Preparation of a Green Sustained-Release Microcapsule-Type Anti-Icing Agent for Asphalt Pavement and Its Application Demonstration Project

Abstract

Salt-storage additives (SSAs) were added to the asphalt mixtures during the construction stage, and the formed anti-icing asphalt pavement (AIAP) played an active and smart role in continuous snow melting, which could avoid traffic accidents and provide positive support for winter road maintenance in cold areas. In this study, a novel and economical green sustained-release microcapsule salt-storage anti-icing agent was prepared by using solid waste porous sustained-release skeleton loading organic acetate salt as the core material and styrene-acrylic-acrylate copolymer P(AA-MA-BA-St) as the wall material, which have less corrosiveness and extended the release time. By comparing the physical properties of different solid waste porous carriers and corrosion inhibitors, the blast furnace slag and NaHCO₃ were selected as the sustained-release skeleton and corrosion inhibitors. The optimal conditions of the synthesis of vesicle wall materials were investigated: 3.8 wt % acrylic acid polymerized at 110 °C with 3 wt % AIBN and for 3.5 h, and the relative ice-snow melting capacity of the prepared sustained-release microcapsule-type anti-icing agent (SMAA) product was 90.8%. The best proportion of the SMAA used to replace a part of the equal mass of mineral powder in the SMA-13 asphalt mixtures was 5.5 wt %, and it could satisfy the requirements of road performance. Moreover, we applied the SMAA product to the 5 cm thick surface layer of SMA-13 of the section K64 + 992 ∼ K65 + 193.641 over the main line ramp at the Sizhuang Toll Station of Beijing-Xiong’an highways to construct AIAP. Compared with adjacent sections of the road without SMAA in winter snowfall, the pilot test section has a very good melting effect. This study contributes to the development of long-acting environment-friendly materials for SSAs to reduce the cost of winter road maintenance, and the obtained product has very promising prospects for practical applications.

1. Introduction

In the cold winter weather, the problem of the formation of ice or accumulation of snow happened now and again in the highway and bridge traffic, which has a negative impact on the traffic operation efficiency, driving vehicle safety, and our property.¹ When the road and bridge surface was covered with snow and ice, the friction between the tire and pavement surface was greatly reduced, and the adhesion coefficient was only 12.5% ∼ 25% of the common road surface, which led to the deterioration of the stability and the braking performance of the running vehicles.² Especially, when the snow and ice were crushed into imperceptible “black ice” for the driver, the safety of vehicles and people would not be guaranteed, resulting in numerous traffic accidents.³ Thus, it calls for the development of better and more timely methods for snow and ice control on pavements.

In the past decades, the main methods to keep the asphalt pavement clear of snow and ice in the worldwide research field are as follows: mechanical removal, artificial removal, abrasives, salt applications before and after snow precipitation, salt-storage additives, internal or external heating technologies, physical modifications of pavement surface, high-friction anti-icing polymer overlays, etc.⁴ Mechanical removal is a traditional method to clear snow or ice on the pavement, but it will leave irreversible damage such as scratches and cracks on the road surface and shorten its service life.⁵ Artificial snow removal affects traffic efficiency and has the disadvantages of high labor intensity and low work efficiency. Chloride-based salt snow melting agent usually was applied to remove snow; however, it was also accompanied by some negative effects such as the suspended particles in the air, pollution of water sources, poisoning surrounding organisms, and corrosion of ancillary facilities around roads.⁵ Another advanced technology of thermal pavement generally has high construction cost, disadvantages of high energy consumption, and short pavement life.⁶

With the development of road materials, in-situ anti-icing technology by incorporating salt-storage additives in the asphalt pavement to release salt chemicals from the interior to the surface of asphalt pavement under traffic loading and upon exposure to ice or snow, has attracted much research attention.⁷ In-situ anti-icing asphalt pavement can effectively prevent or weaken the adhesion between ice/snow and road surface and prevent the formation of black ice, intelligent timely response to the situation freezing conditions of pavement surface,⁴ which can greatly reduce the reliance on the use of chemicals for maintaining the level of service. In the 1960s, Germany, Switzerland, Japan, and other developed countries took the lead in developing the technology of snow/ice removal on salt-storage asphalts pavement.⁸ Various salt-storage additives have been developed with a series of names such as WinterPave,⁹ Verglimit,¹⁰ Iceguard,¹¹ Maflon,¹² IceBane,¹³ ZFSY,¹⁴ and Grikol,¹⁵ etc. Most of these anti-icing products contain 30–70 wt % NaCl, CaCl₂, or MgCl₂ active ingredients with a particle size less than 5 mm. In most cases, the salt-storage additives were added to asphalt mixtures to replace the traditional aggregates and then paved and compacted in thin surface overlays or wearing course, following conventional methods to achieve active snow removal function.¹⁶ Many researchers have investigated and demonstrated that the anti-icing asphalt pavements have excellent snow/ice control performance, higher snow melting capacity, a lower bond strength between ice/snow and asphalt pavement, and higher friction resistance between tire and pavement.¹⁷

However, the anti-icing effectiveness and longevity of such novel pavement are yet to be determined for practical engineering applications and usually have the risk of degradation of mechanical or durability properties, especially moisture damage resistance, due to the presence of released salt.⁷ On the other hand, most part of salt-storage snow/ice melting agent’s active components are chloride-based salts which not only cause corrosion to ancillary facilities around roads and bridges but also cause soil deterioration, plant poisoning, and water pollution and other hazards.¹⁸ Furthermore, most salt storage additives have difficulty in controlling the release rate of the encapsulated salt to extend the anti-icing service life (usually 1–3 years). To address these problems, Shi and Zhang prepared a salt-storage additive CaCl₂-zeolite/p-epoxy, which was zeolite containing calcium chloride (CaCl₂) encapsulated with a microporous epoxy layer.¹⁶ The anti-icing asphalt pavement, featuring good anti-icing capacity, low-temperature anti-icing effectiveness, superior anti-icing longevity, and enhanced mechanical properties. However, the downside of this additive is the use of calcium chloride.

Along similar lines, in this paper, from the perspective of environmental protection, long-term sustained-release, and economy, we designed a novel green sustained-release microcapsule type anti-icing agent (SMAA, a low-cost salt-storage additive), and the preparation process of this anti-icing agent, as shown in Scheme 1. First, the porous material was soaked in the saturated organic salt solution, and then, the organic–inorganic snow melting agent core material was prepared after drying and grinding. Then, the surface of the organic–inorganic salt-storage snowmelt agent was coated with polymer containing hydrophilic groups, and the microcapsule type anti-icing agent with the sustainable release effect was obtained. The wrapped polymer on the surface contained hydrophilic carboxyl groups, which could form a water channel to transmit the snowmelt agent, control the release rate, and achieve the effect of long service life. The snowmelt agent was made of sodium acetate (non-chlorine salt) prepared from the waste liquid of biomass pyrolysis, and the carrier used porous solid waste; all these above realized the environmental protection and economic characteristics of this additive. We investigated in detail the best conditions to prepare the SMAA, the conventional performance of the SMAA product (heat resistance index, relative density, nominal maximum particle diameter, water content, and its corrosion rate of carbon steel), anti-icing performance, and road performance. The dynamic water scouring test was carried out to evaluate the longevity of the asphalt mixtures with SMAA. More importantly, we applied this snow melt agent in the actual highway section and achieved good anti-icing effect and service life.

Scheme 1. Preparation Process of SMAA.

2. Experimental Section

2.1. Materials

Snow melting agent sodium acetate (home-made CH₃COONa) was prepared from biomass pyrolysis waste and sodium carbonate according to previous studies.^19,20 Porous solid waste material fly ash (FA) was from Hebei Xibaipo Power Generation Co. Ltd., blast furnace slag powder (BFSP), and steel slag micronized powder (SSMP) were from the Aosen Iron & Steel Co. Ltd and further processed by Hebei Baicheng Building Materials Co. Ltd. Sodium bicarbonate (NaHCO₃, AR), sodium acetate (AR), acrylic acid (99%), methyl acrylate (99%), butyl acrylate (99%), polyethylene glycol (98%), isopropanol (98%), azo diisobutyronitrile (AIBN, 98%), triethylamine (98%), silane coupling agent KH-570, nano-silica SiO₂, and emulsifier OP-10 were all purchased from the Energy Chemical Corp. Shanghai and used without any further treatment.

In this research, SBS (I-D)-modified asphalt, produced by China Sea Asphalt company Co., Ltd., was applied as the binder of all test specimens, and its technical indicators are shown in Table S1.

The basalt crushed stone was used as the coarse aggregate and machine-made sand was used as the fine aggregate, as shown in Tables S2 and S3, respectively. The basalt crushed stone was taken from Hebei Lingshou County Xiao North Building Materials Co. Ltd., and the machine-made sand was taken from Hebei Yixian Xian Chuang Building Materials Co. Ltd. The adhesion of aggregate was up to level 5 and water absorption of aggregate was less than 1%.

The mineral powder was produced by Hebei Laishui Jinglai Building Materials Co. Ltd. In order to improve the adhesiveness of the mineral powder with asphalt, the mineral powder was made by alkaline rocks and the passing percentage of 0.075 mm sieve pore was above 83.6%. The hydrophilic coefficient was less than 1, as shown in Table S4.

Lignocellulose can be used as a stabilizer in Stone Matrix Asphalt (SMA) to give a reinforcing effect to the asphalt mixtures. The lignocellulose used in this study was from Hebei Shenpeng Chemical Co. Ltd., which was gray flocculent, dry, and free of clumps, impurities, and stains. The basic performance parameters are shown in Table S5, and the addition amount was 0.3 wt % of the total mass of asphalt mixtures.

2.2. Preparation of the Organic–Inorganic Snow Melting Agent Core Material

Typical steps for preparing core materials are as follows: first, saturated organic CH₃COONa snow melting agent solution was prepared. Then, 6 g (3 wt %) of OP-10 surfactant, 2 g (1 wt %) of nano SiO₂ dispersant, and 10 g (5 wt %) of corrosion inhibitor were dissolved in 200 g of saturated CH₃COONa solution. Subsequently, 545 g of BFSP porous solid carrier was put into the above solution, and the mixtures were stirred vigorously for 2 h to make the porous carrier fully adsorb the snow melting agent, and the mixtures were paste-like. Finally, the mixtures were fully dried at 130 °C for 24 h to constant weight, and the dried sample was crushed by a pulverizer until it can all passes through a square-hole sieve with a fineness of 0.6 mm, i.e., the organic–inorganic anti-icing agent core material solid powder with a fineness of 0.6 mm or less was obtained.

According to the previous research, the OP-10 surfactant could improve the activities of the porous carrier and sodium acetate and made the carrier more easily absorb sodium acetate; the recommended mixing ratio was 3∼4 wt %.²¹ The effect of nano SiO₂ was to disperse the system more evenly, and the recommended mixing ratio was 1∼2 wt %.²² In the relevant reference, the optimal amount of the corrosion inhibitor was 5 wt % of the weight of the snowmelt agent.²³ In order to make the carriers fully absorb sodium acetate, we tested the carriers and saturated CH₃COONa solution with different mass ratios (3:1, 2.5:1, 2:1, and 1.5:1). When the CH₃COONa solution has a high ratio, a large amount of salt would be precipitated on the solid surface after adsorption and drying operation. When a very small amount of salt precipitated (2.5:1), it was proved that the carrier has just fully absorbed sodium acetate at this time with proper proportion. Considering economy and the product effect, the addition ratio of OP-10 was 1 wt % and nano SiO₂ was 3 wt % of the CH₃COONa solution.

2.3. Polymerization of the Styrene–Acrylic–Acrylate Copolymer P(AA-MA-BA-St) Wall Material Emulsion

Acrylic acid (2 g), methyl acrylate (25 g), butyl acrylate (18 g), and styrene (8 g) were combined to form a mixture solution of reactive monomers, together with silane coupling agents KH-570 (0.424 g, 0.8 wt % of all the monomers weight) and initiator AIBN (1.59 g, 3 wt % of all the monomers weight) dropwise by a peristaltic pump into isopropyl alcohol (50 g) for 3 h under stirring at 110 °C with a reflux unit. The reaction solution was stirred continuously for 0.5 h after the end of the dosing operation. The temperature of the reaction system then lowered to 40 °C, 4 g of triethylamine was added into the system, and stirring was continued for 20 min to get the P(AA-MA-BA-St) copolymer wall material emulsion.

Investigation of the optimal reaction conditions such as the acrylic monomer content, reaction temperature, reaction time, and initiator dosage was performed as the same polymerization procedure as above.

2.4. Preparation of the SMAA

Organic–inorganic anti-icing agent core materials (159 g) prepared in procedure 2.2 and with 6 g of water were put into the wall material emulsion prepared by procedure 2.3 (the ratio of the total mass of the monomer in the emulsion and the mass of the anti-icing agent was 1:3), well mixed the above mixture, mechanically stirred for 0.5 h, and left to stand for 0.5 h. Subsequently, the paste-like mixture was dried at 110 °C for 12 h, following the grinding operation. Finally, the ground solid powder was sieved to obtain the 0.15 mm nominal maximum particle size SMAA.

2.5. Preparation of Asphalt Mixtures

The Stone Matrix Asphalt 13 (SMA-13) composition mixtures were widely used as a type of asphalt mix gradation for highway asphalt pavement and exhibited excellent wear resistance, long durability and compaction, fatigue resistance, high and low temperature stability, and cracking resistance.²⁴ This project was based on the pavement of the Beijing-Xiong’an highways, and the upper layer gradation mainly uses SMA-13 asphalt pavement designed by the Marshall test according to “Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering JTG E20-2011” as the control, as shown in Tables 1 and 2. SMA-13 grading was used in this study, as shown in Figure S1. In the control mixture, the asphalt content was 5.94 wt %, and the filler (mineral powder and SMAA) usage was 10 wt %. In this project, some of the mineral powder filler was replaced by the SMAA displacement method (replaced the equal mass of mineral powder with the chemical filler).

Table 1. Mixing Ratio of Minerals of SMA-13 Asphalt Mixtures.

aggregate	coarse aggregate 10∼15 mm	fine aggregate 5∼10 mm	machine-made sand	mineral powder and SMAA
mixing ratio/%	36	39	15	10

Table 2. Marshall Test Results of Asphalt Mixtures.

test indicators
asphalt aggregate ratio/%	voidage/%	voids in mineral aggregate (VMA)/%	aggregate voids filled with asphalt (VFA)/%	Marshall stability (MS)/KN	flow value (FL)/mm
5.94	3.5	17	79.5	10.5	3.0

2.6. Specimen Preparation

The basic procedure of asphalt mixtures was as follows: the aggregates are put into the mixing pot and stirred for 30∼40s and heated at about 180 °C. Then, the liquid asphalt binder with about 180 °C was added into the mixtures and blended for 60 s, following with the mineral powder and SMAA, and blended for another 60s, resulting in a well-coated and evenly distributed mixture. Subsequently, the hot mixtures were compacted 50 times on both sides at 180 °C in the steel frames to prepare the experimental specimens, such as the Marshall specimen (101.6 mm × 63.5 mm), a block (300 mmm × 300 mmm × 50 mmm), and a prismatic (250 mm × 30 mm × 35 mm), according to the standard JTG E20-2011 (Standard test methods of bitumen and bituminous mixtures for highway engineering), which is the national industry standard of China.

2.7. Characterization and Measurements

The polymer wall material structure was determined by ¹H NMR spectroscopy (Bruker, DPX-400) with CDCl₃ as the solvent at room temperature. A PE-170 spectrometer was used to collect Fourier transform infrared (FT-IR) spectra of obtained polymers and SMAA products at 300 MHz. The thermogravimetric analysis (TGA) was carried out for characterizing the thermal stability of prepared SMAA using a PerkinElmerTGA-7 analyzer in N₂ with a heating rate of 10 °C/min. Transmission electron microscopy (TEM, JEOL’s JEM-2100, Japan) was performed at an accelerating voltage of 200 kV, and a scanning electron microscope (SEM, S-4800-I, HITACHI, Japan) was used to characterize the morphology features of the obtained SMAA product. The element composition of the sample was characterized by an X-ray diffractometer (XRD) (SmartLab9KW, Smart Lab, Japan). Quantification of the adsorption capacity of solid waste carriers for organic salts was performed by X ray fluorescence (XRF).

The monomer conversion of polymeric reaction systems was measured using a weight measurement method, the calculation equation is as follows:

1

where m₁ is the weight of the polymer after freeze-drying and Soxhlet extraction operations for a given mass of emulsion and m₂ is the weight of the solid mixtures after freeze-drying for the same mass of emulsion.

The specific viscosity (η_sp) of 0.5 g/dL polymer (after freeze-drying and Soxhlet extraction operations) solution in 96 wt % H₂SO₄ at 30 ± 1 °C was measured using an Ubbelohde viscometer (Shanghai Shenyi Glass Products Co. Ltd., China). The intrinsic viscosity (η_IV) was calculated from the single point measurement using the following equation:

2

where c is the polymer concentration in the acid solution.

The estimation of the effective sustained-release time of the anti-icing asphalt pavement in the ideal state was calculated according to the previous study with a simple mathematical model.²⁵ Also, very simplistically, only three factors were considered: the total mass of soluble salt in the pavement, the precipitation rate of salt, and the local annual average precipitation days. The effective sustained-release time was estimated by

3

where T is the effective sustained-release time (year), m is the mass per square meter of soluble salt in the pavement (g/m²), v is the average precipitation rate (g/(m² h)) of salt per square meter of pavement, and t is the local average annual precipitation days (day). The statistics of the local climatic conditions was queried to get the t parameter. m was calculated by the thickness of the anti-icing asphalt pavement, the density of anti-icing mixtures, the SMAA mixing amount, and the content of soluble salts in SMAA. The final parameter v is the precipitation rate of salt determined indirectly by the salt concentration difference of the solution-immersed Marshall specimen.

The measurements of the surface area and pore size were carried out using N₂ adsorption–desorption isotherms at −196 °C with a surface area analyzer (TriStar II 3020), and the samples were pretreated at 150 °C under a N₂ atmosphere for 4 h. The Brunauer–Emmett–Teller (BET) equation was used to calculate the BET surface area (S_BET) with the instrument’s software. The instrument’s software also provided the total pore volume (V_T) and the cumulative desorption area (S_C). The average pore diameter (d_a) was calculated using the ratio 4V_T/S_BET and the cumulative desorption diameter (d_c) using the ratio 4V_T/S_C.

Material properties related to the anti-icing agent such as appearance, density, particle size, water content, heat resistant index, carbon steel corrosion rate, and anti-icing performance-related properties such as freezing point, pH, salt release, moisture absorption rate, and snow melting capacity were characterized and assessed in accordance with the national industry specifications of China JT/T 1210.2-2018 (Deicing and thawy material used in the asphalt mixture-Part 2: salinization-based material)²⁶ and GB/T 23851-2017 (snow-melting agent).²⁷

For the asphalt mixture specimen with SMAA, the rutting test, freeze–thaw splitting test, adhesion test, cantabro test, immersion marshall test, durability test, high-temperature stability, and bending test were carried out according to the Chinese industry test specifications of JTG F40-2004 (Technical Specifications for Construction of Highway Asphalt Pavement),²⁸ JTG E42-2005 (Test Methods of Aggregate for Highway Engineering),²⁹ and JTG E20-2011 (Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering).³⁰

In order to obtain accurate experimental data, five parallel tests were conducted for all relevant experiments and analyses. The results presented in this paper are the average value of all the experimental results by removing anomalies.

3. Results and Discussion

3.1. Screening the Components of the SMAA

In order to reduce the environmental damage of the chlorine-based salt snowmelt agent and reduce the cost of the organic salt snowmelt agent, sodium acetate was prepared from straw pyrolysis waste liquid and used as the core component of the microcapsule type anti-icing agent. Porous materials are usually used as the snow melt agent carrier to achieve salt storage and play a sustained release role. Power plants and steel mills have a large amount of solid waste, and these materials possess a porous structure. For environmental-friendly waste utilization, we used the solid waste material as the carrier of the snow melt agent. Furthermore, we designed the styrene-acrylic-acrylate copolymer as the wall material to modify the functional aggregate, porous solid waste contenting CH₃COONa, by the surface coat treatment. This wall material contains hydrophilic and hydrophobic groups, which can further control the release of the snowmelt agent intelligently and achieve longevity performance. In order to obtain the most economical, long-lasting, and best performance of SMAA, we measured the snowmelt ability of the homemade-CH₃COONa, screened the solid waste carrier, and explored the polymerization conditions of the wall material.

3.1.1. Performance of the Homemade-CH₃COONa

The snow melting agent precipitated to the road surface to absorb the moisture in the air would result in deliquescence and form a solution with a low freezing point. Because of the difference in chemical potential between solid ice and salt solution, solid ice and snow would melt continuously. Therefore, the freezing point, relative snowmelt ability, pH value, and dissolution rate of homemade-CH₃COONa were tested according to the standard GB/T 23851-2017, and other snowmelt agents were also tested for comparison, as shown in Table 3. In the table, we can obviously see that the homemade-CH₃COONa has the lowest freezing point, its dissolution rate at an average level, and the snowmelt capacity is 106.3% relative to NaCl. Moreover, the pH value of the homemade-CH₃COONa is 6.67, which meets the requirements of 6∼9 in the specification and shows that it has good compatibility with asphalt mixtures (pH: ∼8). All these data can verify that the excellent green homemade-CH₃COONa organic salt can be used as the core component for the anti-ice agent for asphalt pavement.

Table 3. Property Comparison of the Snow Melt Agents.

salt type	freezing point (°C)	dissolution rate (γ/g min–1)	relative snowmelt capacity (%)	pH
NaCl	–10.9	10.71	100	6.86
CaCl2	–21.2	18.69	113.2	7.64
MgCl2	–11.4	19.61	53.6	6.86
KCl	–6.3	11.85	95.5	8.30
(CH3COO)2Ca	–14.8	4.89	43.8	6.31
CH3COOK	–5.8	30.58	38.6	5.76
homemade-CH3COONa	–23.5	19.85	106.3	6.67

3.1.2. Screening of the Porous Solid Waste Carrier

In this research, we adopted the solid porous material to load the organic salt snowmelt agent, which could not only store the salt with sustainable releasing but also replace the mineral powder gradation in asphalt mixtures to play the role of the skeleton to support and ensure the service life of asphalt pavement. We choose three solid wastes, fly ash (FA), blast furnace slag powder (BFSP), and steel slag micronized powder (SSMP), which are from power plants and steel mills and are already applied in various building materials in other fields to the suitable one.³¹ The adsorption capacity of the material is positively correlated with its own specific surface area. When the material has a large specific surface area, the adsorption capacity of the material is stronger.¹⁴ The salt-storage porous carriers should have a large specific surface area and a well-developed pore structure; therefore, the BET surface area and average pore diameter of these solid wastes were tested and analyzed. In addition, zeolite powder, a common excellent natural porous material, was selected for further comparison. The results obtained from BET analysis are shown in Table 4 and Figure 1. The blast furnace slag powder and fly ash have large specific surface areas 0.45 and 0.42 m²/g, respectively, which are even larger than that of the common zeolite powder (0.41 m²/g). The cumulative desorption area (S_C) and cumulative desorption diameter (d_c) can better reflect the adsorption performance of the material. The BFSP and FA exhibit excellent performance of S_C (67.46 m²/g, 83.27 m²/g) and d_c (71.15 Å, 48.04 Å), which is far better than their performance in zeolite powder. The average pore diameter (d_a) of BFSP is 1.8 times that of the zeolite powder. On the other hand, we used the SEM technology to observe the microscopic morphology of the solid waste carriers. As shown in Figure S2, the BFSP has abundant pores and the SSMP has a relatively undeveloped pore structure, which is in accordance with the BET results. These carriers were immersed in saturated CH₃COONa solution for 4 h, followed by filtering and drying, and the element content of Na was analyzed by XRF. As illustrated in Figure S3, the BFSP-adsorbed more sodium elements achieve 11.7 wt %. Considering the economic cost factor, we can determine that the BFSP which has a large surface area and an average pore diameter can be the most suitable porous carrier to load the CH₃COONa snowmelt agent.

Table 4. Physical Properties of the Solid Waste and Zeolite Powder.

carrier	SBET (m2/g)	SC (m2/g)	VT (cm3/g)	dc (4VT/Sc) (Å)	da (4VT/SBET) (Å)
BFSP	0.45	67.46	0.12	71.15	10.7
FA	0.42	83.27	0.10	48.04	9.6
SSMP	0.37	52.13	0.04	30.69	4.3
zeolite	0.41	49.84	0.06	48.15	5.9

Figure 1.

Comparison of (a) the cumulative desorption area (S_C) and (b) the cumulative desorption diameter (d_c) of different porous carriers.

3.1.3. Screening of the Corrosion Inhibitor

CMA snowmelt agents are strong alkali and weak acid salts, so their aqueous solution ionizes into alkaline, and the OH^– produced will form a passivation film on the metal surface to inhibit metal corrosion.¹³ To protect the metal facilities around the road, we decided to add additional alkali compounds (NaOH, NaHCO₃, and Na₂CO₃) to the anti-icing agent to increase the pH value of the solution and make the metal enter the passivation zone to achieve further corrosion inhibition. Deionized water and NaCl solution were used as the blank control and the control group, respectively. According to the American Society for Testing Materials (ASTM) Designation G31–37, the corrosion of the steel plate was measured by the repeated hanging method.²⁷ Photographs of steel sheets before and after the corrosion test are shown in Figure S4, and the results of the corrosion rate (ν) and the corrosion inhibition rate (η) are collected in Table 5. The results indicate that alkaline strength is positively correlated with the corrosion inhibition of metals, and NaHCO₃ has the same effect as NaCO₃. However, NaOH is more dangerous when used in large quantities. On the other hand, with comprehensive cost considerations, NaHCO₃ is the best corrosion inhibitor.

Table 5. Results of the Corrosion Rate and the Corrosion Inhibition Rate.

corrosion inhibitor	corrosion rate (ν, mm/year)	corrosion inhibition rate (η, %)
H2O	0.08	0
NaOH	0.01	87.5
NaHCO3	0.02	75
Na2CO3	0.02	75
NaCl	0.36
specification	≤0.10

3.1.4. Investigation of the Polymerization Conditions of the P(AA-MA-BA-St) Copolymer

We synthesized the P(AA-MA-BA-St) copolymer, as shown in Scheme 2, and the structure of the obtained polymer after freeze-drying and Soxhlet extraction operations was determined by NMR technology, as shown in Figure 2. We can see that the peaks at ∼3.4 ppm assigned to the methylene of triethylamine at ∼4.0 ppm are ascribed to the methene protons adjacent to the ester group of butyl acrylate, the peaks of methyl protons in the methyl acrylate appeared at ∼3.6 ppm, and the peak of the benzene ring in styrene appears at ∼7.1 ppm. All the above results suggest that we have synthesized the target polymer successfully.

Scheme 2. Reaction Route of the P(AA-MA-BA-St) Copolymer.

Figure 2.

¹H NMR spectra of the P(AA-MA-BA-St) copolymer in CDCl₃.

However, the conditions of the free radical polymerization reaction are very important for obtaining high molecular weight polymers. Because long polymer chains (shown as Scheme 2) can better wrap the salt storage function carrier core material, we studied the effect of reaction temperature, reaction time, and initiator dosage on the monomer conversion and polymer intrinsic viscosity to determine the best reaction conditions of obtaining a higher molecular weight polymer. As seen in Figure 3a, with the increase of initiator contents (percentage of total monomer mass), the monomer conversion rate and the polymer intrinsic viscosity showed an increasing trend (at 110 °C, 3.5 h), but when the content was 3.5 wt %, the monomer conversion rate decreased to 94.1%. Therefore, 3 wt % is the best initiator content. With the increase of reaction temperature (3 wt % AIBN, 3.5 h), the monomer conversion and polymer intrinsic viscosity increased, as displayed in Figure 3b. However, if the temperature was too high, the reaction system would undergo violent polymerization and the gel appeared and the polymer intrinsic viscosity decreased (120 °C, 1.01 dL/g vs 110 °C, 1.11 dL/g). Therefore, the suitable reaction temperature was 110 °C. It can be seen in Figure 3c that although the monomer conversion rate and polymer intrinsic viscosity increased with the extension of reaction time, the growth was not obvious after 3.5 h and the polymerization rate slowed down. Therefore, from the perspective of the pilot test, the optimal reaction time is 3.5 h.

Figure 3.

(a) Initiator, (b) reaction temperature, and (c) reaction time as the function of the monomer conversion rate and the polymer intrinsic viscosity.

In this paper, the prepared P(AA-MA-BA-St) copolymer used as the wall material, which possesses hydrophilic group −COO^–, can form an ion transport water channel. Therefore, we can control the content of the −COO^– group in the polymer to control the release rate of the sustained-release snowmelt agent. However, the percentage of the hydrophilic group should not be too high, which will make the moisture absorption rate of sustained-release anti-icing material too high and the release and dissipation of the snowmelt agent too fast. We changed the contents (0, 1.9, 3.8, 5.7, and 7.6 wt %; percentage of total monomer mass) of acrylic acid in the polymerization reaction and studied their effects on water uptake and ice-melting capacity of the microcapsule snowmelt agent. The results shown in Table 6 and Figure 4 illustrated that, with the increase of the acrylic acid content, the ice-melting capacity and water uptake of the microcapsule type anti-icing agent show an increasing trend. This can be attributed to the increased contents of −COO^– hydrophilic groups in the system, which improved the hydrophilicity of the copolymer wall material. However, when the content is 5.7 wt %, the water uptake is 0.8 wt %, which is beyond the specification value (<0.7 wt %). Thus, 3.8 wt % acrylic acid content was the best choice (5.2% of the total monomer molar amount), which also exhibit excellent snow/ice melting performance (90.8%). Finally, the optimum polymerization process was determined as AA (2 g): MA (25 g): BA (18 g): St (8 g), AIBN (3 wt %), reaction temperature of 110 °C, and reaction time of 3.5 h.

Table 6. Effect of Acrylic Acid Contents on the Properties of the SMAA.

content (wt %)	0	1.9	3.8	5.7	7.6
water uptake (WU, wt %)	0.1	0.2	0.5	0.8	1.1
relative snow-melt capacity (ω, %)	5.6	36.2	90.8	101.7	105.4

Figure 4.

Relationship between acrylic acid content and water uptake and relative snow-melt capacity.

3.2. Characterization of the Prepared SMAA

3.2.1. FTIR, SEM, and TGA Analysis

The P(AA-MA-BA-St) copolymer emulsion prepared during the investigation of the polymerization conditions of the P(AA-MA-BA-St) copolymer was used to coat the organic–inorganic snow melting agent core material (using homemade-CH₃COONa as the snowmelt agent and blast furnace slag powder as the porous carrier), and the obtained SMAA was white powder, as shown in Figure 5a. FTIR was applied to characterize the structure of SMAA to ensure that the P(AA-MA-BA-St) copolymer was coated on the surface of the CH₃COONa-functioned BFSP carrier. The results in Figure 5b show that the characteristic peaks at 1740 and 1450 cm^–1 that appeared correspond to the bending and tensile vibrations of the ester group. The broad peaks at 2952 cm^–1 attributed to the vibration of −CH₃ and −CH₂– groups. The peaks at 1450 cm^–1 correspond to the stretching vibration of C–H in the polymer chain. Thus, the main absorption peaks of the P(AA-MA-BA-St) copolymer were both present in the composite material, proving that the P(AA-MA-BA-St) copolymer coated the functioned carrier successfully.

Figure 5.

(a) Photograph and the (b) FTIR spectra of the obtained SMAA.

SMAA was incorporated into asphalt mixtures by replacing part of mineral powder in asphalt pavement aggregates. For the SMA-13 asphalt mixture mixing process conducted in a high temperature of 185 °C, the thermal stability of the materials was significantly important, especially, the wall material of SMAA was the polymer material. We used a thermogravimetric analyzer (TGA) to characterize thermal stability. As shown in Figure S5, at 27∼60 °C, a slight mass loss of 6.9% occurs, which was caused by the evaporation of water in the material. At the stage of 60 to 200 °C, the mass of the sample material changed very little, and 200∼300 °C was the degradation stage of the polymer skeleton of the wall material. Continuous mass loss was from 300 °C and the wall material degraded; therefore, the product could meet the asphalt mixing requirements of 185 °C high operation temperature.

After being modified by the P(AA-MA-BA-St) copolymer, the surface of BFSP becomes smoother and the aperture becomes smaller which can better realize the sustained release function (Figure 6a). Moreover, the SMAA product has an irregular granular state and a large specific surface area, which meets the conditions of being a filler for asphalt mixtures. According to the chemical composition analysis of BFSP (Table S6), there was very little Fe₂O₃ (0.48 wt %) in the BFSP, then it would not accelerate the oxidative of the asphalt and has no effect on its cohesiveness. As Figure 6b, c presents, when blending the SMAA product to the asphalt mixtures, asphalt showed excellent cohesiveness, and all kinds of aggregates were well bonded together. Meanwhile, the aggregates also have a good dispersion in the asphalt mixtures (Figure 6b).

Figure 6.

Representative SEM micrographs microporous of (a) SMAA with BFSP as the porous carrier and (b, c) asphalt mixtures with SMAA.

3.2.2. Conventional Performance of the SMAA Product

According to the requirements of snow/ice melting salinized materials for road use issued by the Ministry of Communications in 2018, the heat resistance index, relative density, nominal maximum particle size, moisture content, and corrosion rate of carbon steel of the SMAA product were tested, and the results are listed in Table 7.

Table 7. Properties of the SMAA Product.

property	index	specification values	test method
heat resistance index (%)	0.19	≤5	JT/T1210.2
relative density	2.71	≥2.50	JT/T1210.2
nominal maximum particle diameter (mm)	0.15	≤0.3	JTG E42
water content (%)	0.4	≤1	T0332
corrosion rate of carbon steel (mm/year)	0.02		JT/T 973–2015

The heat resistance index determined by salt dissolved out the concentration difference between samples heated to 210 °C for 1 h and untreated samples. The index of SMAA is 0.19%, which conforms to the requirements of the specification within 0.5%, indicating that the self-developed SMAA product was stable in heat resistance at 210 °C for 1 h, which is consistent with TGA results. After mixing with other asphalt mixture components at 185 °C, it can still maintain better sustained-release performance, which ensures the service life of the salt storage asphalt pavement.

In the salt-storage asphalt mixtures, part of the mineral powder in the mixtures was replaced by the anti-icing agent. Previous studies have studied the influence of MFL and V-260 on asphalt mixtures.³² Snowmelt materials of equal mass were often directly added to the mixture. Nevertheless, when the density of MFL was less than that of mineral powder, the packing volume increases; furthermore, the void fraction of the asphalt mixtures decreases, and the flowability of the asphalt mixtures becomes poor and may even clump. Therefore, the design ratio of the asphalt mixtures and the snowmelt material are not suitable for each other, and it will affect the high temperature stability of the asphalt mixtures and the performance of the snowmelt agent.

Through the specification test measurement (JT/T1210.2), the relative density (kerosene as a reference) of the sustained-release microencapsulated snow/ice melting agent developed in this subject is 2.71, while the density of mineral powder in the SMA-13 asphalt mix grade selected in this subject is 2.78, and the relative density of SMAA is 97.5% that of mineral powder, which meets the requirement of ≥2.50 of the specification.

Water content is a parameter used to characterize the hydrated state of the material, which is expressed by water content per unit mass of the sample. This test was carried out based on the highway engineering aggregate test regulation T0332,²⁹ and the water content of SMAA is 0.4%, which is far below the specification index 1%. It is worth noting that the material needs to be stored under dry and ventilated conditions because of the hydrophilic groups contained in the microcapsule walls.

The corrosion rate of carbon steel is one of the indicators for testing the environmental performance of road snowmelt materials. According to the carbon steel corrosion rate test method in GB/T 23851–2017 “Snow-melting agent”,²⁷ two groups of deionized water and NaCl solution were used as control groups to compare the carbon steel corrosion rates of homemade-CH₃COONa functioned carrier core materials, SMAA without corrosion inhibitor and final SMAA products. The photographs of the steel sheet corrosion experiment are shown in Figure S6. Corrosion test results of carbon steels are displayed in Table 8 and Figure 7, the corrosion rate of the steel sheet is the highest with NaCl, and loading CH₃COONa into the carrier and encapsulation in microcapsules can improve corrosion resistance (0.21 vs 0.13 mm/year). Adding corrosion-resistant agents to SMAA products can further reduce the corrosion rate to only 0.02 mm/year far more below the deionized water control blank group, which has a very significant corrosion inhibition effect, and the corrosion inhibition rate achieved as much as 75%. Thus, our product exhibits excellent sustained release and corrosion resistance performance.

Table 8. Corrosion Rate Test Results of Carbon Steel.

corrodent	corrosion rate (ν, mm/year)	corrosion inhibition rate (η, %)
deionized water (1)	0.08	0
NaCl (2)	0.36	–350
homemade-CH3COONa functioned carrier (3)	0.21	–162.5
SMAA without corrosion inhibitor (4)	0.13	–62.5
SMAA product (5)	0.02	75

Figure 7.

Corrosion test results of carbon steels.

The above test results suggest that the appearance of the material is no clumps of powder. The heat resistance index is 0.19%, which has good heat resistance. The relative density and particle size can meet the requirements of fine aggregate properties and asphalt mixture gradation. The water content of the material is 0.4%, which meets the requirements of the specification on the water content of powdery snowmelt materials. Due to the hygroscopic property of the material, it needs to be dried and ventilated for preservation. By the contrast test, the corrosion rate of the SMAA product is only 0.02 mm/year, and the corrosion inhibition rate is 75% high value, with excellent corrosion resistance.

3.2.3. Anti-Icing Capability of Asphalt Mixtures

The freezing point of asphalt mixtures is one of the important indexes to measure the properties of salt storage materials for asphalt pavement.¹⁸ The freezing point of asphalt mixtures refers to the corresponding temperature when the ice and snow begin to freeze on the asphalt pavement and the bond strength is less than 0.1 MPa.²⁶ The freezing point of different types of snowmelt materials is different. Usually, the lower the freezing point, the more significant the ice inhibition ability and snowmelt effect.

The test block of 100 mm × 100 mm × 50 mm was dropped with an appropriate amount of water (completely immersed but without excess water) fit together closely with the drawing head (bonded epoxy resin and nonwoven fabric and fully absorbed water) and then was put into a constant temperature and low temperature chamber of −5 °C for 4 h. The drawing tester was set to carry out the drawing test at a speed of 13 mm/min and read the bond strength at the time of bonding surface failure. If the bond strength difference between the two was less than 0.1 MPa, the temperature at this point was the freezing point of asphalt mixtures adding SMAA; otherwise, the temperature was adjusted, and the test process is shown in Figure S7. We tested the freezing point of asphalt mixtures in different SMAA ratios (0, 2, 4, 6, and 8 wt %), and the results are displayed in Table 9. According to the test results, the freezing point of ordinary asphalt mixtures is −2.5 °C, indicating that in snow and ice winter, the asphalt pavement is prone to exhibit the icing phenomenon under the condition of water storage or snow, resulting in decreased adhesion of asphalt pavement, posing a threat to driving safety. The freezing point of the mixtures reached −21.3 °C when the SMAA ratio was 6 wt %.

Table 9. Freezing Point Test Results of Asphalt Mixtures with Different Ratios of SMAA.

proportion of SMAA (wt %)	0	2	4	6
freezing point of asphalt mixtures (°C)	–2.5	–9.8	–17.5	–21.3

There are a wide variety of evaluation methods for testing the ice melting capacity of functional deicing and snow melting materials,^33,34 but no fixed evaluation method has been established yet. According to the two existing standards of snow melt material in China,^26,27 the snow melting capacity of the material was tested using the melting amount as an evaluation index. A certain mass of ice block was placed on the Marshall specimen for 2 h at −5 °C, and the ratio of the quality difference before and after the ice mass (M₁) to the initial mass of ice mass (M) was determined as the snow melting capacity, and the results are shown in Table 10. The test result shows that when the SMAA ratio in asphalt mixtures increases, the melting ice ability is higher. When the proportion of SMAA was 4 wt %, the snow/ice melting capacity was 46.9%, there was no binding between the Marshall specimen and the ice block, partial contact interface appeared liquid water, known that the SMAA worked, and has a better melting ice effect. When the proportion of SMAA was 6 wt %, the snow/ice melting capacity reached 71.6%, the ice remains less, and the snow/ice melting performance was excellent.

Table 10. Snow Melting Capacity Results of Asphalt Mixtures with Different Ratios of SMAA.

proportion of SMAA (wt %)	M (g)	M1(g)	σ (%)	specification (%)
0	39.5	0.2	0.51	≥20
2	40.0	7.9	19.7
4	39.7	18.6	46.9
6	39.8	28.5	71.6

Whether the salt asphalt pavement can maintain a proper amount of salt release affects the service life of the salt storage pavement to a large extent. When the salt releases too quickly, the effect of ice inhibition and snow melting effect is good, but it will shorten the service life of the pavement to a considerable extent and increase the maintenance cost of the pavement. Moreover, it will also affect the soil and organisms around the road to some extent. When the release rate of salt is too slow, the function of melting snow and suppressing ice cannot be achieved. The salt release property of asphalt mixtures with different SMAA was measured according to the current specifications of road snow melt materials.²⁶ Marshall specimens were immersed in 1 L of ultrapure water at 25 °C for 10 min and 24 h to measure the salt concentration difference. It can be seen from the test results (Figure 8a) that, with the same content (4 wt %), the salt dissolution rate of the Marshall specimen formed with the SMAA product met the standard requirements ≤0.4%, while with porous salt storage carrier without polymer modification (SSC) was higher, showed that the polymer surface modification significantly reduced the salt sustained-release rate. When the proportion of the SMAA product was 6 wt %, the salt dissolution rate was only 0.35%, which meets the standard requirement of 0.4%.

Figure 8.

(a) Salt dissolution rate and (b) hygroscopicity of the asphalt pavement mixtures.

In an ideal state, the change in the salt solution concentration is converted to the precipitation rate of the salt, which can calculate the effective sustained-release time with laboratory evaluation, which will only be very rough approximations. A certain amount of SMAA existed on the surface after the asphalt mixture specimens formed, and this part of salt directly dissolved in the solution, leading to a high salt precipitation rate at the beginning of the test and tended to be stable after 24 h.²⁵ With 6 wt % SMAA mixing ratio, the salt precipitation rate was about 0.23 g/(m² h), which can be the v value for the anti-icing asphalt pavement with the same proportion of SMAA. In this study, the anti-icing asphalt pavement has a bulk volume density of 2.67 g/cm³. The SMAAA quality accounts for 6% of the total mass of the asphalt mixtures, and the soluble salt content of the SMAA was 15.5%. The thickness of the pavement was 5 cm. Therefore, the soluble salt mass per square meter of the anti-icing pavement was calculated to be 1241.55 g. This project is relied on Jingxiong expressway, Baoding City, Hebei Province, China; thus, we find that the average annual precipitation days in this region was about 29 days. Then, the effective sustained-release time of the anti-icing pavement is calculated to be T = m/24vt = 1241.55/24 × 0.23 × 29 = 7.76 years. As a result, ideally, we predict that the effective service time of the anti-icing asphalt pavement in Jingxiong expressway, Hebei province, is 7∼8 years. It should be noted that the environment will be more complex in a real application; there are many complicated factors affecting the service life of the anti-icing pavement, for example, the anti-icing effectiveness of such an overlay is expected to diminish well before all the encapsulated CH₃COONa is exhausted, the salt releasing rate (v) of the anti-icing overlay in the field will likely increase over time, the leaching of CH₃COONa out of the overlay can occur during the dry days as well if the overlay is wet in the interior (due to moisture ingress from the preceding rainy or snowy days), and so on. Therefore, it is an overly simplified estimation that ignores all the intricate factors that could considerably shorten the service life of such functional pavement in the field environment. The real service life of the anti-icing product needs further observation in the actual road section.

In addition, the pH value of SMA-13 asphalt mixtures was 8.2, and after adding 6 wt % SMAA, the pH value was 7.8, which met the requirements of specification 7∼9, indicating that the SMAA product has little influence on the pH of asphalt mixtures.

Hygroscopicity is an important index to evaluate the deliquescence capacity of snow melt materials. According to the requirements of the specification, the moisture absorption rate should be less than or equal to 0.7%. The deliquescence phenomenon of snow melt salt materials in the salt-storage pavement was always happening. If the moisture absorption rate of the snow-melt material is too high, the salt dissipation rate increases accordingly, which will shorten the service life of the anti-icing asphalt pavement. Meanwhile, resulting in a continuous accumulation of moisture in the air on the surface of the salt-storage asphalt pavement road, the water stability of the road surface decreased; furthermore, the pavement service life decreased. If the moisture absorption rate of the snow-melt material is too low, the salt will not be released, and the effect of the material cannot be achieved. The dry Marshall specimens were placed in a constant temperature and humidity box under curing for 24 h with 25 °C and 85% humidity, and the mass increase rate was the moisture absorption rate. Figure 8b illustrates that the asphalt mixtures with the salt-storage porous carrier, which did not encapsulate the P(AA-MA-BA-St) copolymer coating, have a relatively high moisture absorption rate (0.83%), while with the SMAA product, the moisture absorption rate is only 0.64% even when the proportion is 6 wt %, which could meet standard requirements. In conclusion, the optimal blending ratio of the SMAA in SMA-13 asphalt mixtures is 6 wt %, which can be used in the snow and ice environment to achieve better anti-icing performance, while ensuring driving safety and the service life of the road surface. Furthermore, we will verify the engineering properties of asphalt mixtures incorporating the SMAA additive.

3.3. Engineering Properties of Asphalt Mixtures Incorporating the SMAA Additive

The SMAA product was mixed into asphalt mixtures by the proportion of 0, 2, 4, and 6 wt % to test the influence on asphalt mixtures road performance and determine the optimal dosage of self, ice-melting agent for SMAA. The main test basis was Chinese industry standard JTG F40-2004 “Technical Specification for Construction of Highway Asphalt Pavement published by the Ministry of Communications”²⁸ and JTG E20-2011 “Test Regulations for Asphalt and Mixture of Highway Engineering.”³⁰

3.3.1. Rutting Resistance

The ability of the asphalt mixtures to resist rutting damage, that is, the high-temperature stability of the asphalt pavement, is one of the important road performances. The damage to the asphalt pavement was permanent after the rutting formed, and direct repair is not effective. The sunken part of the asphalt mixture surface layer at the rut decreases in thickness, which easily generates water, resulting in a decrease of pavement strength, and the water seepage will have a negative impact on the strength of the roadbed. Subsequent vehicle traffic will continue to damage the middle and lower layers of asphalt and even the basic level of the road, so it is necessary to re-lay the asphalt pavement for repair, which greatly increases the cost of highway maintenance. Therefore, the rut test was used to evaluate the high temperature stability of asphalt mixtures containing the SMAA product. Rut specimens were prepared by the wheel rolling method and their dynamic stability was tested by the automatic rutting testing machine, as shown in Figure S8.

According to the “Climate Zone for Asphalt Pavement Performance”, the rut test environment was set as 60 °C and 0.7 MPa. The test machine was loaded for 1 h and the rut variables were read at 45 min(t₁) and 60 min(t₂). That is, the rut passes that can be withstood for per rut depth (1 mm) at high temperatures, and the results are presented in Figure 9. According to the test results, under the same oil-stone ratio, with the increase of the SMAA ratio, the dynamic stability of asphalt mixtures shows an overall trend of decreasing, but the decreasing range is limited. According to our specification requirements, the highways section in Hebei belongs to the hot area in summer, and the dynamic stability requirement is not less than 2800 times/mm. The test results showed that the dynamic stability of rutting specimens prepared by SMAA asphalt mixtures was all more than 5000 times/mm, which met the requirements of the specification. When the proportion of SMAA was 2 wt %, the dynamic stability of asphalt mixtures increased 1%, compared to the one without SMAA. Because the ore synthesis grading changed after the SMAA replaced part of the ore powder, the high temperature stability of asphalt mixtures increased. When the mixing ratio was 6 wt %, the dynamic stability of asphalt mixtures decreased by 18.7% compared with that of ordinary asphalt mixtures. The surface of the SMAA coated with the polymer packaging material will cause a slight decrease in asphalt adhesion, which in turn leads to the decrease of the high temperature stability of asphalt mixtures. However, the high temperature stability was less affected, and all the samples were higher than the standard use requirements.

Figure 9.

Results of the rut test for high temperature stability.

3.3.2. Low-Temperature Cracking Resistance

Temperature shrinkage crack is one of the main forms of asphalt pavement cracking.³⁵ The asphalt pavement was cold and shrunk with lower ambient temperature, resulting in temperature shrinkage cracks. Pavement cracks first affect the appearance of the road surface, and the road continuity will also be destroyed, resulting in subgrade bearing capacity decline and new crack expansion. Furthermore, water can penetrate the road base through cracks, resulting in the reduction of subgrade bearing capacity. Serious roadbed damage can lead to roadbed deformation, pavement collapse, and other problems, which will accelerate pavement failure. The trabecular bending test was used to evaluate the low temperature stability of SMAA asphalt mixtures. The maximum strain of trabecular failure was used as the evaluation index of low temperature stability. The ambient temperature of the test was −10 °C. The rut specimen size was 250mm × 35mm × 30mm. The experiment process is shown in Figure S9, and the results are presented in Table 11.

Table 11. Low-Temperature Trabecular Bending Test Results.

SMAA proportion (wt %)	0	2	4	6	specification
flexural-tensile strain (με)	4255	4230	3967	3817	≥2500

Compared with the ordinary asphalt mixtures without SMAA, the maximum flexural tensile strain of the SMAA asphalt mixtures was decreased, and the maximum bending tensile strain was decreased by 10.1% when the SMAA ratio was 6 wt %. When mineral powder was replaced by SMAA in asphalt mixtures, the bonding force between the microcapsule polymer film and asphalt was lower than that between asphalt and mineral powder. The adhesion between aggregates in the mixtures decreases, and the shear resistance of asphalt will also decrease at low temperatures, which will lead to the decrease of the low-temperature crack resistance of asphalt mixtures. According to the requirements of Chinese industry specification on the low-temperature cracking resistance of the modified asphalt pavement, the maximum bending strain of asphalt pavement is required more than 2500 με in cold winter area which exists warmer region. Obviously, in the test results, the low-temperature cracking resistance of asphalt mixtures meets the requirements under the three mixing ratios.

3.3.3. Moisture Susceptibility

When there is water on the asphalt pavement or the water freeze–thaw cycle occurs in winter, the water left in the asphalt pavement surface and the pavement gap will continuously produce the circulation effect of dynamic water pressure and negative pressure suction under the continuous driving load. Free water gradually permeates into the asphalt mixtures, resulting in asphalt adhesion reduction or even disappearance. The asphalt mixtures would become loose, causing grain dropping and aggregate shedding, finally forming many pits, which will seriously affect the smoothness of the pavement and driving safety.³⁶ The moisture susceptibility of asphalt mixtures is the key factor of asphalt pavement resistance to moisture damage. Over the years, researchers from various countries have put forward many research methods and evaluation indexes in the study of moisture susceptibility of asphalt mixtures. Currently, the commonly used moisture susceptibility test methods mainly include the following five: the freeze–thaw splitting test, the vacuum-saturated Marshall test, the immersion rut test, the immersion Marshall test, and the ESC (Environmental Conditioning System).³⁷ These tests evaluated the moisture susceptibility of asphalt mixtures from different views and indexes. The moisture susceptibility of SMAA asphalt mixtures was studied by the freeze–thaw splitting test and the immersion Marshall test³⁸ based on the actual engineering application and the current standards. The experiment process can be found in Figures S10 and S11. Eight Marshall specimens of SMA-13 asphalt mixtures with different SMAA proportions (0, 2, 4, and 6 wt %) were prepared for each group to carry out the immersion Marshall test and the freeze–thaw splitting test. The stability ratio of Marshall specimen cured for 48 h (MS₁) and 30 min (MS) in a constant temperature water tank (60 °C) was determined as residual stability (MS₀). The splitting strength ratio (TSR) of the specimens after freeze–thaw cycles (−18–60–25 °C) was measured by the freeze–thaw splitting test with the specimens immersed in water at 25 °C for 2 h.

As shown in Figure 10a, with the increase of the SMAA addition ratio, the residual stability of asphalt mixtures showed a downward trend and decreased to 87.6% when the SMAA proportion was 6 wt %. When the SMAA proportion was 4 wt %, the stable degree was improved, and the residual stability was also increased slightly. Because after Marshall specimens with a high proportion of SMAA were soaked in a 60 °C water bath for 48 h, part of CH₃COONa precipitated from the mixtures, and the ion contact between asphalt and the microcapsule surface resulted in decreasing adhesion between asphalt and aggregates. Therefore, the better addition ratio of SMAA should be about 4 wt %, although the snow melting effect was better with a higher addition ratio of SMAA. Furthermore, the higher content of SMAA additives will also cause a decrease in the resistance ability of asphalt mixtures to water damage, poor water stability performance of asphalt pavement, and easily produce cracks, pits, and other diseases.

Figure 10.

Results of (a) the immersion Marshall test and (b) the freeze–thaw splitting test.

As shown in Figure 10b, after adding SMAA, the freeze–thaw splitting strength ratio (TSR) is higher than that of asphalt mixtures without SMAA, but it decreases slightly when the ratio reaches 6 wt %. SMAA was used as the filler to replace the mineral powder in the aggregate, after being soaked in a constant temperature water bath at 60 °C, and the asphalt mixtures were damaged by water with stability decrease. In the process of the freeze–thaw splitting test, the Marshall specimen was first put into a vacuum and then through freeze–thaw cycles which closely simulated the working environment of the asphalt pavement in winter. As a functional material with a low freezing point, the snow melting agent itself has a certain resistance ability to the cold environment after adding asphalt mixtures, while the resistance ability of ordinary asphalt mixtures is slightly lower than that of the former in the cold environment,¹⁷ which leads to the increase of TSR of SMAA asphalt mixtures. Combined with the results of the two moisture susceptibility experiments, the better addition ratio of SMAA is 4 wt %, which has good resistance to water damage.

3.3.4. Longevity of the Asphalt Mixtures and Practical Engineering Applications

Durability is also a key factor in determining whether an anti-icing agent can be practically applied. During the service of the pavement, the water in the void of the asphalt pavement surface was pumped under the action of traffic load, which can also accelerate the damage rate of the pavement and reduce the service life of the pavement. As a filler, SMAA replaced mineral powder and was added into asphalt mixtures. With the suction action of the vehicle load pump and capillary action, the water channel was provided for the precipitation of the core material of ice/snow melting components, as shown in Figure 11.

Figure 11.

Photographs of the working principle and practical engineering applications of the sustained-release microcapsule-type anti-icing agent.

However, if the durability of SMAA asphalt mixtures was short, it can even cause loss of part of the filler in the asphalt mixtures.¹³ Therefore, the dynamic water scouring test was designed to evaluate the durability of the asphalt pavement. Dynamic water was used to scour the 4 wt % SMAA asphalt mixture Marshall specimens at a flow rate of 10 L/min with different times (0, 10, 20, 30, 60, and 90 min). The freeze–thaw splitting strength ratio of the Marshall specimen after and before being tested (residual strength ratio, TSR) was used as the evaluation index, and the asphalt mixtures without the SMAA were used as the comparison test, as shown in the results collected in Figure 12. The residual strength ratio of asphalt mixtures with SMAA was higher than that without SMAA. The residual stability of the Marshall specimens decreased with the extension of the test time, but the sample with the SMAA after 30 min exhibited a faster decline rate. When the scouring time was 60 min, the residual strength ratio of the Marshall specimen with 4 wt % SMAA was 90.3% of the TSR without scour, and the residual strength of the Marshall specimen without the SMAA was 95.7% of the TSR without scour. At 60 min, the TSR of the SMAA Marshall specimen was 97.8% of that without the SMAA. The results showed that the asphalt pavement intensified the material damage after the dynamic water scour. The self-made SMAA asphalt pavement had a slightly lower anti-dynamic water scour damage ability than the ordinary asphalt pavement, but the influence of the SMAA on the durability of asphalt mixtures was within the controllable range.

Figure 12.

Durability test results.

When the content of the SMAA was insufficient, the effect of anti-icing performance on the asphalt pavement was also insufficient. According to the results of the previous experiments, the snow melting rate of SMAA asphalt mixtures was 46.9% when the mixing ratio was up to 4 wt % and met the requirements of the specification. After the road performance test, it was found that more than 6 wt % will lead to road durability and road performance decline, even though the anti-icing capacity achieved 71.6%. Therefore, it is necessary to determine a moderate dosage considering the performance of anti-icing, road performance, and economy. Above all, it is determined that in engineering applications, the dosage range of the SMAA is 5.5 wt %.

Relying on the BeiJing-Xiong’an highways, 5.5 wt % SMAA was added to the 5 cm thick SMA surface layer of cross-main ramp K64 + 992 ∼ K65 + 193.641 at the Sizhuang toll Station to replace the same amount of mineral powder. The test section was paved in May 2021, and snow arrived at the end of 2021 and 2022. In the real scene photographs, the roads with and without SMAA had a significant contrast effect with anti-icing performance, and the follow-up performance monitoring of the road section continues. In light snow weather, there is almost no need for auxiliary snow removal operations, and the road itself actively melts the snow and ice on the pavement. In heavy snow, the road can be separated from the ice layer. The snow and ice layer on the road can be destroyed by rolling vehicles, reducing the phenomenon of vehicle skidding, and ensuring the winter road. The price of SMAA will be reduced by more than 35% compared with the commonly used internal blended anti-icing agents, such as MFL, V-260, and MicroPCMs (Phase Change Materials) for the porous sustained-release skeleton was solid waste and the acetate salt was obtained from biomass pyrolysis waste streams. During snow and ice weather, the cost of clearing snow and ice which is about 120,000 RMB per day could be saved. Unclosed highways can generate 940,000 RMB per day. Meanwhile, the anti-icing pavement surface with active snow and ice removal function can reduce the incidence of traffic accidents by 75% compared with conventional road conditions, and the number of casualties in traffic accidents can also be significantly reduced, saving unpredictable property losses and lives. The service life of a normal highway is 8–15 years; thus, from the analysis of the effective sustained-release time, product cost, saving cost of snow and ice removal, income generation in highways with rain and snow weather, and the reduced traffic accidents, our products have high economic and practical application value.

4. Conclusions

In this paper, we designed and prepared novel, green sustained-release microcapsules SSA for anti-icing asphalt pavement. First, the anti-icing capacity of CH₃COONa obtained by biomass pyrolysis waste was verified by the freezing point and the relative snow melting capacity test. The blast furnace slag was selected as the porous carrier of the snow melt salt for it has a large specific surface area and diameter determined by the BET test. The optimal technological conditions of the preparation of P(AA-MA-BA-St) copolymer microcapsule walls were determined. The structure of the polymer was determined by NMR, and the results of FTIR and SEM showed that the polymer successfully encapsulated the porous solid waste which was loaded with CH₃COONa. Subsequently, the conventional performance of the SMAA product, including the heat resistance index (0.19%), relative density (2.71), nominal maximum particle diameter (0.15 mm), water content (0.4%), and its corrosion rate of carbon steel (0.02 mm/year) were tested and all the test values met the specification. The anti-icing capacity of asphalt mixtures with different SMAA proportions, such as the freezing point and the snow melting capacity were investigated by introducing SMAA to SMA-13 to replace part of the mineral powder, and all indicators met the requirements of the specification with the addition ratio 6 wt %. Rutting resistance, the low-temperature cracking resistance test, the immersion Marshall test, and the freeze–thaw splitting test were applied to explore the influence of SMAA addition ratios on the engineering application performance of SMA-13. Finally, the longevity test of SMA-13 with SMAA was carried out by dynamic water scouring, and the results met the standard requirements. With all the results, the SMAA addition ratio was determined to be 5.5 wt % and applied on the Beijing-Xiong’an highways, which achieved good anticoagulant effect in winter. The obtained SMAA made full use of various wastes, realized low carbon emission, and has obvious environmental and low-cost characteristics. The preparation processes of SMAA were simple and convenient to realize the scale-up production. This study provided a new idea and viewpoint for the development of the snow melt agent for the anti-icing asphalt pavement.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07212.

• SBS modified asphalt technical indicators; coarse aggregate test results; fine aggregate test results; mineral powder and blast furnace slag powder carrier test results; lignocellulose test results; SMA-13 design gradation; representative SEM micrographs microporous of BFSP, FA, and SSMP; photographs of steel sheets before and after the corrosion test; TGA curves of the SMAA product; corrosion test of carbon steel; pull-out test to ensure the freezing point of asphalt mixtures; rut test for high temperature stability; low-temperature cracking resistance test; immersion Marshall test; and freeze–thaw splitting test instrument (PDF)

The authors declare no competing financial interest.