## 1 Introduction  
There is a general consensus in the literature that recycled concrete aggregate (RCA) is more porous and heterogeneous than natural aggregate. High-quality RCA can be obtained from waste concrete via a crushing process. This usually entails three to seven steps, including the elimination of foreign substances, rebar, and residual mortar (RM). During the crushing process, the RM quantity adhering to the RCA is altered. The primary properties adversely influenced by the adhered RM are density, absorption, etc. [1]. In particular, adhering RM in RCA results in greater porosity and consequently greater water absorption of RCAs [2], where the porosity is represented by the water absorption [3]. Usually, RCA with lower strength resulted in higher porosity in the aggregate and a newly made interfacial transition zone (ITZ). This, in turn, influences the properties of the concrete that is produced afterward. As a result of the increase in the RM, the physical concrete characteristics are impacted, including the compressive strength, Young's modulus, flexural strength, drying shrinkage, thermal expansion coefficient, freeze-thaw resistance, etc. [4; 5; 6; 7]. Thus, many researchers have carried out various experimental studies on the use of RCA, such as using RCA source derived from precast concretes, the two-lift paving method, modification of mixing processes, new mix design approaches, etc. [8; 9; 10; 11; 12]. One ideal way of acquiring high-quality RCA is to derive it from precast concrete [13; 14; 15] or concrete sleepers. The main advantage of retaining RCA from sleepers is the possibility of producing reliable products and reducing sorting costs. Furthermore, several research groups have investigated the material properties of high performance RCA concrete railway sleepers and found that, in comparison to the use of ordinary concrete, adequate material properties could be obtained [16; 17; 18]. According to Federal Highway Administration (FHWA) data [7], the use of RCA on bases of new pavements is currently allowed in 41 state departments of transportation (DOT) in the USA. Among these states, 11 states use RCA for paving concrete mix. Moreover, two-lift concrete pavements have been successfully constructed in the USA with the bottom layer containing low-quality recycled aggregate concrete [19; 20; 21]. It was pointed out by [PERSON] et al. [22] that two-lift construction using recycled materials in the bottom lift can have the highest positive impacts from a social and environmental perspective. [PERSON] and [PERSON] [23] proposed a two-stage mixing approach (TSMA), while [PERSON] and [PERSON] [24] suggested a triple- mixing procedure to manufacture strong and durable RCA concrete. TSMA divides the mixing process into two parts: initial mixing of all the aggregates and half of the required water and final concrete mixing with the other half of the required water and cement. It was observed that the strength, shrinkage, creep and permeability properties of RCA concrete were enhanced by adopting TSMA [23; 25]. The triple-mixing process divides the mixing process into three parts: coating coarse aggregates mixed by application of additive and a certain amount of the water required for coating, adding cement with fine aggregate, and final concrete mixing with the remaining water and plasticizer. It was observed that the density, compressive and splitting tensile strength, and water absorption capacity properties were improved. New modified mix proportioning methods for producing RCA concrete have been proposed by a few researchers [4; 5; 26]. The equivalent coarse aggregate mass (ECAM) method was proposed by [PERSON] et al. [26]. The main concept is that the attached mortar is treated as part of the sand. Test results show that the compressive strength values of the RAC mixes (up to 50% of RCA replacement ratios) were comparable with the compressive strength of the control natural concrete mixes, while slump decreased with the addition of RCAs.