Flexural test of precast high-strength reinforced concrete pile prestressed with unbonded bars arranged at the center of the cross-section

nfth

otoShin4-95-897ki-A

Received 1 December 2009Revised 2 September 2011Accepted 2 September 2011Available online 4 November 2011

exural capacity was developed. The main structural characteristics of the developed pile include (1) the

damaged concrete-pile foundations in reclaimed ground have beenconducted to clarify the damage-process mechanism therein.Based on a three-dimensional numerical simulation, Uzuokaet al. [5] reported that precast, prestressed concrete piles in a liq-ueed soil failed during this severe earthquake due to a lack of

used in these piles, it cannot contribute to increasing the exuralstrength of the pile. In this paper, a new method for increasing theexural strength of concrete piles was developed, wherein unbond-ed prestressing steel bars are incorporated at the center of the cross-section of the reinforced concrete piles. As shown in Fig. 1b, in com-parison to conventional concrete piles that are subject to small axialforces, the neutral axis of the developed pile is much closer to thecentroidal axis because of the prestress that is provided by the unb-ondedprestressed steel bars; thus, the compression region increasesin cross-section and can improve the exural strength of the pile. In

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]

Engineering Structures 34 (2012) 259270

Contents lists available at

Engineering

lseku.ac.jp (M. Suzuki).1. Introduction

In the seismic design of concrete bridge systems, a plastic hingemust be introduced at the bottom of each bridge pier rather than atthe pile foundation. This is an important concept in capacity designto help guarantee the rehabilitation of the bridge after a largeearthquake [1,2]; however, if concrete bridge systems areconstructed on strata (such as very soft soil) that can experiencesoil liquefaction in a severe earthquake, it is difcult to preventyielding of the pile foundation. Numerous structures with precastconcrete-pile foundations in reclaimed ground were seriouslydamaged by the 1995 HyogokenNambu Earthquake in Japan[3,4]. Field investigations and numerical analyses of some of the

exural strength and ductility capacity. In order to maximizepost-event operability and minimize the repair costs of bridges, in-creased attention should be paid to improving the exural strengthand ductility capacity of precast concrete piles that are driven intoliqueable soil.

Fig. 1adepicts the straindistribution in a concretepile that is sub-jected to a bending moment at the point where the strain of the ex-treme compression ber reaches 0.0035. Because the dead load-induced axial force that acts on the pile foundations of most bridgesin Japan is so small, as shown in Fig. 1, the neutral axis of the pile thatis subjected to the bending moment becomes closer to the extremecompressionber as thebendingmoment increases. Therefore, eventhough concrete with a compressive strength of over 100 MPa isKeywords:Precast pileHigh-strength concreteHigh-strength steelFlexural strengthCarbon-ber sheet0141-0296/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.engstruct.2011.09.007neutral axis is constantly near the centroidal axis of the pile, even if the longitudinal reinforcement yieldsdue to a exural moment, because the pile has a high axial compressive force that is induced by pre-stressed steel bars, and hence, the concrete in the compression region can contribute to increasing theexural strength of the pile; and (2) the exural strength of the pile increases because the high-strengthconcrete is conned by high-strength spirals and carbon-ber sheets in combination with concrete inll-ing, and, together, these modications provide a sufciently high lateral-connement pressure.The results of bending tests demonstrate that the proposed prestressed reinforced concrete pile with

carbon-ber sheets and concrete inlling had a much higher exural capacity than a conventional precastconcrete pile. In addition, an analytical approach is presented that can be used to obtain the relationshipbetween the bending moment and the curvature of the proposed pile. Even if concrete bridge systems areconstructed on strata that can experience soil liquefaction, such as very soft soil, bridge foundations thatuse the proposed piles could remain undamaged under the design seismic action.

2011 Elsevier Ltd. All rights reserved.Article history: In this study, a prestressed reinforced concrete pile that uses high-strength material to increase the pilesFlexural test of precast high-strength reiunbonded bars arranged at the center of

Mitsuyoshi Akiyama a,, Satoshi Abe b, Nao Aoki c, MaDepartment of Civil and Environmental Engineering, Waseda University, 3-4-1 Okubo,b Technical Research Institute, Obayashi Corporation, 4-6 Shimokiyoto, Kiyose, Tokyo 20cEast Nippon Expressway Company Limited, 3-3-2 Kasumigaseki, Chiyodaku, Tokyo 100dDepartment of Civil and Environmental Engineering, Tohoku University, 6-6-06 Arama

a r t i c l e i n f o a b s t r a c t

journal homepage: www.ell rights reserved.orced concrete pile prestressed withe cross-section

yuki Suzuki d

juku-Ku, Tokyo 169-8555, Japan58, Japan9, Japanza-Aoba, Aobaku, Sendai 980-8579, Japan

SciVerse ScienceDirect

Structures

vier .com/ locate /engstruct

.003

03

e dis

tra

trai

l for

g Stthe developed pile, concrete with a compressive strength of100 MPa, a longitudinal reinforcement with a yield strength of700 MPa and a transverse reinforcement with a yield strength of1450 MPa was used. In addition, carbon-ber sheets were used toprevent spalling of the cover concrete. The effects of test variables,such as the number of longitudinal bars, the prestress level andthe presence or absence of carbon-ber sheets, on the exuralstrength of the proposed pile were investigated through a series ofstatic bending tests. An analytical approach that can be used to ob-tain the relationship between themoment and curvature of the pro-posed pile is presented.

This paper primarily investigates exure of the developed pileunder monotonic loading with special emphasis on the exuralstrength; hence, the effects of the experimental variables on theductility capacity and residual displacement of the pile are outsidethe scope of the current investigation. These effects will be exam-ined in a future study using cyclic load tests.

2. Prestressed reinforced concrete piles using high-strengthmaterials and carbon-ber sheets

A total of 17 piles, each 400 mm in diameter and 4000 mm long,were tested under a static bending test. Table 1 summarizes thematerial properties of the proposed piles. Examples of a cross-sec-tion of the proposed pile are provided in Fig. 2. In order to produce

0

0.0

Small axial stress

Axial stress provided by unbonded prestressing steel bars

xb,1 < xb,2 where, xb,1 and xb,2 are thbending

(a)

(b)

s

s

Fig. 1. Effect of the piles axia

260 M. Akiyama et al. / Engineerinthe proposed pile, (1) a hollow pile was molded from high-strengthconcrete using centrifugal force. The centrifugal forcewas applied asshown in Fig. 3 for 15.5 min. All of the piles were removed from thesteelmold after being steam-cured for the rst 12 h andwere subse-quently air-cured until testing. Then, (2) prestressing steel barswitha sheathwere inserted into the center of the pile. (3) For some spec-imens, concrete inlling (denoted Con-B in Fig. 2) was placed inthe hollowof the pile,whereas (4), for other specimens, carbon-bersheets were attached to the pile surface. Finally, (5) prestress wasprovided by tightening the nutswith a torquewrench before releas-ing the jack under themonitoring of a strain gauge thatwas attachedto the prestressing steel bars. Some of the specimens did not haveconcrete inlling and/or carbon-ber sheets. The sheath was notlled with grout in any of the piles.

As shown in Fig. 2, because the ratio of the area of the concretecover to that of the core concrete inCon-A is not small, it is importantto prevent spalling of the concrete cover that is used to increase theexural strength, even after the cover is damaged. Over the past dec-ade, there has been an increasing interest in the use of ber rein-forced polymer (FRP) in the repairing, retrotting, strengthening,and new construction of concrete components. FRP can provide lat-eral conning pressure to the internal concrete of FRP, and themodeling of sections that are conned by FRP reinforcement hasbeen examined [68]. In this study, unidirectional carbon-bersheets were used to investigate the effect of conning the concretecover with such sheets on the resultant exural strength of the pile.The carbon-ber sheet can only affect the piles response to a cir-cumferential stress. Therefore, the carbon-ber sheets crack in re-sponse to tensile stress that is caused by a bending moment.

Previous studieshave shownthat the axial load level signicantlyaffects the exural behavior, especially the ductility, of high-strength reinforced concrete columns [911]. This should be consid-ered in determining the target prestress level of specimens, which isexpressed as the axial force that is provided by the unbonded pre-stressing steel barsdividedby the cross-sectional area of thepile. Be-cause, in terms of capacity design, a bridge foundation would bedesigned to remain undamaged under design seismic action, in thisstudy target prestress levelsweredeterminedby taking into accountonly the number and the specied tensile strain of prestressed steelbars. As shown in Table 1, target prestress levels ranged from 0 to21.0 MPa. To insure clearance between prestressing steel bars, therenumber of prestressing steel bars arranged at the center of the cross-section must be no more than three for the pile with a diameter of400 mm. In addition, themanufacturer of the prestressing steel barsspecies that the maximum tensile strain during providing the pre-stress should be less than 65% of its designed yielding strain. There-fore, the maximum prestress level provided by three prestressing

Small axial stress 5

5 Axial stress provided by unbonded prestressing steel bars

xb,1

xb,2

tances from the extreme compression fiber to the neutral axis of

in

n

ce on the strain distribution.

ructures 34 (2012) 259270steel bars for a pile with a diameter of 400 mm is approximately21 MPa. Because the prestress was provided just before testing, thelong-term loss of prestress in the proposed pile could not be evalu-ated from this experiment.

The main structural characteristics of the developed pile arethat (1) the neutral axis is constantly near the centroidal axis ofthe pile, even if longitudinal reinforcements yield due to a exuralmoment; this occurs because the pile has a high axial compressiveforce that is provided by the unbonded prestressed steel bars. Thus,the concrete in the compression zone can contribute to increasingthe exural strength of the pile. (2) In addition, the strain of theprestressed steel bars that are subjected to a bending moment isso low that it is elastic until the ultimate state of the pile isreached; this is because the unbonded prestressed steel bars arearranged at the center of the piles cross-section. Even thoughthe prestressed steel bars cannot resist the bending moment, theresidual displacement can be reduced by the prestressed steel barsafter the load is removed. (3) High-strength longitudinal bars andspirals are used to increase both the exural and shear strengthsof the pile. (4) The exural strength of the pile increases due tothe fact that the high-strength concrete is conned by high-strength spirals and carbon-ber sheets and due to the use of

ssing

(MPa) (mm) (%)

ere

y th

cons

g Stconcrete inlling; these modications together provide a suf-ciently high lateral-connement pressure.

For maximum exural strength, the proposed pile should have

Table 1Test specimens.

Notation ofspecimena

f 0c0b (MPa) Longitudinal bar Diameter of prestre

steel bar (mm)Con-A

Con-B

fylc

(MPa)Diameter qgd

(%)

D4-1 94.1 787 22.2 2.9 32.0D4-2 D4-3 107 D4-4 D4-5 114 D4-6 D4-7 110 796 31.8 5.9D4-8 D4-9 108 41.0 787 22.2 1.8D4-10D4-11 102 37.0 778 40.0D4-12D4-13 98.7 37.2 787D4-14 97.3 38.1D4-15 796 31.8 3.8D4-16 88.0 39.7 778 22.2 1.8D4-17

a D4 indicates a diameter of 400 mm.b The average compressive strength that was obtained from three cylinders that w

molded by centrifugal force and concrete inlling, respectively, as shown in Fig. 2.c Yield strength of the longitudinal bar.d Ratio of the area of the longitudinal bar to the cross-sectional area.e The prestress level, which was calculated as the axial force that was provided bf Yield strength of the spiral.g Spacing of the spiral.h Volumetric ratio of the spiral.i Type-Ameans that carbon-ber sheets were attached to the concrete in the

carbon-ber sheets were attached to the entire pile.

M. Akiyama et al. / Engineerinboth concrete inlling and carbon-ber sheets; however, pileswithout concrete inlling and/or carbon-ber sheets were testedin order to investigate the effect of these modications on thebehavior of the pile. All piles have a sufcient number of high-strength spirals to exhibit a exure failure mode. In order to permitoptimal shear design of the proposed piles, the effects of the num-ber and yield strengths of spirals on the shear strength of the pro-posed pile should be investigated.

3. Experimental procedure

3.1. Specimen properties and materials

The arrangement of the longitudinal bars and unbonded pre-stressed steel bars in the proposed pile are shown in Fig. 2. Asshown in the gure, the spacing of the unbonded prestressed steelbars and the specied thickness of the Con-A depends on the diam-eters of the unbonded prestressed steel bars. Table 1 depicts theaverage thickness of Con-A, measured after the hollow pile wasmolded by the centrifugal force.

Type I Ordinary Portland cement was used in all of the concretemixtures. Crushed gravel was used as the coarse aggregate, where-in the maximum aggregate size, Gmax, was 15 mm. For the concretethat was used in the Con-A, silica fumes were used to obtain highstrength, workability and the reduction of ne-particle segrega-tion. Highly owable concrete was used as the concrete inlling.The concretes compressive strength, f0c0, was measured as theaverage of three identical cylinders, each of which had a diameterof 100 mm and a height of 200 mm. These cylinders were testedunder axial loading at the time the corresponding pile was tested.

The yield strengths of the longitudinal bars and spirals areshown in Table 1. The yield strengths of the unbonded prestressedsteel bars were 1150 and 1230 MPa for the 32- and 40-mm-diam-eter bars, respectively. The tensile strength of the carbon-bersheet was 4620 MPa; a single layer with a sufcient overlap lengthto anchor the sheet (=100 mm) was used. The carbon-ber sheets

tant-moment region + 150 mm, as shown in Fig. 3, whereas Type-B means that0.0 1440 60 1.29 75.6 20.6 79.6 0.0 78.9 9.8 81.8

20.3 80.4 19.5 120 0.65 84.9 19.8 60 1.29 83.1 20.3 120 0.65 80.4 12.6 60 1.29 89.6 12.1 120 0.65 89.4 6.3 60 1.29 66.2

12.7 68.9 Type-A21.0 66.6 20.4 83.7 Type-A13.4 78.0 Type-B13.7 66.9 Type-A20.1 66.4 Type-A

100 mm in diameter and 200 mm in length. Con-A and Con-B indicate concrete

e prestressed steel bars divided by the sum of the areas of Con-A and Con-B.Prestresslevele (MPa)

Spiral Thickness ofCon-A (mm)

Carbon-bersheeti

fysf sg qsh

ructures 34 (2012) 259270 261were bonded onto the concretes surface with epoxy resin. Theminimum curing time for bonding of the carbon-ber sheets beforethe test was 10 days.

3.2. Testing procedure and instrumentation

Each specimen was tested under a monotonically increasingload until failure using a four-point bending setup, as shown inFig. 4. Deection was measured using ve linear variable-differen-tial transducers (LVDTs). The deection distribution was approxi-mated by a cubic function under the boundary condition thatdeection on the supports is zero. The averaged curvature couldbe obtained by differentiating the approximated deection func-tion for the constant-moment region.

The cracking behavior of the pile without the carbon-bersheets was visually observed. Electrical strain gauges were at-tached to the following surfaces: concrete, prestressing steel bars,longitudinal bars, spirals and carbon-ber sheets. When prestresswas applied to the pile, the strain gauge on the prestressed steelbars was controlled such that a specied prestress was obtained.After the steel bar-provided prestress was introduced, the mea-sured compression strains of the concrete and longitudinal barswere almost equal to the values that were computed from the gi-ven prestress.

4. Experimental results and discussion

4.1. General observations

Because the specimens had a sufcient number of spirals, all thepiles exhibited a exural failure mode only within the constant-moment region, even though minor shear cracks were observed

Concrete infilling (Con-B)

400

240 115

80 80

Direction of loading

400 260 137

70 70

Longitudinal bar

Concrete molded by centrifugal force (Con-A)

Prestressing steel bar

Spiral

Carbon-fiber sheet

Units: mm

Direction of loading

Units: mm

Concrete molded by centrifugal force (Con-A) Prestressing steel bar

Spiral Longitudinal bar

(a) Sample cross-section of a proposed pile that uses prestressed steel bars where each has a diameter of 32 mm.

(b) Sample cross-section of a proposed pile that uses prestressed steel bars where each has a diameter of 40 mm.

Specimen: D4-1

Specimen: D4-14

Fig. 2. Sample cross-sections of the proposed piles.

Metallic mold for creating the pile

Step 1250 rotations per minute for 6 min.



Step 41,000 rotations per minute for 3 min.

Step 51,550 rotations per minute for 0.5 min.

Fig. 3. Molding the pile using centrifugal force.

262 M. Akiyama et al. / Engineering Structures 34 (2012) 259270

500

000

500

rime

g Stoutside the constant-moment region. Fig. 5 shows the crack pat-terns of piles D4-3 and D4-5, which were constructed without car-bon-ber sheets or concrete inlling, at the yield point of thelongitudinal bars and at the maximum loading point. These twospecimens had almost the same strength concrete and steel barsbut different amounts of applied prestress. As the prestress appliedto the pile was increased, the number of cracks decreased. Thecrack length of the pile with prestress that was provided by theprestressed steel bars was shorter than the crack length of the pilewithout prestress because the pile with a high prestress had amuch larger compression zone in the cross-section.

The piles without carbon-ber sheets and concrete inllingexhibited brittle behavior after the concrete cover in the con-stant-moment region was cracked and spalled. The piles withoutcarbon-ber sheets but with concrete inlling did not show brittlebehavior even after the concrete cover was spalled. The longitudi-nal bars in the piles with a wide spacing of spirals buckled afterspalling of the concrete cover. The maximum loads on these spec-imens were observed during spalling of the concrete cover. Thepiles with carbon-ber sheets exhibited ductile behavior. Thesepiles experienced maximum loading during the rupture of the car-bon-ber sheets due to the expansion of the cover concrete. Fig. 6

Prestressing steel bar

Fig. 4. ExpePile

1,260

500

Nut

2,

Steel plate 150

M. Akiyama et al. / Engineerindepicts an example of the appearance of the constant-moment re-gion at the point of maximum loading of specimens D4-5 (withoutconcrete inlling and carbon-ber sheeting) and D4-16 (with con-crete inlling and carbon-ber sheeting).

4.2. Effects of the test variables on the exural strengths of theproposed piles

Fig. 7 depicts the relationship between the load and deectionat the midspans of piles D4-3, D4-4 and D4-5. These piles did nothave carbon-ber sheets or concrete inlling. The three pilesexhibited almost the same material strengths and structural prop-erties but were subjected to different amounts of prestress. Theoccurrence of specic events, such as the rst yielding of the lon-gitudinal bar and spalling of the cover concrete, is indicated inFig. 7. The tests conrmed that the load at cracking increased withincreasing prestress; however, the peak loads of these three pileswere almost the same, and the capacity of the pile with the highestprestress, pile D4-5, decreased by about 30% after spalling of theconcrete cover. Because the cover concrete of this pile had a largecompressive strain, spalling of the cover occurred before yieldingof the longitudinal bar.Fig. 8 depicts the results for piles D4-9, D4-11 and D4-13, whichhad concrete inlling but no carbon-ber sheets. The tests con-rmed that concrete inlling can prevent a sudden decrease in loadafter spalling of the concrete cover. It also conrmed that the max-imum loads of these piles did not depend on the amount ofprestress.

Fig. 9 depicts the effects of different prestressing levels on theexural strength of piles D4-14 and D4-16. These piles had bothconcrete inlling and carbon-ber sheets, and the only differencebetween them was the magnitude of the initial prestress. Becausethe cover concrete was conned by the carbon-ber sheets, it wasnot spalled by the compressive stress that was caused by the bend-ing moment. As shown in Figs. 7 and 8, unlike piles without car-bon-ber sheets, the use of a concrete cover in piles with carbon-ber sheets can contribute to an increased exural strength. As aresult, these piles had larger maximum loads. Because the neutralaxis of a pile without initial prestress or with a low prestress levelis closer to the extreme compression ber, the tensile strain of pre-stressing steel bars in such a pile increases as the bending momentincreases. Even for piles without initial prestress, before loadingthe nuts were tightened with a torque wrench to the point just be-fore the tensile strain of the prestressing steel bars would begin to

LVDT

1,000 1,260

4000

500 500

Carbon-fiber sheet

-kN actuator

500

Units: mm

150

ntal setup.

ructures 34 (2012) 259270 263increase. As described hereinafter, this increase in the tensile straincould result in there being no difference in the compressive axialforce provided by the prestressing steel bars among piles as thebending moment increases. Therefore, as shown in Figs. 79, theprestress level does not have an appreciable inuence on exuralstrength; however, Naaman and Alkhairi [12] pointed out thatthe increment in the tensile stress of unbonded tendons dependson the specimen length. In their experiments, shorter specimensshowed larger increments of tensile stress. The effect of the initialprestress level on the exural strength of piles with differentlengths should be investigated.

The effects of the use of carbon-ber sheeting on the exuralstrengthsof pilesD4-13andD4-14are shown in Fig. 10. Theonlydif-ference between these two piles is the presence or absence of car-bon-ber sheets. Pile D4-14, which had carbon-ber sheets,possessed amuchhigherexural strength.Asmentionedabove, uni-directional carbon-ber was used in the test; therefore, this sheetcan only serve to reinforce the circumferential stress of the pile. Be-cause the proposed pilewith carbon-ber sheets and concrete inll-ing had a much higher exural strength than did the conventionalprecast concretepile (seeAppendix), it is expected that theproposedpile will be able to prevent yielding of the pile foundation understrong earthquake excitation (Japan Road Association [13]).

he l

g StThe effect of the amount of longitudinal bars on the piles ex-ural strength is indicated in Fig. 11. If the piles had neither concreteinlling nor carbon-ber sheets, the differences in their maximumloads were not very large, as shown in Fig. 11a, and this is becausespalling of the cover concrete occurs early due to the large pre-stress. When spalling of the cover concrete is prevented by the

Fig. 5. Crack patterns at the yielding of t

264 M. Akiyama et al. / Engineerinpresence of a carbon-ber sheet, the amount of longitudinal barsaffects the exural strength, as shown in Fig. 11b. Fig. 12 depictsthe effect of spiral spacing on exural behavior. From Fig. 12a, itcan be seen that it is not important to have a smaller spiral spacingfor piles without concrete inlling because these piles exhibit brit-tle behaviors after the concrete cover has become spalled. Even inpiles to which concrete inlling was added to prevent brittlebehavior, the buckling of the longitudinal bars of piles with largerspiral spacings was clearly observed, and such buckling is indicatedby the sudden load drops depicted in Fig. 12b.

Fig. 7 through Fig. 12 present data that are related to the ductil-ity capacity of the piles. Because the proposed pile has an increasedexural strength, it must also have sufcient shear strength to pre-vent a brittle failure mode. As shown in Fig. 12, it is also necessaryfor the proposed pile to have a smaller spiral spacing in order toprevent buckling of the longitudinal bars. Although the pile is de-signed to prevent yielding of the pile foundation and not as asource of hysteretic energy dissipation, further research is neededto identify the optimal combination of prestress level, concrete andrebar strengths, in addition to the amount of carbon-ber sheetingthat is needed to insure the adequate ductility of piles under strongexcitation.

Fig. 13 depicts the relationship between the load and strain, ez,on a carbon-ber sheet at the midspan of pile D4-12. Based onexperimental tests of concrete columns with carbon-ber sheetingunder concentric loading, Kawashima et al. [14] reported thatstrain on the carbon-ber sheets increased at a great rate whenez was larger than 0.0010.002 because of the expansion of theinternal concrete. Similarly, in the measurements, the impact ofez on the carbon-ber sheets of the pile increased sharply afterreaching a value of 0.0010.002, and the longitudinal barsyielded. This result indicates that a high conning pressure wasprovided to the internal concrete by the carbon-ber sheets andthat it contributed to increasing the exural strength of the pile.

5. Analytical evaluation of the experimental results

ongitudinal bar and at a maximum load.

ructures 34 (2012) 259270The momentcurvature relationships presented in this studyare derived from a cross-section layer model that takes into ac-count the constitutive laws of the materials. In order to describethe compression behavior of concrete, the stress-averaged strainmodel presented by Akiyama et al. [15] is used. In previousstressstrain models [1620], the experimental longitudinal strainthat was used was expressed by the change in gauge length dividedby the original gauge length. In tests that have been reported in theliterature, the total length or diameter of the specimen is used asthe gauge length. This strain should be dened as the averagedstrain because the core concrete has a certain fracture zone, andthe descending stress-averaged strain curve depends on the gaugelength. Based on the constant-fracture-energy concept for com-pression, Akiyama et al. developed a formalized stress-averagedstrain model that uses the compressive fracture energy and effec-tive conning pressure. This model is applicable to reinforced con-crete columns that consist of concretes with compressive strengthsof up to 130 MPa and transverse reinforcement yield strengths ofup to 1450 MPa. Regardless of the gauge length and cross-sectionaldimensions, this model agrees well with most test results reportedin the literature.

The effective conning pressures are given by:

pe ke;vqw f s;c at a peak stress f cc of the confined concrete1

p0e ke;vqw f y;h at 0:5 f cc after the peak stress 2

where f s;c Es 0:45ec0 6:39ke;vqwfc0

0:881( ) fyh 3

g StM. Akiyama et al. / Engineerinec0 0:0028 0:0008kb 4

Kb 40fc0 1:0 5

Fig. 6. Appearance of a constant-moment region at a maximum loading.

Deflection at the midspan (mm)

Load

(kN)

D4-3 (fpe = 0N/mm2)D4-4 (fpe = 9.8N/mm2)D4-5 (fpe = 20.3N/mm2)

Yielding of the longitudinal bar Spalling of the cover concrete

0 50 1000

400

800

1200

Fig. 7. Relationship between the load and deection in a pile without carbon-bersheets or concrete inlling.400800

1200

Load

(kN)

D4-11 (fpe = 6.3N/mm2)


D4-9 (fpe = 12.6N/mm2)D4-13 (fpe = 21.0N/mm2)

ructures 34 (2012) 259270 265fc0 0:85f 0c0 6

ke;v 2s02 10s0ds 15d2s15d2s 1 qcc

for a circular cross-section 7

where qw is the area ratio of the transverse reinforcement, which isdened as the total cross-sectional area, As, of the spirals with spac-ing s divided by the area, sd, where d is the core dimension that ismeasured from the center to center of a spiral; ke,v is the effectiveconnement coefcient; fs,c is the stress in the spiral at the peakstress; qcc is the ratio of the area of the longitudinal steel to the areaof the core; ds is the core dimension measured from center to centerof a spiral in the circular column; s is the clear spacing between spi-rals; fyh is the yield strength of the spiral; and f 0c0 is the compressive

0 50 1000


Fig. 8. Relationship between the load and deection in a pile without carbon-bersheets and with concrete inlling.

D4-16(fpe = 13.7N/mm2)D4-14(fpe = 20.4N/mm2)


Load

(kN)

0 50 1000

400

800

1200

Yielding of the longitudinal bar

Fig. 9. Relationship between the load and deection in a pile with carbon-bersheets and concrete inlling.

N)

400

g St

Lo

ad (k

400 Yielding of the longitudinal bar Spalling of the cover concrete

D4-13 1,200

D4-14

800

266 M. Akiyama et al. / Engineerinstrength of plain concrete that has been measured from a cylinderwith a diameter of 100 mm and height of 200 mm.

The compressive fracture energy, Gf,c, is given by:

Gf ;c Gfc0 1 157 pefc0

77:3 pe

fc0

2( )8

Gfc0 134 93:3kb 9In this study, the effective conning pressures pe and p0e in Eqs.

(1) and (2) were modied by taking into consideration whether ornot the pile had carbon-ber sheets and/or concrete inlling. Whena pile without a carbon-ber sheet had concrete inlling, the origi-nal equations indicated in Eqs. (1) and (2) were used to obtain theeffective conning pressure for the spiral-conned concrete in re-gions (b) and (c) in Fig. 14, whereas the concrete cover in region (a)in Fig. 14 was treated as plain concrete.

0 Deflection at the midspan (mm)

0 50 100

Fig. 10. Effect of carbon-ber sheeting on the exural strength.

Yielding of the longitudinal bar


0 50 100 0 50 100

1,200

Load

(kN)

800

400

D4-6 D4-8

D4-16D4-15

(b)(a)


Fig. 11. Effect of longitudinal bar number on the exural strength.1,200

Load

(kN)

800


D4-6 D4-5


D4-10 D4-9

Buckling of the longitudinal bar of D4-10

(b)(a)ructures 34 (2012) 259270When a pile did not have both a carbon-ber sheet and concreteinlling, the concrete in region (a) of Fig. 15 was treated as plainconcrete. The effective conning pressure of the concrete in region(b) of Fig. 15 was modied to take into account the reducedconning pressure due to the presence of the hollow core usingthe factor f, as proposed by Kohashi et al., wherein this was basedon their experimental results with hollow reinforced concrete col-umns under concentric loading [21,22]. The effective conningpressure of a pile lacking a carbon-ber sheet and concrete inllingwas determined as follows:

pe fke;vqwfs;c at the peak stress f cc of the confined concrete10

p0e fke;vqwfyh at 0:5 f cc after the peak stress 11


0 50 100 0 50 100

Fig. 12. Effect of spiral spacing on the exural strength.

0

Gauge B

Gauge C

Gauge A

Load

(kN)

Gauge A Gauge B Gauge C

Yielding of the longitudinal bar Occurrence of damage of strain gauge

Strain of the carbon-fiber sheet0.015

400

800

1,200

0.005 0.010

Fig. 13. Relationship between the load and the strain of the carbon-ber sheets inpile D4-12.

f 2:0 tD

1 eF1100Ps 0 < t D2

12

F1 1fyh=200 F22 F2 13

F2 2:0 f 0c0 60F2 4 f 0c=30 60 < f 0c0 < 120

14

Ps As=t s 15where t and D are the thickness and diameter of the pile,respectively.

For concrete columns with carbon-ber sheets under concentricloading, some models for estimating the conning pressures thatare provided by the carbon-ber sheets have been presented [68,14]. Based on equations presented by Kawashima et al. [14],

Region (a)

Region (b)

Regions (a) and (b): Con-A

Fig. 15. Cross-section of a pile without concrete inlling or a carbon-ber sheet.

Region (c)

Region (a)

Region (b)

Regions (a) and (b): Con-A Region (c): Con-B

Fig. 16. Cross-section of a pile with concrete inlling and a carbon-ber sheet.

Region (a)

Region (b)

Region (c)

Regions (a) and (b): Con-A Region (c): Con-B

Fig. 14. Cross-section of a pile with concrete inlling and without a carbon-bersheet.

0 0.5 1 1.50

50

100

Axial strain (%)

Axi

al S

tres

s (M

Pa)

Stress-averaged strain relation of concrete in Region (b) in Fig. 15 without concrete infilling or a carbon-fiber sheet.

Stress-averaged strain relation of concrete in region (b) in Fig. 14 with concrete infilling and without a carbon-fiber sheet

Stress-averaged strain relation of concrete in region (b) in Fig. 16 with concrete infilling and a carbon-fiber sheet

Fig. 17. Relationship between the stress and averaged strain in conned concrete.

M. Akiyama et al. / Engineering Structures 34 (2012) 259270 2671000

0

2

4

6

D4-11 (fpe = 6.3N/mm2)

Incr

emen

t of S

tress

(MPa

)

Deflection (mm)50

D4-9 (fpe = 12.6N/mm2)

D4-13 (fpe = 21.0N/mm2) Fig. 18. Effect of the initial prestress on the increment of stress in a prestressedsteel bar during loading.

the effective conning pressure applied to the concrete in region(a) of Fig. 16 is given as follows:

pe qCFeCFtECF at the peak stress f cc of the confined concrete16

p0e qCF fCF at 0:5 f cc after the peak stress 17

qCF 4n tCF

D18

where qCF is the area ratio of the carbon-ber sheets, eCFt is thestrain at the peak stress (=0.0015), ECF and fCF are the modulus ofelasticity and tensile strength of the carbon-ber sheet, respec-tively, n is the number of carbon-ber sheets that are wrappedaround a pile, and tCF is the thickness of a carbon-ber sheet.

The effective conning pressure applied to the concrete in re-gions (b) and (c) in Fig. 16 is provided by the carbon-ber sheetsand spirals and is given as follows:

pe ke;vqwfs;c qCFeCFtECFat the peak stress f cc of the confined concrete 19

p0e ke;vqwfyh qCF fCF at 0:5 f cc after the peak stress 20The stress-averaged strain model that was proposed by Akiy-

ama et al. [15] requires the use of the element length Lm overwhich the compressive fracture energy Gf,c is dissipated. In the ana-lytical evaluation of the concentric compression test results, Lmmust be the same as the gauge length. Gf,c is kept constant in thestrain-localized element, regardless of Lm, under the condition thatLm is larger than the compressive fracture zone. Therefore, the

Experimental result Computed result

Given by concrete in region (b) in Figs. 14 to 16

Given by longitudinal bars

Given by concrete in region (a) in Figs. 14 to 16

Given by concrete in region (c) in Figs. 14 and 16

M

omen

t (kN

m)

0

400

800 4-4D2-4D1-4D

D4-5 D4-6 D4-7

D4-9 D4-10 D4-11

0

400

800

0.1

800

D4-3

D4-8

D4-12

-15

0.1

Computed result assuming without prestressing

268 M. Akiyama et al. / Engineering Structures 34 (2012) 2592700

400

0

400

800 D4-13

D4-14

D4

0.1

0.1 Curvatu

Fig. 19. Comparison of the experimental and co

D4-16

D4-17 re (m-1)

mputed momentcurvature relationships.

post-peak portion of the stress-averaged strain curve changes withLm, and it is steeper for larger values of Lm; however, there havebeen no reports describing how to determine Lm in the analyticalevaluations of concrete components that have been subjected tosimultaneous axial loading and bending. In order to minimize thedifferences between the experimental and computed results ofthe momentcurvature relationship, Lm was set to 500 mm.Fig. 17 depicts the relationship between the stress and averagedstrain of the concrete in region (b) of Figs. 1416 withLm = 500 mm. It was conrmed that having both concrete inllingand the carbon-ber sheets enhanced the strength and ductilitycapacity of the conned concrete.

Even if the stress-averaged strain relation of the tensioned con-crete is considered in the section analysis, this may provide a neg-ligible contribution to the relationship between the moment andcurvature, that is, the tensile strength of the concrete is ignored.A bi-linear model was used to demonstrate the steels stressstrainrelationship. Because the unbonded prestressed steel bars wereplaced at the center of the cross-section, the prestressed steel barsdid not resist the bending moment. In the exural analysis, thebending moment that was carried by the prestressed steel barswas ignored.

Fig. 18 depicts the relationship between the increment in thestress of the prestressed steel bars and the deections of D4-9,D4-11 and D4-13 during loading. The increment in stress of theprestressed steel bars that occurred before the ultimate exuralload was reached was not negligible for the pile with the lower

7.10 and 5.75 MPa, respectively. In comparison to the initial pre-stress of these piles, the increments are signicant. In addition,as shown in Fig. 12b, the piles with fewer spirals (i.e., widerspiral spacings), such as D4-8 and D4-10, exhibited buckling ofthe longitudinal bars. The results for these specimens did notindicate good correlations between the experimental and analyt-ical results. It should be noted that if the tensile strain of theprestressed steel bar does not increase with the bending mo-ment, a higher exural strength could not be expected for thepile with a lower prestress level, as shown in the computed re-sults for D4-1, D4-3, and D4-11. The effects of the test parame-ters on the increase in the tensile strain of the prestressed steelbars need to be examined. For the specimens with higher pre-stress levels and smaller spiral spacings, the analytical andexperimental momentcurvature curves agree very well.

The data presented in Figs. 17 and 19 conrm that the carbon-ber sheets and concrete inlling signicantly affected the exuralstrengths of the piles. Even though the carbon-ber sheeting thatwas used in this experiment cannot itself resist the bending mo-ment and the contribution to the momentcurvature relation thatis provided by concrete inlling [region (c)] is small, as shown inFig. 19, the utilization of carbon-ber sheets and concrete inllingcan provide the conning pressures to concrete in regions (a) and(b) and improve the behavior of concrete in regions (a) and (b). As aresult, the piles with carbon-ber sheets and concrete inlling (i.e.,D4-12, 14, 15, 16, and 17) show better structural performances.Fig. 19 shows the computed results for the piles with carbon-ber

0

M. Akiyama et al. / Engineering Structures 34 (2012) 259270 269prestress level, as shown in the gure; however, because this incre-ment could not be quantied in this study, the sectional analysiswas conducted by assuming that the initial compressive force thatwas provided by the prestressed bars acted as an external axialforce on the centroid. For the pile with the lower prestress level,this will produce an underestimate of the exural strength.

Fig. 19 depicts a comparison of the experimental and com-puted momentcurvature relationships. The increments in thestress of the prestressed steel bars of D4-3 and D4-11 were

0

250

500

0.00 0.05 0.1

Mom

ent (

kN m

)

(a)

5-4D

Experimental results Computed results (Lm=300 mm)Computed results (Lm =500 mm)Computed results (Lm =700 mm) Curvature (m-1)

Fig. 20. Effect of Lm on the computedsheets and concrete inlling under the assumption that piles donot have prestressing steel bars. These results show that it is nec-essary for the proposed piles to have prestressing steel bars to in-crease the exural strength.

The element length, Lm, cannot be determined based onmechanical considerations. Fig. 20 shows the effect of Lm on thecomputed momentcurvature relationship. With longer Lm, thestress-averaged strain relation in the post-peak region proposedby Akiyama et al. [15] has a steeper descent if the conning

0

250

500

0.00 0.05 0.10

Mom

ent (

kN m

)

(b)

9-4D

Experimental results Computed results (Lm =300 mm) Computed results (Lm =500 mm) Computed results (Lm =700 mm) m(erutavruC -1)

momentcurvature relationship.

pressure applied to the concrete is low. Therefore, the computedresult with Lm = 700 mm for pile D4-5 lacking a carbon-ber sheetand concrete inlling exhibits brittler behavior, as shown inFig. 20a. Since the stress-averaged strain relation has no descend-ing branch and becomes independent of Lm as the conning pres-sure increases, the computed momentcurvature relations withLm = 300, 500, and 700 mm are almost the same as those shownin Fig. 20b. Although the best t with the experimental resultsfor piles with diameters of 400 mm can be obtained by setting

(DAM105, A-D22) with a diameter of 400 mm are 3.9%, 0.47%, 4.1and 105 MPa, respectively. It should be noted that the prestressingtendons of a conventional pile are bonded with concrete and arenot arranged at the center of the pile but, instead, are arrangedalong the circumference of the spiral, like longitudinal bars. Thebending moment at the rst yield of a longitudinal bar is suggestedby the manufacturer to be 170 kN m. If this pile is tested underload by using a similar four-point setup, as shown in Fig. 4, the cal-culated load at the rst yield of the longitudinal bar is 270 kN.

270 M. Akiyama et al. / Engineering Structures 34 (2012) 259270Lm = 500 mm, the appropriate magnitude of Lm to reproduce theexperimental results may depend on a piles diameter and struc-tural details. Further investigation is needed to determine Lm. Theanalytical method to obtain the relationship between the bendingmoment and the curvature will be used in the seismic design of theproposed pile.

6. Conclusions

(1) In order to prevent the yielding of a pile foundation due to asevere earthquake, the design of a new precast, high-strength, and reinforced concrete pile that was prestressedwith unbonded bars that were arranged at the center of pilecross-section was presented in this study.

(2) The effects of concrete strength, number of longitudinal bars,prestress level and presence or absence of concrete inllingand carbon-ber sheets on the exural strength of the pro-posed pile were experimentally investigated. The test resultsindicate that it is necessary for the proposed pile to haveboth concrete inlling and carbon-ber sheeting to increasethe pile exural strength and prevent brittle failure.

(3) Sectional analyses of the proposed piles were conducted toobtain the momentcurvature relationships. The effectiveconning pressures were estimated by taking into consider-ation the presence or absence of carbon-ber sheets andconcrete inlling. By using the stress-averaged strain rela-tionship for the conned concrete with the effective conn-ing pressures, the analytical and experimental momentcurvature relationships agreed well with one another.

(4) Further research is needed to investigate the effects of com-bined axial and shear forces on the ductility capacity andplastic hinge behavior of the proposed pile. In these tests,the structural details of pile heads that have been embeddedin concrete footings should be examined so that design fea-tures can be identied that will prevent the failure of pile-concrete footing joints that are subjected to cyclic loading.

Appendix A. The exural strengths of conventional piles thatare used in Japan

In Japan, the area ratio of the longitudinal bars, volumetric ratioof the spirals, prestress level and specied concrete strength for aconventional precast, prestressed, and reinforced concrete pileReferences

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Flexural test of precast high-strength reinforced concrete pile prestressed with unbonded bars arranged at the center of the cross-section1 Introduction2 Prestressed reinforced concrete piles using high-strength materials and carbon-fiber sheets3 Experimental procedure3.1 Specimen properties and materials3.2 Testing procedure and instrumentation

4 Experimental results and discussion4.1 General observations4.2 Effects of the test variables on the flexural strengths of the proposed piles

5 Analytical evaluation of the experimental results6 ConclusionsAppendix A The flexural strengths of conventional piles that are used in JapanReferences

Documents

Flexural test of precast high-strength reinforced concrete pile prestressed with unbonded bars arranged at the center of the cross-section