Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005) Chen and Teng (eds)

© 2005 International Institute for FRP in Construction

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THE EFFECTS OF SHEAR KEY AND U STRIP ON THE FLEXURAL BEHAVIOR OF CFS STRENGTHENED RC BEAMS

H. S. Choi1, C. Y. Lee2, S. T. Yi1, S. W. Lee3 and G. Heo4 1 Department of Civil Engineering, Chung Cheong University, South Korea. Email: yist@ok.ac.kr

2 CareCon Company Inc., South Korea. 3 Department of Architecture Engineering, Chung Cheong University, South Korea.

4 Department of Applied Materials, Chung Cheong University, South Korea. ABSTRACT It is generally known that the bonding strength of reinforced concrete (RC) flexural members strengthened with fiber sheet composites is sufficient, and bonding failure does not occur until the sheet fails. However, many researchers have been reported that bonding failure sometimes occurs before sheet failure, even though the bonding length is sufficient. However, for some RC flexural members, bonding failure could happen before the fracturing of the sheet itself, at early loading stages. This study was carried out to evaluate the effectiveness of shear keys and U strips on the flexural behavior of RC beams. The ply numbers of carbon fiber composite sheet (CFS), location of the shear key, and existence or non-existence of the U strip were selected as the main test variables. The test results showed that the behavior of a beam having shear keys near the supports, non-existent U strip, and one-ply CFS along the overall length and the other ply along the center half-length was mostly improved. KEYWORDS CFS, shear key, U strip, strengthening, reinforced concrete (RC) beams. INTRODUCTION As construction technology develops and the world population increases, the safety of infrastructure increasingly becomes a focus of public interest. Also, construction flaws, environmental changes, and maintenance difficulties have increased public awareness of the safety and serviceability of concrete structures. With the development of new construction materials (Malek and Saadatmanesh 1996), such as carbon fiber composite sheets (CFS), aramid fiber composites, and glass fiber composites, the repairing and strengthening of aged concrete structures has been increasingly studied in recent years (Sen et al. 1994; Arduini and Nanni 1997; Lees and Burgoyne 1999; Triantafillou 1992). In addition, a number of long-term studies have been reported, but their focus has largely been on the material aspects of concrete infrastructure, such as chemical resistance to alkalis or creep (Den Uijl 1991). In the past, reinforced concrete (RC) column structures were retrofitted using steel jackets or lateral hoop or spiral reinforcements (Mander et al. 1988a, 1988b). However, recently, a quick and simple solution of rehabilitating these deteriorated concrete members by wrapping them with CFS has become popular. When these repairing and strengthening materials are used in concrete structures, however, two assumptions are primarily adopted: (1) the bond strength between the fiber sheet and the concrete surface is sufficient, and (2) debonding failure occurs when the sheet’s stress reached to its tensile strength. Meanwhile, many researchers (Hau 1999) have reported that, for RC flexural members having a sufficient bonding length, bonding failure could happen before the fracturing of the sheet itself, at early loading stages. If this is correct, bonding failure is a problem to be solved beforehand. Currently, however, some researchers (Jones et al. 1980; Swamy et al. 1987) have taken a core interest in the increment of the tensile or flexural strength. In this study, the effects of the shear key and U strip on bonding strength were experimentally evaluated to improve the flexural behavior of RC members confined with CFS. Namely, for the beam specimens, the ply number and length of the CFS, the location and existence or non-existence of the shear key, and the existence or non-existence of the U strip were selected as the primary test variables.

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FLEXURAL EXPERIMENTS FOR RC BEAMS STRENGTHENED WITH CFS Test Variables and Specimen Properties The specimens used in this study were as follows: (1) a specimen without CFS, (2) specimens strengthened with one or two plies of CFS along the overall length of the specimen, (3) specimens reinforced with one ply along the overall length and with one or two additional plies along the center half-length, (4) a specimen with shear keys near either center or end supports, and (5) a specimen with U-strip reinforcements near both end supports. The specimen numbers and reinforcing details are shown in Table 1. All tested specimens were 150 mm × 250 mm square sections. The length between the supports and overall length were 1800 mm and 2200 mm, respectively. In the tensile and compressive portions of the specimens, the concrete cover was 30 mm. The shapes and sizes of the specimens are shown in Fig. 1. Figure 2 shows details of the shear key and U strip. The reinforcing bars of D10 (yield strength fy = 300 MPa) were used as the tensile, compressive, and shear reinforcements; the design strength of the concrete was 28.0 MPa.

Table 1 Details of the specimens Specimen No. No. of ply Bonding

length Location

of shear keyExistence of U strip

Remarks

CF 0 - - No Ref. specimen CF1 1 140 - No

CF1-SE 1 140 End No CF1-SC 1 140 Center No

CF2 2 280 - No CF2-SE 2 280 End No

CF1.5-SE 1+0.5 250 End No CF1.5.5-SE 1+0.5+0.5 360 End No

CF1.5-U 1+0.5 250 - Yes

100 8@100=800 400 8@100=800 1002200

25 100 25150

3019

030

250

D10

D10

CFSCFS

(unit; mm)

A

A

A-A Section

Figure 1 Details of the specimen

15

7

Φ9

CFS

15

150

250

150

(unit; mm)

(a) Shear key (b) U strip

Figure 2 Details of the shear key and U strip In Table 1, the specimen numbers using the symbol CF and the following numbers represent the materials used (carbon fiber composite sheet) and ply numbers, respectively. In addition, the symbols SE and SC after the

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hyphen refer to the locations of the shear key: either in the ends or at the center neighborhoods of the specimens. Also, the symbol U corresponds to the use of the U-strip reinforcements. In preliminary tests, the flexural characteristics between the different specimens were apparently different. Accordingly, in this study, one concrete beam per condition was prepared, and three-point flexural tests were performed. In Fig. 2, the CFS and the concrete surface were bonded using epoxy resins. The CFS plies (H Company) and epoxy resins used in these tests are sold commercially in South Korea. For these materials, the product numbers are NR72 and PR67+PH33, respectively. The locations of the shear key and U strip were 50 and 30 mm from both supports of the beams to the center direction, respectively. The width of the CFS used for the U strip was 200 mm. Specimen Properties and Materials The concrete mixture proportion selected for the specimens is listed in Table 2. Type I Portland cement was used in the mixture. Crushed gravel was used as the coarse aggregate, and the maximum aggregate size (Gmax) was 25 mm. In addition, an AE admixture and vibrator were used to improve the workability and consolidation of the concrete. All specimens and test cylinders were cast vertically on a level surface. The specimens were removed from the molds after 24 hours and were dry-cured under wet burlap towel until testing. The cylinders were tested at an age similar to the concrete used for the column specimens. The concrete compressive strength was determined based on the average of three identical φ 100 × 200 mm cylinders from the same batch (see Table 2).

Table 2 Concrete mixture proportions Unit weight (kg/m3) '

cf (MPa)

maxG (mm)

w/c (%)

s/a (%) W C G S

Ada

(%)

29.4 25 45.3 45.6 189 417 796 904 2.5 a AE admixture (ratio of cement ratio)

Testing Procedure and Instrumentation A load was applied using a hydraulic universal testing machine (UTM) with a capacity of 20 MN. The overall view of a test specimen set-up is shown in Figure 3. This was achieved using displacement-controlled testing and the displacement increment was 0.2 - 0.3 mm/min to approximately 80% of the maximum load. However, after this point, the displacement increments were gradually reduced to measure the softening behavior.

Figure 3 Overall view of test set-up The deformation of the specimens was obtained using a linear variable differential transducer (LVDT) located vertically at the center of each specimen. Strain in the tensile and compressive reinforcing bars was measured using electrical resistance strain gauges bonded to the reinforcing bars until the specimen failed. In addition, the applied loads were continuously measured using a load cell. Crack patterns and other observational data were also recorded during all tests.

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ANALYSIS OF TEST RESULTS Figure 4 depicts, case by case, the comparison between the different load-displacement curves obtained in this study. Figure 4(a) shows the load-displacement curves with the ply number of the sheets. From this figure, it is noted that the maximum load and the corresponding brittleness at the bonding failure of the sheet increases as the ply number increases. The bonding failure began just after the maximum load was placed, and after showing a little resistance, the final bonding failure occurred. After the bonding failure, it showed a lower strength value than the unstrengthened beam. However, this value was recovered in a short time and shows similar behavior to the unstrengthened beam. This phenomenon was observed in all of the specimens tested in this study.

(a) (b)

(c) (d)

(e)

Figure 4 Load-displacement curves

Figure 4(b) shows the comparison between the load-displacement curves with the location and existence or non-existence of the shear key when one ply is bonded along the overall length. The specimen with a one-ply sheet and shear key at 50 mm from both supports shows similar behavior to the specimen with a one-ply sheet and without a shear key. However, the ductility improved greatly, and after untying the shear key, the load and displacement values increased a little because the sheet resisted the external load. Meanwhile, when the

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specimen with shear keys at 25 cm from the middle section to end directions was compared to the control specimen, the load and displacement values had improved a little in the beginning stage of the loading; however, the difference decreased as soon as the load increased. Namely, to obtain improved reinforcing results, the shear key should be located near the supports. Also, it should be noted that its role has changed from reinforcement to the initial notch when it is located near the middle section of the specimen. Figure 4(c) shows the comparison between load-displacement curves with existence or non-existence of the shear key when two plies are bonded along the overall length. As the ply number increases, as illustrated in Figure 4(a), load-displacement capacities increase. Also, when the same number of plies is used, if a shear key is added near the supports, the maximum load and ductility is apparently improved. Figure 4(d) shows the load-displacement curves with the ply number of the sheets when the shear key is located in both supports of the specimen. When only one ply is used, the load and displacement values are inevitably increased when compared to the control specimen. When the specimen reinforced with two plies along the overall length is compared to the specimen with only one ply along the overall length and the other ply along the center half-length of the specimen, the former has more advantage than the latter for the initial behavior. However, for the maximum load and ductility, the latter has more benefits. This phenomenon occurs because making the reinforcement perfectly with two plies at the preparation of the shear key is difficult. When the specimen reinforced with one ply along the overall length and another ply along the center half-length was compared to the specimen additionally reinforced with one ply along the center half-length, the initial slope of the curve of the latter specimen increased; however, the ductility and the strain corresponding to the maximum load decreased. From this, it has been noted that reinforcing with more than two plies is not efficient. Figure 4(e) shows the load-displacement curves with the effect of the shear key and U strip located near the supports when the specimens have one ply along the overall length and the other ply along the center half-length of the specimen. Namely, after reinforcing with one ply along the overall length and the other ply along the center half-length, the specimen with U strips at 30 mm from both supports compared to the specimen with shear keys at 50 mm from supports. It is noted that the ductility of both specimens is similar, but the maximum load of the specimen with a U strip is smaller. Table 3 shows the maximum load values and the corresponding displacement values. It shows a significant increase for specimen number CF1.5-SE. Namely, the influence of the U strip was not more significant than that of the shear key. Furthermore, the second ply placed along the center half-length of the specimen does not show any apparent improvement.

Table 3 Test results for loads and displacements Maximum value Specimen No.

Load (kN) Displacement (mm) CF 49.872 27.065

CF1 61.603 11.164 CF1-SE 68.453 14.258 CF1-SC 47.589 12.610

CF2 72.687 9.408 CF2-SE 87.024 12.285

CF1.5-SE 88.817 13.026 CF1.5.5-SE 87.847 9.444

CF1.5-U 80.997 12.718 CONCLUSIONS Based on the discussion of the test results for the flexural behavior of RC members confined with CFS, the following conclusions are made: (1) The behavior of a beam where shear keys are located near the supports and with CFS of one ply along the

overall length and the other ply along the center half-length is the most improved. Where the influence of the U strip was not significant than that of shear key.

(2) The ductility of the beams strengthened with CFS, before failure, improved. When the specimen failed, however, it showed more brittle behavior than that of unstrengthened beams.

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FURTHER EXPERIMENT In this study, the U strip was bonded after reinforcing with one ply along the overall length. However, Tan (1999) has been suggested that the FRP for shear strengthening should be applied first. Accordingly, authors have a mind that there can be different results and have a plan to evaluate the effect of bonding sequence on the flexural behavior of CFS strengthened RC beams. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by Carecon Company Inc. and Chung Cheong University of South Korea. The contributions of Hong-Shin Kwak and students at Chung Cheong University are greatly appreciated during the experimental process. REFERENCES Arduini, M. and Nanni, A. (1997). “Behavior of pre-cracked RC beams strengthened with carbon FRP sheets”,

Journal of Composites in Construction, 1(2), 63-70. Choi, K.S., You, Y.C., Lee, J.Y., and Kim, K.H. (2003). “An experimental study to prevent debonding failure

on RC beams strengthened by GFRP (glass fiber reinforced polymers)”, Proceedings of the Korea Concrete Institute, 15(1), 531-536 (in Korean).

Den Uijl, J.A. (1991). “Mechanical properties of ARAPREE”, Part 4, Creep and stress-rupture. Report 25-87-31 for HBG N.V. and AKZO, Stevin Laboratory, TU Delft, The Netherlands.

Hau, K.M. (1999). “Experiments on concrete beams strengthened by bonding fiber reinforced plastic sheets”, Master of Science in Civil Engineering Thesis, The Hong Kong Polytechnic University.

Jones, R., Swamy, R.N., Bloxham, J. and Bouderbalah, A. (1980). “Composite behavior of concrete beams with epoxy bonded reinforcement”, The International Journal of Cement Composites 2, (2), 91-107.

Lees, J.M. and Burgoyne, C.J. (1999). “Experimental study of influence of bond on flexural behavior of concrete beams pretensioned with aramid fiber reinforced plastics”, ACI Structural Journal, 96(3), 377-385.

Malek, A.M. and Saadatmanesh, H. (1996). “Physical and mechanical properties of typical fiber and resins”, Proceedings of the First International Conference on Composites in the Infrastructure, University of Arizona, Tucson, 15-17.

Mander, J.B., Priestley, M.J.N., and Park, R. (1988a). “Theoretical stress-strain model for confined concrete”, Journal of Structural Engineering, ASCE, 114(8), 1804-1825.

Mander, J.B., Priestley, M.J.N., and Park, R. (1988b). “Observed stress-strain behavior of confined concrete”, Journal of Structural Engineering, ASCE, 114(8), 1827-1849.

Sen, R., Spillett, K., and Shahawy, M. (1994). “Fabrication of aramid and carbon fiber pretensioned beams”, Concrete International, 16(6), 45-47.

Swamy, R.N., Jones, R. and Bloxham, J.W. (1980). “Structural behavior of reinforced concrete beams strengthened by epoxy-bonded plates”, The Structural Engineer, 65A(2), 59-68.

Tan, K.H. (1999). “Towards a cost-effective application of FRP reinforcement in structural rehabilitation”, Proceedings of the Seventh East Asia-Pacific Conference on Structural Engineering & Construction, Kochi, Japan, 65-73.

Triantafillou, N.P. (1992). “Strengthening of RC beams with epoxy-bonded fiber-composite materials”, Materials and Structures, 25, 201-211.