retrofitting rc circular columns using cfrp sheets as

Symposium on Infrastructure Development and the Environment 2006 7-8 December 2006, SEAMEO-INNOTECH University of the Philippines, Diliman, Quezon City, PHILIPPINES

RETROFITTING RC CIRCULAR COLUMNS USING CFRP SHEETS AS CONFINEMENT Jason Maximino C. ONGPENG Assistant Professor, Dept. of Civil Engineering, DLSU- Manila

Abstract: Reinforced concrete is used in common infrastructures. In the design process, deformed steel bars are used to compliment with the properties of concrete. These steel bars are used as lateral and longitudinal reinforcement to prevent shear and tensile failure. But due to overloading and updates on seismic codes in columns or bridge piers, retrofitting becomes dominant in order to improve the compressive strength and ductility of reinforced concrete columns. One way of retrofitting existing column structure is the application of Carbon Fiber-Reinforced Polymer (CFRP) Sheets to be wrapped around the circumference of circular column. The retrofitting method will result to an increase in compressive strength and ductility. This strength and ductility enhancement is done by the confinement effect of the CFRP sheets and the contribution of steel ties present on the existing column in resisting lateral expansion. From references, the interrelationships of the confinement effect done by the newly applied CFRP sheets and existing steel ties present in concrete columns are complex. Further experimental data should be made in order to study the effects of both materials when used as confinement in retrofitting structures. The study dealt with experiments on retrofitting designed concrete cylinders of size 180mm diameter x500mm height by wrapping around CFRP of 75mm-strips, one-ply, two-ply with varying amount of steel bars present. A total of 94 concrete cylinders were made and retrofitted from materials available in local conditions. With these data, analysis on the strength enhancement and common modes of failure will be investigated. The goal of the research is in conjunction with the current problems on strengthening existing structures on how structural designers and constructors efficiently and effectively use CFRP. Key Words: Carbon Fiber-Reinforced Polymer, Confinement, Retrofit

1. INTRODUCTION Confinement is the most commonly used method in retrofitting existing column structures. Conventionally, columns in the Philippines are mostly made out of reinforced concrete (RC). This concrete structure comprises of longitudinal steel bars and transverse lateral steel bars. The longitudinal steel bars are required for biaxial resistance while the lateral steel bars for shear resistance. The advantages of confinement in improving the performance of RC columns have been recognized in modern structural design codes especially in seismic design. Seismic codes specify special provisions for concrete such as the minimum requirements of the amount and spacing of transverse reinforcement, including critical locations where transverse reinforcement should be placed. With these requirements from National Structural Code of the Philippines (NSCP), the presence of

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confinement in transverse ties developed in concrete columns by providing lateral steel reinforcement in the form of closed ties, hoops or spirals around the longitudinal steel forming an inner concrete core are pre-existing in RC columns. In retrofitting RC columns wherein the lateral steel ties are present, an additional material may be used to increase the strength and ductility of an RC column. These materials may be steel jackets, or fiber reinforced polymer (FRP) sheets. The different materials vary differently in construction and methodology. Reinforced concrete columns that are confined by both steel reinforcement and additional confining materials are sometimes referred to as “hybrid RC columns”. This type of column has now become common in existing buildings and bridges. What are FRPs? Fiber-reinforced polymers (FRP) are fiber composites. These materials have been slow in development compared to other civil engineering materials. Before its application in civil engineering field, it is predominantly used in aerospace and marine industries. Over the past three decades, engineers became fascinated by its mechanical properties together with their customized fabrication techniques. FRP material systems are composed of fibers embedded in a polymeric matrix that exhibit several properties which is suitable for structural reinforcing elements such as confinement in RC columns. Compressive strength enhancement due to confinement is reflected by an increase of the peak stress of the unconfined compressive strength (f’co) to the confined compressive strength (f’cc). Most of the proposed stress-strain equations (e.g., Mander et al 1988a; Saatcioglu and Razvi 1992; Hoshikuma et al 1997; Saadatmanesh et al 1994; Samaan et al 1998; Saafi et al 1999; Spoelstra and Monti 1999; Li et al 2003; Hosotani and Kawashima 1999; Li and Fang 2004, Ongpeng and Oreta 2005) for confined concrete require the value of f’cc. Hence, the accurate prediction of the peak stress (f’cc) or ultimate confined compressive strength of RC columns is important when the capacity of structures is estimated. Various laboratory tests have been conducted to calibrate these confinement models with experimental data and determine important factors, which affect confinement particularly the confined ultimate compressive strength. Although parameters related to transverse reinforcement by steel or CFRP, such as yield or tensile strength of transverse reinforcement, transverse steel or CFRP volumetric ratio, transverse steel spacing and hoop pattern, are the main factors which increases the compressive strength of concrete. Some experiments revealed that other factors such as the unconfined compressive strength of concrete and longitudinal reinforcement also contribute to the positive effects of confinement. Experimental destructive testing was made in the course of this study. There were 94 concrete specimens of sizes 180mm diameter by 500mm height tested in uni-axial compression machine. The specimens were designed with varying amount of steel ties and CFRP sheets. From these data, analysis and comparison of existing models in hybrid columns were used.

2. CONFINEMENT IN RC COLUMNS Different behavior between steel and CFRP was observed due to the stress-strain relationship of each material shown in Figure 1. Carbon Fiber-Reinforced Polymer is elastic up to failure while steel has an elastic-plastic region. These different material

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properties contribute to a complex interrelationship between the two confining materials. In Figure 2, the geometric dimensions used as parameters in circular columns are shown. The position of lateral steel and longitudinal steel bars are within the core diameter, d, while the wrapping of CFRP in concrete is made by applying epoxy in the outer column diameter, D. Cross-section CFRP D Axial Stress

d

Steel Elevation Plain concrete L Axial Strain

Figure 1. Stress-strain of different materials

Figure 2. Diagram of Confining materials in concrete

There are two values of unconfined compressive strength of concrete. One is given as the unconfined compressive strength (f’c) of a cylinder of standard size and another is the unconfined compressive strength of the actual size (f’co) of a column. Most experiments provide these two values of unconfined strength except for Hosotani and Kawashima (1999) were f’c was not available. It can be observed experimentally that the two values are very close; hence in the absence of any information about either one of the two values, an estimate of the one of the values can be done or it may be assumed that f’c = f’co. In existing empirical models such as Hosotani and Kawashima(1999), and Li and Fang(2004), the ultimate compressive strength, f’cc, due to both confining materials is expressed as a superposition of the effect of steel ties and CFRP as given in Eqn.(1). f’cc = f’co + k1x fls + k2 x flCFRP

(1)

Where k1 = concrete strength enhancement coefficient due to steel, k2 = concrete strength enhancement coefficient due to CFRP, fls = effective lateral confining stress due to steel, and flCFRP = effective lateral confining stress due to CFRP. Shown in Figure 3 is the Free Body Diagram of the confining materials.

Steel ties

p CFRP d fFRP tL

fys As

fys As fFRP tL

Figure 3. FBD of steel ties and CFRP

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Hosotani and Kawashima (1999) derived an empirical equation for f’cc by regression using their experimental data and developed a bilinear stress-strain relationship for confined RC columns with steel ties and CFRP. On the other hand, Li and Fang (2004) used MohrCoulomb failure envelope in tri-axial loading similar to the mechanism of soil to modify the L-L model by Li et al. (2003). Hybrid modeling becomes very complex when interrelationship of the existing steel ties and CFRP are present column structure. Neural Network modeling was done by Ongpeng and Oreta (2005) to come up with a model named as SCC9-7-1B with acceptable prediction in the ultimate confined compressive strength, f’cc. The nine input parameters were: D- outer diameter d-core diameter L-height ρs-volumetric ratio of lateral steel bars ρcc-volumetric ratio of longitudinal steel bars ρFRP- volumetric ratio of CFRP fys- yield strength of steel ties fFRP- yield strength of CFRP f’c- unconfined compressive strength of size 150mm diameter by 300mm height or the actual size of concrete specimen. Neural network model deals with large size of matrices and needs to perform time consuming computations to arrive at the solution. From the SCC 9-7-1B model, a new model was made by simulating as many data possible for a regression analysis in the form of Eqn.(1). This model is shown below in Eqn(2) and named as modified SCC model. f’cc = f’co + 3.41 fls + 1.03 flCFRP

(2)

Comparison was made using the thirty-four available data from Hosotani and Kawashima (1999), and Li and Fang (2004). Shown in Table 1 are the Pearson Coefficient of Regressions, R. It shows that the prediction of the Modified SCC model gives closer values in predicting the ultimate confined compressive strength. Table 1. Comparison on the prediction of the models using Coefficient of Regression Hosotani and Modified SCC Hybrid Model Li and Fang (2004) Kawashima (1999) model Coefficient of Regression, R

0.9709

0.9706

0.9829

Consequently, it can be observed that Modified SCC Model performed well than the two listed models on Table1.

3. EXPERIMENTAL METHODOLOGY In this study, ninety-four specimens of sizes 180mm diameter by 500mm height were fabricated and tested. This experiment has the following configuration as described in Figure 4. Material properties used in the experiment are shown in Table 2. The configuration shown in Figure 4c and Figure 4d were made to provide solutions on the following issues:

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1. The construction methodology of wrapping the whole RC column with a full sheet of CFRP becomes difficult especially when the column is tall. 2. The availability of the CFRP sheet roll having its width limited to 600mm and length 50m is uneconomical especially on tall columns where in longitudinal overlapping should be done for the development on bond length necessary to fulfill fully wrapped structure

Material

Table 2. Material properties used in the experiment Diameter Length Nominal Tensile Elastic Modulus, E (mm) Strength (MPa) (mm) (MPa)

Lateral steel reinforcements

8

480

230

200

Longitudinal reinforcements

12

450

270

200

Carbon Fiber: SikaWrap® Hex230C

-

-

3650

231000

180 mm 25mm

130mm

25mm

S

500mm

S

FIG.4a

50mm

75mm 75mm 75mm 75mm 75mm

FIG.4c

180 mm

180 mm

25 mm 75mm 100mm 50mm 35 and 70 mm overlap

75mm 100mm 75mm

25 mm

FIG.4b

FIG.4d

Figure 4. (a) Configuration on the lateral steel ties placed on the core diameter (b) Configuration on the fully wrapped CFRP (c) Configuration on the 75mm-CFRP plies with a spacing of 75mm (d) Configuration on the 75mm-CFRP plies with a spacing of 75mm

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The specimens were categorized as two types: Type-F and Type-P. Type-F represents the specimens which were fully wrapped by CFRP as can be seen in Figure 4b, while Type-P represents the specimens which were partially wrapped as shown in Figure 4c and Figure 4d. Furthermore, Type-F specimen name follows the format: F – (unconfined compressive strength) – (Spacing of steel ties) – (ply/ies of CFRP used) – (specimen letter). While Type-P specimen name follows the format: P – (unconfined compressive strength) – (spacing of steel ties) – (spacing of 75-mm width of CFRP) – (ply/ies of CFRP used) – (specimen letter). For example, specimen F-30-120-2-A means fully wrapped CFRP were used with unconfined compressive strength of 30 MPa, 120mm spacing for the steel ties, using two plies of CFRP, and the first specimen out of three total identical specimens. In wrapping CFRP to the concrete specimens cured for 28 days, the fibers were oriented 90° with respect to the longitudinal axis of the concrete column. In the preparation of the epoxy matrix, the resin and hardener ratio is 4:1 and was hand mixed for at least 5 minutes. The overlap of CFRP is 35 mm and 70 mm for one- and two-layer of CFRP respectively. Lastly, the wrapped concrete cylinders were cured at room temperature for at least 7 days before it was subjected to uni-axial compressive test at AZTEC concrete testing center.

4. DATA AND RESULTS Shown in Figure 5 and Figure 6 are the stress-strain diagrams of the three specimens that have no steel ties and 40-mm spacing of steel ties, respectively, with increasing amount of CFRP ply used from zero to two plies. It can be observed that the confinement effect of using CFRP and steel ties had increased the compressive strength and the average longitudinal strain that represents the ductility of the specimen. 60

50

40 Stress (MPa) 30 F-27-none-0-A 20 F-27-none-1-A 10 F-27-none-2-A 0 0.00

0.10

0.20

0.30

0.40 0.50 0.60 Average Longitudinal Strain

0.70

0.80

0.90

Figure 5. Stress-Strain Diagram of Specimens without steel ties Shown in Table 3 are the average increases in compressive strength. The results can be observed as follows: 1.) Specimens with no steel ties having 1-ply of CFRP strips spaced 75mm or 100mm, and concrete with 1-ply of fully wrapped CFRP had an increase in strength from 35% to 65% (GroupA)

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2.) Specimens having spacing of steel ties 120mm and no steel ties at all with 2plies of fully wrapped CFRP had an increase in strength from 65% to 95% (GroupB) 3.) Specimens having its spacing of steel ties 40mm and 80mm with 2-plies of fully wrapped CFRP had an increase in strength from 95% to 120% (GroupC) 70 60 50 40

Stress (Mpa)

30 20

F-27-40-0-A

10

F-27-40-1-A F-27-40-2-A

0 0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

Average Longitudinal Strain

Figure 6. Stress-Strain Diagram of Specimens with steel ties Table 3. Average Increase in Compressive Strength Using 27 MPa Unconfined Compressive Strength Steel Ties’ Spacing (mm)

None

40

80

120

Increase

%

( MPa )

increase

No CFRP

0

0

1-ply of 75mm spaced CFRP strips

13.67

50.63

A

1-ply of 100mm spaced CFRP strips

13.63

50.48

A

1-ply of CFRP

10.23

37.89

A

2-ply of CFRP

24.18

89.56

B

No CFRP

2.14

7.93

1-ply of 75mm spaced CFRP strips

12.71

47.07

A

1-ply of 100mm spaced CFRP strips

10.95

40.56

A

2-ply of 75mm spaced CFRP strips

16.03

59.37

A

1-ply of CFRP

19.24

71.26

B

2-ply of CFRP

32.35

119.81

C

1-ply of CFRP

11.83

43.81

A

2-ply of CFRP

27.38

101.41

C

1-ply of CFRP

7.74

28.67

2-ply of CFRP

20.01

74.11

Specifications

Class

B

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Where: Group A – 35 to 65% increase from fc’ Group B – 65 to 95% increase from fc’ Group C – 95 to 120% increase from fc’

Figure 7. Common Failure Mode of Specimens using 75mm CFRP strips Shown in Figure 7 are the common failure mode crushing (left) and cracking (right). It can be observed that the strips of CFRPs rupture when crushing is evident on the failure mode, while CFRPs are still intact and did not rupture when the failure is in cracking.

5. CONCLUSIONS Retrofitting RC circular columns using CFRP sheets as confinement becomes very complex from design to implementation. The prediction of strength enhancement is very crucial especially from the aspect of economy. It was found out that for concrete with no steel ties, using CFRP strips produces 35% to 65% strength enhancement, same as when 1ply of fully wrapped CFRP was used. Further experiments of varying thickness and spacing of the CFRP strips are recommended to come up with a model for predicting the strength enhancement. A complex interaction between the lateral steel reinforcements and the CFRP strip exists and this needs further investigation.

ACKNOWLEDGMENT This research is supported by the University Research Coordination Office (URCO) of De La Salle University – Manila, Philippines.

REFERENCES De Lorenzis, L.(2001). “A Comparative Study of Models on Confinement of Concrete Cylinders with FRP Composites”, Chalmers University of Technology De Lorenzis, L. and Tepfers, R. (2003). “A Comparative Study of Models on Confinement of Concrete Cylinders with Fiber-Reinforced Polymer Composites,” Journal of Composites for Construction, ASCE, 7(3), August 1, 2003, 219-237

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Saatcioglu, M. and razvi, S.R. (1992), “Strength and Ductility of Confined Concrete,” Journal of Structural Engineering, ASCE, 118(6), 1590-1607. Sakai, J et al.(2000). “Effect of Tie Spacing on Stress-Strain Relation of Confined Concrete”. Journal of Structural Engineering, JSCE, 46(3), 757-766 Sakai, J. (2001). “Effect of Lateral Confinement of Concrete and Varying Axial Load on Seismic Response of Bridges”. Doctor of Engineering Dissertation, Department of Civil Engineering, Tokyo Institute of Technology, Tokyo Samaan, M. et al. (1998). “Model of Concrete Confined by Fiber Composites,” Journal of Structural Engineering, ASCE, 124(9), 1025-1031 Spoelstra, M.R. and Monti, G. (1999). “FRP-Confined Concrete Model,” Journal of Composites for Construction, ASCE 3(30, 143-150)

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