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 ver-sion of Eurocode 2 will also include steel fibre reinforced con-crete 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 quanti-tatively compared and a simple approach of conversion is out-lined. It turns out, that the flexural strength in 3-point bending tests is lower, compared to 4-point bending tests, whilst resi-dual 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 con-trast, 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 "Stahl-faserbetonrichtlinie" about 2% to 7% higher residual flexural strengths are measured in 3-point test. As a conversion ap. proach the residual flexural strengths tested in 3-point bending can be reduced by 10 % to 30%. For some fibre types deter. mination of a specific conversion factor can be worth. For high fibre tensile strengths and special fibre geometries specific tests are recommended.