Project topic

In practice, ultra-high performance concrete (UHPC) is usually used in flowable to very flowable form. For this case, the draft of the DAfStb guideline "Ultra-high performance concrete" stipulates that the beams on which the residual flexural tensile strength of the steel fiber-reinforced UHPFRC is determined in 3-point tests in accordance with DIN EN 14651 should be produced by filling the formwork from one end face. Earlier investigations of the fiber orientation and distribution had suggested that a characteristically heterogeneous fiber orientation occurs in the beam cross-section of the beams produced in this way. This raised the question of whether filling the formwork from one end can guarantee reliable and reproducible results.

Research project V 507 therefore aimed to investigate the effects of different methods of filling the formwork with regard to the homogeneity of the fiber orientation. Furthermore, it should be determined to what extent a modified procedure affects the residual bending tensile strength as a key performance characteristic of steel fiber reinforced UHPFRC.

The test program comprised two series of 6 bending beams with w/h/l = 150 mm/150 mm/550 mm made of steel fiber reinforced UHPFRC. The steel fibers were 13 mm long and had a diameter of approx. 0.20 mm. The fiber content was 1.5% by volume in each case. The maximum grain size of the aggregate(_Dmax = 0.5 mm and 8 mm) and the production method of the bending beams were varied.

Three beams of each series were produced according to the method currently specified in the draft DAfStb guideline "Ultra-high performance concrete" (filling from one end face of the beam formwork) (specimen type E). For the remaining three beams, the formwork was filled in approx. 6 layers using a funnel with a rectangular outlet (specimen type L). The funnel was held about 1 cm above the already filled concrete and moved from one side to the other at the flow rate of the concrete. Table 1 gives an overview of the test program.

All 12 beams were tested in 3-point tests according to DIN EN 14651. Subsequently, an approx. 50 mm thick slice was sawn out of the still intact area of each beam, close to the cross-section in which the bending failure had occurred (Fig. 1), and the fiber distribution and orientation were optoanalyzed using the FiDiOr analysis software.

Three of the 12 slices were also analyzed using micro-computed tomography (µCT). With this method, the position and orientation of each individual fiber within a three-dimensional sample space can be displayed and geometrically defined.

Test evaluation

The 3-point tests were analyzed in the form of load-deflection curves, as shown in Figure 2 for specimen type L as an example. The data were used to calculate residual flexural tensile strengths for the center deflections of _δ1 = 0.47 mm, _δ2 = 1.32 mm, _δ3 = 2.17 mm and _δ4 = 3.02 mm and additionally for the maximum of the load-deflection curve according to Eq. (1).

In Eq. (1), _Fj is the force recorded in the test at the center deflection _δj in N, l is the span of the beam in mm, b is the width of the specimen in mm and _hsp is the distance between the tip of the indentation and the top of the specimen in mm.

From the data of the optoanalytical investigations and the investigations using µCT, the number of fibers _nf, the mean volume-related fiber orientation coefficient _ηV according to Eq. (2) and the "effective" fiber content ρf_,ef according to Eq. (3) were determined for the cross-sectional areas of the beams and for various sections of the reconstructed volume structure of the slices examined in µCT.

In Eq. (2) and Eq. (3), _nf is the number of detected fibers in the evaluated area, _ηi is the fiber orientation coefficient of fiber i, _Φf is the nominal fiber diameter, Φf2_,i is the length of the major elliptical axis of fiber i,_Af is the area of the fiber cross-section and_Ac is the area of the concrete cross-section of the evaluated surface.

The visualization of the distribution of the fiber orientation coefficient _ηV and the "effective" fiber content ρf_,ef within a section was carried out in the form of contour diagrams, as shown in Figure 3 as an example for specimen type E of series 2. Furthermore, the mean distribution of the "effective" fiber content over the height of the beam cross-sections (Fig. 4) and the relative frequency of the fiber orientation coefficient were evaluated. Figure 5 compares the distributions of the relative frequency of the fiber orientation coefficient η, which result for one cross-section after evaluation of the examination using µCT and the optoanalytical examination, using two samples as an example.

Summary of the results and conclusion

In the 3-point tests, differences in the load-deformation behavior were determined, which can primarily be attributed to the influence of the maximum grain diameter. Higher residual bending tensile strengths were found for the fine-grain UHPFRC than for the coarse-grain UHPFRC (Fig. 2). The maximum load for the fine-grain UHPFRC occurred at larger center deflections. In contrast, the softening of the coarse-grain UHPFRC was faster than that of the fine-grain UHPFRC. Due to the more discontinuous course of the load-deflection curves, the coefficients of variation for the fine-grain UHPFRC were usually significantly higher than the corresponding values for the coarse-grain UHPFRC. With regard to a possible influence of the manufacturing method, however, the results of the 3-point tests did not reveal any clear trend. For the beams produced with the same maximum grain size, standardization to the fibre content effective in the respective beam cross-section in the tensile direction resulted in comparable residual bending tensile strengths or the differences were within the natural scatter range.

The optoanalytically determined characteristic values of the beam cross-sections (number of fibers _nf, mean fiber orientation coefficient _ηV, "effective" fiber content ρf_,ef) as well as their scatter could not be correlated with the manufacturing method of the beams. However, an obvious influence of the maximum grain diameter could also be determined here. Higher mean fiber orientation coefficients _ηV were found for the fine-grain UHPFRC than for the coarse-grain UHPFRC, as well as a tendency towards greater scatter for all three characteristic values. For the coarse-grain UHPFRC, the distribution of the relative frequency of the fiber orientation coefficient was closer to the result for a spatially random orientation of the fibers. For the fine-grain UHPFRC, the fibers were increasingly oriented in the longitudinal direction of the beam (Fig. 5).

An effect of the production method on the fiber orientation could only be determined for the concreting direction, i.e. for the direction normal to the longitudinal axis of the beam. Here, the evaluation of the µCT investigations for the beam produced in layers with a hopper provided a significantly higher mean fiber orientation coefficient _ηV than for the beam cast from one end face. However, a different fiber orientation normal to the longitudinal axis of the beam theoretically has no influence on the result of the 3-point test, and no such influence could be determined in the 3-point tests.

A systematically heterogeneous distribution of fiber orientation in the beam cross-section, as had been observed in earlier investigations, could not be determined either for the beams produced in layers with a funnel or for the beams cast from one end face. Although the test specimens produced in the present research project also showed a varying fiber orientation across the cross-section (Fig. 3, left), a correlation between the production method and the homogeneity of the beams could not be derived from the data.

In general, a slight increase in fiber content towards the bottom of the formwork was observed in most beam cross-sections (Fig. 4). The beams produced in layers with the hopper appeared to be slightly less affected by this. By rotating the beams by 90° for testing in the 3-point test, the heterogeneity of the fiber distribution, which indicates sedimentation of the fibers, is almost completely compensated for. Effects of a locally varying fiber content within the cross-section on the test results are therefore not to be expected.

As was shown in the evaluation of the data from the µCT investigation for different cross-sections within a sample volume, the number of fibers _nf and thus also the "effective" fiber content ρf_,ef vary considerably in the longitudinal direction of the beam even at short distances. In this respect, the residual bending tensile strengths obtained in 3-point tests can strictly speaking only be reliably correlated with the results of an optical analysis or µCT examination if the number of fibers and the "effective" fiber content are determined for the actual fracture cross-section and not for a cross-section close to the fracture cross-section.

In the vast majority of the analyzed sections - regardless of the evaluation method - an "effective" fiber content ρf_,ef was determined, which in some cases was significantly below the nominal fiber content (Fig. 4). For the fine-grain UHPFRC, ρf_,ef = 1.02 to 1.13 % by volume (optical analysis) and ρf_,ef = 0.85 to 1.03 % by volume (data from µCT examination) were the most significant underruns. The cause of this could not be determined. Even in earlier investigations, a deviation from the nominal fiber content, in some cases considerable, was found in individual series.

The comparison of the results of the optoanalytical tests and the µCT tests showed an overall good agreement between the two evaluation methods. Only for fibres that were oriented approximately normal to the considered cutting plane, the optical analysis tended to provide slightly too low fibre orientation coefficients, as the cross-section of the fiber detected in the cut surface was identified as more elliptical (Fig. 5). For the fine-grain UHPFRC, the mean fiber orientation coefficient _ηV differed most significantly between the two evaluation methods due to the high proportion of fibers aligned predominantly in the longitudinal direction of the beam. Apart from this, the results of the optical analysis and the µCT examination can be regarded as equally reliable.

In summary, the results of this study suggest that filling the beam formwork from one end face and filling it in layers with a funnel provide equally reliable and reproducible results.

As the elementary parameters of the beams produced using both methods do not differ significantly, it is recommended for the sake of simplicity to retain the filling of the beam formwork from one end face as currently envisaged in the draft of the DAfStb guideline "Ultra-high performance concrete" for flowable to very flowable consistency and to dispense with the use of a funnel as an additional aid.

Publications

HECK, L.; SCHLEITING, M.; LEUTBECHER, T., 2024. computed tomography versus optoanalytics: Determination of fiber distribution and orientation in steel fiber reinforced UHPFRC. Beton- und Stahlbetonbau. 119(10), 777-791. doi:10.1002/best.202400040

LEUTBECHER, T.; HECK, L.; SCHLEITING, M., 2022. Testing of UHPFRC according to DIN EN 14651 - On the influence of specimen production on the test results. Concrete and reinforced concrete construction. 117(6), 410-421. doi:10.1002/best.202200027
Supporting Information: Data S1. Results of the optoanalytical investigation
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The project is funded by the Federal Ministry of Education and Research (BMBF) as part of the "Sustainable Universities" program. The aim of the funding is to develop and implement innovative concepts for environmentally friendly and resource-conserving campus design.

Important partners in the project are the city of Siegen, which provides support in the areas of mobility and climate protection, and the Institute for Environmental Research NRW, which provides scientific analysis and expertise. Siegen's municipal utilities are also involved in the implementation of sustainable energy solutions.