Fatigue model for prestressed concrete decks

Abstract

A high number of concrete bridges that were built in the Netherlands around half a century ago and which had a transversely prestressed thin deck, nowadays raise a series of questions regarding their structural integrity. This is because their service life has been affected over time by a substantial rise in the number of vehicles that transit them and the considerable increase in the loads applied by these vehicles upon the structure. The concern amplifies when it is highlighted that these structures were designed according to old codes that were valid during that period, codes that have been substantially modified in the course of time. Having all this that needs to be taken into account, it is of high interest to determine how safe these structures are today. One phenomenon that in time would have left its mark on the structural integrity of these bridges is fatigue. This can be defined as the deterioration in time due to a repetitive loading. A large array of studies were dedicated to the topic of fatigue of concrete, aiming at a better understanding of its behaviour, impact and consequences upon a structure. However, throughout this extensive research that also analysed failed structures, fatigue was never found to be the primary mechanism that led to failure, but its contribution was acknowledged and, as a consequence, investigations on this topic are still of current interest. Of great importance is also the phenomenon of compressive membrane action (CMA) that is characteristic to concrete slabs and has a beneficial influence on the structure, leading to an increase in the capacity, being highly influenced by the degree of restraint At Delft University of Technology, a couple of research programs that were performed recently tried to shed some light on both of the previously described topics. The study done on fatigue gave promising results, with its outcome being found to fit together with other researches performed worldwide. A follow-up study based solely on fatigue will start in the near future, this having as objective the impact of the loading sequence. In anticipation of this new study, in order to see to what extent the previous results can be replicated numerically and future ones forecasted, a FEA model for fatigue assessment and prediction was created and its accuracy was evaluated, this being the scope of the Master’s Thesis. For the development of the finite element model, departure points had to be set for each main phase. For the construction and calibration phase, the point of departure meant the use of a two-dimensional model for describing the behaviour of the real life structure, namely the evolution of stresses, deterioration and failure mode. For the second phase, the one related to replicating and assessing the fatigue damage, the departure point meant assuming a uniform alteration in the material properties due to the cyclic loading, modification considered at a structural level.