## 2 Materials and Methods  
### Bitumen, Aggregates, Coal Bottom Ash  
Bitumen grade 80/100 was selected to be used in this study. The bitumen used was supplied by Asphalt Technology Sdn Bhd, located in Port Klang, Malaysia. The bitumen used acts as a lubricant during compaction and a visco-elastic binder of high viscosity in service. The properties of bitumen 80/100, such as its penetration, softening point, and viscosity, were tested in accordance with ASTM D 5 [46], ASTM D 36 [47], and ASTM D 4402 [48], and are presented in Table 1. The results indicate that the binder satisfies the standard requirement of binder grade 80/100 with penetration and softening point values ranging from 80 to 100 (0.01 mm) and 45 to 52 \({}^{\circ}\)C, respectively. In terms of viscosity, an increase in temperature decreased the viscosity of the binder. At 135 \({}^{\circ}\)C and 165 \({}^{\circ}\)C, the viscosity of the binder was 263 and 151 mPas, respectively, which satisfy the standard requirements set by JKR.  
The crushed aggregates utilised in this study consist of both coarse and fine aggregates, as well as mineral fillers, which range from 20 nominal-size aggregates to quarry dust. All specimens were prepared using a Portland cement mixture with a weight ratio of 2%. Figure 1 depicts the aggregate distribution employed in this study. The SMA20 mixture has been designed using the mid-point distribution of aggregate gradation. The aggregate utilised in this study was obtained from Kajang Rock Quarry Sdn. Bhd, Selangor, Malaysia.  
In order to assess the effect of CBA on pavement performance, the proportion of CBA utilised as a substitute for fine aggregate was varied to different percentages:--0%, 10%, 20%, 30%, 50%, 70%, and 100%--by weight of the total aggregate. The bottom ash was obtained from the Jimah Power Plant, Negeri Sembilan, Malaysia. Tables 2 and 3 display the basic physical characteristics of the CBA and the concentration of trace elements in the CBA when it was measured on a dry basis, respectively. According to the findings, it was concluded that the CBA had a higher water absorption of 22%, which was greater than the water adsorption value specified in Standard Specification for Road Works: Section 4--Flexible pavement (JKR/SPJ/2008-S4) for fine aggregate [49].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Properties** & **Binder Grade** & **Standard** \\  & **80/100** & **Requirement** \\ \hline Penetration (25 \({}^{\circ}\)C, 100 g, 5 s) (0.01 mm) [46] & 89 & 80–100 \\ Softening point (ring and ball) (\({}^{\circ}\)C) [47] & 46 & 45–52 \\ Viscosity at 135 \({}^{\circ}\)C (mPa s) [48] & 263 & Max 3000 \\ Viscosity at 165 \({}^{\circ}\)C (mPa s) [48] & 151 & Max 3000 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical properties of binder.  
Figure 1: SMA20 aggregate gradation.  
Table 3 displays the concentration of trace elements in the CBA on a dry basis (mg/kg). CBA is a byproduct of coal combustion that contains trace elements, including arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), selenium (Se), and silver (Ag), among others. According to the Environmental Quality (Scheduled Wastes) Regulation 2005, bottom ash is categorised as scheduled waste SW104. This refers to dust, slag, dress, or ash that contains specific elements such as aluminium, mercury, cadmium, thallium, arsenic, chromium, nickel, vanadium, lead, beryllium, antimony, copper, tellurium, or selenium. However, it does not include slag from iron and steel factories [50]. Table 3 demonstrates that the concentration of all trace elements in the CBA used in this study meet the minimum requirements established by the Malaysian Department of Environment.