**1. Introduction**

In this century with booming industries, the enormous exploitation and consumption of energy results in the ever-increasing concentration of carbon dioxide worldwide, causing the greenhouse effect and extreme climate changes. According to the statistics from the National Oceanic and Atmospheric Administration (NOAA), the average temperature has increased by up to 0.18 ◦C per decade since 1981. This has caused the snow in the polar regions to melt and subsequently raised the sea level [1]. Furthermore, the concentration of carbon dioxide started at about 265 ppm in 1850, reached 385 ppm in 2009, and even ascended to 407.4 ppm in 2018. The vicious circle of greenhouse gas emission has a tremendous impact not only on metropolises but also environments. Under the circumstances, building envelopes and pavements with high heat capacities bring on and even worsen the urban heat island effect.

Nowadays, to mitigate the urban heat island effect, many studies have proposed to utilize reflective coating of asphalt concrete and have recorded its ambient temperatures

**Citation:** Li, Y.-F.; Yang, P.-A.; Wu, C.-H.; Cheng, T.-W.; Huang, C.-H. A Study on Radiation Cooling Effect on Asphalt Concrete Pavement Using Basic Oxygen Furnace Slag to Replace Partial Aggregates. *Sustainability* **2021**, *13*, 3708. https://doi.org/10.3390/ su13073708

Academic Editor: Jorge de Brito

Received: 22 February 2021 Accepted: 23 March 2021 Published: 26 March 2021

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and internal temperatures at different depths, exploring the principle of cooling with its spectral characteristics [2,3]. One study changed the color of asphalt concrete to discuss its reflection characteristics [4]; some even improved the reflectivity across specific wavelengths, mainly aimed at visible and near infrared light [5–7]; others made multi-layer structure coatings and compared their cooling benefits [8]. Furthermore, adding titanium dioxide with different weight ratios and particle sizes could improve the reflectivity of coating [9,10]. Hollow glass microspheres were substituted for fillers inside the asphalt concrete, optimizing the proportion for the best cooling effect with a simulated solar heating test [11]. This replacement could improve concrete's emissivity and also reduce thermal conductivity [12]. The lower thermal conductivity materials painted on the asphalt concrete were capable of preventing heat from passing through the coating [13]. The conductive multi-layer structures contributed to diverse applications, such as a radiation-cooling pavement in summer and heat preservation on permafrost [14,15].

In terms of building envelopes, inorganic coating made from mineral powders has been painted on steel plates and concrete specimens. The temperatures and heat fluxes of the coated and uncoated surfaces and interiors were measured in the laboratory and in field tests and compared. The results showed that this coating had an obvious radiation cooling effect and grea<sup>t</sup> insulation ability owing to the emissivity and reflectivity. Furthermore, the adhesion and weather resistance were maintained at a certain level [16].

Apart from thermal properties, the mechanical properties of pavements need to be examined as well for practical use. Basic oxygen furnace slag (BOFS) is a byproduct of the steelmaking process. The converter is a by-product of the steelmaking process. A proper ratio for partially replacing aggregate with BOFS should be favorable for the development of bearing capacity and tensile strength when preparing asphalt or recycled concrete [17–21]. Furthermore, based on a two-year on-site test, it was found that the substitution of asphalt concrete entailed a longer life span with less damage in comparison with the original one [22]. Due to the roughness, multi-angularity, and rigidity of BOFS, the substitution concrete had better skid resistance, binding with bitumen, and also abrasion resistance [23]. Even when adopting the warm-mix or hot-mix asphalt approaches, this concrete still showed a grea<sup>t</sup> capability for resistance to deflection [24].

In this study, thermal and mechanical properties tests were done on the asphalt concrete specimens, in which aggregates were partially replaced by BOFS. Our previous research found that BOFS has a high far-infrared emissivity. After absorbing solar radiation, these specimens were capable of emitting the stored energy across the sky window (8–13 μm) to the upper sky, reducing energy accumulation and retention in the atmosphere, thereby achieving the goal of urban heat island (UHI) mitigation. The surface skid resistance is needed for the development of a radiation-cooling pavement. Ceramic particles or fine sand particles were added to increase the pavement roughness and the British pendulum number (BPN). BOFS is a byproduct of steelmaking that can not only achieve the goal of radiation cooling but also meet the needs of resource sustainability.

#### **2. Materials and Their Properties**

BOFS, a byproduct of steel industries, has lower thermal conductivity compared to natural aggregates. Under the same heat source and with an identical time duration, BOFS has a slower temperature-raising rate. Due to the greater hardness, BOFS applied to asphalt concrete has a bearing capacity comparable to natural aggregates.

In this study, five specimens were designed and prepared, including one general asphalt concrete specimen, and the stone aggregates of the asphalt concrete specimens were replaced by BOFS at proportions of 45 wt.%, 55 wt.%, 65 wt.%, and 75 wt.% respectively. The compositions of the aggregates used in the asphalt concrete specimens are shown in Table 1, and they are dense-graded and conform to ASTM D3515 [25]. The BOFS was also sieved with different sieve sizes. Then, we used BOFS aggregates to replace stone aggregates; the replacement percentage of every sieve size was the same. The content of

asphalt to the aggregate mixture was 5.6 wt.%, meeting the specification of between 2 wt.% to 10 wt.%. The naming rules and descriptions of the specimens are shown in Table 2.


**Table 1.** Passing percentages of aggregates in dense-graded asphalt concrete specimens.

**Table 2.** Naming and description of the specimens.

