## 5 Strength Correction Factor for Concrete Core Strength
These pores within the structure may develop a critical damage interface. Once the drill bit penetrates the concrete member, the high speed rotating impact deeply affects the damage acceleration of concrete around the pores then the integrity of concrete worsens compared to the standard concrete cylinder. There are a few research data on strength correction factors between the concrete core and the standard cylinder, and also few data on high strength concrete. Because of the uncertainty in the strength evaluation of the specimens obtained from the extracting process of the cores, the strength correction factors between the concrete core and the standard cylinder should be further investigated, and an empirical strength relationship needs to be researched for the maintenance of high strength concrete structures. From Figure 8, as the concrete strength went to mix 1, the average strength ratio was increased. Thus, the damage in extracting core may be reduced rather than that of stronger specimen of mix 3. This can be explained that higher power and rotating speed is needed for mix 3 specimens so it may affect even more damages inside concrete. \begin{table}
\begin{tabular}{l l l l l} \hline \hline \multirow{2}{*}{**Mix Type**} & \multirow{2}{*}{**Concrete Block No.**} & \multicolumn{2}{l}{\(f_{cor}\) (MPa)} & \\ \cline{3-5}  & & **30 mm** & **50 mm** & **100 mm** \\ \hline Mix 1 & 1 & 44.7, 31.6 & 56.3, 45.0, 51.0 & 48.9, 51.7 \\ - & 2 & 52.8, 38.6 & 67.3, 46.2 & 62.3, 56.9 \\ - & 3 & 32.8, 39.8 & 36.9, 37.1, 38.2 & 57.1, 48.4, 54.7 \\ - & Average & 40.1 & 47.3 & 54.3 \\ \hline Mix 2 * & 1 & 36.2, 31.3 & 32.0, 32.1 & 46.4, 53.6 \\ - & 2 & 40.7, 37.0 & 42.5 & 35.0, 33.3, 40.1 \\ - & 3 & 39.9, 41.6 & 37.5, 35.3, 40.1 & 32.4, 35.8, 44.8 \\ - & Average & 37.8 & 36.6 & 40.2 \\ \hline Mix 3 & 1 & 49.8, 94.8 & 43.8 & 55.3, 54.3 \\ - & 2 & 80.3, 50.3 & 55.8, 67.7, 54.0 & 78.7, 76.2 \\ - & 3 & 42.7, 41.9, 55.3 & 44.9, 42.9, 44.5 & 54.2, 58.8 \\ - & Average & 59.3 & 50.5 & 62.9 \\ \hline \hline \end{tabular}  
* Mix 2 is not included for the evaluation of strength correction factor.  
\end{table} 
Table 8: Compressive core strength of concrete according to the mix types and core diameters. Figure 9 classifies the average compressive strength according to the three different core diameters used in this study. Core specimens made from mix 2 exhibited out of strength results, which might be caused by some problems in the extracting process. Figure 10 shows the average strength correction factor (\(\gamma\)) defined as the ratio of the core strength (\(f_{core}\)) to the cylinder specimen strength (\(f_{c}^{\prime}\)). Mix 2 specimens were excluded when calculating this strength correction factor, due to the reason stated above. Therefore, the strength correction factor is obtained by following expression as \(\gamma=f_{core}/f_{c}^{\prime}\). The average \(\gamma\) was obtained as 0.73 for 30 mm, 0.79 for 50 mm, and 0.94 for 100 mm. The \(\gamma\) equation can simply convert the compressive strength of concrete core specimens to the compressive strength of the concrete cylinders at 28 days. This correlation equation is convenient to use, without needing destructive data on the in situ concrete cylinder, and the strength correction factor can be continuously updated as the tested dataset increases. This equation can be used, in combination with smaller in situ core specimens, as a maintenance strategy for evaluating the designed compressive strength of high strength concrete within NPP structures. Figure 8: Compressive core strength of concrete and strength ratio for the three types of mix proportions. Figure 9: Compressive strength of concrete according to the diameters of 30, 50, 100 mm.