Influence of the existing stress state on the pull-out test for concrete structures

In the last years, prestressed concrete structures, particularly bridges, have been increasing in popularity in consequence of its versatility and the multiple advantages that they have in terms of durability, resistance and design. Inducing a state of tension and deformation, the prestressing technique improves the structural behavior, and with a low amount of steel cables allows obtaining a higher load carrying capacity. The advantages of dealing with prestressed elements extend to the possibility of reducing the dimensions of the transversal section and increasing the length of the spans between pillars. Nevertheless, the performance of prestressed structures depends on the prestressing level, related to the real state of its components, and this could be considered as a drawback which can lead into a brittle and catastrophic failure. Due to the nature of the technique, there are present some losses of prestress from the construction phases and during its serviceability life. The reductions due to friction, anchorage slip, creep, shrinkage and relaxation of the steel are mathematically predicted at the design stage through multiple theories that describe their development. The reliability on the calculations is high enough not to be worried in the future. On the other hand, there are other causes of loss such as the corrosion of the steel cables, that is not considered during the design because it is not expected to happen. This type of deterioration is strictly related to the aggressive environmental conditions. Therefore, it demands the entire attention through a constant monitoring, since steel cables are potentially well protected because of the construction methodology and the issues are not clearly visibly. The aim of the thesis lies into the assessment of the prestressing level in prestressed structures, using the pull – out test, which is a non-destructive technique commonly used for the in-situ determination of concrete strength. The test gives the tensile force allowing the extraction of a screw ion the structural element surface and can be correlated to the concrete strength. The idea behind this work is that the result of the pull-out test is influenced by the surrounding stress state, so that the extraction force will depend on the prestressing state. This conjecture is herein investigated through numerical analyses by means of the Finite Element Method (FEM). The modelling procedure is achieved by exploiting the software LUSAS. Different geometries are considered in order to maximize the effect of prestressing on the pull-out test, thus identifying the most suitable setup to carry out the test. Furthermore, the numerical model is complemented by several experimental tests, performed at the laboratory of the Department of Structural, Geotechnical and Building Engineering of Politecnico di Torino, by means of several load combinations.