## 1 Introduction  
Nuclear power plants (NPPs) have been promoted as a green power source because they are a stable supply of electricity and diminish greenhouse gases relative to other power sources. Recently, environmental events like earthquakes or typhoons have threatened the safety of NPPs; there has already been a big disaster at the Fukushima Daiichi NPP in Japan. Even though there is a serious risk of radiation, increasing the use of NPPs is regarded as a common strategy to satisfy the enormous need for electricity in the future. One possible way of preventing the disastrous leakage of radiation from NPP failure is to build stronger plants, by using innovative materials such as high strength concrete (HSC). In South Korea, one of the more proactive nations in the research and development of NPP facilities, innovative research associated with high strength concrete has been consistently performed; since 2010, researchers have strived to achieve a target concrete compressive strength of 42 MPa at 28 days [1]. Increasingly, NPPs are being constructed using HSC, which naturally creates a need for an appropriate maintenance method for NPPs. After all, during the NPP service life the structural condition should be monitoredby investigating the compressive strength of concrete. One of the main techniques for the estimating compressive strength of concrete is to apply a non-destructive test (NDT) [2; 3]. Another possible strength-assessing method is the core strength test, which is a partially destructive test. Two significant non-destructive methodologies for estimating the compressive strength of concrete are the rebound hammer (RH) test and the ultrasonic pulse velocity (UPV) method. These estimating methods are simple and effective. Thus, they have been used to estimate concrete compressive strength since 1960 [4; 5]. Recently, research on a new estimating equation for NDTs has been carried out [6; 7; 8]. For NDT methods, it was reported that the rebound or ultrasonic velocity could be substantially impacted by void condition, age and the humidity of the concrete [9]. Accordingly, large or small variations can exist in the estimation results and a statistical approach should be employed to develop the estimation equation. Due to its simplicity and practicality, most of the approaches for estimating the compressive strength of concrete are usually based on the statistical regression method. This is widely used because it can obtain a simple, deterministic equation from the tested data. Previous studies have pointed out that using the multiple linear regression method [10; 11; 12] and the combined method [13; 14; 15] in combination with ultrasonic pulse velocity and rebound hammer values (or including other influencing factors) can provide more accurate and reliable prediction of the compressive strength of concrete. Another advanced approach, using an artificial neural network can effectively improve the reliability of the estimating result by excluding the out of data [16]. Even though there have been many research efforts that aimed to estimate the strength of high strength concrete, few of the proposed equations use data from the rebound hammer test and ultrasonic pulse velocity method [17]. Thus, there is still not enough experimental data for estimating the compressive strength of HSC that is stronger than 40 MPa. The core strength test is a partially destructive test that extracts the core specimen from the massive concrete member. For normal strength concrete, some in situ core tests were carried out [18; 19]. However, such tests are rare for high strength concrete members. Because there are still hazard problems during the extracting process of the cores, some codes like ASTM and British Standards (BS) strongly recommend a minimum core diameter of 100 mm, satisfying the condition that the diameter of the core is at least three times larger than the maximum coarse aggregate size in the concrete mixture [20; 21]. For a few special cases, some standards allow smaller diameters, down to 50 mm [22; 23].