BEHAVIORS OF CONCRETE COLUMNS CONFINED WITH BOTH SPIRAL AND FIBER COMPOSITES

Transcription

1 13 th World Conerence on Earthquake Engineering Vancouver, B.C., Canada August 16, 24 Paper No. 351 BEHAVIORS OF CONCRETE COLUMNS CONFINED WITH BOTH SPIRAL AND FIBER COMPOSITES JungYoon Lee 1, YoungJun Oh 2, JiSun Park 3 and Mohamad Y. Mansour 4 SUMMARY Concrete columns conined with highstrength iber reinorced polymer (FRP) composites can enhance the strength as well as the ductility o such structures. In recent years, the use o FRP composites to repair and strengthen existing reinorced concrete (RC) structures has been widely used. When the columns o existing RC structures are wrapped with FRP composites, the core concrete o such columns is conined not only by the FRP composites but ao by the existing steel reinorcing ties (or spira). Thereore, it is necessary to understand correctly the compressive response o concrete conined with both steel spira and FRP composites in order to predict the behavior o such RC columns. Since the behavior o the reinorcing steel spiral ties and the FRP composites are dierent rom each other, the behavior o concrete columns conined with both steel spiral and FRP composites is expected to be dierent rom that o concrete columns conined with only steel spiral or FRP composites. In this paper, 24 RC smalcale columns were tested under pure axial compression in order to study the response o RC columns conined with steel spiral ties and FRP composites. Two main parameters were considered in this investigation: the coninement type (one material (FRP composites or steel spiral) versus two materia (FRP composites and steel spiral)) as well as the lateral conining pressure. The test results showed that the compressive response o concrete when conined with two materia (steel spiral ties and FRP) was quite dierent rom the compressive response o concrete when conined with only one material. The study ao compares the experimental results to the predicted results obtained using current constitutive relationships ound in the technical literature. INTRODUCTION Fiber Reinorced Polymer (FRP) is a relatively new class o composite material manuactured rom ibers and resins and has proven eicient and economical or the development and repair o new and deteriorating structures in civil engineering. The reasons why FRP are increasingly used as 1 Assistant Proessor, Department o Architectural Engineering, Sungkyunkwan University, SouthKorea, jylee@skku.ac.kr 2 Structural Engineer, Wonwoo Structural Engineering Co., Seoul, SouthKorea 3 Researcher, Building Research Dept., Korea Institute o Construction Technology, GoyangSi, GyeonggiDo, SouthKorea 4 Research Assistant Proessor, Department o Civil & Environmental Engineering, University o Houston, USA

2 strengthening materia o RC structures are its high strengthtoweigh ratio, high corrosion resistance, resulting in easier application in conined space, reduction in labor costs, and ease in site handling. When wrapped with FRP composites, axially loaded concrete columns exhibit higher strengths and ductility as well as larger energy dissipation capacities than unconined concrete columns. Thus extensive research was carried out by many investigators; such as Karabinis and Rousakis 1, Campione and Miraglia 2, Karbhari and Gao 3, Ahmad et al. 4, and Demers M, and Neale 5, Mirmiran et al. 6,7 to name a ew; to assess and predict the compressive response o concrete columns wrapped with FRP. For example the model proposed by Mirmiran et al. 6,7 to describe the compressive stressstrain curves o reinorced concrete wrapped with FRP consisted o two regions: the irst (initial) region was similar to that o unconined concrete while the second region considered the eect o FRP coninement and is based on the coninement model originally proposed by Richart et al 8. While the model that was originally developed Mander et al 9 to predict the compressive stressstrain relationships o concrete conined by steel ties was used by Monti et al. 1 to predict the response o FRPconined concrete in compression by taking into account the eect o the lateral strain in the model ormulation. Similarly to Monti s model, Saai et al. 11 proposed a model in which the eect o the FRP coninement was accounted or by modiying the lateral strain o concrete. When the columns o existing RC structures are wrapped with FRP composites, the core concrete is conined by two dierent materia the steel spira and the FRP composites as shown in Fig. 1(a). I both conining materia exhibit the same behavior, the conining eects (the stressstrain relationships, strength, and ductility) o concrete conined with both materia can be predicted by simply superimposing the lateral conining pressure ( l ) o each material. However, the tensile stressstrain curves o FRP composites and Stress o concrete Large coninement eect Small coninement eect Steel yielding Coninement by spiral (a) Concrete column Spiral Stress o concrete FRP Unconined concrete Strain o concrete (b) Stressstrain relations o concrete conined by spiral Constant coninement eect Unconined concrete Coninement by FRP Stress o concrete Unconined concrete Steel yielding Coninement by spiral and FRP Strain o concrete (d) Stressstrain relations o concrete conined by both spiral and FRP Strain o concrete (c) Stressstrain relations o concrete conined by FRP Fig. 1 Stressstrain relationships o conined concrete

3 steel spira are dierent as shown in Fig. 2. The typical stressstrain curves o steel bars consists o an initial linear elastic portion, a yield plateau in which strain increases with little or no increase in stress, and a strainhardening range. Thus, or the case o steel transverse reinorcement, the constant conining pressure can be assumed when the steel is yielding and the stressstrain relationships o steelconined concrete gradually decreases ater the yielding o the steel as shown in Fig. 1(b). Unlike the stressstrain curve o steel spiral, FRP exhibits a stressstrain curve: linear elastic up to inal brittle rupture when subjected to tension, as shown in Fig. 2. Thereore, the stressstrain relationships o FRPconined concrete gradually increase and suddenly drop down when FRP composites rupture as shown in Fig. 1(c). Stress o materia The stressstrain curves o concrete conined with both steel and FRP composites (both materialconined concrete) are inluenced with the lateral conining pressures o both conining materia. I the lateral conining pressure ( ) o steel is smaller than the lateral conining pressure ( l ) o FRP composites, the slope o the stressstrain curves o both materialconined concrete decreases ater steel yielding. However, this slope is stier than that o the only steelconined concrete as shown in Fig.1(d). Such conined concrete ai when FRP composites rupture. Fig. 2 Stressstrain relationships o steel and FRP composites When the columns o the existing RC structures are repaired with FRP composites, the core concrete o the columns is conined by both materia (steel spira / steel hoops and FRP composites) because the FRP composites wrap the existing columns that have been already conined with steel spira or hoops. Although many researches have been carried out to study the compressive response o FRP wrapped RC columns, ew attempts were done to investigate the eect o FRP composites and steel ties coninement on the axial compressive response o concrete. Thus, in this paper 24 RC smalcale columns were tested under axial compression till ailure to assess the eect o both conining materia on compressive response o such columns. TEST PROGRAM Steel yielding Test specimens 24 concrete cylinders 15 mm in diameter and 3 mm in height were tested under axial compression till ailure. Two parameters were considered in the investigation: the types o conining material (FRP or/and steel ties) and the lateral conining pressure. The 24 cylinders were divided into our series: S, S6, S4 and S2. Each series consisted o 6 cylinders. While the S series was conined only with FRP sheets, the S6, S4 and S2 series were conined with both FRP and steel spiral ties. φ5.5 bars were used as steel spiral ties in series S2, S4 and S6. The pitch o the steel spiral ties in series S2, S4 and S6 was 2 mm, 4 mm and 6 mm, respectively. Two letters (S or steel and F or iber) were used to designate the type o coninement used or each cylinder in each series. The numeric digits next to the S and F letters were used to designate the pitch o the steel spiral ties in centimeters and the number o FRP layers used, respectively. The S letter was used to Steel FRP composites Strain o materia

4 designate the steel coninement while the letter F 5 was used to designate the FRP composites. The Spiral number o FRP layer varied orm one cylinder to 4 CFRP the other within a series. For example, specimen SF indicates that this cylinder was not conined 3 with steel spiral hoops nor FRP sheets, while S2F4 indicates that this cylinder was conined with steel 2 spiral ties having a pitch o 2 cm and with 4 layers 1 o FRP sheets. One concrete batch was used to cast the 24 cylinders at the same time. Ater curing the cylinders, primer was applied to the surace o the Axial Strain o lateral coninement cylinders and then the FRP sheets were placed. Fig. 3 Stressstrain relationships o conining The average concrete cylinder strength o each materia specimen at the time o parent cylinder test as well as the material properties o the steel spira and the FRP o each cylinder are shown in Table 1. In Table 1, c ' is the compressive strength o unconined concrete, ε co is the compressive strain o unconined concrete corresponding to c ', sy is the yield stress o the steel spiral, s is the pitch o the steel spiral, y is the tensile strength o CFRP, E s is the modulus o elasticity o CFRP, and t is the thickness o CFRP. Fig.3 shows the stressstrain curves o the steel spiral and CFRP composites. Axial Stress o lateral coninement Table 1 Material properties o cylinders ' Specimen c ε co s sy y E s (%) (mm) (GPa) SF Ä SF Ä SF Ä SF Ä SF Ä SF Ä S6F S6F1 S6F2 S6F3 S6F4 S6F5 S4F S4F1 S4F2 S4F3 S4F4 S4F5 S2F S2F1 S2F2 S2F3 S2F4 S2F t (mm)

5 The lateral conining pressures ( and l ) o steel and CFRP are calculated by Eqs.(1) and (2), respectively. Each lateral pressure o test cylinders is shown in Table 2. 2A sp sy Table 2 Lateral pressure o cylinders = (1) ds s Specimen l Specimen 2 ty t l = (2) SF.. S4F 6.25 D SF S4F where, A sp is the sectional area o steel spiral, d s SF S4F SF S4F is the distance between the centres o the spiral, and D is the diameter o cylinder. SF4 SF S4F4 S4F S6F S2F In Table 2, while the o all cylinders (S6F1 S6F S2F S6F S2F S6F5) in the S6 series is smaller than l, the S6F S2F o the S4 series is approximately the same to l o the concrete conined with one layersheet CFRP (S4F1), and the o the S2 series is almost S6F4 S6F S2F4 S2F the same to l o the concrete conined with two layersheet CFRP (S2F2). l Measurements The overall dimensions and arrangement o reinorcement are shown in Figs.4 and 5. Six linear displacement transducers (LVDT) were attached to the cylinder as shown in Fig. 4. Three LVDTs were used to measure the axial deormation while the other three were used to measure the lateral deormation o each cylinder. In addition, strain gauges were attached to the suraces o the steel spiral ties to record the strains o the transverse steel bars. The load was applied monotonically till ailure o the specimen. The applied load, the corresponding LVDTs, and the strain gauge readings were recorded automatically through a data logger at speciied load interva. The test was terminated when the value o the applied load dropped to about 9% o the maximumrecorded load in the postpeak descending branch. 15mm 15mm 75mm 15mm 75mm 15mm 15mm 3 LVDTs attached at 12 degree on cylinder to measure axial strain 3 LVDTs attached at 12 degree on cylinder to measure lateral strain Fig. 4 Locations o LVDTs Fig.5 Locations o strain gauges

6 TEST RESULTS The axial compression stressstrain curves o the 24 RC cylinders are shown in Figs. 6 (a) through (d). The test results o specimen S6F3 are not shown due to errors in the data logger. The igures show that when concrete is till in the elastic stage, the stiness o unconined and conined concrete is almost identical. In this stage, the axial compressive strain is still relatively small and in turn the resulting lateral strain is even smaller to engage FRP or steel ties to be eective as conining materia. Once the stressstain curve o concrete becomes nonlinear, the strength o concrete is ound to increase as the amount o FRP wrapping increases and as the pitch o the steel spiral ties decreases. Reerring to Fig. 6 (a), the compression strength and the corresponding strain were ound to increase as the thickness o the FRP sheets increases. On the other hand, when concrete is conined by both steel spira and FRP sheets, it was ound that the compressive strength and corresponding strain do depend on the thickness o FRP sheets (or number o layers) as well as the volume o spiral ties SF SF1 SF2 SF3 SF4 SF (a) S series S6F S6F1 S6F2 S6F4 S6F (b) S6 series S4F S4F1 S4F2 S4F3 S4F4 S4F S2F S2F1 S2F2 S2F3 S2F4 S2F (c) S4 series (d) S2 series Fig. 6 Axial stressaxial strain curves o test concrete cylinders

7 Table 3 shows the compressive strength and the maximum strain o the test cylinders. While the compressive strength o the conined concrete increases almost proportionally with increasing the lateral conining pressure, the maximum strain does not increase proportionally to l, but depends on the ratio o to l. The observed increase in axial strain over the unconined cylinder ranged rom 417 to 1292 percent or the CFRPconined specimens in the S series, whereas rom 1583 to 1792 percent or the both materialconined specimens in the S2 series. In Table 3, cu is the compressive strengh o specimen, ε cu is the maximum axial strain o cylinder corresponding to cu, ε co is the compressive strain o unconined concrete corresponding to c '. Figure 7 shows the axial stress versus the lateral strain curves o the test cylinders. The igure shows that the lateral strain( ε l ) suddenly increases ater the axial stress o the cylinder arrives at the compressive strength o unconined concrete ( c ' = MPa). The slope o the curve ater c ' = MPa increases proportionally to the lateral conining pressure. The maximum lateral strains o the cylinders, that have more l than, was almost the same as εl.11regardless o the thickness o CFRP sheets, wheres the maximum lateral strains o the specimens, S6F, S4F, S2F, S2F1, and S2F2, that have more than or similar, were much greater than.11. to l Table 3 Compressive strength and maximum strain o the tested concrete cylinders Specimens cu ε cu cu / c ' εcu / ε SF SF SF SF SF SF S6F S6F S6F S6F S6F S4F S4F S4F S4F S4F S4F S2F S2F S2F S2F S2F S2F co STRESSSTRAIN CURVES OF BOTH MATERIALCONFINED CONCRETE Stressstrain curve o conined concrete Figs. 8 (a) through (c) should be considered to study the eect o the coninement type (FRP vs. steel spira) on the compressive axial stressstrain curves o concrete. Fig. 8 (a) shows the compressive stressstrain curves o specimens SF3 and S2F1. SF3 is conined only three layers o FRP sheets while S2F1 is conined with both FRP (2 layers) and steel spira (pitch is 2 cm). The total conining pressure o both considered specimens SF3 and S2F1 is MPa and 19.8 MPa, respectively. This total conining pressure was obtained by simply summing the conining pressure o the FRP composites and the steel spiral ties. Although the total conining pressure was almost the same or considered specimens, Fig. 8 (a) shows that specimen S2F1 exhibited more ductile behavior when compared to specimen SF3. To urther study the eect o the coninement type on the compressive stressstrain curve o concrete, Figs. 8

8 Axial Stress, c SF SF1 SF2 SF3 SF4 SF S6F S6F1 S6F2 S6F4 S6F Lateral Strain, ε l Lateral Strain, ε l (a) S series (b) S6 series S4F S4F1 S4F2 S4F3 S4F4 S4F Lateral Strain, ε l S2F S2F1 S2F2 S2F3 S2F4 S2F Lateral Strain, ε l (c) S4 series (d) S2 series Fig. 7 Axial stresslateral strain curves o test concrete cylinders (b) and (c) should be reerred to. Here ao and similarly to Fig. 8 (a), the specimens considered in Figs. 8 (b) and (c) have almost the same values o total conining pressure. The value o the total conining pressure was almost 26 MPa in Fig. 8 (b) and 33 MPa in Fig. 8 (c). Comparison o Figs. 8 (a) through (c) show that as the value o the total conining pressure increases, the concrete compressive strength increases but the ductility decreases. As ar as the eect o the type o conining pressure, Figs. 8 (a) through (c) show that higher strength, larger ductility and higher energy dissipation capacity o concrete can be obtained when using both steel spira and FRP as conining materia.

9 1 8 =.MPa =19.8MPa l =14.81MPa =6.61MPa l SF3 S2F1 6 Steel yielding Steel yielding SF4 8 S2F2 6 =.MPa =26.5MPa l =14.81MPa =13.2MPa l (a) SF3 and S2F1 (b) SF4 and S2F2 12 Steel yielding SF5 S2F =.MPa =33.1MPa l =14.81MPa =19.8MPa l (c) SF5 and S2F3 Fig. 8 Comparison o the axial stressaxial strain curves o cylinders Compressive strength o conined concrete Fig. 9 shows the normalized concrete strength o concrete versus the total conining pressure. The normalized strength o concrete was obtained by dividing the concrete strength o each specimen by the concrete strength o specimen SF. The igure shows that as the concrete strength increases linearly as the total conining pressure increases. To urther investigate this trend, Fig. 1 shows the compression stressstrain curves o 4 specimens: SF, S4F, SF4, and S4F4. The strength o specimen S4F is larger than SF; the dierence in strength between these two specimens is denoted by s in Fig. 1 and represents the increase in strength o specimen S4F when compared to SF due to the presence o steel spira. Similarly, the increase in strength o specimen SF4 when compared to SF is denoted by in Fig. 1, and is due to the presence o 4 layers o FRP material. The igure ao shows the increase in strength o specimen S4F4, denoted by s +.

10 cu / ck l SF S4F SF4 S4F s s + Fig. 9 Compressive strength versus conining pressure o test cylinders curves Fig. 1 Comparison o the axial stressaxial strain o cylinders, SF, S4F4, SF4, and S4F4 Maximum compressive strain o conined concrete Fig. 11 shows the normalized concrete peak cylinder strain versus the total conining pressure. The normalized peak strain o concrete was obtained by dividing the concrete peak strain o each specimen by the concrete peak strain o specimen SF. Unlike the linear relationship between the concrete strength and the total conining pressure (reer to Fig. 9), Fig. 11 shows some scatter in the test results. Thus to study how the conining pressure aect the peak concrete strain, Fig. 12 should be reerred to. The igure shows the compressive stressstrain curves o specimens SF, S2F, SF4, and S2F4. the igures clearly shows that the increase in the peak compressive strain o specimen S2F4 can not be obtained by simply adding the increase in the compressive peak strain o specimen S2F to that o SF4. ε / ε cu co S2F5 S2F S2F1 S2F3 S2F4 S2F2 S4F S4F2 S4F ε s + ε ε ε s SF S2F SF4 S2F l Fig. 11 Maximum strain versus conining pressure strain o test cylinders Fig. 12 Comparison o the axial stressaxial curves o cylinders, SF, S2F, SF4, and S2F4

11 BEHAVIOR OF CONFINED CONCRETE Predicting o the stressstrain curve o conined concrete The behavior o the stressstrain curve o both materialconined concrete is inluenced by the ratio o to l. The stresses o the cylinders o S series and S6 series, that have more l than, increase linerly ater arriving at the compressive strength o unconined concrete ( c ' = MPa), and suddenly drops down when CFRP composites rupture. On the other hand, the stresses o the specimens, S4F1, S2F1, S2F2, and S2F3, which have similar amounts o l and, increase with little or no increase ater the steel spiral yields. Figure 13 compares the observed stressstrain curve o S2F2, that has l = 14.81MPa and = 13.2MPa, to the curves calculated by the current mode. Mander et al. 9 proposed Eq.(3) to predict the stressstrain curve o concrete conined with steel spiral. r = ( x r)/( r 1 + x ) (3) c cu where x = εc / εcu, r = Ec /( Ec E2m), and E2 m = cu / εcu The stressstrain curve plotted by Eq.(3) Axial Strain, ε shows that the slope ( E 2 ) o the curve ater c c ' = MPa is almost the same to the secont Fig. 13 Comparison between observed and predicted axial stressaxial strain curves modulus ( E c ) o elasticity o concrete beore c ' = MPa. However, the E 2 value o CFRPconined concrete, such as the cylinders o S series, is much smaller than the E c value. Figure 13 shows Eq.(3) based on the test results o steel spiralconined concrete overestimates the stress o both materialconined concrete ater c ' = MPa. This discrepancy increases as the l value increases. Lam et al. 12 proposed Eq.(4) to calculate the compressive stressstrain curve o FRPconined concrete. For εt εc εcu c = cu + E2l εc (4) where ε t = 2 cu /( Ec E2l) and E2l = 2 l / εcu. To calculate the stressstrain curve o concrete conined with both materia, the sum o l and is substituted or l. Eq.(4), that was originally proposed to predict the stressstrain curve o FRPconined concrete, underestimates the stress o both materialconined concrete ater arriving at the compressive strength o unconined concrete, because E 2 o both materialconined concrete, S2F2, is greater than that o FRPconined concrete. Furthermore, the slope o the curve o S2F2 increases with little or no increases ater steel spiral yielded, while the slope o the curve calculated by Eq.(4) increases linearly until CFRP composites rupture as shown in Fig. 13. Saai et al. 11 predicted the axial stressaxial strain curve o FRPconined concrete by increasing the lateral strain o concrete ( ε l ) For columns with CFS Steel yielding For columns with spiral For columns with spiral and CFS Test Eq.(3)Mander Eq.(4)Lam Eq.(5)Saai

12 c l( εl) = c' c '.84 c εc = εco 1 + (537εl + 2.6) 1 c ' (51) (52) The irst step in the calculation procedures o Eqs.(51) and (52) involves the selection o the lateral strain o concrete ( ε l ). The second step involves the calculation o the lateral conining stress ( l ) corresponding to ε l. The stressstrain curves o conining materia, such as steel or FRP composits, obtained rom material tests are used to calculate l. The third step requires the calculation o ε c and c by Eqs.(52) and (51), respectively. Compared to Eqs.(3) and (4), the calculation procedures o Eqs.(51) and (52) is more complicate and time consuming. However, because taking into account the characteristics o the stressstrain curves o conining materia, Eq.(5) predicts the stressstrain curve o both materialconined concrete with reasonable agreement as shown in Fig. 13. The slope o the curve calculated by Eq.(5) decreases ater steel spiral yields like that o S2F2. Consequently, in order to predict with accuracy the stressstrain curves o both materialconined concrete, there is a need to propose a simple model that includes the characteristics o the stressstrain curves o conining materia. Compressive strength o conined concrete The compressive strength o concrete conined with both materia is approximately the same to the sum o the increments o the compressive strength o concrete conined with one material(steel or FRP). Several mode were proposed to calculate the compressive strength o conined concrete. Mander et al. 9 proposed Eq.(6) to predict the compressive strength o spiralconined concrete. cu l = c ' c ' l c' (6) Lam et al. 12 calculated the cu o FRPconined concrete using Eq.(7). l cu = c '1 + 2 (7) c ' Saai et al. 11 proposed Eq.(8) to predict the cu FRPconined concrete. cu l = c ' c '.84 (8) To calculate the cu o concrete conined with both materia using Eqs.(6) through (8), the sum o l and is substituted or l in the equations. o cuexp / cucal Eq.(6)Mander Eq.(7)Lam Eq.(8)Saai l Fig. 14 Comparison between observed and predicted axial strength Figure 14 show the comparison between the observed shear strength o test cylinders and the calculated

13 ones by Eqs.(6) through (8). This igure shows the ratio o the experimental compressive strength to predicted compressive strength ( cu exp / cu cal ) o the 24 tested cylinders, as a unction o the lateral compressive pressure ( + l ). The predictions o Eqs.(6) through (8) estimated the observed test results or all values o + l with reasonable agreement. The mean values o the compressive strength ratio ( cu exp / cu cal ) as given by Eqs.(6) through (8) o the 24 test beams are.8,.99, and.9%, respectively, The variance values o ( cu exp / cu cal ) as given by Eqs.(6) through (8) are 17.%, 9.3% and 12.2%, respectively, Maximum axial strain o conined concrete Unlike the prediction o the compressive strength, there is a wide discrepancy between the maximum axial strain( ε cu ) o both materialconined concrete and the sum o the ε cu values o CFRPconined concrete and spiralconined concrete as shown in Fig. 11. The ε cu value o both materialconined concrete is approximately the same to the larger ε cu value between CFRPconined concrete and spiralconined concrete. Mander et al. 9, Lam et al. 12, and Saai et al. 11 proposed the equations (9), (1), and (11), respectively, to calculate the maximum strain o conined concrete. cu εcu = εco c ' l εcu = εco c ' cu εcu = εco 1+ ( 537ε o + 2.6) 1 c ' (9) (1) (11) cuexp / cucal ε ε l Eq.(9)Mander Eq.(1)Lam Eq.(11)Saai Fig. 15 Comparison between observed and predicted axial strains cuexp / cucal ε ε S2 Series Eq.(9)Mander Eq.(1)Lam Eq.(11)Saai l Fig. 16 Comparison between observed and predicted axial strains or S2 series

14 Figure 15 shows the ratio o the experimental compressive axial strain to predicted compressive axial strain ( εcu exp / εcu cal ) o the 24 tested cylinders, as a unction o the lateral compressive pressure ( + l ). The igure shows that the predicted axial strains o Eq.(9) underestimate the observed axial strains or + l 3MPa. The mean and variance values o the maximum axial strain ratios ( εcu exp / εcu cal ) o the 24 cylinders as predicted by the Eq.(9) are 1.23 and 28.5%, respectively. On the other hand, the prediction o the maximum axial strain ratio as given by the Eq.(11) not conservative when compared to the results o the Eqs.(9) and (1). The variance values o εcu exp / εcu cal as given by Eq.(1) and Eq.(11) o the 24 test cylinders are 29.7% and 49.2%, respectively. This discrepancy between the observed and predicted axial strains is because the ε cu value o both materialconined concrete is not the same to the sum o the ε cu values o CFRPconined concrete and spiralconined concrete, but is inluenced by the larger ε cu value between CFRPconined concrete and spiralconined concrete. Figure 16 shows εcu exp / εcu cal versus + l o the cylinders o S2 series. Even though the + l o S2F5 is more than two times that o S2F1, the maximum strains o both cylinders are almost the same. The igure shows that the maximum axial strain ratio ( ε exp / ε decreases with increasing o + l. cu cu cal ) CONCLUSIONS In this paper, the behavior o the stressstrain curve o concrete conined with both CFRP and steel spiral was observed by 24 cylinder tests. The observations o the test results led to the ollowing conclusions: 1) The maximum axial strain o concrete conined with both CFRP and steel spiral was approximately the same to the larger maximum strains between CFRPconined concrete and spiralconined concrete. The current mode, that depends on the sum o the lateral conining pressures ( + l ), overestimated the maximum strains o the both materialconined concrete when + l is high. 2) The stressstrain curve o concrete conined with both CFRP and steel spiral was inluenced by the ratio o to l and the stressstrain curves o lateral conining materia. The slope o the stressstrain curve o both materialconined concrete that had more than l decreased ater steel spiral had yielded. Thus in order to predict with accuracy the stressstrain curve o both materialconined concrete, it is need to propose a model considering the eects o the ratio to l and the stressstrain curves o lateral conining materia. 3) The compressive strength o concrete conined with both CFRP and steel spiral was approximately the same to the sum o increments o the compressive strength o CFRPconined concrete and spiralconined concrete. ACKNOWLEDGEMENTS The writers grateully acknowledge that this work was supported by Korea Reach Foundation (KRF22 3D4).

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