# Moisture Stability of Hard Sandstone Asphalt Mixture Based on APT with MMLS3

Authors: Zhang Ermao; Yang Liming; Yang Datian
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

To evaluate the moisture stability of a hard sandstone asphalt mixture, a testing section of a hard sandstone asphalt mixture was prepared in the laboratory. The accelerated pavement testing (APT) with the one-third-scale model mobile load simulator (MMLS3) was conducted in this testing section under temperature and water-coupled conditions. As the APT number increased, the elastic modulus of the hard sandstone asphalt mixture pavement gradually decreased. The average elastic modulus was 22.99 GPa after 160000 APT cycles. The bulk specific density of the sample rolled by the MMLS3 increased, and the residual indirect tensile strength ratio was 88.7%. Therefore, the hard sandstone asphalt mixture had good resistance to moisture damage, and hard sandstone aggregates can be used in a hot mix of asphalt.


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## Body

## 1. Introduction

Moisture damage is one of the diseases of asphalt mixture pavement [1]. The main external cause of moisture damage in asphalt pavement is the coupled effect of water, temperature, and vehicle loads. The main internal causes of water damage in asphalt pavement are the properties of the aggregate and asphalt [2]. High temperatures reduce the asphalt viscosity, water erodes the asphalt film on the aggregates, and repeated loads accelerate the moisture damage of asphalt mixtures.The indirect tensile strength ratio is used to evaluate the moisture stability of asphalt mixtures in the standard method [3, 4]. Based on a certain sinusoidal waveform [5–9], water moves in the core in a compacted asphalt mixture sample, which simulates hydraulic scouring. After this moisture conditioning, the indirect tensile strength or dynamic modulus of the asphalt mixture sample is measured [10].An immersion rutting test is a procedure in which a rolling tire compacts asphalt mixture specimens in the presence of water and measures the rutting depth after each rolling cycle. The rutting depth curves versus rolling cycles are then processed to determine the asphalt mixture’s moisture stability. For this immersion rutting test, the Hamburg wheel-track test (HWTT) and an asphalt pavement analyzer (APA) are used to evaluate the moisture susceptibility of asphalt mixtures [11–14]. Twagira and Jenkins [15] used a one-third-scale model mobile load simulator (MMLS3) and tensile strength retention to evaluate the moisture damage and reveal the performances of bitumen stabilized materials.The aggregate used in the hot mix asphalt is a nonrenewable resource, especially for individual aggregate types that are gradually becoming scarce, such as basalt aggregate and diabase aggregate. To solve the shortage problem of aggregate in asphalt mixtures, hard sandstone aggregate is being used in asphalt mixture pavement. Ouyang et al. [16] used sandstone aggregate to study the highway performances of sandstone asphalt mixtures. Metcalf and Goetz [17] found that sandstone as an aggregate type can be satisfactorily used in asphalt pavement. Zhang and Li [18] used sandstone aggregate to produce sandstone concrete as the road base.In this study, hard sandstone aggregate from an aggregate plant in Guangxi was used. To examine the moisture stability of the hard sandstone asphalt mixture, a testing section of the hard sandstone asphalt mixture pavement with a length of 4.2 m, a width of 1.4 m, and a thickness of 0.04 m was paved in the laboratory. Then, accelerated pavement testing (APT) with the MMLS3 under hot-water conditions was performed on the hard sandstone asphalt mixture pavement. The elastic modulus of the asphalt mixture pavement was measured with a portable seismic property analyzer (PSPA) after the APT. Finally, some core samples at the wheel and nonwheel track locations were drilled, and their indirect tensile strengths and bulk specific densities were measured. Then, their indirect tensile strength ratios were computed to evaluate their moisture stability. The materials and corresponding test methods used in this study are summarized as a flow chart, as shown in Figure1.Figure 1
Flow chart of study materials and corresponding test methods.


## 2. Materials

### 2.1. Coarse Aggregate

In this study, the coarse aggregate was a type of hard sandstone aggregate that has been rarely used in asphalt mixtures. Its properties were measured by the Test Methods of Aggregate for Highway Engineering (JTG E42-2005) [19], as shown in Table 1.Table 1
Properties of coarse aggregate.
PropertiesUnitRequirementsResultsCrushing value%≤2623.4Los Angeles abrasion%≤2825.0Bulk relative density5–10 mm—≥2.602.71510–15 mm2.714Water absorption5–10 mm%≤2.00.4510–15 mm0.43Flat and elongated particles (%)>9.5%≤1211.1<9.5≤1812.8Washing method <0.075 mm%<10.9Durability/soundness%≤126


### 2.2. Fine Aggregate

The fine aggregate was a limestone manufactured sand, whose properties are shown in Table2.Table 2
Properties of fine aggregate.
PropertiesUnitTechnical requirementsResultsApparent relative density—≥2.502.62Durability/soundness%≤120.9Mud content%≤31.8Methylene blue valueg/kg≤2516Angularitys≥3045


### 2.3. Filler

The filler was a limestone grinding powder, whose properties are shown in Table3.Table 3
Technical properties of filler.
PropertiesUnitTechnical requirementsResultsApparent densityg/cm3≥2.502.679Water content%≤10.3Hydrophilic coefficient—<10.75Particle size<0.6 mm%100100<0.15 mm%90–10098.8<0.075 mm%75–10087.4


### 2.4. Asphalt

Styrene-butadiene-styrene (SBS) modified asphalt was used, whose properties are shown in Table4.Table 4
Technical properties of SBS modified asphalt.
PropertiesUnitTechnical requirementsResultsPenetration depth at 25°C, 100 g, 5 s0.1 mm40–6050Penetration index—≥01.152Softening point (ring and ball method)°C≥6085.0Ductility at 5 cm/min, 5°Ccm≥2039Specific relative density at 15°C——1.060Kinematic viscosity at 135°CPa·s≤31.2Flash point temperature°C≥230303Solubility in trichloroethylene%≥9999.54Elasticity recovery at 25°C%≥7581Rolling thin-film oven test at 163°CWeight change%±1.0−0.105Retained penetration%≥6567Ductility at 5°Ccm≥1519


## 2.1. Coarse Aggregate

In this study, the coarse aggregate was a type of hard sandstone aggregate that has been rarely used in asphalt mixtures. Its properties were measured by the Test Methods of Aggregate for Highway Engineering (JTG E42-2005) [19], as shown in Table 1.Table 1
Properties of coarse aggregate.
PropertiesUnitRequirementsResultsCrushing value%≤2623.4Los Angeles abrasion%≤2825.0Bulk relative density5–10 mm—≥2.602.71510–15 mm2.714Water absorption5–10 mm%≤2.00.4510–15 mm0.43Flat and elongated particles (%)>9.5%≤1211.1<9.5≤1812.8Washing method <0.075 mm%<10.9Durability/soundness%≤126


## 2.2. Fine Aggregate

The fine aggregate was a limestone manufactured sand, whose properties are shown in Table2.Table 2
Properties of fine aggregate.
PropertiesUnitTechnical requirementsResultsApparent relative density—≥2.502.62Durability/soundness%≤120.9Mud content%≤31.8Methylene blue valueg/kg≤2516Angularitys≥3045


## 2.3. Filler

The filler was a limestone grinding powder, whose properties are shown in Table3.Table 3
Technical properties of filler.
PropertiesUnitTechnical requirementsResultsApparent densityg/cm3≥2.502.679Water content%≤10.3Hydrophilic coefficient—<10.75Particle size<0.6 mm%100100<0.15 mm%90–10098.8<0.075 mm%75–10087.4


## 2.4. Asphalt

Styrene-butadiene-styrene (SBS) modified asphalt was used, whose properties are shown in Table4.Table 4
Technical properties of SBS modified asphalt.
PropertiesUnitTechnical requirementsResultsPenetration depth at 25°C, 100 g, 5 s0.1 mm40–6050Penetration index—≥01.152Softening point (ring and ball method)°C≥6085.0Ductility at 5 cm/min, 5°Ccm≥2039Specific relative density at 15°C——1.060Kinematic viscosity at 135°CPa·s≤31.2Flash point temperature°C≥230303Solubility in trichloroethylene%≥9999.54Elasticity recovery at 25°C%≥7581Rolling thin-film oven test at 163°CWeight change%±1.0−0.105Retained penetration%≥6567Ductility at 5°Ccm≥1519


## 3. Aggregate Gradation and Optimal Asphalt Aggregate Ratio

In this paper, the optimal asphalt aggregate ratio was determined by the Marshall mix design method [20].According to the Technical Specifications for Construction of Highway Asphalt Pavement (JTG F40-2004) [20], for hot areas in the summer with heavy traffic, the air void content (AV) of an AC-13C asphalt mixture ranges from 4% to 6%.In this paper, the coarse aggregate with three size ranges (3–5 mm, 5–10 mm, and 10–15 mm) was used. The coarse aggregate of 3–5 mm was a limestone aggregate. Through aggregate sieve analysis and the specification requirements, the aggregate gradation was optimized as shown in Table5 and Figure 2. Hence, the combined gradation is composed of 5% filler, 29% fine aggregate, 16% coarse aggregate of 3–5 mm, 26% coarse aggregate of 5–10 mm, and 24% coarse aggregate of 10–15 mm, whose percentages are the ratio of the mass of various aggregates to the mass of blended aggregates.Table 5
Combined gradation.
Sieve size (mm)Mass percentage passing through the sieve (%)Gradation0.0750.150.30.61.182.364.759.513.216Upper limit of gradation815202838506885100100Lower limit of gradation457101524386890100Middle limit of gradation61013.51926.5375376.595100Combined gradation5811162434507695100Figure 2
Aggregate gradation curve.The bulk specific density, Marshall stability (MS), flow value (FL), air void content (AV), voids in the mineral aggregate (VMA), and voids filled with asphalt (VFA) were determined and are shown in Table6 and Figure 3. In Figure 3, the horizontal axis corresponds to the asphalt aggregate ratio.Table 6
Marshall testing results.
Asphalt aggregate ratio (%)Bulk specific densityAV (%)VMA (%)VFA (%)MS (kN)FL (mm)3.52.3806.4216.2960.5821.712.014.02.3985.7016.0564.4723.932.974.52.4135.2815.9766.9424.333.575.02.4125.0016.5469.7722.914.095.52.3824.7616.8471.7319.944.56Figure 3
Design plots for Marshall mix design.
(a)(b)(c)(d)(e)(f)First, from Figure3, the following points were determined:(a)
asphalt aggregate ratio,a1, at the maximum bulk specific density(b)
asphalt aggregate ratio,a2, at the maximum MS(c)
asphalt aggregate ratio,a3, at the midpoint of the specified AV range (4%–6%)(d)
The asphalt aggregate ratio,a4, at the midpoint of the specified VFA range (65%–75%)Second, the average of the four asphalt aggregate ratios was computed, as follows:(1)OAC1=a1+a2+a3+a44=4.7%+4.5%+5.0%+5.1%4=4.83%.Third, the range of the asphalt aggregate ratio,Cmin–Cmax, was determined, based on the Technical Specifications for Construction of Highway Asphalt Pavement (JTG F40-2004) [20] (excluding VMA). The average of the minimum and maximum values were computed, as follows:(2)OAC2=OACmin+OACmax2=3.8%+5.0%2=4.40%.Finally, the average ofC1 and C2 were calculated, as follows:(3)OAC=OAC1+OAC22=4.83%+4.40%2=4.6%.The optimal asphalt aggregate ratio was 4.6%.


## 4. Construction of Hard Sandstone Asphalt Mixture Pavement

To study the moisture stability of the hard sandstone asphalt mixture, a pavement sample of the hard sandstone asphalt mixture was constructed in the laboratory. This hard sandstone asphalt mixture pavement had a thickness of 0.04 m, length of 4.2 m, and width of 1.4 m. The construction procedure of the pavement sample is shown in Figure4.Figure 4
Construction flow chart for the hard sandstone asphalt mixture testing section.In the laboratory, workers removed the old asphalt mixture layer, then sprayed a seal coat, and finally, sprinkled the hot asphalt as the binder on the base. Because the mixing pot used in the laboratory only mixed 30 L of asphalt mixture at a time, about 500 kg of the hard sandstone asphalt mixture was mixed several times. The asphalt mixtures mixed each time were packed in a white iron bucket, which were then stored in an oven at 180°C, as shown in Figure5.Figure 5
Mixing the hard sandstone asphalt mixture.Based on previous construction experience for AC-13C, the loose paving coefficient for AC-13C was 1.2. The hard sandstone asphalt mixtures were placed on the binding layer, as shown in Figure6. After the hard sandstone asphalt mixtures were placed, about a 5-ton road roller-compacted them to a 40-mm thickness, as shown in Figure 7.Figure 6
Paving the hard sandstone asphalt mixture.Figure 7
Compacting the hard sandstone asphalt mixture.


## 5. Accelerated Pavement Testing

### 5.1. One-Third Scale MMLS3

APT refers to “the controlled application of a prototype wheel loading, at or above the appropriate legal load limit to a prototype or actual, layered, structural pavement system to determine pavement response and performance under a controlled, accelerated accumulation of damage in a compressed time period. The acceleration of damage is achieved by means of increased repetitions, modified loading conditions, imposed climatic conditions, the use of thinner pavements with a decreased structural capacity and thus shorter design lives, or a combination of these factors. Full-scale construction by conventional plant and processes is necessary so that real-world conditions are modeled” [21].The MMLS3 [22] can apply 7200 wheel loads per hour. The maximum wheel load was 2.7 kN (2.9 kN for short periods) on the 300-mm-diameter pneumatic tire wheels. The maximum tire pressure was 700 kPa (800 kPa for short periods). The MMLS3 included an electronic wheel load calibration unit, one spare load wheel, eight spare guide wheels, spare drive belts, a tool kit, and a canvas cover, as shown in Figure 8.Figure 8
Schematic diagram of MMLS3.


### 5.2. Wet Heater System

The water heater unit was used to circulate and heat the water to be applied onto the material to be tested. It can be used in conjunction with a set of spray nozzles to spray hot water onto the pavement in the field or a slab in the laboratory while it is being trafficked by the MMLS3. In this study, the hard sandstone asphalt mixture pavement was mainly heated by spraying hot water at 80°C, which resulted in hot and humid environmental conditions, as shown in Figure9.Figure 9
MMLS3 and water heater unit.


### 5.3. Testing Locations

The testing pavement was divided into four segments to measure their elastic moduli separately, as shown in Figure10.Figure 10
Measurement locations for PSPA.


### 5.4. PSPA

The elastic moduli of the hard sandstone asphalt mixture pavement were measured by the seismic wave method, namely using a PSPA [23], as shown in Figures 11 and 12. Because the hard sandstone asphalt mixture pavement was heated by hot water, the elastic moduli measured by the PSPA were transformed into the elastic moduli at 25°C using the following equation [24]:(4)E25=Et1.35−0.01404t,where E25 and Et are the modulus at 25°C and temperature t (°C), respectively.Figure 11
PSPA.Figure 12
PSPA time records and dispersion curves.


### 5.5. Experimental Plan

The numbers of passes during the APT were 0, 5000, 10000, 20000, 40000, 80000, and 160000. After the APT, the elastic moduli of four testing segments were measured. Finally, the core specimens were drilled at the wheel track and nonwheel track. Their bulk specific density and the indirect tensile strength were measured in the laboratory.


## 5.1. One-Third Scale MMLS3

APT refers to “the controlled application of a prototype wheel loading, at or above the appropriate legal load limit to a prototype or actual, layered, structural pavement system to determine pavement response and performance under a controlled, accelerated accumulation of damage in a compressed time period. The acceleration of damage is achieved by means of increased repetitions, modified loading conditions, imposed climatic conditions, the use of thinner pavements with a decreased structural capacity and thus shorter design lives, or a combination of these factors. Full-scale construction by conventional plant and processes is necessary so that real-world conditions are modeled” [21].The MMLS3 [22] can apply 7200 wheel loads per hour. The maximum wheel load was 2.7 kN (2.9 kN for short periods) on the 300-mm-diameter pneumatic tire wheels. The maximum tire pressure was 700 kPa (800 kPa for short periods). The MMLS3 included an electronic wheel load calibration unit, one spare load wheel, eight spare guide wheels, spare drive belts, a tool kit, and a canvas cover, as shown in Figure 8.Figure 8
Schematic diagram of MMLS3.


## 5.2. Wet Heater System

The water heater unit was used to circulate and heat the water to be applied onto the material to be tested. It can be used in conjunction with a set of spray nozzles to spray hot water onto the pavement in the field or a slab in the laboratory while it is being trafficked by the MMLS3. In this study, the hard sandstone asphalt mixture pavement was mainly heated by spraying hot water at 80°C, which resulted in hot and humid environmental conditions, as shown in Figure9.Figure 9
MMLS3 and water heater unit.


## 5.3. Testing Locations

The testing pavement was divided into four segments to measure their elastic moduli separately, as shown in Figure10.Figure 10
Measurement locations for PSPA.


## 5.4. PSPA

The elastic moduli of the hard sandstone asphalt mixture pavement were measured by the seismic wave method, namely using a PSPA [23], as shown in Figures 11 and 12. Because the hard sandstone asphalt mixture pavement was heated by hot water, the elastic moduli measured by the PSPA were transformed into the elastic moduli at 25°C using the following equation [24]:(4)E25=Et1.35−0.01404t,where E25 and Et are the modulus at 25°C and temperature t (°C), respectively.Figure 11
PSPA.Figure 12
PSPA time records and dispersion curves.


## 5.5. Experimental Plan

The numbers of passes during the APT were 0, 5000, 10000, 20000, 40000, 80000, and 160000. After the APT, the elastic moduli of four testing segments were measured. Finally, the core specimens were drilled at the wheel track and nonwheel track. Their bulk specific density and the indirect tensile strength were measured in the laboratory.


## 6. Results and Discussion

### 6.1. Elastic Modulus of Hard Sandstone Asphalt Mixture

As shown in Figure10, there were two segments completely rolled by the MMLS3. The first and fourth segments were only partially rolled. Hence, the elastic moduli of the second and third segments are discussed here. The elastic moduli of the hard sandstone asphalt mixture pavement segments are shown in Table 7.Table 7
Elastic moduli of the hard sandstone asphalt mixture pavement segments.
APT cycles0500010000200004000080000160000Segment 2117.1357.1030.421.4035.3725.5022.60Segment 344.1014.5358.9511.1330.6725.8723.37According to Grubbs’ statistical method [25], the elastic modulus of 117.13 GPa in Segment 2 was an outlier. This outlier was excluded, and the elastic modulus of the hard sandstone asphalt mixture pavement ranged from 11.13 to 57.1 GPa and decreased to 22.60–23.37 GPa after 160000 loading cycles, as shown in Figure 13.Figure 13
Relationships between the elastic moduli and the number of APT cycles for two segments.From Figure13, as the number of loading cycles increased, the elastic moduli gradually decreased.


### 6.2. Bulk Specific Density and Indirect Tensile Strength

After the hard sandstone asphalt mixture pavement was rolled repeatedly by the MMLS3, some core specimens were drilled from the wheel track of the MMLS3, as shown in Figure13. In addition, other core specimens were drilled from sites that were not rolled by the MMLS3, as shown in Figure 14.Figure 14
Core specimens.The bulk specific densities and the indirect tensile strengths of these core specimens were measured in the laboratory. These results are shown in Table8. The bulk specific density increased and the indirect tensile strength decreased after the APT. The cause of the bulk specific density increase was that the air void content was smaller in the hard sandstone asphalt mixture pavement rolled repeatedly by the MMLS3. Under conditions with water, temperature, and load coupling, microdamage could occur in the hard sandstone asphalt mixture, which made the indirect tensile strength smaller. However, the residual indirect strength ratio was still 88.7%, which fully meets the technical requirements of the moisture stability of hot mix asphalt [17].Table 8
Bulk specific densities and indirect tensile strengths of two types of core specimens.
NumberBulk specific densityIndirect tensile strength (MPa)Rolled specimensNonrolled specimensRolled specimensNonrolled specimens12.3692.3411.3071.41722.3562.3351.2301.29732.3512.3261.2431.40842.3342.3371.1261.40352.3422.3241.1791.28362.3492.3341.1081.39972.3642.3331.3191.39482.3722.3291.2491.50192.3642.3241.4271.518Average value2.3562.3311.2431.402To verify the significance of the effect of the wet-heated APT with the MMLS3 on the bulk density and the indirect tensile strength, the bulk specific density and the indirect tensile strengths of the two types of core samples were analyzed by thet-test method, as shown in Tables 9 and 10. According to the Pearson correlation coefficient of 0.04764 shown in Table 9, the linear correlation between the two types of samples was good. The one-tailed and two-tailed P (T ≤ t) values were both smaller than the significance level of 0.05, which verified that there was a significant difference between them. Hence, the APT with the MMLS3 further compacted the hard sandstone asphalt mixture.Table 9
t-test for the bulk specific densities of two types of core specimens.
t-test: paired two sample for meansSampleSample 1Sample 2Mean2.3562.3314Variance0.00016333.628×10^−05Observations99Pearson correlation coefficient0.04765Hypothesized mean difference0df8t Stat5.242P (T ≤ t) one-tailed0.0003909t critical one-tailed1.860P (T ≤ t) two-tailed0.0007818t critical two-tailed2.306Table 10
t-test for the indirect tensile strengths of two types of core specimens.
t-test: paired two sample for meansSampleSample 1Sample 2Mean1.2431.402Variance0.010046.112×10^−03Observations99Pearson correlation coefficient0.5053Hypothesized mean difference0df8t Stat−5.263P (T ≤ t) one-tailed0.0003807t critical one-tailed1.860P (T ≤ t) two-tailed0.0007614t critical two-tailed2.306According to the Pearson correlation coefficient of 0.5052 shown in Table10, the linear correlation between the two types of samples was a subpositive correlation. The one-tailed and two-tailed P (T ≤ t) values were both smaller than the significance level of 0.05, which verified that there was a significant difference between them. Hence, under conditions with water, temperature, and load coupling, the APT with the MMLS3 could accelerate the damage to the hard sandstone asphalt mixture.


## 6.1. Elastic Modulus of Hard Sandstone Asphalt Mixture

As shown in Figure10, there were two segments completely rolled by the MMLS3. The first and fourth segments were only partially rolled. Hence, the elastic moduli of the second and third segments are discussed here. The elastic moduli of the hard sandstone asphalt mixture pavement segments are shown in Table 7.Table 7
Elastic moduli of the hard sandstone asphalt mixture pavement segments.
APT cycles0500010000200004000080000160000Segment 2117.1357.1030.421.4035.3725.5022.60Segment 344.1014.5358.9511.1330.6725.8723.37According to Grubbs’ statistical method [25], the elastic modulus of 117.13 GPa in Segment 2 was an outlier. This outlier was excluded, and the elastic modulus of the hard sandstone asphalt mixture pavement ranged from 11.13 to 57.1 GPa and decreased to 22.60–23.37 GPa after 160000 loading cycles, as shown in Figure 13.Figure 13
Relationships between the elastic moduli and the number of APT cycles for two segments.From Figure13, as the number of loading cycles increased, the elastic moduli gradually decreased.


## 6.2. Bulk Specific Density and Indirect Tensile Strength

After the hard sandstone asphalt mixture pavement was rolled repeatedly by the MMLS3, some core specimens were drilled from the wheel track of the MMLS3, as shown in Figure13. In addition, other core specimens were drilled from sites that were not rolled by the MMLS3, as shown in Figure 14.Figure 14
Core specimens.The bulk specific densities and the indirect tensile strengths of these core specimens were measured in the laboratory. These results are shown in Table8. The bulk specific density increased and the indirect tensile strength decreased after the APT. The cause of the bulk specific density increase was that the air void content was smaller in the hard sandstone asphalt mixture pavement rolled repeatedly by the MMLS3. Under conditions with water, temperature, and load coupling, microdamage could occur in the hard sandstone asphalt mixture, which made the indirect tensile strength smaller. However, the residual indirect strength ratio was still 88.7%, which fully meets the technical requirements of the moisture stability of hot mix asphalt [17].Table 8
Bulk specific densities and indirect tensile strengths of two types of core specimens.
NumberBulk specific densityIndirect tensile strength (MPa)Rolled specimensNonrolled specimensRolled specimensNonrolled specimens12.3692.3411.3071.41722.3562.3351.2301.29732.3512.3261.2431.40842.3342.3371.1261.40352.3422.3241.1791.28362.3492.3341.1081.39972.3642.3331.3191.39482.3722.3291.2491.50192.3642.3241.4271.518Average value2.3562.3311.2431.402To verify the significance of the effect of the wet-heated APT with the MMLS3 on the bulk density and the indirect tensile strength, the bulk specific density and the indirect tensile strengths of the two types of core samples were analyzed by thet-test method, as shown in Tables 9 and 10. According to the Pearson correlation coefficient of 0.04764 shown in Table 9, the linear correlation between the two types of samples was good. The one-tailed and two-tailed P (T ≤ t) values were both smaller than the significance level of 0.05, which verified that there was a significant difference between them. Hence, the APT with the MMLS3 further compacted the hard sandstone asphalt mixture.Table 9
t-test for the bulk specific densities of two types of core specimens.
t-test: paired two sample for meansSampleSample 1Sample 2Mean2.3562.3314Variance0.00016333.628×10^−05Observations99Pearson correlation coefficient0.04765Hypothesized mean difference0df8t Stat5.242P (T ≤ t) one-tailed0.0003909t critical one-tailed1.860P (T ≤ t) two-tailed0.0007818t critical two-tailed2.306Table 10
t-test for the indirect tensile strengths of two types of core specimens.
t-test: paired two sample for meansSampleSample 1Sample 2Mean1.2431.402Variance0.010046.112×10^−03Observations99Pearson correlation coefficient0.5053Hypothesized mean difference0df8t Stat−5.263P (T ≤ t) one-tailed0.0003807t critical one-tailed1.860P (T ≤ t) two-tailed0.0007614t critical two-tailed2.306According to the Pearson correlation coefficient of 0.5052 shown in Table10, the linear correlation between the two types of samples was a subpositive correlation. The one-tailed and two-tailed P (T ≤ t) values were both smaller than the significance level of 0.05, which verified that there was a significant difference between them. Hence, under conditions with water, temperature, and load coupling, the APT with the MMLS3 could accelerate the damage to the hard sandstone asphalt mixture.


## 7. Conclusion

In this study, the moisture damage of a hard sandstone asphalt mixture was investigated using APT with an MMLS3. The elastic modulus of the hard sandstone asphalt mixture pavement as well as the bulk specific densities and indirect strengths of the specimens drilled from rolled and nonrolled sites were analyzed by thet-test method. The elastic modulus initially increased and then gradually decreased. The average elastic modulus was 22.99 GPa after 160000 APT cycles.It was also found that the bulk specific density increased and the indirect tensile strength decreased after the APT with the MMLS3. The bulk specific density increased by 1.1%. The indirect tensile strength ratio of the two types of core samples was 88.7%. Based on a pairedt-test, the bulk specific densities and indirect tensile strengths of the two types of core samples were significantly different at the significance level of 0.05. Hence, the hard sandstone asphalt mixture had a good ability to resist moisture damage.In the future, it would be necessary to further track the changes in the highway performance of hard sandstone asphalt mixture pavement in the field and further carry out fatigue tests of hard sandstone asphalt mixtures. In addition, the differences between the hard sandstone aggregate, limestone aggregate, basalt aggregate, and diabase aggregate will be investigated. These results will be presented in successive publications.



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