Prefabricated composite bridges a study of dry deck joints

Sammanfattning: This thesis deals with prefabricated composite bridges in general, and prefabricated concrete deck elements with dry joints in particular. As outlined in Paper I and Chapter 2 prefabrication has several advantages over in situ construction, and has hence been discussed for decades in the construction business. Further, the house building sector has taken large steps towards a more industrialized approach, in which prefabrication, lean thinking and Building Information Modelling (BIM) are all important components. Numerous studies have also examined the applicability of such an approach in the bridge sector, and several types of prefabrication techniques have been tested. Nevertheless, in many countries the bridge sector seems to lag far behind in the general shift towards more industrialized construction processes. One of the reasons for the relatively slow progress may be the fact that bridges are often unique objects with unique specifications and constraints. This hinders the standardisation that is oftenregarded as a key to industrialised construction. Chapter 2-3 and Paper I, presents evidence from a literature review together with information gathered from a Workshop, attended by bridge designers and researcher in Europe and the US, that prefabricated deck elements are still quite rarely used in bridge construction. Deck elements with dry transverse joints are even rarer. Few examples have been reported. In addition, the degree of prefabrication and the rate of progress towards more industrialised construction processes seem to vary substantially from one country to another. However, as described in Chapter 3 and Paper II, a prefabricated concrete deck element system with dry joints has been developed in Sweden for constructing composite bridges. The transverse joints are completely dry, and all forces are transferred by contact pressure between concrete surfaces. This implies that no tensional forces can be transferred over the transverse joints. Shear forces are transferred by overlapping concrete shear keys, designed as a series of male-female connections. The research presented in this thesis is focused on the structural behaviour of this deck element system. In order to investigate this, laboratory tests have been performed as well as field monitoring. Results of large-scale laboratory tests, presented in Chapter 4 and Paper V, show that a bridge of this type is less stiff than a similar bridge with an insitu cast deck slab. The concrete elements’ contributions to stiffness are negligible in sections with hogging moments, but make some contribution to global stiffness in sections with sagging moments. At moderate load levels, the interacting concrete area is much smaller than in a similar in-situ cast section. This is believed to be due to the combined effects of small gaps in the joints and continuous in-situ cast concrete in the injection channels. After the channels have been injected, existing gaps will be more or less permanent, since the in-situ cast concrete must be compressed up to a certain limit before the rest of the joint will be closed. Destructive testing showed that the differences in stiffness and stresses between a deck of this type and an in-situ cast bridge deck are much smaller in the ultimate limit state. In this case it could even be reasonable to design a cross-section according to Eurocodes, neglecting effects of the joints. As shown in Chapter 5 and Paper III, the overlapping shear keys are a critical detailing in this deck system. Therefore, they were tested in the laboratory to determine how they fail and evaluate their load capacity. The tests revealed two failure modes. The first is a rather ductile failure, activating the shear reinforcement. This was the expected failure mode for shear keys of this design. The second failure mode observed was a quite brittle failure in the concrete covering layer. It has only been observed in small-scale tests, and might be related to the test set-up. Nevertheless, overlapping of the rebars in the male-female shear key connection is strongly recommended to assure the robustness of shear transfer if failure occurs in the concrete covering layer. To complement the laboratory tests, a single span bridge was monitored in the field (Chapter 6 and Paper IV). The bridge was built in 2000, using the prefabricated deck system that this thesis is focusing on, and was tested in both 2001 and 2011. The tests, and subsequent Finite Element analyses, showed that under moderate loading the interacting concrete area is smaller than for a similar in-situ cast bridge. No significant long-term effects were observed, except that under eccentric loading the distribution of the deflection between the girders decreased slightly during the 10 years between tests. This indicates that the joint gaps may have narrowed and at least partly closed during this time. Chapter 7 summarises the research and presents recommendations for dealing with general issues related to the design and construction of a bridge of this type. The design methods are generally the same as for a conventional composite bridge with an in-situ cast deck slab. However, the Eurocodes require some modification for the design of prefabricated deck elements with dry joints, particularly regarding global analysis and the resistance of cross-sections. Finally, conclusions, a general discussion and suggestions for further research are presented in Chapter 8.