Experimental investigation into the static and fatigue behavior of polymer concrete reinforced with GFRP rods

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1 Experimental investigation into the static and fatigue behavior of polymer concrete reinforced with GFRP rods C.M.L. Tavares, M.C.S. Ribeiro, D. Monteiro, P.P. Camanho, A.J.M. Ferreira Instituto de Engenharia Mecânica e Gestão Industrial, Faculdade de Engenharia da Universidade do Porto, Leça do Balio, Portugal ABSTRACT: Polymer concrete structures are being widely used in industrial applications and building industry. The good mechanical properties and corrosion resistance make their use attractive in many situations. In this paper, static fatigue behavior of polymer concrete beams reinforced with GFRP is investigated. Three-point bending tests were performed on a number of prismatic beams. Unreinforced and reinforced concrete beams were tested. Fatigue loading was applied through a sine load with various stress levels. Curves S-N were obtained and the fatigue performance of polymer concrete beams was assessed. 1 INTRODUCTION Development of petroleum chemistry produced various polymer materials that are being widely used as advanced materials in the automobile, aerospace and mechanical industries. In building Industry, polymer concrete (PC), has been introduced as a new material and became popular due to many characteristics overcoming conventional concrete properties such as strength level and hardening speed (Yeon et al. 1995). In some possible future applications, polymer concrete structures will be subjected to fluctuating or repeated cyclic loads. There have been several research works concerning fatigue behavior of conventional construction materials such as cement concrete and steel (Walker, 197; Suresh, 1991). However, little information is available concerning fatigue behavior of polymer concrete. In order to achieve a safe life design for polymer concrete structures, fatigue characterization is essential. Structures may be subjected to several millions of cycles based on their applications and environmental loadings. Low cycle fatigue deals with concrete receiving up to about one thousand cycles such as structures subjected to earthquakes. High cycle fatigue is related with structures subjected to about ten million cycles such as airport pavements and bridges. Super high cycle fatigue deals with structures expected to survive anywhere from ten to five hundred millions as offshore structures (Hsu,1981). Based on limited test data, it has been reported that polymer concrete was superior to conventional cement concrete in flexural fatigue strength (Hsu, 1984). Mebarkia also investigated the fatigue properties of un-reinforced polymer concrete subjected to flexural cyclic loading and it was concluded that the fatigue failure strain was independent of the applied stress level. The flexural fatigue strength at two million cycles was estimated to be 52% of the initial strength (Mebarkia, 1993). The aim of this research work is to investigate the static fatigue properties of polymeric concrete beams reinforced with GFRP rods, under flexural cyclic loading at room temperature. 2 EXPERIMENTAL PROGRAM 2.1 Preparation of specimens Polymer concrete with the binder formulation and mix proportions given in Table 1 was mixed and moulded to prismatic beams specimens (5 4 6mm 3 ). This optimised formulations is the result of a Taguchi analysis (Ferreira et al, (a), (b)). Table 1. PC formulation and mix proportion. PC formulation Resin Epoxy resin EPOSIL/551 Sand Foundry sand Resin content 2% Charge content % The chosen epoxy resin (EPOSIL-551) has low viscosity (5-6 mpa.s) and 7 MPa of flexural strength. The foundry sand used in this study has a very uniform grain and a D 5 of 342 microns (5% of sand particles are less than this size). The sand was previously dried before mixed with the resin.

2 Mechanical properties of PC formulation, as well as the mechanical characteristics of GFRP rebars are presented in Table2. Table 2. Mechanical properties of PC and GFRP rebars PC Properties GFRP Properties Compression Tensile Strength 82 MPa Strength 1 MPa Flexural Strength 38.7 MPa Flexural strength 1 MPa Compression Elasticity Modulus 11.5 GPa Tensile Elasticity Modulus 4 GPa Fifteen beam specimens were cast without reiforcement; fifteen specimens were cast reinforced with one GFRP rod of 4 mm diameter; and fifteen with one GFRP rod of 6mm diameter. GFRP rods were previously sand papered in order to promote the adhesion with PC. Reinforced beam specimens were prepared using the moulds illustrated in figure 1, specially designed for the effect. The reinforcement was placed at a constant depth from the bottom (8mm). All beam specimens were allowed to cure for one day at room temperature and then at 6ºC for seven hours (post-cure treatment). Figure 2 shows the final aspect of one reinforced beam specimen used in this study. characterize the velocity effect in the flexural strength and flexural elastic modulus. To characterize the development of cracks, threepoint bending tests, with 54mm flexural span were also carried out in an INSTRON testing machine, with displacements control, at rate of 1mm/min. Figure 2. Final aspect of one beam specimen of polymeric concrete reinforced with GFRP rod. 2.3 Fatigue tests Nine beams specimens both un-reinforced and reinforced with GFRP rods ( 4mm and 6mm) were tested in fatigue flexure (three-point bending) with 54mm flexural span. The tests were carried out in an INSTRON testing machine, with load control, adapted to apply cyclic loads (Fig.3). Figure 1. Moulds specially designed to produce the reinforced beam specimens. 2.2 Static flexural tests Six beams with both reinforced with GFRP rods ( 4mm and 6mm) and un-reinforced were tested in static three-point bending with 54mm flexural span. The tests were carried out in an INSTRON testing machine, with displacement control, at two different rates: 1mm/min and 1mm/min. Data obtained with these static tests was necessary to Figure 3. Set-up of INSTRON testing machine for flexural fatigue tests. The controlled load had a simple sinusoidal variation with 3.5 Hz of frequency and.5 kn of amplitude. The Stress level, S, is defined as the ratio of P max to P f, where P f is the static maximum load (S = P max /P f ). For each stress level, three specimens of

3 each type of beam were tested in fatigue. The stress levels selected were 8%, 7% and 6% of failure load. The number of cycles at failure, N f, was recorded by a counter installed on the testing machine, up to the maximum of one million. Experimental results and discussion 2.4 Static flexural test results The following figures show the load displacement curves obtained from the static tests. Figure 6 Load displacement curves obtained from the Reinforced Beam f= 4 mm v = 1 mm/min Unreinforced beams v = 1mm/min reinforced beams with GFRP rods of φ = 4 mm static tests with test velocity of 1mm/min Figure 4 Load displacement curves obtained from unreinforced beams static tests with test velocity of 1mm/min. Figure 7 Load displacement curves obtained from the reinforced beams with GFRP rods of φ = 4 mm static tests with test velocity of 1mm/min. 6 5 Reinforced beams f = 6 mm v = 1mm/min 4 4 Unreinforced Beam v = 1 mm/min Figure 8 Load displacement curves obtained from the reinforced beams with GFRP rods of φ = 6 mm static tests with Figure 5 Load displacement curves obtained from unreinforced beams static tests with test velocity of 1mm/min. 5 4 Reinforced beam f = 6 mm v = 1 mm/min Reinforced beam f = 4 mm v = 1mm/min test velocity of 1mm/min Figure 9 Load displacement curves obtained from the reinforced beams with GFRP rods of φ = 6 mm static tests with test velocity of 1mm/min.

4 The effect of test velocity at maximum load is shown in table 3. Table 3 -. Effect of test velocity in the ultimate load. 6 Reinforced beam f = 6 mm V = 1 mm/min Test velocity Unreinforced φ = 4 mm φ = 6 mm Pmax (N) Stdv % Pmax (N) Stdv % Pmax (N) Stdv % 1mm/min mm/min The figures 1 and 11 show the crack propagation for the reinforced beam Reinforced beam f = 4 mm V = 1 mm/min Figure 11 Crack propagation for the beam reinforced with GFRP rods of 6 mm diameter 2.5 Fatigue test results The figures 12 to 15 show the fatigue test results..9 Unreinforced Beam.8 Pmax/Pr.7.6 Figure 1 Crack propagation for the beam reinforced with GFRP rods of 4mm diameter Figure 12 Fatigue results for the un-reinforced beams.

5 .9 Reinforced beam f = 4 mm 3 CONCLUSIONS Pmax/ Pr Pmax/Pr Figure 13 Fatigue results for the beams reinforced with GFRP rods of 4mm diameter Pmax/Pr Figure 14 Fatigue results for the beams reinforced with GFRP rods of 4mm diameter Figure 15 - Fatigue test results Reinforced beam f = 6 mm E+3 1.E+4 1.E+5 1.E+6 1.E+7 reinforced d=4 mm unreinforced reinforced d=6 mm In relation to the static flexure tests the following conclusions can be made: The maximum load supported by the beams with and without reinforcement of 4mm diameter rod is similar and equal to 3 kn The displacement at rupture of the reinforced beams is 5 times greater than the displacement at rupture obtained for the unreiforced beams. The beams reinforced with a 6mm diameter rod have a 3% higher ultimate load than the one obtained for the unreinforced beams and for the reinforced with 4 mm diameter rod, but no conclusion can be drawn as far as displacement is concerned due to the rod slippage As far as crack propagation is concerned for the static tests, the introduction of 4mm rods increases ductility. Using the 6 mm rods there is an improved fracture behaviour of the beams because it introduces a slow propagation of cracks growing under mixedmode conditions Between 1 and 1 mm/min there is no velocity effect in the mechanical properties For the fatigue tests, one can conclude that: The presence of 4 mm rods increase the number of cycles to rupture for 8% and 7% of the maximum load - the reinforced beams can withstand 1 million cycles without rupture The beams with a 6mm rod didn t break even in the worst situation 8% of maximum load and 1 million of cycles These results, far from complete, are already very promising in what concerns the fatigue performance of FRP-reinforced polymer concrete structures. ACKNOWLEDGEMENTS The support of Fundação para a Ciência e Tecnologia under project Development of New Polymer Concretes, contract C/ECM/12277/1998, is gratefully acknowledged.

6 REFERENCES Yeon, K.S.; Kim, K.W.; Lee, Y.S. and Kim, K.W Maturity of polyester polymer concretes. Symposium on Properties and Test Methods for Concrete-Polymer Composites; Proc., Oostende-Belgium, 6 July 1995: Walker, E.K Effects of environments and complex load history on fatigue life. ASTM STP-462:43-47 Suresh, S Fatigue of materials. Cambridge: Cambridge University Press. Hsu, T Fatigue of plain concrete. ACI Journal 78: Hsu, T Flexural behaviour of polymer concrete beams. Dissertation. University of Texas. Mebarkia, S.A Mechanical and fracture properties of high strength polymer concrete under various loading conditions and corrosive environments. Doctoral Dissertation, Faculty of the Department of Civil and Environmental Engineering, University of Houston. A.J.M. Ferreira, C.M. Tavares, M.C. Ribeiro Flexural properties of polyester resin concretes -, Journal of Polymer Engineering, Freund Publishing House. V.2, nº6, (a), p A.J.M. Ferreira, C.M. Tavares, M.C. Ribeiro, M. Figueiredo, A. A. Fernandes, Influence of Material Parameters in the Mechanical Behaviour of Polymer Concrete, Mechanical and Materials in Design, Orlando, USA, May, (b).