1. Introduction
Asphalt mortar can generally be regarded as a remaining part of removing coarse aggregate particles from an asphalt mixture structure. The asphalt mortar contains fine aggregates, mineral filler, asphalt binder and air voids, among others. It also can be referred to as a fine asphalt mixture (FAM) [
1]. The asphalt mortar as a filling material between coarse aggregate gaps in asphalt mixture shows a significant relationship with the deformation, fatigue, cracking and healing properties of the asphalt mixture. However, there are no unified methods for the design and fabrication of asphalt mortar. Moreover, the key design indicators of asphalt mortar, including nominal maximum aggregate size (NMAS), asphalt binder content and air void content, greatly affect the performance of asphalt mortar.
For the NMAS of asphalt mortar, it is believed that the aggregate particle size in asphalt mortar does not exceed 1.18 mm [
2]. Three asphalt mortars with an NMAS of 4.0 mm, 2.0 mm and 1.18 mm, respectively, have been studied, and the fatigue performance results of asphalt mortars with an NMAS of 2.0 mm are most similar to those of asphalt mixtures [
3]. To date, there are no unified regulations of the NMAS limits in asphalt mortar. Therefore, the NMAS of asphalt mortar needs to be determined based on the model computation precision and image processing accuracy. It also should consider the specific testing conditions of target asphalt mortar specimens.
The content of asphalt binder in asphalt mortar should be consistent with the amount of asphalt mortar distributed among coarse aggregates in an asphalt mixture to ensure that research on the rheological performance of asphalt mortar and its correlation with asphalt mixture have practical significance. To ensure the real state of the asphalt mortar in its asphalt mixture, the asphalt binder content can be determined by the test methods of asphalt extraction and asphalt ignition combustion for the produced asphalt mixture specimen [
4,
5]. However, these methods consider the fine aggregate in the asphalt mixture and do not consider the asphalt mortar coated on the surface of the coarse aggregate. The asphalt binder content of asphalt mortar can be determined by the theory of asphalt film thickness in asphalt mixtures. Based on an assumption [
2], 8.0% of the asphalt binder content in asphalt mortar can guarantee an asphalt film coating on a fine aggregate surface with an average thickness of approximately 10 μm. Karki [
6] has also used an asphalt film thickness of 12 μm to determine the asphalt binder content in the asphalt mortar, and it is consistent with the true condition of the mortar portion of the asphalt mixture. In addition, the asphalt binder content in asphalt mortar can be better determined by the specific surface areas of aggregate particles. This method is simple and widely used [
7]. To overcome the large deviation in the specific surface area method in modified asphalt mortar, Ng et al. [
8] have considered the different asphalt binder absorptivity properties of fine aggregates to improve the calculation of asphalt binder content in asphalt mortar. The asphalt binder content for the asphalt mortars with different asphalt binder properties and fine aggregate types can be well determined.
The mechanical behaviour of an asphalt mortar is significantly affected by its air void content. As air voids in asphalt mixtures are distributed between asphalt mortars and coarse aggregates, some researchers have considered all air voids to be in the asphalt mortars [
9]. Then, the amount of air voids in asphalt mortar is consistent with that in its asphalt mixture. The air void content in asphalt mortar can be determined according to the compaction density of the asphalt mixture. The mass and volume of asphalt mortar are determined by removing the coarse aggregate and its absorbed asphalt binder, and then the actual density of asphalt mortar is obtained [
10]. Researchers have analysed the linear shear moduli of asphalt mortars with different air void contents and found that the shear moduli of asphalt mortars increase with decreasing air void content. If the air void content decreases by 1%, the shear moduli of asphalt mortars increase by approximately 7% [
11].
The NMAS, asphalt binder content and air void content are the most important control indicators in the design and preparation of asphalt mortar. However, from recent research, there are no clear or unified methods for determining these control indicators. According to different theoretical assumptions and calculation models, the calculated control indicators are different. Therefore, the preparation of asphalt mortar should simulate its status in an asphalt mixture to the greatest extent to accurately analyse and explore the asphalt mixture’s mechanical performance through asphalt mortar test results.
When the asphalt mixture shows inelastic or nonlinear behaviours, the asphalt mortar might exhibit microcracking or deformation, which is closely related to the overall mechanical performance of asphalt mixture [
9]. Compared with asphalt binder, the scale of asphalt mortar is closer to that of the asphalt mixture [
12]. Moreover, asphalt mortar is relatively close to the location where most fatigue and damage occur in asphalt mixture [
2]. Therefore, as an intermediate material in asphalt mixture structure, the behaviour of asphalt mortar presents great correlation with its asphalt binder and asphalt mixture.
The evaluation methods and indicators of asphalt mortar can be determined for reference to the related laboratory tests of asphalt binders and asphalt mixtures. The complex shear modulus
G* can be used as an evaluation index of the high-temperature viscoelastic properties of asphalt mortar. Similar to the asphalt binder and asphalt mixture, the complex shear modulus
G* is the ratio of stress and strain applied to asphalt mortar under dynamic load conditions. According to the size of asphalt mortar, the
G* index value of asphalt mortar can be accurately obtained through a frequency scanning test conducted by dynamic shear rheometer (DSR) [
13]. In the literature, a 0.01% strain is recommended as the linear viscoelastic boundary limit of asphalt mortar. It is believed that when the strain of asphalt mortar is less than 0.01%, asphalt mortar exhibits linear viscoelastic behaviour during loading [
14,
15]. Generally, asphalt mortar can be used as a model input to effectively predict the related properties of asphalt mixtures [
16], which significantly reduces the test time and material cost.
To date, the relevant research on the high-temperature properties investigation are mainly focused on the asphalt binder and the asphalt mixture; the evaluation of the high-temperature response is mainly investigated through laboratory tests under dynamic loading modes to analyse the creep–recovery properties of the asphalt binder and the asphalt mixture [
17,
18]. The Federal Highway Administration (FHWA) has formed a multiple-stress creep–recovery (MSCR) test to evaluate the high-temperature performance of asphalt binders by adding stress cycles based on a repeated creep and recovery (RCR) test [
19]. The MSCR test is commonly used in high-temperature viscoelastic performance tests and research evaluations for asphalt binder [
20,
21]. It has become one of the most promising methods for evaluating the high-temperature behaviour of asphalt binders. Recent studies have used the MSCR test to test the performance of modified asphalt binders [
22,
23]. It has been found that the MSCR test has the advantage of reflecting the elastic and viscoelastic properties for evaluating the rutting potential of modified asphalt binder. The cumulative strain of a composite modified asphalt is significantly reduced during the MSCR test. The composite modified asphalt is epoxy resin (ER) and crumb rubber powder (CRP) mixed into styrene–butadiene–styrene (SBS)-modified asphalt binder [
24]. In addition, based on the MSCR test results, the urea formaldehyde–epoxy resin (UFE) microcapsule incorporation can improve the viscoelasticity, high-temperature stability and permanent deformation resistance of SBS-modified asphalt binder [
25]. The MSCR test makes the asphalt binder fully exhibit creep–recovery mechanical properties in a high-temperature condition through repeated loading–unloading, which can verify its viscoelastic response well [
7,
26].
For the asphalt mortar, there are few standard laboratory experiments on the high-temperature performance of the intermediate-scale materials (i.e., asphalt mortar) and its relationship to asphalt mixtures. Because of the high contents of asphalt binder in the asphalt mortar and the feasibility of asphalt mortar samples for DSR equipment, the performance of asphalt mortar can be investigated for reference to the test methods of asphalt binder (i.e., MSCR test). The test procedures should be modified further in order to satisfy the characteristics of the asphalt mortar. The asphalt mixture behaviour can be well predicted by the properties of all its components, including asphalt binder, mineral filler, fine aggregate, coarse aggregate, etc. and their interactions. When the measured properties of asphalt mortar were adopted, the performance of asphalt mixture can be predicted regardless of the interactions of fine aggregate, mineral filler and asphalt binder in the asphalt mortar. In addition, the prediction model can be simplified significantly.
Therefore, in this paper, the high-temperature rheological properties of asphalt mortar were investigated by a modified MSCR test. In addition, a multisequence repeated loading (MSRL) test was used to investigate the high-temperature performance of the asphalt mixture. A prediction model for the high-temperature performance of asphalt mixtures based on the rheological response of asphalt mortar and properties of coarse aggregates was established. The correlation between asphalt mortar and its asphalt mixture was well investigated. The results of this study provide a new perspective for predicting the performance of asphalt mixtures. The research flowchart of this study is presented in
Figure 1.