Project topic

Depending on the geometry and production method, very different fiber orientations can occur (locally) in components made of fiber-reinforced concrete. As a result, the fiber-reinforced concrete no longer behaves isotropically, but exhibits different tensile and compressive load-bearing behavior depending on the direction of loading. This aspect is of particular relevance for ultra-high performance concrete (UHPFRC), as UHPFRC usually has a high fiber content in the application. Until now, there has been a lack of meaningful data to quantify the anisotropy in the compressive load-bearing behavior due to the fiber reinforcement.

As part of a series of tests, the influence of fiber orientation and fiber content on the fracture behavior and compressive strength of fine-grain and coarse-grain UHPFRC was therefore investigated in detail. The test program comprised a total of 5 series of compressive strength tests on cylinders with h/d = 200 mm/100 mm and cubes with d = 100 mm made of fiber-reinforced UHPFRC. The study focused on fiber contents between 1.5% by volume (slender fibers) and 3.5% by volume (dense fibers). In addition, the maximum particle diameter of the aggregate _Dmax and the fiber length _lf were varied within the typical value range for UHPFRC.

In addition to 9 cylinders (Cyl) and 12 cubes (Cube A), three beams with l/b/h = 750 mm/100 mm/100 mm were concreted in standard formwork for each series (Fig. 1, left). The beams were filled from one end of the formwork in order to provoke a predominantly unidirectional fiber orientation in the flow and longitudinal direction of the beams due to the flow process in conjunction with the existing formwork walls. A further 12 cubes (Cube B) with an edge length of around d = 100 mm were sawn out of the middle section of the beams.

The cylinders as well as 6 of the cubes produced in standard formwork (Cube A1) and 6 of the cubes sawn out of beams (Cube B1) were loaded in the concreting direction. The remaining cubes (Cube A2 and Cube B2) were tested perpendicular to the concreting direction. Figure 1, right, shows the loading direction for the four groups of cubes and qualitatively illustrates the orientation of the fibers.

The exact fiber orientation was determined for all series at different cut surfaces of the test specimens by optoanalytical investigations. Measurements using a non-destructive induction measurement method to determine the fiber content and mean proportional orientation of the fibers in the three spatial directions supplemented the optical analyses.

The results show that the compressive strength with predominantly unidirectional alignment of the fibers in the plane perpendicular to the loading direction increases significantly with increasing fiber content. In contrast, the influence of the fiber content is low when the fibers are predominantly unidirectional in the direction of loading. Depending on the direction of loading, the compressive strength of the cube differs by up to 26 N/mm² or 14 %. The difference between cube and cylinder compressive strength increases with increasing fiber content (Fig. 2). For example, the difference between cylindrical and cube compressive strength of test specimens produced in standard formwork is approx. 5 to 7 N/mm² with a fiber content of 1.5 % by volume and approx. 11 to 13 N/mm² with a fiber content of 2.5 to 3.5 % by volume.

Unless a more differentiated regulation is sought, taking into account the influence of the fiber content, the authors suggest that the compressive strength to be achieved when testing on the cube should be conservatively defined as fck_,cube = fck_,cyl + 15 N/mm². The concrete strength classes are then C130/145, C150/165 and C175/190.

A special reduction of the cylindrical compressive strength to take into account an "unfavorable" fiber orientation in components appears unnecessary based on the results of this study.

Publications

LEUTBECHER, T., 2022. Influence of fiber orientation on the mechanical properties of ultra-high performance fiber-reinforced concrete | Influence of fiber orientation on the mechanical properties of ultra-high performance fiber-reinforced concrete. In: Congress documents 66th BetonTage: Sustainable building with concrete. Ulm, June 21-23, 2022 Concrete Plant and Precast Technology. 88(6), 72. ISSN 0373-4331

LEUTBECHER, T.; RIEDEL, P., 2022. Influence of fiber orientation on the compressive strength of steel fiber reinforced UHPFRC. In: BERGER, J., ed. Innsbrucker Bautage 2022: Commemorative publication on the 60th birthday of Univ.-Prof. Dr.-Ing. Innsbruck: STUDIA Universitätsverlag, pp. 287-298. Innsbruck - Solid Construction and Bridge Construction, Volume 7. ISBN 978-3-99105-024-7

RIEDEL, P.; LEUTBECHER, T., 2021. Effect of Fiber Orientation on Compressive Strength of Ultra-High-Performance Fiber-Reinforced Concrete. ACI Materials Journal. 118(2), 199-209. ISSN 0889-325X. doi:10.14359/51730417

RIEDEL, P.; LEUTBECHER, T., 2020. Influence of fiber orientation and fiber content on the compressive strength of ultra-high strength concrete. Concrete and reinforced concrete construction. 115(10), 789-800. ISSN 0005-9900. doi:10.1002/best.202000020

LEUTBECHER, T.; RIEDEL, P., 2020. Compressive strength classes and performance classes of ultra-high-performance concrete (Part 2) | Compressive strength and performance classes for ultra-high performance concrete (Part 2). Betonwerk und Fertigteil-Technik/Concrete Plant and Precast Technology. 86(10), 46-53. ISSN 0373-4331

LEUTBECHER, T.; RIEDEL, P., 2020. Compressive strength classes and performance classes of ultra-high-performance concrete (Part 1) | Compressive strength and performance classes for ultra-high performance concrete (Part 1). Betonwerk und Fertigteil-Technik/Concrete Plant and Precast Technology. 86(9), 46-54. ISSN 0373-4331