https://doi.org/10.14311/APP.2022.33.0452 Acta Polytechnica CTU Proceedings 33:452–456, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague DOWELS EFFECTIVENESS INVESTIGATION BETWEEN ULTRA HIGH PERFORMANCE FIBER REINFORCED CONCRETE AND REINFORCED CONCRETE Spyridon A. Paschalisa, ∗, Andreas P. Lampropoulosb a School of Engineering and Computing, University of West London, UK b School of Architecture, Technology and Engineering, University of Brighton, UK ∗ corresponding author: Spyros.Paschalis@uwl.ac.uk Abstract. In the present study, the performance of Reinforced Concrete (RC) beams which were strength- ened with Ultra High Performance Fiber Reinforced Concrete (UHPFRC) and dowels at the interface was investigated. RC beams with a length of 2.2 m strengthened with UHPFRC layers at the tensile side. Before the application of the layers, the interface between RC and UHPFRC was roughened. During the testing, the interface slips between UHPFRC and RC were recorded using Linear Variable Differential Transformers (LVDTs). The beams were tested under four-point flexural test. The results of the present study indicated that the dowels at the interface reduce the slips at the interface, delay the formation of cracks and result in higher load carrying capacity. Keywords: Dowels, interface characteristics, reinforced concrete beams, steel connectors, strength- ening, UHPFRC. 1. Introduction Safety of structures is of paramount importance espe- cially in case of old substandard structures or struc- tures damaged by earthquakes or other accidental ac- tions. Existing strengthening and retrofitting meth- ods present crucial disadvantages which are related to the high cost, the difficulty in the application and the low performance of the technique. The present study investigates the effectiveness of Ultra High Per- formance Fiber Reinforced Concrete (UHPFRC) as strengthening material using dowels at the interface between Reinforced Concrete (RC) members and UH- PFRC layers. The are several studies in the literature investigat- ing the properties of the UHPFRC [1–4]. The last years research has been focused on the application of UHPFRC as repair and strengthening material. An analytical model for the prediction of the response of composite UHPFRC and RC elements was devel- oped by Habel et al. [5]. UHPFRC was applied as repair material from Bruhwiler and Denarie [6], while Lampropoulos et al [7] developed a numerical model that predicts the performance of RC beams which were strengthened with UHPFRC. Paschalis et al. [8], conducted full scale tests to investigate the ef- fectiveness of UHPFRC for the strengthening of RC beams. In this study, RC beams strengthened with UHPFRC layers, with and without the use of steel bars in the layers. The addition of steel bars in the layer resulted in high load carrying capacity of the strengthened beams. However, high values of slips at the interface between UHPFRC and concrete were recorded. In the present study the performance of RC beams which were strengthened with UHPFRC layers and dowels at the interface has been investigated. This is a topic which has not been investigated in detail and needs further investigation. 2. Experimental Program Six beams have been tested in the present study. Two RC beams prepared without any layer and used as control beams. In two beams, UHPFRC layers added at the tensile side and in two beam dowels added at the interface between UHPFRC and RC. The reinforcement and the dimensions of the beams without any reinforcement are presented in Figure 1. In Figure 2, the dimensions of the layers and the po- sition of the dowels are presented. The dowels were designed based on Greek Retrofitting Code [9]. 3. Preparation of the Specimens Fine sand, GGBS, Silica Fume, Superplasticizer, High strength cement and steel microfibers were used in the mix design. In addition, a high percentage of fibers (3 Vol.%) was incorporated in the mix to achieve strain hardening. The interface was rough- ened using a needle scaler and the dowels placed in position using a drill. The preparation of the beams which were strengthened with dowels is presented in Figure 3. 4. Properties of the materials Cubes with 10 cm sides, tested in compression ac- cording to BS EN 12390-3:2009 [10]. The compressive 452 https://doi.org/10.14311/APP.2022.33.0452 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 Dowels Effectiveness Investigation Figure 1. Control Beam. Figure 2. Strengthened beams using dowels. Figure 3. Preparation of the beams. strength of concrete which was used for the prepara- tion of the beams was 31 MPa and the compressive strength of UHPFRC layers was 136 MPa. The ten- sile characteristics of UHPFRC identified with a di- rect tensile test. The results are presented in Figure 4. Based on the average curve of Figure 4, the tensile strength was 11.5 MPa and the modulus of elasticity 50 GPa. 5. Testing of the beams In Figure 5, a strengthened beam with layer before the testing is presented. As can be seen, LVDTs were place at the interface between UHPFRC and RC to record the slip. In Figure 7, the average experiments results are placed. The Results of Figure 7 indicate the effectiveness of dowels. In both techniques, the stiffness of the beams was increased. However, with the addition of dowels a higher load carrying capacity was achieved. In this case, the maximum load was increased by 22.5%. On the contrary, an increase of only 1.5% was achieved with the addition of layers only. In Figure 8, the beams after the testing are pre- sented. The control beams failed with a major crack at the tensile side (Figure 8a). The failure mode of the two beams which were strengthened with UHPFRC 453 Spyridon A. Paschalis, Andreas P. Lampropoulos Acta Polytechnica CTU Proceedings Figure 4. Stress-strain curve from the testing of UHPFRC in tension. Figure 5. Experimental setup for the testing of the beams. Figure 6. a) LVDTs on side 1 b) LVDTs on side 2. 454 vol. 33/2022 Dowels Effectiveness Investigation Figure 7. Results from the testing of beams. Figure 8. Failure mode a) Control Beam b) Beam strengthened with layer c) beam strengthened with layer and dowels. layers was not the same. In one beam the bond at the interface was strong and a major crack at the layer progressively propagated to the RC beam. In the second beam, a de-bonding at the interface oc- curred (Figure 8b). Finally, identical was the failure mode of beams which were strengthened with layers and dowels. A major crack appeared in the layers and propagated to the RC beam (Figure 8c). During the testing it was noticed that dowels re- sulted in the delay of the formation of cracks. This was also evident from the results of Figure 7. The cracking on the control beams started at value of load 5 kN, on the strengthened beams with layers at 15 kN and on beams with layers and dowels at 24 kN. The results from the measurements of slips at the interface are presented in Table 1. Due to the fact that a local debonding commenced in one of the beam which was strengthened with UHPFRC layer, the reading of this beam were ignored. LVDT Layers 1 Dowels 1 Dowels 2 2 0.06 0.04 0.01 3 0.11 0.01 0.18 4 0.36 0.05 0.06 7 0.03 0.03 0 8 0.18 0.14 0.01 Table 1. Reading of the LVDTs. From the results of Table 1, it can be noticed that almost in all positions, the values of slip for the beams with dowels at the interface, were significantly lower 455 Spyridon A. Paschalis, Andreas P. Lampropoulos Acta Polytechnica CTU Proceedings compared to the slips of the beam without dowels. This indicates the better bonding at the interface when dowels are used. 6. Conclusions The main aim of the present study was to investigate the effectiveness of dowels at the interface between UHPFRC and RC. Full scale tests were conducted and RC beam with a length of 2.2 m strengthened with UHPFRC layers, with and without dowels. The slips at the interface were also recorded. Based on the experimental results the following conclusions can be drawn: • Dowels delay the formation of the cracking • Dowels result in a better bonding • Dowels result in a higher load carrying capacity Based on the findings of the present research, the use of dowels should be considered in cases that better bonding and better performance of the strengthened elements is required. References [1] L. Ferrara, N. Ozyurt, M. di Prisco. High mechanical performance of fibre reinforced cementitious composites: the role of casting-flow induced fibre orientation. Materials and Structures 44(1):109-28, 2010. https://doi.org/10.1617/s11527-010-9613-9. [2] S. Paschalis, A. Lampropoulos. Fiber content and curing time effect on the tensile characteristics of ultra high performance fiber reinforced concrete. Structural Concrete 18(4):577-88, 2017. https://doi.org/10.1002/suco.201600075. [3] S. Paschalis, A. Lampropoulos. Ultra High Performance Fiber Reinforced Concrete Under Cyclic Loading. ACI Materials Journal 113(4):419-427, 2016. [4] S.-T. Kang, J.-K. Kim. The relation between fiber orientation and tensile behavior in an Ultra High Performance Fiber Reinforced Cementitious Composites (UHPFRCC). Cement and Concrete Research 41(10):1001-14, 2011. https: //doi.org/10.1016/j.cemconres.2011.05.009. [5] K. Habel, E. Denarié, E. Brühwiler. Structural Response of Elements Combining Ultrahigh-Performance Fiber-Reinforced Concretes and Reinforced Concrete. Journal of Structural Engineering 132(11):1793-800, 2006. https://doi.or g/10.1061/(asce)0733-9445(2006)132:11(1793). [6] E. Bruhwiler, E. Denarie. Rehabilitation of concrete structures using Ultra-High Performance Fibre Reinforced Concrete The Second International Symposium on Ultra High Performance Kassel, Germany, 2008. https://infoscience.epfl.ch/reco rd/182195/files/010.pdf. [7] A. P. Lampropoulos, S. A. Paschalis, O. T. Tsioulou, et al. Strengthening of reinforced concrete beams using ultra high performance fibre reinforced concrete (UHPFRC). Engineering Structures 106:370-84, 2016. https: //doi.org/10.1016/j.engstruct.2015.10.042. [8] S. A. Paschalis, A. P. Lampropoulos, O. Tsioulou. Experimental and numerical study of the performance of ultra high performance fiber reinforced concrete for the flexural strengthening of full scale reinforced concrete members. Construction and Building Materials 186:351-66, 2018. https: //doi.org/10.1016/j.conbuildmat.2018.07.123. [9] Greek Organization for Seismic Planning and Protection, Greek Retrofitting Code (GRECO), Athens, 2013. [10] BS EN 12390-3:2009. Testing hardened concrete- Part 3: Compressive strength of test specimens, 2009. 456 https://doi.org/10.1617/s11527-010-9613-9 https://doi.org/10.1002/suco.201600075 https://doi.org/10.1016/j.cemconres.2011.05.009 https://doi.org/10.1061/(asce)0733-9445(2006)132:11(1793) https://infoscience.epfl.ch/record/182195/files/010.pdf https://doi.org/10.1016/j.engstruct.2015.10.042 https://doi.org/10.1016/j.conbuildmat.2018.07.123