EFFECT OF SHORT METALLIC FIBERS IN MIXED REINFORCED HRFRC BEAMS : AN EXPERIMENTAL STUDY

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1 EFFECT OF SHORT METALLIC FIBERS IN MIXED REINFORCED HRFRC BEAMS : AN EXPERIMENTAL STUDY A.Si-Larbi,E.Ferrier,P.Hamelin Laboratoire Mécanique, matériaux & structures, Université Claude Bernard, Lyon I, Domaine scientifique de la Doua, 696 Villeurbanne Cedex, France. Abstract The four tested beams were frame worked with steel rebars or mixed CFRP-steel armatures according to Eurocode. The first beam that combines high performance concrete without short-fibers and steel framework permits to verify the design and to identify the bending behavior. The mechanical test done on the second beam in HPFRC permits to measure the contribution of the short-fibers in the tensile and the shear zone parts of the beam. In this case, no transverse framework is used following the design. For the two last beams, the objective is to confirm the possibility to use some Carbon Fiber Reinforced Polymer framework mixed with usual steel framework in order to keep ductility in the structure. The paper discusses the results obtained and compares the efficiency of such composite beams with usual RC beam. 1. Introduction The objectives are here to outline the possibility to design reinforced concrete beams using mixed steel-cfrp framework and ultra high performance fiber reinforced concrete (HPFRC) material. These materials show a particular interest for the civil engineering because of their high mechanical properties in compression and because of their better durability. The addition of metallic short fibers in the cementeous matrix permits to get a ductile material with increased tensile properties. Nevertheless the use of tensile rebars is even necessary for structural members applications (P. Rossi, [1, ]). The HPFRC find numerous applications in the civil engineering field in the cases of architectural elements [] and of the prestressed structures [3]. The use of a concrete reinforced with short fibers permits to limit the steel rebars numbers used in usual reinforced concrete structure. For structure such as beams, ultra high performance concrete (HPC) is used with steel prestresses concrete techniques in order to increase the compression level in concrete. The higher cost of the material associated to a complex manufactured method (application of prestressed), limit the development of such material. To reduce the cost, 153

2 it is necessary to search for classical and technical solutions of reinforced concrete structure. The first studies carrying on reinforced concrete beams using HPFRC exhibit the performances and the limits of these structures: - firstly, the addition of metallic fibers to the concrete matrix permits to reduce and above all to eliminate the transverse stirrups [4]. - secondly the gains of performances are important. - on the other hand, the tests of beams in HPFRC "over reinforced by steel rebars" [5, 6], failed by concrete crushing in the compressed part with a brittle failure mode. While reducing the quantity of tensile rebars, it is possible to get a ductile failure by steel yielding and a weak level of compression of the top fiber (about 4% of the ultimate compression concrete strength). It does not permit to valorise the HPFRC. The development of this new kind of concrete for the structures made of reinforced concrete requires an optimisation of the design. It aims at getting "ductile" structures and to reach a level of strain to significant failure of the part compressed of the beam. In this objective, it is possible to use more efficient materials in tension that steel such as high strength CFRP. These rebars reinforcement obtain by pultrusion of carbon fibers in combination with epoxy matrix have very interesting mechanical properties just as well to the level of their rigidity (16 GPa) that of their ultimate strength (5 MPa). The inconvenience of the carbon fibers for civil engineering structures is a brittle mechanical behavior. This behavior does not correspond to the mechanical behaviors of a steel reinforced concrete beam. Also the combination of CFRPs framework and steel rebars will permit to get a set of tensile longitudinal reinforcements more effective presenting a yielding mechanical behavior. The beams obtained finally present a ductile and safety behavior. The addition of short metallic fibers in the HPC permits to remove the transverse steel rebars and contribute to the reduction of the cost of production of these structures. In order to optimise the behavior of the beams in HPFRC reinforced by mixed armatures, it is first necessary to identify the mechanical behavior of these structures and to compare them with reference beams made of usual reinforced concrete. In order to differentiate the effect of the rebar s material and the addition of short metallic fibers, we first design a HPC beam reinforced by steel rebars. Then, the study of a HPC reinforced by short metallic fibers permits to identify the influence of the fibers on the behavior of the structure. The following step corresponds to the introduction of mixed armatures with the aim to evaluate the relevance of this association. The criteria to evaluate the gains and the levels of performance of the multi-material beams are the analysis of the ultimate behavior (gains of load, mechanisms of failure ) and the moment-curvature response of the structure.. Material and tests specimens description.1 Properties of the high performance concrete The HPFRC retain for this study is formulated by Lafarge. The beams have been cast in moulds by the team of the Central Laboratory of Research of Lafarge. The concrete was placed in three layers and was vibrated internally and externally. All beams and 154

3 control specimens were cast and cured under similar conditions. The beams and specimens were kept covered under polyethylene sheets for 8 days until 4 hours before testing. Axial compressive tests have been done on 1 specimens allowing to obtain the ultimate strength of concrete (14 MPa) and the Young s modulus (5 GPa).. Mechanical properties of the reinforcements The steels used for rebars present an average yield strength of 55 MPa and a Young modulus of 1 GPa). The CFRP reinforcements have a brittle elastic behavior with an ultimate strength of 5 MPa and a young s modulus of 16 GPa..3 Beams characteristics A design according to the rules of the Eurocode permits to define the areas of reinforcemnets kept by this study (Fig. 1). Unit in mm A 1 spaced 1 mm 5 A 3 As = 3.35 % Beam 1 Unit in mm B 5 B 3 As= 3.35 % (Beam ) As= 3. % (Beam 3) Beam, 3 and CFRP rebars Steel rebars section AA Fig. 1 Beams description 7 section BB 3 meter span beams have been designed according to the EC with a required failure of the steel/composite rebars and a structural dead load of kn, leading to an ultimate moment of 115 knm. Four mm diameter steel rebars are necessary for beam 1 and two 5 mm diameter steel rebars are mixed with two 1 mm CFRP rebars for beam, 3 and 4. The shear resistance is verified while keeping a concrete tensile strength of 1 MPa and according to Rilem recommendations [8]. Four beams based on this prior 155

4 design have been tested (Fig. 1). The first beam permits to verify the design and to identify the bending behavior of the HPC beams. The mechanical test done on the second beam in HPFRC allows evaluating the contribution of the short metallic fibers. In this case, no shear steel framework is display. The last two beams have been tested in order to observe the efficiency of CFRP rebars. Especially, the fourth beam shows a reduced concrete area in the upper part (Fig. ) in order to increase the compression ratio of the concrete and then the contribution of the rebars. The four-point loading is made of two equally concentrated loads, acting each at 35 cm of the middle span i.e at 1.15 m of the supports (Fig.1). The tests are driven loading by successive loading steps of.5 kn. 3. Experiment and results analysis 3.1 Instrumentation of the beams Electrical strains gauges of 1 ohms and a 1 mm grid length have been bonded on the concrete as well as on the longitudinal steel rebars at the mid-span of the beam. Besides, displacement measurement device are bonded on the concrete with one regular interval of mm in width and mm in height in compressed area (Fig. ). Four bending point Displacement measuring device Strains gauges* 7 3 m 5 Deflection sensors 3 * strains gauges bonded on the lower part of the framework Fig. Experimental device The analysis of these set of data permits to obtain the Navier diagrams and the momentcurvature curves. A 1 mm LVDT displacement sensor is placed at the mid-span to measure the deflection. For each level of loading, the values of the displacements and the strains gauges are monitored. 3. Experimental results Several analysis can be done in order to evaluate the effects of the short metallic fibers and of the mixed rebars. First, the evolution of the mid-span displacement will give information about stiffness and structure ductility. Then the analysis of material strain allows to focus on the efficiency of each. The last part concerns the study of the failure mode of each of the beams. The table 1 gives the values of the tests results obtained. 156

5 Table 1: Beam tests result Cracking load (kn) Yielding load (kn) Failure load (kn) Mid-span deflection (mm) Failure mode Beam Tensile rebars Beam Tensile rebars Beam Tensile rebars Beam Concrete in compression 3..1 Evolution of the mid span displacement The different load-displacement curves don't present any significant discepency all along the loading (Fig. 3). Indeed, the evolution of the mid span displacement corresponds to the three stages of behavior of a reinforced concrete structure. A first stage correspond to the behaviour of a non cracked beam. The second stage drives to the cracking of the beam. Indeed, the cracking decreases the inertia and therefore the bending stiffness of the area. The last stage of behavior corresponds to the yielding of the tense longitudinal armatures. Table : Beams materials strain values for several level of loading Beam 1 Beam Beam 3 Beam 4 craking Upper strain value ( m/m) Rebars strains ( m/m) yielding Failure craking yielding Failure Concerning the beam 3 (mixed reinforcements), it is important to note that the third stage of behavior is modified. The steel reinforcements yielded and the CFRP rebars are elastic, they bear a supplementary effort. A gain of load of 5 % is gotten then for the beam 3. The use of steel-cfrp mixed reinforcements permits to get a structural behavior therefore is equivalent to strain hardening material. The table represents the evolution of the strains of the top fiber and the lower reinforcements all along the loading. The table 5 gives the strains values for 3 load levels corresponding to the occurring of cracks in the concrete in tension, to the steel rebars yielding and to the failure. 157

6 3 5 Load (kn) 1 Beam 1 Beam Beam 3 Beam Mid span displacement (mm) Fig. 3: Evolution of mid-span deflection as a function of loading 3.. Strains evolution The load-strain curves of the Fig. 4 illustrate the linear behavior until failure for the concrete or for the steel rebars. It is also important to note that for the beams 1 and 3 the maximal strains recorded on steel have a value superior to the 1 conventional allowable strain. For the beam 1 a strain of 14 has been recorded for example before the failure of steel in tension. It is again important to note that for the beam 3, the answer in load-strain is of elastic-yielding-hardening type. It permits the increase of the beam area moment. Concerning the strains of the top fiber of the concrete compressed, a significant difference exists between the concrete HPBF of the beam and the HP concrete non-reinforced of the beam 3. Indeed for the same level of loading, the strain of the top fiber is raised more for the HPFRC (beam ) that for the HP concrete (Fig. 4). 3 Beam 3 5 Beam 3 Beam 4 Beam 1 Beam 4 Beam 1 Load (kn) Beam 5 Beam strain gauges 1 5 strain gauges Strain ( m/m) rebars Beam Beam 1 steel rebars Beam steel 3 steel rebars Beam 3 CFRP rebars Beam 4 steel rebars Beam 4 CFRP rebars Beam 1 Beam Beam 3 Beam 4 Fig. 4 Evolution of material strain during the loading This difference is probably explained by a structural effect (position of the neutral axis, beam section equilibrium) corresponding to a modification of the tensile mechanical law of the concrete. The analysis of the Navier diagrams confirms this observation subsequently. In order to analyse the origin of the behavior differences noted between 158

7 the different beams, it is important to calculate the evolution of the position of the neutral axis and the beam curvature. Displacement measurement device placed on the side of the beams in the compressive part allows to assess the variation of length and therefore the compressive strains in several points of the beam height. This analysis allows to assess the curvature and the position of the neutral axis according to the applied load Diagrams of Navier-Position of neutral axis The position of the neutral axis (z u ) and the value of the curvature ( ) are obtained for every level of loading (Fig. 5) ,4 mm Beam depth (mm) 1 5 mm 166,3 mm 16, mm 146,4 mm Neutral axis Beam 1 Beam Beam 3 Beam Load (dan) Fig. 5: Neutral axis position in function of loading It is possible to notice that the curve is linear through the beam depth that is in conformity with an ideal beams bending behavior. The diagrams defined for every level of loading, the position of the neutral axis and the value of the curvature. For every beams, this curvature is identical in the compression and tension parts that permits to verify that the classic assumption of the reinforced concrete, when the mixed armatures or the HPFRC are used, are kept. As far as the evolution of the curvature is concerned, we can note similarities between the beams 1 and 3. The curvature of the beam follows the one of the beams 1 and 3 up to 3 knm. Beyond this value the curvature of the HPFRC beam increases more rapidly than the one of the HPC beams without short metallic fibers and of the HPFRC beams with mixed armatures. This can be explained by the lower neutral axis in the case of the beam. Indeed, the results analysis permits to observe the variation of the position of the neutral axis (z u, Fig. 5). Behaviour of beams, 3, 4 present difference with the beam 1. The effect on the structure is to modify the neutral axis position. The efficiency of the metallic short fibers on the flexural bending behaviour of the beam is put in evidence and must be quantified. 159

8 3..4 Failure mode and cracking pattern of the beams Concerning the beam 1, the behaviour is nearly linear with a low ductility; its failure results from the yielding of steels rebars. The rate of maximal concrete stress in compression (superior fibers) is of 5 % of the ultimate concrete stress. All along the loading, vertical cracks regularly spaced (in the zone of vertical stirrups) are observed for the beam 1. The beam to 4 does not present any shear concrete cracks pattern. The failure occurs in the tension part following the apparition of one macro cracks in central zone of constant moment. The strain rate is then of 8 % of the ultimate concrete strain. The same observations are made for the beam 3, with a superior cracking height corresponding to a higher position of the neutral axis. Concerning the beam 4, the reduction of compressed area permits to get the crushing of the concrete in compression. The HPFRC is used then to the maximum of its performances in compression. In conclusion, the ductility and the failure mode of the HPFRC beam can be obtained by a suitable design. 4. Analyse : influence of the short metallic fibers on the beam mechanical behavior The experimental study permitted to verify that the sections are plane and remain plane. Concerning the compressed stress, because of the quasi-elastic compressive behaviour of HPFR concrete, a distribution of the stress triangular is kept (Fig. 1). The tensile stress is shared between steel and the tensile concrete. The distribution of the stress within the concrete in tension is in agreement with the scientific recommendations of Rilem [8]. The equilibrium of the section of reinforced concrete beam is assured if the external moments (M sd ) are equal to the sum of the moments of the concrete in compression (M c ), of the tensile steels (M s1 and M s ) and to the moments of the fibers (M f ). M sd Mc Ms1 M f Ms (1) For each value of loading and curvature increment, knowing the mechanical behaviour law of the concrete in compression, it is possible to calculate the corresponding stress ( c ). zu zu zu M c bw c z dz bw E c z dz bw E c z dz () While knowing the experimental curvature, the stress-strain material relation and by a iterative calculation process, it is possible to get for every value of loading the values of the internal moments corresponding to the balance of the section (rel 3, 4, 5). Ms1 As 1 s1 ( d1 z) As 1 Es( s) s1 ( d1 z) As 1 Es( s) ( d1 z) (3) Ms As s ( d z ) As Es ( s) s ( d z ) As Es ( s) ( d z ) (4) Concerning the moment undertaken in part by the short fiber reinforced concrete; an expression of calculation is proposed by Rilem recommendations (rel 5). h w h bw (5) M f bw f z dz f 1 w dw w h zu 16

9 with : height of the crack, w,: maximal opening of the crack under an increment of loading, b, h,: basis and depth of the beam, f : tensile stress of the concrete. bw c c z u z u h d d1 A s A s1 s z Fig. 6: Strain and stress distribution in a HPFRC beam The calculation of the moment undertaken by the fibers requires the knowledge of the values of the crack opening (w), of the tensile stress within concrete ( f ), which is difficult to have with no specific measurements. On the other hand, it is possible to get this value as subtracting the sum of the internal moments defined by relation 6 to the external moments applied to the beam. M M M M M ) (6) f sd ( c s1 s f z F As1 F As.h 1 Beam external moment (kn.m) Beam Beam 3 Beam Tense metallic short fibers moment (KN.m) Fig. 7 Tense steel fibers concrete moment in function of external moment. Indeed the sum of the whole internal moments is equal to the external moments for the beam in HPFRC, on the other hand there is a difference among for the three beams in HPFRC. This gap corresponds to the value of the moment undertaken by the short metallic fibers (Fig. 7). It is important to observe the linear variation of the corresponding moment with the progressive cracking of the concrete. It is important to outline that the moment undertaken in tensile part by the short metallic fibers represents 1 % of the value of the applied moment at failure. 5. Conclusion The structural tests presented in this paper put in evidence the possibility to use ultra high performance fiber reinforced fibers concrete for the design of reinforced concrete 161

10 structure. The main hypotheses of the steel reinforced concrete may be applied to this kind of structures. The use of CFRP-steel mixed rebars permits in an elastic stage to keep a bending stiffness comparable to the beams reinforced by traditional reinforcements. On the other hand the use of mixed armatures increases by 5 % the failure load. The ductility of the structure is also increased thanks to the strain yield hardening behavior due to the combination of the mixed reinforcements. Finally the results of this study confirm the potentiality of this kind of structure. Indeed, while using a traditional concrete (with a compressive strength of 4 MPa), a beam section of 19x315 mm is necessary to reach one moment of failure of 115 knm (beam 1, and 4) and a beam section of 1x35 mm to reach knm (beam 3). The use of HPFRC permits to decrease of 4 % the weight of the structure whereas its association with CFRP reinforcements in carbon permits to reduce the same weight of 5 %. The performance of the material permits to consider the optimisation of the section in order to improve its the performances of it and get lighter structures with higher performances. Acknowledgements The authors would like to thanks the Lafarge societies (LCR) and Etandex for their technical support and the supplies of the materials having permitted to achieve this program of research. References [1] P. Rossi, Ultra-High Fibre reinforced concretes (HPFRC): An overview, Proceeding of the fifth International Rilem Symposium,PRO 15, Rilem Publications p [] Chanvillard G., Characterisation of fibre reinforced concrete mechanical properties : A review. Proceeding of the fith International Rilem Symposium PRO 15. Rilem Publications : 9-5 [3] Chong Hu. Casanova P. Delalande F. Mix design of very high strength steel fiber reinforced concrete (VHS SFRC for tunnel liner. Proceeding of the fith International Rilem Symposium PRO 15, Rilem Publication : [4] Aïtcin P C. Richard P., The pedestrian/bikeway bridge of Sherbrooke. Proceeding of 4 th international symposium on utilisation of high-strength/high performance concrete Paris. : [5] Casanova P. Rossi P. Scaaller I. Can steel fibers replace transverse reinforcement in reinforced concrete beams. ACI Materials Journal 1997; 94 (5): [6] Qian C. Patnaikuni I. Properties of high-strength steel fiber-reinforced concrete beams in bending, Cement and Concrete Composites 1999; Vol. 1: [7] Ashour S., Wafa F.F. Flexural behaviour of high-strength fiber reinforced concrete beams. ACI structural Journal 1993 ; 9 (3) :79-87 [8] Final recommendations of Rilem TC 16-TDF. Materials and structures 3 ; Vol 36 :