High-temperature stability, water stability, and ice melting properties of salt-storage bituminous mixture AC-13

This study investigated the road performance and ice melting performance of the AC-13 bituminous mixture prepared with the equal volume replacement of mineral powder from a self-made salt-storage bitumen pavement additive. The results show that adding salt storage bitumen additive improves the bituminous mixture’s high-temperature stability and reduces the bituminous mixture’s water stability. When the replacement rates of the salt storage additive are 50%, 75%, and 100%, the Tensile Strength Ratio (TSR) decreases by 3.35%, 5.95%, and 8.11%, respectively. The dynamic stability (DS) also increases by 4.21%, 7.30%, and 5.62%, respectively. The melting ice rate and salt precipitation tests demonstrate that the salt storage bituminous mixture with a replacement rate of 100% achieves a 36% ice melting rate and 0.3% salt precipitation at -5°C, meeting the standard requirements.


Introduction
Salt-storage bituminous mixture is a mixture that uses salt-storage bitumen pavement additives to replace part or all the mineral filler in traditional bituminous mixtures.The salt-storage bituminous mixture has the function of melting ice and snow actively.There are essential differences between saltstorage additives and ordinary mineral fillers regarding chemical composition, density, etc. Correspondingly, salt-storage additives used in the salt-storage bituminous mixture, the road performance will also be different.Therefore, the variation of pavement performance of salt-storage bituminous mixture has always been the focus of research, and a series of achievements have been made.
Wu et al. [1] investigated the influence of MFL on bituminous mixtures, and the findings indicated that the inclusion of MFL leads to a decrease in the road performance of these mixtures., especially water stability.Sun [2] prepared a salt-storage bituminous mixture with a self-made slow-release saltstorage filler and found that the water damage resistance of the salt-storage bituminous mixture decreased.Liu [3] conducted an experiment where he substituted snow-melting and ice-suppressing materials for the filler in asphalt.The objective was to examine how this replacement affects the performance of bituminous mixtures under varying temperature conditions and their water stability and other road properties.Guo et al. [4] studied the effect of salt chemicals and high elastomer content on salt-treated bitumen wear layers.Various studies have shown that replacing mineral powder with environmentally friendly deicing materials can improve the high-temperature stability of salt-treated bituminous mixtures but adversely affects its water stability and low-temperature performance [5][6][7][8] .
This study used an equal amount of self-made salt storage additive instead of mineral powder to prepare a bitumen salt storage mixture AC-13.Rutting tests evaluated the high-temperature stability of salt-treated bituminous mixtures.Marshall impregnation and freeze-thaw cycle gap tests were performed on bituminous mixtures.The melting ice rate and salt precipitation tests were conducted on the salt-storage bituminous mixture to directly evaluate the ice and snow melting effect of the saltstorage bituminous mixture.

Materials
SBS modified bitumen was used as the bitumen binder.Limestone mineral powder was used as filler.The salt-storage bitumen pavement additives were homemade.Their specific indicators are shown in Table 1.
Crushed limestone gravel was used as the coarse aggregate, and limestone chips were used as the fine aggregate in the bituminous mixture.All the aggregates were sieved to single particle size for later use.
Table 1.Indexes of salt-storage bitumen pavement additive.

Preparation of salt-storage bituminous mixture AC-13
The mineral mixture used in the AC-13 bituminous mixture is continuously densely graded, and the gradation curve is shown in Figure 1.According to the Standard Test Method for Asphalt and bituminous mixtures for Road Construction (JTG E20-2011, China), the optimal asphalt aggregate content of AC-13 bituminous mixture was determined to be 5.0% through the Marshall test.

Figure 1.
Grading curve of AC-13.The salt-storage bituminous mixture was prepared by replacing the mineral powder with an equal volume of salt-storage additives (replacement rates are 50%, 75%, and 100%, respectively).The asphaltaggregate ratio remained unchanged.The salt-storage additives must be mixed with the mineral powder and added to the mixing pot simultaneously.

Pavement performance tests of bituminous mixture AC-13
According to the guidelines outlined in (JTG E20-2011, China), the laboratory performed rutting and bending tests on small beam specimens to assess their properties for highway engineering.To evaluate the water stability of the bituminous mixture, the immersion test and freeze-thaw splitting test were also carried out using the Marshall method.

Test on thaw performance of salt storage bituminous mixture AC-13
Using the standard reference "Deicing and thaw material employed in the bituminous mixture (JT/T 1210.2-2018,China)", we directly assess the ice melting effectiveness of the bituminous mixture stored with salt through experiments such as the ice melting rate test and salting out test.Meanwhile, the freezing point and conductivity tests indirectly indicate the ice melting and ice blocking capabilities of the bituminous mixture stored with salt.The test process of the melting ice rate test is shown in Figure 2. To begin, we measured 40 mL of distilled water and placed it in a crystallization dish with a diameter of 100 mm.The dish was then placed in a refrigerator at -20°C to freeze the water into ice cubes for later use.The Marshall sample, measuring 101.6 × 63.5 mm in size, was allowed to cool to room temperature and placed inside an environmental box set to -5°C for 4 hours.Subsequently, the ice, known as the initial mass m 0 , was removed from the freezer and promptly placed on top of the sample.After the sample was kept in an environmental chamber at -5°C for 2 hours, the ice was removed, and its remaining mass was determined as m 2 .The percentage of the remaining mass of the ice to the initial mass is calculated according to Formula (1), that is, the ice melting rate.
Where σ is the ice melting rate (%).The initial mass of the ice is represented as m 0 .The residual mass of the ice after being maintained at a temperature of -5°C for 2 hours is denoted as m 2 .
Figure 3 illustrates the salt precipitation examination for bituminous mixtures, which aims to evaluate the salt discharge of the salt-storage bituminous mixture sample using the soaking technique.To conduct the test, the following procedures are undertaken: First, we take 1000 ml of distilled water, put it into a 2000 ml beaker, and keep the temperature at 23°C in a constant temperature water bath; Marshall test pieces of bituminous mixture after cooling and demoulded were slowly placed into beakers at room temperature for further heat preservation.The salinity concentration of the solution after soaking for 10 min and 24 h was measured with the salinity measurement module of the magnetic DDSJ-308F conductivity meter, and the salt precipitation of test pieces was calculated according to Formula (2).
Where C S is the salt precipitation of test specimen (%); C 24 is the solution salt solubility of specimen at constant temperature for 24 h (%); C 0 is the solution salt solubility of specimens at constant temperature for 10 min (%).

Results and discussion
Figure 4 shows the rutting test results of the salted bituminous mixture.Compared with ordinary AC-13 asphalt mixture, the dynamic stability (DS) of salt-storage asphalt mixture has somewhat improved.The DS of the bituminous mixture for salt storage had observed a rise of 4.21%, 7.30%, and 5.62% when the replacement rates were 50%, 75%, and 100%, respectively.This indicates that incorporating additives specifically for salt storage into the bitumen can improve the intercalation among aggregates, assuming the grading and oil-stone ratio remain constant, which is conducive to improving structural integrity.In addition, salt-storage asphalt pavement additives increase resistance to rut deformation at high temperatures and improve dynamic stability.
As can be observed from Figure 5(a), the immersion residual stability (MS 0 ) of salt-stored bituminous mixture specimens decreases with the increase of substitution rate.When the substitution rate is 100%, the MS 0 of the salted bituminous mixture drops from 90.3% of the ordinary AC-13 mixture to 86.05%.The MS 0 of the salted bituminous mixture samples with a 50% and 75% substitution rate decreased by 1.88% and 3.21%, respectively, but still met the specification requirements.
The freeze-thaw splitting test results of the salt-storage bituminous mixture shown in Figure 5(b) are similar to those of the immersion Marshall test.The strength ratio (TSR) of salt storage bituminous mixture decreases with the growth of the substitution ratio.The TSR of salt-preserving bituminous mixtures with replacement rates of 50%, 75%, and 100% was reduced by 3.35%, 5.95%, and 8.11%, respectively, and still met the lower limit requirements of the specification.
The above data analysis shows that the incorporation of salt-based bitumen pavement additives hinders the water stability of the mixture.Cracks can easily occur between aggregate and bitumen mortar in a freeze-thaw environment.After water intrusion, NaCl solution is formed with snow melt salt (NaCl) in salt storage bitumen pavement additive.Compared with water, NaCl solution is more likely to infiltrate the aggregate, so it is also more likely to penetrate the interface between bitumen and aggregate.This will reduce the bitumen aggregate adhesion and accelerate the peeling of the bitumen film on the aggregate surface [4,8] .Additionally, submerging the salt-storage asphalt mix in water releases snowmelt salts.There are still gaps in the mix.After water enters the gap, it freezes and expands at low temperatures, accelerating water loss [9][10][11][12][13] .2. The melting rate (σ) of the salt-storage bituminous mixture is calculated to be 36%, which is not less than 20% of the salt chemical materials in the JT/T 1210.2-2018standard.This shows that the salt storage bituminous mixture with an exchange rate of 100% has good ice melting performance at -5°C.

ICAMIM-2023
The results of the salt precipitation test of the salt-storage bituminous mixture are shown in Table 3.It is calculated that the salt precipitation amount of the salt-storage bituminous mixture is 0.3%.According to the standard, the fluxing rate of salt chemical raw materials cannot be greater than 0.4%.The results show that within 48 hours, the snow melt salt precipitation in the bituminous mixture is very small.The short-term precipitation of snow melt salt can meet the requirements of melting snow in the early stage of winter snowfall and ensure that the long-term effect of snow melt and ice suppression in the later stage is not affected.

Conclusions
In this study, the salt-storage bituminous mixture AC-13 was prepared with the equal volume replacement of mineral powder from a self-made salt-storage bitumen pavement additive.We will explore the performance of salt-storage bituminous mixtures in terms of road performance and icemelting capabilities.The key findings obtained are outlined below: (1) Adding salt-storage bitumen pavement additive increases the dynamic stability and the resistance to rutting deformation.When the replacement rate is 75%, the ordinary AC-13 bituminous mixture exhibits a dynamic stability that is 7.30% lower compared to the salt-storage bituminous mixture.
(2) The addition of salt-storage bitumen pavement additives is not conducive to the water stability of the bituminous mixture.The MS 0 of immersion water and the TSR of the freeze-thaw splitting test decrease with the increase of the replacement rate.

Figure 2 .
Figure 2. Test process diagram of melting ice rate test.

Figure 3 .
Figure 3. Test process diagram of salt release test.

Figure 4 .
Figure 4. Results of the rutting test.

Figure 5 .
Figure 5. Results of the (a) Marshall immersion test (b) and the Freeze-thaw split test.Table 2. Results of the melting ice rate of salt-storage bituminous mixture.

Table 2 .
Results of the melting ice rate of salt-storage bituminous mixture.

Table 3 .
Results of the salt precipitation of salt-storage bituminous mixture.