Acta Polytechnica CTU Proceedings https://doi.org/10.14311/APP.2025.53.0007 Acta Polytechnica CTU Proceedings 53:7–12, 2025 © 2025 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague PRODUCTION OF STEEL-CONCRETE COMPOSITE COLUMNS FOR BLAST TESTS Adam Číteka,∗, Karel Hurtiga, Martin Kryštova, Vojtěch Šulcb, Marek Foglarb, David Číteka a Czech Technical University in Prague, Klokner Institute, Šolínova 7, Prague 166 08, Czech Republic b Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, 166 29 Prague 6, Czech Republic ∗ corresponding author: adam.citek@cvut.cz Abstract. The resistance of structures to explosive impacts is a highly relevant issue in modern engineering. During an explosion, structures are subjected to extreme dynamic loads, which necessitate advanced materials and reinforcement strategies. UHPFRC, with its evenly distributed steel fibres, exhibits exceptional mechanical properties that make it well-suited for blast-resistant applications. Combining UHPFRC with steel reinforcement is expected to significantly mitigate structural damage under explosive loading. This article focuses on the process of production of testing steel-concrete column specimens under axial compressive load for evaluating their blast resistance. The initial section describes the test elements and the objectives of the experiment. The following section outlines the design and material properties of the concrete mixtures used. Subsequently, the production process, including formwork preparation and casting, is detailed. Finally, the results achieved from ongoing testing are briefly introduced. Keywords: Concrete, ultra-high performance fibre-reinforced concrete, blast resistance, steel-concrete composite. 1. Introduction Explosions are most often associated with industrial accidents or acts of terrorism. In recent years, the in- creasing threat of terrorist or war attacks, particularly in developed countries, has highlighted the critical need for designing structures capable of withstanding the extreme overpressure loads generated by explo- sions [1, 2]. While steel-concrete composite structures are widely used globally, primarily for their economic efficiency, their behaviour under blast loading has not been thor- oughly investigated. During an explosion, normal- strength concrete is highly susceptible to severe dam- age and collapse, necessitating reinforcement design to mitigate such risks. Explosions generate extreme local loading that is difficult to predict, requiring reinforcement to be designed with a high level of com- plexity [3]. The use of UHPFRC offers a promising solution, as its dispersed steel fibres provide uniform reinforcement throughout the entire volume of the element [4, 5]. When combined with steel components in a composite structure, this material has the poten- tial to achieve significantly greater blast resistance compared to conventional steel-concrete composites using normal-strength concrete [4, 6, 7]. This article is focused on the process of production of steel-concrete column specimens intended to ex- perimental blast resistance tests. In the end of the article a summary of the findings from the ongoing testing is shown. These tests represent only a part of a complex research project focused on the blast re- sistance of steel-concrete structures. The experimen- tal design, numerical analysis, and data evaluation were conducted by the Faculty of Civil Engineering at the Czech Technical University (CTU), which also serves as the lead institution for the entire research project. The optimization of concrete mixtures, mate- rial property testing, and production of experimental specimens were performed by the Klokner Institute (KI). 2. Description of specimens A total of ten column specimens were designed and prepared for the experiment, grouped into three funda- mental types. Type 1 represented a plain steel column with no infill and it was used as a reference. Type 2 was a composite steel-concrete column filled with normal-strength concrete (NSC) with a compressive strength of 30+ MPa and type 3 represented com- posite steel-concrete columns filled with ultra-high- performance concrete (UHPC) with a compressive strength of 120+ MPa. All columns were based on hot-rolled HEA 220 steel profiles fabricated from structural steel grade S355. Each steel profile was sealed with steel plates at both ends to ensure uniform load distribution and struc- tural integrity. Additional side plates were attached to the specimens to assist with handling and positioning 7 https://doi.org/10.14311/APP.2025.53.0007 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en A. Čítek, K. Hurtig, M. Kryštov et al. Acta Polytechnica CTU Proceedings Figure 1. Detailed description of the steel parts of the specimens. during the experiment. These side plates, however, had no structural function and did not influence the test results. To ensure composite action between the steel column and the infill concrete shear connectors in the form of bend reinforcement were welded onto the steel column. To replicate realistic boundary conditions, steel spherical joints were installed at both ends of the columns. These joints simulated ideally pinned con- nections, allowing for uniform stress distribution of the axial compressive load across the specimen’s cross- section. Additionally, the spherical joints facilitated accurate representation of pinned boundary conditions in subsequent numerical analyses. The following Table 1. shows description of the specimen. Figure 1 illustrates the cross-section and longitudinal section of the test specimen. Specimen Specimen description 1a, 1b, 1c Steel only column 2a, 2b, 2c NSC-filled steel column 3a, 3b, 3c UHPFRC-filled steel column Table 1. List of prepared specimens and their de- scription. 3. Material properties As previously stated, a key objective of this part of the experimental program was to compare the performance of only steel columns with NSC-filled and UHPFRC-filled columns. The mix designs were adopted from the previous part of the experimental program, which focused on the blast resistance of com- posite slabs, where concrete mixtures for NSC and UHPFRC had already been developed and optimized by Klokner Institute [8]. The mixture is characterized by a very low water- cement ratio, self-compacting character, and excep- tional material properties. Reinforcing was provided by dispersed steel fibres in volume of 1.5 % of the total volume. This fibre volume ensured required mechan- ical properties while maintaining good workability. The key material properties of the UHPFRC mixtures at 28 days are presented in Tables 2 to 4. Since the casting had to be performed over two days, two sets of results are presented in the table. In case of NSC specimens the same concrete mixture as in the previous part of the experimental program was used [8]. The key material properties of the NSC mixtures at 28 days are presented in Tables 5 and 6. Since the casting had to be performed over two days, two sets of results are presented in the table. 4. Production The prepared steel columns fabricated from steel sec- tions were transported to Klokner institute, where the concrete casting for the samples was carried out. Sam- ples were casted in horizontal position with flanges upright. This arrangement allowed the column flanges to serve as lost formwork, eliminating the need for additional formwork preparation. The casting process needed to be divided in phases, because only one side of the columns could be casted at the time. Between the castings, a two-day technological break was re- quired, after which the samples were rotated to the other side and the second part of the casting was be carried out. In case of NSC, the concrete was compacted using an immersion vibrator (NSC). Because of the self- 8 vol. 53/2025 Production of steel-concrete composite columns for blast tests Material Flow Bulk Average speed of UZ signal [km s−1] Modulus of temperature test density beam cylinder elasticity cylinder [°C] [mm] [kg m−3] 150/150/700 mm 150/300 mm 150/300 mm [MPa] 25.0 285×290 2 430 4.64 4.62 44.9 28.3 240×245 2 410 4.61 4.58 44.7 Table 2. Material properties of UHPFRC. Compressive strength [MPa] Flexural strength [MPa] beam cube cylinder cylinder beam beam 40/40/160 mm 100 mm 100/200 mm 150/300 mm 40/40/160 mm 150/150/700 mm 176.5 144.3 132.4 129.8 33.3 13.5 152.0 134.3 131.2 122.0 29.1 11.7 Table 3. Material properties of UHPFRC – compressive and flexural strength. Flexural tensile strength Residual flexural strength Direct tensile strength Residual direct tensile strength at crack initiation for deflection of [MPa] at crack initiation ffc,tm,res,i for deflection of [MPa] ffc,tm,fl [MPa] 0.5 mm 3.5 mm 5.0 mm ffc,tm,cr [MPa] 0.5 mm 3.5 mm 11.2 12.6 11.7 9.1 8.5 6.4 4.3 9.6 11.5 10.5 9.1 5.8 5.8 3.8 Table 4. Material properties of UHPFRC – direct tensile and flexural tensile strength. Material Slump Bulk Modulus of Average speed of UZ signal [km s−1] temperature test density elasticity cylinder beam cylinder [°C] [mm] [kg m−3] 150/300 mm [MPa] 150/150/700 mm 150/300 mm 22.1 100 2 370 33.0 4.57 4.53 22.3 110 2 360 33.1 4.55 4.54 Table 5. Material properties of NSC. Flexural strength beam Compressive strength [MPa] 150/150/700 mm cube cube cylinder cylinder [MPa] 100 mm 150 mm 100/200 mm 150/300 mm 4.03 60.7 55.4 52.3 49.5 3.95 59.1 50.2 50.2 47.3 Table 6. Material properties of NSC – flexural and compressive strength. compacting character of the mixture, the samples from UHPFRC needed not to be compacted. Few days after the first part of casting it was necessary to rotate each sample around its longitudinal axis to the opposite position, so it was possible to cast the second side of the section. The casting process was processed in the Klokner’s Institute laboratories. The UHPFRC specimens were cast in two batches with a total volume of 0.45 m3. Similarly, the NSC specimens were cast in two batches with a total volume of 0.5 m3. Following casting, the concrete surfaces were treated with an evaporation retardant and covered with plastic foil. The surfaces were treated with water during the first two days after casting. After 28 days, the specimens were trans- ported to the University of Pardubice, where the blast tests were performed. In Figures 2 and 4 process of casting of the test samples is shown. All the prepared specimens can be seen in Figure 3. Figure 2. Process of casting of the test specimens. 9 A. Čítek, K. Hurtig, M. Kryštov et al. Acta Polytechnica CTU Proceedings Figure 3. Process of casting of the test specimens. Figure 4. Process of casting of the test specimens. 5. Experimental test As it was mentioned above all specimens were trans- ported to University of Pardubice, Institute of En- ergetic Materials, where blast experiments were per- formed. For the purposes of these tests on columns sub- jected to axial loading, it was necessary to design a specialized testing apparatus that would met all the requirements for durability, functionality, and easy handling of the specimens. The axial loading was designed corresponding to a corner-column loading of multiple-story building as 1 300 kN in characteristic values. After multiple iterations, the final arrangement was determined to be a statically indeterminate frame, loaded by prestressing induced by a hydraulic jack. The frame was arranged vertically to avoid parasitic bending. The hydraulic jack was positioned at the bottom of the frame, protected by a steel box to pre- vent any damage. The testing apparatus is shown in the following Figure 5. Before the experiment, the elements were positioned vertically in the testing apparatus. The values of the Figure 5. Testing apparatus. Specimen Type Axial load no. [kN] 1a Type 1 (steel only) 1 300 2a Type 2 (steel with NSC infill) 1 300 2b 100 3a Type 3 (steel with UHPFRC infill) 1 300 3b 100 Table 7. Material properties of NSC – flexural and compressive strength. axial compressive loads applied to the elements using a hydraulic press are provided in Table 7. The outer face of the steel flange in the middle of its height was loaded with a contact explosion of 0.8 kg of Semtex 1A moulded to rectangular shape. The blast loading was constant throughout all specimen. During the experiment the time history of displacement of each 10 vol. 53/2025 Production of steel-concrete composite columns for blast tests specimen flange was recorded with the use of 4 chan- nels of photonic-doppler velocimetry (PDV). Also, the testing frame was equipped with strain gauges on both pillars to check axial load in specimens a and to measure residual load after the blast. After the explosive tests was performed, the specimens were also subjected to visual examination. In this phase, it was decided to test only 5 out of the total 10 specimens. Experiments results from the first set were used to further complex and detailed evaluation. This evaluation promising some option to make changes and optimization for the second set of specimens, which should be tested in near future. The design of the testing apparatus and evaluation of the test results was carried out by the Faculty of Civil Engineering at the Czech Technical University (CTU). 6. Results A key finding from the ongoing evaluation of test re- sults is the positive impact of infill concrete on the overall resistance of the steel profile. Steel specimens without concrete infill exhibited significant damage. During the explosion, steel fragments from the flange in direct contact with the charge were torn off. These fragments also caused a damage on the flange on the opposite side of the element (Figure 6). Specimens with concrete infill experienced overall significantly smaller damage. Concrete infill prevented fragmenta- tion of the steel flange in contact with explosive. The concrete infill experienced of crushing and spalling on the sides of the specimen in the area closest to the explosion. Unlike the steel only specimens without concrete infill, almost no visible damage occurred to the opposite flange. Figure 6. Side view of specimen no. 1a. Tests results also shown positive effect of excellent mechanical properties of UHPFRC. Concrete infill made of NSC (Figure 7) experienced considerably bigger damage than UHPFRC infill (Figure 8). The effect of axial compressive force was not evalu- ated, as not all unloaded samples were tested in the first set of the experiments. This evaluation will be possible after all the samples have been tested. 7. Conclusion This part of research project was focused on behaving steel-concrete columns subjected to contact blast load- ing. The emphasis of the experimental tests was fo- Figure 7. Side view of specimen no. 2a. Figure 8. Side view of specimen no. 3a. cused both on comparing performance chosen types of steel-concrete composite columns under axial compres- sive load. The axial load 1 300 kN simulates a corner- column loading of multiple-storey building. To fulfil project goals three types of column specimens were produced. One type was steel only, the other two were steel-concrete specimens with NSC and UHPFRC in- fill. The emphasis of this article is description of pro- duction of the test specimens which was carried out by Klokner Institute (KI). Both UHPFRC and NSC mixtures had been optimized in the previous part of the experimental program. In the end of the article a summary of the findings from the ongoing testing are described. Overall, better blast resistance was achieved with specimens with concrete infill. Concrete infill pre- vented fragmentation of steel profile. The positive effect of the excellent mechanical properties of UH- PFRC on blast resistance can be observed. Specimens with UHPFRC experienced considerably smaller dam- age than specimens with NSC infill. The results from the first completed set of tests will be used for the fur- ther numerical assessment and possible optimization of test set-up for the second part of specimens. Test conception, numerical assessment and evaluation of results was carried out by Faculty of Civil Engineering of CTU. Acknowledgements The article was written with the support of the grant project of the Czech Science Foundation – project No. 22-33039S. References [1] J. Liu, J. Wei, J. Li, et al. A comprehensive review of ultra-high performance concrete (UHPC) behaviour under blast loads. Cement and Concrete Composites 11 A. Čítek, K. Hurtig, M. Kryštov et al. Acta Polytechnica CTU Proceedings 148:105449, 2024. https://doi.org/10.1016/j.cemconcomp.2024.105449 [2] C. Zhao, X. Lu, Q. Wang, et al. Experimental and numerical investigation of steel-concrete (SC) slabs under contact blast loading. Engineering Structures 196:109337, 2019. https://doi.org/10.1016/j.engstruct.2019.109337 [3] H. Draganić, G. Gazić, D. Varevac. Experimental investigation of design and retrofit methods for blast load mitigation – a state-of-the-art review. Engineering Structures 190:189–209, 2019. https://doi.org/10.1016/j.engstruct.2019.03.088 [4] S. Abbas, M. Nehdi, M. Saleem. Ultra-high performance concrete: Mechanical performance, durability, sustainability and implementation challenges. International Journal of Concrete Structures and Materials 10:271–295, 2016. https://doi.org/10.1007/s40069-016-0157-4 [5] M. Foglar, R. Hajek, M. Kovar, J. Štoller. Blast performance of RC panels with waste steel fibers. Construction and Building Materials 94:536–546, 2015. https://doi.org/10.1016/j.conbuildmat.2015.07.082 [6] M. Foglar, R. Hajek, J. Fladr, et al. Full-scale experimental testing of the blast resistance of HPFRC and UHPFRC bridge decks. Construction and Building Materials 145:588–601, 2017. https://doi.org/10.1016/j.conbuildmat.2017.04.054 [7] R. Hajek, J. Fladr, J. Pachman, et al. An experimental evaluation of the blast resistance of heterogeneous concrete-based composite bridge decks. Engineering Structures 179:204–210, 2019. https://doi.org/10.1016/j.engstruct.2018.10.070 [8] A. Citek, M. Krystov, K. Hurtig, et al. Production of steel-concrete composite UHPFRC elements for experimental tests of the blast resistance. Key Engineering Materials 976:113–119, 2024. https://doi.org/10.4028/p-oyi5pK 12 https://doi.org/10.1016/j.cemconcomp.2024.105449 https://doi.org/10.1016/j.engstruct.2019.109337 https://doi.org/10.1016/j.engstruct.2019.03.088 https://doi.org/10.1007/s40069-016-0157-4 https://doi.org/10.1016/j.conbuildmat.2015.07.082 https://doi.org/10.1016/j.conbuildmat.2017.04.054 https://doi.org/10.1016/j.engstruct.2018.10.070 https://doi.org/10.4028/p-oyi5pK Acta Polytechnica CTU Proceedings 53:7–12, 2025 1 Introduction 2 Description of specimens 3 Material properties 4 Production 5 Experimental test 6 Results 7 Conclusion Acknowledgements References