A large part of the bridges in the Netherlands have been built in the 50s and 60s of the last century. These bridges have to be renovated because they are reaching the end of their designed lifespan and they are not designed for the more and heavier current traffic This is especially relevant for the steel bridges because this could lead to problems due to fatigue. A common method for renovating the bridge is by using a FRP bridge deck, which has a high strength to weight ratio and have good fatigue resistance. Since this is a relatively new method, a lot of research has been done for connection methods between the FRP bridge deck and the steel superstructure of the bridge. Two example of these connectors are the Lindapter Hollo bolted connector and the iSRR connector, which are both blind-bolted connectors. The iSRR connector uses steel reinforced resin to create an efficient, slip-resistant connection. This a relatively new connection method, therefore there is not a lot of knowledge about the long-term behavior of this connector. To investigate this, the iSRR connector has been installed alongside the Lindapter Hollo bolted connector at the Ulsderbrug in Groningen. These preloaded connectors are designed in such a manner that they only transfer tensile forces. These bolted connectors are monitored to compare both the short-term and long-term behavior. In addition to the monitoring a finite element model is made, to investigate to predictability of the bolt forces in the iSRR connector.
From the monitoring can be concluded that the iSRR connector shows less loss of preload over a period of 250 days, retaining 50.6% of the original preloading force. Compared to 42.7% for the Lindapter Hollo bolted connector. These losses of preload are significantly impacted by a rubber interlayer that is present between the clamping plate and the FRP bridge deck. Due to the relaxation of this rubber layer, only 77.5% of the applied preloading force is present one day after applying. The force variations, of up to 0.12 kN, due to traffic loads are relatively small and similar for both connector types. These forces are also affected by the rubber interlayer. Imperfections at these layers cause one bolt to experience force cycles up to 1.2 kN, which are ten times higher than for the other bolts. The influence of the stiffness of the rubber interlayer has been investigated using the finite element model. From this model can concluded that a lower stiffness of this layer leads to higher forces in the connectors. This model has been used to investigate the extrapolate the bolt forces for the most unfavorable stiffness of the rubber. Even for this conditions the lifespan of the bolts and the connectors is not affected by the forces due to traffic loading.
In conclusion, the results show that the iSRR bolts retains more of the applied preloading force over a long period than the Lindapter Hollo bolts. Short-term behavior has been similar between the bolts but has significantly been impacted by a rubber interlayer between the clamping plate and the FRP bridge deck. For further research it would be interesting to optimize the positioning of the rubber layer and investigate how this impacts the results.

Steel-concrete composite (SCC) slabs are extensively used in construction, particularly in multi-story buildings, due to their efficiency and structural performance. SCC slabs combine the strengths of steel and concrete, where steel provides tensile strength and concrete offers compressive resistance. This integration results in lightweight, robust structures that save costs and allow for taller buildings with larger spans.
New designs of steel-concrete composite slabs have to be extensively tested before the new designs are approved. It is therefore important that information is gathered during testing. Strain measurements are often done on composite slabs, for example, strain gauges have been applied to the steel sheeting and distributed optical fibers (DOFs) have been applied to the reinforcement rebars. However, there have never been distributed strain measurements on the steel sheeting of steel-concrete composite slabs. This research aims to gain new insights into the structural performance and behaviour of deep steel-concrete composite slabs using distributed strain measurements.
SCC slabs often have embossed regions of steel sheeting, these embossments or indentations transfer shear forces between the steel sheeting and the concrete. DOFs were applied on the embossed parts of the steel sheeting in the direction of the span in four composite slabs. The strain data showed oscillations in the strain values, and that the embossed/indented parts of the steel sheeting were in tension, while the flat areas were in compression. Local deformations, due to the geometry of the embossments, caused the oscillations.
The N.A. position was another critical focus of the study, which was determined using fiber data at five locations on the middle rib of three slabs. The N.A. was determined for three loading stages: initial, elastic, and plastic. The fibers captured the upward movement of the N.A. from the steel sheeting into the concrete during the initial and elastic stages, as expected according to theoretical calculations of the N.A. position. However, in the plastic stage, the N.A. shifted downward, back into the steel sheeting, instead of continuing its upward trajectory, as expected. This unexpected shift was attributed to partial shear interaction, where the steel sheeting and concrete began acting separately, leading to separate strain
profiles.
Buckling of the steel sheeting was observed in two slabs, Slabs 10 and 11, with fibers providing critical insights into the timing and extent of buckling. In Slab 11, where four fibers were located in the buckling zone, two fibers showed early signs of buckling in the top flange at 97% and 98.3% of the peak load. The strain patterns indicated buckling before the peak load was reached, offering valuable data on the onset of this failure mode. For Slab 10, buckling was detected after the peak load.
Lastly, the research explored the use of DOFs for crack detection. Fibers were applied to the bottom of the ribs on one slab to identify crack locations based on strain data. While cracks typically cause localized peaks in strain, the results revealed limitations in the accuracy of crack detection using DOFs on steel sheeting.
In conclusion, this research demonstrates the potential of DOFs for strain measurement in steel-concrete composite slabs, offering valuable insights into strain distribution, N.A. position, and buckling behaviour. However, the crack detection capability of DOFs, particularly when applied to steel sheeting, requires further refinement to improve accuracy and reliability.