Shota OOTANI

Effect of Drying Shrinkage of Concrete on Stress Transfer Properties at Joints of Composite Structures

Takumi SIMOMURA

Steel-concrete composite structures are rational structural forms that demonstrate superior performance that cannot be achieved alone by using materials with properties suited to their actions. Among them, a composite structure is a structural form composed of two or more structural materials that can be regarded as behaving as a single unit. The knowledge of the properties of the materials used in the members and the static response of the joints is well established, and it is possible to design the structure to ensure safety. On the other hand, knowledge on time-dependent behavior is limited, and discrepancies between measured and calculated values have been reported for actual bridges. In recent years, it has also become clear that there exists misalignment behavior between members due to drying shrinkage of concrete and sustained stress due to prestressing, which is a concern as a factor that increases the deflection behavior of bridges. Therefore, in this paper, the effects of drying shrinkage of concrete on time-dependent deformation and stress transfer behavior of joints are investigated for a composite structure using headed studs as prestresses. The test specimens were placed in a four-stage, one-stud arrangement to accelerate shrinkage and constrain shrinkage by studs only, and the volume surface area ratio of concrete was increased compared to that of a typical test specimen. The experimental cases were a combination of static and sustained loading with and without shrinkage before the loading test.
The preloading shrinkage behavior was expected to be small due to the restraint of the studs, but contrary to expectations, it was comparable to that of the free-shrinkage specimens. This is due to the predominance of time-dependent deformation caused by local compressive forces near the studs rather than tensile forces generated by the restraints. In fact, the static test results suggest that shrinkage prior to loading may increase the bearing capacity of the structure but decrease its misalignment stiffness. The combined effects of pre-loading shrinkage and sustained loading resulted in a decrease in bearing capacity. This suggests that the effect of shrinkage may reduce the misalignment stiffness of the joints, making them more susceptible to deformation. Next, the results of the sustained tests showed a change in the misalignment behavior depending on the presence or absence of drying conditions. It was also confirmed that the response of the time-dependent component under non-drying conditions had a generally linear relationship with the load level. In contrast, the measured misalignment displacement under dry conditions showed an increase in time-dependent behavior when the load level was increased. This may be due to the influence of local stresses in the vicinity of the studs from before the loading due to drying shrinkage of the concrete.