Effect of aggregate contact characteristics on densification properties of asphalt mixture

Construction and Building Materials 204 (2019) 691–702

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Effect of aggregate contact characteristics on densification properties of asphalt mixture Jiange Li a, Peilong Li a,b,⇑, Jinfei Su a, Yu Xue a, Wenyu Rao a,c a

Highway School, Chang’an Univ., Xi’an, Shaanxi 710064, China Key Laboratory of Road Structure & Material Ministry of Transport, Chang’an Univ., Xi’an 710064, China c Guangdong Province Communications Planning & Design Institute Co., Ltd, Guangzhou 510507, China b

h i g h l i g h t s
An aggregate contact device (ACD) was developed to evaluate the aggregate contact characteristics.
Two parameters were obtained according to the contact test curve.
The lubrication effect of asphalt binder in high temperature was considered.
The relationships between the aggregate contact characteristics and densification properties of asphalt were discussed.

a r t i c l e

i n f o

Article history: Received 2 September 2018 Received in revised form 1 January 2019 Accepted 3 January 2019

Keywords: Asphalt mixture Aggregate contact Densification property Lubrication effect Contact force Required energy

a b s t r a c t The densification properties of asphalt mixture are an external manifestation of aggregate contact action, including the friction, interlocking and dislocation of aggregates as well as the cohesion of asphalt binder during compaction. The asphalt binder also has a lubrication effect on the contact action at the high temperature. To investigate the effect of aggregate contact characteristics on the densification properties of asphalt mixture, an aggregate contact device was developed and aggregate contact tests were conducted on different conditions. Two parameters, F that refers to the maximum contact force composed of friction and interlocking force and E that refers to the required energy during the whole test, were gained according to the contact test curve. Considering the lubrication of asphalt binder at the high temperature, the MF and ME refer to the dry mixture, while the AMF and AME refer to the asphalt mixture. The densification parameters were obtained by means of analyzing the densification curves. Then the relationships between the parameters of aggregate contact and those of densification of asphalt mixture were discussed. The results indicate that the aggregate contact action composed of friction and interlocking force increases with the enlargement of nominal maximum aggregate size (NMAS) of the asphalt mixture, and the values of MF and ME increase with the increase of NMAS. The asphalt binder has a significant lubrication effect on the aggregate contact at the high temperature. The contact action of asphalt mixture with a dense gradation is lubricated enough is prone to be compacted. Comparing to test temperature and asphalt content, the gradation of mixture and its NMAS are more easily to affect the contact action, which has the significant effect on the densification properties of the asphalt mixture. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction As a type of multiphase granular material, the micro effects of aggregate interfaces such as friction, interlocking, dislocation and cohesion of asphalt mixture affect its macro mechanical properties and durability. Those micro effects derived from aggregate contact

⇑ Corresponding author at: Key Laboratory of Road Structure & Material Ministry of Transport, Chang’an Univ., Xi’an 710064, China. E-mail address: [email protected] (P. Li). https://doi.org/10.1016/j.conbuildmat.2019.01.023 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

action have a directly influence on the densification properties during asphalt pavement compaction and the rutting resistance under traffic loading [1–3]. It could be found that there is a relationship between mechanical response of asphalt mixture and contact action of its coarse aggregates [4,5]. Haskett et al. [6] analyzed the shear friction and aggregate interlock behavior across sliding planes of mineral aggregates. Gao et al. [7] studied that the effects of inter-particle effect and imperfect on the dynamic modulus of asphalt mixture were analyzed by a micromechanical model. Zhang et al. [8] characterized the microstructure of asphalt

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mixture, fabricating specimens via Superpave Gyratory Compaction, by the means of X-ray CT scanning. Li et al. [9,10] discussed the relationship between deformation property and aggregate slip behavior of asphalt mixture. The best deformation resistance of asphalt mixture could be obtained with its largest slip energy index. And the slip energy index had a well linear correlation with the aggregate contact index (ACI) and voids filled with asphalt (VFA), reflecting the contact characteristics of coarse aggregate at a meso level. Tashman et al. [11] qualitatively illustrated the frictional effect among the aggregates, but not researched the methods and indicators to evaluate the frication. The effects of asphalt binder on the friction had not been analyzed quantificationally. The asphalt binder has a lubrication effect on the interface of aggregates in the asphalt mixture at the higher temperature, which affects the aggregate movement during the compaction [12]. Majidi et al. [13] found that the temperature had a positive influence on densification of modified asphalt mixture, as well as the lower value of porosity was expected for the mixture compacted or vibrated at the higher temperature. Wu et al. [14] suggested that the compaction effort as a design factor should be concerned during the asphalt mixture design, considering the aggregate property and asphalt binder had the effect on its densification property. As the key process of asphalt pavement construction, the quality control of compaction had been researched for many years. The performance of asphalt mixture is affected significantly by the quality of compaction. Insufficient compaction often leads to a premature permanent deformation, excessive aging and moisture damage [15]. It is well known that the gradation, property of aggregate and asphalt binder have an obvious influence on the slope and intercept values of the densification curves of asphalt mixture [16,17]. From the micro perspective, the density of asphalt mixture depends on contact states among aggregates. Specifically, each aggregate is in the stable contact state after rolling and migrating. The stability of aggregates would be reduced in adverse condition, then it could be easy to cause the asphalt pavement rutting. There is no doubt that the contact characteristics of aggregate is closely related to the densification properties, which affects the construc-

tion quality of asphalt pavement and its high temperature performance. But, very little is known regarding what the real relationship is between the aggregate contact characteristics and densification properties of asphalt mixture and whether the asphalt mixture can be improved using the contact characteristics to achieve a better performance. This research aims to study the effects of aggregate contact characteristics on the densification properties of asphalt mixture. The aggregate contact device (ACD) was developed and the new tests of aggregate contact were conducted, which were used to investigate the contact characteristics of aggregates in the asphalt mixture. Two test parameters, F (MF/AMF) and E (ME/AME), were gained according to the contact test curve. The lubrication effects of asphalt binder on the contact characteristics were discussed with different test conditions. The compaction properties of asphalt mixture were obtained by the means of analyzing the densification curves. The relationships between aggregate contact characteristics and densification properties of asphalt mixture were discussed to reveal its mechanical properties during the densification process. The flow chart of this research as shown in Fig. 1. 2. Material 2.1. Binder The asphalt binder and its properties have a profound influence on the characteristics of the asphalt mixture [18]. The Shell bitumen (90# in terms of penetration grading) was used in this study. Its properties were listed in Table 1. 2.2. Aggregate and gradations Limestone was used as coarse and fine aggregates. Their specific gravities with each sieve size are shown in Tables 2 and 3. The ground limestone was utilized for filler that had the density of 2.713 g/cm3. Those conventional continuously dense-graded asphalt mixtures, AC-13, AC-16 and AC-20, which were defined in the Chinese Technical Specification for Construction of Highway

Fig. 1. Flow chart of the research process.

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along the guide rail with the STM as shown in Fig. 2(d). As indicated in Fig. 3, the three bins of the ACD were filled with aggregates or asphalt mixture during the test process. With the mid bin moving up, the contact interaction of aggregates happened and spread on the contact and glide plane as shown in Fig. 3. Considering that test error could be produced based on the dilatancy effect, lateral restraint was changed into elastic situation during the test.

Table 1 Properties of asphalt binder. Properties Penetration (25 °C, 5 s, 100 g)/0.1 mm Ductility (5 cm/min,10 °C)/cm Ductility (5 cm/min,15 °C)/cm Softening point (R&B)/°C Viscosity (135 °C)/Pas Viscosity (175 °C)/Pas Density (15 °C)g/cm3 Film heating test Mass lost/% (163 °C, 5 h) Penetration ratio/% Residual ductility (5 cm/min)/cm

Value 88.6 79.5 >100 46 0.383 0.078 1.034 0.065 61.2 47.3

Methods ASTM D5 ASTM D113 ASTM D113 ASTM D36 ASTM D4402 ASTM D4402 ASTM D70 ASTM D1754 ASTM D1754/D5 ASTM D1754/D113

3.2. The method of aggregate contact test The aggregate contact properties of dry mixture and asphalt mixture with gradation of AC-13, AC-16 and AC-20 were conducted by the ACD. Comparing with asphalt mixture, the dry mixture is just without asphalt binder. Four test temperatures (110 °C, 125 °C, 140 °C and 155 °C) and five asphalt contents for each gradation were chosen for the purpose of studying the impaction of temperature and asphalt content on the contact characteristics of aggregates of asphalt mixture. The optimal asphalt content of AC-13, AC-16 and AC-20 were 4.8%, 4.5% and 4.0%, respectively. Other asphalt contents of each gradation were increased and decreased by 0.3% or 0.5% on the basis of the optimal asphalt content. Before the aggregates were filled in the ACD, two steel sheets were inserted into the contact windows in the left and right. At the same time, lateral restraint was changed into rigid situation. Then invariable volume of three bins were constructed. After that, the aggregates of mixture were filled into bins respectively. Afterwards, the cover plates were fastened on these bins. And next, the ACD was connected to the STM with the upper and lower fixtures. Two sheets were pulled out form the ACD that contact plane in the windows were generated meanwhile. Subsequently, lateral restraint was changed into elastic situation making the test to be started precisely. A certain velocity was taken during the test. At the same time, the test data could be recorded and saved automatically by the computer. The test would be stopped immediately after 80 mm displacements of the mid bin. Considering that asphalt binder is sensitive to temperature, it is important to pay attention to the temperature during the aggregate contact test of asphalt mixture [20,21]. Before the aggregates of asphalt mixture were filled into the ACD, the ACD should be preheated in oven at each test temperature. After that, the ACD with full of aggregates was connected to the STM, and kept at each setting temperature for 30 min in the constant temperature oven of STM. Then, six tests were conducted at least with the same conditions.

Table 2 The bulk-specific gravity of coarse aggregate AASHTO T-85. Sieve size/mm

19

16

13.2

9.5

4.75

Bulk-specific gravity/(g/cm3)

2.711

2.717

2.742

2.734

2.764

Asphalt Pavement Chinese National Specification JTG F40-2004 (2004), were applied in this research. Their gradations are shown in Table 4. In order to obtain a strict gradation, all aggregates were sieved into meticulous sizes that met each size in Table 4, and then aggregates of each size were mixed into the gradations.

3. Test method and evaluation parameters 3.1. Aggregate contact test device The macroscopic mechanical properties of asphalt mixture are manifestations of their material properties and aggregates contact interaction by a microscopic view [19]. Aggregates are squeezed and misaligned under the load. The aggregate contact, including interlocking force and friction, is generated during the loading. In order to learn the aggregate contact of asphalt mixture, the aggregate contact device (ACD) was developed and tested on the electrohydraulic servo testing machine (STM) as shown in Fig. 2. As shown in Fig. 2(a) and (b), the ACD includes mid bin (with isolator and cover plate) and two side bins (with cover plate, side plane and lateral restraint) in the left and right sides. Two square contact windows (80mm  80 mm) are symmetrically designed on both sides of the mid bin. The lower part of contact window each side of mid bin is integrated isolator, which could be used to restrain aggregates in the side bins during the test. After mid bin was filled with aggregates or asphalt mixture as the same way like side bins shown in Fig. 2(c), the device was connected to the STM with the fixture, and then the mid bin was pulled up at a certain velocity

3.3. Parameters of aggregate contact test After a large number of pilots, the loading velocity of 15 mm/min and the data sampling frequency of 60 times per

Table 3 The bulk-specific gravity of fine aggregate AASHTO T-84. Sieve size/mm

2.36

1.18

0.6

0.3

0.15

0.075

Apparent-specific gravity/(g/cm3)

2.73

2.77

2.773

2.765

2.717

2.715

Table 4 Aggregate gradation. Gradation

AC-20 AC-16 AC-13

Passing rate of each sieve size (mm)/% 26.5

19

16

13.2

9.5

4.75

2.36

1.18

0.6

0.3

0.15

0.075

100

95 100

83 95 100

72 81 95

61 70 76.5

41 48 53

30 34 37

22.5 24.5 26.5

16 17.5 19

11 12.5 13.5

8.5 9.5 10

5 6 6

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(a) Structure diagram

(b) ACD device

(c) Side bins with aggregates

(d) Loading process

Fig. 2. The aggregate contact device.

second could be obtained. 14,400 data were sampled and drawn as a curve for one test (see Fig. 4). Obviously, the load value shows fluctuations with increasing of displacement in uniform loading mode. From Fig. 4, it can be seen that there is a marked difference between the dry mixture and asphalt mixture. On the same ordinate scale, the curve of dry mixture fluctuates violently while the asphalt mixture shows a smooth curve. It demonstrates that asphalt binder has a significant lubrication effect on contact interaction in a high temperature. While the mid bin moving up, the aggregates in the contact windows have changed the contact situation into slide situation. The load value refers to the friction and interlocking force that need to be overcome for all contacted aggregates. Every peak of the curve representing a higher level of contact interaction

between those aggregates. As indicated in Fig. 5, two test parameters, F and E, were gained according to the contact test curve that can be used to analyze the contact characteristics of aggregates with different test conditions. They could be calculated with the sampled data by the following formula.

F ¼ MaxðF 1 ; F 2 ; F 3 . . . F i . . . F n Þ E¼

n X F i  80 n i¼0

ð1Þ ð2Þ

where n is the number of sampling, which is related to the loading velocity and sampling frequency, while Fi is a sample of the data. The maximum contact force, including friction and interlocking force, could be indicated by the F (MF/AMF) during a test. The required energy that was utilized to overcome the contact force during the whole test could be indicated by the E (ME/AME). Specifically, the MF and ME refer to the dry mixture, while the AMF and AME refer to asphalt mixture. 3.4. Densification parameters of asphalt mixture

Fig. 3. Sectional view of the schema of aggregate contact test.

The same mix design and test conditions as aggregate contact test, Asphalt mixture was compacted by gyratory compactor, considering that the gyratory compaction tend to simulate the field compaction better than other methods [8,22,23]. The test setting was Nini = 8, Ndes = 100 and Nmax = 160, respectively. After the specimen was fabricated, its densification data and curve were gained to be utilized to analyze its densification properties by parameters of K1, K2 and CEI. K1 reflects the compactibility of asphalt mixture in process from Nini to Ndes, while K2 refers to the process from Ndes to Nmax. CEI represents the required energy

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4. Results and discussion 4.1. Contact characteristics of dry mixture Aggregate contact tests were conducted on three dry mixture for different gradations mentioned in Table 4. Six parallel tests were conducted on each gradation. MF and ME were gained by means of analyzing the contact curves, and shown in Figs. 6 and 7, respectively. As illustrated by Figs. 6 and 7, MF and ME of different gradations, AC-13, AC-16 and AC-20, increasing with the increase of nominal maximum aggregate size (NMAS). For a dry mixture without the cohesion that is provided by the asphalt binder, the MF comes from aggregates interlocking force as well as friction produced by aggregates migration. The aggregate contact action was enhanced by the reason of the large size aggregates in the dry mixture with a large NMAS. These results perhaps suggest that not only was the probability of aggregates interlocking increased, but the effect of aggregate morphology on the friction was enlarged as well.

Fig. 4. The typical contact test curve.

4.2. Contact characteristics of asphalt mixture

Fig. 5. Parameters of the curve.

to compact the asphalt mixture in the process from Nini to Ndes. They could be calculated with the data by the following formula.

K1 ¼

K2 ¼

cN des  cN ini lgN des  lgN ini cN max  cN des Nmax  Ndes

Z CEI ¼

Ndes

aNb dN

ð3Þ

As a temperature-sensitive material, the mechanical response of asphalt mixture is directly affected by the test temperature [24]. Essentially, its mechanical properties are the macroscopic performance of aggregate contact action and adhesion of asphalt binder [25,26]. It is evident that there is a lubrication effect in the aggregate interfaces at high temperature. Due to the influences of different test temperature or asphalt content on the aggregate contact action are different and complicated, four test temperatures and five asphalt contents for each gradation, AC-13, AC-16 and AC-20 respectively, were conducted in this study. The results of those tests are shown in Fig. 8. From Fig. 8, it can be seen that there is a difference among the change tendency of AMF for different test temperatures and asphalt mixtures. For AC-13, AC-16 and AC-20, however, the AMF values reduce gradually with the increase of asphalt content or test temperature. At a given temperature for each asphalt mixture, AMF values decrease gradually with the increase of asphalt content. It decreases gently before the optimal asphalt content, and then has a noticeable decline. With the increase in temperature, the cohesion of asphalt binder suffers from attenuation and the lubrication effect in the aggregate interfaces becomes stronger, which are the cause of the decline of the AMF.

ð4Þ

ð5Þ

0

where a and b are the parameters of densification curve of each asphalt mixture. They are related with the gradation, binder content and test temperature. cini , cdes and cmax is the densification ratio of asphalt mixture after the design compaction times was completed. They could be calculated with the data by the following formula.

cx ¼

hdes Gmb  hx Gmm

ð6Þ

Where x is compaction times, and hx is the height of the specimen after one compaction time, while hdes is height of the specimen after 100 compaction times. Gmm is the maximum specific gravity, while Gmb is the bulk specific gravity of the asphalt mixture.

Fig. 6. MF of different gradation.

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Fig. 7. ME of different gradation.

As evident from the Fig. 4, the contact action of aggregates was stronger without the lubrication of asphalt. After aggregate was covered with asphalt, it had a lubrication effect on contact interaction at high temperature. The low content of asphalt has a little lubrication effect, while the cohesion needs to be reckoned. With the increase of asphalt content, the more lubrication and complicated cohesion on the aggregate interfaces, which led to the AMF,

consisting of friction, interlocking force and cohesion, declines slowly with a smooth curve. Due to the test temperatures were high, the cohesion was not as powerful as it at a low temperature, while the lubrication of asphalt was dominant with enough content. Therefore, the AMF has an obvious decline because of enough content of asphalt binder has more lubrication effect than the optimum content. As shown in Fig. 9, the AMFs of three dense asphalt mixtures, AC-13, AC-16 and AC-20 with their optimum asphalt content respectively, were conducted at different temperatures. The AMF values of three mixtures decrease gradually with the increase of the test temperature. For these three mixtures, AMF of AC-20 is largest while the value of AC-13 is smallest at a given temperature. As a dense-graded asphalt, the mechanical strength of AC-13 is more sensitive to the cohesion of the binder rather than friction and interlocking force after having compacted. Due to the asphalt mixtures used in the ACD were loose and uncompacted at the higher temperature, as well as the loading velocity was slow correspondingly, however, the MFs/AMFs were more related to the friction and interlocking force on the interface of aggregates rather than the cohesion of the binder. For the same gradation of mixture at a given test temperature, the AME values that refer to the energies afforded by the STM go down generally with the increase of the asphalt content (see Fig. 10). With the increase of asphalt content, the more lubrication on the interfaces of aggregates were achieved, which making aggregates to move easily and reducing the input of energy. It is known that the temperature has a significant influence on the cohesion of the binder, which plays a role of contact characteristics among the aggregate interfaces [27]. The viscosity of asphalt is

Fig. 8. Changes of AMF with asphalt content.

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value of AC-13 goes up on the contrary. There is no significant change any more with AME value of AC-16 after the optimum asphalt content. More fine aggregate and mineral powder in the dense gradation mixture resulted in mixture segregation with enough asphalt. It seems that the aggregate gradation in the contact window was affected by the agglomeration of fine aggregate and mineral powder, which made more energy to be needed during the test. As can be seen in Fig. 11, the AME value of three mixtures, mixing with their optimum asphalt content, all decline with the increase in temperature. For each mixture, the energy using to overcome the contact action of those aggregates reduced with the increased of lubrication at the higher temperature. It is could be inferred that the more energy to be needed with a large NMAS at the given temperature. 4.3. Lubrication effect of asphalt on aggregate contact action

Fig. 9. Changes of AMF with test temperatures.

smaller while the lubrication of binder is larger at a higher temperature. For the same gradation of mixture at a given asphalt content, the AME values decrease generally with the increase of the test temperature. Theoretically, the lubrication effect of asphalt is largest at the highest temperature. However, it as well as the viscosity of the binder could become more complicated by the reason of the reaction of asphalt and mineral powder at a higher temperature (155 °C). With the increase of asphalt content, the AME value of three mixtures decrease before their respective optimum asphalt content. Then the AME value of AC-20 goes down continuously, while the

It is clear that there is a lubrication effect on the interface of aggregates, which making the AMF (AME) are smaller than MF (ME). The b refers to the lubrication of reducing the AMF value compared to MF, and the a refers to the lubrication of between AME and ME. From Fig. 12, it can be seen that a and b of three asphalt mixtures increase mildly with the increase of asphalt content before the optimum value, then all go up remarkably somewhat. For a given asphalt mixture, the value of b and a all increase with the increase in temperature. MF is a maximum value of the sharp curve, and the AMF is a maximum value of the smooth curve. But, ME and AME value are equal to the area enclosed by the curve and X axis. Compared to the a, the value of b is more sensi-

(a) AC-13

(b) AC-16

(c) AC-20 Fig. 10. Changes of AME with asphalt content.

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0.59 to 0.64 when the applied asphalt content of AC-20 was from 3.4% to 4.6%. It shows that the more lubrication effect of asphalt on the contact action of aggregates with the larger NMAS at a higher temperature. 4.4. Effects of aggregate contact on compaction

Fig. 11. Changes of AME with test temperatures.

tive to lubrication of asphalt. It also is more appropriate to reflect the fluctuation of the curves, which shows that the value of b is always larger than a in the Fig. 12. The b increased from 0.54 to 0.61 when the applied asphalt content of AC-13 was from 3.8% to 5.8%. The b increased from 0.55 to 0.62 when the applied asphalt content of AC-16 was from 3.9% to 5.1%. The b increased from

Asphalt mixture is a kind of anisotropic multiphase granular materials, whose compaction is the consequence of particle rolling and migratory permutation of aggregates. The mechanical property of asphalt mixture during compaction is related to the contact friction that is caused by particle extruding or slippage, as well as the cohesion that is provided by the asphalt binder [28,29]. The different factors, including the gradation, asphalt content and compaction temperature, have a directly influence on the contact action of aggregates, which is a critical issue that concerns the compaction and construction quality of asphalt mixture [30–32]. The asphalt mixture sample were obtained on different test conditions using the gyratory compactor. The evaluation indexes of compaction, K1, K2 and CEI, were gained by means of analyzing the densification curve. The relationships and its regression relation between densification and contact parameters are shown in Fig. 13 and Table 5, respectively. The relationships between contact characteristics of dry mixture and compaction properties as indicated in Fig. 13(a)–(c), focusing on the influence of aggregate

(a) AC-13

(b) AC-16

(c) AC-20 Fig. 12. Lubrication effect of asphalt on aggregate contact action.

J. Li et al. / Construction and Building Materials 204 (2019) 691–702

gradation on the contact characteristics and densification parameters. Its regression relation and correlation coefficients R2 as listed in Table 5, which have a normal temperature (None in the Table 5). The relationships between contact characteristics of asphalt mixture and compaction properties as shown in Fig. 13(d)–(i),

(a) Changes of MF and ME with K1

and its regression relation and correlation coefficients R2 as listed in Table 5, which have the test temperature from 110 °C to 155 °C. From Fig. 13 and Table 5, it can be seen that the correlation coefficients R2 between F, including MF and AMF, and densification parameters are larger than it between E, including ME and AME,

(b) Changes of MF and ME with K2

(c) Changes of MF and ME with CEI

(d) Changes of AMF with K1

699

(e) Changes of AME with K1

Fig. 13. Relationships between densification and contact parameters.

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(f) Changes of AMF with K2

(g) Changes of AME with K2

(h) Changes of AMF with CEI

(i) Changes of AME with CEI Fig. 13 (continued)

Table 5 Regression relation between densification and contact parameters. Densification parameters

Test temperature/°C

MF or AMF

ME or AME

Regression relation

Correlation coefficient R2

Regression relation

Correlation coefficient R2

K1

None 110 125 140 155

y = 62.706x + 887.288 y = 4.137x + 203.741 y = 4.420x + 203.302 y = 5.106x + 204.885 y = 6.307x + 207.247

0.93 0.84 0.84 0.79 0.91

y = 999.132x + 28929.029 y = 281.343x + 13584.416 y = 300.567x + 13824.558 y = 347.187x + 13932.209 y = 108.938x + 11822.157

0.92 0.83 0.84 0.78 0.47

K2

None 110 125 140 155

y = 4449.329x + 291.081 y = 176.805x + 178.796 y = 190.164x + 176.705 y = 218.867x + 174.130 y = 264.305x + 169.019

0.99 0.90 0.92 0.85 0.94

y = 71496.303x + 18130.328 y = 12022.430x + 12158.135 y = 12931.164x + 12015.917 y = 14882.949x + 11840.840 y = 3722.748x + 11127.490

0.99 0.89 0.90 0.85 0.32

CEI

None 110 125 140 155

y = 0.025x + 273.543 y = 0.001x + 165.263 y = 0.001x + 162.101 y = 0.002x + 157.549 y = 0.002x + 148.373

0.95 0.66 0.69 0.61 0.76

y = 0.396x + 19120.615 y = 0.090x + 11237.904 y = 0.097x + 11022.860 y = 0.109x + 10713.296 y = 0.046x + 10755.225

0.96 0.66 0.68 0.61 0.65

and densification parameters, which is marked as listed in Table 5 at the temperature of 155 °C, especially. The power correlation can be used to describe the relationship between MF and compaction parameters, and the correlation coefficients R2 of are more than 0.93, 0.99 and 0.95, while the others that refer to the AMF are less

than it, respectively. As illustrated by Figs. 6 and 7, there was a positive correlation between the MF and NMASs that refer to AC-13, AC-16 and AC-20. Therefore, the MF was used to analyze the influence of gradation with different NMAS on the compaction properties as shown in Fig. 14.

J. Li et al. / Construction and Building Materials 204 (2019) 691–702

Fig. 14. Contact and densification characteristics of mixtures with different NMAS.

Fig. 15. Contact and densification characteristics of AC-13 with different asphalt content.

701

upper threshold value of the contact action, which is associated with force of changing the aggregate’s spatial position. During the compaction, the upper threshold value of contact action of the aggregates was needed to be overcome by the loading force, then the aggregates’ spatial position had changed and the dense structure was fabricated. The large MF value could account for that asphalt mixture was difficultly compacted, which had a small value of K1. It was also linked to the large CEI that was needed to make the asphalt mixture meet the objective void. The previous research shows that the K2 was related to the performance of rutting resistance after opening of the traffic. From the Fig. 14, however, the value of K2 increases with increases of the MF. The AC-20 has both a large CEI and K2, indicating that it is difficult to compact and has a poor performance of rutting resistance. For a dense gradation mixture compacted completely, its contact action was more depend to the adhesion and cohesion than interlocking force and friction in the higher temperature. Comparing with fine gradation, AC-20 has much coarse aggregates and smaller specific surface area resulting in worse adhesion. The contact and densification characteristics of AC-13 with different asphalt content and temperature as shown in Figs. 15 and 16, respectively. From the Figs. 15 and 16, it is can be seen that the b increases while the AMF decreases gradually with the increases of the asphalt content or temperature. Furthermore, there is a remarkable rise with the b or fall with the AMF after the optimum asphalt content or temperature. Correspondingly, the K1 and K2 increase but the CEI decreases with the increases of asphalt content or temperature that are all linked to the lubrication effect value of b. It appears that there is a negative correlation between AMF and densification characteristics. The small AMF value could account for the large K1 and small CEI of asphalt mixture during the compaction. In the process of further densification of the mixture, the K2 increases with the decreases of AMF stem from the increases of lubrication of asphalt, which gives rise to reduce the rutting resistance. For the dense gradation, the contact action of aggregates, including the interlocking force, friction and adhesion, plays a key role of densification of the asphalt mixture. In addition, the lubrication of asphalt shows a striking effect on the contact action with enough asphalt content in higher temperature. The contact action was reduced to make the aggregates moving and rolling easily by the means of lubrication, which means that the asphalt mixture was easily compacted. The correlation coefficients R2 between MF and K1, K2 and CEI are 0.93, 0.99 and 0.92, respectively. Those values are all larger than the correlation coefficients R2 between AMF and densification parameters. It confirms that the gradation and its NMAS is more easily to affect the contact action, which has the significant effect on the densification characteristics of the asphalt mixture.

5. Conclusion

Fig. 16. Contact and densification characteristics of AC-13 with different temperature.

From the Fig. 14, it can be seen that the MF value increases gradually with the NMAS increases, and the K2 and CEI also increase while the K1 declines gradually. The large MF value reveals a large

The self-developed test device was used to perform aggregate contact test to reveal the aggregate contact and the lubrication effect of asphalt binder on it in a higher temperature. Combining the gyratory compaction test, the effects of aggregate contact characteristics on the densification property of asphalt mixture were studied. As a type of multiphase particulate material, the aggregate contact action composed of interlocking force and friction was enlarged by the enlargement of NMAS of the asphalt mixture. The values of MF and ME increase with the increase of NMAS. The asphalt binder has the significant lubrication effect on the aggregate contact in the high temperature. For a dense gradation, the lubrication reinforces and the AMF and AME all reduce with the increase of asphalt content or temperature.

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The compaction of asphalt mixture is the consequence of particle rolling and migratory permutation of aggregates. During the compaction, the interlocking and friction of aggregates were overcome to roll and move the aggregates under the loading force. The asphalt mixture with a dense gradation, which contact action is lubricated enough, is prone to be compacted. Comparing to the test temperature and asphalt content, however, it confirms that the gradation of mixture and its NMAS are more easily to affect the contact action, which has the significant effect on the densification characteristics of the asphalt mixture. Conflict of interest None. Acknowledgments This work was supported by the National Natural Science Foundation of China (51878061 & 51768062), Applied Basic Research Project the Ministry of Transport of China (2014319812151), the Natural Science Basic Research Plan in Shaanxi Province of China (2017JM5099) and the Special Fund for Basic Scientific Research of Central Colleges (300102218405 & 300102218513). The authors gratefully acknowledge their financial support. References [1] P. Cui, Y. Xiao, B. Yan, M. Li, S. Wu, Morphological characteristics of aggregates and their influence on the performance of asphalt mixture, Constr. Build. Mater. 186 (2018) 303–312. [2] X. Cai, K.H. Wu, W.K. Huang, C. Wan, Study on the correlation between aggregate skeleton characteristics and rutting performance of asphalt mixture, Constr. Build. Mater. 179 (2018) 294–301. [3] Y. Liu, Y. Huang, W. Sun, H. Nair, D.S. Lane, L. Wang, Effect of coarse aggregate morphology on the mechanical properties of stone matrix asphalt, Constr. Build. Mater. 152 (2017) 48–56. [4] T. Pan, E. Tutumluer, H. Carpenter Samuel, Effect of coarse aggregate morphology on permanent deformation behavior of hot mix asphalt, J. Transp. Eng. 132 (7) (2006) 580–589. [5] I. Etsion, Y. Kligerman, Y. Kadin, Unloading of an elastic–plastic loaded spherical contact, Int. J. Solids Struct. 42 (13) (2005) 3716–3729. [6] M. Haskett, D.J. Oehlers, M.S. Mohamed Ali, S.K. Sharma, Evaluating the shearfriction resistance across sliding planes in concrete, Eng. Struct. 33 (4) (2011) 1357–1364. [7] X. Gao, Z. Fan, J. Zhang, S. Liu, Micromechanical model for asphalt mixture coupling inter-particle effect and imperfect interface, Constr. Build. Mater. 148 (2017) 696–703. [8] C. Zhang, H. Wang, Z. You, X. Yang, Compaction characteristics of asphalt mixture with different gradation type through superpave gyratory compaction and X-ray CT scanning, Constr. Build. Mater. 129 (2016) 243–255. [9] P. Li, Z. Ding, W. Rao, Evaluation of deformation properties of asphalt mixture using aggregate slip test, Int. J. Pavement Eng. 17 (6) (2016) 542–549. [10] P. Li, J. Su, S. Ma, H. Dong, Effect of aggregate contact condition on skeleton stability in asphalt mixture, Int. J. Pavement Eng. (2018), https://doi.org/ 10.1080/10298436.2018.1450503.

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