# Ultrasonic Non-Destructive Testing of Accelerated Carbonation Cured-Eco-Bricks
## 4 Discussion of Results
### UNDT (Ultrasonic Pulse Velocity Test) and DT (Compression Test)
#### 4.1.2 DT (Compression Test)  
A minimum compressive strength of 3 MPa for non-structural bricks was the target strength in this study [54]. Figure 6 below shows the DT results of the samples tested with UNDT from Figure 5. Like the plots in Figure 5, the maximum strength was recorded for the WC samples compressed with 1.9 MPa, carbonated for 72 h, and soaked in water for another 72 h. Furthermore, 72 h carbonation was considered the optimum because, between 48 and 72 h carbonation, a marginal increase in strength was recorded. Hence, extending the carbonation duration beyond 72 h may not yield a remarkable increase in the strength and could lead to reduced strength which then translates to a waste of time and energy [69]. Table 2 below is a summary of the results obtained for the two testing methods, namely UNDT (P-wave) and DT (CS), the different compression pressures of 1.2, 1.6, and 1.9 MPa, and the three different curing approaches (HC, NWC, and WC) considered in this study. Figure 5: UNDT results for 0% TRC samples. Additionally, UNDT and DT tests were performed on the samples produced with 5 and 10% TRC samples carbonated for 30, 48, and 72 h. Notably, a 1.9 MPa compression pressure was considered since it gave the maximum strength for the 0% TRC samples. After the carbonation durations, one set of samples (NWC) was tested, and the other set was subjected to post-carbonation curing (WC) for 72 h. From the UNDT results shown in Figure 7, the maximum velocities were associated with the samples subjected to post-carbonation curing (WC). Furthermore, in Figure 8, the 5% TRC samples carbonated for 72 h and water-cured (WC) had a maximum CS of 4 MPa. Though the strengths recorded for the 5% TRC samples were lower than the 0% TRC samples, the 5% TRC WC sample is reported to be suitable for non-load-bearing bricks for small buildings [54]. The lower strengths were due to reduced bonding and increased porosity within the rubberized matrix. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Compression** & **P-Wave (m/s)** & **CS (MPa)** & **CO\({}_{2}\)****Curing Duration—Hydration Condition** \\ \hline \multirow{8}{*}{1.2} & 1721 & 1.9 & 72 h HC \\  & 1813 & 2.5 & 30 h NWC \\  & 2145 & 3.9 & 30 h WC \\  & 2134 & 3.3 & 48 h NWC \\  & 2159 & 3.9 & 48 h WC \\  & 2260 & 3.2 & 72 h NWC \\  & 2300 & 4.2 & 72 h WC \\ \hline \multirow{8}{*}{1.6} & 1800 & 2.5 & 72 h HC \\  & 1885 & 3 & 30 h NWC \\  & 2255 & 4.3 & 30 h WC \\  & 2238 & 4.9 & 48 h NWC \\  & 2311 & 5.2 & 48 h WC \\  & 2354 & 5.2 & 72 h NWC \\  & 2437 & 5.5 & 72 h WC \\ \hline \multirow{8}{*}{1.9} & 1880 & 3.4 & 72 h HC \\  & 2138 & 4.5 & 30 h NWC \\  & 2300 & 5.2 & 30 h WC \\  & 2330 & 5.4 & 48 h NWC \\  & 2414 & 5.7 & 48 h WC \\  & 2486 & 6.3 & 72 h NWC \\  & 2550 & 7.2 & 72 h WC \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of UNDT and DT results considering varying compression pressures. Figure 6: DT results for 0% TRC samples. Table 3 is a summary of the results obtained for the two testing methods, namely UNDT (P-wave) and DT (CS), the varying TRC, and the three different curing approaches (HC, NWC, and WC) considered in this study. From plots 7 and 8 above, it can be observed that a direct correlation exists between the UNDT and DT results. Since a direct correlation exists, models can be generated to establish this correlation. Research has also shown that different theoretical models have been developed to demonstrate the correlation between the wave velocities and CS in samples [26; 37; 70]. However, it is important to note that these correlations depend on factors such as the type of cementitious material, water cement/binder ratio, specimen size, curing duration, methods employed in testing the samples, and types of aggregate [39]. In this study, since the WC samples had the highest CS, linear, exponential, logarithmic, and power models were developed based on the results obtained for the WC samples.