## 1 Introduction  
Basic oxygen furnace slag (BOF) is a steelmaking by-product. China's annual output of BOF slag reaches 2 million tons [1], and most of the BOF slag treatment methods involve conveyance to a disposal site for landfill which consumes space; as well, water osmosis from BOF slag causes environmental pollution. There have been several studies on the feasibility of employing other applications. [PERSON] and [PERSON] 2002 [2] pointed out that BOF slag has similar properties to natural aggregate, and has no any compromising effects on asphalt cement. [PERSON] et al. 2006 [3] investigated the applications of BOF for coarse aggregate asphalt mixture, which showed a better construction performance than natural aggregate. In addition, BOF slag can be applied to road construction. BOF slag has a coarser surface texture than that of the conventional aggregate; the adhesive force between BOF slag and asphalt binder improved the asphalt mixture. [PERSON] et al. 2012 [4] used a freeze-thaw experiment to compare reused BOF asphalt mixture, and found that it still had better resistance to water sensitivity when compared to basalt aggregate. In order to improve the performance of asphalt pavement, various laboratory tests have been used to evaluate the feasibility at different conditions. Stone mastic asphalt (SMA) has a high stone content, which forms a gap-graded skeleton-like stone structure. It provides deformation resistance and durability, making it suitable for heavily trafficked roads. The voids of the mixture's structure are filled with a high asphalt mastic and filler. The stone skeleton of SMA has a high angularity that improves the interlocking packing and stone-on-stone contact among aggregates [5; 6]. Porous asphalt (PA) has crushed coarse aggregate without a significant proportion of fine aggregate, and it has high drainage and noise-abrasion properties. The performance of PA mixtures regarding rutting resistance and moisture susceptibility is not as efficient as that of dense graded mixture [7; 8]. In addition, reclaimed asphalt pavement (RAP) is a useful alternative to virgin materials because it reduces the need to use virgin aggregate [9]. However, [PERSON] 2004 [10] showed that the hardness of the total asphalt cement in the mixture increases with the RAP content, while rejuvenating agents were added to the mixture in order to improve the recovered binder performance. Based on the excellent performance of SMA and PA mixtures, many studies used BOF slag as aggregate in SMA and PA mixtures. [PERSON] et al. 2009 [11] added BOF into a porous asphalt (PA) mixture, and found that BOF could improve the skid resistance, water sensitivity, rutting resistance, and sound absorptivity. When used for BOF slag stone mastic asphalt (SMA) mixtures, BOF slag presents high rutting resistance, high-temperature stability, and resistance of moisture damage to improve the stability of high-temperature permanent deformation and low-temperature brittleness, effectively increasing the stability of mixtures [12; 13]. [PERSON] et al. 2007 [14] indicated that BOF slag could be characterized as a porous material and could improve the absorption of the asphalt binder in asphalt mixture. The permanent deformation produced at elevated temperatures for asphalt pavement was reduced. In addition, a newly rehabilitated asphalt pavement has been compacted, and a period of time is required for the mixture to gain strength. The pavement could open to traffic only after it has been cooled and achieved adequate strength [15; 16]. Criteria for opening pavements to traffic have generally been based on time or strength. [PERSON] and [PERSON] 2011 [17] investigated the thermal temperature effect of BOF slag substituted in a reclaimed asphalt mixture, and found that BOF slag could decrease the cooling effect, which means that BOF slag mixture has a temperature preservation effect. However, few studies have been conducted on the cooling effect of different aggregate gradation on asphalt mixtures, and substantiating the 'open to traffic' criterion.