# Verification of Actual Prestressing in Existing Pre-Tensioned Members
###### Abstract  
In the case of prestressed concrete structures, information about the actual state of prestressing is an important basis for determining their load-carrying capacity as well as remaining service life. During the service life of the prestressed concrete structure, the initial level of prestressing is inevitably reduced as a result of the actions of various factors. These reductions of prestressing force are considered as prestress losses, which are influenced by construction stages, used materials, prestressing technology, or required length of service life. Available standards enable the determination of the expected values of prestress losses. Ultimately, their calculation is part of the design procedure of every prestressed concrete structure. However, aging and often neglected infrastructure in Europe is also exposed to factors, such as environmental distress, that are not considered in standard calculations. Therefore, verified and reliable methods for determining the actual state of prestressing are needed. This paper presents an experimental program of an indirect method for the evaluation of the value of prestressing force in seven prestressed concrete sleepers. Particularly, the non-destructive saw-cut method as a pivotal object of this study is performed and assessed. Furthermore, the Barkhausen noise technique is used as a comparative method. Subsequently, the experimental campaign is supported by the numerical analysis performed in the ATENA 3D software. Finally, the experimentally determined values of residual prestressing force are compared to the expected level of prestressing according to Eurocodes. prestressed concrete; prestressing force; assessment; non-destructive testing; saw-cut testing; Barkhausen noise technique +
Footnote †: journal: Applied Sci. 0000-0001-59  
long-span bridges or the reduction of cross-sectional dimensions of the member. Moreover, in contrast to reinforced concrete, compressive stress in prestressed concrete results in a crack-free structure with preserved stiffness, lower deflections, and better protection against environmental distress. However, the design process is more complicated, and a higher level of quality control is required [1; 2; 5]. Two prestressing technologies are recognized. In the case of pre-tensioned concrete structures, the member is cast around the straight strands or wires that are tensioned between stands. The prestressing force is transferred into the pre-tensioned structure through the bond between the steel reinforcement and concrete. This technology finds its application in short-span or medium-span bridges, structures, or railway sleepers. The second prestressing technology is so-called post-tensioning. In this case, the prestressing force is applied by jacking steel tendons against an already-cast member, so the prestressing is transferred through built-in anchorages. Moreover, if the space between the tendon and the duct is injected with cement grout, strands are protected against corrosion and the prestressing is transferred also through the bond between the steel strands or wires and concrete. This technology enables the design of long-span structures and curved tendons and thus provide suitable redistribution of internal forces by changing the eccentricity of the prestressing force [2; 6]. In Slovakia, the first attempts to introduce prestressed concrete bridges date back to the second half of the previous century, so currently, these structures are approaching 60 years of service [7]. Generally, their present structural condition is not appropriate. This fact is closely related to their maintenance, inadequate inspection, external loads, and careless conceptual design [8; 9; 10]. In the coming decades, many aged bridges will require the evaluation of their load-carrying capacity and remaining service life. This will result in the application of reliable methods for the evaluation of residual prestressing force.