Fatigue Analysis of Bitumen Modified with Composite of Nano-SiO2 and Styrene Butadiene Styrene Polymer

Since fatigue cracking is caused in the middle-temperature conditions due to the stresses from heavy traffic and as the bitumen plays a very important role in controlling this failure, therefore, in recent years, the production of the modified bitumen that can give a good performance in the middle temperatures has always attracted the interest of researchers. One of these bitumen modifiers is the styrene butadiene styrene (SBS) polymer. Due to the phase separation of bitumen and polymer, aging and oxidation, this polymer may not exhibit expected field performance at middle temperatures. Therefore, in this research, it is attempted to analyze the middle-temperature performance using the combination of nano-SiO2 and SBS polymer in the bitumen modification. In this paper, the addition of SBS and nano-SiO2 to the base bitumen resulted in the reduction of the complex modulus, phase angle, storage modulus and loss modulus at middle temperatures, thereby improving the potential of fatigue failure resistance. In general, considering the requirement for the rotational viscosity value up to 3 Pa.s at 135 °C and also, regarding the economic issues in choosing a lower percentage, the combination of 4.5% SBS + 3% nano-SiO2 is selected as the optimal composite.


INTRODUCTION
itumen is usually the byproduct in the crude oil production process, which, therefore, may not have all the specifications required for the production of asphalt mixtures [1]. Asphalt mixtures are subjected to the temperature changes and stresses caused by the passing heavy vehicles, which increase the growth of rutting, fatigue and lowtemperature cracking failures. The bitumen plays an important role in the occurrence of these failures. Therefore, in order to achieve the pavements of longer service life, the modification of bitumen is inevitable [2]. Every year, different modifiers are used to improve bitumen properties and behavior. Among these modifiers, in recent decades, the polymers and nanomaterials are increasingly used to improve the rheological behavior of bitumen [3][4][5][6][7][8]. The modification of bitumen with Styrene Butadiene Styrene (SBS) polymer improves the resistance to rutting, fatigue cracking, and low-temperature cracking [9,10], but involves the phase separation and is degraded under the ultraviolet light, as well as oxygen [11,12]. Research results show that modifying the polymer bitumen with nano-materials improves the high-and low-temperature performance, aging, oxidation and phase separation [13,14,3,4,5]. There are some limited studies on the effect of polymer nano-composites; hence, this paper analyzes the middle-temperature performance of the SBS additive and the combined SBS and nano-SiO 2 additive in 10, 15, 20, and 25 o C to the bitumen PG 64-16 as the most common bitumen in Iran. For this purpose, the bitumen performance, dual composite of bitumen and SBS, and triad composite of bitumen, SBS and nano-SiO2 were evaluated based on the softening point, needle penetration, ductility, rotational viscosity, RTFO mass loss, and dynamic shear rheometer (DSR) tests.

Materials
itumen PG 64-16 was prepared from Pasargad Oil Company of Tehran, SBS Polymer (LG 501) was prepared from LG Company, South Korea, and nano-SiO2 was prepared from Degussa Company, Germany. The chemical composition of the base bitumen and the physical and chemical characteristics of SBS and nano-SiO 2 are given in Tab

Preparation of samples
To prepare the combined bitumen, SBS and nano-SiO2 samples, the high shear mixer was used at 175 °C and 4000 rpm for two hours. On this basis and in order to perform all the tests, 1 kg from each of the samples was prepared including the base bitumen, compound of bitumen and SBS polymer as 2. 1952 500 log 20 50 log 120 where the Pen 25 is the needle penetration at 25 °C (0.1 mm) and SP is the temperature of the softening point of bitumen samples (°C).

Dynamic shear rheometer test
Dynamic shear rheometer (DSR) test (ASTM D7175) was conducted using the controlled stress method at constant frequency (10 rad/s) and 10, 15, 20, and 25 °C for different SBS, nano-SiO2 composites with base bitumen on which the short-term aging process was carried out on an RTFO machine (ASTM D2872). The complex modulus (G*), phase angle (δ), storage modulus (G'=G*×cosδ) and loss modulus (G"=G*×sinδ) were determined for bitumen samples. In order to improve the resistance potential to fatigue, the energy loss should be reduced as much as possible. The energy loss in each loading cycle is directly proportional to G*×sinδ parameter. For the bitumen to be acceptable in the fatigue test at a given temperature, the value of G*×sinδ should be less than or equal to 5000 kPa for the aged bitumen with RTFO [15]. This means that it is desirable to reduce the values of G* and δ.

Physical Properties
he results of softening point, needle penetration, PI, ductility, rotational viscosity and RTFO mass loss for various composites of SBS and SBS/nano-SiO2 with base bitumen are given in Tab. (4). These tests were selected for the purpose of fatigue analysis of the bitumen modified by SBS polymer and nano-SiO 2 among the tests of physical properties of the bitumen. As for investigating the performance at the intermediate temperatures i.e. at temperatures T between 10 and 25 Celsius degrees, where failure of the asphalt mixture occurs due to the fatigue, analysis of these tests is appropriate for this purpose. According to Tab. (4), the addition of SBS and SBS/nano-SiO 2 to bitumen generally increases the softening point, PI, and rotational viscosity and decreases the needle penetration. Reducing the needle penetration, increasing the softening point, PI and viscosity will reduce the bitumen's sensitivity to temperature changes and improve its performance at middle temperatures. Considering the effect of SBS polymer, it could be due to the creation of 3D networks in the bituminous environment after creation of long and dispersed chains of SBS polymer. So that the polystyrene and polybutadiene chains would increase the strength and flexibility of the modified bitumen, respectively, which result in reduced fluidity and increased elasticity. Concerning the nano-SiO2 effect, it is due to the interaction between nano-SiO2 material, bitumen and SBS polymer. So that nano-SiO 2 by adsorption, establishes intense adhesion with the bitumen and polymer which leads to increased hardness and concentration of bituminous mortar. This in turn reduces the bitumen needle penetration and increases the softening point, PI and RV values. Reduced the needle penetration and increased softening point, PI and RV lead to reduced sensitivity of the bitumen to variation in temperature which is good for it. In the SBS with higher than 4.5 wt%, the reducing trend of the needle penetration and increasing trend of softening point and PI is mitigated which is due to the dominating polymer phase. Also this process is seen in threefold compound consisting of bitumen, SBS polymer and nano-SiO 2 , where for nano-SiO 2 with higher than 4 wt% , the reducing trend of penetration degree and increasing trend of softening point and PI are mitigated which could be due to saturation of the bituminous mortar by nano particles. So that the balanced phase network of bitumen and SBS polymer is disturbed when using too much nano-SiO2.  The maximum levels of SBS that can provide the ductility greater than 100 [15] are the composites with the SBS level of 4.5% by weight of bitumen. The appropriate bitumen temperature for mixing and compaction is the temperature equivalent to RV values of 0.17±0.2 and 0.28±0.3, respectively, and the maximum temperature for heating of the bitumen is equal to 176°C [15]. According to Tab. (4), adding SBS polymer to the base bitumen from 2.5 to 6 wt% of bitumen causes continuous increase of RV, but it does not exceed 3 Pa.s at 135°C temperature. In these compounds the maximum RV value at 135°C is equal to 1.37 for a compound of base bitumen and SBS polymer with 6 wt% of the bitumen. Also, addition of nano-SiO2 to the bitumen containing SBS polymer with 4.5 wt% of the bitumen, causes increase in the RV value. This value exceeds the limit of 3 Pa.s for the compounds of 4.5%SBS+4%nano-SiO2 and 4.5% SBS+5% nano-SiO2. Therefore, on this basis the maximum value of nano-SiO 2 is equal to 3 wt% of the bitumen in the compound of bitumen modified by SBS polymer with 4.5 wt% of the bitumen. By increase in the temperature, the RV values is reduced. For the compound of bitumen and 4.5% SBS and the threefold compound of bitumen, SBS polymer and nano-SiO2, the mixing and compaction temperatures are both greater than 165 °C which are too close to 176°C and create problems in terms of applicability. Thus, to resolve this problem it is recommended to use admixtures that reduce viscosity such as Fischer-Tropsch Wax. As seen in Tab. (4), adding SBS polymer to bitumen and also adding nano-SiO2 to polymeric bitumen containing SBS polymer with 4.5 wt% of the bitumen, the weight loss is nearly constant and its value in all the compounds is less than 1 wt%.

Complex modulus, phase angle, and storage modulus
The results of complex modulus, phase angle and storage modulus for various composites of SBS, nano-SiO2 with base bitumen are presented in Figs. (1-3), respectively. According to the figures, adding SBS and nano-SiO2 to the base bitumen continuously decreases the complex modulus, phase angle and storage modulus at 10 to 25 °C with the constant frequency of 10 rad/s, indicating the improvement in fatigue resistance potential and middle-temperature performance. Concerning the results of complex modulus at intermediate temperatures (temperatures between 10 and 25 Celsius degrees), as the base bitumen, modified bitumen with 4 wt% of SBS polymer and different threefold combinations of bitumen, nano-SiO 2 and 4.5 wt% of SBS polymer, had passed the short term and long term processes of aging prior to the dynamic shear rheometer test, and the aging process of RTFO+PAV could result in further hardness of the bitumen by change in the molecular structure of the bitumen, therefore, the higher the share of bitumen, the higher would be the complex modulus. Thus, adding SBS polymer and also adding nano-SiO 2 to the polymeric bitumen with 4.5 wt% SBS polymer at intermediate temperatures could reduce the complex modulus. As the reduction in complex modulus at the intermediate temperatures, leads to increase of resistance to fatigue failure, therefore the performance of modified bitumen at intermediate temperatures is improved. As seen in Fig. (2), it is observed that adding SBS polymer and nano-SiO 2 to the SBS polymeric bitumen at temperatures between 10 and 25 Celsius degrees and at constant frequency of 10 radians per second, the phase angle is continuously reduced. This could be due to interaction between nano-SiO 2 , bitumen and SBS polymer, so that nano-SiO 2 particles by adsorption develop enhanced adhesion with the bitumen and polymer compounds which has resulted in reduced phase angle. As stated before, reduction in δ value reduces loss in viscosity and therefore improves the performance at intermediate temperatures of modified bitumen samples.

Loss modulus (fatigue resistance)
The values of the loss modulus (G"=G*×sinδ) for various composites of SBS, nano-SiO2 with RTFO-aged base bitumen are shown in Fig. (4). As seen in the figure, by adding SBS and nano-SiO 2 to the base bitumen, the loss modulus is reduced at 10 to 25 °C with the constant frequency of 10 rad/s, which results in improved fatigue resistance potential and middletemperature performance of the modified bitumen samples. All different composites of SBS, nano-SiO2 with the base bitumen at temperatures 10 to 25 °C and constant frequency of 10 rad/s have the G*×sinδ values less than 5000 kPa, among which two composites of 4.5% SBS + 3 % nano-SiO2 and 4.5% SBS + 4% nano-SiO 2 have the minimum values. As seen in Fig. (4)  CONCLUSION y adding nano-SiO2 and SBS to the base bitumen, the needle penetration is decreased and the softening point, PI, and rotational viscosity are increased. These results decrease the sensitivity of various composites to temperature changes, reduce the fatigue cracking and improve the performance at the middle temperatures. In all of the various composites of SBS and nano-SiO2 with base bitumen, the ductility at 25 °C was found to be more than 100 cm, indicating the good performance at middle temperatures. The maximum rotational viscosity of bitumen samples at 135 °C is considered equal to 3 Pa.s. All bitumen samples, except for 4.5% SBS + 4% nano-SiO 2 satisfied this requirement. All composites were controlling the maximum mass loss in the RTFO test, which is 1%, indicating the resistance to oxidation and short-term aging. By adding nano-silica to the SBS polymer bitumen, the complex modulus, phase angle, storage modulus, and loss modulus at 10 to 25 °C and constant frequency of 10 rad/s were constantly reduced, resulting in the improved fatigue resistance and middle-temperature performance in the bitumen samples. In general, according to the requirement of rotational viscosity up to 3 Pa.s at 135 °C and also, regarding the economic issues in selecting a lower percentage, the combination of 4.5% SBS + 3% nano-SiO 2 is selected as the optimal composite.