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NEAR SURFACE MOUNTED REINFORCEMENT FOR SHEAR STRENGTHENING OF CONCRETE ELEMENTS

P. Sabol1 - S. Priganc2

Abstract: This paper presents the results of experimental research focused on the shear strengthening of reinforced concrete structures with NSM - Near surface mounted reinforcement. In recent years, research in this area focuses on combination of epoxy and FRP (fiber reinforced polymer) materials suitable for strengthening concrete elements. A traditional material -stainless steel- was also used in this study but in non-traditional T- cross section in terms of strengthening of concrete elements.

Keywords: Fiber reinforced polymer (FRP), materials, externally bonded reinforcement (EBR)

1 INTRODUCTION

In practice, it is often required to increase or restore the resistance of reinforced concrete structures. There are several quick and effective solutions for strengthening that can be classified into two groups. In first one EBR (externally bonded reinforcement) method is reinforcement glued to the surface of strengthened members. The second method, NSM involves reinforcement embedded into the glue filled grooves cut out within the cover layer of strengthened members. Both methods use modern materials such as epoxy adhesives and reinforcement from the fiber reinforced polymers. The decisive factor for effective strengthening is full transfer of stresses between the additional reinforcement and concrete elements. Stresses transfer efficiency increases with enlarging of the surface involved in this transfer. Based on these facts the idea was developed (Sabol 2014) to take advantage of both methods where proposed cross-section of reinforcement with T shape maximizes contact with surface bonded parts while at the same time embedded web of in the groove allows better anchoring of reinforcement. Another aim of the above mentioned research was to verify use of commercially available material –

stainless steel, the characteristics of which can compete with expensive modern materials mainly on price, corrosion resistance and stiffness.

2 RESEARCH PARAMETERS

2.1 Concrete beams

The beams dimensions 150x250x1500 mm and design of reinforcement determined their failure in shear. Beams were reinforced only with longitudinal reinforcement placed on the bottom of the beam in two layers with a total of 6 pieces with a diameter of 12 mm and concrete cover of 20 mm (Figure 1). Reinforcing bars were made of steel B420 (fyk = 420 MPa, ft = 453.6 MPa, Es = 200 GPa). Two bars of 8 mm diameter of steel were placed on top of the beam fixed at the support ends by a stirrup, diameter of 6mm. Strength characteristics of concrete beams on the day of testing were fck, cyl = 29.12 MPa, fctm = 2.89MPa and Ec = 33.42 GPa.

26

36

24

164

20

2626

250

49 49

150

20

cover 20 mm

1 φ12 - B420B 2 φ8 - B420B 3 φ6 - B420B

1

2

32 3

1

100650

1500

A

B - B

A

24

20

20

37 37

150

20

1

2

A - A

BB

Figure 1 Beam reinforcement arrangement

INTERNATIONAL JOURNAL OF INTERDISCIPLINARITY IN THE ORY AND PRACTICE

ITPB - NR.: 8, YEAR: 2015 – (ISSN 2344 - 2409)

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2.2 NSM reinforcement, grooves and adhesive

For additional reinforcement we used two different kinds of materials, moreover reinforcing bars had different cross-section. The first one was stainless steel (grade 1.4301) hereinafter referred to as S.STEEL. This reinforcement had T cross section with dimensions 20x20x4 mm. The T cross section was connected to concrete element through the web and the underside of the flange (Figure 2). S.STEEL reinforcement had tensile strength Rm = 649 MPa, nominal yield strength Rp0.2 = 412 MPa and modulus of elasticity Es = 200 GPa.

The second type of reinforcement was FRP strip with carbon fibers CarboDur S512 from Sika Company, hereinafter referred to as CFRP. This reinforcement had a square cross-section with dimensions 1.2x14 mm obtained by longitudinal cutting of 50 mm wide strip. CFRP reinforcement had tensile strength ft = 3100 MPa and modulus of elasticity Ef = 165 GPa, these data were declared by the manufacturer.

Structural adhesive Sikadur 30 from Sika Company was used for bonding of additional reinforcement. Hardened glue, after 24 hours at 23° C, had tensile strength ftg = 22.5 MPa and modulus of elasticity Eg = 9.6 GPa. These data were declared by the manufacturer. Grooves for embedding additional reinforcements were created by cutting, using an angle grinder and diamond cutting disc. Dimensions of the grooves had been derived from the dimensions of the additional reinforcement taking into account the requirements for the adhesive thickness of approximately 1 mm. In both cases the groove

depth was 16 mm, whereas the thickness of the concrete cover was 20 mm (Figure 2).

1.2 1.2 1.2

1416

1.2

1 14

161

20

3.6

Figure 2 Dimensions of the grooves and shape of additional reinforcement

2.3 Scheme of placing NSM reinforcement

Concrete beams with a deficit of shear resistance were strengthened shear region (Figure3). Series of beams consisted of four species, differing in strengthening ratio of the shear field with a width 450 mm given by a load and supporting elements of the beam. The beam with an indication CON was not strengthened and served as a reference sample. The beam with an indication CFRP 150 was strengthened by CFRP reinforcement at axial distance of 150 mm with a strengthening ratio ρadd = 0.0015, similarly the beam S.STEEL 150 was strengthened by T sections S.STEEL reinforcement with a strengthening ratio ρadd = 0.0130 and beam S. STEEL 112 was strengthened by T sections S.STEEL at axial distance 112.5 mm with a strengthening ratio ρadd = 0.0174.

100 450 400 450 100

250

1500

100 400 100

250

1500

100 400 100

250

1500

150 150 150150 150 150

112.5 112.5 112.5 112.5 112.5112.5112.5112.5

F/2 F/2

F/2 F/2

F/2 F/2

CON(control)

S.STEEL 112(strengthened)

S.STEEL 150or CFRP 150(strengthened)

Figure 3 Arrangement of additional reinforcement

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3 TESTING OF BEAMS

3.1 Test settings and equipment

Testing of beams was carried out under four-point bending (Figure 3) using pressing machine DrMB300. Beam was placed on the steel supports and the load was applied over the steel plates to avoid local damage of the beam. The load was applied in steps 5, respectively10 kN in cycles of 2 to 3 minutes, during which deflection and cracks formation were recorded. Deflections were measured with a dial gauge indicator located on the bottom edge of the beam at its mid span. Crack width was measured by using a standard foil crack width gauge and their formation was captured by using non-contact measurement by photogrammetric system GOM Aramis. With respect to available version GOM Aramis allows monitoring of the area of 210x170 mm (Figure 4). Illustration of scheme of cracks (Figure 10) was prepared based on detailed photos (Figure 6-9) taken during the test.

Figure 4 Non-contact measurement by photogrammetric system GOM Aramis

3.2 Results

Strengthening effect was reflected in the significant increase of strength and stiffness of the tested beams (Figure 5). Failure in shear occurred in all cases. Reference beam CON failed at overall load force 126.41 kN increase of resistance in strengthened beams was almost twice. The CFRP 150 beam failed at load force 216.48 kN, S.STEEL beam 150 at load force 205,41 kN and S.STEEL beam 112 at load force 224.13 kN. Beside to increasing of resistance of beams was also effect of strengthening reflected in the width of CSC (critical shear crack). CSC was the first shear crack formed on the beams particularly crucial to the failure of the beam. In all cases CSC was formed between the second and third rod of additional reinforcement (Figure 7-9).

deflection at midspan w [mm]

CFRP 150

S.STEEL 150

S.STEEL 112

CON

0 1 2 3 4 5 60

20

40

60

80

100

120

140

160

180

200

220

240

120 kN

240 kN

240 kN

220 kN

load

forc

e F

[kN

]

Figure 5 Load-deflection curves

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Figure 6 Beam CON after failure

Figure 7 Beam CFRP 150 after failure

Figure 8 Beam S.STEEL 150 after failure

Figure 9 Beam S.STEEL 112 after failure

CON

F/2 F/2

34°

CFRP150

1234

F/2 F/2

40°

S.STEEL150

1234

F/2 F/2

52°

S.STEEL112

12345

F/2 F/2

56°

Figure 10 System of cracks after failure of beams For beam CFRP 150 width of CSC 1.0 mm was recorded, for the other two beams strengthened with S.STEEL reinforcement width of CSC recorded was only 0.4 mm (Figure 11). Maximum width of CSC was found in the place its formation in about half of shear span beam at half height. Slope of CSC in the control beam was the lowest namely 34°, in the case of CFRP 150 beam was slope of

CSC 40°. In case of beams with S.STEEL reinforcement with increased strengthening ratio increased also slope of CSC to 52° and 56° respectively (Figure 10). In the case of CFRP 150 beam was captured by GOM Aramis eye invisible branching of the CDC crossing the NSM reinforcement (Figure 12).

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width of CSC wk [mm]

defle

ctio

n at

mid

span

w [m

m]

CFRP 150

S.STEEL 150

S.STEEL 112

00

0,2 0,4 0,6 0,8 1,0

1

2

3

4

5

6

Figure 11 Development of crack width of CSC in strengthened beams

Figure 12 Branching of the CDC in NSM system with CFRP reinforcement

Together with the significant increase in

beam deflection was an indication of failure, also pulling of additional reinforcement. Slip or debonding of reinforcement did not occur in this test (Figure 13-14).

Figure 13 Pulling of CFRP and S.STEEL reinforcement

Figure 14 Pulling of S.STEEL reinforcement

Summary of above mentioned data is presented in the following Table 1.

Table 1 Summary of the results

F – maximum load by the failure of beam w – deflection at midspan by the failure of beam ∆F– increasing of the load capacity ρadd –strengthening ratio wk – maximum crack (CSC) width α – angle of slope CSC

4 CONCLUSIONS

The results of the laboratory tests have shown high efficiency of the NSM strengthening system, manifested by significant increase in strength and stiffness of strengthened beams as well as the restrictions in widths of CSC at failure of the beams. Generally, it can be said that the use of stainless steel reinforcement with T-cross section is more preferable as the increase of resistance ranged from 83 to 100%, as well as width of CSC was

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from only 0.4 mm. Slip or debonding of reinforcement did not occur in this test and the tensile strength of concrete seems to be the weakest element of strengthening system.

REFERENCES

[1] Sabol, P. 2014. Stress and strain of strengthened concrete members at elevated temperatures, Dissertation thesis, Kosice: SvF TU Kosice.

AUTHORS ADDRESSES

1 P. Sabol

Technical University of Košice, Košice, Slovakia 2 S. Priganc

Technical University of Košice, Košice, Slovakia