Vergleich von 3-Punkt- und 4-Punkt-Biegezugversuchen zur Bestimmung der Nachrissbiegezugfestigkeit

Comparison of 3-point and 4-point bending tests for the determination of the residual flexural strength. In order to determine the performance of „steel fibre reinforced concrete”, the German guideline ”Stahlfaserbeton“ demands 4-point bending tests on unnotched beams. The expected version of Eurocode 2 will also include steel fibre reinforced concrete. It will however, demand 3-point bending tests using notched beams. In this article, systematical differences of these tests methods are discussed. Furthermore, residual flexural strengths of directly comparable test series are quantitatively compared and a simple approach of conversion is outlined. It turns out, that the flexural strength in 3-point bending tests is lower, compared to 4-point bending tests, whilst residual flexural strengths are higher. Moreover, multiple cracking in the tensile zone of 3-point bending tests occurs, if at all, only using high performance fibres and high fibre contents. In contrast, multiple cracking may occur regularly in 4-point bending test. Comparing the single fracture approach in 3-point bending tests to the smeared crack approach of the German ”Stahlfaserbetonrichtlinie“ about 2 % to 7 % higher residual flexural strengths are measured in 3-point test. As a conversion approach the residual flexural strengths tested in 3-point bending can be reduced by 10 % to 30 %. For some fibre types determination of a specific conversion factor can be worth. For high fibre tensile strengths and special fibre geometries specific tests are recommended. © 2021, Ernst und Sohn. All rights reserved.