TEKSTILOM – EKSPERIMENTALNO ISTRAŽIVANJE COMPARATIVE ANALYSIS OF BEHAVIOUR OF REINFORCED CONCRETE BEAMS USING BARS AND TEXTIL – EXPERIMENTAL RESEARCH

Dinamičan razvoj tehnologije materijala doveo je do primjene različitih novih materijala za poboljšanje svojstava nosivosti i trajnosti betonskih nosača. Jedan od pristupa jeste primjena sistema zaštite obložnog dijela betonskog nosača (zaštitni sloj betona). U ovu svrhu, razvijeni su polimerni materijali armirani karbonskim, staklenim ili aramidnim vlaknima (FRP – Fiber Reinfored Polymer). Novi pravac razvoja ovih sistema jeste primjena tekstilom armiranih sitnozrnih betona, gdje se koriste tekstili s vlaknima u više pravaca. Fabrički proizvedena ravninska tekstilna struktura sačinjena je od vlakana upletenih na razne načine, kao što su tkanje, pletenje, filcovanje ili štrikanje. Za potrebe istraživanja i razvoja ovog sistema ojačanja betonskih greda, formirana su dva istraživačka centra u kojima se sprovode projekti pod nazivom „Textile Reinforced Concrete (TRC) -Technical Basis for the Development of a New Technology” (SFB 532) RWTH Aachen University i „Textile Reinforcements for Structural Strengthening and Repair” (SFB 528) Technische Universitat Dresden. U ovim istraživačkim centrima sprovode se istraživanja mehanizama trajnosti, prionljivosti i kapaciteta nosivosti. Pored dva navedena velika projekta, postoji i niz projekata koji se odnose na tekstilom armirani beton, a sprovedeni su u Izraelu, Sjedinjenim Državama, Grčkoj, Belgiji, Ujedinjenom Kraljevstvu i Kanadi [3]. Utvrđeno je da količina i raspored tesktilne strukture imaju značajan uticaj na ponašanje tekstilom armiranog betona. Istraživanja su posvećena:


INTRODUCTION
The dynamic development of material technology has led to the application of various new materials for the purpose of improving the load-bearing properties and durability of concrete girders. One of the approaches is the application of the protection system at the surface layer of the concrete girders(protective layer of concrete). Polymer materials reinforced with carbon, glass or aramid fibres (FRP -Fibre Reinforced Polymer) have been developed for this purpose, A new direction of development of these systems is the use of textile reinforced fine-grained concrete, where textiles with fibres have been used in several directions. The factoryproduced flat textile structure is formed of interlocking fibres in various ways such as weaving, knitting, felting or knitting. Two research centres were formed for the purpose of a research and development of this system where projects entitled 'Textile Reinforced Concrete (TRC) -Technical Basis for the Development of a New Technology' (SFB 532) RWTH Aachen University and 'Textile Reinforcements for Structural Strengthening and Repair'(SFB 528) Technical University of Dresden. These research centres have carried out a research about the durability mechanisms, adhesion and load-bearing capacity. In addition to these two major projects, there are a number of projects related to textile reinforced concrete, which have been implemented in Israel, USA, Greece, Belgium, the UK and Canada [3]. It was determined that the amount and arrangement of the textile structure have primjeni tekstilnih materijala znatno većeg modula elastičnosti u odnosu na beton, kako otvaranje pukotina u betonu ne bi uzrokovalo značajnu redukciju krutosti konstruktivnog elementa, trajnosti tekstilnog materijala, ponašanju tekstilnog materijala pod dugotrajnim opterećenjem, prionljivosti s betonom, te smanjenju troškova izrade tekstilnih mreža. Istraživanjima je utvrđeno da alkalno otporna staklena vlakna, karbonska i aramidna vlakna ispunjavaju potrebne zahtjeve. U radu [13] dat je pregled fizičko-mehaničkih i deformacionih karakteristika pojedinih vlakana.

PROGRAM EKSPERIMENTA (EXPERIMENTAL PROGRAM)
S ciljem utvrđivanja mehanizma nosivosti greda armiranih armaturnim šipkama i tekstilom, eksperimen-a significant influence on the behaviour of textile reinforced concrete. Researchers are dedicated to the application of textile materials with significantly higher modulus of elasticity compared to the concrete, so that openings of the cracks in the concrete are unlikely to cause a significant reduction in stiffness of structural element, durability of textile material, behaviour of textile material under long-term load, adhesion with the concrete and reduced costs of textile meshes production. Studies have shown that alkali-resistant glass fibres, carbon and aramid fibres meet the necessary requirements. In the paper [13], physical-mechanical and deformation characteristics of individual fibres have been presented.
The use of alkali-resistant glass in civil engineering began in the 1970s [3]. Depending on the fineness of the fibres, the strengths are up to 1400 N/mm 2 , the linear elastic elongation is up to 2%, and the modulus of elasticity ranges from 70 to 80 kN/mm 2 . In the experimental program presented in the paper textile meshes with alkali-resistant glass fibres were used. A production of textile meshes has begun for the purpose of achieving a stable form of reinforced concrete with textile. In the paper [3] has been described an idea of reinforcing textile meshes during production, and achieving stability of textile mesh through impregnation and adding additional support and elements for stiffening.  [13]. Research have shown that the reinforcement effects of using textile meshes depends on the main fibre orientation of the textile meshes, the number of textile layers of the meshes, the installation process when it comes to achieving adhesion textile mesh or basis and an adequate shrinkage of the textile mesh girder. Increases have been obtained when it comes to the bearing capacity of reinforced concrete beams for shearing and bending, bearing in mind that the percentage of increase depends on the previously mentioned effects. In the papers [6], [8], [10], [11] and [14] have been presented numerical models for the calculation of reinforced concrete elements reinforced with textile mesh.
This paper presents a research program with the aim of determining possible applications of textile mesh reinforcement within the cross section, which would allow the development of semi-prefabricated reinforced concrete beams with a composite cross-section. Research with a similar idea has been presented in the paper [9].
Testing presented in [1], [6], [8] and [9] showed that the orientation of textiles is important for the final results in terms of load-bearing capacity and crack opening mechanism. Therefore, textile meshes were installed in such way that the main load-bearing fibres in the mesh are placed in the direction of the longitudinal axis of the beam model, which is a favourable orientation from the aspect of bearing capacity to bending. In order to achieve the best possible adhesion of concrete and textile mesh, during the installation of concrete meshes were left open from the top. The final arrangement of CRC model reinforcement is shown in Figure 1. The method of reinforcing the CRC model is presented in Figure 3.During the creation of the TRC model, textile mesh is reinforced with stirrup made out of wire diameter of 3 mm for the purpose of ensuring the stability of the textile mesh during installation of concrete. Figure 3. The method of reinforcing the CRC model Tokom ispitivanja, mjereni su ugibi, dilatacije (izduženja i skraćenja) gornjeg i donjeg površinskog sloja grede i širine pukotina. Raspored mjernih mjesta prikazan je na slikama 4 i 5.
During the testing, deflections, dilatations (elongation and shortening) of the upper and lower surface layer of the beam and the width of cracks were measured. The layout of the measuring points is shown in Figures 4 and 5. Measurement of deflection is done with the measurement accuracy of 0.01 mm, with a dilatation of deformation of 0.01 mm measuring accuracy and measuring bases 100 and 300 mm.
The load intensities of the beams were adopted on the basis of the calculation of a simple reinforced concrete beam of geometrical characteristics and spans presented in Figures 2 and 3. The full load of the beam, at which the bearing capacity is reached with a safety factor of 1.0, is F = 27kN. The models were loaded in 12 phases with load intensities of F = 2, 4, 6, 8, 14, 21, 27, 36, 45, 51, 54 kN. After reaching the load of F = 8 kN, the model was unloaded after each phase of the load in order to monitor the area of development of plastic deformations. The model was loaded with a hydraulic press. The model test setup is presented in Figure 6. U radu se predstavljaju rezultati mjerenja ugiba i praćenja mehanizma otvaranja pukotina. Na osnovu tih rezultata, moguće je donijeti zaključak o ponašanju betonskih greda armiranih armaturnim šipkama i tekstilnim mrežama. Na slikama 7 i 8 prikazani su radni dijagrami silaprogib ispitanih RC modela za mjerna mjesta U2 i U4

TEST RESULTS
The paper presents the results of measuring deflection and monitors the crack opening mechanism. Based on these results, it is possible to draw a conclusion about the behaviour of reinforced concrete beams using reinforced bars and textile meshes.
The fitting efficiency of the working diagrams is determined by the coefficient of determination R2. The coefficients of determination have satisfactory values. It is evident, as shown in diagrams, that there is a slightly higher scattering of the results during measuring the deflection at the measuring points which are in one third of the span (measuring point U4), measuring points far from the place of application of the load, compared to the measuring points near the application of force at half the span (measuring point U2). Here, the effect of heterogeneity of concrete structure, i.e. stochastically variable ratio of aggregate volume and cement stone volume along the beam is visible. The uniformity of the RC model behaviour can be seen in Figure 7. Figures 9 and 10 show the working force-displacement diagrams of the tested CRC models for the same measuring points U2 and U4. Iz radnih dijagrama na slikama 9 i 10 vidljivo je da je kod CRC modela veće rasipanje rezultata, što je posljedica kompozitnosti presjeka, otežanih uslova ugradnje betona i stepena prionljivosti tekstila i betona duž grede.
The working diagrams in Figures 9 and 10 show that the CRC model has a higher scattering of results, which is the consequence of the cross-sectional composition, difficult conditions for concrete installation and the degree of adhesion of textiles and concrete along the beam.
By testing the TRC model, the load-bearing capacity of the beam was approximately 50% higher compared to the non-reinforced concrete beam. Figure 11 presents the working diagrams of the TRC model. Figure 11. TRC models, measurement point U2
The comparison of the force-displacement diagrams of the RC, CRC and TRC models is shown in Figure 12. The presented working diagrams of the RC and CRC models show similar behaviour of conventionally reinforced beams (RC) and beams reinforced with reinforcing bars and textiles (CRC). The determined average fracture force intensity is the same for CRC models and RC models.
Besides measuring deflection and dilatation, it has been monitored the mechanism of crack opening and the failure mode of the tested models. In the RC and CRC models, the first cracks opened at the same force intensity (F = 21kN). However, different fracture (failure) mechanisms have been registered in the RC model and the CRC model. In the RC model, the fracture occurred through the opening of a dominant vertical crack in the middle of the span, or by failure at the bending (Fig. 13), while in the CRC model the fracture occurred through the opening of the dominant oblique crack (Fig. 14).
Za intenzivniju primjenu tekstilnih mreža za armiranje unutar poprečnog presjeka betonske grede, iskustva istraživača u ovoj oblasti eksperimenta pokazuju da je potrebno riješiti tri ključna problema: − tehnologiju ugradnje betona unutar tekstilne mreže; − stabilnost tekstilne matrice (tekstilne mreže) tokom opterećenja; − prionljivost betona i tekstilne mreže. Jedan od pristupa za ostvarenje adekvatnog mehanizma nosivosti betonskih greda armiranih armaturnim šipkama i tekstilnim mrežama, promovisan i u radu [9], jeste izvođenje poluprefabrikovanih betonskih greda s prefabrikovanim obložnim dijelom grede Equal widths of vertical cracks in the middle of the range were registered on the RC and CRC model (width 0.3 mm)by the intensity of force F = 40kN.As intensity of the forces increased, at the same time increased the width of the vertical cracks on RC model up to registered width of 1 mm before the fracture. On CRC models with increasing force intensity, there was no increase in crack width. Thus, in the CRC model, the fracture occurred by shear failure. Textile mesh increased the bending load of the beam, but the orientation of the main fibres of the mesh in the direction of the longitudinal axis of the girder did not contribute to shear load, which is the conclusion of the research in [1], where it is stated that the mesh is perpendicular to the longitudinal axis.

CONCLUSION
By testing the RC model and the CRC model, a fracture force of equal intensity was obtained. However, the difference in the failure mechanism of the RC model and the CRC model indicates that the use of textiles has increased the load-bearing capacity of the beam compared to classically reinforced concrete beams. This increase was not quantified due to the scope of the experimental program, which was limited to a number of models for initial testing. Namely, no combination of textile nets with different directions of the main textile fibres and different percentages of reinforcement with transverse reinforcement was made in order to increase the shear capacity. However, the possibility of applying textile nets to simultaneously increase durability and load-bearing capacity is clear.
For more intensive application of textile reinforcement nets within the cross section of the concrete beam, the experience of researchers in this field shows that it is necessary to solve three key problems, − concrete placing technology within the textile mesh, − stability of the textile matrix (textile mesh) during loading and − adhesion of concrete and textile mesh. One of the approaches for achieving an adequate load-bearing mechanism of reinforced concrete beams using reinforced bars and textile meshes is presented in the paper [9], and that is the construction of semi-armiranim tekstilnom mrežom, ojačanom mrežom od čelične žice i unutrašnjim dijelom grede koji se izvodi na licu mjesta. Prijedlog poprečnog presjeka poluprefabrikovane betonske grede prikazan je na slici 15. prefabricated concrete beams with prefabricated beam cladding reinforced with textile mesh, reinforced steel wire mesh and inner part on the site. The cross-sectional proposal of the semi-prefabricated concrete beam is shown in Figure 15.
The following studies will be carried out for the purpose of developing an experimental program, and technology of adequate coupling of the prefabricated and monolithic part of the concrete beam and the verification of the physical and mechanical characteristics and deformation parts of the beam cross-section in Figure 15.
In modern civil engineering, textile meshes are used to increase the durability of the cladding part of reinforced concrete beams and subsequent reinforcement of beams through external application, coating (wrapping) of beams. Experiences presented in the papers are listed in the references. In addition, the tests presented in this paper have shown that it is possible to use textile meshes within the cross section with the development of installation technology.