Retrofitting Techniques

8/12/2019 Retrofitting Techniques

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RETROFITTING TECHNIQUES

INTRODUCTION:

Retrofitting is the process of modifying the existing building with the addition of new

technologies. Due to the increase in economic and environmental constraints, the currenttrend is to upgrade deteriorated and obsolete structures rather than replacing them with new

buildings. It increases the performance of the structure and reduces the vulnerability of

damage.

RETROFITTING TECHNIQUES:

Jacketing External bonding External reinforcement Fiber-reinforced polymer (FRP) Externally Bonded Simcon Laminates Section enlargement External post-tensioning systems

JACKETING:

Jacketing is the most popularly used method for strengthening of building columns.The most common types of jackets are steel jacket, reinforced concrete jacket, fiber

reinforced polymer composite jacket, jacket with high tension materials like carbon

fiber, glass fiber etc. The main purposes of jacketing are:

1. To increase concrete confinement by transverse fiber reinforcement, especially for

circular cross-sectional columns,

2. To increase shear strength by transverse fiber reinforcement,

3. To increase flexural strength by longitudinal fiber reinforcement provided.

Rectangular jackets typically lack the flexural stiffness needed to fully confine theconcrete. However, circular and oval jackets may be less desirable due to:

(i) Need of large space in the building potential difficulties of fitting in the jackets

with existing partition walls, exterior cladding, and non-structural elements and

(ii) Where an oval or elliptical jacket has sufficient stiffness to confine the concrete

along the long dimension of the cross-section is open to question.


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While jacketing a beam, its flexural resistance must be carefully computed to avoidthe creation of a strong beam-weak column system. In the retrofitted structure, there

is a strong possibility of change of mode of failure and redistribution of forces as a

result of jacketing of column, which may consequently causes beam hinging. The

location of the beam critical section and the participation of the existingreinforcement should be taken into consideration.1

Jacketing of beam may be carried out under different ways; the most common areone-sided jackets or 3- and 4-sided jackets. At several occasions, the slab has been

perforated to allow the ties to go through and to enable the casting of concrete.

The beam should be jacketed through its whole length. The reinforcement has alsobeen added to increase beam flexural capacity moderately and to produce high joint

shear stresses. Top bars crossing the orthogonal beams are put through holes and the

bottom bars have been placed under the soffit of the existing beams, at each side of

the existing column. Beam transverse steel consists of sets of U-shaped ties fixed to

the top jacket bars and of inverted U-shaped ties placed through perforations in the

slab, closely spaced ties have been placed near the joint region where beam hinging

is expected to occur.1

EXTERNAL BONDING TECHNIQUE:

This method consists of bonding steel plates or steel flat bars to the structuralelements and it is widely used in strengthening of bridge structures.

The bonding of the steel plates or steel flat bars to the concrete members is ensured bythe use of epoxy adhesives and in some cases, additional fastening is provided by

means of dowels or bolts glued to the holes drilled in the concrete members.

In the case of RC slabs strengthening this method is used to augment the membersbending resistance. Therefore, the steel plates or steel flat bars can be applied to thebottom or upper faces of the reinforced concrete slab to ensure the bending resistance

(positive or negative bending moments zones).3

One of the disadvantages of this method is that it can be applied only to the relativelysound structures. In case of severe concrete deterioration and major cracks of the RC

member other methods should be considered.3


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EXTERNAL REINFORCEMENT:

The use of external unbonded reinforcement retains many of the merits of unbondedpost tensioning but dispenses with the need for specialist stressing operations andexpensive tendons and fittings. Less clearance would be required for the end

anchorage systemsince access for jacks is not needed. The installed system is easily

inspected and poor workmanship or corrosion of external reinforcement can be easily

checked and monitored. Corrosion protection systems similar to those used for

external prestressing may be used and the system has the advantage of using a larger

cross sectional area of a relatively low strengthmaterial and is therefore less

vulnerable to corrosion, accidental damage and vandalism. It is also compatible with

principles of conservation which require that a structure be returned to its original

condition after any interventions. It avoids the potential problems of workmanship,

weather sensitivity and chloride contamination that are associated with epoxybonding.

The potential of retrofitting using external bars anchored only at the ends of

simplysupported flexural elements, Figure 1, has been demonstrated for flexural

modes of failure (Cairns & Rafeeqi, 2003). Strength enhancements of the order of

100% have been shown to be feasible.2

External unbonded reinforcement remains a viable retro-fitting technique forstrengthening of reinforced concrete beams when installation is carried out with the

beam under load. External unbonded reinforcement alters the pattern of strain in a

beam, and changes structural action from purely flexural to that of a flexure/tied arch

hybrid. The compressive stresses related to the arch action enhance the shear strength

of the existing beam. The amount of external reinforcement should beaccurately

designed in order to have a ductile behavior with the flexural strength slightly lower

than theshear strength. Either analytical tools or numerical FE programs can be

utilized for a proper design ofexternal reinforcement, even though it is possibleto

conclude that, if the existing member is properlydesigned for shear, adding a

percentage of externalreinforcement approximately equal to the one of thebonded

(existing) reinforcement, the failure modedoes not change.2


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CASE STUDY:

FLEXURE AND SHEAR BEHAVIOR OF RC BEAMS STRENGTHENED BY

EXTERNAL REINFORCEMENT

F. Minelli & G.A. Plizzari

University of Brescia, Brescia, ItalyJ. Cairns

Heriot-Watt University, Edinburgh, UK

1 INTRODUCTION AND BACKGROUND

There is a significant and growing need for the strengthening of existing reinforced concrete

(RC) structures. Structural deterioration may have taken place, a change in use could result in

more onerous loading, or design requirements in building codes may change. There is a need

for techniques that can provide cost effective solutions to both the design and implementation

of strengthening measures. The technique is illustrated diagrammatically in Figure 1. High

yield threaded bars are applied to both sides of a RC beam, close to the soffit level of the

beam. The bars pass through yokes at the ends of the beam, where they are anchored by

locknuts. On all but short spans there are benefits from use of deflectors to avoid a reduction

in effective depth as the beam deflects.

Figure 1. Description of the technique herein investigated

No significant prestress is

required and only sufficient force to avoid appreciable sagging of the bars is applied. External

bars can thus easily be installed by hand. The use of external unbonded reinforcement offers

the potential of providing a more cost effective and less disruptive solution to the problem of

strengthening simply supported RC beams in comparison with existing methods such as:

Epoxy bonded steel or composite plates;

An additional concrete layer either as an overslab or to the beam soffit;

provision of additional load paths e.g. an auxiliary support;

post tensioningeither with bonded or unbounded tendons.


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.

The use of the system is, however, limited to strengthening of simply supported beams and

slabs at this time, even though recent studies were conducted for extending this novel

technique to multi-support beams (Cairns & Coakley, 2008).

The concept of retro-fitting unbonded external reinforcement developed from observationsmade in a study of reinforced concrete beams when concrete around bars is broken out during

repair actions. It showed that quite substantial exposure and unbonding of the main bars

might cause little reduction in strength (Cairns & Zhao, 1993) or even an enhancement in

ultimate strength of beams deficient in shear (Cairns, 1995).

Shear strength is generally considered to be a function of aggregate interlock, dowel action of

longitudinal bars, strength of concrete in the compressive region, and the contribution of any

links present. In accordance with this concept, the addition of unbonded external bars may be

considered to impart a longitudinal force in the beam which enhances the contribution of both

aggregate interlock (by reducing crack widths) and the concrete compression zone (by

increasing neutral surface depth). Several design codes allow enhancements to shear capacity

in the presence of axial forces. The consequences of a change from bonded to unbonded

reinforcement can alternatively be thought of as a change in the way a member carries load.

Without bond, the force in reinforcement must be constant along the length of the beam and

the lever arm of the beam varies with bending moment. The neutral axis therefore takes the

form of an arch, with concrete resisting shear/flexure predominantly by compression rather

than shear stresses. It might be expected that this change from a purely flexural mode of

failure towards a tied arch/flexure hybrid would be accompanied by an increase in shear

capacity. A comprehensive study by Grant (2002) developed an approach consistent with

conventional code rules, in which a numerical model was used to predict longitudinal force in

external bars, and this force was then used to estimate upper and lower bounds to shear/

flexure strength enhancement using expressions in

design codes. In a previous contribution by the Authors (Cairns et al., 2005) some

experimental tests carried out by Grant (2002) were numerically studied using the Vec-Tor

finite element software, which is based on the Modified Compression Field Theory (MCFT)

(Vecchio & Collins, 1986). Figure 2 shows the details of the two beams analyzed

numerically, which represent two limit situations as far as link reinforcement is concerned.

Figure 3 describes the experimental and numerical load-displacement curves for the two

beams: the VecTor program proved to give quite accurate and reliable modeling of the

strength, ductility, stiffness and post-cracking strength of the members (Cairns


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et al., 2005). Moreover, an extensive parametric study was undertaken to assess the influence

of the amount of external reinforcement on the collapse mechanism and on ductility. It was

primarily evidenced that flexural strength of the beams should not be higher than its shear

strength to avoid a brittle failure. In order to avoid a shear collapse, the external rebars should

be accurately designed to sufficiently increase the load capacity and assure a flexure failure.A greater percentage of external reinforcement could determine a slight increase in ultimate

load but could bring the member to an undesirable shear failure. Figure 4 exhibits a

comparison between the numerical plots of two beams having the same transverse

reinforcement but different external reinforcement (one having a double the area of the

other): when the flexural strength of the beam is less than the shear strength, a ductile

behavior can be observed; this requires the area of external reinforcement to be limited in

order to allow bars to yield just before shear failure. In doing so, a small decrease of bearing

capacity is observed but the overall ductility (due to a flexural collapse), that can be achieved

is considerably higher. Results available into the literature evidence the enhanced shear

behavior of beams with unbounded rebars (Cairns, 1995).

FIBER REINFORCED POLYMER (FRP):

Fibre reinforced polymer (FRP) composites consist of high strength fibres embeddedin a matrix of polymer resin. Fibres typically used in FRP are glass, carbon and

aramid.

The mechanical properties of composites are dependent on the fibre properties, matrixproperties, fibre-matrix bond properties, and fibre amount and orientation.6

A composite with all fibres in one direction is designated as unidirectional. Since it ismainly the fibres that provide stiffness and strength composites are often anisotropic

with high stiffness in the fibredirection(s). In strengthening applications,

unidirectional composites are predominantly used.

For structural applications, FRP is mainly used in two areas.


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CASE STUDY

STRUCTURAL RETROFITTING OF REINFORCED CONCRETE BEAMS

USING CARBON FIBRE REINFORCED POLYMER

BY YASMEEN TALEB OBAIDAT

Experimental WorkInvestigation of the behaviour of FRP retrofitted reinforced concrete structures has in the last

decade become a very important research field. In terms of experimental application several

studies were performed to study the behaviour of retrofitted beams and how various

parameters influence the behaviour.

The effect of number of layers of CFRP on the behaviour of a strengthened RC beam was

investigated by Toutanji et al. [3]. They tested simply supported beams with different

numbers of CFRP layers. The specimens were subjected to a four-point bending test. The

results showed that the load carrying capacity increases with an increased number of layers of

carbon fibre sheets.

Investigation of the effect of internal reinforcement ratio on the behaviour of strengthened

beams has been performed by Esfahani et al. [4]. Specimens with different internal steel ratio

were strengthened in flexure by CFRP sheets. The authors reported that the flexural strength

and stiffness of the strengthened beams increased compared to the control specimens. With a

large reinforcing ratio, they also found that failure of the strengthened beams occurred in

either interfacial debonding induced by a flexural shear crack or interfacial debonding

induced by a flexural crack.

A test programme on retrofitted beams with shear deficiencies was done by Khalifa et al.

[5]. The experimental results indicated that the contribution of externally bonded CFRP to the

shear capacity of continuous RC beams is significant.

There are three main categories of failure in concrete structures retrofitted with FRP that

have been observed experimentally, Esfahani et al. [4], Ashour et al. [6], Garden and

Hollaway, [7], Smith and Teng, [8]. The first and second type consist of failure modes where

the composite action between concrete and FRP is maintained. Typically, in the first failure

mode, the steel reinforcement yields, followed by rupture of CFRP as shown in Figure 3(a).

In the second type there is failure in the concrete. This type occurs either due to crushing of

concrete before or after yielding of tensile steel without any damage to the FRP laminate,

Figure 3(b), or due to an inclined shear crack at the end of the plate, Figure 3(c). In the third

type, the failure modes involving loss of composite action are included. The most recognized

failure modes within this group are debonding modes. In such a case, the external


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reinforcement plates no longer contribute to the beam strength, leading to a brittle failure if

no stress redistribution from the laminate to the interior steel reinforcement occurs. Figures

3(d)- (g) show failure modes of the third type for RC beams retrofitted with FRP. In Figure

3(d), the failure starts at the end of the plate due to the stress concentration and ends up with

debonding propagation inwards. Stresses at this location are essentially shear stress but due tosmall but non-zero bending stiffness of the laminate, normal stress can arise. For the case in

8 Figure 3(e) the entire concrete cover is separated. This failure mode usually results from the

formation of a crack at or near the end of the plate, due to the interfacial shear and normal

stress concentrations. Once a crack occurs in the concrete near the plate end, the crack will

propagate to the level of tensile reinforcement and extend horizontally along the bottom of

the tension steel reinforcement. With increasing external load, the horizontal crack may

propagate to cause the concrete cover to separate with the FRP plate. In Figures 3(f) and (g)

the failure is caused by crack propagation in the concrete parallel to the bonded plate and

adjacent to the adhesive to concrete interface, starting from the critically stressed portions

towards one of the ends of the plate. It is believed to be the result of high interfacial shear and

normal stresses concentrated at a crack along the beam. Also mid span debonding may take

concrete cover with it.


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EXTERNALLY BONDED SIMCON LAMINATES

One promising new development uses steel fibermats to reinforce the concretematrix. The new approach called SIMCON (Slurry Infiltrated Mat

CONcrete)produces concrete components with extremely high flexural strength [3].Since SIMCON is manufactured using pre-made continuous fiber mats, delivered in

large rolls, fiber placement is substantially simplified.

Earlier, development SIFCON (Slurry Infiltrated Fiberous Concrete) was used in thefield for retrofit and new construction. However, high placement cost and lack of

fiber uniformity associated with manual distribution of discontinuous fibers have

prevented its widespread field use.

All the above limitations can be overcome using SIMCON, which exhibits theimproved features similar to SIFCON at a much lower fiber volume fraction. Since it

is manufactured using pre-made continuous fiber mats, delivered in large rolls, fiber

placement is substantially simplified.

1.SIMCON laminates properly bonded to the tension face of RC beams can enhancethe flexural strength substantially. The strengthened beams exhibit an increase in

flexural strength of 45.45 percent for laminates having volume fraction 5.5 percent

and aspect ratio 300 and 400, 89.09 percent for volume fraction 5.5 and aspect ratio

400, and 100 percent for volume fraction 5.5 percent and aspect ratio 300.

2. At any given load level, the deflections are reduced significantly therebyincreasing the stiffness for the strengthened beams. At ultimate load level of the

control specimens, the strengthened beams exhibit a decrease of deflection up to 87

percent.

3. All the beams strengthened with SIMCON laminates with optimum volumefraction 5.5 percent and aspect ratio 300, 400, and 400 and 300 experience flexural

failures. None of the beams exhibit premature brittle failure.

4. A flexible epoxy system will ensure that the bond line does not break beforefailure and participate fully in the structural resistance of the strengthened beams.

5. Among the three different volume fraction and aspect ratio of bonded SIMCONlaminates, the strengthened beam RB1 of volume fraction 5.5 percent and aspect

ratio 300 exhibit 100 percent increase in flexural strength when compared to the

control specimen and has close agreement with the experimental, theoretical

calculations (section analysis) and numerical (ANSYS) results.


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it posses very high mechanicaland adhesive strength properties most desirable for civil

engineeringapplications. Hand compaction and gravity feedingwere used to produce thorough

penetration of slurry into thepreplaced steel fibers. Curing of SIMCON laminates

wasaccomplished by covering with plastic sheets for 24 hours,followed by water submersion

for 28-days after the curingperiod. The completed SIMCON laminates of size1252502950 mm has one volume fraction and three aspectratios, viz: Vf = 5.5 percent

and aspect ratio 300, Vf = 5.5percent and aspect ratio 400 and Vf = 5.5 percent and cocktail

aspect ratio of 300 and 400 (Fig. 2). Total of 4 beams,1252503200 mm in size, were cast

and tested in the laboratoryover an effective span of 3000 mm. Of the above fourbeams, one

beam was used as control specimens (CB1), andthree beams (RB1, RB2 and RB3) were

strengthened withbonded SIMCON laminates (1252502950 mm) at the bottomunder

virgin condition and tested until failure. The detailsof test beams are presented in Table 1.

The beams are under reinforced section [8], reinforcedwith 2-12 # at bottom, 210 # at top

using 6mm dia stirrups@ 150 mm c/c (Fig. 3). M 20 concrete and Fe 415 grade

steel are used.The soffit of the beams were sand blasted to remove thesurface laitance and

then blown free of dust using compressedair. After surface preparation, the adhesive

componentswere mixed thoroughly and applied to the surface usinga trowel. The SIMCON

laminates already cast wereplaced over the beam and held in position by dead weights.

The strengthened beams were tested after the interval of 14-days. The coin tap was conducted

to identify areas of debond, if any. Beams were tested in four point bending as

shown in Fig. (4).

Fig. (4). Test Set Up for Static Loading.


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The test results on the strength and deformation properties of the control specimen and

strengthened beams are reported in Table 2 and 3.

Summary of Test Results

SECTION ENLARGEMENT

Section enlargement is one of the methods used in retrofitting concretemembers.

Enlargementconsists of the placement of reinforced concrete jacketaround the existing

structural member to achievethe desired section propertiesand performance. With section

enlargement slabs can be enlarged toincrease their load-carrying capacity or stiffness. A

typical enlargement isapproximately 58 cm forslabs.The strengthening by section

enlargement can be performed in two Ways

Strengthening by adding the new reinforcement and new concretelayer to the bottomof the structural element.

Strengthening by adding the new reinforcement and new concretelayer to the top faceof the RC member.

The main disadvantages of such system are the increase in theconcrete member size obtained

after the jacket is constructed and the need toconstruct a new formwork.


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EXTERNAL POST-TENSIONING

It is very effective in increasing the flexural and shear capacity of concrete members. It can be

applied to reinforced and pre stressed concrete members. The technique is applied to RC slabs to

correct the excessive deflections and cracking. The repair system supplements minimal additional

load to the structure thus being an effective economical strengthening technique. The post-tensioning

forces are delivered by means of standard pre stressing tendons or high-strength steel rods, usually

located outside the original section. The tendons are connected to the structure at anchor points,

typically located at the ends of the member. End-anchors can be made of steel fixtures bolted to the

structural member, or reinforced concrete blocks that are cast in situ. The desired uplift force is

provided by deviation blocks, fastened at the high or low points of the structural element.9

Before the strengthening technique can be applied necessary repairs to the structural members mustbe performed. The existing cracks must be repaired by means of epoxy injecting or other known

methods. If there are existing spalls patching must be done, because this repairs must ensure that the

pre stressing forces are distributed uniformly across the section of the member.9

Concrete, precast, wood, and steel beams can be strengthened with external or encased post-

tensioning tendons. Each structure or strengthening scenario is different, the Engineer must

use judgment when applying the information contained herein. Prior to determining the

tendon force, the Engineer should establish the tendon profile (straight line or parabolic) and

the required uplift loading. The Engineer should also decide whether the tendons will be

external or encased. Similarly, any fire or corrosion protection requirements should be

recognized. Any height limitations affecting the tendon eccentricity should be identified. The

Engineer in most cases should work with an experienced contractor when formulating the

construction procedures for the strengthening. Fig. 27 shows the strengthening of a beam

with external tendons. An equal number of tendons are typically used on each side of the

beam. Tendon force and eccentricity are adjusted until an optimum solution for the required

uplift force is obtained. Generally, tendon eccentricity is established by the structures

geometry, however small adjustments can be made by adjusting the vertical location of the

tendon anchorage. Profiled or harped tendons are typically strands while straight tendons can

either be bars or strands. Strengthening tendons encased in concrete will generally have a

parabolic profile. The profile can be easily accomplished with dowels inserted into the

existing beam. When new concrete and existing concrete are to act integrally, the Engineer

must design the details of attaching the new concrete to the existing for proper transfer of


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forces. Minimum amounts of non-prestressed reinforcement in new concrete, dowels to

existing concrete, bonding agent, and roughness of the existing concrete are some items to be

considered in the design of the interface between the new and existing concrete. When using

external tendons, the support points and anchorages must be analyzed (4.1.2 and 4.1.3 discuss

supports and anchorages respectively). For beams, support points can be made with steelpipes or brackets. Steel pipes can be placed against the beam soffit extending out on either

side to support the tendon or the existing beam may be cored to insert the pipe. Pipes need to

be designed to transfer the load to the structure. Brackets at the soffit of the beams offer more

flexibility than pipes. Tendon geometry is easily adjusted with brackets and they can be

placed at multiple locations along the beam. Fig. 28 shows an example of a beam bracket and

Fig. 29 shows how multiplebeam brackets can be used to establish tendon.10


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REFERENCES:

1.Bentz, E.C. Sectional Analysis Of Reinforced Concrete. Phd thesis, Department Of Civil

Engineering, University Of toronto, 2000, 304 P.

2.British Standard Institution, Bs8110 Structural Use Of Concrete part 1, Bsi, London, 1985.

Cairns, J. Strength In Shear Of Concrete Beams With Exposed reinforcement, Proceedings

Of Ice Structures & Buildings,110, May 1995, 176185.

3. Cairns, J., Carpi, R. & Plizzari, G. Strengthening Of R.C.Beams Using External

Reinforcement: Effect Of Load At installation. Proceedings 10th International Conferenceon

Structural Faults & Repair, London, 2003. Engineering technics Press, Edinburgh. 2003. 13

P.

4. Cairns, J. & Coakley, E. Behavior Of Continuous Concretebeams During Repair Breakout.

Proceedings Int Conf Onstructural Faults & Repair, Edinburgh, June 2008. Ecspublications,

Edinburgh 2008.

5. Cairns, J. & Grant, J.F.D. External Unbonded Reinforcement:A Novel Retro-Fitting

Technique. Proc First Int Conf Oninnovative Materials And Technologies For Construction

And Restoration, Lecce, June 2004, 2, 256274. Isbn88-207-3678-0.

6. Piggott, M.: Load Bearing Fiber Composites, 2nd Edition. Kluwer Academic Publishers,

Boston/ Dordrecht/ London. 2002.

7. Karbhari, M.: Frp International. The Official Newsletter Of The International Institutefor

Frp In Construction. 2004; 1(2).

8. Toutanji, H., Zhao, L., And Zhang, Y.: Flexural Behavior Of Reinforced Concrete Beams

Externally Strengthened With Cfrp Sheets Bonded With An Inorganic Matrix.Engineering

Structures. 2006; 28: 557-566.

9.External Post-Tensioning Retrofitting And Modelling Of Steel-Concrete Box-Girder

Bridgesbursis.O.A, Bonellia.B, Mamminoa.C, Pucinottir.D, Tondinin.Ea,B,Edepartment Of

Mechanical And Structural Engineering, University Of Trento, Italyc S.I.Ge.S.


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S.A.Spovegliano (Tv), Italyd Department Of Mechanics And Materials, Mediterranean

University Of Reggio Calabria, Italy

10.Repair And Retrofit Using External Post-Tensioningby Karen J. Barchas

Retrofitting Techniques

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