Project topic

Project description

As is usual for steel fibre reinforced concrete, the verification models of the draft of the DAfStb guideline "Ultra-high performance concrete" also use representative values of the centric post-cracking tensile strength to characterize the tensile behaviour of the cracked fibre-reinforced ultra-high performance concrete (UHPFRC). The centric post-cracking tensile strength is defined as the maximum value of the nominal concrete tensile stress that can be absorbed under axial load in the fully cracked state. This strength parameter is to be derived as simply as possible from a post-cracking tensile strength to be determined in the 3-point bending test.

To identify the conversion factor to be used here, a total of 42 3-point bending tests based on DIN EN 14651 (Fig. 1), 12 4-point bending tests based on the DAfStb guideline "Steel fiber reinforced concrete" (Fig. 2) and 168 direct tensile tests on tensile specimens cut out of beams (Figs. 3 and 4) were carried out in 7 series. Three different UHPFRC systems were tested. These had maximum grain diameters of 0.5 mm, 3 mm and 8 mm and were reinforced with fiber contents of 0.5, 1.0 or 2.5 % by volume. The dimensions of the fibers were _lf/df [mm] = 9/0.175 and _lf/df [mm] = 17/0.20. The fresh concrete consistency was flowable to self-compacting.

The post-cracking flexural tensile strengths and centric post-cracking tensile strengths were highest with a fiber content of 2.5% by volume. Overall, higher strengths were achieved with the longer fibers than with the shorter fibers. There was also a certain influence of the maximum grain diameter. Since the maximum activation of longer fibers only occurs at greater slip, the concretes with longer fibers achieved both the centric post-cracking tensile strength and the post-cracking flexural tensile strength at greater deformations than the concretes with shorter fibers.

The axial tensile tests on the tensile specimens cut out of beams (Fig. 4) yielded different centric post-cracking tensile strengths for the different areas of a beam cross-section, indicating a systematically heterogeneous fiber distribution and orientation within a beam cross-section. Tensile samples taken from the beams on the non-switched filling side had

the lowest centric post-cracking tensile strengths. The reason for this may be the lack of a favorable influence of a formwork edge on the orientation of the fibers and the turbulent flow behavior of the viscous fresh concrete observed during filling into the formwork.

If the centric post-cracking tensile strength averaged over the beam cross-section and the post-cracking flexural tensile strength (maximum of the bending stress-deflection relationship) are compared, a fairly wide range of results is obtained for the different concretes. On the one hand, this is due to the varying degrees of inhomogeneity of the beam cross-sections for the different concretes. On the other hand, the ratio values for concretes with shorter fibers are generally higher than for concretes with longer fibers, since beams with longer fibers have a greater rotational capacity and therefore develop higher flexural load-bearing capacities with the same centric post-cracking tensile strength due to constriction of the flexural compression zone. The latter can be compensated for by evaluating the post-cracking bending tensile strength not as the maximum of the bending stress-deflection relationship, but for all beams at the same central deflection.

The recommendation for the DAfStb guideline "Ultra-high performance concrete" therefore provides for the centric post-cracking tensile strength to be determined in accordance with fib Model Code 2010 on the basis of the post-cracking bending tensile strength determined in a 3-point bending test in accordance with DIN EN 14651 at _CMOD1 = 0.5 mm or _δ1 = 0.47 mm. If information on the fiber distribution and orientation within the beam cross-section is missing, the conversion should be carried out conservatively with a factor of 0.37. The use of a 4-point bending test based on the DAfStb guideline "Steel fiber reinforced concrete" is not recommended, as the course of the load-deflection relationship for unnotched beams, especially with strengthening behavior, varies greatly depending on the crack pattern.

Publications

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

LEUTBECHER, T., 2020 Classification of ultra-high performance concrete: Background and test methods. In: Congress documents 64th BetonTage: Concrete construction of the future. Neu-Ulm, February 18-21, 2020. Betonwerk und Fertigteil-Technik/Concrete Plant and Precast Technology. 86(2), 72. ISSN 0373-4331

LEUTBECHER, T.; REBLING, J., 2019. Predicting the postcracking strength of ultra-high performance fiber reinforced concrete by means of three-point bending tests according to EN 14651. Structural Concrete. 20(6), 2081-2095. ISSN 1464-4177. doi:10.1002/suco.201900070

LEUTBECHER, T.; REBLING, J., 2018. Experimental investigations to derive the centric post-cracking tensile strength of UHPFRC from flexural tests. Bauingenieur. 93(11), 463-472. ISSN 0005-6650. doi:10.37544/0005-6650-2018-11-81