STRAINS OF ECCENTRICALLY COMPRESSED RC COLUMNS STRENGTHENED

WITH CFRP MATERIALS

T. Trapko 1, M. Musial 1 and M. Kaminski 1 1 Institute of Building Engineering, Wroclaw University of Technology

Pl. Grunwaldzki 11, 50-377 Wroclaw, Poland. Email: tomasz.trapko@pwr.wroc.pl

ABSTRACT This paper presents the results of experimental investigations of strains in eccentrically compressed RC columns strengthened with CFRP sheets and strips. The main objective of the investigations was to evaluate the influence of longitudinal CFRP reinforcement intensity and the manner of transverse CFRP reinforcement construction on strains and strengthening effectiveness of eccentrically compressed RC columns strengthened with CFRP strips and sheets. Six columns strengthened with CFRP materials and three control columns without strengthening were performed and investigated to reach stated objectives. The investigations were conducted on model specimens of columns with dimensions of 200x200 mm (cross-section) and 1500 mm (height). The specimens were subjected to immediate axial or eccentric compression. The specimens were strengthened with an external CFRP reinforcement in two manners – longitudinal CFRP strips and transverse bands made of CFRP sheet or longitudinal CFRP strips and a continuous jacket (on whole height of column) made of CFRP sheet. In the investigations maximum possible intensity of CFRP strengthening with one layer of strips was applied ρL=2,53%. The intensity of the external reinforcement of columns is described with a strengthening level ρL (the area of longitudinal CFRP strips – AL in relation to the area of concrete in the element – AC). It was proved with the conducted experiment that longitudinal CFRP strengthening restricts strains on edges and causes the redistribution of internal forces in a cross-section. The redistribution appears with linear character of strains till failure. KEYWORDS CFRP, columns, eccentric compression, strain, strengthening. INTRODUCTION The objective of the conducted experimental investigations was the analysis of the influence of eccentricity of load on strains and effectiveness of strengthening of RC columns strengthened with CFRP strips and sheets. The columns were different in construction of the external reinforcement: longitudinal CFRP strips and transverse bands made of CFRP sheet or longitudinal CFRP strips and a continuous jacket (on whole height of column) made of CFRP sheet. Load was applied in different ways as an axial or eccentric load inside the core of a cross-section (eccentricity: h/12 and h/6). To achieve stated goals six elements strengthened with CFRP materials and three control elements without strengthening were performed and investigated. In the investigations program following parameters were assumed: Changeable parameters: (1) eccentricity of load: A1 – axial compression, A2 – h/12, A3 – h/6, (2) transverse strengthening: B1 – transverse bands (SikaWrap Hex 230C),

B2 – continuous jacket (S&P C Sheet 240), Regular parameters: (3) modulus of elasticity of CFRP strips: C1 – 210 GPa (Sika CarboDur type M),

C2 – 200 GPa (S&P CFK-Lamellen 200/2000), (4) intensity of longitudinal strengthening: D1 – ρL=2,53%. Two types of strengthened elements were designed on the basis of the parameters shown above. These elements were subjected to immediate axial and eccentric compression (according to table 1): (A) type A specimens: the intensity of longitudinal strengthening ρL=2,53%, strips: Sika CarboDur type M,

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transverse bands – SikaWrap Hex 230C sheet, (B) type B specimens: the intensity of longitudinal strengthening ρL=2,53%, strips: S&P CFK-Lamellen 200/2000, continuous jacket – S&P C Sheet 240.

Table 1. Programme of experimental studies Eccentricity 0 (axial load) h/12 h/6

Specimens type SbwA_0 SwzA_16 SwzA_32 SbwB_0 SwzB_16 SwzB_32

PREPARATION OF TEST SPECIMENS

Columns The investigations were conducted on the models of columns. The dimensions of cross-section were 200x200 mm and height was of 1500 mm (Figure 1). The longitudinal steel reinforcement was four bars ∅12, a grade of steel was A-IIIN with yield strength of fyd=420 MPa (PN-B-03264, 2002), the transverse steel reinforcement was stirrups ∅5,5,a grade of steel A-I with yield strength fyd=210 MPa (PN-B-03264, 2002). The distance between stirrups was reduced at the ends of the elements and it was equal of 1/3 of the main distance. The parts of the columns with the reduced distance between stirrups were the length of 200 mm (the width of cross-section of the column). The stirrups were provided with semicircular hooks as a bonds and tied on the longitudinal bars. Each stirrup was rotated in relation to the previous with angle of 90°. The edges of the specimens were chamfered with dimensions 10x10 mm. The columns were provided with steel frontal plates to guarantee the surfaces of load being parallel. The longitudinal internal reinforcement was passed through the plates. It was welded to the plates and cut off before the investigations. The way of construction of the ends of the columns assured equal load of concrete and steel reinforcement. Moreover, it prevented the longitudinal steel bars from pushing into the concrete.

Figure 1. Performed specimens

Concrete mix The specimens were performed in an industrial plant. A type CEM I 42,5R (PN-EN 197-1, 2002) portland cement was used. The fine aggregate used was river sand 0÷2 mm. The coarse aggregate was a river gravel of 2÷8 and 8÷16 mm. Mix proportions for the concrete mix used in this study are shown in table 2 (on the basis of the industrial plant recipe). The ingredients of the concrete mix were measured with a scale. The main investigative elements (columns) and the specimens (cubes and cylinders) to investigate the properties of the concrete were made of the same batch of concrete mix.

Table 2. Concrete mix proportions Portland cement 360 kg/m3

Fly ash – 20% of the cement content 72 kg/m3 Sand 0÷2 mm 524 kg/m3

Gravel 2÷8 mm 989 kg/m3 Gravel 8÷16 mm 495 kg/m3

Superplasticizer – 0,5 % of the cement content 1,9 l/m3 Water 87 l/m3

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Properties of materials Six concrete cubes with dimensions of 150x150x150 mm, six concrete cylinders with dimensions of ∅160x160 mm and six concrete cylinders with dimensions ∅113x350 mm were made. Described specimens and columns were consolidated with internal vibrator in the same way. All specimens were kept in the same temperature and humidity. The day before the investigations of the columns authors tested: (1) mean compressive strength of concrete fcm,cube=61,23 MPa and fcm,cyl=48,58 MPa, (2) mean tensile strength of concrete fctm

spl=3,36 MPa, (3) mean modulus of elasticity of concrete Ecm=34,20 MPa. The longitudinal reinforcement was made of ribbed bars #12 mm, a grade of steel was A-IIIN and a sort of steel was RB500W (PN-B-03264, 2002). Authors selected the specimens randomly and tested properties of steel as follows: (1) mean yield strength fy=550,8 MPa, (2) mean tensile strength ft=644,3 MPa, (3) mean modulus of elasticity Es=202 385 MPa. In case of Sika CarboDur system materials – strips Sika CarboDur type M and sheets SikaWrap Hex 230C, properties as follows were tested in the own investigations (Trapko, 2004): (1) mean tensile strength of CFRP sheet fwm=2936,6 MPa, (2) mean modulus of elasticity of CFRP sheet Ewm=237 600 MPa, (3) mean tensile strength of CFRP strip fLm=3467,5 MPa, (4) mean modulus of elasticity of CFRP strip in tension test ELm=228 227 MPa, (5) mean modulus of elasticity of CFRP strip in compression test ELm=230 410 MPa. EXPERIMENTAL INVESTIGATION METHODOLOGY

Experimental set-up A cylindrical bearing was designed and performed to apply load. Different eccentricities were realizable with the bearing: axial load (Figure 2-a), eccentricity of h/12=16 mm (Figure 2-b) and eccentricity of h/6=32 mm – force on the boundary of the core (Figure 2-c). Support zones of the columns were provided with screwed steel bands with height of 200 mm.

a) axial load b) eccentricity of load h/12=16 mm c) eccentricity of load h/6=32 mm

Figure 2. Cylindrical bearing used in the experiment Instrumentation

b) specimen Sbw_16 b) specimen SwzA_32 c) SwzB_16

Figure 4. Specimens in experimental set-up

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In each load step following values were measured: (1) longitudinal strains εcv and transverse strains εch of concrete with electrical resistance strain gauge rosettes

attached at mid-height on each side of column, (2) longitudinal strains εLv and transverse strains εLh of CFRP stripes with electrical resistance strain gauge

rosettes attached at mid-height on the surfaces of the stripes on each side of column, (3) transverse strains εwh of CFRP sheets with electrical resistance strain gauges attached at the middle part of

column (one gauge on each side of column), (3) strains of the longitudinal steel reinforcement εsv with electrical resistance strain gauges attached to each bar

at the mid-length. The results were registered with PC and measuring system UPM 100 made by Hottinger Baldwin Messtechnik. Loads were applied with a pneumatic press with range of 0÷6000 kN (Figure 4). Experimental investigations were carried out in Building Engineering Institute Laboratory of Wroclaw University of Technology. TEST RESULTS AND DISCUSSIONS Longitudinal strains (for different levels of force N) of the compressed cross-sections strengthened with CFRP strips and sheets are shown on the charts below. Longitudinal strains of reinforcing steel, concrete and CFRP strips are shown on Figures 5÷7 for some load levels including load-bearing capacity Nu (Nu=1720 kN for the specimen type Sbw_16, Nu=2234 kN for SwzA_16 and Nu=2480 kN for SwzB_16). Results for the specimen type Sbw_32 (Nu=1400 kN), SwzA_32 (Nu=1999 kN) and SwzB_32 (Nu=2097 kN) are shown on Figures 8÷10. Strain gauges were arranged in layers in the middle cross-section. It enabled to measure strains presented on the charts. Some gauges attached under CFRP strip or sheet were damaged during the tests, therefore some values were not registered.

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200kN400kN600kN800kN1000kN1200kN1400kN1600kN1720kN

Figure 5. Distribution of longitudinal strain in the middle cross-section – specimen type Sbw_16

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200kN400kN600kN800kN1000kN1200kN1400kN1600kN1800kN2000kN2234kN

Figure 6. Distribution of longitudinal strain in the middle cross-section – specimen type SwzA_16

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Figures 5 and 8 show that when load level is small the whole loaded cross-section is compressed. Specimen behaves like homogenous and stress distribution is linear. With the increase of load the redistribution of stress in cross-section proceeds. The chart of strain takes the convex shape on. Some freedom of strains of outermost fibres causes the decrease of the strains at the more compressed edge. Concrete starts behaving like a plastic material. When stress value reaches compressive strength concrete undergoes crushing.

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]200kN400kN600kN800kN1000kN1200kN1400kN1600kN1800kN2000kN2200kN2400kN2480kN

Figure 7. Distribution of longitudinal strain in the middle cross-section – specimen type SwzB_16

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Figure 8. Distribution of longitudinal strain in the middle cross-section – specimen type Sbw_32

Figures 6 (SwzA_16), 7 (SwzB_16), 9 (SwzA_32) and 10 (SwzB_32) show that use of the external strengthening as a longitudinal reinforcement and composite confinement caused the increase of stiffness of the element. The external composite reinforcement limits the freedom of strains in outermost fibres, which is shown in linear stress distribution in the cross-section. It can be assumed that the cross-section behaves like a homogenous. If force N is applied at the boundary of the core (as in case of the specimen type Sbw_32) then in initial phase of loading strains on the less compressed edge εv=0 (Figure 8). With the increase of force N tensile stress appears in the cross-section. Its range is relatively small. In the phase of failure concrete in tensile area of the cross-section fractures. In case of the strengthened specimens compressed by force at the boundary of the core (eccentricity of load 32 mm) the stress redistribution (caused by the increase of force N) was not as clear as in case of not strengthened specimens. Tensile stress did not appear in the cross-sections. Load-bearing capacity of reinforced concrete columns was reached when stress in the more compressive reinforcing bars was equals the yield stress of steel (Figures 5 and 8). Immediately after that stress in concrete attained its compressive strength. In case of the specimen Sbw_16 by strain εsv2=3,02‰ stress in steel had a value of σs=612 MPa, however for the specimen Sbw_32 by strain εsv2=2,92‰ stress was σs=590 MPa. Yield strength of longitudinal steel reinforcement was fy=550,8 MPa. In strengthened specimens yield strength in the internal steel bars was not reached. It is justified by the limitation of the velocity of the increase of longitudinal strains caused by the external composite reinforcement CFRP.

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Failure of the specimen occurred with the rupture of the transverse composite strengthening and the crushing of concrete at the more compressed edge.

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200kN400kN600kN800kN1000kN1200kN1400kN1600kN1800kN1999kN

Figure 9. Distribution of longitudinal strain in the middle cross-section – specimen type SwzA_32

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Figure 10. Distribution of longitudinal strain in the middle cross-section – specimen type SwzB_32

CONCLUSIONS Conducted experimental studies showed that longitudinal composite strengthening causes limitation of strains at the edge of column and internal forces redistribution in cross-section. The redistribution appears in linear stress distribution till failure. Performed tests and achieved results encourage doing more extensive analyses. Currently authors are planning to conduct the investigations allowing to analyse the influence of the intensity of longitudinal strengthening on load-bearing capacity of eccentrically compressed columns. REFERENCES Kaminski M., Trapko T., Musial M., Bywalski Cz. (2008). “The influence of longitudinal CFRP strips on the

load-bearing capacity of RC columns”. Proceedings of the 4th International Conference on FRP Composites in Civil Engineering. CICE 2008. Zurich, Switzerland, July 22-24. EMPA.

Kaminski M., Trapko T. (2006). “Experimental behaviour of reinforced concrete column models strengthened by CFRP materials”, Journal of Civil Engineering and Management, Vol. XII, Issue 2, June, 109-115.

Kaminski M., Trapko T. (2006). “On effective use of CFRP as strengthening reinforcement of eccentric compressed RC columns”, Inżynieria i Budownictwa, No. 1, 39-43 (in Polish).

Trapko T. (2004). “Load capacity of RC columns strengthening with CFRP strips and wraps”, PhD dissertation, Rapport series PRE 22/04, Institute of Building Engineering, Wroclaw University of Technology, 177 p.

PN-B-03264:2002. Polish Standard. Plain, reinforced and prestressed concrete structures. Analysis and structural design.

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