SEISMIC PERFORMANCE OF CONCRETE COLUMNS REINFORCED BY STEEL-

FRP (FIBER REINFORCED POLYMER) COMPOSITE BAR

Z.Y. Sun 1, G. Wu 1*, Z.S. Wu 2 and M. Zhang 3 1 International Institute for Urban Systems Engineering, Southeast University, Nanjing, China.

E-mail: g.wu@seu.edu.cn 2 Department of Urban & Civil Engineering, Ibaraki University, Hitachi, Japan.

3 Beijing Texida Technology R & D Co., Ltd., Beijing, China ABSTRACT Horizontal low cyclic loading test is conducted on concrete columns reinforced by Steel-FRP (fiber reinforced polymer) composite bar (SFCB) and ordinary reinforced concrete (RC) columns, with axial compression ratios of 0.12, and fiber types and steel/fiber ratios of SFCB as the main variable parameters. Seismic behaviours such as f ailure mode, h ysteretic curve, s tiffness d egradation, an d r esidual d isplacement (deformation r estoring capacity) are thoroughly investigated in the test. The results show that 1) concrete columns reinforced by SFCB have an obvious and stable post-yield stiffness, small residual displacement, as well as better recoverability; 2) post-yield s tiffness should be measured by magnitude and effective length; and 3) the seismic performance o f columns reinforced by steel-BFRP (basalt f iber reinforcement polymer) composite bar (SBFCB) is better than that reinforced by steel-CFRP (carbon fiber reinforced polymer) composite bar (SCFCB). KEYWORDS Steel-FRP composite bar (SFCB); concrete column; post-yield stiffness; residual displacement. INTRODUCTION Serious rust and corrosion tend t o h appen t o common reinforced co ncrete applied in harsh o r exposed environments like irrigation works, harbors and chemical industries (Figure 1). Billions of dollars are spent each year worldwide i n r epairing and s trengthening these concrete s tructures. According t o Nanni (1993), about 1/ 6 of t he bridges are seriously damaged by rust and corrosion of steel bar in the United Stares; economic loss by deterioration of steel bar each year in the U.S. is as much as 70 billion d ollars. I n China, eco nomic loss i n t his as pect i s a lso i ncreasing each y ear (Chen 2003) . Reinforced concrete undergoing c orrosion n ot only gives the appearance of poor performance, but can, in extreme cases, lose its structural integrity. Fiber reinforced polymer (FRP) composites are gaining attention due to their light weight, high strength, and especially, their resistance t o co rrosion. S tudies (Rostasy 1996 , A CI 2003 ) have showed that it is an effective way to use FRP tendons instead of steel bar to resist rust and corrosion problems. However, due to the fact that FRP is a brittle material and has low elastic modulus, concrete structures reinforced by FRP (fiber reinforced polymer) composites usually have weak stiffness, great deflection and brittle failure in using stage (ACI 2003). Steel-FRP composite bar (SFCB), a new material proposed by Z.S. Wu and G. Wu et al (Wu 2006, 2009, and Luo 2009), consists of steel bar as its inner core and longitudinal continuous fiber as its out layer, c ombining t he strengths o f common steel bar (high elastic modulus and good ductility) and FRP (good anti-corrosion ability and large ultimate strength). (1) at initial stage, effects of the steel bar can ensure SFCB has high elastic modulus; (2) FRP is linear elastic while steel bar is elastoplastic, hence after steel bar yielding, FRP will continue to functionate, which generates an obvious post-yield stiffness in stress-strain relationship of SFCB at th is stage (Figure 2a); (3) after rupture of F RP, steel bar continues t o t ake e ffect, ensuring good ductility. Furthermore, the stable post-yield s tiffness o f SFCB makes t he r einforced structures damage-controllable and favorable to achieve performance-based seismic design. Ideal configuration of SFCB can be seen in Figure 2b.

Figure 1. Pier corrosion of a RC

bridge (Hangzhou, China)

481

Transverse winding fiber

Wrapped Longitudinal fiberInner core steel rebar

Low-cyclic reversed horizontal loading test is conducted on one RC contrast column and two SFCB columns with section o f 300×300 mm in t his p aper. S eismic p erformances o f SFCB reinforced co ncrete co lumns ar e researched through this preliminary experimental study.

σ

ε

σy

εy εf εs,max

σfYielding platform of normal steel bar

FRPSteel rebar

SFCB

Stable post-yield stiffness of SFCB after inner steel rebar yielded

(a) Stress-strain relationship of SFCB (b) Longitudinal components of SFCB

Figure 2. Stress-strain relationship and configuration of SFCB

TEST PROGRAM Specimen design The longitudinal reinforcement of RC column is steel bar with 14 mm diameter and 340 MPa yield strength, while these t wo SFCB columns w ere B30S10 and C40S10 respectively. Specimen d esign parameters and properties of column reinforcement can be seen in Table 1 (Wu et al. 2009) and Figure 3. In Table 1, the initial elastic modulus of SFCB E and post-yield stiffness of SFCB E2 refer to the elastic modulus of SFCB before and after the yielding of SFCB inner core steel bar respectively; the equivalent strength of the SFCB when the inner core steel bar of SFCB yielded is regarded as the yield strength; and the strength when the wrapped fiber o f SFCB ruptured is t reated as the ultimate strength. The type of B30S10 means the SFCB are made o f steel bar with 10 m m diameter and 400 M Pa yield s trength longitudinally wrapped by 30 bu ndles of 2400 te x b asalt f iber, where tex is a measurement unit in te xtile i ndustry, r epresenting th e weight o f single bundle fiber per kilometer; C40S10 means the SFCB are made of steel bar with 10 mm diameter and 400 MPa yield s trength longitudinally wrapped by 40 bundles of 12 k c arbon f iber, where 12 k means e ach b undle o f carbon fiber has 12,000 fibers. Commercial concrete was used the specimens, whose tested cubic compressive strength (150×150×150 mm) is 47.27 MPa.

Table 1. Properties of column reinforcement (Wu et al. 2009)

Column number

Reinforcement type

Diameter (mm)

Elastic modulus E (GPa)

Yield strength fy (MPa)

Post-yield stiffness E2 (GPa)

Ultimate strength fu (MPa)

E2/E Elongation rate (%)

C-S14 HRB400 14.00 200.00 400.00 / 400.00 / 15.0 C-B30S10 B30S10 13.19 142 312.4 16.6 691.3 0.117 2.5 C-C40S10 C40S10 12.95 155.5 342.2 30.6 641.8 0.197 1.6

(a) Specimen reinforcement (b) Cast specimens

Figure 3. Column specimen design

SFCB columns

RC column

A-A

250

A A

Steel rebar/SFCB

80@10φ

80@10φ

150@10φ

Reinforcement

482

Test method The a xial c ompression r atio in t his test was 0. 12. Load was applied by center hole jack on the top of the c olumn. Axial c ompression was kept c onstant during t he t est b y a djusting the o il p ressure. Load was a pplied t hrough c ontrolling b oth by load an d displacement (Figure 4): before yielding, the test was controlled by load with level difference of 10 kN and the load was cycled once; after yielding, the test was controlled by horizontal displacement at column end, integral multiples o f yield d isplacements (5 mm i n this te st), and the l oad was cycled t hree t imes till failure. The whole p rocedure o f t he t est was controlled by MTS (Material Test System). Measurement The main measuring contents included: (1) load-displacement curves at column cap, data collected automatically by M TS with d isplacement sensor i n t he ce ntre of its lateral s urface (Figure 4a); ( 2) strain o f lo ngitudinal reinforcement and stirrups; (3) crack appearance and development; (4) failure modes, and so on.

TEST RESULTS

Failure modes All specimens went through flexural failures. Concrete near column base cracked first and then the longitudinal reinforcement or the inner core steel bar of SFCB yielded, followed by partial or to tal rupture of the wrapped fiber of SFCB. Load was applied continuously till the concrete cover at column b ase was cr ushed and began spalling, and the longitudinal reinforcement was bended (Figure 5). Compared with co mmon R C co lumn, t he cr acks o f SFCB columns h ad larger width a nd s pacing, du e t o t he poorer bond be haviour be tween SFCB and co ncrete t han t hat b etween co mmon steel b ar and co ncrete. T his result accorded with the bond test between SFCB and concrete (Zhang 2009). At the stage of large displacement, columns reinforced by SFCB had better crack cl osing ab ility t han co mmon RC columns because of the high strength of FRP. Both RC column and SFCB columns have good ductility, while the latter have obvious post-yield stiffness and smaller residual displacement.

(a) Failure mode (b) Wrapped fiber rupture of SFCB

Figure 5. Specimen failure Hysteretic cure Figure 6 shows the P-Δ hysteretic curves of the columns. The hysteretic curve of common RC column is shaped like a p lump spindle, e xhibiting good ductility a nd energy d issipation capability. Before yielding, the residual displacement in each cycle was small. After yielding, residual displacement increased as the increase of cyclic times and lateral displacement. The post-yield stiffness of column C-S14 was nearly “0” while SFCB columns C-B30S10 and C -C40S10 h ad stable post-yield s tiffness due to th e h igh s trength o f BFRP a nd C FRP. The

t

P (kN) Lateral displacement Δ

1020304050607080

-70-60-50-40-30-20-100

-80

Force control Displacement control

3 times 3 times 3 times 3 times

1Δ2Δ3Δ4Δ

-1Δ0

-2Δ-3Δ-4Δ

Figure 4. Loading program

Rupture of SFCBs longitudinal wrapped fiber

concrete cover crushing and spalling

EastWest

483

hysteretic curve o f SFCB reinforced columns exhibited obvious pinch e ffect a t positive unloading and reverse loading. Under the same loading displacement, the residual displacement of SFCB was much smaller (Figure 6).

-40 -20 0 20 40

-150

-100

-50

0

50

100

150 Yielding platform

Load

(kN)

Lateral displacement (mm)

C-S14 Caculated C-S14 Exp.

-40 -20 0 20 40

-150

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-50

0

50

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Load (k

N)

Lateral displacement (mm)

C-B30S10 Caculated C-B30S10 Exp.

Fracture of wrapped basalt fiber of B30S10

Post-yield stiffness ofBFWR column (longer effective length)

-40 -20 0 20 40

-150

-100

-50

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Load

(kN)

Lateral displacement (mm)

C-C40S10 Caculated C-C40S10 Exp.

Post-yield stiffness of SCFCB column

Fracture of wrapped carbon fiber of C40S10

(a) Column C-S14 (b) Column C-B30S10 (c) Column C-C40S10

Figure 6. Hysteretic cures of columns The main characteristics of the test can be seen in Table 2, where Pcr，Py，Pp，Pu are the crack load, yield load, maximum load, and ultimate load respectively; and Δcr，Δy，Δp，Δu are the corresponding displacement.

Table 2. Test results of the main feature points Column number Pcr (kN) Δcr (mm) Py (kN) Δy (mm) Pp (kN) Δp (mm) Pu (kN) Δu (mm) Δu/Δy

C-S14 48.05 1.21 120.10 5.43 121.65 34.45 99.89 39.99 7.36 C-B30S10 43.64 1.24 84.17 5.26 115.20 29.60 85.60 39.54 7.52 C-C40S10 43.39 1.20 84.05 5.06 112.32 17.15 75.04 39.95 7.90

The crack load and the corresponding displacement of the three columns are close, which means the reinforcement ratio of column has little influence on its crack load. There was not any big difference between the ductility coefficient of RC column and that of SFCB columns, for the little difference in their yield displacement and u ltimate d isplacement. I f t he f iber pr oportion i n t he SFCB is e nlarged, the b earing cap acity o f SFCB columns will be much reduced for the rupture of fiber and thus its ductility should be reconsidered. Skeleton curve and post-yield stiffness Skeleton curve and post-yield stiffness of the columns can be seen in Table 3 and Figure 7, where K1=Py/Δy is the column stiffness at elastic stage; K2=(Pp-Py)/(Δp-Δy)= β1K1 refers to the post-yield stiffness of the columns, which is n early “0” for RC c olumn; K3=(Pu-Pp)/(Δu-Δp)= β2K1 refers to the bearing cap acity degradation of concrete columns during and after the crushing and spalling of column base concrete cover, which is an essential parameter of structures under strong earthquakes; and γ=(Δp—Δy )/Δy refers to the effective length coefficient of post-yield stiffness.

Table 3. Post-yield stiffness of columns

Column number

Axial stiffness ratio of

reinforcement

K1 (Test)

K2 (Calculated)

K2 (Test)

Utilization ratio of K2

(%)

K3 (Test)

β1 (Test)

β2 (Test)

γ (Test)

C-S14 1.00 22.12 0.071 -0.02 / 3.57 0.00 0.16 5.34 C-B30S10 0.63 16.01 1.804 1.27 51.66 2.98 0.08 0.19 4.63 C-C40S10 0.67 16.63 1.571 2.34 36.21 1.64 0.14 0.10 2.39

Common RC column had ideal yielding platform, while SFCB columns had good post-yield stiffness after the inner co re s teel b ar of SFCB yielded. T he bearing cap acity o f C-C40S10 be gan t o decrease af ter l ateral displacement reached 17.15 mm while C-B30S10 did not decrease until the lateral displacement reached 27.03 mm. Column C-B30S10 had larger bearing capacity and better ductility than C-C40S10, which represents better seismic performances.

484

-50 -40 -30 -20 -10 0 10 20 30 40 50-150

-100

-50

0

50

100

150

Rupture of wrapped basalt fiber of B30S10

Rupture of wrapped carbon fiber of C40S10

Load

(kN)

Displacement(mm)

C-S14 C-B30S10 C-C40S10 C-S10

0 5 10 15 20 25 30 35 40 45

0.680.720.760.800.840.880.920.961.00

Stre

ngth

degr

adati

on (λ

)

Ruputer of carbon fiber

Lataral displacement (mm)

C-S14 C-B30S10 C-C40S10

Basalt fiber still works due to its high enlongation rate

Figure 7. Skeleton curves of columns Figure 8. Strength degradation of columns

Strength degradation Under ho rizontal reversed load, strength of t he s pecimen c ontinued t o de grade with t he i ncrease o f l oading displacement. T he s trength d egradation law of each s pecimen can b e r epresented b y t he r atio o f t he b earing capacity in the third cycle to that in the first cycle under each level of load. The strength degradation ratio λj is

,3 ,1/j j jP Pλ = (1) where Pj,3and Pj,1 are the lateral load in the third and first cycle respectively under j level displacement of load. The strength degradation of columns can be seen in Figure 8. (1) As the loading displacement increased, the strength degradation of columns became more obvious. When the displacement at column cap was from 5 mm to 30 mm, the strength degradation was comparatively stable. After the displacement at column end exceeded 30 mm, the strength degradation increased rapidly. (2) Due to the premature rupture of carbon fiber, C-C40S10 had the fastest strength degradation. For the carbon fiber began to rupture after displacement reached 20 mm and was almost all ruptured when displacement reached 25 mm, thus there was only the inner core steel bar taking effect after displacement reached 30 mm, when the strength degradation of C-C40S10 was much the same as that of common RC column (Figure 8). (3) After displacement at column cap reached 35 mm, the concrete cover in the column base began to crush, resulting in more rapid strength degradation. When displacement reached 40 mm, for the high ductility of basalt fiber, C-B30S10 exhibited better strength degradation performance, which means BFRP-wrapped r ebar (BFRW) reinforced co lumns may have b etter p erformances t han c ommon R C c olumns under strong earthquakes. Deformation recoverability (Residual displacement ratio) The deformation recoverability of structure is closely related to its use after earthquakes, rehabilitation degree and cost. The deformation recoverability of the specimen columns is shown in Figure 9. According to the figure, (1) the residual displacement of c ommon RC c olumn increased linearly when the d isplacement at co lumn cap exceeded 10 mm and the loading curve almost parallels with the unloading curve; (2) c ompared with R C c olumn, SFCB columns h ad smaller residual displacement and residual displacement ratio under th e same loading displacement, for the high strength of FRP, which shows t hat SFCB can effectively reduce t he r esidual displacement; (3) th e r esidual displacement ratio o f C-B30S10 was l arger t han t hat o f C-C40S10 at in itial s tage d ue to th eir difference on elastic axial stiffness but smaller in late period due to th e high e longation r ate of b asalt fiber, proving th at b asalt fiber h as more ad vantages t han car bon f iber when a pplied i n concrete columns. Supplementary anti-corrosion test of SFCB and steel bar Whether o r no t SFCB has good a nti-corrosion cap acity d ecides whether i t can replace F RP or steel b ar as a reinforcing m aterial to b e applied in harsh o r e xposed e nvironments. Thus, the anti-corrosion performance of SFCB was experimentally studied in this paper. Corrosion accelerating method was adopted. SFCB was put in the electrochemical corrosion environment, the corrosion of SFCB was observed and compared with that of steel bar under the same condition.

0 1 2 3 4 5 6 7 8 9 10

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ruputer of carbon fiber of C40S10 composite bar

∆u/∆y

Resid

ual d

isplac

emen

t rati

o

C-S14 C-B30S10 C-C40S10

Figure 9. Residual displacement ratio

485

Before specimens were immersed in 5%NaCl solution for several days, the mass o f each bar was measured (accurate to 0 .1g), i ts length by a r uler, and its diameter by vernier caliper (accurate to 0.2 m m). After corroding by e lectrifying f or 96h , t he m ass of specimens, their section areas and lengths were measured. After 96h’ accelerating corrosion, t he exterior surface o f t he steel bar was quite bumpy and its section was much thinner, while SFCB specimens, on the other hand, had no obvious change except the glue layer on the surface was a little whitened. Through weighing the mass b efore an d af ter co rrosion t o d etermine t he co rrosion degree, it c ould be f ound t hat t he mass of steel b ar decreased magnificently but there was almost no difference to the mass of SFCB (Figure 10). ACKNOWLEDGEMENTS The authors would like to acknowledge financial support from the National Natural Science Foundation of China (No. 5 0608015), the National B asic R esearch P rogram o f C hina ( 973 P rogram) ( No.2007CB714200), and National Key Technology R&D Program of China in the 11th Five-Year Period (No. 2006BAJ03B07). CONCLUSIONS

Based on the low cyclic reversed horizontal loading test and electrochemical anti-corrosion test study on SFCB columns and contrast RC column, the following conclusions can be drawn. (1) Compared with common steel bar, SFCB has better anti-corrosion capability. After 96h of electrochemical corrosion accel erating t est, the mass l oss o f SFCB is a lmost e qual to “0”. (2) Under low c yclic reversed horizontal loading, SFCB columns went through flexural failures, consisting of concrete cracking, the yielding of the inner core steel bar of SFCB, partial or total rupture of the wrapped fiber, and the crushing and spalling of the concrete. (3) SFCB columns h ad s table pos t-yield s tiffness a nd it s h ysteretic c urve e xhibited obvious pi nch effect at positive unloading and reverse loading. Since FRP used in SFCB is designable, magnitude and effective length ar e ap plied as two i ndexes to ev aluate t he post-yield s tiffness of SFCB column. (4) The s trength degradation of SFCB columns i s much t he s ame a s t hat o f R C c olumns. While a t la rge d isplacement s tage, BFRP (basalt fiber reinforced polymer) wrapped rebar (BSFCB) columns have the best strength degradation for the h igh d uctility o f b asalt f iber. (5) The unloading residual d isplacement r atio of SFCB columns i s much smaller than that of RC columns under the same loading displacement, which is an important index to evaluate the seismic performance and recoverability of concrete columns. REFERENCES ACI (2003). “Guide for t he design and c onstruction of c oncrete reinforced with FRP bars”, ACI 440. 1R-03,

American Concrete Institute, Detroit, Michigan, USA. Chen, Z .Y. ( 2003). “Safety and durability o f s tructural w orks i n c ivil e ngineering”, China architecture &

building press, Beijing, China, 76-83. (in Chinese) Luo, Y.B., Wu, G., Wu, Z.S., Hu, X.Q., and Tian, Y. (2009). “Study on Fabrication Technique of Steel Fiber

Composite Bar (SFCB)”, Earthquake Resistant Engineering and Retrofitting, 31 (1), 28-34. (in Chinese) Nanni, A. (1993). “FRP reinforcement for concrete structures”, Elsevier Science Publishers. Rostasy, F.S. (1996). “FRP: T he European perspective”, Fiber Composites in Infrastructure, 1st Int. Conf. in

Infrastructure, H. Saadatmanesh and M. Ehsani, eds., 12-20. Wu, G., Wu, Z.S., Luo, Y.B., Wei, H. C. (2009). “A new reinforcement material o f s teel f iber composite bar

(SFCB) a nd its mechanics p roperties”, Proceedings of 9th international symposium on fiber reinforced polymer reinforcement for concrete structures. Sydney, Australia.

Wu, Z .S., W u, G ., and Lv, Z .T. ( 2006). “Earthquake-resistant co ncrete structures r einforced b y steel-FRP composite bar”, China National Invention Patent, Publication No: CN 1936206A. (in Chinese)

Zhang, L.L., Wu, G., Sun, Z.Y., Wu, Z.S. (2009). “Experimental study on the bond behavior between steel fiber composite bar (SFCB) and concrete”, Proceedings of the innovation and sustainability of structures in civil engineering, Guangzhou, China.

0

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200

SBFCB

steel-CFRP composite bar (SCFCB)

SBFCBSCFCBS3S2S1

Mass (g

)

Specimen type

Before corrosion After corrosion

Steel bar

Figure 10. Anti-corrosion test of SFCB

( Mass loss)

486