Abstract
Background
Laboratory research suggested that basic oxygen furnace (BOF) slag-based asphalt mixture was a functional material. However, the BOF slag aggregate's quality was difficult to control when it was heavily used in entity engineering.
Aims
The primary objective of this research was to evaluate the functional performances of asphalt mixture containing BOF slag coarse aggregate (BSCA), which was from an industrialized production line.
Methods
Limestone mixture was a control group. The Marshall method was first adopted to design asphalt mixtures. The performances of limestone asphalt mixture and BOF slag asphalt mixture including fatigue failure resistance and moisture stability were then evaluated and compared.
Results
Results showed that the asphalt mixture containing BSCA possessed better durability, which meant the quality of BSCA from industrialized production lines was well controlled and this BSCA can be heavily used in entity engineering.
Introduction
A road transport system is quite critical to the development of the national economy. A large number of roads are newly constructed and reconstructed every year in China, which put pressure on the supply of natural materials (aggregates and fillers) (1). The use of secondary (recycled) materials in road construction can reduce the demand of natural resources. These secondary materials refer to many aspects such as waste rubber, demolition waste and steel slag (2-3-4). Steel slag includes many types such as basic oxygen furnace (BOF) slag and electric arc furnace (EAF) slag (5). Many developed foreign countries have very high recycling rates of steel slag. About 39%-62% of steel slag was used in road construction in Europe (6), and the use rate of steel slag used as aggregate was up to 98% in the UK (7). BOF was the main type used.
The utilization of BOF slag coarse aggregate (BSCA) in asphalt mixture has been well evaluated. Wu et al used BSCA to prepare stone mastic asphalt (SMA) mixtures (8). Results indicated BSCA improved many performances of SMA such as skid resistance, crack and deformation resistance. Shen et al evaluated the feasibility of BOF slag for substitution of aggregate in porous asphalt mixture (9). The mixture of 100% BSCA substitution was determined to be the optimum substitution percentage. The combination of BSCA and other solid wastes in dense asphalt mixture was also investigated. Results suggested that asphalt mixtures can obtained satisfactory fatigue resistance and moisture stability when proper design was considered (1). Therefore, BOF slag asphalt mixture was a functional material (1, 5, 8, 9).
The annual output of BOF slag is close to 17 million tons in China and its rate of use in asphalt mixture is quite low. The storage of slag has caused many environmental problems (4). The composition of BOF slag is complicated (10), which is unlike the natural stone. The technique used to process natural stone is not quite suitable for processing BOF slag. Unstable specification and poor angularity of BOF slag aggregate produced by current methods in China are two main problems. A modified BSCA production line was built in a large enterprise in Hubei, China. It consisted of combined crushing, screening and cleaning technology. BSCA produced by this modified production line was proven to possess good specification stability and angularity.
The primary objective of this research was to evaluate the functional performances of asphalt mixture containing BSCA from industrialized production. Limestone asphalt mixture was functioned as a control group. Asphalt mixtures were first designed using the Marshall procedure. The performances of limestone asphalt mixture and BOF slag asphalt mixture including fatigue failure resistance and moisture stability were then evaluated and compared.
Materials and Methods
Raw materials
Two mineral mixtures were considered in this research. BOF slag mixture consists of BSCA with size beyond 9.5 mm and limestone aggregate with size below 9.5 mm, and limestone mixture completely composed of limestone aggregate. The basic physical indexes of BSCA, suffered weathering treatment for more than one year, and limestone aggregates are shown in Table I, and they are all within the Chinese specification requirements. Limestone filler with a hydrophilic factor of 0.7 and base asphalt binder with penetration of 68 (0.1 mm at 25°C, 100 g and 5 s), ductility beyond 150 cm (5 cm/min, 15°C), and softening point of 46.4°C, were also used.
The basic physical properties of aggregates
BOF = basic oxygen furnace.
Experimental methods
BOF slag asphalt mixture and limestone asphalt mixture with the maximum nominal size of 26.5 mm were designed by Marshall procedure. In BOF slag mineral mixture, BSCA with size beyond 9.5 mm accounted for 46% by volume. The hybrid gradation curves are shown in Figure 1. It can be seen that the hybrid gradation curve of BOF slag mixture did not coincide with that of limestone mixture. This was because the specific gravity of BOF slag was larger than limestone, and volume control method was used in order to consistently maintain the volume composition of different mixtures. Therefore, the gradation curves for different mixtures were not completely overlapped in the coordinate system of mass passing percent (1, 11).
The hybrid gradations of basic oxygen furnace (BOF) slag mixture and limestone mixture.
The asphalt mixture's fatigue performance was determined by the four-point beam fatigue test, which was conducted according to AASHTO T321 (12). The tested specimens with the dimensions of 380 mm in length, 63.5 mm in width, and 50 mm in height were sawed from laboratory compacted block samples. The specimen was preassembled in the universal test machine (UTM) equipped with a data-acquisition system. A strain-controlled model was adopted and three micro-strain levels (400 μm, 500 μm, 600 μm) were considered. The test temperature was 15°C and loading frequency was 10 Hz. According to AASHTO T321, the loading cyclic number corresponding to 50% reduction in initial stiffness (measured at the 50th cycle) is regarded as the fatigue life of asphalt mixture (12). Three replicates for each mixture at every combination of test conditions were considered.
Dissipated creep strain energy (DCSEf) method was widely used in evaluating the asphalt mixture's moisture stability (13, 14). DCSEf can be determined according to the results of indirect tensile test and MR test. The schematic diagram is shown in Figure 2, where MR is the resilient modulus, εf and St is the Marshall specimen's strain and indirect tensile strength (ITS), respectively, when failure happened, and EE is elastic energy. In this research, freeze-thaw damage model was adopted. The indirect tensile test and MR value test of specimens before and after freeze-thaw damage were conducted according to AASHTO T322 and ASTM D7369, respectively (15, 16). The test temperature was 20°C. Three replicates for each mixture at every test condition were considered. The DCSEf ratio can reflect the asphalt mixture's moisture stability:
The schematic diagram for dissipated creep strain energy (DCSEf) determination.
where DCSEfi is the DCSEf of specimens subjected to freeze-thaw damage for i times, kJ/m3; DCSEf0 is the original DCSEf of specimens. The DCSEf ratios of different asphalt mixture when facing the same moisture damage were prepared.
Results and discussion
Design results of asphalt mixtures
The design results of limestone asphalt mixture and BOF slag asphalt mixture are shown in Table II. The optimum asphalt content (OAC) of BOF slag asphalt mixture was 0.2% larger than value of limestone asphalt mixture. There maybe two factors caused higher asphalt content. One was that BOF slag was a porous materials and pores absorbed some asphalt binder. The other was that BOF slag mixture was slightly difficult to be compacted and some asphalt was used to fill mineral aggregate voids in order to obtain desired air voids of asphalt mixture.
The volumetric properties of designed asphalt mixtures
BOF = basic oxygen furnace.
Fatigue resistance of asphalt mixtures
The power model was effective in analyzing the fatigue behavior of asphalt mixture. The power model in logarithm will be a linear equation, and as following:
The relations between fatigue lives (Nf) and strain levels were shown in Figure 3 with double logarithmic coordinates. It can be seen that the fatigue life of BOF slag asphalt mixture was always higher than the value of limestone asphalt mixture at the same strain level, and the difference enhanced with the increase of micro-strain. Although the reversed results of fatigue lives may be presented at lower strain level such as 100 με, larger strain is more consistent with the actual traffic conditions in China. The values of parameter n in linear equation corresponding to the curve's slopes in Figure 3, which can reflect the attenuation speed of fatigue life with the increase of strain levels. The values of parameter n for BOF slag mixture and limestone mixture were 4.124 and 4.480, respectively. A lower n value meant better fatigue failure resistance. Therefore, the introduction of BSCA from industrialized production in asphalt mixture improved the fatigue life and fatigue failure resistance of asphalt mixture when subjected to high strain.
Bean fatigue test results of different asphalt mixtures.
Moisture stability of asphalt mixtures
The DCSEf and DCSEf ratio results of BOF slag asphalt mixture and limestone asphalt mixture are shown in Figure 4. The original DCSEf of BOF slag mixture without freeze-thaw damage was 13.5% larger than that of limestone mixture. It indicated that BOF slag asphalt mixture obtained better creep resistance. The DCSEf values of BOF slag mixture and limestone mixture both decreased after moisture freeze-thaw damage was carried out. Limestone mixture showed a faster decrease rate of DCSEf, and only 40% of DCSEf was retained after moisture freeze-thaw three cycles, which was 55% for BOF slag mixture. Therefore, BSCA from industrialized production improved the durability of asphalt mixture when serious moisture damage was carried out.
Dissipated creep strain energy (DCSEf) and DCSEf ratio of different asphalt mixtures.
Conclusions
The primary objective of this research was to evaluate the functional performances of asphalt mixture containing BSCA from industrialized production. Limestone asphalt mixture was functioned as a control group. Based on the results discussed above, the following items can be concluded:
Asphalt mixture containing BSCA obtained larger bean fatigue life and better fatigue failure resistance than that of limestone asphalt mixture.
The introduction of BSCA in asphalt mixture improved the durability when asphalt mixture was subject to serious moisture freeze-thaw cycle damage.
Satisfactory functional performances of BOF slag asphalt mixture indicated that BSCA from industrialized production can be heavily used in entity engineering.
Footnotes
Financial support: This work was financially supported by the National Key Scientific Apparatus Development Program (No.: 2013YQ160501), the Natural Science Foundation of China (No.: 51278392), the International Science & Technology Cooperation Program (No.: 2013DFE83100) and the 973 Program (No.: 2014CB932104).
Conflict of interest: None of the authors has financial interest related to this study to disclose.