as the vehicle speed increases beyond the low speed range.
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Fig. 3. Low-speed and high-speed skid resistance as a function of vehicle speed and water film thickness.
The rate of skid resistance deterioration is a function of the macrotexture of the pavement surface. It is therefore of importance to measure SN0 and surface macrotexture of a mix design in order to evaluate its skid resistance performance. The value of low-speed skid resistance can be measured in the laboratory by means of a suitable skid resistance tester. The British pendulum tester (BPT) and the Dynamic Friction Tester (DFT) are the two most commonly used equipment currently in use for this purpose. Unfortunately, both devices are not suitable for the purpose of mechanistic evaluation of pavement surface skid resistance. The contact mode and skid resistance generation mechanism of both BPT and DFT are not the same as that between moving vehicle tires and a pavement surface [43,44]. The friction contact area in BPT testing is restricted to a narrow strip along the front edge of its rubber slider. On the other hand, DFT measures skid resistance using three spinning rubber sliders with their lower flat faces in contact with the test surface. Because of their modes of testing, both forms of tests could display unrealistic results when tested on coarse-textured surfaces, including porous pavement surfaces [38,43,44]. For the purpose of mechanistic evaluation of pavement skid resistance, a more appropriate form of low-speed skid resistance testing device is one that measures skid resistance between a rubber tire and test surface. There are several such test devices currently available, such as the Micro GripTester, T2GO Portable Friction Tester, and the Walking Friction Tester (WFT) [38]. Han et al. [38] conducted laboratory low-speed skid resistance measurements using WFT, BPT and DFT, and found that WFT produced more reliable test measurements than BPT and DFT because of its more realistic mode of friction measurement. The low-speed skid resistance measurements obtained from devices using a rubber test tire, such as WFT, are suitable for mechanistic evaluation of pavement skid resistance. 4.4. Measurement of sound absorption coefficient The total tire-pavement noise generated from a vehicle moving on a pavement is the combined effect of the tire-pavement interaction noise transmitted directly through the air and the noise reflected from the pavement. The noise generated from tirepavement interaction can be estimated by means of appropriate mechanistic models, while the reflected noise can be calculated
by first measuring the sound absorption coefficient of the porous mixture concerned. The reflected noise from a porous pavement consists of the component reflected from the pavement surface as well as the other component reflected after penetrating into the porous pavement. Laboratory sound absorption tests have been performed by researchers to determine the sound absorption coefficients of different pavement materials, including porous pavement materials [45,46]. The ASTM E1050-10 standard procedure [47] using acoustic impedance tubes may be employed for this purpose. Past researchers have found that the frequency range between 630 and 2000 Hz was of practical importance in tire-pavement noise studies [46,48]. Hence, the frequency range of 100–2500 Hz was considered adequate for laboratory sound absorption coefficient determination. In this study, following the recommendations of ASTM E1050-10 standard procedure [47], a 100 mm diameter impedance tube was used for low frequencies below 500 Hz, and an impedance tube of 29 mm diameter for higher frequency. 5. Application of measured data for functional performance assessment 5.1. Surface runoff drainage capacity assessment Most porous road pavements are designed to discharge surface runoff laterally toward roadside drainage systems. Since the main function is to avoid or minimize accumulation of rainwater on the porous pavement surface, the runoff drainage capacity of the pavement needed can be assessed based on its ability to maintain pavement surface water film thickness below a desired level under the design rainfall intensity. Several theoretically derived surface runoff models are available for performing this analysis, including the numerical simulation model proposed by Ranieri et al. [49] and the PAVDRN computer model [50]. Both models provide solutions for lateral flow of runoff toward the edge of a pavement under a steady-state rainfall condition. The former model was derived from the Boussinesq equation for steady flow while the latter model was formulated using a one-dimensional steady-state kinematic wave equation. Both require input data of permeability coefficient, pavement cross slope, porous surface thickness, and rainfall intensity. The PAVDRN model has been widely used for pavement drainage analysis due to its easy availability [51].
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The two models above calculate water-film thickness on a porous pavement surface as an output. A mix design may be selected to provide zero water-film thickness under a design rainfall intensity for a given geometric dimensions of the porous pavement concerned. Alternatively, since the water-film thickness varies from lane to lane in a multi-lane roadway [51], one may allow a small thickness of water film along the outer slow lane while maintaining zero water-film thickness on the fast lanes. In many situations in practice under heavy rainstorm, it is not possible to maintain zero water-film thickness even for the fast lane, and the choice of required permeability for mix design would have to be based on risk analysis of skidding and hydroplaning, as explained in the next section. 5.2. Wet-weather skid resistance assessment Knowing the water-film thickness, pavement surface macrotexture, permeability coefficient and low-speed skid resistance, it is possible to predict the hydroplaning speed and pavement skid resistance at different vehicle speeds. This can be achieved by performing a numerical skid resistance simulation analysis. Different numerical models based on mechanistic theories are available for determining pavement skid resistance using finite-element software, such as ADINA, ANSYS, and ABAQUS [52–55]. The simulation models analyze tire-water-pavement interaction to calculate the skid resistance generated under the influence of pavement surface macrotexture and microtexrture. The simulation analysis is able to generate (i) the hydroplaning speed of a mix design for a given water-film thickness, (ii) the skid resistance of the mix design at any vehicle speed and water-film thickness, and (iii) the braking distance required. The acceptability of the mix design is established on the following basis: (i) Checking the calculated hydroplaning speed against the design speed, (ii) Checking the skid resistance against the corresponding minimum safe skid resistance specified by the authority concerned, and (iii) Checking the braking distance against the safe braking distance [56].
types of tire. Standard test tire can be used in the simulation and sound pressure or sound intensity levels at specified locations can be computed to obtain tire-pavement noise corresponding to either the On-Board Sound Intensity (OBSI) Method [59] or the Close Proximity (CPX) Method [60]. The acceptability of the mix design concerned may thus be determined by comparing with the maximum noise level allowed.
6. Application illustration 6.1. Illustrative example Two porous asphalt mix designs PA-13 and PA-20 were prepared in this study to illustrate the application of the proposed laboratory test data. The steps outlined in the flow diagram of Fig. 1 was followed in the analysis. Table 1 lists the mix proportions and laboratory determined properties of two mix designs for the wearing course of a porous asphalt pavement. For the testing of each mix property, three replicate specimens were prepared. The permeability coefficients of the mixes were determined using the falling-head apparatus shown in Fig. 2. The low speed friction coefficients were measured using the Walking Friction Tester (see Section 4.3). Adopting a design rainfall intensity of 150 mm/h, and knowing the pavement permeability coefficient, the water-film thickness in the outermost lane of a 3-lane one-directional porous pavement was calculated using the PAVDRN model (see Section 5.1). The water-film thicknesses were found to be 2.27 mm and 1.98 mm for PA-13 and PA-20 respectively. With the low-speed friction coefficients given in Table 1, the skid resistance characteristics of the two types of porous pavement surfaces were predicted by applying the ANSYS finite element model developed by Zhang et al. [53], The computed skid resistance characteristics with speed are plotted in Fig. 4. The PA-20 mix produced higher skid resistance than PA-13 due to its better drainage properties. 80
5.3. Tire-Pavement noise assessment Mechanistic simulation models currently available are able to model the tire-pavement noise generated through the interaction of rolling tire and porous pavement. Both Zhang et al. [57] and Yang and Wang [58] respectively developed coupled finite element method-boundary element (FEM-BEM) model to study the noise generated from a tire rolling on different pavement types, including porous pavements. FEM was used to obtain tire vibration under excitations of different texture levels, which in turn was used as the boundary condition input to compute sound pressure levels around the tire using BEM. The simulation models can be applied to obtain tire-pavement noise generated on different porous mix designs under different
Skid Number (SN)
70 60 50 40 30 20 10 0
10
20
30
40
50
60
70
90
80
Vehicle Speed (km/h) Fig. 4. Predicted skid resistance performance of design mixes.
Table 1 Properties of mix designs of example problem. (a) Mix proportions Sieve Size (mm) Mix Type
20 PA-13 PA-20
Binder Content
16
13.2
9.5
– – 100 85 100 95 85 72 5.0% polymer-modified binder PG76-22
4.75
2.36
1.18
0.6
0.3
0.075
45 22
30 18
25 –
20 13
13 9
4 6
(b) Mix properties Mix Design
Nominal Top Aggregate Size (mm)
Percent Air Voids (%)
Permeability Coefficient (mm/s)
Low-Speed Friction Coeff.
Wearing course Thickness (mm)
PA-13 PA-20
12.7 19.6
13.7 21.3
4.2 16.6
0.66 0.69
50 50
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Fig. 5. Sound absorption coefficient spectra of design mixes.
For tire-pavement noise evaluation, the required data of measured sound absorption coefficients of the two mixes are shown in Fig. 5, and the texture level spectra obtained from the measured surface texture of the design mixes are plotted in Fig. 6. The sound absorption coefficients were measured using the acoustic impedance tube method according to the ASTM E1050-10 standard procedure [47]. The texture level spectra were obtained using a laser scanner with a texture measurement precision of 0.1 mm. Next, the tire-pavement noise was estimated by applying the finite element model developed by Zhang et al. [57]. The overall tirepavement noise levels calculated at three different vehicle speeds of 70, 80 and 90 km/h, respectively, based on the microphone positions of the CPX method for the design mixes PA-20 and PA-13 are listed in Table 2. There are little differences between the sound absorption properties of the two mixes, but the finer surface texture of PA-13 (as depicted in the surface texture level plot in Fig. 6) causes lower tire vibrations and hence lower overall dBA
Fig. 6. Surface texture level spectra of design mixes.
Table 2 Calculated CPX noise levels of example problem. Vehicle Speed (km/h)
dBA of Design Mix PA-13
dBA of Design Mix PA-20
70 80 90
95.7 97.9 99.6
97.6 100.1 101.9
than PA-20. These calculated results are in general agreement with field measurements recorded from a field trial conducted in a parallel study using similar porous mixes [61].
6.2. Further remark on application of proposed laboratory testing Although the preceding illustrative example deals with porous asphalt mix design, the proposed procedure is equally applicable to pervious concrete mix design. The proposed procedure for evaluating surface runoff drainage capacity, skid resistance, and tirepavement noise are equally valid for pervious concrete. The proposed laboratory tests presented above can also be applied to assess the long-term functional performance (i.e. drainage capacity, skid resistance and tire-pavement noise) of a porous design mix, including the durability of the functional properties of the design mix. The functional properties of a typical porous pavement tend to deteriorate with time under the actions of traffic loadings. The deterioration of skid resistance of a porous pavement is caused by two major actions: polishing of pavement surface materials, and clogging of pores in the porous pavement structure. The polishing effect affects the low-speed skid resistance directly, while the clogging effect affects the drainage capacity of the porous pavement, thereby causing its high-speed skid resistance to fall. The clogging effect also adversely affects the sound absorption coefficient of the porous pavement, leading to higher reflected noise. To predict the long-term deterioration and durability of drainage capacity, skid resistance and tire-pavement noise reduction properties of a design porous mix, laboratory polishing treatment and laboratory clogging can be performed. Many different laboratory polishing devices and clogging set-ups have been used by pavement engineering professionals and researchers [45,62,63]. Applicable laboratory polishing devices for porous pavement mixtures include the Wehner and Schulze method [34] and the NCAT Three-Wheel Polishing Device (TWPD) [63]. A suitable clogging set-up is one developed by Fwa et al. [33] that permits permeability monitoring during the clogging treatment process. Having performed the desired polishing and/or clogging treatments, the proposed laboratory procedure described in this study can be performed to study the deterioration trend and durability of drainage capacity, skid resistance and tire-pavement noise generation properties of the design mix concerned.
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7. Conclusion This paper has proposed a procedure comprising (i) conducting a set of tests to determine the properties of a design mix related to its drainage capacity, skid resistance and tire-pavement noise performance; and (ii) performing assessment analysis of the corresponding functional characteristics of the mix. The proposed tests are additional to the conventional standard tests in pavement mix design. Specifically, the proposed tests include the following: permeability test using either constant- or falling-head set-up, macrotexture characterization by means of laser scanning, microtexture characterization using low-speed skid resistance testing, and sound absorption test by means of acoustic impedance tubes. These tests have been used by different researchers and professionals for the respective stated purposes, and the equipment required are currently available commercially. The functional characteristics evaluation analysis to be performed using the measured data include drainage capacity evaluation, skid resistance and tirepavement noise prediction. The required techniques and software for the analysis are also within the reach of the pavement community today. Using the proposed laboratory tests and analysis would overcome the major limitation of the current mix design methods of not addressing the functional needs of porous pavements, and significantly enhance the capability of pavement engineers in providing porous mix designs that would satisfy the desired functional requirements of highway pavements. Conflict of interest None. References [1] P.G. Abbott, P.A. Morgan, B. McKell, A Review of Current Research on Road Surface Noise Reduction Techniques, Transport Research Laboratory, Wokingham, UK, 2010. [2] A. Vaitkus, T. Andriejauskas, V. Vorobjovas, A. Jagniatinskis, B. Fiks, E. Zofka, Asphalt wearing course optimization for road traffic noise reduction, Constr. Build. Mater. 152 (2017) 345–356. [3] V. Audrius, C. Donatas, V. Viktoras, A. Tadas, Traffic/road noise mitigation under modified asphalt pavements April 18-21, in: Transportation Research Procedia, Transport Research Arena TRA2016, Amsterdam, 2016, pp. 2698– 2703. [4] C. McGovern, P. Rusch, D.A. Noyce, State Practices to Reduce Wet Weather Skidding Crashes‘‘, Publication FHWA-SA-11-21, U.S. Department of Transportation, Washington D.C, Federal Highway Administration, 2011. [5] G. Dell’Acqua, M. De Luca, F. Russo, R. Lamberti, Analysis of Rain-Related Crash Risk on Freeway Pavements in Porous and Dense Asphalt, Transportation Research Board 91st Annual Meeting Compendium of Papers DVD, Transportation Research Board of the National Academies, Washington, D.C., 2012. [6] J.W. Hall, K.L. Smith, L. Titus-Glover, J.C. Wambold, T.J. Yager, Z. Rado, Guide for Pavement Friction. NCHRP Web-Only Document 108, National Cooperative Highway Research Program, Washington, DC., 2009. [7] D. Pezzaniti, S. Beecham, J. Kandasamy, Influence of clogging on the effective life of permeable pavements, Proc. ICE – Water Manage. 162 (3) (2009) 211– 220. [8] H.M. Imran, S. Akib, M.R. Karim, Permeable pavement and stormwater management systems: a review, Environ. Technol. 34 (17) (2013) 2649–2656. [9] J. Augenstern, T.B. Boving, M. Stolt, Porous Pavement Parking Lot and its impact on Subsurface Water Quality, in: Proceedings of the International Association of Hydrologists, XXXIII Annual meeting, Zacatecas, Mexico, 2004, pp. 1749– 1754. [10] R. Field, H. Masters, M. Singer, Porous pavement: research, development, and demonstration, J. Transp. Eng. 108 (3) (1982) 244–258. [11] J.W. Hall, K.L. Smith, L. Titus-Glover, J.C. Wambold, T.J. Yager, Z. Rado, Guide for Pavement Friction. NCHRP Web-Only Document 108, National Cooperative Highway Research Program, Washington DC, 2009. [12] R.B. Mallick, P.S. Kandhal, L.A. Cooley, D.E. Watson, Design, Construction, and Performance of New-Generation Open-Graded Friction Courses, NCAT Report 00–01, National Center for Asphalt Technology, Auburn, Alabama, April 2000. [13] J.T. van der Zwan, T. Goeman, J.A.J. Gruis, J.H. Swart, R.H. Oldergurger, Porous Asphalt Wearing Courses in the Netherlands: State of the Art Review, Transportation Research Record, No. 1265, Transportation Research Board, 1990, pp. 95–110.
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