Safety concept for ultra-high-performance fiber- reinforced concrete subjected to tension

Compared with (normal-strength) steel fiber-reinforced concrete, the fiber volume fraction in ultra-high-performance fiber-reinforced concrete (UHPFRC) is significantly higher. Consequently, the contribution of the steel fibers to the overall load-bearing capacity is substantial. This enables new reinforcing strategies, exceptional slender structures, a high degree of prefabrication, and superior resource efficiency. However, implementing such novel construction methods in engineering practice requires that structural design is explicitly accounting for the existing performance of the steel fibers. Since the distribution of the steel fibers within the volume inevitably exhibits statistical variation, the safety concept must consider these effects. A particular challenge arises from the fact that the scatter in fiber volume fraction strongly depends on the expected value of the number of fibers: When considering a small cross-sectional area or UHPFRC with low nominal fiber volume fraction, there is a high probability that the number of fibers will be either very high or very low. As the cross-sectional area or the nominal fiber volume fraction increases, the likelihood of such extreme number of fibers decreases noticeably (Fig. 1).

Due to the lack of valid data, currently no approach exists to quantify the described effects for UHPFRC and incorporate them into a progressive safety concept. To close this gap, a research project funded by the German Committee for Structural Concrete (DAfStb) is conducting optical analyses and induction-based measurements on UHPFRC structural elements of various sizes. These analyses, along with the results of building material tests, are being statistically evaluated. The aim of the research project is to derive a design value of the post-cracking tensile strength for UHPFRC, taking into account the size dependence of the scatter. The intention is to incorporate the findings of this work into a future edition of the DAfStb guideline on ultra-high-performance concrete.