# Evaluation of Workability and Mechanical Properties of Bottom Ash Aggregate Concrete
## 4 Results and Discussion
### Compressive Strength at 28 Days (\(f_{cu}\))  
Due to the low strength and porosity of the aggregate itself, CCBA shows relatively low compressive strength compared with NWC. Therefore, in the determination of concrete \(f_{cd}\) in mix design, the type of aggregate also becomes a factor to consider, in addition to \(W\)/C and unit cement contents. When examining the compressive strength (\(f_{cu}\)) of the specimens at 28 days, as shown in Table 4, the L-100-100 mixture shows approximately 1.68 times higher strength than that of the L-0-0 mixture, depending on the type of aggregate used. It also shows a 1.5 times higher strength in group M, and 1.26 times in group H, respectively. It therefore appears that the influence of the type of aggregate decreases as \(W\)/C decreases (that is, as target strength increases). This means that some of the compressive strength reduction caused by the replacement of BAA can be offset by adjusting design factors such as \(W\)/C. Accordingly, to achieve a better \(f_{cd}\) for CCBA, the design of a relatively low \(W\)/C is desirable according to the replacement ratio of BA. In addition, as revealed by a number of previous studies, it is very advantageous in terms of strength development to add BA to concrete as fine aggregate, rather than as coarse aggregate. [6; 10; 11]. In the experiment, the strength deviations between #-0-100 mixtures and #-100-0 mixtures were found to be on average 8.9 MPa (group L), 12.2 Mpa (group M), and 11.9 Mpa (group H), respectively. This is because cracking caused by external force occurred with the cracks passing through the BAA, and thus it acted as a greater weakness in coarse aggregates of large particle sizes [30]. The existing studies on the compressive strength of CCBA, although they are in early stages, have reported conflicting results. Some studies have reported that the use of BAA lowers compressive strength. Among them, [PERSON] et al. [31] reported that when a 50% replacement of fine aggregate and coarse aggregate was used, respectively, the compressive strength at 28 days was reduced from 52 Mpa to 40 Mpa. [PERSON] et al. [32] found that when replacing BAA, the compressive strength drastically declined by up to 40%. On the other hand, [PERSON] and [PERSON] [6], [PERSON] et al. [33], and [PERSON] and [PERSON] [34] reported that mixtures to which BAA is applied showed higher compressive strength than NWC. To explain this, they pointed out the pozzolanic reaction of BA or suggested that the mixing water was reduced due to the porosity of the aggregate. No study, however, has reported results showing that the strength increased when using coarse BAA. In general, the compressive strength of concrete is highly correlated with the \(W\)/C and air contents (\(v_{A}\)) [35; 36]. In particularly, when using mixtures containing LWA such as BAA, \(\rho_{c}\) should also be considered as an important factor. [PERSON] et al. [17; 23] conducted non-linear multiple regression analysis (NLMR) on the basis of many studies, and included 39 A-LWAC mixtures and 308 S-LWAC mixtures. As a result, they proposed an equation for \(f_{cu}\) prediction using parameters such as \(W\)/C, \(v_{A}\), and \(\rho_{c}\). Figure 4 shows a comparison between the equation proposed by [PERSON] et al. [17] and the values obtained from the present experiment. The results of the present experiment, which include results using BAA, show a lower \(f_{cu}\)/\(f_{0}\) than [PERSON] et al., who used artificial LWA. This is attributable to specific physical properties such as the density of the aggregate used, strength, and percentage of voids. To predict the compressive strength of CCBA, which was evaluated in this study, with a good fit to the predicted values (R\({}^{2}\) = 0.87), the equation proposed by [PERSON] et al.