STRENGTHENING EFFECTS OF BONDING AFRP PLATE ON SHEAR …

The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015)

Joint Conference, 14-16 December 2015, Nanjing, China

STRENGTHENING EFFECTS OF BONDING AFRP PLATE ON SHEAR

LOAD-CARRYING CAPACITY OF RC BEAMS FOR SUBMERGED APPLICATION

Yusuke Kurihashi1, Hiroshi Mikami2, Masato Komuro3 and Norimitsu Kishi4

1 Muroran Institute of Technology 27-1 Mizumoto, Muroran, 050-8585, Japan

Email: [email protected]

2 Sumitomo-Mitsui construction Co., Ltd. 518-1 Komagi, Nagareyama, 270-0132, Japan

Email: [email protected]

3 Muroran Institute of Technology 27-1 Mizumoto, Muroran, 050-8585, Japan Email: [email protected]

4 National Institute of Technology, Kushiro College

2-32-1 Otanoshike-nishi, Kushiro, 084-0916, Japan Email: [email protected]

Keywords: RC beam, AFRP plate, Shear strengthening, Submerged AFRP plate bonding method

ABSTRACT

In this study, in order to investigate shear strengthening effect of the AFRP plate bonding method for submerged RC structures, four-point static loading tests of the RC beams were conducted taking bonding application way (in the air or water) and width of the AFRP plate/sheet as variables. From this study, following results were obtained: 1) shear load-carrying capacity of the RC beams can be upgraded applying submerged AFRP plate bonding method proposed here similarly to the case of AFRP sheet bonding in the air; 2) therefore those failure mode can be shifted from the shear failure type to the flexural one; and 3) the strengthening effect due to the submerged bonding of the AFRP plate can be evaluated by means of the conventional design formula with some safety margin. 1 INTRODUCTION

Recently, in Japan, in order to secure safety of the infrastructures for occurrence of a huge earthquake, many seismic strengthening works for existing Reinforced Concrete (RC) structures have been conducted. Until now, Fiber Reinforced Polymer sheet (FRPs) bonding method has been widely used as one kind of the upgrading methods. Usually, a strengthening work was conducted under the dry conditions, even if the structures were constructed under the water. In the case of strengthening the submerged RC structures, surroundings of the structures were dried by pumping out the water after enclosing those with sheet piles. Therefore, strengthening work for the submerged structures will result in a higher cost comparing with that for the structures in the air. As a result, the seismic strengthening works for the existing submerged RC bridge piers have not been progressed as much as expected.

To improve this kind of drawback as much as possible, authors have proposed an FRP plate (FRPp) bonding methods. If the proposed methods were surely established, the RC bridge piers constructed in river and/or sea might be effectively strengthened without any enclosing and drying works. The works for those can be too economical.

Yusuke Kurihashi, Hiroshi Mikami, Masato Komuro and Norimitsu Kishi

From this point of view, the flexural strengthening effects of the proposed methods for submerged RC structures with Aramid FRPp (AFRPp) were investigated conducting out the static four-point loading tests [1]. In these experiments, following results were obtained: 1) the bonding capacity between AFRPp and the concrete surface can be effectively improved by applying a sand coating method to the bonding surface of the AFRPp; 2) the AFRPp may be debonded due to a peeling action of the critical diagonal crack developed in the lower concrete cover near the loading points; and 3) the failure mode of the RC beams strengthened by means of the submerged AFRPp bonding method may be similar to that in the case of strengthened in the air.

However, the shear strengthening effect of the proposed method has never been investigated yet. In this paper, in order to establish a rational seismic strengthening procedure for existing RC structures constructed in river/sea by means of the proposed method, four-point static loading tests for shear strengthened RC beams with proposed method were conducted taking bonding application way (in the air or water) and width of the AFRP plate/sheet as variables. 2 EXPERIMENTAL OVERVIEW

2.1 Specimen

In total seven specimens listed in Table 1 were used in this study, in which bonding application way and width of the AFRP plate/sheet were taken as variables. The nominal name of the beam was designated using bonding application way (A: in the air, and W: in the water), and width (mm) of AFRP plate/sheet. A beam N means a not-strengthened beam. Design values of the load-carrying flexural and shear capacities were calculated following Japanese Concrete Standard with material properties of concrete, rebar, and AFRP plate/sheet listed later. Here, a design value for the shear load-carrying capacity of the beam due to winding AFRP plate/sheet VAF was evaluated following Equation (1), where Aw is cross-sectional area of AFRP plate/sheet, fwyd is tensile strength of AFRP plate/sheet, is angle formed between AFRP plate/sheet and the member axis, z is a length of lever arm (= d/1.15,

Figure 1 Layouts of the reinforcements and the AFRP plate/sheet

Table 1 List of specimens.

Spec- imen

Bonding application

way

Width of AFRP

plate/sheet (mm)

Design flexural capacity

(1) (kN)

Design shear capacity (kN) Shear bending capacity ratio

(2)/(1) 2 × Vc 2 × VAF Total

(2)

N - - 100.4 59.8 - 59.8 0.60 A-20 Air 20 26.6 86.4 0.86 A-40 40 53.1 113 1.12 A-60 60 79.7 139 1.39 W-20 Water 20 26.6 86.4 0.86 W-40 40 53.1 113 1.12 W-60 60 79.7 139 1.39



d is effective depth), and s is length of intervals of winding AFRP plate/sheet.

VAF = Aw fwyd (sinα + cosα) z/s (1)

Layouts of rebar and the AFRP plate/sheet are shown in Fig.1. The beams have a rectangular cross section of 200 × 150 mm (height × width) and a clear span of 1.7 m. Steel deformed rebar with nominal diameter = 19 mm was placed at the upper and lower edges. Stirrups with nominal diameter = 6 mm were placed at intervals of 50 mm except region of the right-hand side of the shear span (hereinafter, test span). Each corner of the section was chamfered with 10 mm in radius. AFRP plate/sheet was winded with intervals of 100 mm in the region of the test span without stirrups as shown in Fig.2. The nominal tensile strength of the AFRP plate/sheet was 392 kN/m as listed in Table 2.

2.2 Mechanical property of adhesives for a submerged application

The mechanical properties of two kinds of adhesives for a submerged application used in this study are listed in Table 3. Both adhesives were of the epoxy resin type that can be used mixing a base resin and a hardening agent. In this study, the putty type adhesive was used for bonding between concrete surface and AFRPp. On the other hand, the liquid type adhesive was used for treating lap joint of AFRPp at the upper side of the section as shown in Fig.1.

Bonding capacity of the putty type adhesive was evaluated referring to the Japanese guideline for evaluating the pull-off strength of continuous fiber sheets with concrete [2] in this study. As a result, it was observed that test was terminated with the tensile failure of concrete before AFRPp being debonded in all cases. The average strength in the pull-off test was 2.6 MPa. This value is greater than the reference value (= 1.5 MPa) for repairing materials for existing concrete structures [3]. Thus, it is confirmed that adhesive used in this study was sufficient bonding capacity in the case of submerged application.

2.3 Outline of submerged bonding method

Bonding procedures for proposed submerged bonding method were as follows: 1) The AFRPp was fabricated impregnating epoxy-resin into AFRPs and then curing; 2) Bonding surface of the RC beam was grinded using disk sander to remove laitance and to

improve the bonding capacity, and then the beam was soaked in the water; 3) The putty type adhesive was prepared mixing a base resin and a hardening argent, and was

formed into a flat plate shape with about 4 mm thickness; 4) The adhesive was applied to one side of the AFRPp, and was pressed to ensure bonding

capacity to the plate; 5) The AFRPp with the adhesive were winded around the predetermined positions of the RC

beam as shown in Fig. 1 and was pressed for perfect bonding to the RC beam;

Table 2 Mechanical properties of AFRP plate/sheet.

Mass (g/m2)

Nominal tensile strength per unit

width (kN/m)

Thickness (mm)

Tensile strength (GPa)

Elastic modulus

(GPa)

Ultimate elongation (%)

280 392 0.193 2.06 118 1.75

Table 3 Mechanical properties of submerged adhesives.

Kind of adhesive Material property Nominal value (MPa) Evaluating method Putty type adhesive

Compressive strength 53.0 JIS K - 6911 Flexural strength 32.4 JIS K - 6911 Tensile strength 15.0 JIS K - 6911

Liquid type adhesive

Compressive strength 40.0 JIS K - 6911 Flexural strength 35.0 JIS K - 6911


6) After five days of curing in the water, the reinforced RC beam was taken out from the water; and

7) Strain gauges were glued on the AFPRp and the RC beam was placed on the testing apparatus.

On the other hand, in the case of AFRPs applying in the air, AFRPs were bonded with widely-used epoxy adhesive.

2.4 Measuring items and material properties

Static four-point loading tests were conducted using a hydraulic jack with 500 kN loading capacity. The measuring items were the surcharge load, the mid-span deflection (hereinafter, deflection), and the axial strain distribution of the AFRP plate/sheet. Development of cracks occurred in the test span was recorded using digital cameras. The material properties of the AFRP plate/sheet are listed in Table 3. At the commencement of this experiment, average compressive strength of concrete was 30.0 MPa. Yield strength of the main rebar was 395 MPa.

3 EXPERIMENTAL RESULTS

3.1 Load-deflection curve

Comparisons of the relationship between shear force and deflection are shown in Fig.2. In this study, assuming cross-section remaining plane during bending, a multi-section method was employed to numerically evaluate the load-carrying behaviour of the beams. Also, the strain of concrete reaching the compressive failure was assumed to be 3,500 μ based on the Japanese Concrete Standards [4]. Confining effect of AFRP plate/sheet was ignored. From this figure, it is observed that the shear force for beam N without strengthening is linearly increased up to 40 kN, after that the force was suddenly decreased. In this experiment, since the shear crack occurred in the test span was widely opened, the beam was failed with the shear failure mode.

On the other hand, in all cases of shear strengthened beams, the shear force was increased even after reaching 40 kN. After the shear force reaching 50 to 55 kN, the flexural stiffness of the beam was drastically decreased. This means that the main rebar was yielded. From these results, it is seen that the shear capacity of the RC beam can be upgraded by means of the proposed submerged AFRPp bonding method, and its failure mode can be shifted from the shear failure type to the flexural one.

Here, beam W-20 was failed with the flexural failure mode even though the shear-bending capacity ratio evaluated using the design values was 0.86 as listed in Table 1. This means that upgrading effect of the AFRP plate bonding method for submerged structures can be evaluated by means of the conventional design formula of Eq. (1) with some safety margin.

Figure 2 Comparisons of relationship between shear force and deflection.



3.2 Crack patterns

Crack patterns developed in the side-surface of the test span of the beams at maximum loading point are shown in Photo 1. From these photos, it is seen that in the case of beam N, one typical arch-shaped crack pattern was formed from the loading point to around the lower rebar near the supporting point, and then the crack was developed horizontally toward the supporting point. In this loading test, this crack was suddenly opened after reaching the maximum loading point.

On the other hand, in the cases of strengthened RC beams, the cracks were occurred from the loading point similarly to the case of beam N, however length and/or width of the cracks tend to be decreased with increasing in the width of AFRPp. This means that the larger the strengthening volume of AFRP plate/sheet is, the greater the shear strengthening effect is, irrespective of bonding application being treated in the air or water. Also, AFRP plate/sheet did not rupture in this experiment.

3.3 Strain distribution of AFRP plate/sheet

Examples of the relationships between strain of AFRP plate/sheet and deflection of the beam are shown in Fig. 3. Here, the results for the cases of beams A/W-40 are shown. In these figures, locations of the winding AFRP plate/sheet were designated from A-1 to A-5 in turns from near the loading point (see Fig. 4). Strain distribution in each AFRP plate/sheet was obtained connecting maximum values occurred in the plate/sheet. From these results, it is observed that the strains obtained in all AFRP

Photo 1 Crack patterns developed in test span of beam at maximum loading point

Figure 3 Examples of relationships between strain of AFRP plate/sheet and mid-span deflection.


plate/sheet were drastically increased at the deflection reaching around 4 - 5 mm. After that, the strains have never increased in spite of the deflection being increased. This means that the shear failure may be restrained due to winding AFRP plate/sheet even if the diagonal cracks were occurred, and then main rebar was yielded.

Magnitude of the strains were quite different among the locations of A-1~5 for beam A/W-40. This implies that the strains in the plate/sheet may be strongly influenced by the location of the crack developing. Also, the strains in the case of beam W-40 tend to be smaller than those in the case of beam A-40. This means that the AFRPp applied in the water may be easier to be debonded than the AFRPs applied in the air.

Figure 4 shows examples of the strain distributions of AFRP plate/sheet at the rebar yielding point for the cases of beams A/W-40. From these figure, it is observed that all strains were less than 0.5%. Comparing the strain distributions with the crack patterns shown in Photo 1, it is seen that the locations of large strain being occurred may be similar to those of the typical crack opening.

From these results, it can be confirmed that opening of the diagonal shear cracks can be effectively restrained due to winding AFRP plate/sheet, irrespective of bonding application in the air or water.

4 CONCLUSIONS

1) Applying submerged AFRP plate bonding method, the shear load-carrying capacity of the RC beams can be upgraded and those failure mode can be shifted from the shear failure type to the flexural one similarly to the case of applying AFRP sheet bonding in the air;

2) The larger the strengthening volume of AFRP plate/sheet is, the greater the shear strengthening effect is, irrespective of bonding application in the air or water; and

3) Strengthening effect of AFRP plate bonding method for submerged structures can be evaluated by means of a conventional design formula with some safety margin.

REFERENCES

[1] Kurihashi Y., Mikami H., Komuro M., and Kishi N. Strengthening effects of bonding AFRP plate on flexural capacity of RC beams for submerged application, Proceedings of EASEC13, Sapporo, Japan, 2013

[2] Japan Society of Civil Engineering. Recommendations for Upgrading of Concrete Structures with Use of Continuous Fiber Sheets, JSCE, 2001.

[3] Japan Society of Civil Engineering. Recommendations for Shotcreting, JSCE, 2006. [4] Japan Society of Civil Engineering. Standard Specifications for Concrete Structures -2002

"Structural Performance Verification" JSCE Guidelines for Concrete No.3, 2005

Figure 4 Examples of strain distributions of AFRP plate/sheet at rebar yielding point

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