## 1 Introduction  
In construction practice, it is often necessary to test new or existing structures in order to assess the quality of the construction materials used. Concrete is one of the most used materials [1; 2; 3; 4]. The study and determination of the main characteristics of concrete, as well as the tracking, evaluation, control and diagnosis of its actual condition is necessary to ensure reliability, durability, safety, security and good service quality in building structures. In practice, it is necessary to evaluate many parameters of concrete, such as its compressive strength, modulus of elasticity, the probability of corrosion, carbonation, homogeneity and changes in the characteristics of concrete occurring with time, as well as evaluations of construction quality and structural integrity through the detection of voids, cracks, honeycomb and other defects. The assessment of concrete's current state [4; 5; 6; 7; 8; 9] most often involves the complex application of various methods in order to obtain the most reliable results. Choosing an appropriate evaluation technique involves seeking a compromise between performance, cost, the accuracy of results and objective limitations. There are many methods for evaluating, measuring, testing and controlling the parameters and characteristics of concrete and they can be used in all phases of construction and at any time in the service of buildings and facilities [3; 4; 9; 10]. Depending on the method of impact used and the condition of the structural elements after the test--whether they are damaged and/or affected--there are two types of methods: destructive and non-destructive. To increase the reliability of test results, it is necessary to comprehensively apply combinations of destructive and non-destructive methods [1; 3; 6; 7; 10; 11; 12; 13; 14; 15; 16]. One of the most important and key properties of concrete is its compressive strength. Traditionally, the determination of compressive strength is carried out by means of destructive tests of cubes with dimensions determined according to a standard and made on site on the day of laying the concrete mix according to EN 12390-2 [17] in the reinforced concrete elements, or with cores taken from the reinforced concrete structures [18; 15; 16; 18]. Destructive methods provide direct and accurate information on the value of compressive strength, but they are more expensive, more labor-intensive and sometimes impossible to apply for reasons of damage limitation and the possibility of weakening the existing reinforced concrete structures. Non-destructive methods (NDM), especially in field testing, have been increasingly used in recent decades due to their rapidity, low cost and easy application [1; 2; 3; 4; 6; 12; 19; 20; 11]. A disadvantage of these methods is that they are indirect. Despite the fact that NDM studies have been carried out by many researchers for decades, achieving an efficient and reliable estimation of concrete parameters remains a controversial issue and is a goal that has been pursued by many specialists. Combining destructive and non-destructive test results, which are not equally sensitive to the various factors influencing compressive strength, is still a matter of discussion. Due to the fact that concrete is a non-homogeneous material, a unique assessment model for all concretes does not exist. For the effective conditional assessment of concrete in recent years, a measuring technique based on the achievements of electronics and computer software is used, which enables long-term observations and the automation of measurements, increasing the accuracy and the ability to collect, store and transfer data. In this article, the author presents experimental results regarding the determination of the compressive strength of concrete samples of different ages--28, 244, 280, 293, 342, 1126 and 1926 days--using destructive and non-destructive methods (the elastic rebound method, the ultrasonic pulse velocity method and the SonReb method).