APPLICATION OF THE FRP FOR CONSTRUCTION AS HIGH DURABILITY MATERIALS IN JAPAN

Transcription

1 Proceedings of US-Japan Workshop on Life Cycle Assessment of Sustainable Infrastructure Materials Sapporo, Japan, October 21-22, 2009 APPLICATION OF THE FRP FOR CONSTRUCTION AS HIGH DURABILITY MATERIALS IN JAPAN Akira Kobayashi, Yuya Hidekuma and Makoto Saito Technical Development Department, Nippon Steel Composite Co., Ltd., Japan ABSTRACT For over 20 years, FRP (Fiber Reinforced Polymer) has been tried to put in practice as reinforcing materials for infrastructures such as bridges. This is because FRP is not only lightweight and high strength material, but also highly durable material. In Japan, FRP rod and grid for internal reinforcement of concrete and FRP cable for tendon were developed first, specifically for newly-built concrete structures. FRP sheets and strips were developed next for repairing and strengthening of existing concrete structures. Nowadays, repairing and strengthening with FRP is widely used for seismic retrofitting of reinforced concrete (RC) piers and improvement of durability of RC slabs. As for newly construction field, partly FRP go into actual use such as pedestrian bridge. However, in most cases, FRP is not yet for actual use because it still needs more experiments and trials. This paper summarizes features of FRP sheets and FRP grids which are widely used as reinforcing material for concrete in Japan. Durability and life cycle cost (LCC) of FRP applications are also discussed. Keywords: FRP sheet, FRP grid, LCC, durability, repairing, strengthening 1 INTRODUCTION For over 20 years, FRP (Fiber Reinforced Polymer) has been tried to be applied to the materials for infrastructure such as bridges. This is because FRP is not only lightweight and high strength material, but also highly durable material. In 1980 s, FRP reinforcements for construction such as FRP rod, cable and grid were developed. After that, researches to apply FRP to the concrete structures became active. FRP was applied to aerospaces and sports field because of its characteristics of lightweight and high strength. However, in construction field, FRP reinforcement was not easily accepted because cost-effectiveness of FRP was inferior to such conventional materials as steel. In the early 1990 s, the researches to use FRP reinforcement such as FRP sheets for repairing and /or strengthening of existing structures became active. The lightweight property and workability of FRP sheets has captured attention because it didn t require heavy machineries and the only human power was required at work site of retrofitting of existing structures. Nowadays, the FRP sheets bonding method has become very popular method for seismic retrofitting of RC bridge piers and strengthening of super structures of bridges in the rehabilitation filed. In long life maintenance, not only initial cost but LCC (Life Cycle Cost) is also important. It is possible to use FRP for improving the durability of structures because FRP itself is highly durable against corrosion. In this paper, characteristics and application examples of FRP sheets and FRP grids in Japan are introduced and their effectiveness is discussed from the view point of LCC. 2 FRP SHEETS 2.1 Outline of FRP sheets bonding method As FRP materials for repairing and strengthening of existing concrete structures, FRP sheets such as carbon fiber sheet (CFRP sheets) (Figure 2.1) and aramid fiber sheet are widely used in Japan. In this method, the dry continuous fiber sheet is bonded to concrete surface using room temperature curing resin and impregnated with the same resin at the same time. After curing, FRP is formed on the concrete surface. In recent years, this method has also been applied for repairing and strengthening of corroded steel structures. There are three types of carbon fiber sheets; high 1

2 ( Tensile strength (MPa strength, high modulus, and middle modulus type. The unidirectional FRP sheet which arranged carbon or aramid fiber in one direction is mainly used for strengthening of concrete structures. Number of plies, the types of the sheet, and types of the fiber are determined according to the design calculation. As the adhesive between FRP and concrete, room temperature curing epoxy resin is widely used. In addition, MMA (Methyl Methacrylate) resin is sometimes used because MMA has excellent curing characteristics such as low temperature curing and rapid-curing. Figure 2.2 shows the change of tensile strength of CFRP sheet which consist of carbon fiber sheet and epoxy resin after accelerated artificial exposure durability test. In this Figure, it was clarified that the tensile strength of CFRP sheet did not decrease even without painting until 2,000 hour of exposure. Carbon fiber blocked the ultraviolet ray so that the inside resin was protected from UV while only outside epoxy resin was deteriorated. However, in order to avoid the deterioration of outside epoxy resin, protective coating should be used. In the case of FRP sheet bonded concrete structures, the durability of the structure itself is increased because FRP sheet blocks intrusion of chloride ions. Table 2.1 shows that the duration of blockage effect recommended for upgrading of concrete structures with use of continuous fiber sheets by Japan Society of Civil Engineers [1]. Table 2.1 Expected period for continuous fiber sheets to block chloride ions (years) Environmental category Number of plies Figure 2.1: Carbon fiber sheet Ordinary environments Environments with harsh climatic conditions Only one sheet 10 5 Two or more sheets Applications of FRP sheets (1) Seismic retrofitting of RC bridge piers In the case of RC bridge piers designed with old Exposure period (hr) Figure 2.2: Tensile test results after accelerated artificial exposure Figure 2.3: Seismic retrofitting of high piers specifications, anchoring length at termination of main reinforcements are often inadequate and also amount of shear reinforcement are often short. The FRP sheet jacketing method is applied in order to prevent fracture at the termination of main reinforcements and to improve the shear capacity and ductility. For the flexural strengthening of column at the termination of main reinforcements, the FRP sheets which have appropriate length at upside and down side from the termination section are attached on concrete surface in vertical direction. For the shear and ductility reinforcing, the circumferential FRP sheets are attached on concrete column. In these cases, the high strength type CFRP sheet is usually used. Figure 2.3 shows seismic retrofitting of RC high piers of express high way with using CFRP sheets [2]. This 60m high RC pier had hollow circular cross section to minimize self weight. The longitudinal reinforcement was curtailed at several levels 2

3 according to the old design specification. In the planning phase, the CFRP sheet jacketing method was compared to concrete jacketing method and steel plates jacketing method. The concrete jacketing method was impossible to apply because the cross sectional diameter of pier after concrete jacketing was too large and the flow of river might be obstructed. It was also difficult to set up concrete form on the riverbed. In the case of steel plate jacketing method, the construction cost was calculated to be expensive because it was necessary to build temporary structure in the river to carry heavy steel panels. Whereas, carbon fiber sheets were very light weight and easy to handle, construction works could be conducted on gondola without any temporary structures in the river. Therefore the carbon fiber sheet jacketing method was adopted. Additionally, for steel jacketing method, painting should be required periodically to avoid corrosion of steel. The maintenance cost would become expensive because this bridge pier has 60 m in height. On the other hand, in the case of CFRP sheet jacketing method, the painting was optional for its appearance, and repainting was not necessary because of good corrosion resistance of CFRP. Therefore, it was considered that the cost of maintenance was small compared to steel plate jacketing method. This lower maintenance cost of CFRP sheet jacketing method was an advantage for reducing Life Cycle Cost (LCC). (2) Strengthening of road bridge RC slabs The fatigue deterioration of road bridge RC slabs has become a serious problem. The deterioration of RC slabs are caused by several reasons such as wheel loads greater than their designed load, ever increasing traffic, inadequate slab thickness, insufficient distribution bar amount, low concrete quality, improper concrete pavement, etc. Fracture mode of RC slabs in the actual bridges is punching shear failure without the fatigue failure of reinforcing bars. Damage in RC slabs of road bridge starts with the generation of single-direction cracking, and leads to development of the twodirection cracking, followed by propagation of fine cracks. Penetration and the formation of slab concrete into blocks is the next step. Finally the slab reaches punching shear failure. Examples of deterioration of slabs are shown in Figure 2.4 (a), (b), and (c) [3]. Photographs of an identical slab were taken at intervals, 15, 22, and 27 Leached lime Bridge axis Additional (a) After 15 years (b) After 22 years (c) After 27 years Main girder stiffening Figure 2.4: Intermittent strengthening and deterioration of the slab Figure 2.5: Punching failure of the slab after 32 years 3

4 2 nd layer carbon fiber sheet Distribution bar direction 2 nd layer carbon fiber sheet Distribution bar direction 1 st layer carbon fiber sheet Main bar direction 1 st layer carbon fiber sheet Main bar direction (a) Full surface bonding (b) Grid bonding Figure 2.6: Strengthening of RC bridge slabs with carbon fiber sheets years after the slab began service. Cracking in concrete slab propagates from one direction (main reinforcing direction) to two directions. After these cracks then become alligator cracks, the width and depth of cracks progress and leakage and efflorescence increase (Figure 2.4 (c)). Finally, the concrete slab separates into pieces and punching shear failure occurs (Figure 2.5). In Japan, there are several strengthening methods for the slabs; an additional stiffening girder method, a steel plate bonding method, a CFRP sheet bonding method, a concrete overlay method, etc. For strengthening of RC slabs with use of CFRP sheets, the first layer of CFRP sheets are attached in the main reinforcing bar direction, and the second layer of CFRP sheets are attached in the distribution reinforcing bar direction on bottom surface of RC slab with epoxy resin. As shown in Figure 2.6, there are two bonding pattern. One is the full surface bonding, another is the grid bonding. The advantages of the CFRP sheet bonding method are no need of stopping traffic on the bridge, free from rust, good resistance to chemicals, and minimum maintenance. Therefore this method is widely used for strengthening of road bridge RC slabs in Japan. By bonding a CFRP sheet to the bottom surface of the RC slabs, it is possible to reduce the slab deflection and also reduce the stress in the tensile reinforcing bars. The CFRP sheet is also effective in constraining the development of cracks in the concrete at the bottom surface of the RC slabs. Therefore, by bonding a CFRP sheet to the bottom surface of the RC slabs that have been damaged, it is possible to prolong the slab fatigue life remarkably. Fatigue tests of bridge slabs strengthened with CFRP sheets were conducted by Matsui et al. using a wheel running machine that is capable of reproducing slab fatigue damage [4]. The schematic diagram of the wheel running machine is shown in Figure 2.7. The slab specimens were supplied from Deflection (mm) Figure 2.7: Wheel Running Machine 100, 150kN 10t 150kN None t th i With CFRP sheets Apply CFRP sheets Number of cycles (x 10 6 ) Figure 2.8: Result of wheel running test 4

5 an existing bridge that had been in service for 26 years. Two plies of middle modulus CFRP sheet were applied to the under surface of the slab. The first ply was applied in the main reinforcing bar direction and the second ply was applied in the distribution reinforcing bar direction. The deflection of the slab at center point as a function of the number of cycles is shown in Figure 2.8. It can be seen that the deflection of the reinforcing slab increased very slowly in comparison with the deflection of the bare slab. No breakage of CFRP sheet and no peeling off of the sheet were observed during the fatigue test. It was confirmed that the CFRP sheet constrained the deflection of the slab and the opening of the cracks, thereby improving the slab fatigue life noticeably. For a road bridge, the life of RC slabs is usually shorter compared with other members, such as main girders. Generally, countermeasures of RC slabs such as strengthening or replacing have been carried out during service life of the bridge. Therefore, when maintenance plans for existing bridges are made, it is necessary to consider the maintenance plan of RC slabs such as evaluation deterioration rate of RC slabs, prediction of life of RC slabs, intervals of maintenance activities, and selections of deterioration countermeasures. Therefore, it is important to evaluate LCC of RC slabs under considerations of life long maintenance cost. Matsui has defined the serviceability limit state of a slab when the live load deflection reaches the theoretical deflection estimated by neglecting the tension side concrete [5]. Based on the definition, the degree of deterioration of the slab after the loading cycles can be evaluated from the following equation D W W 0 S = (1) Wc W0 Where D s : degree of deterioration W : live load deflection at measurement time W 0 : theoretical live load deflection assuming gross cross section effective for bending rigidities W c : theoretical live load deflection assuming cracked section rigidities neglecting tension side concrete. The degree of deterioration is changed from 0 at initial sound state to 1.0 at serviceability limit state according to progress of deterioration as shown in Figure 2.9. It is well known that the deterioration of RC slab is rapidly progressed and risk of punching shear fracture is increased after the degree of deterioration reached to 1.0. The period that the degree of deterioration reached to 1.0 is thought to be in the range of 0.5 to 0.8 of period to ultimate slab life. If the degree of deterioration can be evaluated Live load deflection W c W 0 Service limit state Initial state N d /N f 1.0 Normalized number of cycles N/N f Degree of deterioration Figure 2.9: Relationships between normalized number of cycles and degree of deterioration Live load deflection 0 Strengthening Number of cycles W cb W ca W 0b W 0a (a) Change of deflection of strengthened slab Degree of deterioration 0 Number of cycles (b) Degree of deterioration of strengthened slab Theoretical deflection assuming cracked section rigidities neglecting tensile side concrete W cb : before strengthening, W ca : after strengthening Theoretical deflection assuming gross cross section effective for bending rigidities W 0b : before strengthening, W 0a : after strengthening Figure 2.10: Paradigm of change of deflection and degree of deterioration of reinforced slab 1.0 Service limit state Strengthening 5

6 properly, it is considered that it is possible to predict soundness and residual life of RC slabs. The studies on prediction of deterioration for slabs strengthened with CFRP sheet were conducted by the JSCE research committee on road bridge slabs [6]. Figure 2.10(a) shows relationships between live load deflection and the number of loading cycles, Figure 2.10(b) shows relationships between degree of deterioration and the number of loading cycles. The RC slab is loaded initially without strengthening, and the deflection and degree of deterioration are gradually increased. When the slab is strengthened with CFRP sheets, the deflection and the degree of deterioration are decreased. After strengthening, the increasing speed of the degree of deterioration become slower compared with that of the degree before strengthening. The degree of deterioration of the RC slab which is strengthened with CFRP sheet is calculated according to Eq. (1) as the same method for the RC slabs without strengthening. Where both calculated deflection of W 0 and W c should be estimated in assumption that the CFRP sheets are completely unified bottom surface of the RC slab and the fiber strain of CFRP sheet is proportional to the distance from the neutral axis of the section. According to results of wheel running test of RC slabs strengthened with carbon fiber sheet, the live load defection of RC slab gradually became larger as number of loading cycles, but the increasing speed of live load deflection became smaller compare with the RC slabs without CFRP sheet. Finally, the RC slabs were fractured in punching shear mode as same mode of the RC slabs without strengthening. Figure 2.11 shows the relationships between the degree of deterioration and the number of loading cycles which were obtained from wheel running test of the RC slabs strengthened with CFRP sheets [7]. Specimen 1 and Specimen 2 were strengthened with two types of CFRP sheets having different fiber weight of per unit area, 200g/m 2 and 300 g/m 2 respectively. Namely specimen 2 was strengthened with CFRP sheet which had 1.5 times higher tensile stiffness compared with that of specimen 1. The degree of deterioration and increasing speed of it of specimen 2 were smaller than those of specimen 1. Degree of deteriaration No.1 No.2 0.0E E E E E+06 Number of cycles Figure 2.11: Relationships between number of cycles and degree of deterioration of the RC slabs strengthened with CFRP sheets Equivalent Load Po / Psx 1.0E E E E E E S-N Formula Prolongation of life No3 EA80G No2 t15 non-reinforcement S-N formula No2-t15-S64-non-reinforcement No3-t15-S64-EA80-grid-bonding No4-t15-S64-EA68-grid-bonding Cal.-t18-S68 non-reinforcement No5-t18-S68-EA70-grid-bonding No6-t18-S68-EA60-grid-bonding No1-t22-S96- non-reinforcement SHM1-t18S68-EA67-full surface 600 1t18-S43-EA82-full surface 300 2t18-S43-EA82-full surface No4EA68G Cal.-t18 non-reinforcement Prolongation of life No1t22 nonreinforcement No6EA68G No5EA70G SHM1EA67F 300 2EA82F 600 1EA82F Number of cycles (at Po=150kN) Figure 2.12: The relationship between number of cycles until failure and load P o /P sx 6

7 For specimen 1 strengthened with CFRP sheets, the degree of deterioration was gradually increased as number of cycles was increased. After the degree of deterioration then exceeded 1.0, it rapidly increased, and finally, specimen 1 was fractured in punching shear mode. Therefore, it is assumed that the degree of deterioration which is calculated by Eq. (1) is useful for evaluating the deterioration of the RC slabs strengthened with CFRP sheets and also 1.0 of the degree of deterioration can be considered the service limit. The S-N diagram of RC slabs strengthened with CFRP sheets are indicated in Figure The thicknesses of RC slabs were 150mm and 180mm. The slabs strengthened with several types of CFRP sheet which had various young s modulus and fiber weight per unit area. A normalized S-N curve was developed to facilitate the fatigue life prediction of RC slabs designed in accordance with different specifications by Matsui [8]. The S-N equation is as follow: Log P P sx = Log( N ) + Log( C) (2) Where P: wheel load P sx : Punching shear strength of the effective cross section perpendicular to the main bars of the slab N: Number of cycles until failure C: Constant C= 1.52 This S-N equation of the RC slab without CFRP sheet was shown in Figure 2.12 as solid line. The test results of slabs which were strengthened with CFRP sheets were plotted in upper-right side area of this S- N equation. Therefore it is considered that strengthening using CFRP sheets remarkably prolong the life of RC slabs. It is assumed that the life of RC slabs strengthened with CFRP sheet depends on magnitude of load and several properties of the CFRP sheet and RC slab, such as stiffness of CFRP sheet, thickness of slab, amount of existing reinforcing steel bars, concrete strength, deterioration condition of RC slab before strengthening, etc. Therefore, in order to evaluate the effect of strengthening, it has been important issue to develop S-N equation of RC slab strengthened with CFRP sheets. fibers are impregnated with resin of high resistance to chemical agents and formed into FRP grids. Figure 3.1 shows photograph of several types of FRP grid. This material has many useful characteristics, such as light weight, high strength, and is free from rust and corrosion. The characteristics of FRP grid are as follows: The use of high-performance continuous fibers gives extremely high strength of the axial bar and the FRP grid crossing. This ensures bond and anchorage of FRP grid to concrete. As the FRP grid crossing are in the same plane as the axial bars, concrete coverage can be reduced and the amount of concrete can be saved. Since FRP grid has low specific gravity at , it is light and can be formed into any shapes required on site as well as a flat surface. Thus, it can improve work productivity. Three types of FRP grid namely CFRP, GFRP, and AFRP are commercially available with different combinations of fiber types. Flat panel and curved panel are commercially available. At the grid crossing of FRP grid, fibers are laminated alternately as shown in Figure 3.2, adequate strength is provided by constraint effect of fibers in addition to the adhesive strength of matrix resin. 3.2 Applications of FRP grid (1) Retrofitting of existing RC piers Figure 3.1: FRP grids 3 FRP GRIDS 3.1 Future of FRP grids FRP grid is a continuous fiber reinforcing material consisting of high-performance continuous fibers such as glass, carbon, and aramid. All of the Figure 3.2: Structure of grid crossing 7

8 For the last decade, FRP grid has been widely used in the repairing and strengthening construction fields. Application examples with use of FRP grid are introduced. In the Hokuriku Express-way at Oyashirazu in Niigata Prefecture Japan, CFRP grid CR6-150P (bar number 6-150mm grid spacing) flat panels and curved panels were used for reinforcement of pier, as shown in Figure 3.3. Because express-way at this region is located at very close to the sea-side, pier concrete was deteriorated severely deep inside due to salt damage. For a fear of steel rebar corrosion, as reinforcing material rustproof FRP grid was selected and used in this repairing construction. (2) Footbridge (newly construction) Benten Bridge is a footbridge between seaside and Benten Island at Iwaki-city, Fukushimaprefecture, JAPAN. In Benten Island, there is a historic monument called Namitate Yakushi Temple. It is very popular among citizen and an important sightseeing resource in Iwaki-city. The old bridge which had three span simple prestressed concrete desk slab bridge was built in 1973 (shown in Figure 3.4). However, the corrosion of steel bar, crack and peeling of concrete occurred at superstructure and substructure because this bridge was located at splash zone. After a lapse of 28 years, rebuild of that bridge become required due to serious damage. In building plan of new bridge, a target bridge life was decided to 100 years in spite of splash. Furthermore, the bridge had to be built under the constraint that construction period was three month and work in the sea was permitted only at the low tide hours. After due consideration, five span simple FRP reinforced hollow slab bridge without using steel reinforcement for superstructure and substructure was selected. The precast concretes which manufactured at factory for superstructure and work yard near building site for substructure were installed by using crane. As FRP reinforcement, CFRP grids were applied because of short construction period and crack restraining effect. CFRP grids were bended and assembled temporary at factory (Figure 3.5, 3.6). The appearance of complete bridge is shown in Figure 3.7. The initial cost of construction was almost 1.8 times higher compared with normal PC bridge, because FRP grid was used and the particular method which has precast concrete for substructure was used. The lifetime of the old bridge was less than 30 years in spite of high durability of PC bridge because this bridge was exposed heavily to corrosive environment as splash zone. Therefore, when the lifetime of bridge was determined long term as 100 years, using FRP has a LCC advantage. Figure 3.3: Retrofitting of RC bridge pier using CFRP grids at seaside Figure 3.4: Aspect of Benten Bridge Figure 3.5: CFRP grid of superstructure Figure 3.6: CFRP grid of substructure 8

9 Figure 3.7: The appearance of complete bridge 4 CONCLUSIONS This paper summarized the features of FRP sheets and FRP grids which are widely used as FRP reinforcement for concrete in Japan. Durability and life cycle cost (LCC) are also discussed and the following conclusions were deduced. (1) The retrofitting methods with FRP reinforcement, such as FRP sheet bonding method and FRP grid method, etc. have become popular for the rehabilitations of RC bridge piers, super structures of bridges and other concrete structures because of its advantages such as light weight, good workability and good environmental resistance. (2) The CFRP sheet bonding method is effective to life prolongation of road bridge RC slabs. It is assumed that the deterioration status and residual life of RC slabs can be evaluate by degree of deterioration with using deflection of slabs, even after strengthening with CFRP sheet. In order to evaluate the life of RC slabs strengthened with CFRP sheets, it has been important issue to develop S-N equation of RC slab strengthened with CFRP sheet. (3) The Benten bridge which was constructed without using steel reinforcement was introduced as an application example of FRP for newly construction. In the case of quite severely corrosive environment, using FRP as reinforcement for newl construction has advantage when LCC is considered. REFERENCE [1] JSCE: Recommendations for upgrading of concrete structures with use of continuous fiber sheets, JSCE, pp.63, (2000) [2] K. Osada, N. Yasima, K. Terada and S. Ikeda: Seismic retrofitting of reinforced concrete high piers with carbon fiber sheets, Concrete researce and technology, JCI, Vol. 11, No.3, pp (2000), (in Japanese) [3] M. Okada, S. Matsui and A. Kobayashi: Fatigue test of concrete slabs reinforced with carbon fiber sheets, Proceedings of 11 th Road Engineering Association of Asia and Australia Conference, (2003) [4] S.Matsui, N.Mori, A.Kobayashi and M.uemura: Study of Concrete Slab Reinforcement With Carbon Fiber Sheet, ICCE/3, (1996) [5] Maeda, Y. and Matsui, S. : A study on the evaluation of deterioration of RC slabs of highway bridges, Proc. of symposium on the evaluations of durability for existing bridge structures and its members, Kansai branch of JSCE, (1983), (in Japanese) [6] The JSCE research committee on road bridge slabs: Peformance and maintanace of road bridge slab, JSCE, (2008), (in Japanese) [7] A. Kobayashi, K. Chai, M. Shimonishi and S. Matsui: Fatigue Dulability of RC slabs strengthened using carbon fiber sheet grid bonding method, Proc. of JCI, JCI, Vol.27, No.2, pp , (2005), (in Japanese) [8] S. Matsui: Predicion of bridge lifspan-fatigue life-span prediction of RC slabs of highawy bridges, Safety Engineering, Vol.30, No.6, pp , (1991), (in Japanese) 9