Acta Polytechnica CTU Proceedings https://doi.org/10.14311/APP.2024.47.0063 Acta Polytechnica CTU Proceedings 47:63–65, 2024 © 2024 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague POSSIBILITIES OF REINFORCEMENT OF SUBTLE TRANSLUCENT CONCRETE PANEL Věra Kabíčkováa,∗, Jakub Hájeka, Tomáš Vlacha, Jakub Řepkab a Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, 166 29 Prague, Czech Republic b Czech Technical University in Prague, University Centre for Energy Efficient Buildings, Třinecká 1024, 273 43 Buštěhrad, Czech Republic ∗ corresponding author: vera.kabickova@fsv.cvut.cz Abstract. Textile-reinforced concrete not only has many advantages that lead to less consumption of primary resources, but also enables the creation of thin and subtle elements. The high performance of such elements allows us to create interesting architectural features, such as translucent concrete. This article focusses on the possibility of reinforcing translucent concrete panels. Keywords: Concrete, light-transmitting concrete, textile-reinforced concrete, high-performance concrete. 1. Introduction Concrete is one of the most used building materials, but due to its massive production, concrete has a huge impact on the environment (especially because of the cement and steel used as reinforcement). On the other hand, concrete offers many benefits (such as mechanical properties or the ability to maintain the stability of the internal environment) that make it almost irreplaceable in many cases. This is the reason why it is so important to optimise concrete [1]. There are several ways to improve concrete and reduce its environmental impact – for example, it is possible to partially replace cement using fly ash, sil- ica fume, or slag, reduce the amount of water (that leads to higher strength which allows lower concrete thickness) or replace traditional steel reinforcement with alternative noncorrosive reinforcement that has a lower environmental impact. Furthermore, no con- crete cover is required to protect the reinforcement, which also leads to a lower thickness of the concrete. Reducing the thickness of concrete has an impact on the reduction of used materials, lower load, which means smaller dimensions of the supporting structures, and easier handling [1]. Textile reinforced concrete (TRC) combines the above improvements. It is reinforced by technical tex- tiles made of carbon, alkali-resistant AR-glass, basalt, or natural materials such as flax or hemp, which can be impregnated using a polymer matrix, e.g., epoxy resin. Technical textiles are in the form of orientated rovings and are embedded in a concrete matrix, which is usually high-performance fine-grained concrete [2– 5]. Another material that combines the above improve- ments is fibre-reinforced concrete (FRC), where dis- persed reinforcing fibres (made of steel, polymers or, for example, glass) instead of traditional reinforce- ment. These fibres can contribute to higher strength and ductility, but also to enhance fire resistance [5, 6]. Overall, there are many ways to improve concrete and thus reduce its environmental impact, but it is also possible to combine multiple functions (e.g., load bearing, acoustic, or lighting) in one material. For example, luminescent or acoustic concrete is being developed and light-transmitting concrete is already available (under the name of Litracon, Lucem, or Licrete). Light transmission is ensured by several elements – usually optical fibres, polymers, or glass [7– 9]. Based on that, it was decided to explore the pos- sibilities of light-transmitting concrete reinforcement using alternative reinforcement to reduce the thickness of concrete and thus reduce its environmental impact. Different types of rovings (carbon, AR-glass, and flax) were tested, and a plexiglass (or polymethyl methacry- late – PMMA) elements were chosen to ensure light transmission. 2. Preparation of samples Translucent samples were made with dimensions of the samples 30×150×400 mm for the flexural strength test. There were 4 groups of TRC, 6 samples in to- tal, 4 samples were translucent, and 2 samples were opaque: • U R (2×) carbon roving (Tenax STS40 F13 24K 1600tex) + plexiglass, • U R ref (2× – opaque) carbon roving (Tenax STS40 F13 24K 1600tex), • L R (1×) flax roving + plexiglass, • ARG R (1×) AR glass roving (Cem-FIL® 5325) + plexiglass. A fine-grained concrete mixture was used for the prepa- ration of samples, which is described in the Table 1. 63 https://doi.org/10.14311/APP.2024.47.0063 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en V. Kabíčková, J. Hájek, T. Vlach, J. Řepka Acta Polytechnica CTU Proceedings Reinforcement was prepared as impregnated rovings with epoxy resin. The used rovings are described in the Table 2. [kg m−3] CEM I 42.5 R 650 Silica sand Dmax=1.25 mm 1 200 Silica fume 75 Microsilica 235 Superplasticizer 18 Water 190 Table 1. Concrete mixture. Fibre type T Tensile Elastic strength modulus [tex] [MPa] [GPa] Carbon (Tenax STS40 F13 24K) 1 600 4 000 240 AR glass (Cem-FIL® 5325) 2 400 >1 000 72 Flax 1 680 N/S N/S Table 2. Material characteristics of used rovings [10, 11]. To ensure a 10 mm distance between the plexiglass elements (Figure 1), 10 mm wide wooden bars were placed between them. During the preparation of the translucent TRC samples, the rovings were impreg- nated with epoxy resin, placed in the space where the plexiglass elements are cut and then sprinkled with fine silica sand. After 24 hours, the hardened epoxy resin was able to keep the required distance between the plexiglass elements so the wooden bars could be removed and the same process could be repeated on the other side of the sample (Figure 2). The prepared inserts were placed into the mould and weighted down and then the concrete mixture was poured. All the samples were later demoulded and left in water until the day of testing. Figure 1. Scheme of translucent sample. 3. Experiment and results The compressive test according to CSN EN 12390-3 and the flexural strength test (three-point bending test) according to CSN EN 12390-5 both at the age of 28 days were performed to measure the mechani- cal properties of the concrete mixture. The translu- cent samples were prepared for the flexural strength Figure 2. Elements with one layer of carbon rovings impregnated with epoxy resin and silica sand (right). Figure 3. Force-displacement diagram of the translu- cent samples – rovings (right). test (four-point bending test) with constant loading speed of 1 mm min−1 which was partly according to CSN EN 12390-5. The average compressive strength on the cubes was 110.69 MPa, and the average flexural strength on the prisms was 17.24 MPa. Sample Spill Flexural Average flexural test strength fcf strength fcf ,avg [mm] [MPa] [MPa] U R ref_1 300 18.31 18.00U R ref_2 300 17.70 U R_1 300 18.22 18.79U R_2 300 19.36 ARG R 300 8.42 8.42 L R 300 3.35 3.35 Table 3. Results of the four-point bending test. In Figure 3 force-displacement curves of the translu- cent TRC samples are presented. The translucent TRC samples achieve high flexural strengths. The highest strength is achieved by the carbon-reinforced 64 vol. 47/2024 Possibilities of reinforcement of subtle translucent concrete panel Figure 4. Number of cracks after four-point bending test: translucent TRC sample (left), opaque TRC sample (right). samples, the flexural strength of the translucent sam- ples is even higher than the opaque samples with the same reinforcement – 18.79 MPa and 18.00 MPa (see Table 3). It might be caused by the microcracks which cause that rovings must bear a certain amount of load from the very beginning of loading and can adapt to it (see Figure 4. The flexural strength of glass roving is 2 times lower than carbon roving (18.79 MPa and 8.42 MPa), and the flexural strength of flax roving is even 5.5 times lower than carbon roving (18.79 MPa and 3.35 MPa). After the maximum load, the samples with carbon and flax roving can carry about 4 % of the maximum load, unlike the samples with glass roving, which collapsed immediately. 4. Conclusions All translucent samples achieved optimistic results. The highest average flexural strength had samples reinforced with carbon roving (18.79 MPa, which is even slightly more than opaque samples with the same reinforcement – 18.00 MPa), followed by glass roving (8.42 MPa, which is 2 times lower) and flax roving (3.35 MPa, which is 5.5 times lower). The samples with rovings also show ductility behaviour. In summary, all rovings could be used in certain applications of translucent concrete. Acknowledgements The work on this paper was supported by Czech Science Foundation Grant No. 22-14942K entitled “Possibilities of using natural fibers for the production of hybrid textile reinforcement in concrete”. The authors would like to acknowledge all financial assistance provided to support this research. References [1] P. Hájek. Význam betonu a betonových konstrukcí z hlediska kritérií udržitelné výstavby. Časopis Stavebnictví 2007. [2022-12-16]. https://www.casopisstavebnictvi.cz/clanky- vyznam-betonu-a-betonovych-konstrukci- zhlediska-kriterii-udrzitelne-vystavby.html [2] Co je to textilní beton (TRC). TZB-info [2022-11-13]. https://stavba.tzb-info.cz/beton-malty-omitky/ 18732-co-je-to-textilni-beton-trc [3] T. Vlach. Interakce textilní výztuže a vysokopevnostní betonové matrice. Phd thesis, České vysoké učení technické v Praze, 2021. [2022-11-13]. https://dspace.cvut.cz/handle/10467/98215 [4] W. Brameshuber. Textile Reinforced Concrete - State-of-the-Art Report of RILEM TC 201-TRC, 2006. [2022-11-14]. https: //www.rilem.net/publication/publication/100 [5] P.-C. Aïtcin. Vysokohodnotný beton. 2005. ISBN 80-86769-39-9. [6] N. V. Chanh. Steel fiber reinforced concrete, 2015. [2022-11-13]. https://www.academia.edu/70854322/ Steel_Fiber_Reinforced_Concrete [7] Litracon. [2022-11-13]. http://www.litracon.hu/en [8] Lucem - light transmitting concrete – Lucem GmbH Aachen, Germany. [2022-11-13]. https://lucem.com/ [9] Licrete® – innovative building material composed of high quality concrete and transparent elements. [2022-11-13]. https://licrete.com/ [10] Uhlíkový roving Tenax STS40 F13 24K 1600tex. [2022-11-30]. https: //www.havel-composites.com/cs/produkty/uhlikovy- roving-tenax-sts40-f13-24k-1600tex-12-6217 [11] Cem-fil® 5325 | owens corning composites. [2023-01-02]. https://www.owenscorning.com/en-us/ composites/product/cem-fil-5325 65 https://www.casopisstavebnictvi.cz/clanky-vyznam-betonu-a-betonovych-konstrukci-zhlediska-kriterii-udrzitelne-vystavby.html https://www.casopisstavebnictvi.cz/clanky-vyznam-betonu-a-betonovych-konstrukci-zhlediska-kriterii-udrzitelne-vystavby.html https://www.casopisstavebnictvi.cz/clanky-vyznam-betonu-a-betonovych-konstrukci-zhlediska-kriterii-udrzitelne-vystavby.html https://stavba.tzb-info.cz/beton-malty-omitky/18732-co-je-to-textilni-beton-trc https://stavba.tzb-info.cz/beton-malty-omitky/18732-co-je-to-textilni-beton-trc https://dspace.cvut.cz/handle/10467/98215 https://www.rilem.net/publication/publication/100 https://www.rilem.net/publication/publication/100 https://www.academia.edu/70854322/Steel_Fiber_Reinforced_Concrete https://www.academia.edu/70854322/Steel_Fiber_Reinforced_Concrete http://www.litracon.hu/en https://lucem.com/ https://licrete.com/ https://www.havel-composites.com/cs/produkty/uhlikovy-roving-tenax-sts40-f13-24k-1600tex-12-6217 https://www.havel-composites.com/cs/produkty/uhlikovy-roving-tenax-sts40-f13-24k-1600tex-12-6217 https://www.havel-composites.com/cs/produkty/uhlikovy-roving-tenax-sts40-f13-24k-1600tex-12-6217 https://www.owenscorning.com/en-us/composites/product/cem-fil-5325 https://www.owenscorning.com/en-us/composites/product/cem-fil-5325 Acta Polytechnica CTU Proceedings 47:63–65, 2024 1 Introduction 2 Preparation of samples 3 Experiment and results 4 Conclusions Acknowledgements References