Evaluation of the Stress-Strain Behaviour of UHPFRC as Derived from Analysis of Four-Point Bending Tests
Author: André Robert
Language: English
Abstract
The incorporation of short fibers in UHPFRC (Ultra-High Performance Fiber Reinforced Concrete) prevents brittle failure under tensile loading and allows for further load increase even after cracking of the cross-section. This effect, known as strain hardening, describes UHPFRC’s behaviour under tensile stress. Due to the complex interaction between the fibers and the concrete matrix, the material behaviour is still not fully understood.
Through the evaluation of 4-point bending tests using Digital Image Correlation (DIC), important material parameters such as displacements and strains could be determined at any given time of the experiment. These data made it possible to proof the linear increase in compressive strains with distance from the neutral axis.
Furthermore, an approach was developed to determine the stresses on the compression side. Then, using the equilibrium of forces and equilibrium of moments, the location and magnitude of the acting tensile force, resulting from the tensile stresses within the cross-section, were determined. The results are shown below. The SIA 2052 verification for bending was also applied to the dataset and checked for consistency. This result is also presented below for one of the specimens. Additionally, the SIA proposal for the stress distribution for determining the tensile strength using inverse analysis was verified. The evaluations indicated that the assumed stress distribution matches the experimental results to within approximately 6%, with the application according to SIA being on the conservative side. Finally, a new approach was attempted to determine the cracking moment for the specimen from the DIC data. Initial evaluations have shown that the method is effective.
In conclusion, it can be said that the strain hardening model under tensile loading and the assumptions in the standard were validated through tests with the UHPFRC beams.
