## 1 Introduction
In general, the interpretation and validation of NDT results should involve three distinct phases [10]; processing of collected data, analysis of within-test variability, and quantitative evaluation of the property under analysis. Relevant information can be obtained by the analysis of within-test variability, by comparing the obtained results in a location with the typical one for the NDT method in use, either to provide a measure of the quality control or to detect abnormal circumstances in NDT application [10]. A good planning of a research when inspecting a concrete structure should also include the procedures for data treatment and interpretation of in-situ test results prior to the inspection. When monitoring concrete compressive strength during construction, it is usually sufficient to compare test results with the limits established by trials made at the start of the contract, but in other complex situations, like in old structures, the prediction of the actual concrete compressive strength could be required for calculation design. Depending on the purpose of the research, either for estimation of the in-situ concrete compressive strength for conformity checking, either for design calculations, many questions concerning the conversion between the mean value of compressive strength and the characteristic value or the minimum in-situ design value, or either about the safety factor coefficient to apply, may lead to complex discussions, because of the basic differences between in-situ concrete and the standard test specimens upon which most specifications are based [1; 4]. Most of the NDT give a measure of a property of concrete on the surface or near to the surface that can be related to concrete strength (surface hardness, resistance to penetration of a probe, pull-out force of a 25 mm ring placed at 25 mm depth, pull-off force to extract a cylindrical disk glued to the surface or near the surface to measure direct tensile strength, internal fracture test, among others). The obtained readings of NDT may be correlated with compressive strength experimentally. However, placing, compacting, and curing may turn the concrete in the surface zone unrepresentative of the concrete at deeper levels, and care should be taken to ensure that the correlations adopted are relevant to the circumstances of use. One of the NDT methods that does not cause any damages on concrete surface and can be used to evaluate the interior mass of concrete elements is the ultrasonic pulse velocity test. However, the ultrasonic pulse velocity in reinforced concrete is significantly affected by the presence of steel reinforcement, and this can impair significantly the results [1; 3; 4]. It is worth mentioning that, because of its versatility, the ultrasonic pulse velocity test can also be used in rocks to evaluate its mechanical properties, e.g., for structural diagnosis of old rock masonry of historical heritage building [14]. Likewise, the surface hardness test (type-L) can also be used to estimate the compressive strength in rocks for the same purpose. The number and type of variables affecting the correlations with the concrete compressive strength may differ for each NDT method. Some correlations can include correction parameters to attend for some of these variables in order to broad the range of application of the test, while others are missing with respect to this.