1. Introduction
Road construction is a major part of infrastructure. During road construction, asphalt pavements are widely used on highways due to advantages such as the driving safety and comfort offered by asphalt mixtures [
1]. Several kinds of raw materials are usually used in asphalt pavement, such as asphalt binders, aggregates, and mineral fillers [
2]. The combination of asphalt binder, mineral filler, and fine aggregate consists of asphalt mortar, which mainly determines the final mechanical and durable properties of asphalt pavement. A previous study has reported that the higher temperature anti-rutting or low-temperature anti-cracking properties of asphalt mixtures are highly related to asphalt mortar [
3]. Hence, lots of scientific research has focused on the modifier of asphalt binders and mineral filler of asphalt mortar for improving the properties of asphalt mixture.
Recently, crumb rubber from scrap tires has been wildly used in the field of asphalt modification. The reutilization of the end-of-life tires to modify virgin binder provides a promising way to significantly reduce the number of land-filled tires [
4]. Besides, it is known that rubber can improve the performances of asphalt binders by optimizing the property of asphalt through the swelling, degradation, and a series of complex physico-chemical processes [
5,
6]. According to previous findings, when adding crumb rubber, the low-temperature performance, skid resistance, and fatigue properties of modified asphalt have been improved. Additionally, adding crumb rubber significantly improved the rutting resistance, temperature susceptibility, and moisture susceptibility of asphalt mixture [
7,
8], as well as even long-term pavement service performance characteristics, such as driving safety and comfort [
9]. Therefore, the waste rubber has been added into asphalt as a modifier to prepare rubber modified asphalt (RA) worldwide.
To further improving the performance of asphalt mixtures, mineral fillers such as fly-ash, cement, silica fume, bag-house fines, and hydrated lime have been used in asphalt mixtures for the promotion of rutting resistance and moisture damage resistance [
10]. It has been found that the permanent deformation of asphalt mixtures can be improved by the type and amount of filler [
11,
12,
13]. Steel slag powder can be used as a mineral filler in asphalt mixture [
14]. Lots of research has focused on the reutilization of steel slag as a coarse aggregate to replace the natural mineral aggregates in design and preparation of asphalt mixture due to its high abrasion resistance and skid resistance [
15,
16]. Additionally, excessively reducing the consumption of steel slag depends not only on reutilization as coarse aggregate, but also as a mineral filler in asphalt mixture. Consequently, it can minimize the overall environmental impact of steel slag. However, the feasibility of application of steel slag are still unknown. It is crucial to the design of long-life asphalt pavement to understand the performance and optimal additional amount of filler, especially for a novel mineral filler prepared using solid waste materials.
Generally, for any process of modification of asphalt binders by crumb rubber or mortar by mineral fillers, the rubber or fillers are directly added with asphalt binder before the preparation of asphalt mixtures. Few examples of research have focused on the development of compound crumb rubber mineral filler asphalt composite prior to prepare of asphalt mixtures [
17,
18,
19,
20,
21]. Besides, it has been found that there is an issue of the storage stability of modified asphalt when modifiers such as crumb rubber and mineral fillers are added into asphalt binders. Despite the better performance of modified asphalt by modification, the poor storage stability also raises concern [
22,
23]. There are some modification methods provided by previous literature which showed that the asphalt binders were only modified by high-speed shear and heating device in laboratory [
3,
18]. The poor storage stability problems might be predictable during the modification of asphalt in a mixing plant or the transportation of modified asphalt binders. Many efforts have been devoted to treating the modifiers or fillers before their utilization in the modification of asphalt binders in order to improve storage stability [
14,
24]. Few of these efforts have reported on either continuously modifying asphalt binders by adding crumb rubber and steel slag fillers, or preparing the compound rubber and steel slag filler modified asphalt composite (RSAC) for the purpose of better storage stability and convenient transportation of modified asphalt composite. It is important to investigate the basic and rheological behavior of RSAC when it is in the state of solid particles at room temperature.
Hence, the object of this work is to prepare the compound rubber and steel slag filler modified asphalt composite, as well as to conduct a comprehensive investigation on the rheological properties of RSAC. A base asphalt with crumb rubber was used to prepare the crumb rubber asphalt. Afterwards, the rubberized asphalt was further mixed with steel slag filler to prepare RSAC by granulation. Subsequently, the conventional properties, such as viscoelastic behavior, rheological behavior, storage stability, as well as thermogravimetric analysis were studied.
5. Conclusions
This study investigated the rheological properties of compound rubber and steel slag filler modified asphalt composite (RSAC). Conclusions are drawn as follows.
The RSAC particles were prepared to improve the storage stability and convenient transportation of modified asphalt composites. The viscosity–temperature susceptibility of RSAC was superior to that of modified asphalt binder with only one additive.
Steel slag filler led to a slight reduction in storage stability of the modified asphalt samples, while crumb rubber enhanced storage stability resulting from the degradation of rubber and its consequent reaction with asphalt component. The addition of steel slag filler in rubber modified asphalt led to a decreasing sensitivity of frequency of asphalt materials.
The creep test illustrated a better anti-permanent deformation performance of RSAC can be obtained, which means a better low-temperature performance could become predictable. The complex modulus and viscous-elasticity performance of RSCA can be well-described by the CAM and Burgers models due to their higher coefficient values.