This study analyzed the effects of the filler–bitumen interaction of the content and the meso powder characteristics of the mineral filler on the low-temperature performance of bitumen mastics. Control strategies for the mineral filler content (filler–bitumen ratio (RFB)) were also determined. Panjin #90 bitumen and styrene–butadiene–styrene polymer-modified bitumen were used in the experiment. Four kinds of limestone powder were used, all of which satisfy the Chinese standard for powder particle size but exhibit different meso characteristics. Each kind of limestone powder was used to prepare bitumen mastic samples under five different RFBs. The meso voids in the unit mass (Vg) of the four kinds of mineral filler were tested on the basis of the principle of the Rigden void ratio. The fixed bitumen–free bitumen ratio in the bitumen mastic samples was determined using Vg, bitumen density, and RFB. The low-temperature cohesive strength of the bitumen mastics was used as the control index for critical failure, whereas variation rates of bending creep stiffness at low temperature were used as the control index for fatigue failure. Results showed that the effects of the filler–bitumen interaction of the content and the meso characteristics of the mineral filler are significant and such effects are determined by the fixed bitumen–free bitumen ratio. The optimal fixed bitumen–free bitumen ratio in the bitumen mastics under two low-temperature conditions (−30 ◦C and −10 ◦C) can be determined on the basis of the influence of the fixed bitumen–free bitumen ratio on the critical and the failure control indices. Moreover, RFB can be obtained through reverse calculation. The mineral filler content can therefore be precisely controlled, which is crucial for the rational use of mineral filler and for the improvement of the pavement performance of bitumen mastics at low temperatures.
Recent engineering applications worldwide showed that the addition of mineral filler in bitumen mixture is necessary. The application of mineral filler not only improves the viscosity of bitumen mastics but also prevents segregation in bitumen mixture during mixing, transportation, paving, and compaction. Studies demonstrated that mineral filler can significantly improve the low-temperature strength of bitumen mastics and improve the low-temperature performance of bitumen mixture [1–5]. Mineral filler has therefore been considered a critical component of bitumen mixture. Although mineral filler is extensively applied in bitumen mixture, factors affecting construction, such as reasonable content, meso powder characteristics, and filler-bitumen interaction, still need to be investigated. Further research on these issues is necessary for the rational application of mineral filler.
Mineral filler content is not a sensitive technical problem in terms of using the powder to increase the viscosity of bitumen mastics and preventing segregation in the mixture [6,7]. The mineral filler content becomes a sensitive factor when the low-temperature strength of bitumen mastics is considered. Studies conducted worldwide reveal that the reasonable mineral filler content in bitumen mixture remains an unsolved technical problem because the mechanism in which mineral filler affects the low-temperature strength of bitumen mastics is not uniform. The control indices used in tests are inconsistent, and factors considered during analysis vary [8–10].
In recent years, numerous researchers have focused on the meso powder characteristics of mineral filler and its effect on bitumen mastic strength [11–14]. The meso characteristic is between the macroscopic characteristic and microscopic characteristic. The meso powder parameters of the mineral filler include meso gradation, specific surface area, particle size, length-diameter ratio, and roundness. The meso void features of the mineral filler comprehensively reflect the multiple meso powder parameters. If the meso powder parameters of the mineral filler are substantially different, then the meso void features of the mineral filler will be significantly different . There are additives to blends that act as a filler while their specific surface area, particle size properties are significantly different from the properties of a lime filler. Examples are zeolites: warm mix asphalt additives: Clinoptilolite SBET = 18 m2/g, zeolite Na-P1 SBET = 95 m2/g [16,17]. In a previous study, a particle image analysis system was used to analyze the meso powder characteristics of mineral filler (e.g., meso gradation, specific surface area, particle size, length–diameter ratio, and roundness). The results showed that all of the mineral filler samples obtained from different sources satisfied the specified requirements, but their meso powder characteristics significantly varied , and the mineral filler content influences the pavement performance of bitumen mastics . The rational mineral filler content is therefore an important research subject. The mineral filler content is determined on the basis of bitumen pavement performance indices. The results however also vary because the control indices are different and several of the control indices lack rationality [20,21]. More importantly, the influence of the meso powder characteristics of the mineral filler is not considered at a certain mineral filler content.
Bitumen exists in bitumen mastics in two states, namely, fixed bitumen and free bitumen. The fixed bitumen–free bitumen ratio significantly affects the strength of bitumen mastics. Existing research has fully demonstrated that the content and the meso powder characteristics of mineral filler influence the fixed bitumen-free bitumen ratio in bitumen mastics. Analyzing the filler-bitumen interaction of these two factors is therefore necessary. The results of this study will provide theoretical and experimental foundation for the rational application of mineral filler in the future.
2. Experimental Materials
Two kinds of bitumen, which are used extensively in China, are used in this experiment. One is Panjin 90# bitumen, and the other is styrene–butadiene–styrene (SBS) polymer-modified bitumen. The penetration values of bitumen at 5 ◦C and 25 ◦C were obtained using a penetration tester according to test standards <JTG E20-2001>. A ductility tester and a softening point tester were used to test the ductility and the softening point of bitumen, respectively. The viscosities of bitumen at 60 ◦C and 90 ◦C were determined using a standard viscosity tester based on test standards <JTG E20-2001>. The viscosity tested in this study is dynamic viscosity. Table 1 shows the basic performance parameters of these two kinds of bitumen. All tests meet the standards <JTG E20-2001>.