Decay Characteristics of Mechanical Properties of Asphalt Mixtures under Sizeable Wet Temperature Cycle

Abstract

Asphalt mixtures will inevitably be affected by rainwater and the effect of the wet temperature cycle during a pavement’s life span. Especially in coastal areas such as Guangdong and Hainan in China, asphalt pavement is particularly susceptible to the sizeable wet temperature cycle formed by the high temperature and sudden temperature drop of rainstorms in summer. For this study, we used a homemade sizeable wet temperature cycle environment simulation device to analyze the decay characteristics and the mechanical properties of asphalt pavements in this environment; modified bending and tensile strength and shear strength tests were used to study the decay patterns of shear strength, bending, and tensile strength, and the stiffness modulus of asphalt mixtures with different air voids and different pavement depths under the action of a sizeable wet temperature cycle. In addition, the Grey correlation method was used to analyze the significance of each influencing factor on the decay of mechanical properties, and mathematical fitting was used to establish the prediction equation of the mechanical properties of asphalt mixtures. The results show that with the increase in the number of sizeable wet temperature cycles, the asphalt mixture’s shear strength, flexural tensile strength, and flexural tensile modulus decrease, the degree of decay increases, and the rate of decay gradually slows down. In the case of the same number of sizeable wet temperature cycles, the degree of decay of the asphalt mixtures gradually decreases with increasing depth or decreasing void ratio. After 100 sizeable wet temperature cycles, the maximum values of the decay of shear strength, modulus of strength, and flexural tensile strength were 22.30%, 23.29%, and 32.01%. The importance of the influence of each factor on the decay of the mechanical properties is as follows: the number of sizeable wet temperature cycles > void rate > depth. The prediction equations of the established mechanical properties have a good prediction effect, and the correlation between predicted values and actual values can be up to 0.925. The prediction equations can effectively predict the mechanical properties of asphalt mixtures with different air voids and depths under the action of sizeable wet temperature cycles.

1. Introduction

In China’s southern coastal region, which sees summer rainstorms (before rainstorms where the asphalt pavement temperature can be up to 60 °C or more), for a short period, rainstorms bring the high-temperature asphalt pavement down to 30 °C or less. The asphalt pavement experiences rapid warming and drying in the intervening rainstorms; cooling–warming and drying–rainfall cooling (dry and wet cycle) form the sizeable wet temperature cycle environment in which the asphalt mixture exists. In a sizeable wet temperature cycle environment, the asphalt mixture’s internal temperature drops due to the sharp contraction and produces micro-cracks [1,2]. The infiltration of rainwater along the cracks further aggravates the decay of the mechanical properties of the asphalt mixture [3]. Although modifiers, fibers, rubber, and other additives can be used to improve the effectiveness of asphalt mixtures in terms of their resistance to the wet temperature cycle [4,5], early-stage damage such as severe water damage and cracking [6] still occurs in asphalt pavements in the region [7,8]. Therefore, it is of great significance to analyze the decay law of the mechanical properties of asphalt mixtures under the action of a sizeable wet temperature cycle for the design of asphalt pavements in high-temperature and rainstorm-prone areas in summer and to reduce the early-stage damage caused by the temperature and humidity changes in the asphalt pavements in this area.

To research the effect of temperature and humidity on the properties of asphalt mixtures, Ren et al. [9] and Zhou et al. [10] used the AASHTO T182 test method, in which mixture specimens are submerged in water and immersed for different times at target temperatures in order to simulate the effect of wet temperature coupling. However, this method is mainly used to evaluate the water stability of asphalt mixtures and does not consider the influence of the change in temperature and humidity conditions and cyclic action; based on the high-temperature water immersion test, Lu et al. [11] and Yang et al. [12] adopted high-temperature water immersion and subjected asphalt mixtures to drying cycle treatment to simulate high-temperature and high-humidity conditions. The cycle’s effect on the performance of the asphalt mixture found an excellent linear relationship between the number of wet and dry cycles and the stability decay rate. However, in the wet and dry cycle and sizeable wet temperature cycle under the conditions of the asphalt mixture performance degradation mechanism, there was a big difference between the former and the latter; based on the small amplitude of temperature and humidity, the former was based on reduced asphalt aggregate adhesion caused by small changes in temperature and humidity, while the latter focused on the sharp contraction of asphalt mixtures caused by the sudden drop in temperature from heavy rainfall under high-temperature conditions [13,14,15]. The use of a particular freeze–thaw cycle (AASHTO T 165) test with different freezing and thawing durations has been reported in the literature, and it has been found that the freeze–thaw cycle can affect the splitting strength, compressive strength, and resilience modulus of the asphalt mixture to different degrees. However, the freeze–thaw cycle method is mainly aimed at areas with a lower average annual temperature; the temperature difference between day and night is significant, and areas with a large amount of asphalt pavement in the natural environment, such as the north of China, most frequently experience the freezing and thawing. In the south of China, the summer high-temperature rainstorms do not align with the region in terms of climatic characteristics. In addition, some researchers suggest that the greater the void ratio of the asphalt mixture under the action of temperature and humidity, the more serious the performance damage, and the closer one is to the upper surface of the specimen, the more obvious the internal structural damage is [16,17,18,19]. The existing methods used to evaluate the mechanical properties of an asphalt mixture under the action of temperature and humidity consider the mechanical strength of the specimen as a whole [6,20]. The existing test methods for evaluating the mechanical properties of asphalt mixtures under the action of wet temperature are based on the overall mechanical strength of the specimen, so they cannot reflect the mechanical properties of asphalt mixtures at different depths under the changes in temperature and wet state. In summary, the existing test methods used to simulate the role of wet temperature cannot reflect the environmental characteristics of a sizeable wet temperature cycle; at the same time, the influencing role of wet temperature on the mechanical properties of asphalt mixtures, as well as the decay law, has not yet been studied.

Therefore, considering the limitations of the existing test methods used to simulate the sizeable wet temperature cycle environment in South China, we used a self-made sizeable wet temperature cycle environment simulation device and layer-cut specimens to analyze and establish a prediction equation for the mechanical properties of asphalt mixtures with different air voids under the action of a sizeable wet temperature cycle, depth, and the pavement decay law, and improve the bending and tensile test and shear strength test method. To a certain extent, it makes up for the research defects in related aspects, and provides a more theoretical basis for the design of asphalt pavements in hot and humid areas of the south. Figure 1 shows the flow chart of this study.

2. Materials and Methods

2.1. Materials

The test asphalt was No. 70 matrix asphalt (Penetration Degree Grading), the aggregate was pyrophyllite, and the mineral was a ground lime powder. According to the Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011) [21], the main performance indexes were tested. The test results are shown in

Table 1. Performance index of asphalt.

Items	Density at 15 °C/(g·cm−3)	Penetration (25 °C, 100 g, 5 s)/0.1mm	Ductility (5 °C, 5 cm/min)/cm	Softening Point/°C	Solubility/%	60 °C Power Viscosity/Pa·s	Flash Point/°C
Test Results	1.015	68.4	129	48.3	99.7	226	291
Specification Requirements	Measure value	60-80	≥15	≥46	≥99.5	≥180	≥260
Test method	T 0603	T 0604	T 0605	T 0606	T 0607	T 0620	T 0633

and

Table 2. Basic performance index of aggregate.

Items	9.5–16 mm	4.75–9.5 mm	2.36–4.75 mm	Machine-Made Sand	Mineral Powder	Test Method
Gross volume relative density	3.069	3.053	2.994	/	/	T 0304
Apparent relative density	3.079	3.069	3.023	2.695	2.864	T 0304
Water absorbance/%	0.23	0.29	0.33	/	/	T 0304
Los Angeles Abrasion Loss/%	7.14			/	/	T 0317
Crushing Value/%	8.1		/	/	/	T 0316
Hydrophilic Coefficient/%	/	/	/	/	0.6	T 0353

, which meet the specification requirements. According to the design range of AC-13 grading, a Marshall test was used to compact the Marshall specimen 75 times on both sides. Three kinds of aggregate grading were designed, and their best oil–rock ratios and the corresponding void ratio are determined in

Table 3. AC-13 asphalt mixture designs.

Grading Number	Passing Rate/%										Oil-Rock Ratio/%	Void Ratio/%
	16	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
Lower limit of grading	100	90	68	38	24	15	10	7	5	4	/	/
1#	100	90.5	69	39	25	16	11	7.5	5.2	4.1	4.7	5.9
2#	100	93.5	74	49	33	24	16	12	8	5.5	4.8	5.0
3#	100	99	84	67	49	37	27	19	14	7.8	5.1	4.1
Higher limit of grading	100	100	85	68	50	38	28	20	15	8	/	/

.

2.2. Methods

2.2.1. Sizeable Wet Temperature Cycle Test

Since the existing indoor test device is not able to simulate the sizeable wet temperature cycle environment suffered by asphalt pavement, a sizeable wet temperature cycle environment simulation device was developed in this paper, whose schematic diagram is shown in Figure 2. As can be seen from the figure, the developed device mainly consists of a central control system, rainfall system, temperature control system and blower system. Its operation principle is to set test parameters (the number of sizeable wet temperature cycles, the temperature of the specimen, rainfall amount, rainfall duration, blower length, etc.) through the central control system. The temperature of the specimen is controlled by the temperature control system to reach a certain value after the system is closed. The rainfall system begins with rainfall cooling and then automatically shuts down until the rainfall reaches the set time. The air blower system is initiated to dry the specimen until the air blower time runs out, and at this time, the air blower system will be closed. The environment for sizeable wet and temperature cycles can be simulated indoors by the circulation of a heater, rainfall, and air blower.

Based on the measured road surface temperature and rainfall in Changsha during the hot and rainy months from 5 June to 5 August 2022 (June to September), the sizeable temperature cycle test parameters were determined in this paper. Eventually, the sizeable wet temperature cycle indoor simulation test was carried out under the condition that the rainfall amount was 31.4 mm/h (heavy rain), the rainfall duration was 30 min, the pavement temperature was 59.2 °C with a depth of 2 cm and the blower duration was 15 min. A double-layer rutting specimen served as the testing object, whose inner temperature was monitored by drilling holes and inserting temperature sensors. The drilling position was located 2 cm at the upper corner of the surface of the specimen. The single sizeable wet thermal test process is described as follows: ① Place the specimen and close the environmental box door. ② Turn on the heating system and stop heating when the temperature sensor reaches its set temperature. ③ Turn on the rainfall system and then turn it off after rainfall is simulated for 30 min. ④ Turn on the blower system, and turn it off after blowing for 15 min. The above steps were repeated for the sizeable wet temperature cycle test.

2.2.2. Mechanical Performance Test

(1)

Improvement of shear test

According to existing specifications [21,22], a uniaxial penetration test and triaxial compression test are used for determining the overall shear strength of the asphalt mixture. However, their test results are not capable of reflecting the changes in the shear strength with the depth of the pavement. Therefore, in order to investigate the decay law of the shear strength of the asphalt mixture with the depth of the pavement under high humidity and at high temperatures, this paper improved the test method by cutting the specimen into layers, changing the size of the specimen, and indenter and adding a base layer. Improved test specimens were prepared using the double-layer rutted specimen upper layer cut method. Layers 1–4 were cut at a uniform depth. Each layer had a length, width, and height of 70 mm, 70 mm, and 11 mm. The steel mold base had a length, width, and height of 70 mm, 70 mm, and 40 mm. The interior contained holes 40 mm in diameter. The diameter of the base of the indenter is 20 mm. The test temperature was 15 °C. The loading rate was 10 mm/min. The test process is shown in Figure 3. The shear strength is calculated by:

$$\tau =\frac{P}{2\pi r·h}$$

(1)

where τ is the shear strength (MPa), P is the maximum damage load (kN), r is the indenter radius (mm), h is the height of the specimen, and D is the decay degree of the mechanical properties, which can be calculated by:

$$D=(1-\frac{Measured valued}{Standard valued})\times 100$$

(2)

where D is the decay degree of mechanical strength (%). The standard value of mechanical strength is the average value of mechanical strength from Cut Layers 1 to Cut Layer 4 when the number of sizeable wet temperature cycles is 0.

(2)

Improvement of flexural–tensile test of small beam

According to existing specifications [21], a flexural–tensile test was performed on small beams to evaluate the flexural–tensile properties of the asphalt mixture. However, the test results were primarily used to assess the flexural–tensile properties of the whole rutted specimen, which cannot reflect those of the mixtures at different depths. Therefore, in order to investigate the decay law of the flexural–tensile properties of the asphalt mixture with the depth of pavement under the action of a sizeable wet temperature cycle, the flexural–tensile test was modified in this paper by reducing the beam thickness and increasing the beam width to change the size of the specimen. The cut mode of the modified specimens was the same as that of the shear test. The length, width, and height of the final shear specimen were 90 mm, 50 mm and 11 mm, respectively, and the span diameter was 70 mm. The test temperature was 15 °C and the loading rate was 10 mm/min. The test process is specified in Figure 4. The flexural–tensile strength and modulus were calculated with reference to specification [21]. The decay degree was calculated by Equation (1).

3. Mechanical Property Decay Laws

3.1. Decay Law of Shear Strength

The relationship between the shear strength of the asphalt mixture and its decay degree with the number of sizeable wet temperature cycles at different air voids (5.9%, 5.0%, and 4.1%) and depths (Cut Layers 1 to 4) is shown in Figure 5. The figure shows the following:

(1)

Under the action of a sizeable wet temperature cycle, the shear strength of the asphalt mixture with different air voids and depths gradually decreased with the increase in the number of wet temperature cycles. In the case of a constant void rate and number of sizeable wet temperature cycles of the same asphalt mixture, the decay degree of shear strength gradually decreased with the depth; if the pavement depth and the number of sizeable wet temperature cycles of the same asphalt mixture remained constant, the larger the void rate, the more severe the decay of shear strength. Therefore, the shear strength of Cut Layer 1 of the mixture with a void ratio of 5.9% suffered the most significant decay (22.30%), while that of Cut Layer 4 of the mixture with a void ratio of 4.1% had the smallest decay (10.14%), with a difference of 12.16% between the two.

(2)

Under the action of sizeable wet temperature cycle, the growth rate of the shear strength decay of asphalt mixture constantly changed. Specifically, in the first 50 sizeable wet temperature cycles, the shear strength of asphalt mixture with different air voids and depths attenuated rapidly. After 50 sizeable wet temperature cycles, decay degree was between 7.87% and 18.41%; when the number of cycles reached 100, decay degree was in the range of 10.14~22.30%. The above results indicate that with the continuous increase in the number of sizeable wet temperature cycles, its impact on the mechanical properties of asphalt mixture decay gradually weakened, and the decay degree of shear strength gradually decreased.

3.2. Decay Law of Flexural-Tensile Strength

The relationship between the flexural–tensile strength of asphalt mixture and its decay degree with the number of sizeable wet temperature cycles at different air voids (5.9%, 5.0%, and 4.1%) and depths (Cut Layers 1 to 4) is shown in Figure 6. It can be seen from Figure 6 that:

(1)

Under the action of sizeable wet temperature cycles, the flexural–tensile strength of asphalt mixtures of different air voids and depths was gradually reduced with the increase in the number of sizeable wet temperature cycles. In the case of the constant void rate and the number of sizeable wet temperature cycles of the same asphalt mixture, as the depth increased, the decay degree of the flexural–tensile strength gradually decreased; if pavement depth and the number of sizeable wet temperature cycles of the same asphalt mixture remained constant, the larger the void ratio, the more prominent the decay of flexural–tensile strength. Therefore, when there were 100 sizeable wet temperature cycles, the flexural–tensile strength of the asphalt mixture with a void ratio of 5.9% was reduced most significantly (32.01%), while that of the asphalt mixture with a void ratio of 4.1% suffered the slightest decay (17.95%), 14.06% lower than the former.

(2)

Under the action of the sizeable wet temperature cycle, the growth rate of the decay of the flexural–tensile strength of the asphalt mixture constantly changed. Specifically, the flexural–tensile strength of the mixture decreased significantly in the first 75 sizeable wet temperature cycles. The flexural–tensile strength of the mixture with different air voids and depths decayed by between 16.06% and 28.44% after 75 sizeable wet temperature cycles. When the number of sizeable wet temperature cycles was between 75 and 100, the decay degree of the flexural–tensile strength of the mixture with different depths and air voids gradually decreased, with a growth rate between 1.73% and 3.90%. Because the upper section of the specimen was directly affected by sizeable rainfall and cooling, resulting in the most notable temperature contraction, it saw the most emergence of microcracks and the gradual development of vertical cracks [23]. In addition, water gradually penetrated the lower section of the mixture along the cracks, thus accelerating the growth of lower micro-cracks and the reduction in bending strength, ultimately resulting in a more significant void ratio. The closer the asphalt mixture was to the upper surface, the faster the flexural–tensile strength decreased. With the decrease in depth and void ratio, the decay of the flexural–tensile strength of the mixture gradually slowed down. However, regardless of the void ratio and depth of the asphalt mixture, the sizeable wet temperature cycle is an essential factor in its internal temperature contraction stress and the emergence of microcracks. Therefore, with the increase in the number of sizeable wet temperature cycles, the flexural–tensile strength continued to decrease until cycles were repeated up to 75 times. The decay of flexural–tensile strength gradually slowed down, as the sizeable wet temperature cycle has a limited effect on the decay of flexural–tensile strength.

3.3. Decay Law of Strength Modulus

The relationship between the strength modulus of the asphalt mixture and its decay degree with the number of sizeable wet temperature cycles at different air voids (5.9%, 5.0%, and 4.1%) and depths (Cut Layers 1 to 4) is shown in Figure 7. It can be seen from Figure 7 that:

(1)

Under the action of sizeable wet temperature cycles, the flexural–tensile modulus of asphalt mixtures with different air voids and depths decreased with the increase in the number of sizeable wet temperature cycles. In the same sizeable wet temperature cycle and for the same depth of the asphalt mixture, the decay in the strength modulus gradually increased with the increase in void ratio, and in the case of the same void ratio, the decay of the strength modulus gradually decreased with the increase in depth. The above results show that under the action of a sizeable wet temperature cycle with a smaller air voids and greater depth, the less the strength modulus of the asphalt mixture would be affected.

(2)

With the increase in the number of sizeable wet temperature cycles, the decay degree of the strength modulus of the asphalt mixture with different air voids and depths constantly changed. Specifically, in the first 75 sizeable wet temperature cycles, the decay degree of the strength modulus of the asphalt mixture increased almost linearly. After 75 sizeable wet temperature cycles, the strength modulus of the asphalt mixture with different depths and air voids decayed by between 10.08% and 20.77%. However, with the increase in the number of sizeable wet temperature cycles, the strength modulus decayed increasingly significantly, but its growth rate considerably decreased. When the number of sizeable wet temperature cycles reached 100, the strength modulus decreased by between 11.70% and 23.29%.

4. Prediction of Mechanical Properties

As the flexural–tensile modulus can better characterize the change in the fatigue process of the asphalt mixture and can be derived from the fatigue life of the mixture [24,25], the flexural–tensile modulus was selected as an index for evaluating the mechanical properties of the asphalt mixture under the action of sizeable wet temperature cycles, and correspondingly, a prediction model was established in this paper.

4.1. Importance Analysis of Influencing Factors Based on Grey Correlation

In order to study the influence of each factor on the decay of the flexural–tensile modulus of the mixture, appropriate variables were selected in the establishment of the prediction equation for the mechanical properties of asphalt mixtures under the action of sizeable wet temperature cycles. The Grey System Theory Modeling software (7.0) and Grey correlation method were employed to analyze the importance of the influence factors, and the decay degree of strength modulus was selected as the reference sequence and the influencing factors as the comparative sequence. The analysis results are shown in

Table 4. Grey correlation results.

Reference Sequence	Comparison Sequence
Degree of decay D	Number of sizeable wet temperature cycles	Void ratio	Depth of cut
	0.783 (1)	0.619 (2)	0.527 (3)

. As can be seen from Table 4, the number of sizeable wet temperature cycles was the main factor affecting the decay of the strength modulus, followed by the void ratio and the depth of the cut layer. However, the grey correlation ratio of the three influencing factors was 1.48:1.17:1, indicating that each factor had a significant effect on the decay of the strength modulus, which should not be ignored in the establishment of the prediction model.

4.2. Establishment of Prediction Model

According to previous analysis results, the number of sizeable wet temperature cycles, the void ratio and the cut layer were selected as variables to establish the prediction equation for the strength modulus of the asphalt mixture under the action of sizeable wet temperature cycles. The fitted equation is shown in Equation (3), and the correlation coefficients are shown in

Table 5. Coefficients of fitted equations.

Lamination	Void Ratio/%	a	b	c	d	R2
1	4.1	73.2	1785.43	−6.19	1785.43	0.974
2	4.1	29.39	1433.6	−5.42	716.8	0.993
3	4.1	14.71	1076.02	−4.89	358.67	0.987
4	4.1	8.64	842.63	−4.44	210.66	0.964
1	5.0	86.47	1729.45	−7.12	1729.45	0.934
2	5.0	34.77	1390.89	−6.88	695.44	0.958
3	5.0	17.48	1048.58	−6.69	349.53	0.985
4	5.0	10.30	823.84	−6.36	205.96	0.985
1	5.9	84.97	1699.44	−8.33	1699.44	0.952
2	5.9	34.107	1364.29	−7.68	682.14	0.955
3	5.9	17.11	1026.72	−7.3	342.24	0.952
4	5.9	10.1	808.4	−6.84	202.1	0.961

.

$$SB=a\cdot VV+b\cdot F+c\cdot T+d$$

(3)

where S_B is the strength modulus (MPa), VV is the void ratio (%), F is the cut layer, T is the number of sizeable wet temperature cycles, and a, b, c, and d are all correlation coefficients.

As shown in Table 5, the correlation coefficient of the regression equations under different conditions was greater than 0.95, and the regression equations were well correlated. On this basis, the comprehensive integrated correction coefficient K_VF of the void ratio and the cut layer was introduced. The prediction equation of the strength modulus with a void ratio of 4.1% for Cut Layer 1 was used as the basis, and the coefficients of the equations under the conditions of different cut layers and air voids were corrected and integrated, as shown in Equation (4). Subsequently, the prediction model parameters with a void ratio of 4.1% for Cut Layer 1 were used to calculate the equations with different cut layers and air voids, and the relationship between the relevant parameters was fitted. Finally, the integrated correction coefficient calculation equations for the prediction equations of the strength modulus of the AC-13-graded asphalt mixture under a sizeable wet temperature cycle are obtained in Equations (5)~(8).

$$SB=K_{VF1}\cdot VV+K_{VF2}\cdot F+K_{VF3}\cdot T+l$$

(4)

$$K_{VF1}=282.6\cdot VV-23.19\cdot F+488.7$$

(5)

$$K_{VF2}=-3317.08\cdot VV-308.53\cdot F+2189.61$$

(6)

$$K_{VF3}=-154.50\cdot VV-0.3344\cdot F+0.805$$

(7)

$$l=\frac{1785.43}{(0.888+2.778\cdot VV)\cdot (0.04825\cdot F^2+0.0565F+0.4575)}$$

(8)

where K_VF is the comprehensive correction coefficient for the void fraction and the cut layer.

4.3. Predictive Model Validation

In order to verify the applicability of the predictive model of the strength modulus established in this paper under the action of a sizeable wet temperature cycle, the values of relevant variables were substituted into Equations (4)~(8), and the corresponding prediction results were calculated and compared with the measured values and subjected to one linear regression using y = kx + b, as shown in Figure 8.

As shown in Figure 8, after the integrated coefficient K_VF of the cut layer and void ratio was introduced, the accuracy of the established prediction model was reduced compared to that of the original model. However, the correlation coefficient between the predicted values was 0.925, which indicates that the prediction model still obtained good prediction results with a high fitting accuracy and reproducibility, which is conducive to the prediction of the strength modulus of the asphalt mixture under the action of a sizeable wet temperature cycle and can provide the basis for related research in the future.

5. Discussion

This paper focuses on the decay characteristics of the mechanical properties of asphalt pavements of different depths in hot and humid areas of southern China caused by sudden temperature drops due to rainfall.

Sudden temperature drops will lead to sharp contractions in the asphalt mixture, resulting in the emergence of microcracks and the degradation of its properties. Past studies tend to favor winter temperature shrinkage and cracking in cold regions, but there is a similar problem in that temperature induces asphalt pavement performance degradation in the coastal areas of southern China. In this region, summer is almost hot and rainy, and asphalt pavements experience rapid cooling and shrinkage in a short period of time. Therefore, there is a temperature gradient from top to bottom of the asphalt pavement in this region, and internal microcracks also gradually expand downward from the upper part where the temperature change is the largest, resulting in a difference in performance degradation at different depths.

In order to study the decay law of asphalt mixtures under the above conditions, a large-scale wet temperature cycle test device was developed independently to simulate the indoor environment of the southern coastal region. The mechanical properties of the specimens were determined by cutting the layers, and the mechanical properties at different depths from top to bottom were analyzed. According to the results, the decay in the mechanical properties of the asphalt mixture at each cut layer firstly rapidly increased and then gradually decreased, which is consistent with the results in some studies on the damage law of asphalt mixture properties under the action of wet temperature coupling [15,26].

Finally, the prediction equations of mechanical properties at different depths under such conditions were established. The established prediction equation is intended to provide more basis for the design of asphalt pavement in this area.

However, the research in this paper is limited by conditions, and the extension of its internal microcracks can be investigated subsequently by using instruments such as ICT.

6. Conclusions and Recommendations

Based on a sizeable wet temperature cycle test and a mechanical performance test of each cutting layer, the following conclusions are drawn by analyzing the factors affecting the mechanical properties of the AC-13 asphalt mixture.

(1)

With the increase in the number of sizeable wet temperature cycles, the shear strength, flexural–tensile strength and strength modulus of the asphalt mixture with different air voids (5.9%, 5.0%, 4.1%) and depths (Cut Layer 1~4) decay more and more severely. When the number of sizeable wet temperature cycles reaches 100, the flexural–tensile strength, strength modulus and shear strength decayed by 32.01%, 23.39%, and 22.30% at the maximum.

(2)

Under the action of the sizeable wet temperature cycle, the decay degree of the flexural–tensile strength, strength modulus, and shear strength first increases rapidly and then decreases with the increase in the number of sizeable wet temperature cycles; the decay degree of the shear strength increases rapidly in the first 50 cycles, and that of the flexural–tensile strength and strength modulus increases rapidly in the first 75 cycles, and later the decay of all the three gradually slows down in subsequent cycles.

(3)

Under the same conditions of a sizeable wet temperature cycle, the smaller the void ratio and the deeper the asphalt mixture, the smaller the decay degree of shear strength, flexural–tensile strength and strength modulus, and vice versa.

(4)

The Grey correlation method was used to analyze the influencing factors of the mechanical property decay of the asphalt mixture under the action of the sizeable wet temperature cycle, the functional relationship between the influencing factors and strength modulus was established, and finally correction coefficients were introduced to optimize the integration of the established functional relationship. The results show that the importance of the influencing factors is in the following order: the number of sizeable wet temperature cycles > gap ratio > depth. The prediction equation can better predict the magnitude of the strength modulus regardless of void ratio, number of sizeable wet temperature cycles and depth, and the correlation between the predicted and actual values can be up to 0.925.

The conclusions presented in this paper are based on selected road surface temperature and rainfall. It is recommended that performance tests of asphalt mixtures with different road surface temperatures and rainfall levels be performed to better understand their behavior. In addition, it is advisable to consider conducting sizeable wet temperature cycle simulations and performance tests on the upper, middle, and lower layers of the pavement structure.