Investigation of Moisture Sensitivity and Conductivity Properties of Inductive Asphalt Mixtures Containing Steel Wool Fiber

.e construction of suitable roads in rainy areas has created problems in the construction process due to the low resistance of asphalt to moisture. To solve this problem, materials are commonly used that make mixtures resistant to moisture; however, these materials may reduce the dynamic resistance of asphalt. .erefore, materials should be used that, in addition to increasing the dynamic resistance, also increase the moisture resistance of asphalt mixtures. One of these materials used in this research is steel wool fiber (SWF), which in addition to creating conductive roads also could have a significant effect on moisture resistance. In this study, the impact of 2%, 4%, 6%, 8%, and 10% SWF on the Marshall stability and moisture sensitivity of mixtures was investigated using the Marshall stability and indirect tensile strength (ITS) tests, respectively. Moreover, using SWF as a conductive fiber, the conductivity properties of asphalt mixtures were explored to find the optimal amount of electrical conductivity. .e results of the Marshall stability test indicated that by increasing SWF contents, the stability of mixtures increased, compared with the base sample, and greater amounts of 6% SWF resulted in the reduction of the Marshall stability. .e results of ITS showed that modification of bitumen by SWF increased ITS and tensile strength ratio (TSR) amounts of mixtures. 6% SWF was the optimal amount for enhancing the resistance of asphalt mixtures to moisture sensitivity. .e results of the electrical resistivity test showed that the resistivity had three phases: high resistivity, transit, and low resistivity. Mixtures containing less than 4% SWF illustrated an insulating behavior, with electrical resistivity greater than 7.62×10Ω.m. At the transit phase, the resistivity of mixtures had a sharp reduction from 7.62×10Ω.m to 6.17×10Ω.m. Finally, 8% SWF was known as the optimal content for the electrical conductivity of mixtures.


Introduction
Damage caused by moisture is one of the prevalent failures in asphalt pavements. Moisture may weaken adhesion between aggregates and bitumen and separate them from each other.
is damage is commonly known as stripping that may cause holes on the road surface and can lead to traffic accidents [1]. e durability of asphalt pavements is affected by these damages because bitumen tends to be stripped from the surface of aggregates and causes aggregates to become bitumen-free. e presence of water in asphalt pavements disrupts the bonding and results in increased moisture sensitivity [2,3]. e modification of bitumen helps to improve the behavior of asphalt mixtures [4][5][6]. Creating small cracks in asphalt pavements causes bigger damages. So, it is possible theoretically to repair cracks in the pavement by adding conductive fibers, enhancing its conductivity in order to enhance its self-healing degree [7]. erefore, this study aims to indicate how the asphalt mixture conductivity and moisture resistance change by adding various amounts of conductive material.
Some asphalt pavements have self-healing properties. e healing phenomenon happens when stress or strain produced is large enough to cause damage. As soon as the loading caused failure is eliminated and the two crack faces are placed in contact, the release of the molecules starts from one side to the other side. e self-healing process continues until the microcracks are completely removed, and the healed materials have a resistance level of the raw material [8,9].
Inductive asphalts (IAs) comprising steel wool fiber (SWF), as a new pavement substance, have some advantages in various areas, including maintenance [10][11][12], functional/ durability characteristics [13,14], and service life [15,16]. e convenience of maintenance and efficiency is the significant advantage of IAs compared with common asphalts. SWFs are added to asphalt mixtures during the first manufacturing phase. Nevertheless, when microcracks are generated or these substances are not added at the manufacturing phase, some additional substances such as healing agents are required to seal and fill cracks in order to enhance healing [17,18]. Bitumen healing is reported greatly sensitive to moisture conditioning [19]. Some studies also indicated that the conductivity of asphalt mixtures depends on the amount of conductive fibers used and an excessive amount of conductive materials can result in the degeneration of the asphalt mixture characteristics [20]. erefore, it is important to study the effect of conductive fibers on the simultaneous improvement of asphalt behaviors such as moisture performance and conductivity properties. e healing mechanism of a crack includes an instant increase in loading transferability because of cohesion and adhesion, as well as following long-time healing because of molecular distribution through the interfaces [21]. One of the exterior factors that influence the healing mechanism is the presence of water absorbed on the faces of cracks. e water infiltration may result in cohesion and adhesion loss and therefore decrease the healing capacity. Moreover, the healing process may be decreased by interruptions of water in the dipole balance of asphalt mixtures by creating a bond with the greatly polar groups of the asphalt molecules or salts and ions which are water soluble and connected to the polar sections of asphalt mixtures [22].
Some studies have investigated the impact of moisture penetration into asphalt mixtures by the use of both computational and experimental approaches [23][24][25][26][27]. It was reported that the presence of moisture could result in damages in asphalt mixtures by displacement, detachment, PH instability, pore pressure, and hydraulic scour [28]. Some studies have tried to recover the moisture characteristics in asphalt mixtures damaged along with the investigation of the healing process [16,19,29]. e presence of moisture was stated harmful for microwave healing and induction of reinforced mixtures by fibers, although it enhanced the heating rate of the mixtures [16]. Moreover, conditioning of mixtures by moisture was illustrated to have a negative effect on the self-healing of bitumens [19]. By contrast, saturated samples in moisture were observed to recover from damages caused fatigue in an allowed sufficient rest time [25,29]. In another study, moisture penetration had a nominal influence on multiple fracture-rehealing behavior [18]. erefore, it is necessary to adopt a solution to improve the moisture and self-healing behaviors of mixtures.
Some studies have been conducted to examine the behavior of IAs containing SWF/steel fiber [7,10,[30][31][32][33][34][35][36]. However, few studies have been conducted to investigate the moisture behavior in these asphalts containing SWF. Liu et al. performed a study to explore the mechanical behavior of porous asphalt using indirect tensile strength (ITS) as well as the conductivity of mixtures by the use of SWF. ey indicated that 10% SWF was the optimal amount to achieve a satisfying ITS as well as an optimum conductivity [37]. Sun et al. investigated the healing characteristics of asphalt mixtures due to moisture damages using steel fiber and illustrated that the heating speed of mixtures with induction heating was improved at the water presence. ey also stated that it is better for the mixtures to be healed in dry conditions compared with the water presence [16]. Li et al. examined the moisture conditioning impact on the healing and mechanical characteristics of asphalt mixtures containing 6% SWF and indicated that there was no corrosion of SWF in asphalt mixtures after the moisture-induced sensitivity test. Moreover, the fracture energy and tensile strength were reduced after the test [36]. e significant point of these research studies is the various size of steel fiber that they used and also another difference between these studies and the present study is in the properties of SWF (e.g., diameter and average length) and asphalt mixtures as well as the tests conducted in this paper (such as electrical resistivity and Marshall stability). e principal goal of the present study was to examine the impact of SWF amounts on the Marshall stability and moisture sensitivity of hot mix asphalt (HMA) in order to determine the optimal amount of SWF. Also, by adding SWF as a conductive fiber, the conductivity properties of asphalt mixtures were investigated to find the optimal amount of electrical conductivity.

Aggregate.
Limestone aggregate was used in this research, and the grading of the aggregate was according to the ASTM standard that the nominal and maximum size of the aggregate is 12.5 mm and 19 mm, respectively. e grading of the aggregate is presented in Figure 1, and the physical features of the aggregate are indicated in Table 1.

Bitumen.
In this research, bitumen with a penetration grade of 60-70 prepared from the Jey Oil Refinery was used as the base bitumen, and its features are indicated in Table 2.

Modification of Bitumen.
Steel wool fiber (SWF) as an electrically conductive fiber with the weight percentages of 2%, 4%, 6%, 8%, and 10% was used to modify the bitumen properties; that is, the density, equivalent diameter, average length, and electrical resistivity of SWF were 7.8 g/cm 3 , 70-130 μm, 4.2 mm, and 7 × 10 −7 Ω.cm, respectively. First, the aggregate was heated for 24 hours at 150°as well as bitumen and fibers were heated for 2 hours at 150°, and they were then mixed for 3.5 minutes at 150°in the mixing bowl with 100 rpm [38]. Figure 2 shows the SWF sample used in this research. Also, SWF used had the same approximate length, but the difference between the modified asphalt mixtures was in SWF weight percentages.

Asphalt Mix Design.
e Marshall mix design method was used in this research to specify the optimal content of bitumen according to AASHTO T245 [39] and based on the

Test Method
In this research, the Marshall stability, moisture sensitivity, and electrical resistivity of asphalt mixtures modified by SWF with 2%, 4%, 6%, 8%, and 10% weight percentages were investigated. First of all, the Marshall stability test was performed to determine the stability of the base and modified mixtures. In this test, each base or modified bitumen (with different percentages of 4.5%, 5%, 5.5%, and 6%) was modified by SWF with percentages of 0%, 2%, 4%, 6%, 8%, and 10%, and 3 samples were constructed for each composition so that finally 72 samples were made for the Marshall stability test. After the Marshall stability test, 48 samples were made for the ITS test to evaluate the moisture sensitivity of mixtures. Also, 18 samples were constructed for determining the electrical conductivity of mixtures. e number of replicates of each test is presented in Table 4.

Marshall Stability Test.
e Marshall stability test, according to ASTM D6927, was applied to examine the physical and mechanical characteristics of mixtures [40]. In the test, by immersing the mixtures in a 60°water bath for 45 min, the mixtures were prepared.

ITS Test.
In this research, the ITS test was applied to examine the behavior of asphalt mixtures to damages caused by moisture based on AASHTO T283. For performing the test by the modified Lattmann, three mixtures were made in dry conditions and three in wet conditions (with distilled water). e saturated percentage of samples should be approximately 70% to 80%. Loading was performed in this test at a 5.08 cm rate per minute until the specimens break. e loading amount was filed at the failure moment. By the use of equation (1), the ITS value of the specimens was obtained [6]: where P is the maximum load at the moment of failure (kN), d represents the mixture diameter (m), and t presents the mixture thickness (m). e moisture sensitivity of the samples with the average ITS ratio of the wet to dry specimens was obtained from the following equation [41]: where ITS wet presents the average ITS value in the wet sample (kPa) and ITS dry represents the average ITS value in the dry sample (kPa).

Electrical Resistivity
Test. e measurements of electrical resistivity were performed at 21°(room temperature). e mixtures were subjected for 8 hours at 40°to eliminate moisture and preclude SWFs from corrosion on the surface of mixtures. A digital multimeter and a resistance tester were utilized for measuring the resistance lower and higher 36 × 10 6 Ω, respectively. Two electrodes made with a rectangular copper plate at the size of 100 × 160 mm were in conjunction with wires and connected with the resistance tester in order to test resistance. ese two were put at both ends of the mixture for measuring the electrical resistance. e end effects were regarded as insignificant, and the electric field was supposed to be fixed. From the second Ohm's law, the electrical resistivity was achieved, according to the following equation [42]: in which L presents the internal electrode distance (m), ρ represents the electrical resistance (Ω.m), R presents the calculated resistance (Ω), and S illustrates the electrode conductive area (m 2 ).

Marshall Stability.
e results of the application of the Marshall stability of mixtures containing SWF are illustrated in Figure 3. As is clear, by increasing SWF contents, results demonstrated that the Marshall stability enhanced up to 14.12 KN that is the maximum amount for samples containing 6% SWF. An increase in SWF content resulted in the reduction of stability of mixtures under compressive stresses, which could be related to the increment in bitumen viscosity in the presence of SWF. As is clear from Figure 3, the viscosity of bitumen had a straight relationship with SWF up to 6%. More than this content, the modulus of asphalt mixtures was increased and it caused to strengthen, but by increasing SWF amounts to 8% or 10%, the viscosity of bitumen was declined and the asphalt mixtures tended to fail and fracture.

ITS.
e results corresponding to the ITS values of the samples under the wet and dry conditions are illustrated in Figure 4. As can be seen, with the cohesion loss of the asphalt mixtures due to the exposure of the samples to moisture, the ITS amounts of the samples in wet conditions were reduced compared with those of samples in dry conditions. On the other, the ITS amounts in both wet and dry conditions were increased by modifying the properties of bitumen with SWF so that by adding SWF, the cohesion and adhesion of mixtures were increased and caused it to obtain a higher moisture resistance over the unmodified mixtures. e ITS value of the mixtures modified with 2%, 4%, 6%, 8%, and 10% SWF was increased by 4.49%, 9.51%, 15.13%, 10.46%, and 6.14% in dry conditions and 14.15%, 31.27%, 47.98%, 33.29%, and 19.27% in wet conditions, respectively, compared with the base sample, and the highest increase in the ITS value was visible in the modified samples with 6% SWF. e results showed that the addition of SWF more than 6% reduced the amount of ITS in dry and wet conditions because adding excessive SWF contents decrease the bitumen film thickness as well as air voids of asphalt samples, resulting in an unappropriated adhesion in the asphalt system [37]. Figure 5 illustrates the TSR amounts of the samples under the dry and wet conditions. ese values indicate the importance of the presence of SWF and its effect on moisture   Advances in Civil Engineering sensitivity. As is clear, the TSR amounts of the modified samples with 2%, 4%, 6%, 8%, and 10% SWF compared with the base sample increased by 9.24%, 19.87%, 28.54%, 20.67%, and 12.38%, respectively. e application of 6% SWF indicated the best impact on enhancing the resistance of asphalt mixtures to moisture sensitivity.
Based on the values obtained from Figure 5, it can be concluded that the TSR values have increased significantly, which may be due to the fact that SWFs with their small diameter have filled some of the pores of asphalt samples and prevented water penetration into them. On the other hand, an enhancement in the TSR may be due to the fact that steel fibers are hydrophobic and do not absorb any water. It should also be noted that the amount of 6% additive was provided as the optimal amount to increase the moisture resistance of asphalt mixtures.

Electrical Resistivity.
e impact of SWF contents on the electrical resistivity of mixtures is presented in Figure 6. e resistivity of mixtures showed three phases. e first phase illustrated a high resistivity of mixtures with SWF contents less than 4%, followed by the transit phase with 4-8% SWF contents and the low resistivity phase with SWF contents more than 8%. Mixtures containing less than 4% SWF illustrated an insulating behavior, with electrical resistivity greater than 7.62 × 10 8 Ω.m. At the transit phase, the resistivity of mixtures had a sharp reduction from 7.62 × 10 8 Ω.m to 6.17 × 10 4 Ω.m. Based on the results in Figure 6,   containing more than 8% SWF did not have much effect on reducing the electrical resistance and as can be seen, the electrical resistivity of the mixture containing 10% SWF was slightly less than the mixture with 8% SWF. Due to the deterioration of the dynamic properties of asphalt mixtures with excessive increase in SWF, 8% SWF was known as the optimal content for the electrical conductivity of mixtures. In fact, the optimal amount of electrical conductivity is the maximum amount that does not show a significant change in the electrical resistance of the mixtures by increasing SWF. Also, in this amount, the asphalt mixture is in its best selfhealing state in case of heating induction. e significant point is that the optimum amount of SWF for modifying the moisture and electrical resistivity of hot mix asphalt mixtures is less than amounts that modified porous asphalt mixtures in the study of Liu et al. [37].

Conclusion
In this research, the performance of moisture and electrical resistivity of asphalt mixtures was investigated under the influence of SWF additive. e significant findings of the research are as follows: (i) e results of the Marshall stability test indicated that by increasing SWF contents, the stability of mixtures increased, compared with the base sample. However, this increment was obvious up to 6% SWF and greater amounts resulted in the reduction of the Marshall stability. (ii) e results of the ITS test showed that modification of bitumen with SWF increased the ITS and TSR amounts of the mixtures. (iii) e ITS value of the mixtures modified with 2%, 4%, 6%, 8%, and 10% SWF was increased by 4.49%, 9.51%, 15.13%, 10.46%, and 6.14% in dry conditions and 14.15%, 31.27%, 47.98%, 33.29%, and 19.27% in wet conditions, respectively, compared with the base sample. (iv) 6% SWF indicated an optimal amount for enhancing the resistance of asphalt mixtures to moisture sensitivity. (v) e results of electrical resistivity of mixtures showed that the resistivity had three phases: high resistivity, transit, and low resistivity phases, which were related to SWF contents less than 4%, 4-8%, and more than 8%, respectively. (vi) Mixtures in the first phase demonstrated an insulating behavior, with electrical resistivity greater than 7.62 × 10 8 Ω.m, and at the transit phase, the resistivity had a sharp reduction. Finally, 8% SWF was known as the optimal content for the electrical conductivity of mixtures.

Data Availability
e data used to support the findings of this study are currently under embargo while the research findings are commercialized. Requests for data, 3 months after publication of this article, will be considered by the corresponding author.  Advances in Civil Engineering 7