Resumen de Theoretical and experimental study of the behaviour at service and failure of partially prestressed concrete beams under flexure and shear
Partially Prestressed Concrete (PPC) is an intermediate design approach between reinforced and totally prestressed concrete, that enables controlled cracks in service phase. Conversely, totally prestressed concrete is design to prevent cracks in service. This design strategy can be useful for structural optimization, as allows for more freedom in the variation of stiffness, crack opening, stress level and resistance. This technique has been studied during the past decades. However, its extensive use has been limited due to the lack of methodologies to control the crack width. This thesis aims to analyse the behaviour of PPC under flexural and shear forces.
In this research, the behaviour of PPC elements in bending and shear is investigated through an an experimental campaign of sixteen tests. The specimens were design to represent the webs of a girder box bridge, hence the I section of the beams. From the sixteen tests, twelve studied the shear behaviour of PPC beams and the influence of web width, prestress ratio, stirrup ratio and lay out. The remaining tests were four-point bending tests. The aim of these tests was to evaluate the flexural behaviour at service and failure by changing the prestress and longitudinal reinforcement ratios.
For each test, the loading protocol performed cyclic loads to analyse the evolution of the behaviour of the specimens at service. The cycles presented three steps corresponding to quasi-permanent, frequent and characteristic load level. Each step was used to analyse and compare the evolution of the main parameters that characterise the behaviour of the beams. After the cycles, the load was monotonically increased until failure.
Several models have been presented to explain the shear resistance mechanism for sections with flanges, with very different results. The Compression Chord Capacity Model (CCCM) presented the best predictions for this experimental campaign.
A photogrammetric technique was developed to analyse the shear crack pattern. This technique obtains from the pictures the crack spacing, the crack angle and the crack width. With this tool, an equation to predict the cot¿¿ at service and failure was obtained. The equation relates the angle of the cracks with the ratio between compression due to prestress and tensile strength and the ratio of stirrups. This technique allowed to obtain the influence of the reinforcements over the crack shear width.
A model to predict the average shear crack width in service was developed using the average strain of the stirrups, the experimental crack spacing and the angle of the cracks. Strains were calculated taking into account the shear contribution of the stirrups.
From the experimental results, it was evidenced that in the shear tests at service, the strains in the stirrups were over the yielding threshold. Therefore, the shear cracks were large. Meanwhile the strains in the four-point bending tests only reached the yielding level in failure, being stable for the service part of the test.
