• Máca, Petr

Abstract

Reinforced concrete structures are increasingly subjected to extreme loading events such as impacts, explosions and earthquakes. Because reinforced concrete is a composite material, good load transfer between concrete and reinforcing steel is required. Such load transfer is provided by an adequate bond. The bond stress-slip relationships were studied under quasi-static and high loading rates ranging from 0.01 mm/s to 10 m/s. Only deformed bars were investigated in this dissertation where deep insight into experimental setup and techniques is provided. The theory of wave propagation through an elastic body is used to analyse the measured results. Bond stress-slip relationships for impact loading were obtained during push-in and pull-out tests. A new specimen geometry which is more suitable for impact testing was proposed in this work. The experimental program included setting up a drop-tower which was produced sufficiently long loading pulse which led to the failure of the bond zone. Also, high rate push-in tests were performed in modified split Hopkinson pressure bar. The used experimental setup and the evaluation process of the recorded data were described. Engineering strains measured on the steel rebar were utilised to evaluate the bond stress, which was thereupon put into relation to the slip. Several aspects that influence the bond strength were discussed both for quasi-static and impact loading. The importance of different slip measurement approaches on the overall bond stress- slip relationship was illustrated. In addition, the influence of the inertia of the specimen on the obtained data was discussed. It was shown that in the case of non-direct measurements, the inertial effects must be considered during the data evaluation process. The main focus of this work was on experimental techniques and evaluation methods. Therefore, only one concrete class with an average compressive strength of 51 MPa was studied. The results show that it is not possible to define the bond stress or slip rate as a single value as they change in time. It was concluded that higher loading rates increase the bond strength. However, this increase is only up to 30%, and it is much lower than it was expected. In all investigated cases, the failure mechanism was caused by shearing off the concrete cantilevers between the steel ribs. No change in failure mode was observed based on the loading rate or type of loading. Nearly negligible influence of loading type was observed. In most cases, the bond resistance for push-in loading was higher in comparison to the pull-out type of loading.

Topics

• impedance spectroscopy
• theory
• strength
• steel
• composite