Function evaluation of asphalt mixture with industrially produced BOF slag aggregate

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.

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 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 50^th 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 (DCSE_f) method was widely used in evaluating the asphalt mixture’s moisture stability (13, 14). DCSE_f can be determined according to the results of indirect tensile test and M_R test. The schematic diagram is shown in Figure 2, where M_R is the resilient modulus, ε_f and S_t 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 M_R 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 DCSE_f ratio can reflect the asphalt mixture’s moisture stability:

DCSE f  ratio =  DCSE fi /DCSE f0 ×100%          Eq.  [1]

where DCSE_fi is the DCSE_f of specimens subjected to freeze-thaw damage for i times, kJ/m³; DCSE_f0 is the original DCSE_f of specimens. The DCSE_f ratios of different asphalt mixture when facing the same moisture damage were prepared.

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.

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:

Log  N f  =  − n × Log ε + K          Eq. [2]

The relations between fatigue lives (N_f) 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.

Moisture stability of asphalt mixtures

The DCSE_f and DCSE_f ratio results of BOF slag asphalt mixture and limestone asphalt mixture are shown in Figure 4. The original DCSE_f 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 DCSE_f 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 DCSE_f, and only 40% of DCSE_f 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.