Function investigation of stone mastic asphalt (SMA) mixture partly containing basic oxygen furnace (BOF) slag

Raw materials

Two types of coarse aggregates, namely, BSCA and basalt (both with size gradations of 4.75-9.5 mm and 9.5-16 mm), were used. Meanwhile, basalt was also applied as fine aggregate. Two BSCA production lines were built in Hubei, China. One was utilized to produce BSCA with size gradations of 9.5-19 mm and 19-31.5 mm, while the other one was designed for the production of BSCA with size gradations of 4.75-9.5 mm and 9.5-16 mm. BSCA received sufficient aging treatment to remove the free lime and easily fractured parts like honeycomb particles contained in slags. The basic physical properties of the aggregates were tested according to Chinese standard (19), and results were shown in Table I. Besides, SBS modified asphalt binder with penetration of 58.5 (0.1 mm at 25°C), ductility beyond 20.0 cm (5 cm/min, 5°C), and softening point of 61.5°C, limestone powder and polyester fiber were used. The basic physical properties of all raw materials satisfied the requirements of Chinese specification (20).

Design of SMA mixtures

SMA mixtures with nominal maximum size of 13.2 mm were designed in this research, and the compositions of each mixture were shown in Table II. A volume control method was used in order to maintain constant volume composition of different SMA mixtures. Aggregates with size gradations of 9.5-16 mm, 4.75-9.5 mm and below 4.75 mm accounted for 45%, 29% and 16% of the total volume of the mineral mixture, respectively. Limestone filler occupied 10% of the total volume of the mineral mixture. In order to obtain satisfied hybrid gradations of mineral mixtures, the proportions of BSCA with different size gradations (9.5-16 mm, 4.75-9.5 mm) were not the same due to specific particle size distribution for each type of BSCA. It also suggested that the mix proportions for different types of BSCA cannot change arbitrarily when the design type of asphalt mixture was determined. The mix proportions used in this research can be copied in the construction of future BOF slag SMA pavement. Therefore, the focus of this research was on the size gradations, not on the proportions of different types of BSCA.

The optimum asphalt content (OAC) was determined using steps as follows: (1) Based on the common range of asphalt contents for SMA mixtures, five asphalt contents (5.4%, 5.7%, 6.0%, 6.3%, and 6.6%) were considered for each SMA mixture; (2) then, each SMA mineral mixture was mixed with these five asphalt contents, respectively; (3) finally, standard Marshall test was carried out for the mixed SMA specimens (21). Afterwards, whether the volumetric parameters of SMA mixture with OAC were qualified or not were examined. The detailed information of each SMA mixture were shown in Table II, where the volumetric parameters met the requirements of Chinese specification (20).

Skid and deformation resistances of SMA mixtures

The slab specimens with a length of 300 mm, a width of 300 mm and a height of 50 mm were prepared for the antiskid test. The BPN values of slab specimens were determined at both dry and wet conditions using the British pendulum tester, shown in Figure 1A. The test process was performed following ASTM E303 (16). The surface of slab specimens ready for the test should be clean and free of loose particles. Four repeated tests were conducted for each specimen under dry and wet conditions, respectively.

The Marshall test (stability and flow value) was conducted according to ASTM D6927 (17). Marshall specimens with a height of 63.5 mm and a diameter of 101.6 mm were prepared by using a standard Marshall hammer. Four specimens for each SMA mixture were prepared, and all specimens were immersed in 60^oC water bath for 30-40 min. A HM-3000 master loader with a constant deformation rate of 50 mm/min was used to test the Marshall specimens.

The dynamic creep test was conducted according to BS EN 12697-25 (18), and modification has been made on the dimension of cylindrical specimens. Specimens with a diameter of 100 mm and a height of 100 mm were cored from double-layer slab compacted by the roller as shown in Figure 1B. Creep test was conducted by UTM-25 as shown in Figure 1C (3), where the applied stress was 0.1 MPa, and the haversine load pulse consisted of a 0.1 s loading period and a 0.9 s rest period. Besides, the test temperature was fixed at 60^oC, and four repeated tests for each SMA mixture were conducted.

Effect of BSCA’s size gradation on the skid resistance

The BPN values of SMA mixtures under both dry and wet conditions were shown in Figure 2. It can be seen that under dry conditions, BSCA with a size gradation of 9.5-16 mm, 4.75-9.5 mm displayed higher BPN values by 6.3% and 3.4%, respectively, compared to the pure basalt. Moreover, BSCA with the combination of these two size gradations, namely, 4.75-16 mm, performed even better than the previously separated BSCA components. However, the increment of BPN value caused by the combination was not the simple sum of the increments caused by their separated components. Under wet condition, similar effect was observed. In comparison to basalt SMA mixture, BSCA with a size gradation of 4.75-9.5 mm, 9.5-16 mm and 4.75-16 mm showed increased BPN values by 2.4%, 4.8% and 9.1%, respectively. Therefore, BSCA played a positive role in enhancing the skid resistance of SMA mixture. This effect was more apparent to coarser BSCA. The combination of BSCA with the two size gradations demonstrated the best skid resistance among all these SMA mixtures.

Effect of BSCA’s size gradation on the deformation resistance

The MQ, the ratio of Marshall stability to flow value, was used as an indicator to reflect the creep deformation resistance of asphalt mixture (22). The Marshall stability and MQ of SMA mixtures were shown in Figure 3. It showed that the BSCA with a size gradation of 4.75-9.5 mm, 9.5-16 mm and 4.75-16 mm increased the Marshall stability of SMA mixtures by 4.2%, 3.3% and 8.3%, respectively. The high alkalinity of BSCA, which was of benefit for the adhesion between steel slag and asphalt binder, may have contributed to it. Both the two types of BSCA (9.5-16 mm and 4.75-9.5 mm) displayed increased MQ, and the increase of MQ caused by the former was about 1.4 times higher than the latter. A larger MQ indicated a higher stiffness, so that the asphalt mixture had a greater ability to spread the applied load and resist creep deformation (23). Therefore, BSCA with a size gradation of 9.5-16 mm played a more significant role in improving the deformation resistance of SMA mixture. Similarly, SMA mixture containing BSCA with the combined size gradations performed the best among all SMA mixtures.

The dynamic creep deformation curve consists of three stages as shown in Figure 4 (3). In the first stage, the strain growth rate rapidly decreases until reaching a constant value, which indicates the start of the secondary stage. The secondary stage lasts for a very long period, and stable increase of total strain can be observed in this stage. However, the situation can be quite different when referring to the third stage. The strain growth rate rapidly increases and the total strain becomes very high in a short time, which finally results in failure. Load repeated cycles recorded at the start point of the third stage is known as the flow number (F_N), which reflects the deformation resistance limit of asphalt mixture.

The F_N of each SMA mixture was shown in Figure 5. It can be seen that SMA mixtures containing BSCA obtained larger F_N than pure basalt SMA mixture. BSCA with a size gradation of 9.5-16 mm also showed more positive effect on increasing F_N than BSCA with a size gradation of 4.75-9.5 mm, and the increase caused by BSCA with a size gradation of 9.5-16 mm was 2.4 times larger. Compared to BSCA with a size gradation of 9.5-16 mm, the superiority of BSCA with combined size gradations was small. It further proved that the effect of coarser BSCA on enhancing the deformation resistance of SMA mixture was much more significant. This may be related to the stable skeleton consisting of coarser particles.