# Assessment of Strength Development at Hardened Stage on High-Strength Concrete Using NDT
## 1 Introduction  
The use of concrete facilitates the construction of buildings and structures that can withstand high compressive loads. The strength of concrete can be attributed to the formation of hydrates (i.e., solid products formed from the reaction between water and cement) that gradually fills the porosity of the material. Furthermore, the compressive strength of concrete is typically evaluated via the destructive testing of cylinder molds that are collected in the field in accordance with the relevant criteria of each country [1; 2; 3; 4; 5; 6].  
The strength of concrete is significantly affected by various environmental factors such as the surrounding climate, temperature, and humidity. Furthermore, during construction, various quality issues may arise, owing to factors such as temperature variation, poor compaction, and careless construction. In consideration of such problems, researchers have generally recommended measuring the compressive strength of concrete by conducting destructive tests on the concrete cores that have been extracted from structures for a more precise diagnosis [7; 8; 9]. The extraction of concrete cores is a useful approach for accurately estimating the compressive strength; however, this approach may lead to problems such as structural instability and the subsequent repair method.  
Thus, several approaches have been developed for structural maintenance through nondestructive tests (NDT) [10; 11; 12; 13]. Such test methods aim to evaluate the structural integrity without the need for destructive tests and primarily evaluate the surface hardness of concrete and the dangers of internal cracking. Furthermore, various studies have been conducted to nondestructively estimate the compressive strength of concrete [14; 15; 16; 17].  
Although many NDT methods are used to estimate the compressive strength of concrete, tests measuring the rebound value and ultrasonic pulse velocity are mainly used when considering the portability of equipment and ease of use. In addition, the rebound and ultrasonic methods are economical. The method based on the rebound value is referred to as an impact method, while the ultrasonic pulse velocity method is a nonimpact method.  
In the context of the application of these two methods, [PERSON] et al. proposed a strength estimation formula based on the rebound value [18; 19; 20; 21; 22; 23], and further studies also reported improved precision in strength estimation through analytical methods [24; 25; 26; 27]. [PERSON] et al. derived the relationship between the compressive strength and ultrasonic pulse velocity [28; 29; 30], and [PERSON] et al. proposed modifications considering the influence of various materials such as cement and aggregate [31; 32; 33; 34; 35].  
These research results are very frequently applied to diagnose structures, as they form the basis of the building criteria laid down by the USA, Europe, Japan, and other countries; however, their reliability is still questionable. In addition, the design strengths of concrete have increased with the corresponding increase in the height of buildings, which has led to the use of various admixtures to improve the concrete strength. Therefore, the accuracy of nondestructive inspection as it relates to the assessment of concrete strength needs to improve.  
In most cases, the existing NDT estimation formula is set as the result of a small mold measured in a laboratory. In the case of an actual structure, it is necessary to clearly examine the relationship thereto because it may be different from the environmental conditions of the laboratory.  
Against this backdrop, herein, we propose two models for the compressive-strength estimation of concrete within a compressive-strength range of 24-60 MPa by using the rebound value and ultrasonic pulse velocity. To this end, we prepared cylinder molds and mock-up members and subjected them to NDT. In addition, we conducted destructive tests to examine the correlation with the compressive strength based on the measured values.