4. Ultimate Strength and Deformation of Concrete Members Strengthened by Jacketing With Various Materials

8/10/2019 4. Ultimate Strength and Deformation of Concrete Members Strengthened by Jacketing With Various Materials

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The Institution of Engineers,

Malaysia

Universiti

Teknologi MARAUniversiti Malaya


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Ultimate Strength and Deformation of Concrete Members

Strengthened by Jacketing with Various Materials

UEDA Tamon

Hokkaido University

Abstract

This paper presents the generic model to predict ultimate strength and deformation of

concrete members with and without jacketing by various materials. The generic

model consists of flexural strength and deformation model and shear strength and

deformation model, which interact each other. The model can predict the post-peak

behavior including fracture of strengthening material. The model clearly indicates the

advantage of material with large fracture strain and no yielding as shear reinforcement.

1. Introduction

Jacketing is the most popular method for strengthening of existing concrete structures,

especially for seismic strengthening. Jacketing does not require much additional space, so

that the functions of existing structures would not be disturbed after being strengthened.

When appropriate material is chosen for jacketing, disturbance during execution can be

minimized.

Jacketing is a type of strengthening, in which strengthening material is externally bonded to

existing structures, however debonding, as an ultimate state, is not a main issue like other

external bonding methods. The important parameter then is the stiffness of jacketing material

at the ultimate state; either peak load or ultimate deformation. In the case of steel jacketing

and concrete jacketing with steel reinforcement, the post-yielding stiffness at the ultimate state

needs to be known. For FRP sheet jacketing, the tension fracture of FRP sheet often causes

the ultimate state because the stiffness of FRP sheet suddenly drops after its fracture.

Therefore, it is necessary to predict the strainof jacketing material at the ultimate state.


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In this paper the generic model to predict load-deformation relationship including post-peak

region is introduced. This model consists of shear and flexure strength model as well as shear

and flexure deformation model. The shear strength model is developed with the concept of

potential shear strength, which is a function of stiffness of shear and tension reinforcement,

including jacketing material, and concrete strength. The shear deformation model is derived

from the truss mechanism. The flexure model is a fiber model with consideration of

confinement effect of shear reinforcement on concrete. The feature of the generic model is

also found in consideration of interaction between shear and flexure model, such as reduction

in neutral axis depth by shear cracking and increase in flexure deformation due to tension shift

induced by truss mechanism. The model can predict the experimental data of cases with

concrete, steel and FRP jacketing.

The paper also emphasizes on advantages of fiber materials for FRP sheet, whose fracturing

strain is large, and which is a natural fiber as green technology. The generic model can

predict the ultimate points of jacketing with large-fracturing strain FRP and natural fiber

reinforced polymers.

2. Generic Model for Load-Deformation Relationship (Jirawattanasomkul, et al 2013)

2.1 Flexural strength and deformation model

To calculate the flexural strength, a section analysis is performed by dividing the section area

into a number of discrete strips, and it is assumed that plane sections remain planes at any

loading level. In this analysis, the increments in strain for the compression at the top fiber are

fixed, and the strain across the depth of the cross-section is assumed to be proportional to the

distance from the neutral axis, as shown in Figure 1.

In the flexural strength model, the enhancement of the flexural strength is a consequence of the

confined concrete stress-strain relationship. Figure 2 shows the concept of the confinement

effect. For a given flexural cross section, Eq. (1) and Eq. (2) give the force and moment

equilibrium conditions, respectively. The corresponding shear force, Vmu, is obtained using Eq.

(3).


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Figure 1 Section analysis

Figure 2 Confinement effects by reinforcement

1 1

n mP A Aci ci sj sj

i j

(1)

1 1

n mM A d A dci ci i sj sj j

i j

(2)

/V M amu (3)

where ciand sjis stress in ithconcrete layer andj

thlongitudinal reinforcement, respectively, di

and dj is distance from top fiber to the centroid of ith concrete layer and j

th steel layer,

respectively, Aci andAsjis area of i

th

concrete layer andj

th

longitudinal reinforcement, Pis axialforce (N), M is moment at the considered cross section (N-mm), ais shear span (mm), and i, j


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= 1,2,3nor m.

As expressed in Figure 1, the strain compatibility equations of the ith concrete and the j

th

longitudinal reinforcement are given by the following equations:

di

ci ct cc ct h

(4)

h dj

sj ct cc ct h

(5)

From the section analysis, the secant modulus of material (Ese, Eje, Ewe, Ece), the effective

strength of concrete (f'ce), strain at the extreme fiber (cc) and neutral axis depth (x) can be

obtained. These values are applied in the shear strength model. However, the neutral axis

depth used in the flexural strength model is not the same as that used in the shear strength

model (Figure 4) because of the shear crack opening. In fact the neutral axis depth is

calculated by Eq. (12), meaning that the assumption of the plane-sections-remains-plane cannot

be applied.

Flexural deformation is calculated by doubly integrating strain obtained by the section analysis.

The tension shift, which is explained by the truss mechanism (see Figure 3 and the section of

Shear strength and deformation model) and causes increase in strain of tension

reinforcement, increases flexural deformation.

Figure 3 Tension shift


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2.2

Shear strength and deformation model

The main limitation of the section analysis is that the effect of the shear strength behavior,

shear crack opening and reduction of the neutral axis depth, on the flexural strength is not taken

into account. The post-peak region of the load-deformation response is dominated by the

shear strength behavior. To account for this, a truss mechanism approach proposed by Sato, et

al (1994, 1996, 1997) is combined with the section analysis to predict the shear strength more

precisely.

Figure 4 illustrates the concept of the shear strength model based on the truss mechanism.

Previous experimental observations by Sato, et al (1994, 1996, 1997) showed that the shear

strength of RC columns depends significantly on the secant stiffness of the flexural and shear

reinforcements. As explained previously, the secant stiffness is obtained from the stress-strain

relationships of materials, which satisfy the compatibility and equilibrium conditions in the

flexural strength model. The shear strength capacity continuously decreases after the yielding

of the shear reinforcement, because the shear reinforcement contribution shows no further

increase. In the generic model, the shear strength model is developed after modifying the

original model by Sato, et al (1994, 1996, 1997).

Figure 4 Shear strength model by Sato, et al (1997)

The total shear strength can be expressed as a sum of the contribution of the concrete (Vc) and

the shear reinforcement (Vs+j), as shown in Eq. (6). This shear reinforcement consists of

contribution from steel shear reinforcement and jacket.


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stress. Their formula (Sato, et al 1997) indicated that the concrete compression depth (xe) is

related to the initial neutral axis depth obtained from the section analysis (xi). Anggawidjaja, et al

(2009) modified formula proposed by Sato et al. by including the influence of fiber

reinforcement on the neutral axis depth as shown in the following equation:

0.4

0.70.420.08

1000

0.12

11 1.25

1 3.2

s se

w we f fe

a Ed

e n

E Ei cc

x ee

x f

(12)

Shear deformation becomes significant after shear cracking and is obtained by the truss

mechanism approach as shown in Figure 5 (Ueda, et al 2004). The original shear deformation

model is expanded for post-yielding range of tension and shear reinforcement. The angle of

diagonal compression strut, varies as the shear deformation evolves and its expression is

obtained based on the experimental results.

Figure 5 Truss analogy for shear deformation

2.3

Verification

Steel jacketing is a more common retrofit technique than concrete jacketing. Aboutaha, et al

(1999) proved that steel jacketing shows several advantages: a smaller increase in the

cross-sectional dimensions, ease and speed of construction, lower cost of structural intervention

and interruption of use, and a smaller increase in additional stiffness to the retrofitted column.

Figure 6 demonstrates the load-deformation responses from the experiment carried out by


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3. Advantages with Large Fracture Strain Materials

The sudden breakage of FRP as reinforcement is a drawback. It indicates that the greater

fracturing strain of material is preferable to achieve better member strength. For members

subjected to high seismic effects, plastic deformation becomes quite high in so called plastic

hinge zone, meaning that material would be subjected to quite high strain. High fracturing

strain can be achieved by only material with high fracturing strain but not material with low

fracturing strain.

According to the generic model proposedby the authors group (Jirawattanasomkul et al 2013),

shear strength of reinforced concrete members depend on stiffness of both tension and shear

reinforcement. The higher the stiffness of tension and shear reinforcement is, the higher the

concrete contribution is. The higher the stiffness of shear reinforcement is, the higher the

shear reinforcement contribution is. The post-peak behavior of concrete members can be

explained as the decrease in potential shear strength (or remaining shear strength) with the

decrease in stiffness of tension and shear reinforcement (see Figure 7). Generally the less

amount of shear reinforcement gives earlier reduction in shear reinforcement stiffness, thus the

post-peak region comes earlier.

(c) Specimen SP3 (d) Specimen SP4

Figure 7 Load-deformation relations (Anggawiddjaja et al 2006)


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Figure 9 Enhancement of shear strength and ultimate deformation by PET jacketing

Figure 10 PET jacket without fracture in hinge zone

4. Concluding Remarks

The generic model for prediction of ultimate strength and deformation of concrete members is

presented. This model has the following features:

(1) It consists of flexural strength and deformation models and shear strength and deformation

models, which interact each other.


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(2) It can predict the post-peak behavior as the reduction in potential shear strength. The

reduction in the shear strength is caused by reduction in stiffness of tension and shear

reinforcement.

(3) It can be applied to any strengthening materials and predict the fracture of reinforcement at

the ultimate state.

(4) It clearly indicates the advantage of strengthening materials with large fracture strain and

no yielding as shear reinforcement (jacket).

5. Acknowledgment

The author is grateful to all the members of the study team on generic model, especially Dr

SATO Yasuhiko, Dr DAI Jian-Guo, Dr ZHANG Da-Wei, Dr Tidarut

JIRAWATTANASOMKUL, Mr Dhannyanto ANGGAWIDJAJA, Mr SENDA Mineo and Mr

NAKAI Hiroshi.

6. References

Aboutaha, R S, Engelhardt, M D, Jirsa, J O and Kreger, M E. Rehabilitation of shear critical

concrete columns by use of rectangular steel jackets. ACI Structural Journal. 96(1), 1999:

68-78.

Anggawidjaja, D., Ueda, T., Dai, J., and Nakai, H. (2006). Deformation capacity of RC piers

wrapped by new fiber-reinforced polymer with large fracture strain. Cement and Concrete

Composites, 28(10), 2006: 914-927.

Anggawidjaja, D., and Ueda, T. PET, high fracturing strain fiber, for concrete structure

retrofitting: shear force and deformation enhancement: experiments, analysis and model. VDM

Verlag Dr. Mller, Germany; 2009: 1-64.

Jirawattanasomkul, T, Zhang, D W, and Ueda, T. Prediction of the post-peak behavior of

reinforced concrete columns with and without FRP-jacketing. Engineering Structures, 56,

2013: 1511-1526.

Sato Y, Ueda T, Kakuta Y. Shear resisting model of reinforced and prestressed concrete beams

based on finite element analysis.Bulletin of the Faculty of Engineering, Hokkaido University.

No 171. 1994:1-17.

Sato Y, Ueda T, Kakuta Y. Shear strength of prestressed concrete beams with FRP tendon.

Concrete Library of JSCE. 27, 1996: 189-208.

Sato Y, Ueda T, Kakuta Y. Shear strength of reinforced and prestressed concrete beams with

shear reinforcement. Concrete Library International of JSCE. 29, 1997: 233-47.


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Ueda, T, Sato, Y, Ito, T and Nishizono, K. Shear deformation of reinforced concrete beam.

Concrete Library International. JSCE. 43, 2004: 9-23.

Ueda, T. Structural performance of members strengthened by FRP jacketing with high

fracturing strain Proc. of the Second Asia-Pacific Conference on FRP in Structures (APFIS),

International Institute for FRP in Construction (IIFC), Seoul, Korea, 2009: 19-28.

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4. Ultimate Strength and Deformation of Concrete Members Strengthened by Jacketing With Various Materials