AN APPLICATION OF HPFRCC AND FIBER NET FOR RECOVERING STRENGTH OF RC MEMBERS DETERIORATED BY CHLORIDE INDUCED CORROSION

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1 BEFIB2012 Fibre reinforced concrete Joaquim Barros et al. (Eds) UM, Guimarães, 2012 AN APPLICATION OF HPFRCC AND FIBER NET FOR RECOVERING STRENGTH OF RC MEMBERS DETERIORATED BY CHLORIDE INDUCED CORROSION Koichi Kobayashi *, Shoya Asano 1, Ryosuke Ohashi 1 and Keitetsu Rokugo 1 * Dep. Civil Eng., Faculty of Engineering, Gifu University Yanagido, Gifu, , Japan ko2ba@gifu-u.ac.jp 1 Dep. Civil Eng., Faculty of Engineering, Gifu University Yanagido, Gifu, , Japan Keywords: HPFRCC, patch repair, chloride attack, beam strength. Summary: This study proposes a patching method using HPFRCC as a repairing technique for the RC member deteriorated by chloride-induced corrosion. The deteriorated cover concrete in RC beams was removed as far as to the backside of the tensile rebar with a water jet, after which HPFRCC was overlaid by shotcreting. The effects of combined use of a fiber net and HPFRCC on the mechanical performance of the beams were also investigated. Patching repair with HPFRCC proved to be effective to recover reduced yield strength and ultimate strength of the deteriorated beam. 1 INTRODUCTION Reinforced concrete (RC) structures are highly durable because steel in concrete is protected from corrosion by a tight passive film formed on the surface of steel due to the high alkalinity of the concrete. However, ingress of chloride into the cover concrete can result in destruction of the passive film and consequent corrosion of the steel. Steel corrosion leads to a reduction of cross-sectional area of steel and spalling of the cover concrete, thereby deteriorating the mechanical performance of the structure. This chloride attack is the most common cause of deterioration. As the deterioration progresses rapidly, structures deteriorated by chloride attack must be properly repaired as soon as possible in order to stop further progress of corrosion and to recover the original functions. Bending cracks are tolerated to some degree in RC structures. Each country has in its design code a permissible crack width in cover concrete specified in consideration of durability. However, cracks allow penetration of aggressive agents into the cover concrete. As demonstrated by several reports [1,2,3], an increase in crack width and the water permeability coefficient have a good correlation, i.e, the water permeability shows a sharp increase with the increase of crack width that is as small as 600μm or lower [1]. Therefore, in considering the durability of RC structures, not only the impermeability of the material itself but also control of crack width should be taken into account. High Performance Fiber Reinforced Cement Composites (HPFRCCs) are materials composed of a cement-based matrix and short reinforcing fibers. It generates numerous cracks under tensile stress, but since the crack width is very small, HPFRCC has high impermeability to chloride, water, and oxygen gas. Cracks generated in typical HPFRCC with polyethylene fiber, for example, are 100μm or lower in width. When applied to the results reported by Wang et al. mentioned above [1], the water permeability coefficient of this material would be only 1% or lower of that of mortar with 0.3mm crack width. Besides, it has a matrix composition similar to that of mortar with a low water cement ratio. Even after the outbreak of rebar corrosion, HPFRCC s high tensile ductility prevents crack propagation and cover spalling, which would accelerate deterioration in normal concrete [4,5,6]. Therefore, HPFRCC is

2 Stress (N/mm 2 ) BEFIB2012: Koichi Kobayashi, Shoya Asano, Ryosuke Ohashi and Keitetsu Rokugo. expected to exhibit high durability against chloride attack if applied as a repair material for RC structures for which tensile cracks are considered tolerable. Based upon the above, the authors of this study have focused on the applicability of HPFRCC as a repair material, and have so far demonstrated the high corrosion proof performance of HPFRCC due to its high chloride impermeability [7]. However, the mechanical performance of an RC member repaired with HPFRCC is yet to be investigated. This study therefore aimed at clarifying the mechanical performance of RC beams deteriorated by rebar corrosion and repaired with HPFRCC. A loading test was carried out using RC beams deteriorated for seven years by chloride attack and then repaired by waterjet removal of cover concrete and HPFRCC shotcreting. It was expected that repair with HPFRCC alone could not fully recover the strength of the deteriorated beams, and therefore this study also proposes a technique of using a synthetic fiber net in combination with HPFRCC for reinforcement purposes. 2 EXPERIMENTAL PROCEDURE 2.1 Materials Table 1 shows the mixture of HPFRCC used as a repair material. A pre-mix fiber mixture consisting of polyvinyl alcohol fiber (9mm in length, 40μm in diameter) and polyethylene fiber (9mm in length, 12μm in diameter) was used. Fig. 1 shows the HPFRCC s stress-strain relationship obtained from a JSCE uni-axial tension test [8]. Since this study simulates HPFRCC patching on RC beams by shotcreting, this specimen for the uni-axial tension test was prepared by shotcreting. This made the ductility of the material very low as shown in Figure 1. The specimens were composed of ordinary Portland cement concrete with a water-cement ratio of To some beams, 12kg/m 3 of chloride ion (19.8kg/m 3 of NaCl) was mixed in the concrete. For tensile reinforcement and shear reinforcement, D10 deformed bars (yield strength: f y =370N/mm 2, tensile strength: f u =496N/mm 2 ) and D6 deformed bars (f y =420N/mm 2, f u =526N/mm 2 ) were used, respectively. Table 1: HPFRCC mixture W/B Unit mass (kg/m 3 ) Fiber (vol. (%) W Binder S %) No.1 No.2 1 No.3 No.4 No Strain (%) Figure 1: HPFRCC mixture Figure 2: High strength polyethylene fiber 2

3 Figure 2 shows a high strength polyethylene fiber net originally developed for preventing cover concrete spalling. The net has a grid pitch of 10mm, a nominal strength per 5 grid (50mm) of 2.7kN, and an elastic modulus of fiber of about 50-70kN/mm Preparation of RC beams Figure 3 shows the dimensions of the beam specimens. The specimen size was 120 x 180 x 1600mm. Seven beams with chloride (CL) and four beams without chloride (noncl) were used for the loading test. All the beams were left outside for seven years and deteriorated (Figure 4). The maximum width of cracks caused by tensile reinforcement corrosion reached 1.5mm. Then, the cover concrete on the tensile side was removed with a waterjet to a target depth of 30mm to reveal the backside of the rebars from five of CL beams and two of noncl beams, respectively (Figure 5). Even though there were tensile rebars that suffered some loss in the cross-sectional area, they were not replaced, nor were any new rebars added. The steel surface was cleaned of corrosion products with a water jet, and coated with a nitrite-based corrosion inhibiter, after which HPFRCC was overlaid by shotcreteing (Figure 6). Furthermore, one fiber net of 120mm x 1600mm was added for two of repaired CL beams (Figure 7). It should be noted here that the corroded stirrups and lateral confinements in the chloride contaminated substrate part were not given any corrosion removal and corrosion proof treatment. The other beams were subjected to the loading test without being repaired by HPFRCC. Table 2 shows the list of specimens. Figure 3: Loading test specimen Figure 4: Cover cracks due to rebar corrosion Figure 5: Cover concrete removal by water jet 3

4 Figure 6: Shotcreting of HPFRCC Figure 7: Set of fiber net in HPFRCC layer 2.3 Loading test Table 2: Beams used in loading test HPFRCC patching Fiber net Measured corrosion loss (%)* noncl noncl noncl-hp-4 Yes - - noncl-hp-5 Yes - - CL CL CL-HP-2 Yes CL-HP-3 Yes CL-HP-5 Yes Yes 6.7 CL-HP-NET-1 Yes Yes 7.5 CL-HP-NET-4 Yes Yes 5.8 * Average corrosion loss of rebars taken out from the shear span A monotonic loading test was carried out. The bending crack width was measured with a microscope in the 400mm-long mid span of the beam. All the crack widths were measured in a specified area in the mid span when the applied load reached 15kN, 30kN or 45kN, or when the displacement at the center span reached 5mm, while only the maximum crack width was measured when the displacement at the center span reached 10mm or 15mm. The test was finished when crushing failure or rebar break occurred in the beam. 2.4 Corrosion loss determination After the loading test, the cover concrete (NC and HPFRCC) was removed and the tensile rebars were taken out from the specimens. They were cut at the loading point and the supporting point. Four rebars from the shear span of each beam were prepared as specimens, since the length and mechanical properties of rebars in the shear span would be least affected by the loading. The rebars were immersed in a 10% di-ammonium hydrogen citrate solution at 60 o C for 24 hours, and the corrosion loss was measured. Table 2 shows the corrosion loss of rebars taken out from the shear span. An average corrosion loss in each CL beam was 5 to 12 %, as shown in Table 2. 4

5 3 RESULTS 3.1 Crack propagation Figure 8 shows the number and width of bending cracks in the beams during the loading test. Figure 9 shows the maximum crack width in each specimen during the loading. Figure 10 shows the distributions of cracks in the beams after the loading tests. Multiple cracks were generated in a dispersed manner in the HPFRCC layer, with the crack width remaining small up to the yielding point of the beam, and in the beams repaired with the combination of a fiber net and HPFRCC, the cracks were even finer and in a greater number. After the yielding of the beams, however, cracks were no more created in both beams repaired with HPFRCC and HPFRCC/fiber net, and instead, only some cracks increased in their width, indicating a local increase in strain. Nevertheless, the maximum crack width before the yielding of the beam was less than 0.1mm and therefore the HPFRCC layer would still have a low permeability at a serviceability limit state. Figure 8: Crack propagation during loading test Figure 9: Maximum crack width in each specimen at each loading step 5

6 (a) CL-6 (b) CL-HP-2 (c) CL-HP-NET Mechanical performance of beams Figure 10: Cracks after the loading test Figure 11 shows the load-deflection relationship of each beam. A comparison between the results of CL-6 and noncl-1 revealed a 15% loss in the strength of the specimens by chloride attack. With the patch repair with HPFRCC, the beam strength was recovered to some extent. The HPFRCC-patched specimens with non-corroded rebars had a higher yielding strength than the specimens without HPFRCC. A unique feature of its load-deflection curve is that the load declines temporarily when the deflection reaches around 10mm after the yielding. This is because of the small tensile ductility of the HPFRCC used in this study. Namely, the multiple fine cracks generated in HPFRCC share the tensile load in the cross sectional area up to the yielding point, whereby the yielding load of the beam is increased. After that, however, as the strain increases, HPFRCC undergoes strain-softening (see Figure 1) and cracks widen (see Figure 9), at which stage the cracks can no longer share the tensile load in the cross section of the beam, hence the drop of the load. 6

7 Load (kn) Load (kn) Load (kn) BEFIB2012: Koichi Kobayashi, Shoya Asano, Ryosuke Ohashi and Keitetsu Rokugo noncl-1 noncl-6 noncl-hp-4 noncl-hp Deflection (mm) CL-6 CL-7 CL-HP-2 CL-HP-3 CL-HP Deflection (mm) CL-HP-2 CL-HP-NET-1 CL-HP-NET Deflection (mm) Figure 11: Load-deflection relationship of beams deteriorated by chloride attack Although the load drops temporarily after the yielding point as mentioned above, it is still higher than that of the unrepaired specimens. As shown in Figure 9, after the deflection has reached a certain level, the crack width in HPFRCC layer increased locally. In this state, HPFRCC showed a strain softening behavior (see Figure 1), and could not bear the tension load anymore. However, the load did not decline even after the crack width almost reached 1mm. This is considered to be due to the high bond strength, rather than tensile ductility, of HPFRCC [9,10]. Namely, as shown in Figure 11, in the ordinary concrete beam, a bond failure occurred between the concrete and the rebar around bending cracks, resulting in a pullout of the rebar from the concrete, because of which the rebar strain increased uniformly over a certain length in the vicinity of the cracks. On the contrary, HPFRCC s high bond strength prevented a pullout of the rebar and resulted in the very local increase of strain that occurred only inside the cracks. It is assumed, therefore, that the stress increased on account of strain hardening of the rebar in the HPFRCC-patched beam, leading to the increase in the beam strength (Figure 12). The beams repaired by a combination of HPFRCC and a fiber net showed relatively a small decline in the load with the increase of deflection after the yielding. The fiber net likely has restricted localized increase in the crack width and strain in the HPFRCC layer. 7

8 Load Stress BEFIB2012: Koichi Kobayashi, Shoya Asano, Ryosuke Ohashi and Keitetsu Rokugo. Normal concrete HPFRCC Bond failure & pull-out Localized high strain d with HPFRCC b c a without HPFRCC a: Increase of stiffness b: Increase of yielding capacity c: Increase of strength d: Increase of ductility by fiber net Deflection Strain Load - defrection relationship of beam Stress - strain relationship of rebar Figure 12 Effects of HPFRCC patching repair on the mechanical performance of RC beam 3.3 Beam strength and corrosion loss Figure 13 shows the bending strength of each specimen and the corrosion loss of the tensile reinforcement. In this paper, the corrosion loss is the average value of four rebars taken out from the shear spans of each beam. The bending capacity of beams intrinsically depends on the properties of reinforcement in the constant moment span. However, the rebar in this region after the loading test has a large residual strain that makes precise determination of corrosion loss difficult. In Figure 12, therefore, the corrosion loss in the shear span was used instead of the corrosion loss in the constant moment span. Despite the use of such substitutes, the general tendency is seen from Figure 12 that the larger the corrosion loss, the lower the beam strength. Figure 13 Relationship between corrosion loss of rebars and beam strength 8

9 The HPFRCC patching repair increased the beam strength. It should be noted, however, that the reinforcement in HPFRCC suffers local, very high strain around the cracks. Therefore, a further investigation is necessary on the ductility of the beams with HPFRCC subjected to more severe crosssectional loss, and on the seismic performance of RC member with HPFRCC under large displacement. 4 CONCLUSIONS This study proposed and investigated a repair method using HPFRCC for the RC member deteriorated by rebar corrosion. In this method, deteriorated cover concrete is removed as far as to the backside of the rebar with a water jet, after which, without replacing or adding rebar, HPFRCC is overlaid by shotcreting. In addition, the effects of combined use of a fiber net and HPFRCC on the mechanical performance of the beams were also investigated. The results are summarized as follows: (1) Cracks are finer and dispersed more evenly in the HPFRCC-patched beam. (2) However, after the yielding of the beam, no more cracks are formed, but the crack width increased locally, causing a local increase in strain. (3) Beams deteriorated by rebar corrosion can recover their strength by HPFRCC patching. (4) Combined use of fiber net disperses fine cracks in the HPFRCC layer. REFERENCES [1] K. Wang, D.C. Jansen, S.P. Shah and A. Karr, Permeability study of cracked concrete, Cement and Concrete Research, 27(3), (1997). [2] C.M. Aldea, S.P. Shah and A. Karr, Permeability of cracked concrete, Materials and Structures, 32, (1999). [3] M. Hoseini, V. Bindiganavile and N. Banthia, The effect of mechanical stress on permeability of concrete: A review, Cement and Concrete Composites, 31, (2009). [4] M. Sahmaran, V.C. Li and C. Andrade, Corrosion resistance performance of steel-reinforced engineered cementitious composite beams, ACI Materials Journal, 105(3), (2008). [5] T. Sato, T. Kanematsu, S. Asano, K. Kobayashi and K. Rokugo, The effect of transverse reinforcement and HPFRCC on reduction of corrosion crack width, Proceedings of the Concrete Structure Scenarios, 10, (2010). (in Japanese) [6] H. Mihashi, S.F.U. Ahmed and A. Kobayakawa, Corrosion of reinforcing steel in fiber reinforced cementitious composites, Journal of Advanced Concrete Technology, 9(2) (2011). [7] K. Kobayashi, T. Iizuka, H. Kurachi and K. Rokugo, Corrosion protection performance of high performance fiber reinforced cement composites as a repair material, Cement & Concrete Composites, 32, , (2010). [8] JSCE concrete committee, Recommendations for design and construction of high performance fiber reinforced cement composites with multiple fine cracks (HPFRCC), (2008). ( committee/concrete/e/hpfrcc_jsce.pdf). [9] S.H. Chao, A.E. Naaman and G.J. Parra-Montesinos, Bond behavior of reinforcing bars in tensile strain-hardening fiber-reinforced cement composites, ACI Structural Journal, 106(6), (2009). [10] W. Koyanagi, K. Rokugo and H. Iwase, The stress-strain relationship of reinforcing bars in concrete and mechanical behavior of RC beams in flexure, JSCE Journal of Materials, Concrete Structures and Pavements, 384/V-7, (1987). (in Japanese) 9