REPAIR OF DAMAGED PRESTRESSED CONCRETE BEAMS USING CFRP … · CFRP fiber repair technique The Tyfo System was designed to meet specific design criteria. The design is based on the

http://www.iaeme.com/IJCIET/index.asp 427 [email protected]

International Journal of Civil Engineering and Technology (IJCIET)

Volume 9, Issue 10, October 2018, pp. 427–440, Article ID: IJCIET_09_10_044

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=10

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

©IAEME Publication Scopus Indexed

REPAIR OF DAMAGED PRESTRESSED

CONCRETE BEAMS USING CFRP FABRIC AND

STITCHING TECHNIQUES

M. Y. Sabra

Civil Engineering Department, Beirut Arab University, Beirut, Lebanon

Y. A. Temsah


O. M . Baalbaki


Z. Abou. Saleh

Civil Engineering Department, Rafic Hariri University, Meshref, Lebanon

ABSTRACT

Since the early 1950’s, prestressed concrete has been used in the construction

concrete structures. Using prestressed concrete offers many advantages such as

larger span and thinner elements size compared to the conventional reinforced

concrete. Repairs of prestressed concrete structures are necessary when existing

tendons are damaged (e.g. corroded, cut, or broken...).

The objective of current paper is to focus on repairing of damaged prestressed

concrete beams using carbon fiber reinforced polymer (CFRP) fabric technique, and

to explore a new technique for the strengthening by stitching the structural elements

using external post-tension strands. This work was conducted in order to investigate

the suitability of such techniques through laboratory experiments by testing simply

supported prestressed beams subjected to bending load. The repaired beams are

compared with the control undamaged beam for evaluation purpose.

The outcome of this paper will lead to a set of guidelines for optimal repair

technique to be used for the repair of damaged presterssed beams.

Key words: Prestressed concrete, CFRP, Stitching, Concrete repair, Load Capacity.

Cite this Article: M. Y. Sabra, Y. A. Temsah, O. M . Baalbaki and Z. Abou. Saleh,

Repair of Damaged Prestressed Concrete Beams Using CFRP Fabric and Stitching

Techniques, International Journal of Civil Engineering and Technology (IJCIET)

9(10), 2018, pp. 427–440.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=10

M. Y. Sabra, Y. A. Temsah, O. M. Baalbaki and Z. Abou. Saleh


1. INTRODUCTION

Many structural members have been in serious need of repair due to aging, poor quality of

materials, faulty construction practices, severe environmental and accidental influences (e.g.

overloads, vehicular impacts, strong earthquakes, fire) changes in use that increase service

loads (e.g. load enhancement beyond the original design values), and increased safety

requirements, T.Alkhrdaji and J. Thomas (2004) [1]. The structural changes of concrete

structures can be achieved using one of many different technique methods such as reduction

in span length, externally bonded steel, external composites (carbon fibers), external or

internal post-tensioning systems, Aravinthan T. and Heldt T, (2010) [2], section enlargement,

or a combination of these techniques. Repair systems must perform in a composite manner

with an existing structure to be effective and to share the applied loads, L. Krauses (2006) [3].

The objective of this study is to investigate a technique that can be applicable for repairing of

damaged prestressed concrete beams. External prestressing was first used in the late 1920’s

and has recently being used in bridges, A. F. Daly and W. Witarnawan (1997) [4], both for

new construction as well as repairing of existing structures. Stitching is a technique of

prestressing concrete structural elements to increase its capacity. However this technique is

not commonly used or well-known.

Strengthening of prestressed concrete using CFRP is a commonly used technique to

enhanced durability, serviceability, and to increase the flexural capacity of damaged girders,

control crack propagation if it is present, and reduce deflections under subsequently applied

load (Schiebel et al. 2001 [5]; Tumialan et al. 2001 [6]; Klaiber et al. 2003 [7]; Reed and

Peterman 2005[8]; Reed et al. 2007[9]). This conventional application of CFRP strips/fabric

is referred to as externally bonded CFRP.

2. EXPERIMENTAL PROGRAM

2.1. Structural Material Properties

A set of three beams were constructed using a ready mixed concrete with an average 28-days

compressive strength of 45 MPa based on standard cylinder. A prestressing low relaxation

ASTMA416 7 wire strands of 1860MPa tensile strength and 12.7mm of diameter was used in

reinforcing each beam. The beams were also reinforced with two upper and lower bars of

10mm diameter, and stirrups of 10mm diameter spaced at 200mm.

2.2. Carbon Fiber Reinforced Polymers (CFRP) Properties

CFRP fabric was adhesively bonded to the damaged prestressed concrete beam using high

strength epoxy to compensate the loss of strength due to damage. The CFRP used in this

research is the Tyfo SCH-41 composite, which consists of Tyfo S Epoxy and Tyfo SCH-41

reinforcing fabric. Tyfo SCH-41 is a custom, uni-directional carbon fabric orientated in the

zero degree direction parallel to the N.A of the beam. The Tyfo S Epoxy is a two-component

epoxy matrix. The material properties are presented in Table 1 and Table 2.

Table 1 Physical and mechanical CFRP properties

Typical dry carbon fiber properties

Property Typical test value

Tensile strength 3.79 GPa

Tensile Modulus 230 GPa

Ultimate Elongation 1.7%

Density 1.74 g/cm3

Minimum carbon fibers weight 644 g/cm2

Weight 710 g/m2 ± 35 g/m2

Repair of Damaged Prestressed Concrete Beams Using CFRP Fabric and Stitching Techniques


Table 2 Epoxy material properties

Epoxy material properties

Properties Testing method Typical test value

Tg ASTM D4065/ EN12614 82o C

Tensile Strength ASTM D638 Type 1 72.4 MPa

Tensile Modulus ASTM D638 Type 1 3.18 GPa

Elongation Percent ASTM D638 Type 1 5.0%

Flexural Strength ASTM D790 123.4 MPa

Flexural Modulus ASTM D790 3.12 GPa

2.3. Testing Program

In this paper three prestressed beams measured 250mm wide x 300mm depth x 3200mm long

were constructed. The beam B1 represents the control beam, and the two others CFB1 and

SB1 are the CFRP and the stitching repaired techniques beams respectively. CFB1 and SB1

were subjected to damage by cutting the strand using core drill machine. The beams

dimensions and reinforcement details are shown in Figure 1.

Figure 1 Beam dimensions and reinforcements

Both beams were tested to failure and compared with the control beam. Figure 2 illustrates

the steps of beams preparation.

Figure 2 (a) Preparing of reinforcement, (b) pretention of the strand, (c) pouring, (d) prestressed beams



2.4. Damaging the specimens

Two beams were subjected to damage, which consisted of coring the beams in the mid span,

using coring cylinder of 2.5 inch diameter. The target of coring is to cut the strand and to

simulate sudden damage and cut in the prestressing steel. Figure 3 shows the site coring.

Figure 3 Coring of beams

2.5. Repairing the Specimens

To strengthen the damaged beams two techniques were implemented, the carbon fiber

reinforced polymers (CFRP) fabric and the stitching techniques.

2.5.1. CFRP fiber repair technique

The Tyfo System was designed to meet specific design criteria. The design is based on the

allowable strain for each type of application and the design modulus of the material.

The repair of beam CFB1 needed 2 sheets of CFRP fiber with 250mm wide x 2000mm

long x 1mm thick. Additional layers of CFRP reduce the debonding strain, and are therefore

proportionally less effective, Jarret L. Kasan et al. (2014) [10]. The following steps were

conducted to achieve the repair by CFRP:

Surface Preparation

In general, the surface was cleaned to be dry and free of protrusions or cavities, which may

cause voids behind the Tyfo composite. For discontinuous wrapping surface of beam was

grinded for bonding. Sharp and chamfered corners were rounded off by grinding see Figure 4.

Figure 4 Grinding the beam

Mixing with Epoxy

The CFRP fabric was cut into 2 pieces of 250mm wide x 2000mm long, and the epoxy was

prepared by pouring the hardener into the epoxy, and mixed thoroughly until the desired

viscosity was achieved, Figure 5.



Figure 4 Fiber fabric sheet and epoxy

Application

The prime coat of epoxy was applied on the substrate by using a roller. An approved hand

method was used. Saturated and applied of subsequent layers of the fabric according to the

specifications and the design requirements. A final coat of thickened Epoxy was applied and

detailed all fabric edges, including splice, termination points and jacket edges. See CFRP

fabric repair in Figure6.

Figure 5 CFRP fabric repair

2.5.2. Stitching Repair Technique

Figure 6 stitching layout

The stitching technique consists of two external post tensioned strands of 1860MPa tensile

strength and 9.53mm of diameter (54.8 mm2 of area). Two strands were used to provide the



necessary prestressing effect, taking into consideration the loss of prestressing force by

friction. The strands inserted through the depth of the beam starting from the top by drilling

two opposite holes for each strand with inclined direction of 45o with the bottom. The strand

continues horizontally in the bottom of the beam to cover the damage cut zone, and then turn

back to the top in the opposite hole as shown in Figure 7.

The strand is gripped at both ends, tensioned from one or two edges and anchored to stress

the concrete. The stitching technique does not require professional staff for installer. Figure 8

illustrates the steps of stitching technique.

(a) (b)

(c) (d)

Figure 7 (a) Drilling, (b) anchoring,(c) post tensioning, (d) stitching technique

2.6. Test set up

Figure 9 shows the test set up, the three beams were simply supported with 3000mm span,

and subjected to flexure with two point loads located at 1/3 and 2/3 of span length from the

support. The load-deformation curve was determined using an automated testing machine

provided with data acquisition unit.

Figure 8 test setup



0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80

Load

KN

Deflection mm

Load vs Deflection

Control

Inelastic elastic

On-line measurements of load and deflection were taken from transducers and transferred

to the computer through the data acquisition unit. Specialized software allowed the on-line

monitoring of the load deflection curve. The load was acting monotonically through a

displacement control method. The load produced single curvature bending in the beam.

3. EXPERIMENTAL RESULTS AND DISCUSSION

3.1. Control Beam B1

Figure10 illustrates the failure pattern and the load-deflection curve which was recorded

during testing. The experiment generated vertical cracks within the beam part, and between

the two point loads and the middle of span.

Figure 9 control beam B1 failure

The control beam B1 exhibited flexural cracks that located near the mid-span and

propagated between the two point loads. It was noticed that the failure was flexural type when

the beam reached the ultimate moment capacity. The load-deflection curve was bilinear and

divided into two parts, elastic part and inelastic part as shown in Figure 11, and it shows that

the control beam failed at load of 120 KN, with deflection of 68mm.

Figure 10 control beam B1 load-deflection curve

3.2. CFRP Fabric Beam CB1

For the CFRP repaired beam CFB1, the cracks started at the mid-span and propagate under

the fiber sheet until a brittle failure was experienced at the ultimate load that can be supported

by the beam; this failure appeared when the fiber sheet pulled away with the concrete from

the specimens as shown in Figure 12.



0

20

40

60

80

100

120

140

160

0 10 20 30 40 50

Load

kN

Deflection mm

Load vs Deflection

CFRPFabric

Inelastic elastic

Figure11 CFRP beam CB1 failure

The CFB1 beam failure was described first by yielding of the internal reinforcement, and

then the concrete crushed and separated from the specimen with CFRP fabric due to tension

failure.

The curve in Figure 13 illustrates the capacity of the CFRP repaired beam which is 140

KN, with 13.5mm deflection, this result indicate the brittle failure and quasi no ductile

behavior was observed.

Figure 12 CFRP beam CB1 load-deflection curve

3.3. Stitched Beam SB1

Cracks in beam SB1 began to form at the mid- span in the tension side during testing, a few

number of cracks appeared before failure occurred at the bottom side of the beam in contact

with the strands as shown in the Figure14.

Figure 13 stitching beam SB1 failure



0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60 70

Load

kN

Deflection mm

Load vs Deflection

Stiching

Inelastic elastic

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60 70

Load

kN

Deflection mm

Load vs Deflection

Control

CFRP Fabric

Stiching

Inelastic elastic

The load deflection curve in Figure 15 reflects the beam behavior, which is affected by the

length of the strand in contact with the bottom of the beam creating the uplift line load. The

load deflection curve is divided into two parts, the elastic part, and inelastic part which exhibit

stiffness reduction caused by the cracks. The damage occurred when the capacity of the

specimen reached 162 KN with 31.5mm deflection. It was noticed a reduction of deflection

due to camber resulting from prestressing. The load displacement curve showed no brittle

failure.

Figure 14 stitching beam SB1 load-deflection curve

3.4. Beams Comparison

Figure 16 exhibits the load-deflection curves of the 3 tested beams, control beam B1, CFRP

fabric beam CFB1, and stitching beam SB1.

Figure 15 Loads deflection curves

It is well noticed the similarities of the behavior in the elastic stage for the three beams,

whereas the behavior of the beams are different in the inelastic stage. Comparing the loads

capacity of specimens, beam CFB1 and SB1 exhibited an increase in load capacity by 16.66%

and 35% respectively from the control beam B1.Comparing to the control beam, it can be

seen in Figure 15 that the failure of beam CFB1 was quasi brittle failure and occurred at a

small deflection. On the other hand, beam SB1 showed better ductility; hence the failure

occurred at a deflection of 31.5mm. Since the ductility helps in preventing sudden



catastrophic failures K. Q. Walsh and Y. C. Kumara (2004) [11], therefore, SB1 beam may be

more suitable in the earthquake zones, L. Panian, M. Steyer & S. Tipping (2007) [12].

The ductility is provided by the reinforcement presented in tension and compression zone

of all the 3 beams, even though the amount of steel was the same, the control beam showed

more ductility than CFB1 and SB1, this is due to the continuity of the prestressing strand in

B1, therefore the repaired beams generated less ductility.

The failure of CFB1was quasi brittle and the CFRP didn’t compensate the loss of

ductility.

The percentage of forces regained using CFRP and stitching repair are shown in the Table 3

Table 3 Repair Type and Percentage of Loading Regained.

Specimen # Type of repair Ultimate

load, (KN)

Percentage of force

regained

B1 Control beam 120 -

CFB1 CFRP 140 116.66%

SB1 Stitching 162 135%

4. THEORETICAL INVESTIGATION

Conventional beam theory was used to predict the capacity of the beams tested in this paper

as followings:

The Control Beam

The theoretical capacity of the beams was calculated using the following criteria:

To calculate the stress in bonded strand at failure stage, Eq.1 (AC1 318-08) was used

(

[

( )]) (1)

= 1750 MPa.

Where fps is the stress at prestressed reinforcement at nominal strength, fpu is the tensile

strength of prestressing tendon, 1860 MPa, f’c is the 28 days of compressive strength of the

concrete,45MPa, is the reinforcement index of the prestressed reinforcement and equal to

, is the steel percentage of the strand, dp is the distance from the compression fibers

to the center of the prestressed reinforcement, d is the is the distance from the compression

fibers to the center of tension reinforcement, , is the reinforcement index for the

compression of steel reinforcement, β1, is the factor relating depth of equivalent compression

block to depth of neutral axis,0.8.

Figure 16 Equilibrium of Tensile and Compressive Couple Forces



From equilibrium system Figure 17, the depth of equivalent a, was calculated as follow

according to ACI-318:

Tst+Ts=Cc+Cs

a=19mm

Where Tst & Ts are tensile forces of strands and bottom reinforcement respectively, Cs &

Cc are compressive forces of upper reinforcement and concrete respectively, b is the width of

the beam, fy is the yield strength of steel reinforcement, 420 MPa, d is depth of the main

reinforcement layer, As is the total cross section area of reinforcing bars, Aps is the cross

section area of the strand, 99mm2, and a is the depth of equivalent rectangular stress block.

The moment capacity will be as shown in Eq.2

M= (

) (

) (2)

M=59 KN.m

Figure 17 Loading setup

To calculate the load capacity, Eq.3 was used, see

Figure 17:

M=PL/3 (3)

P=59 KN load capacity= 2P=118 KN

The CFRP fabric Beam CFB1

Gross Laminate Properties of Tyfo Composite System according to ACI-440.2R-08 are as

follow:

Ultimate tensile strength ffu = 834 MPa, modulus of Elasticity: Ef = 82.0 GPa, ultimate

elongation:

εfu= 0.0085, thickness per layer: tf = 1.00 mm, tensile force produced by the Tyfo SCH-41

Composite System:

Ff = Αf · ff Af = n · tf · wf

where n = 2 layers wf,face = 200 mm < 250 mm

Af =400 mm²

ff = Efd · εfe with εfe ≤ εfd = 0.41 √(fc / n·Ef·tf) and εfd <0.90·εfu = 7.65‰ εfe =5 ‰

ff = Efd · εfe → ff = 410 MPa




From equilibrium system

Figure 18, the depth of equivalent a, was calculated as follow according to ACI-318:

Tf+Ts=Cc+Cs

.

a=26.3mm.


M= (

) (

) (4)

M=65 KN.m


Figure 17:

P=65 KN load capacity= 2P=130 KN

The Stitched Beam SB1

To determine the theoretical capacity of beam SB1, the stress in the strands at ultimate should

first be determined. To calculate the stress in member with unbonded strands and with a span-

to-depth ratio of 35 or less, Eq.5 (AC1 318-08) was used

(5)

= 1446 MPa.




Where fps is the stress at prestressed reinforcement at nominal strength, fpe is effective

stress in prestressing steel (after allowance for all prestress losses), 1068 MPa, is the steel

percentage of the strand, dp is the distance from the compression fibers to the center of the

prestressed reinforcement.

From equilibrium system Figure 20:

Tst+Ts=Cc+Cs

.

a=16.6mm

Where Tst is the tensile force of the external strands, Aps is the cross section area of the

strand, 54.8mm2.


M= (

) (

) (6)

M=64 KN.m


Figure 17:

P=64 KN 2P=128 KN

5. THEORETICAL AND EXPERIMENTAL RESULTS

Table 4 shows the summary of the experimental and theoretical ultimate loads. The nominal

experimental capacity of control Specimen B1 was calculated to be 120 kN. Compared with

the theoretical ultimate load of 118kN, the ratio of the experimental to theoretical ultimate

load is 1.01, indicating that the theoretical prediction is in good agreement with the

experimental value. In CFB1 and SB1 the ratios of experimental to the theoretical ultimate

load are 1.07 and 1.28 respectively, which are appropriate for predicting the ultimate capacity.

Table 4 Experimental and theoretical ultimate loads

Specimens # Experimental

ultimate load,(kN)

theoretical ultimate

load , (kN)

Ratio of experimental to

analytical ultimate load

B1 120 118 1.01

CFB1 140 130 1.07

SB1 162 128 1.26



6. CONCLUSIONS

In this research two prestessed beams CFB1 and SB1 were subjected to damage in the

prestressing steel in order to assess two repair techniques, the CFRP fabric and the stitching.

Laboratory tests were carried out on these beams CFB1, SB1, and B1, where B1 is the

control beam, to evaluate the structural performance of strengthening techniques by

investigate their flexural behavior.

It was concluded that both techniques can be considered as adequate for the intended

purpose, and they do not significantly increase the designed loads to the structural elements. It

was also concluded, based on the experiments in this research, that specimens CFB1 and SB1

were successfully repaired using CFRP fabric, and stitching technique, and these techniques

are able to compensate the damaged strand by increasing the ultimate load of 16.66% and

35% respectively compared with control beam.

However, the stitching technique had offered the following advantages compared to CFRP

fabric technique:

An improvement with ductile behavior when compared to the quasi brittle failure of the beam

repaired by CFRP fabrics. Therefore SB1 beam more suitable in the earthquake zones than

CFRP beam.

Stitching material is readily available at a lower cost.

Stitching is more traditional and can easily apply while CFRP fabrics require highly skilled

labor for proper installation.

The theoretical analysis can be used to properly predict the capacity of the repaired beams.

However, additional investigation should be performed several variables to stitching

technique (such as inclination angle of strand, strand length ….) in order to enlarge the

domain of application.

REFERENCES

[1] Tarek Alkhrdaji, Jay Thomas, “Structural Repair and Strengthening Techniques for

Concrete Facilities”, Journal of Structural Engineer (May 2004).

[2] Aravinthan T. and Heldt T., “Innovative Strengthening Technique using Post-tensioning”,

Austrian Journal of Structural Engineering, 11 (2), pp. 117-128 (2010).

[3] Larry Krauses, Repair Modifications and Strengthening with Post-Tensioning, PTI

Journal, (July 2006).

[4] A.F.Daly and W. Witarnawan, “Strengthening of bridges using external post-tensioning” ,

Road Research Development Project, Published Paper PA 11, East 97, Seoul, Korea, 29-

31 (October 1997).

[5] Schiebel, S., Parretti, R., and Nanni, A.“Repair and strengthening of impacted PC girders

on Bridge A4845, Jackson County, Missouri.” Rep. RDT01-017, Missouri DOT, Jackson

City, MO (2001).

[6] Tumialan, J. G., Huang, P., Nanni, A., and Jones, M. “Strengthening of an impacted PC

girder on Bridge A10062, St Louis County, Missouri.” Rep. RDT01-013, Univ. of

Missouri, Rolla, MO (2001).

[7] Klaiber, F. W., Wipf, T. J., and Kempers, B. J. “Repair of damaged prestressed concrete

bridges using CFRP.” Proc., Mid-Continent Transportation Symp., Center for

Transportation Research and Education, Ames, IA (2003).

[8] [8. Reed, C. E., and Peterman, R. J. “Evaluating FRP repair method for cracked

prestressed concrete bridge members subjected to repeated loadings phase 1.” Rep.

KTRAN: KSU-01-2, Kansas DOT, Topeka, KS (2005).



[9] Reed, C. E., Peterman, R. J., Rasheed, H., and Meggers, D. “Adhesive applications used

during repair and strengthening of 30-year-old prestressed concrete girders.”

Transportation Research Record 1827, Transportation Research Board, Washington, DC

(2007).

[10] Jarret L. Kasan, A.M.ASCE; Kent A. Harries, M.ASCE; Richard Miller, M.ASCE; and

Ryan J. Brinkman, “Repair of Prestressed-Concrete Girders Combining Internal Strand

Splicing and Externally Bonded CFRP Techniques” journal of bridge engineering © asce /

(February 2014)

[11] Keven Q. Walsh and Yahya C. Kumara, “Behavior of Un-bonded Post-Tensioning

anchoring Systems Under Monotonic Tensile Loading”, PTI Journal, (2010).

[12] L. Panian, M. Steyer & S. “Tipping, Post-Tensioned Concrete Walls for Seismic

Resistance”, PTI Journal, Vol 5, No.1 July (2007).

Documents

REPAIR OF DAMAGED PRESTRESSED CONCRETE BEAMS USING CFRP … · CFRP fiber repair technique The Tyfo System was designed to meet specific design criteria. The design is based on the