SP-70s

Shear-Strengthening of Reinforced Concrete Beams Using Small-Diameter CFRP Strands

Judy M. I. Soliman, Tarek K. Hassan and Sami H. Rizkalla

SYNOPSIS: The use of Fibre Reinforced Polymer (FRP) reinforcement has been widely adopted in the construction industry to solve durability issues related to steel corrosion and also to upgrade the capacity of reinforced concrete beams. This paper presents the findings of an experimental program undertaken to examine the use of small-diameter Carbon Fiber Reinforced Polymer (CFRP) strands for shear strengthening of concrete beams. Nine concrete beams were constructed and tested to evaluate the effectiveness of the strengthening scheme. The considered parameters included the size and spacing of the CFRP strands. The research was extended to examine the feasibility of additional anchorage system to delay premature failure due to delamination of the strands. Test results revealed that the use of small-diameter CFRP strands for shear strengthening of concrete beams is simple, easy to install and efficient in increasing the shear capacity by around 23% compared to the control specimen. It was also shown that presence of the longitudinal CFRP strands enhanced the shear behavior of the beams by providing more resistance to the induced diagonal tension and delayed delamination of the strands.

Keywords: behavior, CFRP strands (CS), debonding, delamination, externally bonded, fiber reinforced polymers (FRP), shear, strengthening

Judy M. I. Soliman, Ph.D., is an Assistant Professor at the Department of Structural Engineering at Ain Shams University, Cairo, Egypt. Her research interest includes the bond characteristics of high strength and corrosive resistant steel reinforcement, and the application of advanced composite materials on structures using both cementitious materials and epoxy resins.

Tarek K. Hassan, Ph.D, is a Professor of Concrete Structures at the Department of Structural Engineering, Faculty of Eng., Ain Shams University, Cairo, Egypt. Dr. Hassan received his M.Sc. and Ph.D. from the University of Manitoba, Canada in 1998 and 2002, respectively.

ACI member Sami H. Rizkalla, Ph.D., FPCI, FACI, FASCE, FIIFC, FEIC, FCSCE, is a Distinguished Professor of Civil Engineering and Construction, director of the Constructed Facilities Laboratory, and director of the NSF Center on Integration of Composites into Infrastructure at North Carolina State University, Raleigh, NC. His research interest includes the application of advanced composite materials for structures, structural complexity, structural reliability, and science-based structural engineering.

INTRODUCTION

In the last two decades, the use of Fibre Reinforced Polymer (FRP) reinforcement has been widely adopted in the construction industry to solve durability issues related to steel corrosion and also to upgrade the capacity of reinforced concrete (RC) beams. FRP systems are lightweight, exhibit high tensile strength, are not prone to electro-chemical corrosion, are easy to install, and can be bonded to concrete substrate. These features facilitate handling and help expedite repair or construction [1-2]. The high strength-to-weight ratio of FRP composites makes them more structurally efficient than traditional strengthening materials [3]. Extensive research has shown that FRP systems improve both short and long term flexural behaviour of concrete girders. A significant amount of research has been conducted on flexural and axial strengthening but limited investigations have been conducted on the use of externally bonded FRP for shear strengthening. Several researchers discussed the shear behaviour of concrete girders strengthened with FRP systems and found that the size-effect of test specimens has little influence on the effectiveness of externally bonded FRP and thus empirical design expressions calibrated from small-scale test results should provide reasonable accuracy [4-6]. Previous researches concluded that FRP reinforcement can be bonded around the entire section, to the two sides as well as the soffit of the beam (U-shape), and to the two sides of

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the beam only in a discrete form. It was found that center-to-center spacing between FRP strips is not effective at all for spacing greater than half the effective depth of the specimen [7].

This paper presents the results of an experimental program conducted to evaluate the efficiency of small-diameter CFRP strands for shear strengthening of concrete beams. The proposed small-diameter CFRP strands, provided in sheet configuration as shown in Fi, is a promising alternative strengthening system for concrete structures. The CFRP strands (0.572 mm diameter) are stitched together leaving a gap between the strands allowing each strand to be totally covered by the epoxy adhesive, resulting in an excellent bond mechanism. A total of nine T-section concrete beams were constructed and tested at the Structural Engineering Laboratory of Ain Shams University. The beams were strengthened using small diameter CFRP strands bonded to the concrete surface in two directions to resist diagonal shear stresses. The strands were applied in different configurations and arrangements as will be explained later in this paper. The beams were designed to fail in shear to better study the interaction between the concrete and the externally bonded small-diameter CFRP strands.

EXPERIMENTAL PROGRAM

Test-specimens: The experimental program undertaken to study the effectiveness of the proposed CFRP strands in increasing the shear strength of concrete beams consisted of nine T-shaped concrete beams tested up to failure at the Structural Laboratory of Ain Shams University. All beams had a length of 2.3 m and a total depth of 400 mm. The beams were simply supported with a clear span of 2.00 m. Flange dimensions were 500 mm wide by 120 mm deep. The width of the web was 150 mm. Top reinforcement consisted of four 12 mm diameter steel bars. Six 25 mm diameter steel reinforcing bars were used as bottom reinforcement. The bars were placed on three rows with 25 mm clear spacing between rows. Transverse steel reinforcing bars of 6 mm. diameter and at spaced at 200 mm were used along the beam’s flange. Web transverse reinforcement consisted of 8 mm diameter stirrups spaced at 170 mm along the clear span of the beam. All beams were cast using the same concrete mix. The cylinder concrete compressive strength at 28 days was 40 MPa. The specimens were adequately designed to avoid flexural failure. Table 1 summarizes the mechanical properties of materials used in the current study.

Table 1- Mechanical properties of materials

Material Strength Modulus of Elasticity Ultimate strain of fibersType MPa GPa

Longitudinal Steel Yield strength 400 200 0.0018Ultimate strength 690Transverse Steel Yield strength 240 200 0.0012

Concrete Compressive strength 40 28.5 0.003Small diameter CFRP

Strands Tensile strength 2060 118 0.0175

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Fig. 1 Small-diameter CFRP strands

Shear-Strengthening of Reinforced Concrete Beams Using Small-Diameter CFRP Strands

Test setup and instrumentations: All beams were tested using a three point bending configuration. The span of the beams was kept constant at 2000 mm. The test setup allowed two shear spans of 1000 mm each. The beams were supported on a roller support at one end and a hinged support at the other. One hydraulic jack of 1500 kN capacity was used to apply the load on the top of a rigid steel spreader beam that equally distributes the load along the flange width. A total of four electrical resistance strain gages and three Linear Variable Differential Transformers (LVDTs) were used to monitor the strains and the deflections on the beams, respectively. The four electrical resistance strain gages were attached to the transverse shear reinforcements (stirrups) crossing the critical shear sections at mid-height of the effective depth (d/2). The deflections were measured using three LVDTs. The first LVDT was placed at mid span and the other two were located at quarter span on both sides. The small diameter CFRP strands were externally bonded to the concrete using a local adhesive specialized in bonded steel rebars to concrete. All test specimens were allowed to cure for at least 7 days before being placed on the reaction frame for shear testing.

Strengthening scheme: One beam was tested without strengthening and used as a control specimen. Four specimens were strengthened in shear using externally bonded CFRP strands with different spacing and width of CFRP strands as detailed in Table 2. The remaining specimens had the same shear strengthening configuration but with the addition of two longitudinal CFRP strands 30 mm wide, placed at h/3 and 2h/3, where “h” is the clear depth below the flange. The horizontal CFRP strands were added to delay delamination of the vertical strands and also to allow higher shear capacity by providing the CFRP in both vertical and horizontal directions to resist the diagonal shear stresses. Strengthening details of the test specimens are shown in Fig. 2.

Fig. 2 – Typical elevation of tested beam

Table 2- Strengthening details of test specimensBeam

IDStrip width

(mm.)Spacing between strips

(mm.)Longitudinal strips

of 30 mm.Bo N/A N/A N/A

B5II15 50 150 N/AB5II17 50 170 N/AB3II10 30 100 N/AB3II17 30 170 N/AB5#15 50 150 PRESENTB5#17 50 170 PRESENTB3#10 30 100 PRESENTB3#17 30 170 PRESENT

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TEST RESULTS AND DISCUSSION

All strengthened beams were designed to fail in shear. Shear cracks were marked as they appeared on the specimen along with the corresponding load during the test. Table 3. Presents summary of the test results along with the failure mode of the specimens.

Table 3- Summary of experimental results

GROUP Beam ID

Shear crack

initiation(kN)

Initiation of delamination

(kN)

Failure load(kN)

Max. deflection at mid-span

(mm.)

% Increase in capacity

Mode of failure

CONTROL Bo 200 --- 440 8.2 --- Shear tension failure

GROUP I

B5II15 200 375 460 9.4 5

Delamination of CFRP Strands

B5II17 200 370 450 9.7 2.3B3II10 200 300 520 9.7 18.2B3II17 200 370 510 10.3 15.9

GROUP II

B5#15 250 430 500 8.6 13.6B5#17 270 430 490 8.7 11.4B3#10 270 410 540 9.2 22.7B3#17 230 370 526 8.5 19.5

The control specimen experienced a typical shear tension failure as shown in Fig.3. with shear cracks propagating from the support to the loading point. Two shear cracks were initiated at 200 kN, one at a 45 o angle and the other crack at the loading-support angle. Cracks continued to increase in length and width towards the supports with increased load levels. The critical crack at 45o near the hinged support, lead to a brittle shear failure at an ultimate load 440 kN. The corresponding deflection at mid-span was 8.2 mm. The maximum recorded tensile strain in the stirrups at the onset of shear failure was 0.97 % at the stirrup crossing the shear crack at loading-support angle indicating yielding of the transverse reinforcement as shown in Fig. 4a. Fig. 4b depicts the load-deflection behavior of the control beam.

Fig. 3- Control beam shear cracks propagation and failure

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Fig. 4a- Microstrain readings of control beam Bo Fig. 4b– Deflection profile of control beam Bo

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Shear-Strengthening of Reinforced Concrete Beams Using Small-Diameter CFRP Strands

Beams B5II15 and B5II17 (Fig.5. and Fig.6, respectively) were strengthened with vertical CFRP strands of 50 mm wide and spaced at 150 and 170 mm, respectively. Both beams experienced delamination of the CFRP strands originating from the shear cracks. Initial signs of delamination were observed at a load level around 80% of the failure load for both specimens. Both beams have almost identical behavior compared to the control specimen. The increase in ultimate load carrying capacity was trivial and ranged from 2 to 5% compared to the control specimen as shown in Fig. 7. It shall be noted that despite the premature failure due to delamination for both specimens, the transverse steel reinforcement experienced yielding with tensile strain reaching around 1% at the onset of delamination as depicted in Figs 8 and 9.

Fig. 10 illustrates the effect of reducing the width of the CFRP strands strips from 50 mm as for B5II15 and B5II17 to 30 mm for B3II10 and B3II17 using a spacing of 100 mm, and 170 mm, respectively. It was observed that closely-spaced CFRP strands enhanced the overall behavior of the strengthened beams. Nevertheless, the controlling mode of failure was due to delamination of the CFRP strands as shown in Figs. 11 and 12. This could be attributed to loss of anchorage for this type of reinforcement as the beams were not fully wrapped. Load-deflection behavior as well as the load-tensile strain behavior are shown in Fig. 13 and 14 for specimen B3II10, which experienced the maximum increase in capacity of Group I specimens by 18%.

Fig. 5 – B5II15 shear cracks pattern propagation and failure

Fig. 6 – B5II17 shear cracks pattern propagation and failure

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Fig. 7 – Typical Load-Deflection curve for Bo, B5II15, and B5II17

Fig. 8 - Microstrain readings of B5II15

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Fig. 9 – Microstrain readings of B5II17 Fig.10 – Typical Load-Deflection curve for Bo, B3II10, and B3II17

Fig. 11 – B3II10 shear cracks pattern propagation and failure

Fig. 12 – B3II17 shear cracks pattern propagation and failure

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Fig.13 - Microstrain readings of B3II10 Fig. 14 - Microstrain readings of B3II17

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Shear-Strengthening of Reinforced Concrete Beams Using Small-Diameter CFRP Strands

Based on the observed mode of failure for Group I specimens, it was necessary to fabricate more beams using an additional anchorage system of horizontally-applied CFRP strands. Four more spesimens were construted and tested with the same configuration as that in Group I but with additional CFRP strands placed horizontally at h/3 and 2h/3. In general, presence of the two longitudinal CFRP strands delayed the initiation of the shear cracks, and allowed the strengthened beams marked with “#” to carry more load in comparison with those marked with “II”.

The first shear cracks were observed at 250 kN for B5#15 showing a 25% increase compared to the control specimen. No significant difference in behavior was observed for Group II specimens compared to Group I except for the ultimate carrying capacity. Presence of the longitudinal CFRP strands enhanced the shear behavior of the beams by providing more resistance to the induced diagonal tension and delays delamination of the strands. The ultimate load carrying capacity was found to increase by about 8% compared to Group I specimens. The overall increase in capacity compared to the control specimen ranged from 11 to 23 % depending on the size and spacing of the CFRP strands as given in Table 3. Crack pattern at failure for Group II specimens is given in Fig. 15

Fig. 15 Crack patter at failure for Group II specimens

SUMMARY AND CONCLUDING REMARKS

The current study examined the behavior of reinforced concrete beams strengthened in shear using

externally bonded small diameter CFRP strands. Nine T-section concrete beams were tested under a three point bending setup, including one control specimen. The effectiveness of the strengthening scheme was examined by varying the size and spacing of the strands. Presence of additional anchorage using horizontally-applied strands was enumerated. Based on the results of the experimental program, the following conclusions could be drawn:

1. Use of small-diameter CFRP strands for shear strengthening of concrete beams is simple, easy to install and efficient in increasing the shear capacity by around 23% compared to the control specimen.

2. Delamination of the strands was the controlling mode of failure for all the strengthened specimens due to insufficient anchorage of the strands along the beam.

3. Presence of horizontally-applied CFRP strands at both one third and two thirds the beam height delayed delmination failure mode by about 8%.

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4. The effectiveness of small-diameter CFRP strengthening system increased by reducing the spacing between the strands.

REFERENCES

[1] Triantafillou, T. C. and Plevris, N. 1992. Strengthening of RC beams with epoxy-bonded fibre-composite materials, Materials and Structures, 25, 201-211.

[2] Soliman, J. 2012. Behavior of Reinforced Concrete Beams Strengthened with Externally Bonded Fiber/Steel Reinforced Polymers and Grancrete, Ph.D Thesis, Ain Shams University, Cairo, Egypt.

[3] ACI Committee 440. 2008. Guide for The Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. ACI 440.2R-08. Farmington Hills, MI: American Concrete Institute.

[4] Kuchma, D., Ayoub, A., Mirmiran, A., Bae, S.-W., Belarbi, A., Okeil, A., and Transportation Research Board. 2011. Design of FRP Systems for Strengthening Concrete Girders in Shear. NCHRP Report 678, 120.

[5] Kang, T. H.-K , and Ary, M.I. 2012. Shear-Strengthening of Reinforced & Prestressed Concrete Beams Using FRP: Part II – Experimental Investigation. International Journal of Concrete Structures and Materials, 6(1), 49-57.

[6] Belarbia, A., and Acun, B. 2013. FRP Systems in Shear Strengthening of Reinforced Concrete Structures, Proceedings of the 11th International Conference on Modern Building Materials, Structures and Techniques, Procedia Engineering, 57, 2-8.

[7] Chen, G.M., Teng, J.G., and Chen, J.F. 2012. Process of debonding in RC beams shear-strengthened with FRP U-strips or side strips. International Journal of Solids and Structures, 49, 1266-1282.

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