## 1 Introduction  
Since the middle of last century, concrete has undergone several developments and continued to show to be a remarkably versatile material for many applications in civil constructions. Nowadays, concrete still continues to be the preferential construction material to integrate many structural members for different kinds of constructions and infrastructures, such as buildings, bridges, dams, among others. In particular, many architectural structures have been and still are built using concrete as the key building material to achieve often complex spatial geometries. In this particular case, self-compacting concrete (SCC) has been widely used because it can be placed easily in complicated formwork and with high degree of reinforcement without the need of vibration. In addition, SCC produces a smooth and well-finished surface at the end of concreting, which is an important factor in many architectural structures with exposed concrete. In the last years, maintenance and rehabilitation concerns have also covered concrete as a building material. Its preservation is essential to ensure the stability of the structure, so as not to impair its use nor jeopardize the safety of the users during the intended lifetime. The need to ensure the quality of concrete structures, the verification of aging of older structures and the premature degradation of recent ones justify, because of the associated high costs, special attention to the study of maintenance and rehabilitation issues. Nowadays, if existent concrete structures show unacceptable deterioration, a correct assessment of the actual state of concrete is required to quantify the available safety margin. The effective concrete compressive strength is usually a key parameter that is required to perform this evaluation. Other situations that require an evaluation of concrete strength include the quality control of precast or in-situ concrete application, namely to decide when handling and transport precast units, to evaluate the concrete compressive strength for application of prestress, or to support the decision to remove formwork or temporary supports for structural elements. The use of non-destructive tests (NDT) to evaluate concrete quality and to estimate its in-situ compressive strength has been well-known for some decades [1; 2; 3; 4; 5]. Many of these tests and devices were initially developed for normal vibrated concrete (NVC) of normal strength range. However, in the nineties of the last century, some of these tests and devices were adapted for high-strength NVC [6; 7; 8; 9]. Specific studies concerning the application of NDT in SCC are still scarce [10]. The procedures to apply the most firmly established NDT methods can be found in the normative documents from different countries. However, it is important to mention that such procedures can present small differences between them and the selection of the most appropriate method should be decided previous to test, to avoid divergences when interpreting the results [4; 5]. The range of available tests vary from the most economical, simple, and easy to use (e.g., surface hardness test, ultrasonic pulse velocity test, and pull-out test using Lok-test system), to the most complicated (e.g., pull-off test using Bond-test system with partial coring and pull-out test using CAPO-test system) and expensive ones (e.g., Windsor Probe Test System and Maturity meter). Careful selection of the types of tests to be combined in each situation is critical to achieve both accuracy of results and cost savings [1; 3; 4]. When such tests are applied, it is necessary to evaluate the variables which can affect the test results and the correlations. Some tests are more sensitive and/or reliable than others, but all of them can differ in terms of the within-test variability and repeatability of test results [11; 10; 12; 13].