Laboratorial investigation on effects of microscopic void characteristics on properties of porous asphalt mixture

Construction and Building Materials 213 (2019) 434–446

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Laboratorial investigation on effects of microscopic void characteristics on properties of porous asphalt mixture Bing Yang a, Hui Li a,b,⇑, Hengji Zhang a, Ning Xie a, Haonan Zhou a a b

Key Laboratory of Road and Traffic Engineering of the Ministry of Education, College of Transportation Engineering, Tongji University, Shanghai 201804, China University of California Pavement Research Center, University of California, Davis, USA

h i g h l i g h t s
The number, area and equivalent diameter of voids in porous asphalt were explored.
Void characteristics highly correlated with properties of porous asphalt.
The maximum equivalent void diameter has most significant effect on properties.
Microscopic void characteristics could optimize properties of porous asphalt.

a r t i c l e

i n f o

Article history: Received 25 December 2018 Received in revised form 5 April 2019 Accepted 8 April 2019

Keywords: Porous asphalt mixture Microscopic void characteristics Properties Correlation Maximum equivalent diameter

a b s t r a c t The properties of porous asphalt mixture significantly depend on its microscopic void characteristics. This paper mainly studied the correlation between the properties of porous asphalt mixture and three types of void characteristic parameters obtained through computed tomography (CT). It was found that the microscopic void characteristic parameters presented little influence on the moisture sensitivity resistance at high temperature and cooling performance, but strong correlation with other properties such as raveling resistance, permeability, connective porosity and noise reduction. In particular, the equivalent diameter and area exhibited strong correlation with properties of asphalt mixture. Moreover, the maximum equivalent diameter of void exerted the most significant impact on the properties of asphalt mixture. The investigated results could provide useful insights for optimizing and enhancing properties of porous asphalt mixture using the microscopic void parameters. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction As a low impact development (LID) technology, permeable pavement can reduce the area of impermeable surfaces and lower the negative effects on the environment, such as noise reduction, evaporative cooling, recharging of underground water, and improved driving safety [1–8]. However, due to insufficient structural capacity and weak strength [9–11], permeable pavement materials are usually used for light-duty pavements, such as plaza, and parking lots with porous concrete or asphalt mixture. The properties of permeable pavement materials are not only determined by conventional parameters, such as porosity and percent passing of 4.75 sieve, but also its microscopic void characteristics. At present, void content is commonly used to guide the design of

⇑ Corresponding author at: 4800 Caoan Rd, Shanghai 201804, China. E-mail address: [email protected] (H. Li). https://doi.org/10.1016/j.conbuildmat.2019.04.039 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

porous materials; however, microscopic void characteristics are hardly and used to evaluate the properties of permeable pavement. Effects of void characteristics on properties of permeable pavement have been studied. It was reported that the vertical porosity presented a much better correlation with the actual permeability than the permeability predicted using the average porosity [12]. Cooling mechanisms of permeable pavement are evaporation [13,14], heat resistance [15], or reflection [16,17]. Cooling effect of permeable pavement is highly dependent on evaporation rate [13,14], which is affected by air temperature, water temperature, void content and void size. Compared with impermeable pavement, permeable pavement reduced noise by 3–6 dB(A) [18,19]. Moreover, it was found that permeable pavement could also purify rainwater through zebrafish acute bio-toxicity test [20]. In general, different gradations result in mixtures with different void characteristics. The University of California Pavement Research Center (UCPRC) studied the effects of aggregate size on properties of permeable pavement materials [21].The results presented that permeability tended to increase with increasing aggregate size. Hamburg

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Wheel Tracking Device (HWTD) results showed that the opengraded mixes had less rutting and moisture sensitivity resistance at high temperature under soaked conditions compared to the dense-graded mix under the same conditions. The shear stiffness of porous asphalt mixture with 19-mm nominal maximum aggregate size was somewhat stiffer than the mixes with smaller aggregates. In addition, the raveling loss generally increased with increasing aggregate size [21]. Many researchers have applied nondestructive evaluation by using X-ray computed tomography (CT) to assess civil engineering materials [22,23]. The internal structure and voids distribution of asphalt mixture were achieved by X-ray CT [24–28]. In addition, image processing technology was used to study the void structure of permeable pavement. Jiang [29] conducted laboratory tests and obtained void features of porous asphalt concrete with CT to show a linear relationship between void diameters and properties of porous asphalt concrete, including Cantabro loss, dynamic stability, shear strength, anti-clogging and noise reduction. Allex [30] studied the internal structure of permeable friction course (PFC) by means of CT technology, and found that the voids of fieldcompacted mixtures and specimens under shear gyratory compactor (SGC) presented an uneven distribution in horizontal direction. Erdem [31] collected CT images of asphalt mixture before and after accelerated pavements tests to investigate changes in void and aggregate distribution with image processing and particle tracking methods. Zhou [32] investigated the influence of the size of clogging particle and pores on clogging using CT scanning and image processing, and the experimental results showed that the pore size could determine whether clogging particles can block or pass through the pores or not. Above all, void characteristics are mainly used to study the internal void distribution and evaluate the properties of mixtures. However, the influential mechanism of microscopic void characteristics on mechanical or ecological properties of materials has not been fully explored. This study will focus on the effects of microscopic void characteristics on mechanical and ecological properties of porous asphalt mixture. On the basis of correlation analysis, microscopic void characteristics and common design parameters would be combined to evaluate and enhance the properties of porous asphalt mixture.

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mass of asphalt to mixture in Table 2. Mixing and compaction temperatures of mixture were 160 °C and 185 °C, respectively. Gyratory compactor was used for preparation of specimens. In this study, R Studio [34] had been used to visually analyze the correlation between mixture properties and void characteristic parameters. The correlation visualization of R can help distinguish the significant difference of correlation and simplify data processing.

3. Experiments 3.1. CT scanning test X-ray CT was used to obtain the internal structure images of mixtures due to different X-ray absorption, and 3D structure was generated by computer. Images of cross-section were achieved every 4 mm from top to bottom of each specimen, and a total 11 images of each specimen were selected for image processing. The size of each image was 1200 * 1255 pixels, and the scanning precision was 1 mm per 11.5 pixels. Fig. 1(a) presented original CT images and processed images with threshold method were shown in Fig. 1(b), in which the black represented air voids. The parameters of void characteristics each cross-section were calculated by means of threshold method, including void number, void area, void area ratio, maximum equivalent void diameter (max), average, median, 25% quantile, and 75% quantile of equivalent void diameter. Equivalent void diameter was calculated with Eq. (1), and the definitions of the void parameters listed in Table 3.

D¼

rffiffiffiffiffiffi 4A

p

ð1Þ

where D = equivalent diameter, A = void area each cross-section. It can be clearly seen from Fig. 1(b), with the increase of the nominal maximum aggregate size (NMAS) of asphalt mixture, the number of voids tended to decrease significantly, while the size of voids tended to increase. In addition, the void distribution inside asphalt mixture was very uneven. Due to the precision of threshold, the interface of asphalt film and voids could not be precisely identified. Therefore, when calculating the void parameters, the voids with the equivalent diameter less than 1 mm were ignored.

2. Materials and methods This study used high viscosity asphalt with 8% styrene – butadiene – styrene block copolymer (SBS). Its properties were tested [33] and shown in Table 1. Basalt and limestone were used as coarse aggregates and fine aggregates, respectively. It is very important to obtain clearly distinguishable void differences. Some researchers have prepared porous asphalt mixtures by porosity [29]. In this paper, in order to obtain porous asphalt mixtures with distinguishable void differences, 5 types of gradations were chosen, as shown in Table 2. In addition, the same asphalt film of 13 lm was used to reduce the influence of asphalt film thickness on void structure. Binder content refers to the ratio of

Table 1 Properties of high viscosity asphalt. Test items

Unit

Specifications (JTG E20-2011)

Results

Penetration@25 °C, 100 g, 5 s Penetration Index Ductility@5 °C 5 cm/min, cm Softening Point, TR&B Dynamic Viscosity @60 SHRP PG

0.1 mm – cm °C Pas –

20–40 0.0 20 82 20000 PG 76–22

38 +0.17 22 88 14,532 PG76-22

3.2. Mechanical properties test Three types of Cantabro tests were conducted in this paper, and different experimental procedures were listed as follows: 1) Standard Cantabro Test: the specimens were preserved in water with 20 ± 0.5 °C for 20 h, and then they were removed and put in the Los Angeles abrasion machine to conduct tests; 2) Water immersion Cantabro Test: the specimens were preserved in water with 60 ± 0.5 °C for 48 h, and then removed and placed at indoor laboratory temperature for 24 h, and finally put in the Los Angeles abrasion machine to conduct tests; 3) Aging Cantabro Test: the specimens were preserved in oven with 60 ± 0.5 °C for 7 days, and then removed and placed at indoor laboratory temperature for 24 h, and finally put in the Los Angeles abrasion machine to conduct tests; Hamburg Wheel Tracking Device (HWTD, AASHTO T 324-14) was used to evaluate the moisture sensitivity resistance at high temperature. Water temperature of tests was controlled at 60 °C, and the tests ended when rut depth was up to 10 mm.

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Table 2 Gradations of porous asphalt mixtures. Sieve/mm 19 16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075 Binder content/%

OGFC-5 Percent Passing/%

100.0 100.0 95.0 20.0 5.0 5.0 5.0 5.0 5.0 4.4

OGFC-10 Percent Passing/%

OGFC-13 Percent Passing/%

OGFC-16 Percent Passing/%

OGFC-20 Percent Passing/%

100.0 100.0 70.0 10.0 6.0 6.0 6.0 6.0 6.0 4.9

100.0 100.0 97.0 68.0 28.0 16.0 12.0 8.0 6.0 6.0 6.0 4.9

100.0 90.0 70.0 45.0 12.0 10.0 6.0 6.0 6.0 6.0 6.0 4.6

95.0 90.0 64.0 37.0 10.0 10.0 6.0 6.0 4.0 4.0 4.0 3.2

Fig. 1. Voids distribution each gradation: (a) X-ray CT images; (b) images processed with threshold.

Table 3 Definitions of the void parameters. Void parameters

Definitions

Void number Area Area ratio Max

The number of voids from every cross-section The sum of area of voids from every cross-section The ratio of area of voids to total area of a cross-section The maximum of equivalent diameter of voids from a cross-section The average of equivalent diameter of voids from a cross-section The median of equivalent diameter of voids from a cross-section The 25% quantile of equivalent diameter of voids from a cross-section The 75% quantile of equivalent diameter of voids from a cross-section

Average Median 25% Quantile 75% Quantile

3.3. Permeability test The test method of the permeability coefficient was constant head test method from ‘‘Technical specification for permeable cement concrete pavement” (GJJ/T 132–2009). During the test, the water head difference H had been recorded every 5 s, which

was used to calculate the permeability coefficient based on the Eq. (2). Every specimen was tested for 3 times to calculate the average.

K¼

QL AHT

ð2Þ

where: K = permeability coefficient (cm/s), Q = water output in the period of T (ml), L = the specimen height (cm), A = the crosssection area of the specimen (cm2 ), H = the water head difference (cm), and T = the period of testing time (s). 3.4. Evaporation rate test In this study, evaporation rate was used to evaluate the cooling performance of different asphalt mixtures. The test procedure was as follows: 1) Fill voids with water: put specimens into water with indoor laboratory temperature for 20 h, which made air voids filled with water; 2) Evaporation test: a) put specimens into polyvinyl chloride (PVC) plastic cylinders with 100 mm diameter and 150 mm height; b) injected water to the cylinders when water just covered the surface of the specimen, and placed the cylin-

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ders in the oven at 50 ; c) recorded the change of the total mass over time. The evaporation rate was calculated with Eq. (3).

Ev aporation rate ¼ ðmt2  mt1 Þ=ðt 2  t 1 Þ

4. Results and discussion 4.1. Visualization of overall correlation results

ð3Þ

The results of laboratory tests and void characteristics parameters are shown in Table 4 and Table 5 respectively. Visualization results by means of R Studio are presented in Fig. 2, and the diagonals in the figure show the names of void characteristic parameters and properties of asphalt mixture, the upper right figure displays the scatter plots between every two parameters on the diagonals, and the lower left figure displays the correlation coefficient between every two parameters. From Fig. 2, the maximum correlation coefficients between the void characteristic parameters and the properties of mixture were labeled. As can be seen from the leftmost column, the number of voids was negatively correlated with the equivalent diameter of voids and positively correlated with the area of voids. In addition, the correlation between the number of voids and the equivalent diameter was more significant. In this study, the equivalent diameter included maximum and average, 25% quantile, median and 75% quantile. An interesting result was that all of the equivalent diameter parameters except the maximum appeared strong correlation with the number of voids. According to Fig. 2(a), it can be seen that the permeability coefficient presented strong correlation with the equivalent diameter and the number of voids, but weak correlation with the voids area. In addition, permeability coefficient was negatively correlated with the number of voids and positively correlated with the equivalent diameter. As can be seen from the marked correlation coefficients, the maximum equivalent diameter had the largest correlation coefficient with the permeability coefficient, and the average of equivalent diameter and the 75% quantile also presented fairly strong correlation with the permeability coefficient. The correlation between porosity and three types of void characteristics parameters was weak, indicating that microscopic void characteristic parameters were not enough to evaluate the porosity of asphalt mixture. The connective porosity was highly correlated with the maximum equivalent diameter and was weakly correlated with the number and area of voids. While evaporation rate was weakly correlated with three types of void parameters, but

where mt2 = the total mass of the cylinder at t2, and mt1 = the total mass of the cylinder at t1. 3.5. Connective porosity test The mass of the specimen in water was tested to calculate connective porosity through the Eq. (4).

Vt ¼ ðV  V 0 Þ=V

ð4Þ

where Vt = connective porosity, V = the gross volume of the specimen, V 0 = the volume of aggregate and closed voids, and A = the weight of the dry specimen. rw  1:0g=cm3 。 Porosity of asphalt mixture was calculated with Eq. (5):

 VV ¼



1

cf  100 ct

ð5Þ

where: VV = porosity of asphalt mixture (%); cf = the gross volume relative density; ct = maximum theory relative density. 3.6. Sound absorption coefficient test Standing wave ratio was used to test the sound absorption coefficient of different specimens at different acoustic frequencies. The impedance tube used in this study was from Beijing, China, and a low-frequency tube with 100 mm diameter was used for the test. The test method was found on ‘‘Acoustics-Determination of sound absorption coefficient and impedance in impedance tubes - Part 1: Method using standing wave ratio” (GB/T18696.1–2004). The test frequencies were 200 Hz, 250 Hz, 315 Hz, 400 Hz, 500 Hz, 630 Hz, 800 Hz, 1000 Hz, 1250 Hz, 1600 Hz, and 2000 Hz. The sound absorption coefficient of the test specimen was the average value of the sound absorption coefficient at all test frequencies.

Table 4 Test results of different experiments. Mixture

OGFC-5

OGFC-10

OGFC-13

OGFC-16

OGFC-20

Porosity (%) Connectivity porosity (CP, %) Permeability (cm/s) Evaporation rate (ER, g/h) Sound absorption (SA) HWTD Standard raveling loss (SR, %) Water immersion raveling loss (WR, %) Aging raveling loss (AR, %)

27.1 14.93 0.65 1.05 0.36 290 16.2 20.8 12.3

24.1 13.29 0.68 0.78 0.39 570 27.1 34.2 26.3

20.1 12.83 0.66 0.53 0.2 2960 18.5 21.6 15.3

24.2 17.78 1.80 0.92 0.31 730 31.8 29.5 22.4

26.7 21.56 2.78 1.10 0.29 1290 54.4 88.6 45.4

Table 5 Parameters of microscopic void characteristics. Mixture

OGFC-5

OGFC-10

OGFC-13

OGFC-16

OGFC-20

Number Area ratio (%) Area (mm2 ) Max (mm) Average (mm) Median (mm) 25% Quartile (%) 75% Quartile (%)

276 23.2 1823.40 9.75 2.23 1.73 1.25 2.67

267 21.8 1712.45 10.21 2.40 1.93 1.37 2.94

171 15.6 1224.98 8.94 2.63 2.22 1.51 3.32

108 19.5 1530.75 11.32 3.60 2.93 1.84 4.89

77 16.8 1298.65 13.42 3.77 2.81 1.68 4.99

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was relatively well correlated with the equivalent diameter. The sound absorption coefficient was highly correlated with voids area, but not with equivalent diameter and the number of voids.

In addition, from Fig. 2(a), permeability was highly correlated with connectivity and poorly correlated with porosity, which indicated that connective porosity was more suitable to evaluate per-

Fig. 2. Correlation between voids characteristics and properties: (a) ecological properties; (b) mechanical properties.

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Fig. 3. Void equivalent diameter for different parameters: (a) NMAS; (b) percent passing of 4.75 mm sieve.

Fig. 4. Void number for different parameters: (a) NMAS; (b) percent passing of 4.75 mm sieve.

Fig. 5. Void area and area ratio for different parameters: (a) NMAS; (b) percent passing of 4.75 mm sieve.

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meability due to closed voids included in mixtures. Moreover, porosity presented more strong correlation with evaporation rate, but weak correlation with connective porosity. It should be noted that the specimens were put into water for 20 h and voids were filled with water. Therefore, water of closed voids also evaporated in the test. From Fig. 2(b), Hamburg Wheel Tracking Device (HWTD) results presented strong correlation with voids area, but poor correlation with the equivalent diameter and the number of voids. The correlation between three types of Cantabro tests and the void characteristic parameters was similar, and the maximum equivalent diameter and Cantabro tests achieved the largest correlation coefficient. In addition, the correlation between every two types of raveling loss was very strong, which indicated that the experimental conditions poorly affected the change tendency of raveling loss. In addition, compared with microscopic void characteristics, percent passing of 4.75 mm sieve weakly correlated with mechanical and ecological properties from Fig. 2. Microscopic void

characteristics presented strong correlation with properties of porous asphalt mixture from marked correlation coefficients in Fig. 2. 4.2. Microscopic void characteristics With the increase of the nominal maximum aggregate size (NMAS) of asphalt mixture, the average, median, 25% and 75% quartile of the void equivalent diameter tended to increase from Fig. 3(a), except the maximum equivalent diameter of OGFC-13. As shown in Fig. 4(a), with the increase of the NMAS, the number of voids decreased. The void area and area ratio of the crosssection of the mixture tended to decrease with the increase of NMAS from Fig. 5(a). Compared with NMAS, microscopic void characteristics presented opposite change trend with increasing percent passing of 4.75 mm sieve. Above all, when NMAS increased, percent passing of 4.75 mm sieve decreased, the number of coarse aggregate in the mixture increased and the number of fine aggregate decreased, indicating that the content of coarse aggregate

Fig. 6. Maximum equivalent diameter and porosity: (a) change trend; (b) linear fitting.

Fig. 7. Maximum equivalent diameter and connective porosity: (a) change trend; (b) linear fitting.

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had a significant impact on the internal voids structure and distribution of asphalt mixture. 4.3. Porosity and connective porosity As shown in Fig. 2(a), the correlation between porosity and void characteristic parameters was weak, and the porosity of specimens was more than 20% in this study. Therefore, it could be inappropriate to evaluate the porosity of asphalt mixtures with large porosity by using microscopic void characteristic parameters. In Fig. 6, there is a valley in value at the NMAS of 13 mm. Porosity of 13 mm-NMAS was minimum among 5 types of gradations, and it could be difficult to produce larger equivalent diameter if porous asphalt mixture has a small porosity. Larger NMAS could not achieve greater porosity, and internal voids of porous asphalt mixture were not only determined by porosity. If porous asphalt mixture has few content of air voids and larger size of aggregates, porous asphalt mixture can produce larger size of air voids and larger equivalent diameter. If porous asphalt mixture has few content

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of air voids and small size of aggregates, porous asphalt can produce small size of air voids and small equivalent diameter. Moreover, it can be seen from Fig. 6(a), when NMAS was greater than 5 mm, porosity and maximum equivalent diameter displayed very similar trend. The data with NMAS of 5 mm was ignored in the linear fitting, as shown in Fig. 6(b). The linear fitting results exhibited very strong correlation between porosity and maximum equivalent diameter. Therefore, when NMAS was greater than 5 mm, the larger equivalent diameter will result in the greater porosity. With the increase of NMAS, the gradation becomes coarser and the number of fine aggregate decreases, which leads to a structure formed by coarse aggregate. In other word, bulkier voids appear inside the mixture and the void content of asphalt mixture increases. It can be seen from Fig. 7(a), with the increase of NMAS of asphalt mixture, the connective voids firstly decreased and then increased. From Fig. 2(a), it can be known that connective porosity presented the best correlation with the maximum equivalent diameter. Linear fitting was carried out and shown in Fig. 7(b).

Fig. 8. Maximum equivalent diameter and permeability: (a) change trend; (b) linear fitting.

Fig. 9. Evaporation process with time: (a) cumulative evaporation; (b) evaporation rate.

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Fig. 10. Maximum equivalent diameter and evaporation rate: (a) change trend; (b) linear fitting.

Fig. 11. Sound absorption coefficient with frequency.

Fig. 13. Repetitions to 10 mm rut of different mixtures.

Fig. 12. Sound absorption coefficient and area ratio: (a) change trend; (b) linear fitting.

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The correlation coefficient was 0.96 and the p value was 0.004, which showed a very strong linear correlation. 4.4. Permeability The permeability coefficient of mixtures with different NMAS was measured by constant head test. It can be seen from Fig. 2 (a), except area and area ratio, the permeability coefficient presented strong correlation with other voids parameters, and achieved the largest correlation coefficient with the maximum equivalent diameter. There is a valley in value at the NMAS of 13 mm in Fig. 8(a), which is similar with Fig. 6(a). OGFC-13 had a small porosity and equivalent diameter, which can produce more closed voids and lower permeability. As shown in Fig. 8(a), both of maximum equivalent diameter and permeability displayed very similar change tendency. Linear fitting of them exhibited that the correlation coefficient was 0.96 and p value was 0 from Fig. 8(b), indicating a very significant linear relationship. In other word, with the increase of the maximum equivalent diameter, the permeability coefficient of asphalt mixtures presented increasing tendency. From the above, the number and equivalent diameter of voids were important factors affecting the permeability coefficient of asphalt mixtures. The water flow pathways running fluently formed inside the mixture with greater equivalent diameter. On the other hand, the number of voids negatively correlated with permeability coefficient, which indicated that more voids led to narrower water flow pathways and greater viscous resistance. In addition, weak correlation between connective porosity and porosity was displayed in Fig. 2(a), and the correlation coefficient was only 0.59, which was due to non-connective voids inside the mixture. Therefore, it was not a good method to gain suitable connective porosity by means of porosity, and the porosity could hardly determine the permeability of asphalt mixtures. Moreover, the permeability was strongly correlated with connective porosity, and the connective porosity could be used to improve the permeability coefficient combined with the porosity. 4.5. Evaporation rate According to R visualization from Fig. 2(a), it can be seen that the correlation coefficient between porosity and evaporation rate was 0.97, indicating that porosity was an important factor affecting the evaporation efficiency of porous asphalt mixture, while evaporation rate weakly correlated with connective porosity. It was concluded that evaporation could hardly be affected by closed voids.

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As shown in Fig. 9, evaporation rate test was conducted for 48 h. From the beginning to 10th h, evaporation proceeded on the top half of the specimens and there existed unstable change of evaporation rate due to specimen preparation and uneven distribution of voids. In this period of time, water rose to the surface of the specimen via capillary action and then escaped into air. When the capillarity was not enough to overcome the gravity of water and the viscous resistance, water evaporation process continued at the lower plane and escaped into the air by means of vapor diffusion through the upper voids. From Fig. 9(a), cumulative evaporation curves exhibited significant difference after 10 h. In addition, considering that the voids characteristic parameters were calculated from images of the middle region of the specimen, the average evaporation rate of 10–25 h was selected in this study to study the correlation between evaporation rate and void characteristic parameters. From Fig. 10(a), evaporation rate appeared different change trend from the maximum equivalent diameter. However, when the NMAS was greater than 5 mm, the evaporation rate and maximum equivalent diameter presented similar change trend. In addition, in Fig. 10(a), there is a valley in values at the NMAS of 13 mm. According to water evaporation process, if porous asphalt mixture had a larger equivalent diameter, water quickly escaped into air. If porous asphalt mixture had a small equivalent diameter, water rose via capillary action and escaped into air through vapor diffusion. The equivalent diameter of OGFC-13 was in a middle position. Water firstly escaped into air by vapor diffusion and capillary action, and it could become slow if capillarity was not enough to overcome the gravity of water and the viscous resistance. According to R visualization from Fig. 2(a), the evaporation rate achieved the maximum correlation coefficient with the maximum equivalent diameter, but the correlation coefficient was only 0.71. If OGFC-5 was ignored in the linear fitting, as shown in Fig. 10(b). The marker in the figure referred to OGFC-5, which obviously deviated from the other. Although OGFC-5 contained a large number of voids, they formed narrow water flow pathways leading to stronger viscous resistance. From the results of linear fitting, the correlation coefficient was 0.98 and p value was 0.003, indicating strong correlation. 4.6. Sound absorption The sound absorption coefficient of the specimens at different frequencies was tested by standing wave tube. The test results were shown in Fig. 11. Among the range of 200–1000 Hz, sound

Fig. 14. Repetitions to 10 mm rut and area ratio: (a) change trend; (b) linear fitting.

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absorption coefficient of mixtures presented similar trend. From the visualization results in Fig. 2(a), sound absorption coefficient appeared strong correlation with the area and area ratio of crosssection, which can be found in Fig. 12(a). It can be seen from Fig. 12(b), there was an obvious linear relationship between sound absorption coefficient and the area ratio. The correlation coefficient was 0.91, and p value was 0.005. With the increase of the voids area, sound absorption coefficient of asphalt mixture tended to increase. In other word, with the increase of voids content inside asphalt mixture, greater sound absorption coefficient will produce better noise reduction effect. 4.7. Results of Hamburg Wheel Tracking test As shown in Fig. 13, OGFC-13 had the best ability of moisture sensitivity resistance at high temperature under soaked condition. From Tables 4 and 5, OGFC-13 had minimum porosity and the maximum equivalent diameter of OGFC-13 was less than any other, which could be the reason why OGFC-13 had the best ability of moisture sensitivity resistance. If OGFC-13 was ignored, with the increase of the NMAS, the ability of moisture sensitivity resistance became better. HWTD displayed weak correlation with the number of voids and the equivalent diameter from Fig. 2(b), but negatively

strong correlation with the area ratio from Fig. 14(a). The correlation coefficient between HWTD and area ratio was 0.87, and p value was 0.071 from Fig. 14(b), indicating that larger area or area ratio of voids could result in worse resistance to moisture sensitivity at high temperature. This is due to the fact that the contact area between mixture and water becomes larger with increasing area of voids. Moreover, spalling of asphalt at high temperature under soaked condition would also destroy void structure. 4.8. Results of Cantabro test From Fig. 2(b), it can be seen that 3 types of Cantabro tests results presented similar correlation with the maximum equivalent diameter. As shown in Fig. 15(a), three types of raveling loss tended to increase with the increase of the NMAS. Fig. 15 displayed linear fitting with 3 types of raveling loss and the maximum equivalent diameter. The correlation coefficients of standard, water immersion and aging raveling loss with the equivalent diameter were respectively 0.97, 0.91, and 0.91, indicating that the raveling resistance of mixture was highly correlated with the maximum equivalent diameter. This is due to the fact that the maximum equivalent diameter increases with the increase of coarser aggregate, which results in forming fewer contact area among aggregate

Fig. 15. Microscopic voids parameters and raveling loss: (a) the maximum equivalent diameter and raveling; (b) linear fitting with standard raveling; (c) linear fitting with water immersion raveling loss; (d) linear fitting with aging raveling loss.

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particles. Therefore, the anti-raveling properties of asphalt mixture can become weak without adding the content of asphalt. 5. Conclusion This paper mainly studied the effects of microscopic void characteristic parameters on mechanical and ecological properties of porous asphalt mixture, including raveling resistance, moisture sensitivity resistance at high temperature, permeability, cooling and noise reduction performance. It was found that properties of porous asphalt mixture presented strong correlation with microscopic void characteristic parameters. The following conclusions can be drawn from the results and analyses in this study: (1) With increasing nominal maximum aggregate size, the equivalent diameter tended to increase, while the number and area of voids inclined to decrease, which suggested that the content of coarse aggregate significantly affected structure and distribution of voids. There was strong correlation between the number and equivalent diameter of voids; however, the correlation between the area and the other two parameters was weak. The equivalent diameter appeared the strongest correlation with properties of asphalt mixture. Moreover, compared with microscopic void characteristics, percent passing of 4.75 mm sieve weakly correlated with mechanical and ecological properties. (2) Raveling resistance, permeability, connective porosity and noise reduction presented strong correlation with the microscopic void characteristic parameters, while moisture sensitivity resistance at high temperature and cooling performance weakly correlated with the microscopic void characteristic parameters. Moreover, in this study, evaporation rate tests were conducted using PVC plastic cylinders under 50 oven, which resulted in keeping porous materials and water under a constant temperature. However, the temperature of real pavement decreases from surface to subgrade. Therefore, it might be limited to validly evaluate evaporation rate of porous asphalt mixture based on the results of this paper. (3) In addition to sound absorption coefficient and moisture sensitivity resistance at high temperature, anti-raveling, cooling, permeability and connective porosity were all highly correlated with the maximum equivalent diameter. Therefore, under certain conditions, the maximum equivalent diameter can be used to comprehensively evaluate the main mechanical and ecological properties of porous asphalt mixture. (4) According to the research results of this paper, the influence of microscopic void parameters on total porosity was relatively weak. Permeability coefficient had strong linear correlation with connective porosity, and the maximum equivalent diameter strongly correlated with connective porosity and permeability. Therefore, it would not be suitable to produce porous asphalt mixture with high permeability through increasing total porosity. If the maximum equivalent void diameter or connective porosity can be used and combined with total porosity in the design process of porous asphalt mixture, it could produce beneficial effect. Conflict of interest None. Acknowledgement The grants supporting this research are from Ministry of Science and Technology of the People’s Republic of China, China (Grant No. 2016YFE0108200), the Fundamental Research Funds for the Central Universities, China (Grant No. 22120180093), Science and Technology Commission of Shanghai Municipality, China (Grant

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No. 17230711300), Department of Transportation of Hebei Province, China (Grant No. QG2018-5), and Fund of Shanghai Peak Discipline, China (Grant No. 2016J012309). This paper only reflects the views of the authors, but not the official views or policies of the sponsors.

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