Fatigue Resistance of RCA Modified Asphalt Based on Dielectric Properties

LU Jiabao, WANG Guozhong, HU Jiangsan, LIU Zhongning

(College of Energy and Transportation Engineering, Inner Mongolia Agricultural University, Hohhot 010018, China)

Abstract: In order to study the anti-fatigue performance of RCA modified asphalt (RMA), the performance of RMA and 90 # matrix asphalt with different modifier content were measured by asphalt penetration, ductility, softening point, Brookfield viscosity, rheological index, infrared spectrum and dielectric constant test.This paper discusses the changes of asphalt basic indexes, fatigue properties and asphalt components based on dielectric properties under different modifier contents, and analyzes the grey correlation degree between components and asphalt pavement performance indexes.The results show that the optimum content of RCA modifier is 16.7% of the asphalt quality according to the penetration, ductility, softening point,Brockfield viscosity, viscosity temperature curve and fatigue life.In the phase angle-strain curve, there is disorder in the latter part of the curve.According to the strain (εd) corresponding to the disorder point, a new fatigue failure criterion is proposed and proved.Based on the new asphalt fatigue failure criterion, the fatigue prediction model of asphalt mixture is improved, and the fatigue life predicted by the improved fatigue model is compared with the fatigue life obtained by four-point bending fatigue test.The results show that the proposed new asphalt fatigue failure criterion is reasonable, and the fatigue life predicted by the improved asphalt mixture fatigue prediction model is accurate.The research method of classifying asphalt components based on dielectric properties is simple and effective, and the components have a high correlation with the road performance of base asphalt and modified asphalt.

Key words: dielectric properties; anti-fatigue performance; fatigue failure criterion; RCA modified asphalt

1 Introduction

In recent years, highway traffic has achieved a rapid development, so the research on the road performance of asphalt mixture has become particularly important.WUet alused dynamic shear rheometer(DSR) to study the dynamic rheological properties of supramolecular UV resistant layered dihydroxide(LDHs) modified asphalt.It is found that when the LDHs content is between 3 wt% and 5 wt% of asphalt weight,the high temperature performance and fatigue resistant property of the modified asphalt become better, and the UV aging resistance properties are improved.With the development of research, high modulus modifier and high modulus asphalt mixture gradually appear[1].Zhanget alstudied the high modulus modifier of RPCM, and analyzed it by asphalt temperature scanning and asphalt frequency scanning.The results showed that the addition of RPCM reduced the temperature sensitivity of the asphalt binder and correspondingly increased its stiffness(complex modulus).At dosages greater than 0.30%,RPCM exhibited superiority over SBS in enhancing the complex modulus of the asphalt binder.With the addition of RPCM under increasing temperature, the phase angle generally exhibited an increasing trend.This result suggested that modification with RPCM had the potential to improve the high-temperature elastic properties of the asphalt binder[2].Xuet almade a comparative study between high modulus modified asphalt and SBS modified asphalt.The results showed that high modulus modifier improved the high temperature performance and viscoelasticity of asphalt.Under a certain content, the modification effect is better than SBS modified asphalt[3].

Earlier, French scholars proposed to prepare high modulus asphalt concrete with high modulus modifier.The main research idea is to reduce the deformation of asphalt mixture under vehicle load by improving the modulus of asphalt mixture with high modulus modifier, so as to improve the rutting resistance and fatigue resistance of pavement.Finally, the service life of road surface can be extended[4].Zouet alimproved the design of high modulus asphalt mixture and deepened the understanding of the performance characteristics of high modulus asphalt mixture[5].The design and performance of high modulus modified asphalt and its asphalt mixture were studied.The results show that high modulus modified asphalt has good storage stability, excellent high temperature rutting resistance, low temperature crack resistance,water stability and good comprehensive performance[6].Some scholars have studied the combination of different forms of fibers or long-chain polymers with high modulus modifiers, which has improved its lowtemperature crack resistance and durability to a certain extent[7,8].To observe the micro-morphology of high modulus asphalt binders(HMABs), a scanning electron microscope (SEM) and a fluorescence microscope were employed.The binder morphology showed that the relative viscosity of modified asphalt increased with time; the test results of the mixtures indicated that compared with the original asphalt mixture and the SBS mixture, the HMABs significantly improved the high temperature performance, water stability, moisture-heat synthesis property, and dynamic modulus of mixtures[9].RCA modifier as a kind of high modulus modifier, its main components are natural rock asphalt and nanopolymer materials, because of its good compatibility,high temperature resistance, anti-peeling, anti-water damage, anti-aging performance and other advantages.Some scholars compared the road performance and economic benefit of RCA modifier with SBS modifier.The results show that RCA can significantly improve the high temperature rutting resistance, water damage resistance, low temperature cracking resistance and fatigue life of asphalt mixture.The comprehensive road performance of asphalt mixture is better than SBS modifier.The performance ratio is higher than SBS modifier[10].

In recent years, most of the highway failed to meet the design life of fixed number of year, and more than half of the highway in the use of 10-12 years needs for a large repair, and some highways in the use of 6-8 years need medium-sized repair.The actual service life is much lower than the design life.The emergence of this situation leads to the consumption of a large number of non-renewable resources, and has a considerable impact on the environment and traffic.The capacity of the road and the efficiency of the road network are significantly reduced, and the maintenance costs are greatly increased.The main reason that has a serious impact on the durability and reliability of asphalt pavement is the uncertainty and non-objectivity of the fatigue performance characterization of asphalt mixture[11].Therefore, it is very important to study the fatigue life characterization method of asphalt and asphalt mixture.Based on the fatigue prediction model of asphalt and asphalt mixture, this paper provides a series of prediction models to reduce the difference between the actual road life and the design life.

Common methods of fatigue performance characterization of asphalt mixtures can be divided into four categories.The first type is the traditionalS-Nfatigue equation characterization method.In this method, the repeated stress value of material with fatigue failure is called fatigue strength, and the corresponding number of repeated stress action is called fatigue life.The second is mechanical approximation method, that is, fracture mechanics is used to analyze the fatigue cracking characteristics of asphalt mixture.Lianget alproposed a prediction model of fatigue crack propagation based on fracture mechanics, which artificially divided the fatigue failure process into two stages: crack initiation and crack propagation[12].The third is damage mechanics method, which uses fatigue damage to characterize the fatigue life of asphalt mixture.Some scholars proposed to combine viscoelastic continuous damage model with finite element model to predict the fatigue performance of asphalt pavement.The comparison between measured and predictive results shows that this method can accurately capture the characteristics of mixtures[13-15].The fourth type is energy method,which uses cyclic dissipated energy as damage factor to analyze the fatigue performance of asphalt mixture.

However, most of the fatigue tests used to characterize the fatigue characteristics of asphalt mixtures are expensive and time-consuming.Linear amplitude scanning (LAS) test has been recently introduced to evaluate and characterize the fatigue performance of asphalt binder because of its advantages of shorter test time compared with time scanning.In recent years, it is a very good evaluation index to evaluate the influence of the fatigue performance of asphalt binder on the fatigue performance of asphalt mixture and the actual pavement performance.In addition to the other factors affecting the fatigue performance of asphalt mixture, the performance of asphalt binder fatigue is a good indicator of the fatigue performance of asphalt mixture.In the Strategic Highway Project (SHRP), researchers proposed to use the shear modulus of binder (G*) and its phase Angle (δ)as a mixture fatigue index to evaluate asphalt pavement performance.Sabouriet altested the fatigue resistance of different kinds of modified asphalt by linear amplitude scanning test method, and tested the fatigue resistance of corresponding asphalt mixture by fourpoint bending beam fatigue test.The results show that LAS test is an effective fatigue test for asphalt mixture to predict its fatigue performance[16].

A large number of researchers have systematically studied the high and low temperature and economic benefits of RCA modifiers, but their fatigue properties have not been studied in detail.Therefore, starting with RMA, this paper discusses the penetration, ductility,softening point, Brockfield viscosity and other indexes of asphalt.The fatigue performance of asphalt was studied by LAS test.A new fatigue failure criterion of RMA is put forward based on the disorder of phase angle-strain curve.The correlation between asphalt components based on dielectric properties and asphalt pavement performance is analyzed, and the feasibility of this analysis method is verified.Based on the new fatigue failure criterion of RMA, the phenomenological fatigue model of asphalt mixture was improved, and the improved fatigue model was verified by four-point bending fatigue test of asphalt mixture.

2 Experimental

2.1 Raw materials

90# base asphalt is used as the original asphalt,and its physical properties are shown in Table 1.The amount of RMA modifier is 11.3%, 16.7%, 21.8% and 26.8% (mass ratio), and the laboratory preparation process was as follows: The base asphalt was heated to 135 ℃, a certain amount of RCA modifier was added,and then stirred for 10min.The mixed asphalt was put into the oven and held for 30 min at 175 ℃.After holding for 30 min, the modified asphalt was sheared for 60 min at 175 ℃ and 3 000 r/min with a high speed shears.Indexes of RCA modifier are shown in Table 2.

Table 1 Physical properties of No.90 matrix asphalt

Table 2 Basic index of RCA modifier

2.2 Basic performance test

The penetration, ductility, softening point and Brookfield viscosity of 90# base asphalt and four RMA with different content were tested respectively.Among them, the ductility test condition of 90# base asphalt is 15 ℃, and that of RMA is 5 ℃.The penetration test condition of asphalt is 25 ℃, and the softening point test condition is 5 ℃.No.27 rotor was selected to carry out asphalt Brockfield viscosity test at 100, 120, 135,160, and 175 ℃.

2.3 Linear amplitude scanning test

Dynamic shear rheometer (DSR) was used to study the fatigue performance of 90# matrix asphalt and RMA.Linear amplitude sweep (LAS) test was used in the fatigue test.

2.4 Micro mechanical test

Fourier infrared spectroscopy (FTIR) test was simulated by using hF-03 asphalt rapid analyzer, with a wavelength range of 2 000 to 700 cm-1, a resolution of 4 cm-1, and 32 internal scan.

The dielectric constant test adopts the industrial microwave CT of Yunlu Technology, which can reflect the composition of asphalt by measuring the dielectric constant of different substances in asphalt.The sample size is about 30 g, which can ensure the representative sampling, simple operation and real-time observation.The dielectric constant ranges from 1 to 6.2.

2.5 Asphalt mixture test

2.5.1 Asphalt mixt ure gradation

Using AC-13 mineral aggregate gradation,through Marshall design method, the gradation design of 90# matrix asphalt mixture and RCA optimal content modified asphalt mixture was carried out, and the best asphalt-aggregate ratio was obtained, which were 5.2%and 5.0% respectively.The results are shown in Fig.1.

Fig.1 Grading curve of asphalt mixture

2.5.2 Forming and cutting of asphalt mixture specimen

PReSBOX shear compaction molding instrument of IPC Company was used to shape asphalt mixture specimen.Under the condition of known asphalt mixture density, the height of cuboid specimen of asphalt mixture shear compaction was controlled by adjusting the quality of asphalt mixture.The multifunctional cutting machine of IPC company was used to cut the shear compaction specimens of formed asphalt mixture, and the trabecular specimens with length × width × height of (380 ± 5) mm×63.5 mm × (50± 5) mm were cut.

2.5.3 Asphalt mixture fatigue test

Utm-100 universal testing machine was adopted, four-point bending fatigue test was carried out according toTest specification for asphalt and asphalt mixture of highway engineering[17], and strain controlled haversine loading mode was adopted.Before the fatigue test, the fatigue specimen was placed in an environmental chamber at 15 ℃ for curing for more than 4 h, and then the four-point bending fatigue test was carried out on the cured specimen.

3 Asphalt research results and discussion

3.1 Data analysis of three indicators

As can be seen from Figs.2-4, the penetration and ductility of RMA are both lower than that of 90#base asphalt, and the softening point is greater than that of 90# base asphalt.In Fig.2, with the increase of the dosage of RCA modifier, the penetration gradually decreases.This is because RCA modifier is mainly composed of heavy asphalt components, and its addition leads to a decrease in the proportion of light components in asphalt, which leads to an increase in the hardness of asphalt and a decrease in penetration.When RCA content is between 16.7% and 21.8%, the decrease rate of penetration slows down.

Fig.2 Penetration of asphalt

It can be seen from Fig.3 that, with the increase of modifier content, the ductility of asphalt decreases gradually, which is also due to the change of component proportion, the brittleness of asphalt increases, the elongation decreases, and the flexibility and low temperature crack resistance of asphalt decreases.When RCA content is between 16.7% and 21.8%, the decreasing value of asphalt ductility also presents a gentle trend.

It can be seen from Fig.4 that the softening point of asphalt increases with the increase of the dosage of RCA modifier.The increase of softening point can indirectly indicate that the high temperature stability of asphalt has been improved.It shows that the addition of RCA modifier can improve the high temperature performance of 90# matrix asphalt.The asphaltene in asphalt is not only related to the viscosity of asphalt,but also affects the thermal stability of asphalt.At the same time, when the RCA content is between 21.8% and 26.8%, the softening point is higher and the increasing trend is stable.Considering the asphalt thermal stability, asphalt viscosity and the sensitivity of asphalt to different RCA modifiers, the best RMA content was selected when the RCA modifier content was between 16.7% and 26.8%.

Fig.4 Softening point of asphalt

3.2 Brockfield viscosity data analysis

It can be seen from Fig.5 that, at the same test temperature, with the increase of RCA modifier content, the Brookfield viscosity value of asphalt also increases.According to the change of viscosity at 135℃, with the increase of modifier content, its Brookfield viscosity value is 0.8, 1.08, 2.54 and 4.31 times that of base asphalt respectively.The viscosity-temperature curve is close to a straight line in double logarithmic coordinates, and the parameters after linear fitting with Origin software are shown in Table 3.

Fig.5 Brockfield viscosity under different temperatures

Table 3 Log-log curve fitting parameters of Brockfield viscosity

The growth rate of linear absolute value reaction viscosity with temperature can indirectly reflect the change of temperature sensitivity of RMA with different content compared with 90# matrix asphalt.As can be seen from the above table, it can be seen that with the addition of RCA modifier, the absolute value of slope increases, indicating that the viscosity and temperature sensitivity of asphalt increase.When the RCA content is 11.3%, the absolute value of slope is the largest, that is, the viscosity and temperature sensitivity of asphalt increase most obviously.Compared with the modified asphalt with 11.3%RCA content, the intercept change and the absolute increase of slope of the modified asphalt with the other three mixing proportions are relatively insignificant, and the increase of viscosity and temperature sensitivity of the asphalt is relatively low, and the viscosity and temperature sensitivity of the asphalt are in a stable state when the content of RCA modifier are 16.7%-21.8%.The above phenomenon is mainly due to the fact that RCA modifier changes the original four-component ratio of base asphalt, which leads to the increase of asphalt viscosity and temperature sensitivity.

By analyzing the penetration, ductility, softening point data of asphalt and Brockfield viscosity data of asphalt, it can be concluded that the reasonable optimum blending ratio of RCA modifier is between 16.7%-21.8%.

3.3 Fatigue performance of asphalt

In this part, through the asphalt fatigue test,the change degree of the anti-fatigue performance of RMA with different dosage of modifier is obtained,and the optimum dosage of RCA modifier is selected from 16.7%-21.8% based on the results of penetration,ductility, softening point and Brookfield viscosity data of asphalt.

Through LAS test of asphalt, the complex shear modulus(G*)and phase angle of asphalt (δ)can be measured.Complex shear modulus(G*)includes storage modulus and loss modulus.The storage modulus represents the energy storage of the asphalt binder in the process of denaturation, which is indicated byG'and is the recoverable elastic part of the complex shear modulus (G*).The loss modulus reflects the energy loss in the process of deformation of asphalt binder, which is represented byG" and represents the unrecoverable viscous part of complex shear modulus (δ).When asphalt yields, the corresponding normal stress is called yield stress, the corresponding strain is yield strain, and the phase angle 3 is a measure of the time lag between applied stress and strain response or applied strain and stress response.The tangent value of the phase angle reflects the proportional relationship between the viscous component and the elastic component of the material, and the change of the phase angle can be used to analyze the viscoelastic change law of the material.

3.3.1 Analysis results by normative method

In AASHTO specification, damage is defined by|G*|·sinδ as the internal state parameter of materials,and |G*|·sinδ decays to 35% as the fatigue failure criterion.Through viscoelastic continuous damage model(VECD), damage can be quantified based on the principle of thermodynamics irreversibility, and the relationship between fatigue life and strain of asphalt can be calculated.The calculation process is shown in Formula 1 to Formula 8, and the calculation result is shown in Fig.6.

Fig.6 Calculation results of fatigue life by standard method

where: |G*| is the complex shear modulus; δ is the phase angle;IDis the initial composite shear modulus of the strain interval; γmaxis the strain of a given data point;tis time;C0is the value of |G*|·sinδ at the beginning of 0.1% strain interval;C1andC2are curve fitting coefficients;Dis damage;Dfis the fatigue failure value;fis the loading frequency.

Fig.6(a) is the calculation results of fatigue damage of asphalt at 15 ℃ test temperature.It can be seen from Fig.6(a) that the cumulative rate of asphalt fatigue damage increases with the increase of RCA modifier content.When the time is about 50 second, the accumulated damage is disordered and accompanied by a period of platform area.It shows that the cumulative damage growth rate slows down from the beginning of disorder, which can be understood as the platform area,that is, the asphalt yield stage, after which the asphalt damage increases sharply until it is destroyed.This shows that RCA modified asphalt increases the yield stage of asphalt compared with base asphalt, and the cumulative rate of asphalt damage becomes faster with the increase of RCA content.

Fig.6(b) is the calculation results of asphalt fatigue life.As can be seen from Fig.6(b), the fatigue life of 16.7% RMA under low strain is higher than that of base asphalt, and that under high strain is lower than that of base asphalt.The dependence of 16.7% RMA on strain is obviously higher than that of matrix asphalt,which is consistent with the conclusion of many scholars that the fatigue life of high modulus modified asphalt mixture under low strain is better than that of matrix asphalt mixture.

Based on the above properties, 16.7% can be selected as the optinum content of RCA modifier.

3.3.2 Proposing and verifying the new fatigue failure criterion

Fatigue criterion is an important basis for judging fatigue life.LAS test is widely used as a rapid test method to evaluate and predict asphalt fatigue performance at present, but this test faces is the nonuniversality of failure criterion.

Some scholars has done experiments similar to LAS, and proposed the peak phase angle as the definition of fatigue failure, and verified its rationality[18,19].Using this idea for reference, it is found in the research process that with the increase of oscillating strain in the fatigue failure process of RMA,the phase angle will fluctuate up and down, as shown in Fig.7.This is mainly due to the fact that RCA modifier,besides rock asphalt and polymer materials, also contains some materials that are immiscible with matrix asphalt as dispersed phase dispersed in matrix asphalt,as shown in Fig.8.As a result, the heterogeneity of asphalt is enhanced.When the shear strain reaches a certain level, the deformation of dispersed phase and dispersed medium is not uniform, and the strain lag is not uniform with the amplitude of stress, which leads to the phenomenon that the amplitude of phase angle is disordered under large strain.The change of phase angle with oscillating strain can also indirectly indicate that micro-damage occurred in modified asphalt at this time, so it is theoretically feasible to use it as the criterion of fatigue failure.

Fig.7 Phase angle-oscillation strain diagram

Fig.8 Bitumen topography

The oscillating strain corresponding to the numerical disturbance of asphalt phase angle is regarded as fatigue failure strain εd, and the corresponding |G*|·sinδ is defined asCd.The fatigue failure valueDfin VECD model is improved, and the improved formula is as follows.

where:Ddis the fatigue failure value;C0is the value of |G*|·sinδ at the beginning of 0.1% strain interval;C1andC2are curve fitting coefficients;Dis damage; The fatigue properties of base asphalt and 16.7% RMA are calculated by the new fatigue failure criterion, and the calculation results are as follows.

It can be seen from Fig.9 that the fatigue life of 16.7% RMA under 5% low strain is higher than that of matrix asphalt according to the new fatigue failure criterion.This is consistent with the conclusion that the anti-fatigue performance of 16.7% RMA is better than that of matrix asphalt under low strain.With the increase of strain, the fatigue life of RMA is gradually lower than that of matrix asphalt.Therefore, in the application process of RMA mixture, attention should be paid to adjusting the conversion mode of equivalent axle load.The high strain caused by heavy vehicles will significantly affect the fatigue life of RMA pavement than that of matrix asphalt pavement.

Fig.9 Asphalt fatigue life based on new fatigue failure criterion

3.4 Mechanism research and analysis of RCA modified asphalt

3.4.1 Microscopic mechanism analysis of RCA modified asphalt

The microscopic mechanism of asphalt was analyzed by infrared spectrum test.As shown in Fig.10,with the increase of the content of RCA modifier, the peak of asphalt has changed to some extent, but there is no new peak.With the increase of the content of RCA modifier, the corresponding absorption peak strength in asphalt increases.This change indicates that the RCA modifier does not produce new functional groups in the modification process of base asphalt, which is physical modification, and no new functional groups are produced.

Fig.10 FTIR spectra of modified asphalt with different RCA content

3.4.2 Analysis of dielectric constant results and its correlation with road performance

The research of dielectric properties in the field of asphalt mixture mainly focuses on the prediction of asphalt pavement density.Some scholars predict the mixture density model based on dielectric properties,observation of self-healing situation,etc[20-22].Industrial microwave CT can reflect the composition of asphalt by measuring the dielectric constants of different substances in asphalt, especially the changes of substances in the aging process.The sample is about 30 g, which can ensure the representativeness of sampling,simple operation and real-time observation.

Within the test range, the dielectric constant is divided into 10 parts on average from 1.0 to 6.2, that is, component 1 (the dielectric constant range is 1.0 to 1.52), component 2 (1.53 to 2.04)...component 10 (5.69 to 6.2).Fig.11 shows the test results of the dielectric constants of five kinds of asphalt.

Fig.11 Composition distribution based on dielectric properties

It can be seen from Fig.11 that the addition of RCA modifier obviously changes the component distribution ratio of base asphalt.The more RCA modifier is added, the greater the content of components with larger dielectric constant is.Grey correlation analysis is established between the above 10 components and the road performance index of asphalt,and the calculation results are listed in Table 4.

Table 4 Correlation degree between component and performance index

From Table 4, it can be seen that the correlation degree between each component and different road performance indexes of asphalt is different, indicating that different components have different contribution rates to different road properties of asphalt, and one or more components mainly determine a certain road property of asphalt.From the average correlation degree, the average correlation degree of asphalt fatigue lifeNf(fatigue life of asphalt calculated by new fatigue failure criterion under 1% strain)calculated by the new fatigue failure criterion is above 0.68.It is also proved that the fatigue life of asphalt calculated by the new fatigue failure criterion is reasonable, and the method of evaluating the fatigue resistance of asphalt by dielectric constant is feasible.

Further research can extract the monomer materials corresponding to components 1 - 10 and explore their chemical composition, and then infer the fatigue life prediction model.In the process of asphalt modification, corresponding chemical components can also be added according to the demand of road performance to improve the asphalt performance.

3.4.3 Microscopic mechanism analysis of RCA modified asphalt

The microscopic mechanism of asphalt was analyzed by infrared spectrum test.As shown in Fig.12, with the increase of the content of RCA modifier,the wave peak in asphalt changed to some extent, but no new wave peak was generated.The intensity of the corresponding absorption peak in asphalt increased with the increase of the content of modifier.This change indicates that no new functional groups aregenerated during the modification of 90# base asphalt by RCA modifier.It is physical modification and no new functional groups are generated.

Fig.12 FTIR spectra of modified asphalt with different RCA content

4 Results and discussion

In order to verify the fatigue performance of asphalt mixture according to the new asphalt fatigue failure criterion, AC-13 mineral aggregate gradation was adopted, and the best asphalt-aggregate ratio was determined by Marshall test method, and two kinds of asphalt mixtures, base asphalt and 16.7% RMA,were made respectively.Two kinds of asphalt mixtures are shear-pressed by shear compactor, and the shearpressed asphalt mixture specimens are cut into small beam specimens with length × width × height of (380± 5) mm × (63.5 ± 5) mm × (50 ± 5) mm respectively.Four-point bending fatigue test was conducted on two kinds of asphalt mixtures.The test conditions were 15℃ test temperature, 10 Hz loading frequency, 500, 600,700 and 800 με four loading strains for fatigue test, and three groups of control experiments were conducted for each loading strain to ensure the accuracy of fatigue test.

Through four-point bending fatigue test, referring to specifications and standards, the fatigue times corresponding to the initial stiffness modulus of asphalt mixture falling to 50% are defined as the fatigue life of asphalt mixture.The fatigue life of asphalt mixtures with different loading strain is shown in Fig.13.It can be seen from the figure that the fatigue life of the two asphalt mixtures decreases with the increase of loading strain.

Fig.13 Fatigue life of asphalt mixture under different strain

In order to correspond the strain level of asphalt mixture to that of asphalt, the concept of strain ratio will be adopted in this paper.Some studies indicate that when the strain rate is 80 times, the predicted fatigue life of asphalt based on S- model has a good correspondence with that of asphalt mixture[23].Therefore, this paper chooses 80 as the fatigue strain ratio of asphalt and asphalt mixture.That is, the oscillating strains of asphalt corresponding to four loading strains of asphalt mixture, 500, 600, 700 and 800 με, are 4%, 4.8%, 5.6% and 6.4% respectively.

According to the new fatigue failure criterion,the fatigue life of matrix asphalt and 16.7% RCA modified asphalt corresponding to four kinds of oscillating strains are calculated respectively, and the corresponding relationships are established with the fatigue life of asphalt mixture and the accumulated dissipation energy of asphalt mixture specimens before failure, respectively, as shown in Figs.14 and 15.

From Fig.14 and Fig.15, it can be seen that the fatigue life of asphalt obtained by the new asphalt fatigue failure model is consistent with the law of asphalt mixture life and cumulative damage value of asphalt mixture and has a good correlation (R2is above 0.99).It is indicated that the main role of asphalt in the fatigue performance of asphalt mixture, and at the same time that the new asphalt fatigue failure model proposed in this paper is reasonable.

Fig.14 (a) Fatigue life of base asphalt-fatigue life of base asphalt mixture and (b) Fatigue life of 16.7%RCA modified asphalt - fatigue life of 16.7%RCA modified asphalt mixture

Fig.15 (a) Fatigue life of base asphalt - cumulative dissipated energy of base asphalt mixture and (b) Fatigue life of 16.7%RCA modified asphalt - cumulative dissipated energy of 16.7%RCA modified asphalt mixture

In order to better predict the fatigue life of asphalt mixture by asphalt fatigue performance, some scholars have improved the VECD theoretical model to some extent.The improved model can predict the fatigue performance of asphalt mixture at a given strain level on the basis of asphalt LAS test[24].The fatigue life prediction model of asphalt mixture is as follows.

AandBare the parameters obtained by VECD theoretical model in asphalt LAS test,εis the loading strain (με), andNfis the predicted life of asphalt mixture.Aiming at RMA, this paper combines the new fatigue failure model of asphalt with the fatigue life prediction model of asphalt mixture mentioned above, and obtains the improved fatigue life prediction model of asphalt mixture as follows.The fatigue life prediction model of asphalt mixture is as follows.Among them,AεandBare obtained from the new fatigue failure model of asphalt, andAεis related to oscillating strain, so the loading strain (μ ε) ofεasphalt mixture is converted into oscillating strain (%):

Through the improved prediction model of asphalt mixture, four life times of asphalt mixture under loading strain, 500, 600, 700 and 800 με, are calculated respectively, and the correlation between them and the fatigue life of asphalt mixture obtained by four-point bending fatigue test is analyzed.As shown in Fig.16,for the matrix asphalt mixture, the correlation degreeR2is 0.988; for the 16.7% RMA mixture, the correlation degreeR2is 0.985.Through the correlation between two kinds of asphalt mixtures, it can be seen that the prediction value of fatigue life of asphalt mixture by the improved asphalt mixture prediction model is relatively accurate, and the improved fatigue model is reasonable.The establishment of this model enables the fatigue life of asphalt mixture to be directly predicted by simple and quick asphalt LAS test.It can replace the test of asphalt mixture beams with long time consumption and expensive investment.

Fig.16 New asphalt mixture fatigue model prediction life vs Asphalt mixture life

5 Conclusions

a) According to the penetration, ductility,softening point, Brookfield viscosity, viscositytemperature curve, fatigue life and other parameters of matrix asphalt and RMA with different dosage of modifier, the optimal dosage of RCA modifier is determined to be 16.7% of asphalt mass.

b) Because of its own characteristics, RMA will produce disorder after the peak of phase anglestrain curve.It is feasible to evaluate the antifatigue performance of RMA by using the strain εdcorresponding to the disorder point as the fatigue failure criterion to calculate the fatigue lifeNf.

c) The research method of using industrial microwave CT to classify asphalt components based on dielectric properties is simple and effective, and the correlation between components and road performance of base asphalt and modified asphalt is high.No new wave peaks and no new functional groups were generated during the modification of base asphalt by RCA modifier, indicating physical modification.

d) The research shows that the fatigue life prediction model of asphalt mixture improved by the new asphalt fatigue failure criterion is reasonable.The improved fatigue life prediction model makes it easy to obtain the fatigue life of asphalt mixture.

Conflict of interest

All authors declare that there are no competing interests.