Shear Strengthening of Concrete Members Using NSM Method

Procedia Engineering 65 ( 2013 ) 364 – 369

Available online at www.sciencedirect.com

1877-7058 © 2013 The Authors. Published by Elsevier Ltd.Selection and peer-review under responsibility of University of Zilina, Faculty of Civil Engineering, Department of Structures and Bridgesdoi: 10.1016/j.proeng.2013.09.056

ScienceDirect

Concrete and Concrete Structures 2013 Conference

Shear Strengthening of Concrete Members Using NSM Method Peter Sabola,*, Sergej Priganca

aDepartment of Concrete and Mansory Structures, Institute of Structural Engineering, Faculty of Civil Engineering, Technical University of

Abstract

This article describes the shear strengthening of reinforced concrete structures with one of the latest and most promising method: NSM - Near surface mounted reinforcement. Research in this area started only a few years ago with a combination of epoxy and FRP (fiber reinforced polymer) materials suitable for strengthening concrete elements. In this paper main components of NSM strengthening , design approaches and also numerical models of strengthened concrete beams are also mentioned.

2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of University of Zilina, Faculty of Civil Engineering, Department of Structures and Bridges.

Keywords: Concrete, Shear strengthening, Near surface mounted (NSM), numerical analysis;

1. Introduction

During the durability of the structures some situations can occur which are not considered in the initial design. It can be intended, for example changes in using of the structure and this can lead to a change in load, or it can be a special intervention for example fires, natural disasters or accidents. These situations can result in to stage when a qualified intervention as the rehabilitation and strengthening of structure is necessary. The reconstruction is a complex process that requires professional approach. This process begins with the diagnostics of structure, and continues by design and application of strengthening system. Over the past decade, extensive research has been

* Tel.: +421 55 602 4318

E-mail address: [email protected]

© 2013 The Authors. Published by Elsevier Ltd.Selection and peer-review under responsibility of University of Zilina, Faculty of Civil Engineering, Department of Structures and Bridges

365 Peter Sabol and Sergej Priganc / Procedia Engineering 65 ( 2013 ) 364 – 369

conducted on the strengthening of concrete structures using externally bonded reinforcement (EBR). More recently, NSM FRP reinforcement has attracted an increasing amount of research as well as practical application. In the NSM method, grooves are first cut into the concrete cover of a concrete element and the reinforcement is bonded therein

ased on a limited number of research projects with limited scope. Therefore, this factor negatively affects the production of complex design guidelines and restricts the use of new materials and methods in practice.

2. Shear Strengthening of Concrete structures

Strengthening methods that are based on the principle of retrofitting reinforcement can be divided into two groups, the NSM and EBR (Fig. 1). These methods in conjunction with FRP materials and epoxy adhesives brought an effective way of strengthening concrete structures.

a b c d e

Fig. 1. Examples of Strengthening systems, a - EBR; c,d,e - NSM ; b- combination.

Early applications were realized in the last century (NSM since 1948 [1]; EBR since 1967 [2]), where used materials correspond to this time. Reinforcing elements began as common steel, stainless steel and later by FRP composites, for gluing were initially used cement mortar later epoxy grout and currently mainly epoxy adhesives or modified cement pastes are used. With the arrival of FRP materials first research studies began to appear dealing with the application of these materials for the strengthening of concrete elements, respectively structures. In comparison to the EBR technology the NSM method is preferable in the possibility of a better anchorage of reinforcement in the grooves. NSM reinforcement is less prone to debonding from to concrete substrate; moreover, the reinforcement embedded in grooves is protected against corrosion and UV degradation, high temperatures or vandalism. NSM reinforcement can be more easily pre-stressed.

3. NSM strengthening system

The NSM technique involves the embedment of NSM bars - of circular, square or rectangular cross-section into grooves opened on the concrete surface. The NSM reinforcement is bonded to concrete using groove filler. This section describes three components of the NSM strengthening system: groove filler, groove dimensions and reinforcement.

3.1. Groove filler

The groove filler is the medium for the transport of stresses between NSM reinforcement and the concrete. In terms of structural behaviour, the tensile and shear strength are more relevant mechanical properties. The most common and best performing groove filler is a two-component epoxy. The use of cement paste or mortar in place of epoxy as the groove filler has recently been explored in an attempt to lower the material cost, reduce the hazard to workers, minimize the environmental impact and achieve better resistance to high temperatures and improved thermal compatibility with the concrete substrate.

3.2. Groove dimensions

For round bars, De Lorenzis [3], based on results of bond tests with square grooves (bg= hg

k=bg/db, proposed a minimum value of 1.5 for k for smooth or lightly sand-blasted bars and a minimum value of 2.0 for deformed bars. Parretti and Nanni [4] suggested that both bg and hg should be not less than 1.5db. For NSM


strips, Blaschko [5] suggested that the depth and width of the cut groove should be about 3 mm larger than the height and thickness of the corresponding FRP strip respectively, so to obtain an adhesive layer thickness of about 1-2 mm. Also for NSM strips, Parretti and Nanni [4] recommended that the minimum width of a groove should be not less than 3tf and the minimum depth not less than1.5hf. Groove dimensions for NSM strengthening systems are illustrated in Fig. 2.

CONCRETE REINFORCEMENT ADHESIVE

hg

bg

db

bgbg

hfhf

tftf

Fig. 2. Groove dimensions for NSM strengthening systems.

3.3. Reinforcement

In last time, FRP materials are used for reinforcing or strengthening of concrete members. FRP is an anisotropic material composed at least of two components i.e. fiber and matrix which exhibit different physical and mechanical properties, FRP composite exhibit different characteristics from its components (Fig. 3) [6]. FRP may contain even other components like fillers and additives.

MPa

0

34 - 130

600 - 3000

1800 - 4900 1

2

3

1 FIBERS

2 MATRIX

3 COMPOSITE

0,4 - 4,8 >10

Fig. 3. Stress-strain diagram of FRP components.

Fibers - are reinforcement for composite and provide mechanical strength (carbon, glass, aramid, etc.). Matrix - is the glue that holds the fibers together and provide transfer of forces between them (polymers). Fillers - reduce the cost of FRP and enhance its material properties, like flammability (gypsum, quatz, etc.). Additives - can extend the usability of polymers and extend their durability (bromine, rubber, etc.).

4. Design of shear resistance strengthened concrete section

To date, most of the available design guidelines do not yet contain design models with respect to NSM contribution to the overall shear resistance of strengthened reinforced concrete element. Design codes are mainly oriented to the EBR method, like ACI 440 + ACI 318, fib Bulletin 14 + EC2, and CNR DT 200. Approaches explicitly dealing with NSM strengthening are formulation from De Lorenzis [7-8] and Nanni [9]. Shear resistance is calculated, in accordance with ACI 318-95 by superimposing the contributions provided by the concrete Vc, by the transverse steel stirrups Vs and by FRP reinforcement Vf . Contribution of Vf according to formulation in papers [7-8] is limited by reaching the limit strain in the reinforcement which allows aggregate interlock or by possibility of occurrence debondig failure mod. Recommendation in the paper [9] considers as a limit condition the stage of reaching of the limit strain in the reinforcement which allows aggregate interlocking.


5. Numerical analysis of the NSM FRP strengthened beam

In this section, a numerical analysis of the shear NSM FRP strengthened concrete beam is given. This analysis is based on an experimental project by Barros and Dias [10]. From experimental program, three beams were selected (Fig. 4): the beam A12C without any shear reinforcement, the beam A12S with shear reinforcement - steel stirrups of 6 mm, and the strengthened beam A12VL by CFRP laminates. The cross section of beams is 0.15 x 0.30 m and 1.5 m of a span length. As the groove filler an unknown epoxy resin with modulus of elasticity of 7 GPa was used; therefore, for numerical modeling epoxy resin with similar modulus of elasticity was selected: ER2035 Epoxy Resin, with tensile strength of 50 MPa, modulus of elasticity of 8 GPa and compressive strength of 140 MPa.

F/2

300

F/2 F/2 F/2

F/2 F/2

a

50 501500 (x 150)

A 12 C A 12 S

A 12 VL

150 150 150 150 300 150150150150

6 x 100 300 6 x 100

2 O 6

4 O 12

O 6

Fig. 4. Parameters of the analyzed beams.

Numerical analysis has been executed in software ATENA, developed for the nonlinear analysis of concrete structures. There were different materials considered namely for concrete and epoxy resin Cementitious2 material model was selected, for CFRP laminate Steel Von Mises 3D was used and steel rebars were modelled as discrete reinforcement using material called Reinforcement. For supports and load plates, Elastic 3D was used. For contact between components of strengthened beam, concrete/epoxy and epoxy/CFRP laminate special material Interface was used. Properties of all materials were set according to ATENA theory [12]. For numerical analysis, Hexahedra finite elements for mesh as well as Newton-Raphson method for solution were selected. Due to symmetry, only (the left) half of beams was modelled (Fig. 6).

Table 1. Numerical vs. experimental results of analyzed beams

Type of Beam

Fmax test [kN]

Fmax test [kN]

Fmax ATENA [kN]

Fmax ATENA [kN]

Fmax test / Fmax ATENA

Fmax test / FmaxATENA

A 12 C 116,5 - 129.7 - 0.898 - A 12 S 214.9 98.4 203.2 73.5 1.058 1.339

A 12 VL 235.11 118.61 212.1 82.4 1.108 1.439 Fig. 5 shows that the numerical models can produce high agreement with the test results of beams carried out by

Barros and Dias [10]. Ratio (FmaxTest / FmaxATENA) between the load-carrying capacity of beams A12C, A12S and A12VL (Tab. 1.) were 0.898, 1.058 and 1.108 respectively. Ratios ( FmaxTest / maxATENA) between profitability of shear reinforcing systems A12S and A12VL were 1.339 and 1.439 respectively. Taking the Fmax resulting from the test of A12C beam as a reference value, the steel stirrups provided the 85 increase in Fmax, while CFRP


strengthening system assured the increase 102 . Increasing of Fmax from numerical analysis by ATENA software was for A12S 56.7 and for A12VL 62.3 .

Fig. 5. Load vs. deflection curves.

Fig. 6 shows crack pattern of beams at failure reached by numerical analysis by ATENA software. The width of shear cracks is the largest for beam with any shear reinforcement A12C. In this case, cracks are generally concentrated in one region. For beam A12S, cracks are scattered in wider region. It can be seen the influence of transverse steel reinforcement. For beam A12VL, shear cracks are not so significant as in previous cases. Crack width at mid-span beam indicate that bending failure has occurred in this case.

A 12 C A 12 S A 12 VL

Fig. 6. Crack pattern of beams at failure produced by ATENA.

In the Tab. 2., there are summarized analytical, experimental and numerical results of NSM FRP shear strengthening technique. The contribution of the NSM FRP elements Vf for the shear strengthening are obtained by analytical approach by Nanni et al. [9] and De Lorenzis [7-8]. From these results, it is clear that results, obtained from this series of beams, by available formulations are more similar to results by numerical analysis if compared to results by experimental testing. The good correlation with numerical/test results is obtained by using b=16.1 MPa and f [9] formulation.

The values of the CFRP average strain f b =6.9 MPa are recommended by Nanni [9]. The value of 6.9 MPa for the b was determined from bond tests with round cross sectional FRP bars, by pullout bending tests [13]. The values of b=16.1 MPa and f

0

40

80

120

160

200

240

0 1 2 3 4 5 6 7

Forc

e [k

N]

Deflection at mid span [mm]

Force vs deflection

A 12 C TestA 12 C AtenaA 12 S TestA 12 S AtenaA 12 VL TestA 12 VL Atena


Table 2. Analytical vs. experimental vs. numerical results for NSM technique

Beam Nanni et al.1 Vfana1 [kN] Nanni et al.2 Vfana

2 [kN] De Lorenzis3 Vfana3 [kN] Vftest [kN] VfATENA [kN]

A12VL

23.9 39.6 34.2 59.3 41.2

Vftest / Vfana1 Vftest / Vfana

2 Vftest / Vfana3 Vftest /Vftest Vftest /VfATENA

2.481 1.497 1.734 - 1.439

VfATENA / Vfana1 VfATENA /Vfana

2 VfATENA /Vfana3 VfATENA /Vftest VfATENA /VfATENA

1.724 1.040 1.205 0.695 - 1 - formulation by Nanni et al. where f = 4. b = 6.9 MPa and Ef = 166 GPa 2 - formulation by Nanni et al. where f = 5. b = 16.1 MPa and Ef = 166 GPa 3 - formulation by De Lorenzis where f = 5. b = 16.1 MPa and Ef = 166 GPa

6. Conclusions

Based on the experimental research, numerical analysis and analytical approaches, it is evident that the NSM method is an effective way of shear strengthening of the concrete elements.

Results of numerical analysis carried out by using software ATENA in this study proved good agreement with the results of the experiment executed by Barros and Dias [10].

Significant reserves in the shear resistance of strengthened elements given by actual formulations suggest a need to improve the design rules for efficient design of strengthening concrete structures.

References

[1] S.O. Asplund, Journal of the American Concrete Institute, Vol. 20, No. 6, pp. 396 407.

[2] C.J. Fleming, G.E.M King, 19 RILEM International Symposium, Synthetic Resins in Building Construction, Paris, pp.75-92.

[3] L. De Lorenzis, Strengthening of RC structures with near surface mounted FRP rods. PhD Thesis, Department of Innovation Engineering, University of Lecce, Italy, 2002.

[4] R. Parretti, A. Nanni, Strengthening of RC members using near-surface mounted FRP composites: design overview. Advances in Structural Engeneering, 2004;7(6):469 83.

[5] M. Blaschko, Bond behaviour of CFRP strips glued into slits. In: Proceedings FRPRCS- ; 2003. p. 205 14. [6] Reinforcing Concrete Structures with Fibre Reinforced Polymers: Design Manual No. 3, ISIS Canada, Intelligent Sensing for Innovative

Structures, University of Manitoba, 2006. 151 s. ISBN 0-9689006-6-6. [7] L. De Lorenzis, A. Nanni, -Surface Mounted Fiber-

ACI Structural Journal, Vol. 98, N.1, pp. 60-68. (2001), [8] -Surface Mounted Fiber-Reinforced Polymer Rods and Concrete in Structural

ACI Structural Journal, Vol. 99, No. 2, March-April 2002, pp. 123-132. (2002), [9] Advances

in Structural Engineering, Vol. 7, No. 4. [10] Near surface mounted CFRP laminates for shear strengthening of concrete beams, Cement and Concrete

Composites -292 [11] ening of RC Beams with Near-Surface- 7th International

Symposium on Fiber Reinforced Polymer (FRP) Reinforcement for Concrete Structures (FRP7RCS), Kansas, USA, SP-230-47, 807-824. [12] ATENA Program Documentation, Part 1, Theory, from Vladimir Cervenka, Libor Jendele and Jan Cervenka.Prague, March 14, 2012 [13] Sena-Cruz JM, Barros JAO. Bond between near-surface mounted CFRP laminate strips and concrete in structural strengthening. Journal of

Composite Construction, 2004;8(6):519 27.

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Shear Strengthening of Concrete Members Using NSM Method