## 2 State of the Art of NDTs for Concrete Prestress Evaluation  
This section presents a critical review of current non-destructive techniques (NDTs) that can be used to evaluate the prestressing of internal PT systems, highlighting the advantages and limitations of each technique. The methods discussed include those recommended by the Italian Guidelines for Special Inspection [8], developed based on the North American Federal Highway Administration Guideline [9], and the Highways England CS 465 [10] and CS 464 [11]. Current NDTs for assessing residual prestressing force in existing structures can be categorized into stress release methods and the X-ray diffraction (XRD) method. Stress release methods estimate prestress by measuring the stress relieved when the continuity of a structural member (such as the concrete surrounding tendon ducts, or a wire of the tendon) is mechanically interrupted. These methods include the exposed strand method, the drilled hole method (or stress release coring), the saw-cut method, and the flat-jack test. It is worth noting that all these tests are considered as non-destructive because the affected areas are small in comparison with the dimensions of the structural member and are usually repaired after the test with minimal long-term impact on the overall structure. The exposed strand method [12] estimates the prestressing force in a strand by comparing its load-displacement curve, obtained using a custom instrument applied to the strand, with a calibration diagram. Although the procedure appears straightforward, its accuracy heavily depends on the specific conditions under which the calibration data are collected, such as the strand's dimensions, type, and exposed length. References on the application of this method date back to 1998 [12], with no recent applications found in the literature. Moreover, as the name suggests, this method requires the strand to be exposed. This involves removing the concrete, opening the duct, and extracting the grout to access the strand. After the test, the concrete must then be repaired. Similar to the exposed strand method, the XRD method [13] also requires strand exposure, but it operates through a different physical principle. In this technique, X-rays are shot directly at the wire of the strand to measure the strain within the crystal lattice of the steel, allowing for an estimation of stress based on the material composition and elastic properties. XRD is widely used in materials engineering to analyze the crystalline structure of materials. In recent years, interest has grown in applying it to PT cables. Some exploratory studies [14; 15] have obtained reliable results using XRD for this purpose. Reference [13] reported errors of about \(\pm\)10% for typical working loads (corresponding to about 60% of the yielding strength of steel) and of \(\pm\)30% for low working loads. However, the application of the XRD method to large-scale concrete structures faces substantial challenges. The most significant limitation is its invasiveness and the requirement for direct access to the steel tendons. To use XRD for measuring stress in the prestressing steel, the concrete cover, grout, and often even the duct must be removed to expose the steel strands. This makes it a highly destructive method for practical in situ assessment. Beyond invasiveness, the operational complexity is considerable. XRD equipment is typically laboratory-based, often large and sensitive, making it difficult to transport and operate on site, especially in challenging bridge environments. The technique also involves the management of ionizing radiation, requiring strict safety protocols and specialized training for operators. Its application is generally limited to small, exposed areas, making it impractical for assessing the overall prestress distribution across a large structural element.