Material Composition Design and Anticracking Performance Evaluation of Asphalt Rubber Stress-Absorbing Membrane Interlayer (AR-SAMI)

To promote the application of the asphalt rubber stress-absorbing membrane interlayer (AR-SAMI), the material composition design and the evaluation of antireflective cracking effect of AR-SAMI need to be studied. In this paper, conventional asphalt tests, dynamic shear rheometer (DSR) test, and fatigue cracking test were conducted to evaluate the high and low temperature performance, elastic recovery property, and antifatigue performance of SK90# raw asphalt, asphalt rubber, and SBS-modified asphalt. +e AR-SAMI’s material composition design method based on the interlaminar shear strength was put forward. +e influence of the asphalt application rate and aggregate application rate on interlaminar shear strength was also discussed to determine the optimum material composition. +e fatigue cracking test was designed based on the Hamburg rutting instrument, and the cracking resistance of AR-SAMI was analyzed.+e results indicate that asphalt rubber is the suitable binder for SAMI.+e application rates of asphalt and aggregate have significant impact on the interlaminar shear strength of AR-SAMI. +e optimum binder application rate of asphalt rubber and aggregate application rate are 2.2 kg/m and 14 kg/m, respectively, for AR-SAMI. +e fatigue cracking life and fatigue fracture life of composite specimens increase obviously after AR-SAMI is paved. +e increasing range of fatigue life because of the use of AR-SAMI is up to 30% under the dry condition of 15°C.+e decreasing range of fatigue life caused by water reaches as high as 50%.+e fatigue life falls sharply when the temperature increases from 15°C–25°C to 35°C–45°C.


Introduction
Large amounts of cement concrete pavements were built in China from the early 1970s to the late 1980s.ese pavements are just in the period of large-and medium-sized maintenance [1].Among the measures which are employed to deal with the old cement concrete pavement, the paving asphalt layer is one of the most common treatment methods [2].Asphalt overlays can not only improve the performance of cement concrete pavements but also make full use of the residual strength of the old cement concrete slabs.Meanwhile, the construction of asphalt overlays has little influence on the traffic and environment [3,4].However, there are also some problems about asphalt overlays needed to be solved.e asphalt overlay is likely to generate reflective cracks under the cyclic traffic loading due to the existing joints and cracks in the old cement concrete slab, and the reflective cracks usually propagate to the asphalt surface in a short time [5][6][7].Additionally, the reflective cracks will cause the degradation of pavement performance and affect the appearance of asphalt overlays.e cracks also provide access to road-surface water to infiltrate into the pavement and affect the strength of subgrade, thus causing the reduction of stability and overall strength of pavement [8,9].
To solve the problem of reflective cracks in asphalt overlays paved on the rigid or semirigid pavement, many road engineers carried out lots of researches.Noori et al. [10] demonstrated that STRATA had a significant performance advantage in retarding the propagation of reflective cracks.Norambuena-Contreras and Gonzalez-Torre [11] studied the influence of different geosynthetics on retarding the reflective cracks of asphalt overlays.It was found that the good tensile behavior of geosynthetic cannot assure the function of delaying the propagation of reflective cracks.Kim and Buttlar [12] found that the base-isolating interlayer between asphalt overlays and old concrete slabs can improve the stress state in asphalt overlays.Ogundipe et al. [13,14] carried out researches on the performance of the stress-absorbing membrane interlayer (SAMI) under traffic loading.
e results indicated that the SAMI can effectively delay the generation and propagation of reflective cracks.Tan et al. [15] evaluated the performance of the high viscous stress-absorbing layer by routine laboratory tests and found that the viscous asphalt stress-absorbing layer has an excellent ability to prevent the reflective cracks.
e previous studies show that the stress-absorbing membrane interlayer (SAMI) can not only solve the existing problems of reflective cracks in asphalt overlays but also extend the service life of the composite pavement.In addition, it can reduce the cost of repair and maintenance for pavement, thus decreasing the life-cycle costs of the pavement.However, the studies of SAMI are mostly about its effect on retarding reflective cracks in asphalt overlays but rarely involve the material composition design.Meanwhile, most of the instruments used in previous studies belong to conventional test equipment, which is difficult to obtain the actual conclusions.
In addition, the number of waste tires increased with the development of automobile industries quickly.Waste tires belong to a kind of industrial solid waste which will threaten human health and cause irreversible damage to the environment [16].From the environmental perspective, the waste tires must be utilized [17].One widely used recycling approach for waste tires is to break down the tires to produce rubber powder [18].Rubber powder is the most widely used polymer modifier for raw asphalt [19,20].If the asphalt rubber can be utilized in SAMI, the environmental pollution caused by waste tires can be greatly reduced.erefore, the problems about material composition design and the reflective cracking resistance of AR-SAMI need to be studied systematically.
In this paper, firstly, the conventional asphalt tests, DSR test, and fatigue cracking test of different asphalt binders were conducted to select the appropriate asphalt for SAMI.
en, the composite specimens with and without AR-SAMI were shaped at different application rates of asphalt and aggregate.e interlaminar shear tests at 25 °C and 45 °C were carried out to determine the optimum asphalt application rate and aggregate application rate of AR-SAMI.Finally, the wheel loading of the Hamburg rutting instrument was selected to simulate the effect of vehicles on the road.Based on the Hamburg rutting test, the fatigue cracking tests were carried out under different conditions, and the fatigue cracking life and fatigue fracture life were monitored to evaluate the influence of AR-SAMI in retarding the reflective cracks.

Raw Asphalt, Asphalt Rubber, and SBS-Modified
Asphalt. e raw asphalt used in this paper was SK90#.e main technical indexes of the raw asphalt are listed in Table 1.e asphalt rubber was prepared by SK90# raw asphalt and the rubber powder of 40 mesh in the laboratory, and the mixing amount of rubber powder accounted for 19% by raw asphalt weight.e main technical indexes of rubber powder are listed in Table 2.
e SBS-modified asphalt was also prepared by SK90# raw asphalt and SBS modifier in the laboratory, and the mixing amount of the SBS modifier was 4%.

Aggregate.
e coarse and fine aggregates are basalt aggregates, and the mineral powder is limestone powder.Various technical indexes of aggregates and mineral powder are listed in Tables 3 and 4, which can meet the requirements of Chinese current standards.
According to the international experience, the aggregate of single particle size is usually applied in the AR-SAMI.In this research, the aggregate of single particle size of 9.5-13.2mm is used.
e gradation of SMA-13 is listed in Table 5.

Conventional Asphalt Tests.
e cracking resistance of SAMI is mainly dependent on the performance of the asphalt binder.e conventional indexes such as penetration, softening point, ductility, elastic recovery, and viscosity can reflect the properties of asphalt from different aspects.(25 °C), softening point, ductility (5 °C), and elastic recovery of raw asphalt, rubber asphalt, and SBSmodified asphalt were measured in accordance with the standards of ASTM D5, ASTM D36, ASTM D6084, and ASTM D113, respectively.e viscosity of the three kinds of asphalt was tested by a Brookfield rotational viscometer (model DV-II+) at 135 °C and 177 °C according to the standard ASTM D4402.

Interlaminar Shear Test.
e C30 concrete slab of 5 cm thickness was prepared, and then, the AR-SAMI containing different application rates of asphalt and aggregate was overlaid on the concrete slab, as shown in Figure 1. e composite slabs were formed after the 5 cm SMA asphalt mixture was paved upon the SAMI.en, the core samples were drilled from the composite slabs by a core machine.e diameter of the core sample was 100 mm, as shown in Figure 2. e number of replicates for the interlaminar shear test was three.e shear strength of the core sample was tested by the interlaminar shear instrument and can be calculated by the following equation [21]: where τ is the interlaminar shear strength (MPa), F is the applied load (N), and S is the bond area (m 2 ) (here, 7.85 × 10 −3 m 2 ).
e interlaminar shear test was carried out at 25 °C (room temperature) and 45 °C (high temperature) according to the actual climate condition of Xi'an.And the interlaminar shear tests were conducted at asphalt application rates of 1.8, 2.0, 2.2, 2.4, and 2.6 kg/m 2 .At each asphalt application rate, the samples were prepared and tested at five aggregate application rates of 10, 12, 14, 16, and 18 kg/m 2 .e test results were also analyzed to determine the influence of the asphalt application rate and aggregate application rate on the shear strength of AR-SAMI.

Fatigue Cracking Test.
e composite beam specimen consisted of the C30 concrete slab, AR-SAMI, and asphalt layer.
e thickness of the asphalt layer was 50 mm, the thickness of AR-SAMI was 15 mm, and the concrete slab thickness was 50 mm.e length and width of the beam specimen were 500 mm and 100 mm, respectively.A 10 mm notch was sawn at the centre of the slab through the cement concrete layer to simulate the existing cracks of the old pavement [14].e test loading model and composite beam specimen of the fatigue cracking test are shown in Figure 3.To observe the generation and propagation of reflective cracks clearly, both sides of the asphalt layer and AR-SAMI were painted as white.
e wheel of the Hamburg rutting instrument was selected as a loading device to load on the composite specimen.e number of replicates for the fatigue cracking test was three.
e effect of vehicles on the pavement was simulated by the backward and forward loading of wheels on the test specimen.
e wheel loading of the Hamburg rutting instrument is 0.7 MPa, and the loading frequency is 52 times/min.e fatigue cracking resistance of the asphalt mixture is closely related to the environment temperature.e fatigue damages of the asphalt mixture usually occur between 13 °C and 15 °C, corresponding to the temperature in the spring and rainy seasons of northern China.15 °C is the temperature condition of allowable tensile stress in the specifications for design of asphalt pavement [22].
erefore, the test  e fatigue cracking life is defined as the loading times of the Hamburg rut instrument corresponding to the generation of reflective cracks in the asphalt layer.e fatigue fracture life is defined as the loading times corresponding to the reflective crack propagation to the top surface of the asphalt layer [23].It is thought that the reflective cracking resistance of the composite specimen is better if the fatigue cracking or fracture life is longer.
In this paper, the fatigue cracking tests of composite specimens with and without AR-SAMI under different conditions were carried out.
e process of the fatigue cracking test is shown in Figure 4. e test conditions and programs are listed in Table 6.
During the test process, the camera Nikon D7000 was used to record the generation and propagation of reflective cracks.Before the test, appropriate resolution of the camera should be set.
e selected resolution meets the requirements for reflective crack identification.So after the test, the compaction time of the fatigue cracking test corresponding to the cracking or fracture of composite specimens can be accurately determined by observing videos.en, the fatigue cracking life and fatigue fracture life of composite specimens can be obtained by multiplying rolling time with the loading action frequency of 52 times/min, as shown in the following equations: where N C is the fatigue cracking life, t C is the rolling times of the test wheel corresponding to the cracking (min), and R is the loading action frequency (m 2 ) (here, 52 times/min).
where N F is the fatigue fracture life, t F is the rolling times of the test wheel corresponding to the fracture (min), and R is the loading action frequency (m 2 ) (here, 52 times/min).

Conventional Asphalt Tests.
e conventional tests of raw asphalt, SBS-modified asphalt, and asphalt rubber were carried out, and the test results are listed in Table 7.
It can be seen from Table 7 that the penetrations of raw asphalt, SBS-modified asphalt, and asphalt rubber increase with different scales as the temperature rises.Compared with other two kinds of asphalt, the penetration of asphalt rubber at different temperatures is the smallest.
e PI value of asphalt rubber is the biggest, indicating that the temperature sensitivity of asphalt rubber is the best.e softening point of asphalt rubber is 68.5 °C, which is higher than that of raw asphalt by 40.1% and approaches to that of SBS-modified asphalt.
e change of penetration and softening point implies that the mixing of rubber powder can improve the high temperature performance of raw asphalt.
e ductility can be used to characterize the lowtemperature deformation resistance of asphalt.High lowtemperature ductility indicates that the asphalt is not easy to brittle fracture.
e order of ductility is SBS-modified asphalt > asphalt rubber > SK90# raw asphalt.
e lowtemperature ductility of asphalt rubber increases after the full swelling of rubber powder in raw asphalt at high temperature; namely, the low temperature performance of raw asphalt to resist embrittlement is improved.
e elastic recovery rate of asphalt binder at 25 °C reflects the displacement restoring capacity of asphalt pavement.
e higher the elastic recovery rate, the stronger the displacement restoring capacity of asphalt pavement; namely, asphalt pavement has the better fatigue resistance.e data of the elastic recovery test at 25 °C in Table 5 indicate that SBS-modified asphalt and asphalt rubber have good resilience, which can greatly improve the antifatigue performance of asphalt and extend the fatigue life of the pavement.
At 135 °C and 177 °C, the viscosity order is asphalt rubber > SBS-modified asphalt > SK90# raw asphalt.e viscosity of asphalt rubber at 177 °C is 3.097 Pa•s, which can also meet the design requirements of binder viscosity (1.5 Pa•s∼4.4Pa•s).On the whole, the asphalt rubber has excellent cohesiveness.

DSR Test.
e complex module (G * ) and phase angle (δ) of asphalt rubber, SBS-modi ed asphalt, and SK90# raw asphalt are shown in Figures 5 and 6.
As shown in Figure 5, the G * of the three kinds of asphalt has good linear correlations with temperatures in the semilogarithmic coordinate.e G * values of the three kinds of asphalt decrease with the increasing temperatures and then reach roughly equal at 40 °C.e change ratio of raw asphalt is the fastest, followed by SBS-modi ed asphalt, and the lowest change ratio belongs to asphalt rubber.Figure 6 shows that the δ of asphalt increases gradually with the increasing temperatures.When the temperature is below 25 °C, the δ of SBS-modi ed asphalt is approximately equal to that of raw asphalt.But the δ of SBS-modi ed asphalt is less than that of raw asphalt when the temperature is higher than 25 °C.As for asphalt rubber, the δ remains at a low level with the change in temperature.Clearly, asphalt rubber has lower temperature sensitivity when compared with raw asphalt and SBS-modi ed asphalt.
e G * •sin δ of raw asphalt, SBS-modi ed asphalt, and asphalt rubber at di erent temperatures is shown in Figure 7.
In Figure 7, the fatigue factor (G * •sin δ) of the three kinds of asphalt also has good linear correlations with temperatures in the semilogarithmic coordinate.e G * •sin δ falls gradually with the rise of temperature, which indicates the fatigue resistance of asphalt binder increases gradually.e G * •sin δ of raw asphalt is greater than 5000 kPa at 16 °C, exceeding the fatigue limit of Superpave PG speci cation.e G * •sin δ of SBS-modi ed asphalt is also greater than 5000 kPa at 13 °C, but that of asphalt rubber still remains below 4000 kPa at 13 °C.Obviously, the fatigue resistance of asphalt rubber is the best among the three kinds of asphalt, followed by SBSmodi ed asphalt, and that of raw asphalt is the worst.

Fatigue Cracking Test.
e fatigue cracking tests of composite beam specimens with SAMI containing SK90# raw asphalt, SBS-modi ed asphalt, and asphalt rubber were carried out under the dry condition of 15 °C.e asphalt spraying content of the three kinds of composite beam specimens is 2.2 kg/m 2 , and the gravel spreading amount is 14 kg/m 2 .e in uence of asphalt type on fatigue cracking life and fatigue fracture life is shown in Figure 8.
It can be seen from Figure 8 that the fatigue cracking life and fatigue fracture life of composite beam specimens with SK90# and SAMI is the minimum, followed by SBS-modi ed asphalt, and the life of composite beam specimens with AR-SAMI is the maximum, which indicates the fatigue   Advances in Materials Science and Engineering property of composite beam specimens with AR-SAMI is the best.Based on the above studies, the asphalt rubber has excellent high and low temperature performance, elastic recovery property, and fatigue resistance.High elasticity and antifatigue properties of asphalt rubber can well meet the functional requirements of SAMI.erefore, the asphalt rubber is recommended as the binder for SAMI when the asphalt layer is paved on the old cement concrete pavement.

Material Composition Design Method for AR-SAMI.
e AR-SAMI can delay the propagation of cracks from the bottom layer and prevent water in ltrating from the surface when the interlayer bonding is good.However, the bene cial e ect of AR-SAMI cannot be achieved if the shear strength between the layers is poor.e interlaminar shear test can qualify the interlayer bonding between the overlay and concrete slab.e greater the shear strength value, the better the bonding between layers.Zhou et al. [24] found that as the shear strength increases, the stress concentration at the crack tip decreases.erefore, the asphalt and aggregate application rates of AR-SAMI can be determined based on the interlaminar shear strength.e application rates are determined based on the following criteria: (1) Proper aggregate embedment into asphalt: e proper and reasonable application rates of asphalt and aggregate should not only make the aggregate be well stabilized but also avoid the AR-SAMI forming a weak interlayer in the composite structure.

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Advances in Materials Science and Engineering e percent of embedment traditionally required for aggregate is 70% [25].
(2) Proper bonding between layers: e material composition of AR-SAMI should ensure that the asphalt overlay has su cient shear strength in the composite structure.
en, the optimum application rates of asphalt rubber and aggregate can be obtained by the maximum interlaminar shear strength of AR-SAMI at di erent temperatures.

Determination of Optimum Asphalt and Aggregate
Application Rates for AR-SAMI.Figure 9 illustrates the interlaminar shear strength at 25 °C for AR-SAMI containing di erent application rates of asphalt rubber and aggregate.
As shown in Figure 9, the interlaminar shear strength of AR-SAMI increases rst and then decreases with the increase of aggregate application rate at one certain asphalt application rate.At several asphalt application rates, the aggregate application rate corresponding to the maximum interlaminar shear strength is 14 kg/m 2 .When the aggregate application rate is xed, the interlaminar shear strength of AR-SAMI also increases rst and then decreases with the increase of asphalt application rate.e largest interlaminar shear strength was observed when the asphalt application rate was 2.2 kg/m 2 and the aggregate application rate was 14.0 kg/m 2 .
e interlaminar shear strengths of AR-SAMI containing di erent application rates of asphalt rubber and aggregate at 45 °C are shown in Figure 10.
Figure 10 shows that the interlaminar shear strength trend of AR-SAMI increases rst and then decreases with the increase of asphalt application rate and aggregate application rate.e interlaminar shear strength curve gets its peak at the asphalt application rate of 2.2 kg/m 2 and the aggregate application rate of 14.0 kg/m 2 .
is phenomenon is due to the lever and wedge e ect.e aggregate cannot be coated completely by asphalt, and the asphalt membrane between aggregates is thin when the asphalt application rate is little.e incomplete or thin asphalt membrane cannot assure adequate adhesion between asphalt and aggregates [26], thus resulting in the interlaminar shear strength of AR-SAMI that remains at a low level.Meanwhile, too little asphalt application rate also leads to the insu cient wrapped depth of aggregates by asphalt, which will in uence the waterproof e ect of AR-SAMI.
But when the asphalt application rate is too large or the aggregate application rate is too low, redundant asphalt will become the lubricant which promotes the displacement of aggregates.en, the interlaminar shear strength of AR-SAMI will decline as the asphalt application rate increases.Experience shows that too large asphalt application rate will lead to  Advances in Materials Science and Engineering the bleeding and promote the formation of a weak interlayer [27].Part of asphalt cannot be overlapped by aggregates, and sticking wheels often occur when the road roller compacts on pavement.Seriously, this phenomenon may lead to many di culties to the construction of the pavement.
Based on these results, the asphalt application rate and aggregate application rate were determined to be 2.2 kg/m 2 and 14 kg/m 2 , respectively, for AR-SAMI.

Anticracking E ect of AR-SAMI.
e fatigue cracking tests of composite beam specimens with and without AR-SAMI were carried out.
e tests were conducted at the temperature of 15 °C.e in uence of AR-SAMI with different asphalt application rates on fatigue cracking life and fatigue fracture life is shown in Figure 11.
Figure 11 shows that the fatigue cracking life (N C ) of the asphalt layer clearly increases because of the existence of AR-SAMI when compared to the control group with no SAMI.
e fatigue cracking life of the asphalt layer increases continually when the asphalt application rate increases from 1.6 kg/m 2 to 2.2 kg/m 2 .After the asphalt application rate exceeds 2.2 kg/m 2 , the increasing range of fatigue cracking life of the asphalt layer decreases slightly.
e fatigue cracking life of the asphalt layer does not continue to increase with the increase of asphalt application rate.ere is no obvious di erence between the fatigue cracking life of AR-SAMI corresponding to the asphalt application rates of 2.2 kg/m 2 and 2.5 kg/m 2 .
After the AR-SAMI is paved, the fatigue fracture life (N F ) of the asphalt layer also increases obviously, and the maximum increasing range reaches 29.1% approximately.e results show that fatigue fracture resistance of the composite structure improves gradually with the increase of asphalt content, but there is no signi cant di erence between the increasing ranges of fatigue fracture life corresponding to asphalt application rates of 2.2 kg/m 2 and 2.5 kg/m 2 .Based on these results, it appears that the asphalt application rate of 2.2 kg/m 2 is appropriate for AR-SAMI.
is is consistent with the optimum asphalt application rate of AR-SAMI determined by the shear test.
Clearly, the AR-SAMI can not only delay the propagation of re ective cracks but also improve the ability of composite structure to resist fatigue fracture.In a certain range, the AR-SAMI has better exibility with more asphalt rubber content.e deformation recovery ability and strain capacity of AR-SAMI with good exibility are excellent, which will ensure longer fatigue life of composite pavement.

Analysis of In uence Factors on Fatigue Cracking
Resistance of AR-SAMI.To analyze the in uence of water and temperature on fatigue cracking resistance of AR-SAMI, the fatigue cracking tests of composite specimens with and without AR-SAMI were carried out by the Hamburg rutting instrument in a water bath condition at 15 °C, 25 °C, 35 °C, and 45 °C.e fatigue cracking life and fatigue fracture life of composite specimens under the condition of dry and water bath at 15 °C are shown in Figure 12.
As shown in Figure 12, water has signi cant in uence on the fatigue cracking life and fatigue fracture life of composite specimens.e fatigue cracking life and fatigue fracture life of composite specimens under water bath condition are lower than those under dry condition.e largest reduction of fatigue life under water bath condition is approximately 50% when compared with dry condition.e adverse e ect of water on fatigue life of the pavement structure is obvious.In addition, it can also be seen from Figure 9 that when the asphalt application rate exceeds 2.2 kg/m 2 , there is no improvement of fatigue life.
is phenomenon is because that the water immersion into the specimens will reduce the adhesive ability of asphalt, resulting in the decrease of overall strength of the composite specimen [28].e free water in ltrating into the asphalt

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Advances in Materials Science and Engineering mixture will generate hydrodynamic pressure under repeated tra c loading, which promotes the water immersion to the interface of asphalt and aggregates [29].en, the stripping of asphalt lm from the aggregate surface or the reduction of bonding strength between asphalt and aggregate will occur under the action of vehicle loading and temperature stress [30], resulting in the deterioration of fatigue resistance of asphalt overlays.e air voids of the asphalt mixture provide the access for the in ltration of water, so they should be strictly controlled when designing the mixture for asphalt overlays [31].e on-site air voids of the asphalt mixture should be less than 6%-8% in case of the in ltration of water [27].e fatigue cracking life and fatigue fracture life of composite specimens with and without AR-SAMI under the conditions of water bath at 15 °C, 25 °C, 35 °C, and 45 °C are shown in Figure 13.
Figure 13 illustrates that the fatigue cracking life and fatigue fracture life increase with the increase of asphalt application rate at 15 °C-25 °C (room temperatures).e di erence in the increasing range is not obvious when the asphalt application rate varies from 0 kg/m 2 to 1.6 kg/m 2 .e fatigue cracking life and fatigue fracture life increase signi cantly when the asphalt application rate exceeds 1.6 kg/m 2 .At 15 °C, the fatigue cracking life and fatigue fracture life at the asphalt application rate of 2.2 kg/m 2 are larger by about 33% and 35%, respectively, than those at 1.6 kg/m 2 .When the temperature is 25 °C, the increasing range of fatigue cracking life and fatigue fracture life between di erent asphalt application rates is familiar with that at 15 °C.With the increase of asphalt application rate, the fatigue cracking life and fatigue fracture life at 15 °C- 25 °C do not change signi cantly when the rate exceeds 2.2 kg/m 2 .Obviously, when the application rate of asphalt rubber is around 2.2 kg/m 2 , the improved e ectiveness of AR-SAMI in cracking resistance is good.
is further demonstrates that the material composition design method based on the interlaminar shear strength for AR-SAMI is feasible.
e fatigue cracking life and fatigue fracture life also increase with the increase of asphalt application rate at 35 °C-45 °C (high temperatures), but the slope of the curve is low, namely, the growth rate of fatigue life is slow.At different asphalt application rates, the fatigue cracking life and fatigue fracture life of composite specimens in high temperature conditions decline signi cantly when compared with those under room temperature conditions.e e ect of AR-SAMI in improving the fatigue cracking resistance of composite pavement under room temperature conditions is superior to that under high temperature conditions.

Conclusions
In this study, the optimization of asphalt binder for the stress-absorbing membrane interlayer (SAMI) was studied.
e optimum of asphalt and aggregate application rates for AR-SAMI was also investigated by using interlaminar shear strength and fatigue cracking tests.e following conclusions may be derived based on the results obtained.
High elasticity and fatigue properties of asphalt rubber can well meet the functional requirements of SAMI.It is a good choice to select asphalt rubber as the binder for SAMI.
e application rates of asphalt and aggregate have signi cant in uence on the interlaminar shear strength of AR-SAMI.According to the interlaminar shear strengths of AR-SAMI at room temperature (25 °C) and high temperature (45 °C), the optimum binder application rate of asphalt rubber and aggregate application rate for AR-SAMI are recommended as 2.2 kg/m 2 and 14 kg/m 2 , respectively.Advances in Materials Science and Engineering e Hamburg rutting instrument is used to load reciprocally on the composite specimen consisting of the asphalt layer, AR-SAMI, and old cement concrete.e fatigue cracking life and fatigue fracture life are monitored in the test process.According to the results, the fatigue cracking life and fatigue fracture life of asphalt overlays clearly increase after AR-SAMI is paved on the old cement concrete.Up to 30% increase in fatigue life was observed because of the use of AR-SAMI.
Water has an adverse influence on the fatigue cracking resistance of composite specimens.About 50% reduction in fatigue life was observed because of the water.e rise of temperature also presents adverse effects on the fatigue life of specimens as well.e fatigue life falls sharply when the temperature increases from 15 °C-25 °C to 35 °C-45 °C.
Advances in Materials Science and Engineering temperature for the fatigue cracking test is 15 °C.At the same time, in order to analyze the influence of moisture and temperature on the fatigue cracking resistance of AR-SAMI, the fatigue cracking tests were carried out in a water bath environment at 15 °C, 25 °C, 35 °C, and 45 °C, respectively.e evaluation indexes of the fatigue cracking test are fatigue cracking life and fatigue fracture life.

Figure 1 :
Figure 1: e laying of SAMI on the concrete slab.

Figure 2 :
Figure 2: e specimen of the interlaminar shear test.

Figure 3 :
Figure 3: e loading model (a) and beam specimen (b) of the fatigue cracking test.

Figure 4 :
Figure 4: e process of the fatigue cracking test.

Figure 13 :
Figure 13: Fatigue cracking test results under the conditions of water bath at di erent temperatures: (a) fatigue cracking life; (b) fatigue fracture life.

Table 2 :
Main technical indexes of rubber powder.

Table 3 :
Technical indexes of basalt aggregates.

Table 4 :
Technical indexes of mineral powder.

Table 6 :
e test conditions and programs of the fatigue cracking test.

Table 7 :
Conventional properties of asphalt.
Figure 12: Fatigue cracking test results under the condition of dry and water bath at 15 °C.