## 4 Verification of the Applicability of the Unified Equation
**} & \multicolumn{2}{c}{**Day 1**} & \multicolumn{2}{c}{**Day 3**} & \multicolumn{2}{c}{**Day 4**} & \multicolumn{2}{c}{**Day 7**} & \multicolumn{2}{c}{**Day 9**} & \multicolumn{2}{c}{**Day 14**} & \multicolumn{2}{c}{**Day 28**} & \multicolumn{2}{c}{**Day 56**} \\ \cline{2-13}  & **S** & **V** & **S** & **V** & **S** & **V** & **S** & **V** & **S** & **V** & **S** & **V** & **S** & **V** \\ \hline PNC1 & 23 & 3900 & & & 48 & 4437 & 51 & 4477 & 55 & 4485 & 56 & 4500 & 59 & 4564 & \\ PNC2 & 14 & 3633 & & & 31 & 4252 & 39 & 4354 & 39 & 4386 & 43 & 4401 & 48 & 4456 & \\ PNC3 & 7 & 3245 & & & 16 & 3972 & 24 & 4092 & 25 & 4193 & 28 & 4241 & 31 & 4290 & \\ PNC4 & 3 & 2739 & & & 11 & 3800 & 15 & 3993 & 18 & 4040 & 20 & 4118 & 22 & 4198 & \\ PNC5 & 23 & 3862 & & & 45 & 4300 & 51 & 4389 & 51 & 4406 & 53 & 4340 & 62 & 4517 & \\ PNC6 & 11 & 3552 & & & 28 & 4147 & 34 & 4293 & 35 & 4295 & 40 & 4326 & 46 & 4389 & \\ PNC7 & 8 & 3310 & & & 18 & 3974 & 23 & 4115 & 24 & 4190 & 24 & 4246 & 29 & 4306 & \\ PNC8 & 4 & 3026 & & & 9 & 3680 & 14 & 3916 & 14 & 3968 & 15 & 4073 & 18 & 4159 & \\ PNC9 & 25 & 4011 & 37 & 4271 & & & 45 & 4470 & & & 49 & 4551 & 57 & 4624 & 62 & 4657 \\ PNC10 & 10 & 3572 & 21 & 3945 & & & 26 & 4202 & & & 32 & 4278 & 36 & 4391 & 41 & 4433 \\ PNC11 & 25 & 3936 & 35 & 4287 & & & 42 & 4430 & & & 43 & 478 & 48 & 4542 & 49 & 4574 \\ PNC12 & 10 & 3440 & 18 & 3971 & & & 24 & 4206 & & & 28 & 4245 & 34 & 4346 & 38 & 4279 \\ PNC13 & 23 & 3990 & 37 & 4355 & & & 50 & 4459 & & & 57 & 4560 & 60 & 4592 & 67 & 4659 \\ PNC14 & 7 & 3385 & 17 & 3904 & & & 24 & 4123 & & & 29 & 4225 & 31 & 4320 & 35 & 4389 \\ PNC15 & 24 & 3915 & 36 & 4301 & & & 50 & 4430 & & & 54 & 4481 & 58 & 4552 & 58 & 4628 \\ PNC16 & 6 & 3261 & 15 & 3898 & & & 23 & 4122 & & & 27 & 4223 & 33 & 4302 & 33 & 4381 \\ PPC1 & 20 & 3967 & 36 & 4285 & & & 46 & 4469 & & & 46 & 4513 Further exploration indicated that estimations using the unified equation seem to have higher percentages of error in concrete specimens at ages less than 3 days. Concrete at ages between 1-3 days exhibits the highest hydration rate, and the influence of different water-to-cementitious material ratios and levels of pozzolanic material content on variations in concrete UPV and strength is also the most conspicuous during this period. Therefore, using the unified equation of the relationship between UPV and the strength evolution index for concrete in this period incurs a relatively high level of uncertainty. For practical applications in construction, the execution of major construction procedures such as applying post tensioning or removing formwork supports requires the concrete to achieve a specific level of strength. The aforementioned construction procedures are usually carried out three days after the concrete is placed. At this time, the strength estimation accuracy and reliability of the unified equation are both high. In particular, in the actual application of the unified equation, only the terminal UPV and strength of the same batch of concrete need to be obtained; the mixture proportions of the concrete and the UPV-strength relationship curve are unnecessary. This method should be easy to execute for the nondestructive testing and quality control of concrete strength in construction projects. Finally, it is necessary to reiterate that the innovation of this research is to find a unified equation to describe the relationship between the UPV and strength evolution index. For practical application, the terminal UPV and strength need to be known first. For a large construction project that takes several years, the terminal UPV and strength of the concrete used in the project can be measured at the early stage of construction (28-day or 70-day age of concrete). In the following years of construction, the unified equation proposed in this study can be used to predict the strength of concrete at different ages for the execution of major construction procedures and quality control.