鋼繊維と横補強筋を併用したコンクリートの圧縮塑性変形挙動

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愛知工業大学研究報告

第17号B 昭和57年 181

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鋼繊維と横補強筋を併用したコ

ンクリートの圧縮塑性変形挙動

小池狭千朗 .谷川恭雄九山田和夫日本

The high rotation capacities are required for reinforced concrete members to secure the seismic safety of reinforced concrete structures against such a large earthquake motion as in Japan.It is well known that their plastic rotation capacities are closely related to the stress. strain behavior of concrete und芭rcompreSSlOn This paper describes the complete stress-strain behavior of steel fiber reinforced concrete confined with lateral reinforcement (confined SFRC). Prismatic column specimens are unia -xially loaded in a new type of high rigidity compression tεsting machine. The stress-strain curves of confind SFRC up to a large plastic strain are measured，and the effects of the volume fraction of steel fiber， the spacing and the yield strength of lateral reinforcement， the casting direction of concrete and the waterc巴m叩tratio on stress-strain relationship are examined in detail INTRODUCTION The plastic rotation capacities of reinforced concrete m印 lbersare closely related to the stress strain curv邑ofconcrete under compression CRef. 1J Th巴authorshave b巴巴nstudying the inelastic behav司 ior of steel fiber reinforced concrete (SFRC) and confined concrete subjected to compressive load for improving the ductility of concrete CRefs. 1-5J . In the present paper， the complete stress-strain curves of steel fiber reinforced concrete confined with lateral reinforcement (confined SFRC) under uniaxial. compression are report巴d. It is well known that SFRC has been applied to structural members subjected to tensile or flexural load in general， because of the excellent properties under such loads. However， SFRC has another one that the ductility under compressive load is very high CRefs. 6 and 7J目Theearlier test results obtained by the authors indicated that the stress-strain curves of SFRC are extremely varied with the casting direction of concrete and that SFRC is applied more effectively to reinforced concrete columns than to reinforced concrete beams CRef. lJー On the other hand. it is also well known that the behavior of concrete confined with lateral reinforce

-ment such as hoop， stirrup， or tie is very ductile und巴r

compression CRefs. 8-11J . In this study， the effects of the volume fraction of steel fiber， the spacing and the yield strength of hoop， the casting direction of concrete， and the water -C巴mentratio on the inelastic behavior of confined SFRC are examined based on the experimental data obtained by prismatic column specimens. 1.EXPERIMENTAL PROCEDURES 1.1 TEST SPECIMENS

The experiment as shown in Table 1 was carried out in ord巴rto examine the stress-strain curves of confined SFRC up to a large compressive strain. As shown in Fig. 1， 15x15x45cm prismatic specimens which are laterally reinforced with mild steel bars (yield strength=2，500kgfcm') or PC wires (proof stressニl1，300kgfcm') were prepared for the ex -periment. The nominal diameter of mild steel bar or PC wire was kept to 6mm. The depth of concrete cover was 0 cm， and the spacings of hoops were 30， 10， 5， and 2.5cm， respectively. The end zone of 7.5cm of the specimens was reinforced with mild steel bars at the spacing of 3.75cm to prevent the local failure. The cylindrical specimens of φ10 x 20cm were also prepared

*

Department of Architecture， Faculty of Engineering， Aichi institute of Technology

*

Department of Architecture， Faculty of Engineering， Mie University.

*

Department of Architecture， Faculty of Engineering， Nagoya University

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182 小池狭千朗・谷川￨恭雄a山田和夫

1.2 FABRICATION AND CURING OF SPE-CIMENS

Ordinary Portland cemeBt， river sand (maximum SlZ巴ニ5mm)，river gravel (size rang己=5-15mm)，and

steel fiber (cross section= 0.35 x 0.6mm， 1巴ngth= 30mm) were used for the fabrication of the specimens The water-cement ratio of concrete (W /C = 0.5， 0.6，

and 0.7)， the volume fraction of steel fiber (V

，

=0，0.75，

and 1.5%) and the casting direction of concrete were varied in the巴xperimentThe specimens were cast in parallel to the loading axis in general， except for some specimens (H-series) cast perpendicularly

The forms of prismatic specim巴nsand cylindrical specimens were removed at the age of 1 day and 2 days， respectively， and then cured in air until the test except for some cylindrical specimens for water curing. Tests were carri巴dout at the age of 4 weeks. 1.3 METHODS OF LOADING AND MEASURE MENT

All the sp巴cimenswere loaded uniaxially in a new type of high rigidity compression testing machine [Ref.1]under the constant strain rate of about1.7 x 10-3jmin. The longitudinal strain was measured by

two differentia! transformers. The measurement lengths were 30cm for prismatic specimens and 18. 8cm for cylindrical specimens.

Complete stress-strain curves of prismatic specimens and cylindrical specimens were recorded up to th巴strainof 15 x 10-3and 10 x 10←3， respctively

2. TEST RESULTS AND DISCUSSION

1n this experiment， the芭ffectsof the vo!ume frac

-tion of steel fib巴r，the spacing and the yield strength of hoops， the casting direction of concrete， and th巴 water-cem巴ntratio on the stress (σ) -strain (ε) curv巴S of confined SFRC were examined in detail

2.1 EFFECT OF VOLUME FRACTION OF STEEL FIBER The effect of the volume fraction of steel fiber on the stress-strain curves of prismatic specimens with different spacing of hoops (S) is shown in Figs.2 (a) thrugh 2 (d). In all the specimens， the stress-strain curves in the stress descending portion become less steep with incr巴asingvolume fraction of ste巴!fiber，

and this effect is more remarkable as the spacing of hoop is increased

Fig.3 and Fig.4 show the eff巴cts of the volume fraction of steel fiber (V，)on th巴compressiv巴streng -th(F，)and the strain at the maximum stress (cm) of prismatic specimens， respectively. As shown in Fig.3， the compressive strength increases with increasing volume fraction of steel fiber except for some specimens. The increase of strength in the sp巴clmen reinforced with PC wire is almost costant， regardless of the spacing of hoops. On the other hand， the increase of strength in the specimens reinforc巴dwith mild steel bars whose spacing is large is similar to that of the specimens reinforced with PC wire， but the specimens whose spacing of steel bars is small do not necessarily show the increase of str巴ngth.

The strain at the maximum stress becomes larger in proportion to the volume fraction of steel fiber， independ巴ntlyof the kind of hoop， and the increasing rate is higher as the spacing of hoop decreases， as shown in Fig.4. The relationship between relative absorbed energy (Ab) and volume fraction of steel fiber (V，)is indicat -ed in Fig.5， where the relative absorbed energy is d巴finedas the ratio of the area enclos巴dby the stress -strain curve of each specimen up to the longitudinal strain of 15 x 10-3 to that of the specimen of S=30cm

and V

，

=O%. As shown in this figure， the relative absorbed energy of concrete b日comeslarger with increasing volume fraction of steel fiber， and the increasing rate is higher as the spacing of hoops increases， that is， the effect of steel fiber on the improvement of ductility of concrete b巴comesmore

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Fig. 1 Test specimens. Table 1 Outline of experiment S巴riesNo W/C Vf Casting direction Spacing of hoop Kind of hoop (%) (%) of concrete (cm) 0-60 60

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PCw汀e 0.75-60 60 0.75 V 30 (P) 1.5-60 60 1.5 10 1.5-70 70 1.5 (H in a part) 5 Mild 1.5-50 50 1.5 2.5 steel bar H-1.5-60 60 1.5 H (N)

[NotesJ羽T/C:Wat巴rcement ratio， V; : Volume fraction of ste巴1liber， V : Specimens cast in parallel to the loading axis， H : Specimens cast perpendicularly to the loading axis

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鋼繊維と横補強筋を併用したコンクリートの圧縮塑性変形挙動 300 n u n u 内 J ﹄ N E u m v -) b S=30cm 一一一V-p 一一…V-N Vf=I.5'1o -f a E 3 1 何 7 0 n u = 三 f T W V

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184 小池狭千甥・谷川恭雄・山田和夫 300 一一一一 V-p V-N 一一一 H-N S=2.5cm n u n u 内 4 N E U ﹃ m v - ) む S=Scm 5=1日cm 0・60

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2.2 EFFECT OF SPACING OF HOOP

The effect of the spacing of hoops on the stress -strain curves of confined SFRC is shown in Figs.6(a} through 6(f).The descending portions of all the stress -strain curves of concrete become less steep with decreasing spacing of hoops， and the improvement of ductility is more remarkable as the volume fraction of steel fiber and the water-cement ratio decrease. The compressive strength(Fc)， the strain at the maximum stress(Em)， and the relative absorbed energy (Ab) are plotted in Figs.7， 8， and 9 against the volumetric ratio of hoops (Pw)， respectively. General -ly， the values of Fc，εm， and Ab increase in proportion to the volumetric ratio of hoops (pw). Fig.10 shows the comparative effects of volume fraction of steel fiber(Vr) and volumetric ratio of hoops (Pw) on the relative absorbed energy (Ab) of confined SFRC， where the contour lines of relative absorbed energy are schematically drawn. The fol -lowing statements can be made from the figure

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185 鋼繊維と横補強筋を併用したコンクリートの圧縮塑性変形挙動 15 CトーペコN ← →p n u n u N E U ¥ m b u h L ₂ ヤ 10 Eコ H

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5 than that with mild steel bars， and this tendency is mor巴remarkableas the volume fraction of stぽ1fiber increases and as the spacing of hoops and the water -cem巴ntratio decrease. 1) Steel fiber is more effective for improving th巴 ductility of concrete than lateral reinforcement when the volume fraction of steel fib巴r(V，)and the volume -tric ratio of hoops (Pw) are less than 1.5%. The value of relative absorbed巴nergyat V，=1.0% and Pw=O% is almost equal to that at V，ニ0%and Pw=1.5%ー 2) The improvement of ductility by steel fiber gradually decreases as the volumetric ratio of hoops mcreases 3) The contour lines are a little convex against the origin. However， the linearly additional effect of steel fiber and lateral reinforcement on the relative absorbed en巴rgycan be roughly expected when their

volume fractions are less than 1.5%. For example， the value of Ab at V

二

，

0.75%and Pw=0.75% is almost equal to that at V

，

=O% and Pwニ1.5%

2.3 EFFECT OF YIELD STRENGTH OF HOOP According to Figs.2， 4， 5， 6， 8， and 9， the specimens reinforced with PC wires show more ductile behavior

2.4 EFFECT OF CASTING DIRECTION OF CONCRETE

The compressive strength and the strain at the maximum stress of the sp巴cimens cast perpendicularly to the loading axis (H-series) are hardly affected by the volume fraction of steel fiber，

as shown in Figs.2 and 3. The strains at the maximum stresses(εm)of such specimens are v巴ry small， compared with those of the specimens cast in parallel to th巴loadingaxis. 1t is indicated in Figs.7 and 8 that th巴increasesof the strength and the strain at the maximum stress by lateral reinforcement are more remarkable in the sp巴cimenscast in parallel to the loading axis (V-1.5-60 series) than in the specimens cast perpendicularly to it (H -1.5-60 series).

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186 小池狭千朗・谷川恭雄白山田和夫

The effect of the casting direction of concrete on th巴stressωstraincurves is shown in Fig.11.The stress -strain curves of V-series specImens are more ductile than those of H-series as mentioned abov巴， and the

Y oung's modulus of H-series speCimens is much higher than that of V-s巴riesspecimens. This may be resulted from the orientation of steel fiber and bleed ing caused by different casting direction CONCLUSIONS In the present paper， the effects of the volume fraction of steel fiber， the spacing and yield strength of hoops， the casting direction of concrete and the water-cement ratio on the complete stress-strain behavior of steel fiber reinforced concrete confined with hoops (confined SFRC) were examined. The test results are summarized as follows :

1) The stress-straIn curve of concrete shows more ductility with increasing volume fraction of steel fiber， and this tendency is more remarkable as the spacing of hoops increases. 2) The efficiency of lateral reinforcement on the improvement of ductility of concrete becomes higher with decreasing volume fraction of steel fiber and water-cement ratio. 3) The effect of steel fiber on the improvement of ductility of concrete is more remarkable than that of lateral reinforcement when the volume fraction of steel fiber (Vf) and the volumetric ratio of hoops (Pw) are less than about1.5%. For example， the value of relative absorbed energy at Vf=1.0% and Pw=O% is almost equal to that at Vf=O% and Pw=1.5% 4) The specimens confined laterally with PC wire are mor巴ductilethan those with mild steel bars， and this tendency becomes more remarkable with increas -ing volume fraction of steel fiber and decreasing volumetric ratio of hoops and w呂ter-cementratio

R忍FERENCES

1) Tanigawa， Y.， K. Yamada and S. Hatanaka， “Inelastic Behavior of Steel Fiber Reinforced

Concrete under Compression，"Proc. of Inter -national Symposium on Advances in Cement-Matrix Composites， Materials Research Society，

Nov. 16-20， 1980， Boston， pp.l07-118.

2) Tanigawa， Y.， K. Yamada and S. Hatanaka， “Statistical Aspects on Mechanical Properties of

Steel Fiber Reinforced Concrete under Compres-sive Loading，"Trans. of Japan Concrete 1n stitute， Vol.l， 1979， pp.215-222

3) Tanigawa， Y.， K. Yamada and S. Hatanaka，

“Size Effect in Mechanical Properties of Steel Fiber Reinforced RC Beams under Flexural Load，“Trans. of ]apan Concrete Institute， Vol.l，

1979， pp.239-246.

4) Kosaka， Y.， Y. Tanigawa and K. Baba“，Effect of Lateral Reinforcement on Mechanical Properties of Mortar，"Review of 20th General Meeting of Cement Association of ]apan， May 1975， pp.192 -193

5) Tanigawa， Y. and Kosaka， Y.，“Hysteretic Stress-Strain Relations of Confined Concrete under Rep日atedCompressive Load，"Review of 33rd Gen巴ralMeeting of Cement Association of ]apan， June 1979， pp.252-254

6) Shah， S. P. and Rangan， B. V.，“Effects of Reinforcement on Ductility of Concrete，"Proc. of American Society of Civil Engineers， Vo1.96， No. ST6， June 1970， pp.1l67-1184

7) Hughes， B. P. and Fattuhi， N.

I

.

“

Stress-Strain Curves for Fiber Reinforced Concrete in Com pression，"Cement and Concrete Research， Vo1.7，

No.2， March 1977，pp.173-184

8) Sargin， M.， S. K. Ghosh and V.K. Handa“，Effects of Lateral Reinforc巴ment upon Strength and Deformation Properties of Concrete，" Magazine of Concrete Research， V 01.23， N o. 75-76， J une -Sept. 1971， pp.99-110 9) Burdette， E. G. and Hilsdorf， H. K汁“Behaviorof Laterally Reinforced Concrete Columns，"Proc of American Society of Civil Engineers， Vo1.97， No.ST2， Feb. 1971， pp.587-602

10) Gangadharam， D. and Reddy， K. Nagi，“Effect of Cover upon the Stress-Strain Properties of Concrete Confined in Steel Binders，"Magagine of Concrete Res巴arch，Vo1.32， N 0.112， Sept. 1980， pp.147-155

11) Muguruma， H.， F. Watanabe， H. Tanaka and S Katsuda，“Study on Confining Effects of Concrete by High Yield Strength Lateral Hoop Reinforce苧

ment，" Review of 33rd General Meeting of Cement Association of Japan， June 1979， pp.263 -265司