## 3 Relationship between UPV and Strength Evolution Index of Concrete
### Influence of Pozzolanic Material and Paste Volume Ratio  
The testing results in the previous section revealed that differences in the S/A and coarse aggregate type did not have a significant influence on the correlation between UPV  
Figure 5: Relationship between UPV and strength for normal concrete specimens with different types of aggregate: (**a**) UPV and strength, (**b**) UPV and strength evolution index.  
and the strength evolution index. This section examines the influence of adding pozzolanic materials and altering the paste volume ratio. The specimens were prepared according to the mixture proportions in Table 3 for fly ash concrete and those in Table 4 for slag concrete. Because a lower evolution index in strength was observed for concrete samples in which pozzolanic materials such as fly ash and slag were added compared with normal concrete, the time points for testing were 3, 7, 14, 21, 28, 56, and 70 days. During analysis, the UPV and strength values of specimens at an age of 70 days were adopted as the terminal UPV and strength values for concrete with pozzolanic material.  
Figure 6a displays the testing results of specimens produced according to the mixture proportions listed in Table 3 for fly ash concrete. As indicated in Figure 6a, the regression curves of the UPV and strength evolution index were nearly identical for fly ash concrete with different paste volume ratios (i.e., 36 and 42%). Figure 6b presents the testing results of specimens produced according to the mixture proportions listed in Table 4 for slag concrete. The regression curves of the UPV and strength evolution index for slag concrete with different paste volume ratios (i.e., 36 and 42%) again nearly overlap. The results of a regression analysis of Figure 6a,b reveal that changes in paste volume ratio did not affect the regression curves of the UPV and strength evolution index of concrete with pozzolanic material. However, the terminal UPV and strength values used for concrete with pozzolanic material were measurement results obtained at an age of 70 days.  
To evaluate the difference between normal concrete and concrete with pozzolanic material, the data of normal concrete specimens (NC1-NC5) with identical paste volume ratios (\(V_{p}\) = 36%) and sand-aggregate ratios (S/A = 45%) were combined with the data of pozzolanic concrete for analysis. Figure 7 displays the relationship between the UPV and strength evolution index for specimens NC1-NC5, FC1-FC5, and SC1-SC5. The terminal UPV and strength data for the normal and pozzolanic concrete were the test results at 28 and 70 days, respectively. Figure 7 demonstrates that the data points for the UPV evolution index had a strong correlation with the data points for the strength evolution index. An exponential function was then adopted for a regression analysis. The optimal fitting curve was obtained as \(\mathrm{S}_{g}=0.0599\mathrm{e}^{(0.0742\cdot V_{g})}\), where \(\mathrm{R}^{2}\) = 0.951. This result indicates that, as long as the UPV and strength values measured at 70 days were adopted as the terminal UPV and strength for the pozzolanic concrete, the relationship between the UPV and strength evolution index of normal and pozzolanic concrete should still have been able to be expressed using a single relationship curve.  
Figure 6: Effects of adding pozzolanic admixture and altering the paste volume ratio on the UPV and strength evolution index: (**a**) fly ash concrete; (**b**) slag concrete.