The effect of confining forces on the resistance of headed bars in tension in Reinforced Concrete elements

The use of headed reinforcing bars has been limited to heavily reinforced structures such as offshore oil platforms, but it offers distinct advantages in ordinary reinforced structures such as reducing the required development lengths, enhancing the confinement of joints, and reducing reinforcement congestion, thereby facilitating the placement of reinforcement bars on site and concreting. The primary mode of failure for headed bars is concrete blowout failure which is more critical as cover and spacing between bars are reduced. FprEN 1992-1-1:2023 includes a new formulation to calculate the maximum tensile stress that can be developed in a reinforcing steel bar for a given head geometry and reinforcement layout. The methodology employed for the provision is inspired by the formulation proposed for the minimum mandrel diameter of hooked bars. But, FprEN 1992-1-1:2023 does not consider the effect of confinement around the bar, which can be present in the form of support or zone of the applied load, however, allows to conduct a more refined analysis to account for the presence of confinement. This motivates the study of the effect of confinement on the resistance of headed bars. This thesis includes the design and execution of an experimental campaign to study the effect of the degree of confinement resulting from the zone of the applied load providing partial confinement and full confinement. Furthermore, the campaign design includes the variation of important parameters involved in the failure mechanism in order to verify the new formulation provided by FprEN 1992-1-1:2023. The headed bars are formed using a threaded portion to which a nut and washer are fixed to provide a support surface for the anchorage. The parameters of the study include the edge distance, the group effect, and the effect of the anchor length. The specimens (24 in all) were built and tested in the Laboratory of Civil Engineering School of the Universidad Politécnica de Madrid. The experimental results found that the specimens with full confinement provided more resistance in comparison with the specimens with partial confinement. The study further investigated the effect of the presence of anchor length on the resistance. The results provided lower resistance than that proposed by FprEN1992-1-1:2023, so the summation of the head capacity plus 90% of the capacity of the anchor length specified in the code model should be revisited. The tests also show that the resistance increases with the increase in cover and with the presence of more bars in the specimens (double layer). The current formulation outlined in FprEN 1992-1-1:2023 appears to underestimate the resistance for small edge covers while demonstrating good results for larger edge covers. Additionally, the incorporation of a double-layered reinforcement arrangement yields higher resistance contrary to what is expected from the FprEN 1992-1-1:2023 model. These results indicate that the model should still be refined.