Engineering Structures 34 (2012) 259–270Contents lists available at SciVerse ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Flexural test of precast high-strength reinforced concrete pile prestressed with unbonded bars arranged at the center of the cross-section Mitsuyoshi Akiyama a,⇑, Satoshi Abe b, Nao Aoki c, Motoyuki Suzuki d a Department of Civil and Environmental Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-Ku, Tokyo 169-8555, Japan Technical Research Institute, Obayashi Corporation, 4-6 Shimokiyoto, Kiyose, Tokyo 204-9558, Japan c East Nippon Expressway Company Limited, 3-3-2 Kasumigaseki, Chiyodaku, Tokyo 100-8979, Japan d Department of Civil and Environmental Engineering, Tohoku University, 6-6-06 Aramaki-Aza-Aoba, Aobaku, Sendai 980-8579, Japan b a r t i c l e i n f o Article history: Received 1 December 2009 Revised 2 September 2011 Accepted 2 September 2011 Available online 4 November 2011 Keywords: Precast pile High-strength concrete High-strength steel Flexural strength Carbon-fiber sheet a b s t r a c t In this study, a prestressed reinforced concrete pile that uses high-strength material to increase the pile’s flexural capacity was developed. The main structural characteristics of the developed pile include (1) the neutral axis is constantly near the centroidal axis of the pile, even if the longitudinal reinforcement yields due to a flexural moment, because the pile has a high axial compressive force that is induced by prestressed steel bars, and hence, the concrete in the compression region can contribute to increasing the flexural strength of the pile; and (2) the flexural strength of the pile increases because the high-strength concrete is confined by high-strength spirals and carbon-fiber sheets in combination with concrete infilling, and, together, these modifications provide a sufficiently high lateral-confinement pressure. The results of bending tests demonstrate that the proposed prestressed reinforced concrete pile with carbon-fiber sheets and concrete infilling had a much higher flexural capacity than a conventional precast concrete pile. In addition, an analytical approach is presented that can be used to obtain the relationship between the bending moment and the curvature of the proposed pile. Even if concrete bridge systems are constructed on strata that can experience soil liquefaction, such as very soft soil, bridge foundations that use the proposed piles could remain undamaged under the design seismic action. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In the seismic design of concrete bridge systems, a plastic hinge must be introduced at the bottom of each bridge pier rather than at the pile foundation. This is an important concept in capacity design to help guarantee the rehabilitation of the bridge after a large earthquake [1,2]; however, if concrete bridge systems are constructed on strata (such as very soft soil) that can experience soil liquefaction in a severe earthquake, it is difficult to prevent yielding of the pile foundation. Numerous structures with precast concrete-pile foundations in reclaimed ground were seriously damaged by the 1995 Hyogoken–Nambu Earthquake in Japan [3,4]. Field investigations and numerical analyses of some of the damaged concrete-pile foundations in reclaimed ground have been conducted to clarify the damage-process mechanism therein. Based on a three-dimensional numerical simulation, Uzuoka et al. [5] reported that precast, prestressed concrete piles in a liquefied soil failed during this severe earthquake due to a lack of ⇑ Corresponding author. Tel.: +81 3 52862694; fax: +81 3 52863485. E-mail addresses:
[email protected] (M. Akiyama), abe.satoshi.ha @obayashi.co.jp (S. Abe),
[email protected] (N. Aoki),
[email protected] (M. Suzuki). 0141-0296/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2011.09.007 flexural strength and ductility capacity. In order to maximize post-event operability and minimize the repair costs of bridges, increased attention should be paid to improving the flexural strength and ductility capacity of precast concrete piles that are driven into liquefiable soil. Fig. 1a depicts the strain distribution in a concrete pile that is subjected to a bending moment at the point where the strain of the extreme compression fiber reaches 0.0035. Because the dead loadinduced axial force that acts on the pile foundations of most bridges in Japan is so small, as shown in Fig. 1, the neutral axis of the pile that is subjected to the bending moment becomes closer to the extreme compression fiber as the bending moment increases. Therefore, even though concrete with a compressive strength of over 100 MPa is used in these piles, it cannot contribute to increasing the flexural strength of the pile. In this paper, a new method for increasing the flexural strength of concrete piles was developed, wherein unbonded prestressing steel bars are incorporated at the center of the crosssection of the reinforced concrete piles. As shown in Fig. 1b, in comparison to conventional concrete piles that are subject to small axial forces, the neutral axis of the developed pile is much closer to the centroidal axis because of the prestress that is provided by the unbonded prestressed steel bars; thus, the compression region increases in cross-section and can improve the flexural strength of the pile. In 2 Axial stress provided by unbonded prestressing steel bars strain Fig. (2) prestressing steel bars with a sheath were inserted into the center of the pile.0 MPa.5 min. FRP can provide lateral confining pressure to the internal concrete of FRP. the concrete in the compression zone can contribute to increasing the flexural strength of the pile. This paper primarily investigates flexure of the developed pile under monotonic loading with special emphasis on the flexural strength. Therefore. (3) For some specimens. which is expressed as the axial force that is provided by the unbonded prestressing steel bars divided by the cross-sectional area of the pile. As shown in Table 1. The sheath was not filled with grout in any of the piles. because the ratio of the area of the concrete cover to that of the core concrete in Con-A is not small. In order to produce the proposed pile. the prestress level and the presence or absence of carbon-fiber sheets. xb. retrofitting. in this study target prestress levels were determined by taking into account only the number and the specified tensile strain of prestressed steel bars. the strain of the prestressed steel bars that are subjected to a bending moment is so low that it is elastic until the ultimate state of the pile is reached. An analytical approach that can be used to obtain the relationship between the moment and curvature of the proposed pile is presented. (4) The flexural strength of the pile increases due to the fact that the high-strength concrete is confined by highstrength spirals and carbon-fiber sheets and due to the use of . carbon-fiber sheets were used to prevent spalling of the cover concrete. (2) In addition. The carbon-fiber sheet can only affect the pile’s response to a circumferential stress. this is because the unbonded prestressed steel bars are arranged at the center of the pile’s cross-section. each 400 mm in diameter and 4000 mm long. (1) a hollow pile was molded from high-strength concrete using centrifugal force. target prestress levels ranged from 0 to 21. To insure clearance between prestressing steel bars. the maximum prestress level provided by three prestressing steel bars for a pile with a diameter of 400 mm is approximately 21 MPa. 2) was placed in the hollow of the pile. unidirectional carbon-fiber sheets were used to investigate the effect of confining the concrete cover with such sheets on the resultant flexural strength of the pile. the developed pile. 2. The centrifugal force was applied as shown in Fig. Effect of the pile’s axial force on the strain distribution. Thus. such as the number of longitudinal bars. this occurs because the pile has a high axial compressive force that is provided by the unbonded prestressed steel bars. All of the piles were removed from the steel mold after being steam-cured for the first 12 h and were subsequently air-cured until testing. concrete infilling (denoted ‘‘Con-B’’ in Fig. 3 for 15. Prestressed reinforced concrete piles using high-strength materials and carbon-fiber sheets A total of 17 piles.1 < xb. Therefore. there number of prestressing steel bars arranged at the center of the crosssection must be no more than three for the pile with a diameter of 400 mm. and the modeling of sections that are confined by FRP reinforcement has been examined [6–8]. hence. This should be considered in determining the target prestress level of specimens. (5) prestress was provided by tightening the nuts with a torque wrench before releasing the jack under the monitoring of a strain gauge that was attached to the prestressing steel bars. the long-term loss of prestress in the proposed pile could not be evaluated from this experiment. there has been an increasing interest in the use of fiber reinforced polymer (FRP) in the repairing. the manufacturer of the prestressing steel bars specifies that the maximum tensile strain during providing the prestress should be less than 65% of its designed yielding strain. Table 1 summarizes the material properties of the proposed piles. / Engineering Structures 34 (2012) 259–270 (a) 0. on the flexural strength of the proposed pile were investigated through a series of static bending tests. The effects of test variables. even after the cover is damaged. Then. a longitudinal reinforcement with a yield strength of 700 MPa and a transverse reinforcement with a yield strength of 1450 MPa was used. the effects of the experimental variables on the ductility capacity and residual displacement of the pile are outside the scope of the current investigation. the residual displacement can be reduced by the prestressed steel bars after the load is removed. Because. even if longitudinal reinforcements yield due to a flexural moment.1 Small axial stress Small axial stress strain xb.2 (b) where.260 M.0035 Axial stress provided by unbonded prestressing steel bars xb. it is important to prevent spalling of the concrete cover that is used to increase the flexural strength. 2. (3) High-strength longitudinal bars and spirals are used to increase both the flexural and shear strengths of the pile. 1. Examples of a cross-section of the proposed pile are provided in Fig. concrete with a compressive strength of 100 MPa. a bridge foundation would be designed to remain undamaged under design seismic action. Because the prestress was provided just before testing. whereas (4). Some of the specimens did not have concrete infilling and/or carbon-fiber sheets.2 are the distances from the extreme compression fiber to the neutral axis of bending 0. These effects will be examined in a future study using cyclic load tests. Previous studies have shown that the axial load level significantly affects the flexural behavior. especially the ductility. In this study.0035 xb. Even though the prestressed steel bars cannot resist the bending moment. the carbon-fiber sheets crack in response to tensile stress that is caused by a bending moment. In addition.1 and xb. Finally. Over the past decade. The main structural characteristics of the developed pile are that (1) the neutral axis is constantly near the centroidal axis of the pile. were tested under a static bending test. As shown in Fig. carbon-fiber sheets were attached to the pile surface. of highstrength reinforced concrete columns [9–11]. In addition. 2. strengthening. in terms of capacity design. and new construction of concrete components. Akiyama et al. for other specimens. 65 1.6 79. measured after the hollow pile was molded by the centrifugal force. The carbon-fiber sheets were bonded onto the concrete’s surface with epoxy resin. Electrical strain gauges were attached to the following surfaces: concrete. The minimum curing time for bonding of the carbon-fiber sheets before the test was 10 days. was measured as the average of three identical cylinders.2 2.8 102 37. For maximum flexural strength. silica fumes were used to obtain high strength.4 13. f0 c0. h Volumetric ratio of the spiral. Deflection was measured using five linear variable-differential transducers (LVDTs).7 20.9 796 31.65 1. Gmax.0 20.3 12.4 Type-A Type-A Type-B Type-A Type-A (%) a ‘‘D4’’ indicates a diameter of 400 mm.9 83. each of which had a diameter of 100 mm and a height of 200 mm. spirals and carbon-fiber sheets.9 108 41. 3. as shown in Fig. As shown in the figure.8 0. was 15 mm. The tensile strength of the carbon-fiber sheet was 4620 MPa. When prestress was applied to the pile. In order to permit optimal shear design of the proposed piles. All piles have a sufficient number of highstrength spirals to exhibit a flexure failure mode. the proposed pile should have both concrete infilling and carbon-fiber sheets. Akiyama et al. ‘‘Con-A’’ and ‘‘Con-B’’ indicate concrete molded by centrifugal force and concrete infilling. 4.6 12. The average compressive strength that was obtained from three cylinders that were 100 mm in diameter and 200 mm in length. prestressing steel bars.7 107 114 110 796 778 Diameter of prestressing steel bar (mm) Prestress levele (MPa) (%) 32. General observations Because the specimens had a sufficient number of spirals. 2.2 38.2 3. the effects of the number and yield strengths of spirals on the shear strength of the proposed pile should be investigated. the strain gauge on the prestressed steel bars was controlled such that a specified prestress was obtained. longitudinal bars.8 1. which was calculated as the axial force that was provided by the prestressed steel bars divided by the sum of the areas of Con-A and Con-B. The yield strengths of the longitudinal bars and spirals are shown in Table 1.0 787 22.8 20. as shown in Fig.9 66. 3. c Yield strength of the longitudinal bar.0 31.1.6 83. b concrete infilling. 2.6 78. / Engineering Structures 34 (2012) 259–270 Table 1 Test specimens. a single layer with a sufficient overlap length to anchor the sheet (=100 mm) was used. Testing procedure and instrumentation Each specimen was tested under a monotonically increasing load until failure using a four-point bending setup. f Yield strength of the spiral.2 68.5 19.2.0 40. respectively. respectively. the measured compression strains of the concrete and longitudinal bars were almost equal to the values that were computed from the given prestress.8 80.9 81. even though minor shear cracks were observed .1 6. For the concrete that was used in the Con-A.1 80. These cylinders were tested under axial loading at the time the corresponding pile was tested. The yield strengths of the unbonded prestressed steel bars were 1150 and 1230 MPa for the 32.7 21.4 66.6 89.0 66. all the piles exhibited a flexural failure mode only within the constantmoment region. wherein the maximum aggregate size.3 12. however. Specimen properties and materials The arrangement of the longitudinal bars and unbonded prestressed steel bars in the proposed pile are shown in Fig.1 Spiral fysf g q h s (MPa) s (mm) 1440 60 1. Crushed gravel was used as the coarse aggregate. these modifications together provide a sufficiently high lateral-confinement pressure.29 Thickness of Con-A (mm) Carbon-fiber sheeti 75. The averaged curvature could be obtained by differentiating the approximated deflection function for the constant-moment region. Experimental procedure 3. After the steel bar-provided prestress was introduced.0 9.0 778 98. as shown in Fig.9 66. The cracking behavior of the pile without the carbon-fiber sheets was visually observed.6 0. Type I Ordinary Portland cement was used in all of the concrete mixtures. i ‘‘Type-A’’means that carbon-fiber sheets were attached to the concrete in the constant-moment region + 150 mm. piles without concrete infilling and/or carbon-fiber sheets were tested in order to investigate the effect of these modifications on the behavior of the pile.4 13.29 0.3 37. 3.and 40-mm-diam- eter bars.1 787 88.1. Notation of specimena D4-1 D4-2 D4-3 D4-4 D4-5 D4-6 D4-7 D4-8 D4-9 D4-10 D4-11 D4-12 D4-13 D4-14 D4-15 D4-16 D4-17 0 b fc0 (MPa) Longitudinal bar c q d g ConA ConB fyl (MPa) Diameter 94.4 89.29 0.0 39. the spacing of the unbonded prestressed steel bars and the specified thickness of the Con-A depends on the diameters of the unbonded prestressed steel bars.29 120 60 120 60 120 60 0.1 787 22.0 20. d Ratio of the area of the longitudinal bar to the cross-sectional area. e The prestress level.261 M.8 22.65 1. g Spacing of the spiral. workability and the reduction of fine-particle segregation. The deflection distribution was approximated by a cubic function under the boundary condition that deflection on the supports is zero.7 78. whereas ‘‘Type-B’’ means that carbon-fiber sheets were attached to the entire pile.8 5.3 19. Experimental results and discussion 4.7 97. The concrete’s compressive strength.8 20. Highly flowable concrete was used as the concrete infilling.4 84.2 1. Table 1 depicts the average thickness of Con-A. 4. 000 rotations per minute for 3 min. Carbon-fiber sheet Longitudinal bar Concrete molded by centrifugal force (Con-A) Prestressing steel bar Spiral Concrete infilling (Con-B) Specimen: D4-14 Direction of loading 137 70 260 70 Units: mm 400 Fig. Concrete molded by centrifugal force (Con-A) Prestressing steel bar Spiral Longitudinal bar Specimen: D4-1 115 Direction of loading 80 240 Units: mm 80 400 (b) Sample cross-section of a proposed pile that uses prestressed steel bars where each has a diameter of 40 mm. Step 3：850 rotations per minute for 2 min.550 rotations per minute for 0. / Engineering Structures 34 (2012) 259–270 (a) Sample cross-section of a proposed pile that uses prestressed steel bars where each has a diameter of 32 mm. Molding the pile using centrifugal force. Fig. Metallic mold for creating the pile Step 1：250 rotations per minute for 6 min. 3.5 min. Step 2：400 rotations per minute for 4 min. Akiyama et al. Step 5：1. 2. Step 4：1.262 M. Sample cross-sections of the proposed piles. . pile D4-5. and the only difference between them was the magnitude of the initial prestress. Fig. outside the constant-moment region. 9 depicts the effects of different prestressing levels on the flexural strength of piles D4-14 and D4-16. The only difference between these two piles is the presence or absence of carbon-fiber sheets. shorter specimens showed larger increments of tensile stress. Because the proposed pile with carbon-fiber sheets and concrete infilling had a much higher flexural strength than did the conventional precast concrete pile (see Appendix). / Engineering Structures 34 (2012) 259–270 2. 6 depicts an example of the appearance of the constant-moment region at the point of maximum loading of specimens D4-5 (without concrete infilling and carbon-fiber sheeting) and D4-16 (with concrete infilling and carbon-fiber sheeting). The tests confirmed that concrete infilling can prevent a sudden decrease in load after spalling of the concrete cover. possessed a much higher flexural strength. As the prestress applied to the pile was increased. which had carbon-fiber sheets. As a result. such as the first yielding of the longitudinal bar and spalling of the cover concrete. The occurrence of specific events. decreased by about 30% after spalling of the concrete cover. spalling of the cover occurred before yielding of the longitudinal bar. this increase in the tensile strain could result in there being no difference in the compressive axial force provided by the prestressing steel bars among piles as the bending moment increases. As described hereinafter. The effects of the use of carbon-fiber sheeting on the flexural strengths of piles D4-13 and D4-14 are shown in Fig. Pile D4-14. and the capacity of the pile with the highest prestress. however. . These two specimens had almost the same strength concrete and steel bars but different amounts of applied prestress. however. before loading the nuts were tightened with a torque wrench to the point just before the tensile strain of the prestressing steel bars would begin to increase. the peak loads of these three piles were almost the same.263 M. 7. 7 depicts the relationship between the load and deflection at the midspans of piles D4-3. 7–9. Therefore. unidirectional carbon-fiber was used in the test. Because the cover concrete was confined by the carbon-fiber sheets. These piles had both concrete infilling and carbon-fiber sheets. as shown in Figs. it is expected that the proposed pile will be able to prevent yielding of the pile foundation under strong earthquake excitation (Japan Road Association [13]). The maximum loads on these specimens were observed during spalling of the concrete cover. These piles did not have carbon-fiber sheets or concrete infilling. 8 depicts the results for piles D4-9. which had concrete infilling but no carbon-fiber sheets. D4-4 and D4-5. is indicated in Fig. 4. therefore. The crack length of the pile with prestress that was provided by the prestressed steel bars was shorter than the crack length of the pile without prestress because the pile with a high prestress had a much larger compression zone in the cross-section. Because the neutral axis of a pile without initial prestress or with a low prestress level is closer to the extreme compression fiber. D4-11 and D4-13. The piles with carbon-fiber sheets exhibited ductile behavior. at the yield point of the longitudinal bars and at the maximum loading point. As shown in Figs. unlike piles without carbon-fiber sheets. the prestress level does not have an appreciable influence on flexural strength. it was not spalled by the compressive stress that was caused by the bending moment. these piles had larger maximum loads. Fig. These piles experienced maximum loading during the rupture of the carbon-fiber sheets due to the expansion of the cover concrete. Experimental setup. The effect of the initial prestress level on the flexural strength of piles with different lengths should be investigated.260 4000 Units: mm Prestressing steel bar Fig. 5 shows the crack patterns of piles D4-3 and D4-5. The piles without carbon-fiber sheets and concrete infilling exhibited brittle behavior after the concrete cover in the constant-moment region was cracked and spalled.260 500 500 1. The tests confirmed that the load at cracking increased with increasing prestress. The three piles exhibited almost the same material strengths and structural properties but were subjected to different amounts of prestress. It also confirmed that the maximum loads of these piles did not depend on the amount of prestress. Fig. Effects of the test variables on the flexural strengths of the proposed piles Fig. 7 and 8. this sheet can only serve to reinforce the circumferential stress of the pile. Fig. the tensile strain of prestressing steel bars in such a pile increases as the bending moment increases. the use of a concrete cover in piles with carbonfiber sheets can contribute to an increased flexural strength. As mentioned above. which were constructed without carbon-fiber sheets or concrete infilling. Akiyama et al. Naaman and Alkhairi [12] pointed out that the increment in the tensile stress of unbonded tendons depends on the specimen length.000-kN actuator Steel plate 150 500 500 150 Carbon-fiber sheet Pile Nut 500 1. the number of cracks decreased. 10.000 500 LVDT 1.2. The longitudinal bars in the piles with a wide spacing of spirals buckled after spalling of the concrete cover. 4. In their experiments. Even for piles without initial prestress. Because the cover concrete of this pile had a large compressive strain. The piles without carbon-fiber sheets but with concrete infilling did not show brittle behavior even after the concrete cover was spalled. 002 because of the expansion of the internal concrete. and such buckling is indicated by the sudden load drops depicted in Fig. 11b. [14] reported that strain on the carbon-fiber sheets increased at a great rate when ez was larger than 0.001–0. Akiyama et al. the impact of ez on the carbon-fiber sheets of the pile increased sharply after reaching a value of 0. 5. Fig. 12a. Kawashima et al. Fig. this model agrees well with most test results reported in the literature. In previous stress–strain models [16–20]. Even in piles to which concrete infilling was added to prevent brittle behavior. 12. Similarly. Regardless of the gauge length and cross-sectional dimensions. on a carbon-fiber sheet at the midspan of pile D4-12. as shown in Fig. the amount of longitudinal bars affects the flexural strength. 12 depicts the effect of spiral spacing on flexural behavior. and the descending stress-averaged strain curve depends on the gauge length. As shown in Fig.h at 0:5 f cc after the peak stress ( where f s. [15] is used. This model is applicable to reinforced concrete columns that consist of concretes with compressive strengths of up to 130 MPa and transverse reinforcement yield strengths of up to 1450 MPa. Based on the constant-fracture-energy concept for compression. Crack patterns at the yielding of the longitudinal bar and at a maximum load. it is also necessary for the proposed pile to have a smaller spiral spacing in order to prevent buckling of the longitudinal bars. The effect of the amount of longitudinal bars on the pile’s flexural strength is indicated in Fig. 13 depicts the relationship between the load and strain. 5. developed a formalized stress-averaged strain model that uses the compressive fracture energy and effective confining pressure. 11a. From Fig. Based on experimental tests of concrete columns with carbon-fiber sheeting under concentric loading. If the piles had neither concrete infilling nor carbon-fiber sheets. ez. 11. This result indicates that a high confining pressure was provided to the internal concrete by the carbon-fiber sheets and that it contributed to increasing the flexural strength of the pile. Analytical evaluation of the experimental results The moment–curvature relationships presented in this study are derived from a cross-section layer model that takes into account the constitutive laws of the materials. In order to describe the compression behavior of concrete. Although the pile is designed to prevent yielding of the pile foundation and not as a source of hysteretic energy dissipation. In tests that have been reported in the literature. Akiyama et al. concrete and rebar strengths. further research is needed to identify the optimal combination of prestress level.c ¼ Es 0:45ec0 þ 6:39 0:881 ) ke.v qw fyh fc0 ð2Þ ð3Þ . and this is because spalling of the cover concrete occurs early due to the large prestress.002. as shown in Fig. the stress-averaged strain model presented by Akiyama et al.c at a peak stress f cc of the confined concrete ð1Þ p0e ¼ ke. / Engineering Structures 34 (2012) 259–270 Fig.v qw f s.v qw f y. in addition to the amount of carbon-fiber sheeting that is needed to insure the adequate ductility of piles under strong excitation. Fig. 7 through Fig. it must also have sufficient shear strength to prevent a brittle failure mode. the differences in their maximum loads were not very large.264 M. This strain should be defined as the averaged strain because the core concrete has a certain fracture zone. When spalling of the cover concrete is prevented by the presence of a carbon-fiber sheet.001–0. in the measurements. and the longitudinal bars yielded. Because the proposed pile has an increased flexural strength. the buckling of the longitudinal bars of piles with larger spiral spacings was clearly observed. 12b. 12 present data that are related to the ductility capacity of the piles. it can be seen that it is not important to have a smaller spiral spacing for piles without concrete infilling because these piles exhibit brittle behaviors after the concrete cover has become spalled. the total length or diameter of the specimen is used as the gauge length. The effective confining pressures are given by: pe ¼ ke. the experimental longitudinal strain that was used was expressed by the change in gauge length divided by the original gauge length. which is defined as the total cross-sectional area.4N/mm2) Load (kN) 800 Fig.7N/mm2) D4-14(fpe = 20. Appearance of a constant-moment region at a maximum loading. sd.v ¼ 0 0 50 Fig.v is the effective confinement coefficient. 400 0 fc0 ¼ 0:85fc0 ð6Þ 2 ke. Relationship between the load and deflection in a pile without carbon-fiber sheets and with concrete infilling. s’ is the clear spacing between spi0 rals. ke. fs. where d is the core dimension that is measured from the center to center of a spiral. 1200 D4-16(fpe = 13.3N/mm2) D4-9 (fpe = 12. As.8N/mm2) D4-5 (fpe = 20. and fc0 is the compressive . 6. / Engineering Structures 34 (2012) 259–270 1200 D4-11 (fpe = 6. Kb ¼ 40 1:0 fc0 2 15ds ð1 qcc Þ ðfor a circular cross-sectionÞ ð7Þ 100 Deflection at the midspan (mm) ec0 ¼ 0:0028 0:0008kb 2s02 10s0 ds þ 15ds ð4Þ ð5Þ where qw is the area ratio of the transverse reinforcement. 400 1200 D4-3 (fpe = 0N/mm2) D4-4 (fpe = 9. Akiyama et al.265 M. ds is the core dimension measured from center to center of a spiral in the circular column.c is the stress in the spiral at the peak stress. 8. of the spirals with spacing s divided by the area. Relationship between the load and deflection in a pile without carbon-fiber sheets or concrete infilling. qcc is the ratio of the area of the longitudinal steel to the area of the core. 7. 9. fyh is the yield strength of the spiral.6N/mm2) D4-13 (fpe = 21.0N/mm2) Load (kN) Yielding of the longitudinal bar Spalling of the cover concrete 800 400 0 0 50 Deflection at the midspan (mm) 100 Fig.3N/mm2) Yielding of the longitudinal bar Load (kN) Yielding of the longitudinal bar Spalling of the cover concrete 0 800 0 50 Deflection at the midspan (mm) 100 Fig. Relationship between the load and deflection in a pile with carbon-fiber sheets and concrete infilling. 200 D4-16 D4-15 Yielding of the longitudinal bar Gauge A Gauge B Gauge C Load (kN) 800 Load (kN) 800 Yielding of the longitudinal bar Occurrence of damage of strain gauge 400 Gauge C 400 Gauge A 0 50 0 100 0 50 Deflection at the midspan (mm) 100 0 0 Gauge B 0. (1) and (2) were used to obtain the effective confining pressure for the spiral-confined concrete in regions (b) and (c) in Fig. 12. the effective confining pressures pe and in Eqs. Gf. Fig. 14. as proposed by Kohashi et al.v qw fyh at 0:5 f cc after the peak stress ð11Þ . 15 was modified to take into account the reduced confining pressure due to the presence of the hollow core using the factor f. 14 was treated as plain concrete. Effect of longitudinal bar number on the flexural strength.c ¼ Gfc0 1 þ 157 pe fc0 2 ) p 77:3 e fc0 ð8Þ Gfc0 ¼ 134 93:3kb ð9Þ p0e In this study. is given by: ( Gf . (a) (b) D4-6 D4-8 Yielding of the longitudinal bar Spalling of the cover concrete 1..22]. Relationship between the load and the strain of the carbon-fiber sheets in pile D4-12.200 (a) D4-13 D4-14 (b) D4-6 D4-5 Yielding of the longitudinal bar Spalling of the cover concrete Yielding of the longitudinal bar Spalling of the cover concrete D4-10 D4-9 Yielding of the longitudinal bar Spalling of the cover concrete 1. 10.200 Load (kN) Load (kN) 800 400 Buckling of the longitudinal bar of D4-10 800 400 0 50 Deflection at the midspan (mm) 0 100 0 0 50 100 0 Deflection at the midspan (mm) 50 100 Fig. Akiyama et al.200 1. (1) and (2) were modified by taking into consideration whether or not the pile had carbon-fiber sheets and/or concrete infilling.v qw fs. When a pile did not have both a carbon-fiber sheet and concrete infilling.01 Strain of the carbon-fiber sheet Fig. 11.005 0. 13. The effective confining pressure of the concrete in region (b) of Fig. Effect of carbon-fiber sheeting on the flexural strength.c at the peak stress f cc of the confined concrete ð10Þ p0e ¼ fke. 15 was treated as plain concrete.015 0. The compressive fracture energy. / Engineering Structures 34 (2012) 259–270 1. the concrete in region (a) of Fig. wherein this was based on their experimental results with hollow reinforced concrete columns under concentric loading [21. the original equations indicated in Eqs.c. When a pile without a carbon-fiber sheet had concrete infilling.266 M. Fig. strength of plain concrete that has been measured from a cylinder with a diameter of 100 mm and height of 200 mm. The effective confining pressure of a pile lacking a carbon-fiber sheet and concrete infilling was determined as follows: pe ¼ fke. whereas the concrete cover in region (a) in Fig. Effect of spiral spacing on the flexural strength. Cross-section of a pile with concrete infilling and without a carbon-fiber sheet. some models for estimating the confining pressures that are provided by the carbon-fiber sheets have been presented [6– 8. respectively.14]. For concrete columns with carbon-fiber sheets under concentric loading. . Cross-section of a pile with concrete infilling and a carbon-fiber sheet. 0 0 0. 16 with concrete infilling and a carbon-fiber sheet Fig.0N/mm2) 0 Region (c) 0 Regions (a) and (b): Con-A Region (c): Con-B Fig.267 M. 14 with concrete infilling and without a carbon-fiber sheet 50 Stress-averaged strain relation of concrete in Region (b) in Fig. 14. 100 Region (b) Axial Stress (MPa) Region (a) Stress-averaged strain relation of concrete in region (b) in Fig. 18.5 1 Axial strain (%) 1. Akiyama et al. Based on equations presented by Kawashima et al. 17. 16. 15. 50 Deflection (mm) 100 Fig. Effect of the initial prestress on the increment of stress in a prestressed steel bar during loading. Regions (a) and (b): Con-A Fig. [14]. Cross-section of a pile without concrete infilling or a carbon-fiber sheet.6N/mm2) 2 D4-13 (fpe = 21.3N/mm2) 4 D4-9 (fpe = 12. / Engineering Structures 34 (2012) 259–270 t D f ¼ 2:0 1 eF 1 ð100Ps Þ 0 < t D 2 Region (a) ð12Þ Region (b) F1 ¼ 1 ðfyh =200 F 2 Þ2 þ F2 ð13Þ 0 ðfc0 60Þ F 2 ¼ 2:0 ð14Þ 0 F 2 ¼ 4 fc0 =30 ð60 < fc0 < 120Þ Ps ¼ As =ðt sÞ Region (c) ð15Þ where t and D are the thickness and diameter of the pile. Relationship between the stress and averaged strain in confined concrete. Region (a) 6 Increment of Stress (MPa) Region (b) D4-11 (fpe = 6. 15 without concrete infilling or a carbon-fiber sheet.5 Fig. Regions (a) and (b): Con-A Region (c): Con-B Stress-averaged strain relation of concrete in region (b) in Fig. the Given by concrete in region (b) in Figs. Lm must be the same as the gauge length. eCFt is the strain at the peak stress (=0.v qw fyh þ qCF fCF at 0:5 f cc after the peak stress Computed result Computed result assuming without prestressing Given by concrete in region (c) in Figs.1 800 D4-13 D4-15 D4-14 D4-17 D4-16 400 0 0. 16 is provided by the carbon-fiber sheets and spirals and is given as follows: pe ¼ ke.c is dissipated. 19.c is kept constant in the strain-localized element. n is the number of carbon-fiber sheets that are wrapped around a pile. Therefore. Akiyama et al. [15] requires the use of the element length Lm over which the compressive fracture energy Gf. Comparison of the experimental and computed moment–curvature relationships.c þ qCF eCFt ECF at the peak stress f cc of the confined concrete p0e ¼ qCF fCF qCF at 0:5 f cc after the peak stress ð17Þ 4n tCF ¼ D ð18Þ where qCF is the area ratio of the carbon-fiber sheets.1 800 D4-5 D4-6 D4-8 D4-7 Moment (kN m) 400 0 0. and tCF is the thickness of a carbon-fiber sheet.1 800 D4-10 D4-9 D4-11 D4-12 400 0 0.0015). 14 to 16 800 ð19Þ Curvature (m-1) Fig. 14 and 16 Given by concrete in region (a) in Figs. ECF and fCF are the modulus of elasticity and tensile strength of the carbon-fiber sheet. under the condition that Lm is larger than the compressive fracture zone. Gf. 16 is given as follows: pe ¼ qCF eCFt ECF at the peak stress f cc of the confined concrete ð16Þ The effective confining pressure applied to the concrete in regions (b) and (c) in Fig.1 ð20Þ The stress-averaged strain model that was proposed by Akiyama et al. regardless of Lm. respectively. 14 to 16 D4-1 D4-3 D4 -2 D 4-4 400 0 0.v qw fs. . / Engineering Structures 34 (2012) 259–270 the effective confining pressure applied to the concrete in region (a) of Fig. Experimental result Given by longitudinal bars p0e ¼ ke. In the analytical evaluation of the concentric compression test results.268 M. 17 depicts the relationship between the stress and averaged strain of the concrete in region (b) of Figs. D4-12. such as D4-8 and D4-10.e. Fig. In addition.05 Curvature (m-1) Fig. Even though the carbon-fiber sheeting that was used in this experiment cannot itself resist the bending moment and the contribution to the moment–curvature relation that is provided by concrete infilling [region (c)] is small.10 and 5. 19 depicts a comparison of the experimental and computed moment–curvature relationships. however. the utilization of carbon-fiber sheets and concrete infilling can provide the confining pressures to concrete in regions (a) and (b) and improve the behavior of concrete in regions (a) and (b). In comparison to the initial prestress of these piles. 20. / Engineering Structures 34 (2012) 259–270 post-peak portion of the stress-averaged strain curve changes with Lm. 18 depicts the relationship between the increment in the stress of the prestressed steel bars and the deflections of D4-9. 19.00 0.e. 14–16 with Lm = 500 mm. as shown in the figure. The data presented in Figs. 16.05 Curvature (m-1) D4-9 0. the prestressed steel bars did not resist the bending moment. cannot be determined based on mechanical considerations.. Fig. the tensile strength of the concrete is ignored.00 0. The increments in the stress of the prestressed steel bars of D4-3 and D4-11 were 7. Lm was set to 500 mm. the piles with fewer spirals (i. 19 shows the computed results for the piles with carbon-fiber sheets and concrete infilling under the assumption that piles do not have prestressing steel bars. as shown in Fig. and it is steeper for larger values of Lm. The increment in stress of the prestressed steel bars that occurred before the ultimate flexural load was reached was not negligible for the pile with the lower prestress level. the stress-averaged strain relation in the post-peak region proposed by Akiyama et al. Fig. as shown in Fig. respectively. For the pile with the lower prestress level. this may provide a negligible contribution to the relationship between the moment and curvature. the piles with carbon-fiber sheets and concrete infilling (i. and 17) show better structural performances. Effect of Lm on the computed moment–curvature relationship.10 . the analytical and experimental moment–curvature curves agree very well. 17 and 19 confirm that the carbonfiber sheets and concrete infilling significantly affected the flexural strengths of the piles. and D4-11. D4-11 and D4-13 during loading. there have been no reports describing how to determine Lm in the analytical evaluations of concrete components that have been subjected to simultaneous axial loading and bending. These results show that it is necessary for the proposed piles to have prestressing steel bars to increase the flexural strength. exhibited buckling of the longitudinal bars. Even if the stress-averaged strain relation of the tensioned concrete is considered in the section analysis. Akiyama et al. 20 shows the effect of Lm on the computed moment–curvature relationship. With longer Lm.10 0 0. It should be noted that if the tensile strain of the prestressed steel bar does not increase with the bending moment.75 MPa. wider spiral spacings). 15. As a result.269 M. The effects of the test parameters on the increase in the tensile strain of the prestressed steel bars need to be examined. 14. Lm. however. Fig. a higher flexural strength could not be expected for the pile with a lower prestress level. D4-3. 0. It was confirmed that having both concrete infilling and the carbon-fiber sheets enhanced the strength and ductility capacity of the confined concrete. the increments are significant. Because the unbonded prestressed steel bars were placed at the center of the cross-section. The element length. the sectional analysis was conducted by assuming that the initial compressive force that was provided by the prestressed bars acted as an external axial force on the centroid. because this increment could not be quantified in this study. Fig. that is. In order to minimize the differences between the experimental and computed results of the moment–curvature relationship. [15] has a steeper descent if the confining 500 500 Moment (kN m) Moment (kN m) (a) 250 (b) 250 Experimental results Computed results (Lm=300 mm) Computed results (Lm =500 mm) Computed results (Lm =700 mm) Experimental results Computed results (Lm =300 mm) Computed results (Lm =500 mm) Computed results (Lm =700 mm) D4-5 0 0. In the flexural analysis. as shown in the computed results for D4-1. A bi-linear model was used to demonstrate the steel’s stress–strain relationship. The results for these specimens did not indicate good correlations between the experimental and analytical results.. For the specimens with higher prestress levels and smaller spiral spacings. the bending moment that was carried by the prestressed steel bars was ignored. this will produce an underestimate of the flexural strength. 12b. [22] Kohashi H. Further investigation is needed to determine Lm.and normal-strength transverse renforcements. Hosotani M.507707. Eng Struct 2006. / Engineering Structures 34 (2012) 259–270 pressure applied to the concrete is low. Kazama M. the structural details of pile heads that have been embedded in concrete footings should be examined so that design features can be identified that will prevent the failure of pileconcrete footing joints that are subjected to cyclic loading. Baum KSS. The analytical method to obtain the relationship between the bending moment and the curvature will be used in the seismic design of the proposed pile. Behavior of high-strength concrete columns under cyclic flexure and constant axial load. Oka F. Conclusions (1) In order to prevent the yielding of a pile foundation due to a severe earthquake. Soils Foundat 1996. Paultre P. prestress level and presence or absence of concrete infilling and carbon-fiber sheets on the flexural strength of the proposed pile were experimentally investigated. Ltd. Frangopol DM. 1995 Hyogoken–Nambu Earthquake. Japan: Maruzen. Soils Foundat 1996. [4] Tokimatsu K.17(6):607–16. By using the stress-averaged strain relationship for the confined concrete with the effective confining pressures. Yamato S. respectively. It should be noted that the prestressing tendons of a conventional pile are bonded with concrete and are not arranged at the center of the pile but. (3) Sectional analyses of the proposed piles were conducted to obtain the moment–curvature relationships. are arranged along the circumference of the spiral. highstrength. ACI Struct J 1991. the computed result with Lm = 700 mm for pile D4-5 lacking a carbon-fiber sheet and concrete infilling exhibits brittler behavior. Matsuzaki M. the analytical and experimental moment– curvature relationships agreed well with one another. Karabinis AI. Hatfield E. the design of a new precast. prestress level and specified concrete strength for a conventional precast. 2002.98(3):395–406. Concrete confined by FRP material: a plasticity approach. the computed moment–curvature relations with Lm = 300.24:923–32.9%. and reinforced concrete pile that was prestressed with unbonded bars that were arranged at the center of pile cross-section was presented in this study.28:1001–8. like longitudinal bars. The effective confining pressures were estimated by taking into consideration the presence or absence of carbon-fiber sheets and concrete infilling. ACI Struct J 1994. J Mater Civil Eng ASCE 2005. Alkhairi FM. Zhang F.107(2):179–88. Stress-strain model for laterally confined concrete. 0. Confinment model for high-strength concrete. John Wiley & Sons. Paultre P. Although the best fit with the experimental results for piles with diameters of 400 mm can be obtained by setting Lm = 500 mm. Nakatsuka T. [17] Cusson D. Eng Struct 2006.125(3):281–9. Inelastic design of low-axially loaded high-strength reinforced concrete columns. 500. Yamamoto H. J Struct Eng ASCE 1995. Mizuno H. (2) The effects of concrete strength. 4. ACI Struct J 2010. 6. Yamato S. doi:10. the appropriate magnitude of Lm to reproduce the experimental results may depend on a pile’s diameter and structural details. Oda K.. as shown in Fig. Sanjayan JG. Appendix A. [18] Bing L. The test results indicate that it is necessary for the proposed pile to have both concrete infilling and carbon-fiber sheeting to increase the pile flexural strength and prevent brittle failure.91(3):336–45.28:1346–53. Carbon fiber sheet retrofit of reinforced concrete bridge piers. [20] Hong K-N. [12] Naaman AE. Tokyo. References [1] Priestley MJ.1:189–200. In these tests. [13] Japan Road Association. Strength-deformation characteristics of cylindrical concrete confined with circular lateral reinforcement. Sento N. [14] Kawashima K. [21] Kohashi H. Since the stress-averaged strain relation has no descending branch and becomes independent of Lm as the confining pressure increases. 20b. Stress-averaged strain model for confined high-strength concrete. J Struct Eng ASCE 1999. Struct Infrastruct Eng 2010. Han SH. The flexural strengths of conventional piles that are used in Japan In Japan.1:219–34.22(3):217–22 (in Japanese). (4) Further research is needed to investigate the effects of combined axial and shear forces on the ductility capacity and plastic hinge behavior of the proposed pile. Three-dimensional numerical simulation of earthquake damage to group-piles in a liquefied ground.97(4):591–601. ACI Struct J 2001. and 700 mm are almost the same as those shown in Fig. . Brungardt P.88(6):683–92. Building damage associated with geotechnical problems Special Issue on Geotechnical Aspects of the January 17. the area ratio of the longitudinal bars.2:124–35. the calculated load at the first yield of the longitudinal bar is 270 kN. Seismic behavior of square high-strength concrete columns. Kiousis PD. Yi S-T. Dang DH. [9] Légeron F. Nat Center Res Earthquake Eng.29:1343–53. A-D22) with a diameter of 400 mm are 3. instead. FRP-confined concrete members: axial compression experiments and plasticity modelling.25:1083–96. Kakurai M. Foundation damage of structures Special Issue on Geotechnical Aspects of the January 17. Reliability-based capacity design for reinforced concrete bridge structures. Taipei. 1995.1 and 105 MPa. Proc Int Workshop Ann Commemor Chi-Chi Earthquake. Eng Struct 2002.1080/15732479. Calve GM. Design specifications of highway bridges. [3] Matsui T. Pro Japan Concrete Inst 1999. Park R. Therefore. [15] Akiyama M.21(3):241–6 (in Japanese). Saatcioglu M. Seible F. Yoneda K. Stress at ultimate in unbonded post-tensioning tendons: part 2 – Proposed methodology. [11] Ho JCM. Eng Struct 2007. [7] Rousakis TC. Seismic design and retrofit of bridges. 20a.47%. Tanaka H. Proc Japan Concrete Inst 2000. Soil Dyn Earthquake Eng 2007. Yashima A. and reinforced concrete pile (DAM105. [5] Uzuoka R. [16] Razvi R. Setunge S. [6] Karabinis AI. Part V seismic design. Taiwan 2000. Suzuki M.270 M. [19] Lokuge WP. 1995 Hyogoken–Nambu Earthquake. Akiyama et al. prestressed.121(3):468–77. Suzuki M. volumetric ratio of the spirals. Nakatsuka T. number of longitudinal bars. Rousakis TC. [10] Azizinamini A. ACI Struct J 2000. If this pile is tested under load by using a similar four-point setup. Stress-strain model for confined high-strength concrete.2010. The effects of material strength on strength-deformation characteristics of cylindrical concrete confined with circular lateral reinforcement. Pam HJ. High-strength concrete columns confined by lowvolumetric-ratio lateral ties. Stress-strain behavior of high-strength concrete confined by ultra-high.27:395–413. [8] Li G. as shown in Fig. The bending moment at the first yield of a longitudinal bar is suggested by the manufacturer to be 170 kN m. Eng Struct 2003. [2] Akiyama M. 4. Experimental study of FRP confined concrete cylinders.