Deriving the design value of the post-cracking tensile strength for ultra-high performance fibre-reinforced concrete taking into account the size effect on the scatter

Research topic

Design and construction with ultra-high performance concrete (UHPC)

Funding and project duration

German Committee for Structural Concrete (Deutscher Ausschuss für Stahlbeton, DAfStb), Research Project V 525
Duration: February 2025 until October 2025

Person in charge

Lennart Heck, M.Sc.

Project description

The aim of the research project is to derive the design value of the post-cracking tensile strength for ultra-high performance fibre-reinforced concrete (UHPFRC). By taking the size effect of the scatter into account, sufficient reliability is to be ensured without compromising the efficiency of execution.

Compared to (normal-strength) steel fibre-reinforced concrete covered by the DAfStb Guideline "Steel Fibre Reinforced Concrete", the fibre volume fraction in UHPFRC is significantly higher. Accordingly, the nominal contribution of the steel fibres to the overall load-bearing capacity is considerable, so that even for members subject to bending, no further reinforcement (e.g. in terms of shear reinforcement, splitting tensile reinforcement, etc.) is required in addition to (pretensioned) prestressing. This enables a significantly less massive construction, a high degree of prefabrication, an increased service life and high resource efficiency. However, implementing new kinds of design and execution in construction practice requires that the nominal performance of the cost-intensive steel fibres can actually be taken into account. In this respect, in addition to reliability, the ability of appropriate design for the material involved and the question of efficiency of execution are directly linked to the question of defining an appropriate design value of the post-cracking tensile strength.

A particular difficulty in assessing a safety concept for steel fibre-reinforced concrete is that the spread of the post-cracking tensile strength depends largely on the size of the tensioned cross-sectional area: regarding small cross-sections, such as those used in building material testing, there is a high probability that the number of fibres in a section is very large or very small, resulting in very high coefficients of variation for the post-cracking tensile strength compared to other strength parameters. As the size of the cross-sectional area increases, it becomes increasingly unlikely that the number of fibres shows an extreme value.

Based on these fundamental considerations, the investigations within the research project will focus on quantifying the effects described. The data will then be used to derive a design value of the post-cracking tensile strength for UHPFRC . The concept will be based on the principles of the DAfStb Guideline "Steel Fiber Reinforced Concrete" and will take into account the size effect on the scatter.