## 5 Strength Correction Factor for Concrete Core Strength  
For the maintenance of concrete strength evaluation, the concrete core specimen is generally considered to be the direct representative specimen for evaluating the compressive strength of concrete within an in situ structural member. According to the specifications British Standard Institution (BSI) [21], International Union of laboratories and Experts in Construction Materials, Systems and Structures (RILEM) [5] and Architectural Institute of Japan (AIJ) [4] for NDT, a non-destructive test is recommended for estimating the compressive strength of concrete members, based on the correlation of the strength obtained by core strength tests. One of the most significant concerns in using a core specimen is that it is easily damaged by the high speed drill bit during the extracting process of the cores [41]. The core strength test essentially partially damages the target concrete member. A concrete core diameter of 100 mm is generally used, however, the practical concrete core size is usually limited by the installation of internal reinforcement, such as pre-stressed tendon. Another limitation is the position where the core concrete can be extracted from: it is necessary to minimize partial damage in parts of the member where the stresses concentrate. In these conditions, a smaller core size specimen is a useful option, because it is causes much less partial damage so that the structural integrity of concrete member can be guaranteed after the extracting process of the cores. The use of a small-size core specimen must be quite effective in the maintenance of NPP structures made of HSC. When estimating the compressive strength of HSC using a small-size core specimen, the strength correction factor between the smaller diameter specimen and the 100 mm concrete cylinder should be evaluated. Figure 7 shows the three diameters of core specimen that were evaluated in this study. Concrete core specimens were obtained from a square concrete block without steel reinforcement. The block is 600 mm long, 600 mm wide and 200 mm thick, as previously shown in Figure 2.  
Table 8 summarized compressive core strength of concrete according to the mix types and core diameters. Considering that the maximum size of the coarse aggregate is 20 mm, a minimum core size of 30 mm is reasonable for investigating the compressive strength of concrete using a compressive test. As the diameter size of the specimen decreases, the compressive strength of the core specimens also decreases. Figure 7: Extracted core specimens for diameter of 30, 50, 100 mm. This trend is in agreement with previous research, including an observed higher coefficient of variation (COV) [25]. This strength discrepancy is deeply related with degree of coarse aggregate. Coarse aggregate is the source that enables excellent interlocking mechanisms within concrete members. For instance, 25 mm diameter coarse aggregate is usually used for concrete mixing. If the concrete core diameter decreases to below 25 mm, the probable chance of having 25 mm aggregate within the core specimen also decreases. Accordingly, the estimated compressive strength of the concrete core specimen must be lower, as there is less coarse aggregate to resist against the compressive loading. The minimum size of the core diameter will have to be appropriately determined by considering the maximum coarse aggregate used in the target concrete structures for core strength tests. Figure 8 exhibits the strength correction factor between concrete core specimens and the 100 mm standard concrete cylinder. For 100 mm specimens, the strength correction factor between the core and cylinder ranged from 0.92 to 1.02. However, the 30 and 50 mm core specimens had lower evaluated strength than those of the concrete cylinder specimens. One possible reason for the strength reduction is that water particles inside the concrete are gradually eliminated due to drying and proceeding hydration mechanism so that a number of micro voids remain.