5.4 EXPERIMENTAL S RC B M S /FRP S

Intermediate Crack Debonding of Plated RC Beams Experimental Investigation on Moment Redistribution

- 351 -

5.4 EXPERIMENTAL STUDIES ON RC BEAMS WITH NEAR SURFACE MOUNTED STEEL/FRP STRIPS

Through the tests performed on beams with externally bonded plates, it was found that, despite the

premature debonding behaviour of EB plated beams moment redistribution was still present; however

the amount of moment redistribution achieved was considerably less than that of unplated beams.

Another form of retrofitting technique is near surface mounting, which is less susceptible to premature

debonding, and hence should allow greater moment redistribution to occur. Because of a lack of

existing experimental research on full scale continuous beams with NSM strips, in this research a test

program was conducted where nine continuous RC beams with near surface mounted (NSM) steel or

CFRP strips of various dimensions were tested. The specific aim of these tests was to both

demonstrate and measure moment redistribution in NSM plated flexural members, and to examine

how debonding can affect the moment redistribution behaviour. In this Chapter, the specimens, the

test set-up and the material properties are first described, followed by thorough descriptions of the

results from each test. Finally a summary of the test results are presented in the journal paper

included in Section 5.4.5, along with a comparison between the results for the different beams to

illustrate the effectiveness of the various plating systems.

5.4.1 GEOMETRY OF TEST SPECIMENS

The test program was divided into two series: ‘NS’ and ‘NB’; the NS test series consisting of six

specimens with the slab shaped cross-section given in Figure 5.73, and the NB test series consisting

of three specimens with the beam shaped cross-section shown by Figure 5.74. All specimens were

two-span continuous beams as illustrated in Figure 5.75, which were plated with CFRP or steel strips

of length Lp=2200mm in the hogging region over interior support only. Note that all the specimens

have the same reinforcement along the length of the beam, such that the tensile reinforcement in the

hogging region was much less than the tensile reinforcement in the sagging region, to ensure that the

plated hogging region reached its moment capacity first; this then allowed the hogging region to shed

moment, or redistribute moment, to the sagging region as the static moment was being increased,

thereby, increasing the sagging moment should the ductility of the hogging region allow. All beams in

the NS test series had W10 stirrups placed at 1200mm centre to centre (c/c) to hold the longitudinal

bars in position, while the specimens in the NB test series had W10 stirrups at 70mm centre to centre

to prevent shear failure.


- 352 -

2Y12

4Y16

W10 @ 1200c/c

120

375

36

38

[All dimensions in mm]

20mm clear cover

(a) (b)

2Y12

4Y16

38

bp

sp NSM strips

tp 36

Figure 5.73 Cross-sectional details of NS test series: (a) sagging region; (b) hogging region

(b) (a)

2Y12

3Y24

2Y32

36 bp

85

46

sp

tp

2Y12

3Y24

2Y32

36

85

46

240

220

W10 @ 70c/c

2Y12

3Y24

2Y32

dp

bp

tp (c)

46


20mm clear cover

Figure 5.74 Cross-sectional details of NB test series: (a) sagging region; (b) hogging region for beams

with tension face strips; (c) hogging region for beams with side face strips

East West

NSM strips

L=2400 L=2400 100 100

5000

1200 1100 1100 1200

internal bars

Load P

h


Load P

Figure 5.75 Two span continuous beam specimens


- 353 -

The purpose of this test program was specifically to study moment redistribution of NSM plated

flexural members and not to demonstrate the effectiveness of the strengthening method. In order to do

this, the plated beams were designed so that premature plate debonding was delayed or prevented

where possible such that the maximum plate strain and hence ductility could be achieved. In

designing the beams, it was assumed that premature debonding failure, such as plate end (PE)

debonding, critical diagonal crack (CDC) debonding, and intermediate crack (IC) debonding, that

occurs in EB plated beams (Oehlers and Seracino 2004) may also occur in NSM plated beams. Due

to the lack of design rules available for this relatively new retrofitting technique, test beams were

designed using analysis techniques for externally bonded plates (Oehlers and Seracino 2004;

Mohamed Ali et al. 2001; Smith and Teng 2003), with slight modifications to allow for the NSM strips,

in order to avoid brittle debonding failure of the beams. PE debonding was prevented by terminating

the plate beyond the point of contraflexure and onto the compression faces of the sagging regions.

CDC debonding was prevented by ensuring that a critical diagonal crack, associated with the concrete

component of the vertical shear capacity Vc, did not occur prior to plate debonding; because of the

CDC requirement, the slab shaped cross-section was used for the NS test series. Note that in the NS

test series, minimal shear reinforcement is used as shear is of minor concern in slabs, and stirrups

cannot prevent CDC debonding (Oehlers and Seracino 2004). Since beam shapes are more prone to

CDC debonding than slab shapes, as previously discussed in Section 5.3.1, large amounts of internal

longitudinal reinforcement was used in the NB test series to postpone the formation of critical diagonal

cracks, as Vc is dependent on the ratio of the internal reinforcement. However, it needs to be

emphasised that this test program was specifically designed to study moment redistribution, so that

the failure mechanism was of little importance.

The geometrical properties of the NSM strips used in the nine specimens tested are given in Table

5.9, where sp, tp and bp are the centre to centre spacing, thickness and width of each strip

respectively. The main variable in each test series was a plate property. In the NS test series, all

beams were 375mm wide by 120mm deep and the strips were near surface mounted to the tension

face over the interior support as shown in Figure 5.73 and Figure 5.75. Each NSM specimen was

prepared by first saw cutting grooves perpendicular to the concrete surface of the depth dg and width

bg given in Table 5.10. An FRP or steel strip was then embedded into each groove using adhesive as

shown by the illustration in Figure 5.76. For Specimen NS_F4, NS_S2 and NB_F3, where double

strips were used, two CFRP or steel strips were first glued together before embedding into the

grooves. Specimen NS_F1 had five strips and NS_F2 had two strips with a thickness tp of 1.2mm

spaced evenly over the tension face of the beams at spacing sp, and NS_F3 had one strip of


- 354 -

tp=1.2mm of CFRP at mid-width. Specimen NS_F4 had 2 CFRP strips of thickness 1.2mm which were

glued together with the thickness tp and Young’s modulus Ep measured directly from specimens taken

from the plated beam. Specimen NS_F1 used strips of widths bp=20mm, and NS_F2 to NS_F4 used

strips of width bp=15mm. Specimen NS_S1 had four high yield steel strips 1mm thick and 19mm wide,

spaced evenly at 75mm centres over the tension face of the beam. Specimen NS_S2 used four strips

with the same geometry as NS_S1, but with two 1mm thick strips glued together, with the thickness tp

measured directly from specimens taken from the plated beam, i.e. the glued strips were tested to

obtain the actual material properties. The two glued strips were spaced at 125mm centres.

In test series NB, all the beams were 240mm deep and 220mm wide, and CFRP strips 15mm wide

and 1.2mm thick were near surface mounted to different surfaces of the beam over the interior

support. For specimen NB_F1, two 1.2mm CFRP strips were applied on the two sides of the beam,

one on each side at a depth dp of 60mm from the tension face of the beam, as illustrated in Figure

5.74c. Specimen NB_F2 had two 1.2mm CFRP strips near surface mounted to the tension face of the

beam at 73mm centre to centre such as shown in Figure 5.74c. Specimen NB_F3 had four 1.2mm

CFRP strips, with two strips glued together, and were near surface mounted to the tension face of the

beam at 73mm centres, where the thickness tp was measured directly from specimens taken from the

plated beam.

Table 5.9 Geometrical properties of NSM strips

Specimens material No. of strips sp (mm) tp (mm) bp (mm)

NS_F1 CFRP 5 62 1.22 20.5

NS_F2 CFRP 2 125 1.24 15.5

NS_F3 CFRP 1 188 1.25 15.4

NS_F4 CFRP 2(glued)a 188 2.95d 15.2

NS_S1 steel 2 75 0.93 19.1

NS_S2 steel 2 x 2(glued)b 125 2.05d 19.1

NB_F1 CFRP 2 60c 1.25 14.8

NB_F2 CFRP 2 73 1.24 15.2

NB_F3 steel 2 x 2(glued)b 73 2.77d 15.0

a Two 1.2mm CFRP strips were first glued together before embedding into the grooves

b Two 0.9mm steel strips were first glued together before embedding into the grooves

c 1 strip near surface mounted to each of the 2 side faces at a depth of 60mm from tension face d measured thickness of the glued strips


- 355 -

Table 5.10 Groove dimensions

Specimens No. of grooves dg (mm) bg (mm)

NS_F1 5 21.7 3.30

NS_F2 2 16.8 3.40

NS_F3 1 15.9 3.57

NS_F4 1 15.9 5.01

NS_S1 4 21.0 3.40

NS_S2 2 21.0 5.24

NB_F1 2 16.1 3.55

NB_F2 2 16.5 3.68

NB_F3 2 16.5 5.43

bg

plate

dg bp

groove

adhesive tp

Figure 5.76 Illustration of grooves with embedded strips

5.4.2 TEST SETUP AND INSTRUMENTATION

The test set-up of the beams in both the NS and NB series are same as the externally bonded beam

tests discussed in Section 5.3.2, where concentrated loads P were applied at mid-span of the sagging

regions as in Figure 5.4 such that an elastic distribution of moment with an assumed constant flexural

rigidity, the maximum hogging moment would be 20% greater than the maximum sagging moment.

Deflections were measured under the applied loads P, load cells were placed at the applied loads and

at the west support, so that the distribution of forces could be determined directly. The test set-up,

moment distribution, and the test rig are shown by Figure 5.4, Figure 5.5 and Figure 5.6 respectively.

Nine strain gauges (SG) were attached to a NSM plate distributed from the central support along the

length of the strip to measure plate strains as well as to detect IC debonding. The strain gauges were

attached to the width of the strips prior to embedding the strips into the grooves. For all the test

beams, only one strip, usually the one positioned at the centre, had nine strain gauges placed over the

length of the beam at 200mm spacings from the centre support. The other strips each had 3 strain

gauges positioned at 200mm spacings over the centre support. The positions of the strain gauges for


- 356 -

the beams in the NS test series are shown in Figure 5.77 to Figure 5.81, and in Figure 5.82 to Figure

5.84 for the NB test series, where the number below each black mark denotes the strain gauge

numbering. Except for specimen NS_F1, all test beams in both series had a strain gauge (SGC)

placed on the compression face of each span near the interior support in order to measure the

maximum concrete compressive strain in the beam.

375

400 200 200 200 200 200 200 400

7 8 9 10 11 12 13 14 15

5 64

2 31

16 17 18

19 20 21

West East

interior support

strain gauge

strip 3

strip 1

strip 2

strip 4

strip 5

Figure 5.77 Strain gauges positions for beam NS_F1

3751 2 3 4 5 6 7 8 9

10 11 12

West East

400 200 200 200 200 200 200 400

200 200

interior support

strain gauge

strip 1

strip 2

SGC14

197200

SGC13

Figure 5.78 Strain gauges positions for beam NS_F2


- 357 -

3751 2 3 4 5 6 7 8 9

West East

400 200 200 200 200 200 200 400

interior support

strain gauge

NSM strip

SGC11

107.5107.5

SGC10

Figure 5.79 Strain gauges positions for beam NS_F3 and NS_F4

SGC20107.5107.5

SGC19

375

400 200 200 200 200 200 200 400

7 8 9 10 11 12

13 14 15

5 64

2 31

16 17 18

West East

interior support

strain gauge

strip 2

strip 1

strip 3

strip 4

Figure 5.80 Strain gauges positions for beam NS_S1

375

1 2 3

4 5 6 7 8 9 10 11 12West East

400 200 200 200 200 200 200 400

200 200

interior support

strain gauge

strip 2

strip 1

SGC14107.5107.5

SGC13

Figure 5.81 Strain gauges positions for beam NS_S2


- 358 -

2401 2 3 4 5 6 7 8 9

West East

400 200 200 200 200 200 200 400

interior supportSOUTH (Side)

24018 17 16 15 14 13 12 11 10

West East

400 200 200 200 200 200 200 400

interior supportNORTH(Side)

h=

h=

strip 1

SGC12107.5107.5

SGC11

strip 2

strain gauge

Figure 5.82 Strain gauges positions for beam NB_F1

2201 2 3 4 5 6 7 8 9

West East

400 200 200 200 200 200 200 400

interior supportTop view

121110strain gauge

strip 2

strip 1

SGC14107.5107.5

SGC13

b=


220

1 2 3 4 5 6 7 8 9

West East

400 200 200 200 200 200 200 400

interior supportTop view

121110

strain gauge

strip 2

strip 1

SGC14107.5107.5

SGC13

b=



- 359 -

5.4.3 MATERIAL PROPERTIES

The concrete properties of specimens in the NS and NB test series are given in Table 5.11 and Table

5.12 respectively, where Ec, ft and fc are the Young’s modulus, tensile strength and cylinder

compressive strength respectively.

Table 5.11 Concrete properties for beams in NS test series

Age Sample No. Ec (MPa) ft (MPa) fc (MPa) Density (x10-3 g/mm3)

1 40530 3.51 38.78 2.34

2 38937 4.40 36.03 2.32

3 38472 3.92 36.00 2.33

306a

(Days)

Average 39313 3.94 36.94 2.33

1 37657 4.05 37.26 2.32

2 38398 3.86 37.88 2.32

3 37473 3.85 36.43 2.32

364b

(Days)

Average 37842 3.92 37.19 2.32

Average 38578 3.93 37.06 2.33

a Concrete age before test

b Concrete age after test

Table 5.12 Concrete properties for beams in NB test series

Age Sample No. Ec (MPa) ft (MPa) fc (MPa) Density (x10-3 g/mm3)

1 37287 3.40 34.88 2.33

2 37451 3.52 32.43 2.31

3 36116 - 33.33 2.29

173a

(Days)

Average 36951 3.46 33.55 2.31

1 31370 3.30 36.32 2.32

2 30681 3.85 37.44 2.31

3 29559 4.12 35.55 2.30

307b

(Days)

Average 30537 3.76 36.44 2.31

Average 33744 3.61 34.99 2.31

a Concrete age before test

b Concrete age after test

Tensile tests were performed on the internal reinforcing bars used in the test specimens to obtain the

yield fy and fracture frac strengths of the bars as given in Table 5.13. Being mild steel, the bar’s

Young’s modulus Eb can be assumed to be 200GPa.


- 360 -

Table 5.13 Material properties of reinforcing bars

Bars Sample no. fy (MPa) fu (MPa)

1 561 650

2 554 642

3 558 647 Y12

Average 558 646

1 565 656

2 567 659

3 565 657 Y16

Average 566 657

1 600 699

2 600 698

3 600 701 Y24

Average 600 699

1 576 684

2 579 685

3 579 684 Y32

Average 578 684

The material properties of the near surface mounted plates were obtained from tensile tests and are

given in Table 5.14. For FRP plates, the plates do not yield, and so, the on-axis (longitudinal) Young’s

modulus was measured directly. For the double CFRP and steel strips used in specimens NS_F4,

NS_S2 and NB_F3, their thickness tp, yield strength fp.y, fracture strength fp.frac, and Young’s modulus

Ep were measured directly from specimens that were taken from the plated beam i.e. the glued strips

were tested to obtain the actual material properties.

Before near surface mounting the plates, the steel strips were sand blasted to remove rust or loose

particles, and any grease or oil was removed by applying non-inflammable solvents, and the CFRP

strips were cleaned with any grease or oil removed using a solvent prior to application. Each concrete

specimen was prepared by saw cutting grooves perpendicular to the concrete surface of dimensions

given in Table 5.10. The grooves were then injected with epoxy adhesive to provide the necessary

bond with the surrounding concrete. Araldite K340 and MBrace Laminate adhesive were the epoxy

resins used for the steel and CFRP plates respectively with the properties given in Table 5.8. The FRP

or steel strips were then inserted into the grooves ensuring that they were completely covered with

epoxy as illustrated in Figure 5.76.


- 361 -

Table 5.14 Properties of NSM strips

Material Sample no. tp (mm) fy (MPa) fu (MPa) Ep (GPa)

1 0.89 840a 934 191258

2 0.89 837a 937 180538

3 0.89 835a 929 177034 Steel

average 0.89 837 a 933 182943

1 1.98 700 841 162883

2 1.95 700 861 170402

3 2 700 838 170880

Steel (double stripb)

average 1.98 700 846 168055

1 1.23 - 2761 173260

2 1.22 - 2892 174970

3 1.23 - 2736 172273 CFRP

average 1.23 - 2796 173501

1 2.77 - 2141 128553

2 2.82 - 2541 151841

3 2.82 - 2311 140028

CFRP (double stripb)

average 2.80 - 2331 140141

a Proof stress b 2 strips glued together; properties measured directly from glued specimens

5.4.4 TEST RESULTS

In the following section, descriptions of the behaviour of the beam as loads were gradually applied,

and analyses of the test results are presented for each test specimen. In the beams tested, it was

often difficult to define exactly when failure occurred in these continuous beams, as concrete crushing

often occurred in the sagging region while the hogging region remained relatively strong. As a result,

the maximum moment in the hogging region Mhog and the load applied continued to increase, while

the maximum moment in the sagging region Msag reduced gradually. Because the beams were plated

over the hogging regions, the plate strain was also increasing, until rapid reduction in applied load

occurred, either due to severe concrete crushing or debonding of the plates. Here in this context,

failure is defined as when the applied load reduces. All beams were tested under load control until the

maximum applied load was obtained, and thereafter, displacement control was adopted based on the

deflections ∆ measured at midspan.


- 362 -

5.4.4.1 BEAM NS_F1 (5 X CFRP1.2MM)

The beam had five strips of 20mm x 1.2mm CFRP near surface mounted to the tension face of the

beam over the hogging region; the numbers in Figure 5.85 to Figure 5.89 refer to the applied

concentrated loads P at each span as shown in Figure 5.4, and the dotted lines denote the positions

of the NSM strips. The flexural crack marked A in Figure 5.85 was a pre-existing crack that formed

during test preparation. Flexural cracking was first observed at an applied load P of 11.8kN, with an

average reaction force R of 3.8kN at the external supports. This caused a maximum moment at the

hogging Mhog and sagging Msag regions of 5.04kNm and 4.56kNm respectively. Upon further loading, a

lot of flexural cracks formed over the hogging region as shown in Figure 5.85. These intermediate

cracks which intercepted the plate caused slip to occur at the plate/concrete interfaces, and hence,

resulted in the formation of small diagonal ‘branching’ or herringbone cracks at E and F around the

intermediate flexural cracks B and C respectively in Figure 5.85. These branching cracks are

equivalent to the horizontal IC interface cracks that form in beams with externally bonded plates

(Section 5.3) as both are caused by the sliding between the plates and the adjacent concrete, and will

therefore be referred to as IC interface cracks in the following context. The first IC interface crack

formed at P=23.7kN (R=7.9kN, Mhog=9.5kNm, Msag=9.36kNm).

Figure 5.85 Beam NS_F1: Flexural cracking and IC interface cracking (P=28kN)

As the load continued to increase, more IC interface cracks formed, propagating away from the root of

the intermediate crack such as G and H which branched out from flexural crack I in Figure 5.86.

Comparing the crack distributions at P=28kN in Figure 5.85 with that at P=45kN in Figure 5.86, it can

be seen that most of the flexural cracks formed along the beam at small applied loads, and as the load

east west

Interior support

A

E

C B F


- 363 -

was further increased, mostly branching or herringbone cracks formed which signified slip propagation

as shown in Figure 5.86.

Figure 5.86 Beam NS_F1: Further development of herringbone cracks (P=45kN)

Due to the strong bond that occurred between the NSM plates and the adjacent concrete, premature

debonding was delayed allowing large loads to be applied to the specimen, which eventually led to

shear failure near the west applied load in Figure 5.87 at P=63.3kN (R=21.1kN, Mhog=25.3kNm,

Msag=25.3kNm). Near failure of the beam, minor concrete crushing was also observed in the sagging

region next to the loading plates. This suggests that the flexural capacity may have been reached in

the sagging region at failure. Figure 5.88 shows the distribution of the herringbone cracks at beam

failure, where it can be seen that although many branching cracks have developed which indicated

that there was slip at the plate/concrete interfaces, severe debonding was not evident. From the test,

a maximum moment redistribution of 13.9% was achieved at P=42.8kN (R=14.5kN, Mhog=16.6kNm,

Msag=17.4kNm). Based on the measured plate strain and from full interaction analysis, it is estimated

that the tensile bars yielded at the maximum hogging moment position at a load of 44.7kN (R=15kN).

The distribution of flexural cracks in the hogging region upon failure is shown in Figure 5.89, where

irregular crack spacing was observed. At failure, an average crack spacing of 76mm was measured,

where approximately 8 flexural cracks formed over the hogging region of each span, with the last

flexural cracks in the region, J and K in Figure 5.89, at approximately 430mm from the interior support.

H

east

Interior support

I

G

west


- 364 -

Figure 5.87 Beam NS_F1: Shear failure in sagging region (P=66.3kN)

Figure 5.88 Beam NS_F1: Hogging region at shear failure(P=66.3kN)

Figure 5.89 Beam NS_F1: Flexural crack distribution at failure (P=66.3kN)

The variation of the moment, at the position of the maximum hogging and sagging moments, with the

mean deflection under the applied loads at mid-spans, are plotted in Figure 5.90 for specimen NS_F1.

In region A, when the beams were still linear elastic, the hogging moment was roughly equal to the

sagging moment as would be expected from an elastic analysis, since only small amounts of CFRP

Interior support west

P

east west

Interior support

east west

Interior support

J

K


- 365 -

was used in the hogging region. After herringbone cracks occurred at point B in Figure 5.90, due to

the lower stiffness, the moment in the hogging region Mhog is less than that in the sagging region Msag.

The maximum moment redistribution was achieved at point C. Minor concrete crushing was first

observed in the sagging region at E, followed by shear failure in the sagging region at F, which caused

a reduction in both Msag and the applied load P. However, as the hogging region has not yet failed,

more moment is redistributed to the hogging region, hence a small increase in Mhog was observed

(region G in Figure 5.90) as the displacement continues to increase. It is worth noting that the

reduction in P and hence Msag after failure at F was gradual initially until severe sliding action occurred

along the critical diagonal crack, which resulted in a rapid reduction in load as well as the moments in

the specimen at region H in Figure 5.90.

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45displacement (mm)

Mho

g (k

Nm

)

A

1st herringbonecrack

max %MRB

C D

baryield

conc crushing in sag

E

Msag

Mhog

F

shearfailure

insag

G

H

M (

kNm

)

Figure 5.90 Beam NS_F1: Moment vs displacement

Figure 5.91 shows the plate strains measured along the NSM strips as the moment over the interior

support Mhog increased, where the positions of the strain gauges SG are given in Figure 5.77. The

dotted line A represents Mhog at which shear failure occurred in the sagging region, and (C) is the SG

at the interior support. Steady increases in plate strains were observed, with no sudden reduction or

increase in plate strain, which indicates sudden loss of bond force due to severe debonding did not

occur. The average plate strain at the position of maximum hogging moment, i.e. at the interior

support, was 0.0042 when the maximum percentage moment redistribution was achieved. In the test,

a maximum plate strain εp.max of 0.0072 was recorded upon failure at SG20 i.e. strip 5 at the interior

support (Figure 5.77). After shear failure occurred in the sagging region, as the hogging region had


- 366 -

not yet failed, a small increase in Mhog (region G in Figure 5.90), as well as the plate strain (Figure

5.91), was observed until rapid reduction in the applied load took place.

0

5

10

15

20

25

30

-2000 0 2000 4000 6000 8000strain (x10-6)

Mho

g (k

Nm

)

SG1 SG2(C) SG3 SG4SG5(C) SG6SG7 SG8SG10 SG11(C) SG12 SG13SG14 SG15SG16 SG17(C) SG18 SG19SG20(C) SG21

A

εεεεp.max

Figure 5.91 Beam NS_F1: Moment vs plate strain

The variation of the maximum hogging moment in the beam Mhog as a proportion of the maximum

sagging moment in the beam Msag is shown in Figure 5.92. From an elastic analysis in which EI is

assumed to be constant, Mhog/Msag = 1.2 which is shown as line A in Figure 5.15. The abscissa in

Figure 5.15 is the applied static moment, Mstatic given by Equation 5.2, as a proportion of the ultimate

maximum static moment, (Mstatic)u = (Msag)u + (Mhog)u/2, based on nonlinear full interaction analysis of

the ultimate capacity of the hogging and sagging sections, (Mhog)u and (Msag)u, and ignoring IC

debonding in the case of the hogging region; hence the upper limit of Mstatic/(Mstatic)u = 1.0. For the

plated beam considered, (Mstatic)u=42.9kNm. The line marked B is the maximum redistribution

assuming the sagging region achieved its theoretical moment capacity Msag=(Msag)u, while the capacity

of the hogging region is the moment experimentally measured at failure Mhog=(Mhog)fail; and the line

marked C is the maximum redistribution after plate debonding that is when (Mhog)u is the theoretical

ultimate capacity of the unplated section.

When the load was first applied Mhog/Msag approached 1.2 in region D in Figure 5.92 with a slight

divergence because the beam was bedding or settling down under very small loads. Soon after,

Mhog/Msag reduced gradually and the divergence from Mhog/Msag equal to 1.2 signifies moment


- 367 -

redistribution. First sign of flexural cracking was observed at point E. This was shortly followed by the

appearance of the first herringbone crack at F. The maximum moment redistribution occurs when the

divergence of Mhog/Msag from 1.2 is the greatest, which is at point G in Figure 5.92. Yielding of the

tensile bars in the hogging region then follows immediately at point H. Minor concrete crushing in the

sagging regions near the applied loads was observed at I, soon after, shear failure in the sagging

region of the west span occurred at J, which coincided with the maximum plate strain. As the hogging

region has not yet failed, more moment is redistributed to the hogging region, which resulted in an

increase in Mhog/Msag at K.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1

Mstatic/(Mstatic)u

Mh

og/M

sag

A

test results

B

C

1st flexural crack

1st debonding conc crush insagging region

elastic (EI constant)

shearfailure

max MR

DE

FG

H

I

J

K

Figure 5.92 Beam NS_F1: hogging-moment/sagging-moment

Figure 5.93 shows the variation of the maximum hogging Mhog and sagging Msag moments as the

applied loads P increased. (Mhog)el and (Msag)el are the hogging and sagging moments obtained based

on elastic analyses at constant EI. The greater the divergence from the elastic moments means that

more moment is being redistributed. It can be seen from Figure 5.93 that the beam behaved elastically

until the first flexural crack occurred, after which small amounts of hogging moment were redistributed

to the sagging regions. More moment was redistributed when debonding occurred as observed by the

formation of herringbone cracks (Figure 5.86). Minor concrete crushing in the sagging region occurred

before the beam failed due to shear near the applied load.


- 368 -

0

5

10

15

20

25

30

35

0 20 40 60 80load (kN)

Mho

g (k

Nm

)

0

5

10

15

20

25

30

0 20 40 60 80load (kN)

Msa

g (

kNm

)

1st debonding

shear failure

(Mhog)el

(Msag)el1st flexural

crack

concretecrushing

Figure 5.93 Beam NS_F1: Maximum hogging and sagging moments

The variation of percentage of moment redistribution %MR calculated using Equation 5.1 is shown in

Figure 5.94 for different Mstatic applied. Initially, before flexural cracking, the beam behaved elastically

such that there is zero moment redistribution. The discrepancy of results at A is because the beam

was bedding or settling down under very small loads. As the applied load, and hence Mstatic increased,

the %MR increased up to a maximum of 13.9% at B. Upon further loading, small reductions in %MR

were observed, and when concrete crushing occurred in the sagging region at C, the reduction

became more rapid. Eventually at point D shear failure occurred in the sagging region at 11% of

moment redistribution.

0

2

4

6

8

10

12

14

16

0 0.2 0.4 0.6 0.8 1

Mstatic/(Mstatic)u

% M

omen

t red

istr

ibut

ion

A

1st flexuralcrack

1st debonding

max MR

B

C

D

shear failure

Figure 5.94 Beam NS_F1: percentage of moment redistribution


- 369 -


The beam had two strips of 15mm x 1.2mm CFRP near surface mounted to the tension face of the

beam over the hogging region. The flexural crack marked A in Figure 5.95 was first observed at an

applied load P of 5.8kN (R=1.9kN, Mhog=2.4kNm, Msag= 2.28kNm). Upon further loading, a lot of

flexural cracks formed over the hogging region at irregular crack spacings as shown in Figure 5.95.

These intermediate cracks which intercepted the plates caused slip to occur at the plate/concrete

interfaces, and hence, resulted in the formation of herringbone cracks at C around the intermediate

flexural crack B in Figure 5.95. This signifies IC interface cracking, with the first IC interface crack

formed at P=17.8kN (R=6.2kN, Mhog=6.48 kNm, Msag=7.44kNm).


As the load continued to increase, more IC interface cracks formed, propagating away from the roots

of the intermediate flexural cracks as indicated by the arrows in Figure 5.96. Note how in between two

adjacent flexural cracks, such as A-D and A-B in Figure 5.96, IC interface cracks propagated in both

directions. The reverse direction of propagation is also indicated by the reversal in inclination of the

cracks. These are the classical herringbone formation of cracks associated with shear cracking in

concrete components. Hence there is reversal in slip and bondstress, between the two flexural cracks

and a point of zero slip occurs between the flexural cracks. It was found in the tests that most of the

flexural cracks formed along the beam at small applied loads, and as the load was further increased,

mostly branching or herringbone cracks formed which signified slip propagation as shown in Figure

5.96.

Interior support

A B

C


- 370 -

Figure 5.96 Beam NS_F2: Further development of herringbone cracks (P=35kN)

Upon further loading, the IC interface cracks that formed in between adjacent flexural cracks were in

an opposite direction to those formed previously, as shown by the crossing of the IC interface cracks

between flexural cracks A-D, A-B and B-E in Figure 5.97, which clearly showed that debonding cracks

reversed in direction and started propagating in a single direction towards the plate end, that is from A

to G and A to F in Figure 5.97. This indicates that slip and bond reversal no longer exist between

cracks A and E that is, there is no point of zero slip between the cracks. Due to the curvature in the

beam and the large perpendicular stiffness of the plate, it was noticed that at P=62.9kN a layer of the

FRP fibre detached from the plate surface.

Figure 5.97 Beam NS_F1: Hogging region at beam failure (P=64kN)

east

west

Interior support

D A B

east west

Interior support

D A B G E F

F G


- 371 -

Although much debonding was observed near beam failure as can be seen in Figure 5.97, this did not

cause a reduction in the applied load as significant amounts of bond were still present, especially

between the last flexural cracks F and G to the plate ends. Failure was caused by severe concrete

crushing in the sagging region near the east applied load at P=63kN (R=23.2kN, Mhog=19.8kNm,

Msag=27.9kNm) with a maximum plate strain of 0.0129 recorded at failure. Close to failure of the

beam, minor concrete crushing was also observed in the hogging region, however this did not reduce

the strength of the region. As the testing was continued under displacement ∆ control after failure

occurred in the sagging region, much of the moment in the sagging region Msag was redistributed to

the hogging region Mhog, which has not yet failed. This caused increases in Mhog, and hence the plate

strain, while Msag and the applied load gradually reduced. As a result, rapid debonding propagation

occurred in the east span, i.e. from F to H and F to J in Figure 5.98. A close up of the herringbone

cracks formed between crack F in Figure 5.98 and the east plate end is given in Figure 5.99, where it

can be seen through the inclination of the cracks that the plate is pulling through, similar to that

observed in pull tests.

Figure 5.98 Beam NS_F2: Debonding propagation after sagging region failure (∆ =67mm)

Figure 5.99 Beam NS_F2: Debonding from last flexural crack F after sagging region failure (∆ =67mm)

east

Interior support

D A B E F G

H

J

F G

J

F


- 372 -

As the beam displacement continued to increase, the plate strain increased until, at a deflection at

midspan ∆ of 67mm, where a maximum plate strain of 0.0154 was achieved, the plate strain began to

reduce and rupture of the plate soon followed at a deflection of 76.5mm as shown in Figure 5.100.

Figure 5.100 Beam NS_F2: Plate rupture after sagging region failure(∆ =76.5mm)

From the test, a maximum moment redistribution of 30.9% was achieved at P=61.8kN (R=22.9kN,

Mhog=19.2kNm, Msag=27.5kNm) with a maximum recorded plate strain of 0.012 prior to beam failure.

Based on the measured plate strain and from full interaction analysis, it is estimated that the tensile

bars yielded at the maximum hogging moment position at a load of 24.7kN (R=8.7kN). The distribution

of flexural cracks in the hogging region upon failure is shown in Figure 5.97, where irregular crack

spacing was observed. At failure, an average crack spacing of 85mm was measured, where

approximately 4 flexural cracks formed over the hogging region of each span, with the last flexural

cracks in the region, F and G in Figure 5.97, at approximately 348mm from the interior support.


mean deflection under the applied loads at mid-spans, are plotted in Figure 5.101 for specimen

Interior support

A

K

K

F

F

G

G


- 373 -

NS_F2. It can be seen that near the start, in region A, when the beams were still linear elastic, the

hogging moment was roughly equal to the sagging moment as would be expected from an elastic

analysis, since only small amounts of CFRP were used in the hogging region. After cracking occurs at

point B in Figure 5.101, due to the lower stiffness, the moment in the hogging region Mhog is less than

that in the sagging region Msag. Concrete crushing failure occurred in the sagging region at point C,

which caused a reduction in both Msag and the applied load. However, as the hogging region has not

yet failed, more moment is redistributed to the hogging region, hence a small increase in Mhog, as well

as the plate strain, was observed (point D in Figure 5.101) as the displacement continues to increase.

After concrete crushing occurs in the sagging region, the applied load reduces until at point E major

debonding cracks were found to propagate along the beam, causing gradual reduction in Mhog while

the plate strain continued to increase until eventually rupture of the plates occurred.

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80

MhogMsag

mom

ent(

kNm

)

displacement (mm)

BA

Cconc. crushing failure

in sag.

D E

majordebonding plate rupture

MhogMsag



support Mhog increased up to beam failure, where the positions of the strain gauges SG are given in

Figure 5.78. The dotted line A represents Mhog at which concrete crushing failure occurred in the

sagging region, and (C) is the SG at the interior support. Sudden increases in plate strains were

observed at B due to the formation of flexural cracks near SG4, SG6, SG10 and SG12. The average

plate strain at the position of maximum hogging moment, i.e. at the interior support, was 0.0120 when

the maximum percentage moment redistribution was achieved. In the test, a maximum plate strain

εp.max of 0.0129 (average of the two strips) was recorded over the interior support at concrete crushing

failure. After concrete crushing failure occurred in the sagging region, as the hogging region had not

yet failed, a small increase in Mhog (region D in Figure 5.101) with rapid increases in plate strains at

S7, SG8, and SG12 was observed, as marked by C in Figure 5.102, due to the debonding


- 374 -

propagation in the east span as shown in Figure 5.98 and Figure 5.100. Eventually strip 1 (Figure

5.78) ruptured causing sudden reduction in Mhog and plate strain. Compared to beam NS_F1 (Figure

5.91), the increase in plate strain for NS_F2 tended to be more rapid and much higher plate strains

were obtained along the strips.

0

2

4

6

8

10

12

14

16

18

20

22

-4000 1000 6000 11000 16000strain (x10-6)

Mho

g (k

Nm

)

SG10SG11(C) SG12SGC13SGC14

0

2

4

6

8

10

12

14

16

18

20

22

-4000 -2000 0 2000 4000 6000 8000 10000 12000 14000strain (x10-6)

Mho

g (k

Nm

)

SG1SG2SG3SG4SG5(C) SG6SG7SG8SG9

A

B

B

B

B

A C

C

C

SG5

SG4

SG6

SG7

SG8

SG3

SG9

SG11

SG10SG12

SG14




assumed to be constant, Mhog/Msag = 1.2 which is shown as line A. The line marked B is the maximum

redistribution for Msag=(Msag)u and Mhog=(Mhog)fail; and the line marked C is the maximum redistribution


- 375 -

for unplated sections. After flexural cracking occurred at D, much divergence of Mhog/Msag from line A

was observed, which signified moment redistribution. The amount of moment redistribution gradually

increased after debonding began at E. Eventually, the maximum moment redistribution was achieved

at F, and soon after, concrete crushing in the sagging region near the applied load caused the applied

load to reduce, that is failure occurred. As the hogging region has not reached its flexural capacity,

more moment was redistributed to the hogging region after the sagging region failed, which caused

Mhog/Msag to increase until plate ruptured at I. It is worth noting that the maximum moment

redistribution (point G) achieved was close to the maximum moment redistribution allowable for the

plated beam (line B), and failure occurred at Mstatic/(Mstatic)u close to 1.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1Mstatic/(Mstatic)u

Mho

g/M

sag

A

test resultsB

C

1st flexural crack

1st debonding plate rupture


conc crushingfailure (sag)

max MR

bar yield

D

E F

GH

I




on elastic analysis of constant EI. It can be seen that after flexural cracking and plate debonding

occurred at points A and B respectively, much of the hogging moment was redistributed to sagging

region, causing Msag to increase as Mhog reduced. However when concrete crushing occurred in the

sagging region at C, this reduced Msag and hence, increased Mhog, which had yet to reach its ultimate

capacity, as shown by points D and E respectively; until the plate ruptured at F.


- 376 -

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70load (kN)

Msa

g(kN

m)

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70load (kN)

Mho

g(k

Nm

)

1st debonding

platerupture

(Mhog)el

(Msag)el 1st flexuralcrack

concretecrushing

AB

C

DE

F



Figure 5.105 for different Mstatic applied. As the applied load, and hence Mstatic increased, the %MR

increased up to a maximum of 30.9% at C, after which a small reduction in %MR was recorded.

However when concrete crushing occurred in the sagging region at D at 30.1%MR, this caused rapid

reduction in %MR due to the increasing Mhog (Figure 5.104) as denoted by region E in Figure 5.105,

until eventually plate ruptured at F.

0

5

10

15

20

25

30

35


% M

omen

t red

istr

ibut

ion

A

1st flexuralcrack

1st debonding

max MR

B

C D


E

F platerupture



The beam had one strip of 15mm x 1.2mm CFRP near surface mounted to the tension face of the

beam over the hogging region. The flexural crack marked A in Figure 5.106 was first observed at an

applied load P of 5.9kN (R=2kN, Mhog=2.4kNm, Msag= 2.3kNm). As more flexural cracks formed and


- 377 -

intercepted the plate, it induced slip at the plate/concrete interfaces, and hence, resulted in the

formation of herringbone cracks at C and D around the intermediate flexural cracks A and B in Figure

5.106 at P=17.8kN (R=6.2kN, Mhog=6.48 kNm, Msag=7.44kNm).


As the load continued to increase, more flexural cracks formed causing IC interface cracks to

propagate away from the roots of the intermediate cracks as shown by C and D in Figure 5.107. From

the inclination of the herringbone cracks that formed in between adjacent cracks such as between

cracks A and B, it can be seen that IC debonding propagated in both directions, that is the bondstress

and slip reversed directions in between cracks as indicated by the arrows in Figure 5.107.

Figure 5.107 Beam NS_F3: Further development of herringbone and flexural cracks (P=39kN)

Interior support

A B

C D

east west

Interior support

A B

C D

east west


- 378 -

Upon further loading, the IC debonding between adjacent cracks began to propagate more dominantly

in a single direction towards the plate end, as evident from the crossing of the IC interface cracks

between flexural cracks i.e. cracks A and B, and the region marked J in Figure 5.108. A close up of

the debonding propagation of region J in Figure 5.108 is illustrated in Figure 5.109. This indicates that

slip and bond reversal no longer exist between the adjacent cracks that is, there is no point of zero slip

between the cracks. Due to the curvature in the beam and the large perpendicular stiffness of the

plate, it was noticed that at P=60kN a layer of the FRP fibre detached from the plate surface.

Figure 5.108 Beam NS_F3: Hogging region at beam failure (P=61kN)

Figure 5.109 Beam NS_F3: Debonding propagation (P=61kN)

Failure was caused by severe concrete crushing in the sagging region near the applied load at

P=61.4kN (R=23kN, Mhog=18.9kNm, Msag=27kNm) with a maximum plate strain of 0.0148 recorded at

Interior support

A B

C D

east

west F E G H

J

J

A F G


- 379 -

failure. Minor concrete crushing was also observed in the hogging region, however this did not reduce

the strength of the region. As the testing was continued under displacement ∆ control after failure

occurred in the sagging region, Msag, Mhog, and P gradually decreased while the deflection and the

plate strain continued to increase. Unlike beams NS_F1 and NS_F2, the hogging moment Mhog did not

increase after the beam failed. This was because the flexural capacity of the hogging region had

already been reached as indicated by minor concrete crushing near the interior support. As the plate

strain continued to increase with deflection, this caused severe debonding to occur in the west span

as shown by K in Figure 5.110. A close up of the severe debonding in the east span (K in Figure

5.110) is given in Figure 5.111, where it can be seen through the inclination of the cracks that the

plate was pulling through, which caused some of the concrete to detach from the beam. As the beam

displacement continued to increase, the plate strain increased until at a deflection at midspan ∆ of

50.44mm, where a maximum plate strain of 0.015 was recorded, after which plate strain began to

reduce due to major debonding (Figure 5.110).

Figure 5.110 Beam NS_F3: Debonding after sagging region failure

Figure 5.111 Beam NS_F3: Debonding between SG3 and SG4 after sagging region failure

A B

C D

east

west F E G H

K

Interior support

K

H

K

H


- 380 -

From the test, a maximum moment redistribution of 32.3% was achieved at P=61.1kN (R=23kN,

Mhog=18.6kNm, Msag=27kNm) with a maximum recorded plate strain of 0.0137 prior to concrete

crushing failure in the sagging region. Based on the measured plate strain and from full interaction

analysis, it is estimated that the tensile bars yielded at the maximum hogging moment position at a

load of 14.7kN (R=4.9kN). The distribution of flexural cracks in the hogging region upon failure is

shown in Figure 5.108, where irregular crack spacing was observed. At failure, an average crack

spacing of 114mm was measured, where approximately 3 flexural cracks formed over the hogging

region of each span, with the last flexural cracks in the region, E and H in Figure 5.108, at

approximately 311mm from the interior support.



NS_F3. After cracking at point B, the moment in the hogging region Mhog became less than that in the

sagging region Msag indicating that moment redistribution had occurred. The maximum moment

redistribution was obtained at C followed by concrete crushing failure in the sagging region at point D.

After failure, Msag and the applied load reduced, while Mhog remained roughly constant as the

displacement continues to increase, which indicates that the hogging region has reached its capacity.

Eventually at point E, when major debonding cracks occurred (K in Figure 5.110), this caused a rapid

reduction in Mhog as shown in Figure 5.112.

0

5

10

15

20

25

30

0 10 20 30 40 50 60

displacement (mm)

Mom

ent

(kN

m)

A

1st herringbonecrack

max %MR

B

C

D

baryield

conc crushingin sag

Msag

Mhog

E

majordebonding



- 381 -



Figure 5.79. The dotted line A represents Mhog at which concrete crushing failure occurred in the

sagging region, and (C) is the SG at the interior support. A sudden increase in plate strain was

observed at B due to the formation of flexural crack near SG4. The plate strain at the position of

maximum hogging moment, i.e. at the interior support, was 0.0137 when the maximum percentage

moment redistribution was achieved; and a maximum plate strain εp.max of 0.0148 was recorded over

the interior support at concrete crushing failure. After concrete crushing failure occurred in the sagging

region, Mhog remained roughly constant while plate strains increased at S4, SG5, and SG6, as marked

by C in Figure 5.113. Eventually major debonding occurred propagating from SG6 to SG3 (Figure

5.110), which caused the plate strains at SG4, SG5 and SG6 to reduce (D in Figure 5.113), and SG3

to increase at E in Figure 5.113.

0

2

4

6

8

10

12

14

16

18

20

-5000 -2500 0 2500 5000 7500 10000 12500 15000strain (x10-6)

Mho

g (k

Nm

)

SG1 SG2SG3 SG4SG5(C) SG6SG7 SG8SG9 SGC10SGC11

A

SG11

SG10 SG1&SG9

SG7

SG3

SG6SG4

SG5

at max MR

εp.max

B

C

D

E

C

DD



sagging moment in the beam Msag is shown in Figure 5.114. Line A represents the elastic analysis in

which EI is assumed to be constant i.e. Mhog/Msag = 1.2. The line marked B is the maximum


for unplated sections. After IC debonding initiated at D, much divergence of Mhog/Msag from line A was

observed, which signified moment redistribution. Eventually, the maximum moment redistribution was

achieved at E, and soon after, concrete crushing in the sagging region near the applied load caused


- 382 -

the applied load to reduce, that is failure occurred at F. It is worth noting that the maximum moment

redistribution (point E) achieved was closed to the maximum moment redistribution allowable for the

plated beam (line B), and failure occurred at Mstatic/(Mstatic)u close to 1.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Mstatic/(Mstatic)u

Mho

g/M

sag

A

test results

B

C

1st flexural crack

1st debonding

conc crush insagging region


max MRD

F

E




on elastic analysis of constant EI. It can be seen that after flexural cracking and plate debonding

occurred at points A and B respectively, much of the hogging moment was redistributed to sagging

region, hence causing Msag to increase as Mhog reduced. However when concrete crushing occurred in

the sagging region at C, this gradually reduced Msag and Mhog until major plate debonding occurred,

which caused Mhog to reduce rapidly (point D).

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

load (kN)

Mho

g (k

Nm

)

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

load (kN)

Msa

g (k

Nm

)

1st debonding

majordebonding

(Mhog)el

(Msag)el 1st flexural

crack

concretecrushing

AB

C

D



- 383 -



increased up to a maximum of 32.3%, soon followed by concrete crushing failure in the sagging region

at 31.4%MR. After failure, %MR remained roughly constant until major debonding occurred which

caused Mhog to reduce rapidly (Figure 5.115), and hence, resulted in a rapid increase in %MR as

denoted by region A in Figure 5.116.

0

5

10

15

20

25

30

35

40

45


% M

omen

t red

istr

ibut

ion A

1st flexuralcrack

1st debonding

max MR


major debonding


5.4.4.4 BEAM NS_F4 (1 X 2CFRP1.2MM)

This beam was near surface mounted with two strips of 1.2mm thick CFRP which were glued together

before embedded into the concrete. The crack propagation leading to intermediate crack IC

debonding is illustrated in Figure 5.117 to Figure 5.120; the numbers in the figures refer to the applied

concentrated loads P at each span. The flexural crack marked A in Figure 5.117 first occurred

immediately over the hogging support at an applied load P of 3.05kN (R=1.1kN, Mhog=1.08kNm, Msag=

1.29kNm). Upon further loading at P=5.9kN (R=2.25kN, Mhog=1.68kNm, Msag= 2.7kNm), this crack,

which intercepts the plate, caused slip to occur at the plate/concrete interfaces, and, hence, resulted

in the formation of small diagonal ‘branching’ cracks at B around the intermediate flexural crack.

These branching cracks are equivalent to the horizontal IC interface cracks that form in beams with

externally bonded plates (Oehlers et al. 2004a) as both are caused by the sliding between the plates

and the adjacent concrete, and will therefore be referred to as IC interface cracks in the following

context.


- 384 -


Figure 5.118 Beam NS_F4: Propagation of IC interface cracks (P=55kN)

Figure 5.119 Beam NS_F4: At debonding failure (P=58kN)

A

B C

A

D

E

E

F G

I NSM CFRP strip

Centre support

westeast

A

A

E

E

H B F

I NSM CFRP strip

Centre support

west east

I J

I

NSM CFRP strip

Centre support

west east


- 385 -

Figure 5.120 Beam NS_F4: Plate completely debonded

As the load continued to increase, more IC interface cracks formed, propagating in both directions

such as from B to C and B to D in Figure 5.117. Note how in between the two flexural cracks A and E

IC interface cracks propagated in both directions, from B to C and F to G. The reverse direction of

propagation is also indicated by the reversal in inclination of the cracks. These are the classical

herringbone formation of cracks associated with shear cracking in concrete components. Hence, there

is reversal in slip and bondstress between the two flexural cracks and a point of zero slip occurs

between the flexural cracks. It was found in the tests that most of the flexural cracks formed along the

beam at small applied loads, and as the load was further increased mostly branching or herringbone

cracks formed which signified slip propagation as shown in Figure 5.118. Compared to Figure 5.117,

more IC interface cracks are present in Figure 5.118 which propagated predominantly towards the

plate ends, as shown by the arrows in Figure 5.118.

As mentioned previously, at small loads points of zero slip occurred between flexural cracks, such as

A and E in Figure 5.117, as the interface cracks were found to propagate from B to C and from F to G.

However, as more load was applied, the IC interface cracks that formed near F were in an opposite

direction to those formed previously, as shown by the crossing of the IC interface cracks encircled at

H in Figure 5.118. This clearly shows that debonding cracks reversed in direction and started

propagating in a single direction towards the plate end, that is from B-F in Figure 5.118. This confirms

that slip and bond reversal no longer exist between cracks A and E that is, there is no point of zero

slip between the cracks.

On further loading in Figure 5.119, the IC interface cracks gradually propagated beyond the last

flexural crack I in the hogging region, moving towards the plate end as shown by J in Figure 5.119.

Although the plate did not completely debond in Figure 5.119, the excessive slip at the concrete/plate

interface caused a reduction in the applied load, which signified IC debonding failure at an applied

load P=59.4kN (R=22.4kN, Mhog=17.4kNm, Msag=26.9kNm) with a maximum plate strain of 0.0084

west


- 386 -

recorded at the interior support. As the beam displacement continued to increase after failure,

eventually complete debonding of the plate occurred in Figure 5.120 due to both the weakened bond

caused by severe debonding as well as the large curvature that developed in the strips as a result of

the high stiffness and the orientation of the strips. It is interesting to note how the branching cracks J

in Figure 5.119 that formed from the last flexural crack, crack I, in the west span were similar to those

observed in pull tests, which suggests that the NSM strip was pulling through from crack I towards the

west plate end.

From the test, a maximum moment redistribution of 34.8% was achieved upon failure. Based on the

measured plate strain and from full interaction analysis, it is estimated that the tensile bars yielded at

the maximum hogging moment position at a load of 41.7kN (R=15kN). The distribution of flexural

cracks in the hogging region upon failure is shown in Figure 5.118. At failure, an average crack

spacing of 112mm was measured, where approximately 3 flexural cracks formed over the hogging

region of each span, with the last flexural cracks in the region at approximately 335mm from the

interior support.


mean deflection under the applied loads at mid-spans, are plotted in Figure 5.121. The maximum

hogging moment and the maximum plate strain was found to occur at the same applied load at point C

in Figure 5.121, after which debonding failure of the beam immediately followed. After debonding

failure occurred in the hogging region, as the sagging region was still very strong, more moment was

redistributed to the sagging region, which resulted in an increase in Msag, shown by point D in Figure

5.121, as the displacement of the beam further increased.

0

5

10

15

20

25

30

0 10 20 30 40 50

MhogMsag

Mo

me

nt (

kNm

)

displacement (mm)

A

C

D

max εp;debonding failure

MhogMsag



- 387 -



Figure 5.79. Note that SG1, SG4 and SG9 were faulty at the time of testing. The dotted line A

represents Mhog at IC debonding failure occurred in the sagging region, and (C) is the SG at the

interior support. In the test, a maximum plate strain εp.max of 0.0.00838 was recorded over the interior

support immediately prior to debonding failure (B in Figure 5.122). As the beam deflection continued

to increase after debonding failure, the NSM strip was pulling through from the interior support

towards the plate end (J in Figure 5.119). This caused the plate strain in the east span to reduce as

denoted by C in Figure 5.122, while the plate strains in the west span rapidly increased (D in Figure

5.122), until the plate completely debonded at E. It is worth noting that the maximum plate strain

obtained in NS_F4 was much less than that for specimen NS_F2 which also had two NSM CFRP

strips (Figure 5.91) owing to premature debonding failure of the beam.

0

2

4

6

8

10

12

14

16

18

20

-4500 -3000 -1500 0 1500 3000 4500 6000 7500 9000strain (x10-6)

Mho

g (k

Nm

)

SG2 SG3SG5(C) SG7SG8 SG9SGC10 SGC11

A

SGC11

SGC10

SG7

SG3

SG2

SG5

εp.max;max MR

BD

C

SG9

D

C

CC

E E






for unplated sections. Because flexural cracking (D in Figure 5.123) and IC interface cracking (E in

Figure 5.123) occurred whilst the beam was still settling, this caused discrepancy in the results. The

amount of moment redistribution gradually increased after debonding began at E. At point G, the

maximum moment redistribution and plate strain were obtained, and debonding failure immediately


- 388 -

followed. The excessive slip at the plate/concrete interface at debonding failure caused the moment in

the hogging region, and hence, the applied load to reduce, and more moment was redistributed to the

sagging region, resulting in reductions in Mhog/Msag until the plate completely debonded at H.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1

Mstatic/(Mstatic)u

Mho

g/M

sag

A

test results

B

C

1st flexural crack

1st debondingplate completely

debonded


debonding failure;bar yield

DE F G

H

max MR; εp.max




on elastic analysis at constant EI. It can be seen that after flexural cracking and plate debonding

occurred at points A and B respectively, much of the hogging moment was redistributed to the sagging

region, causing Msag to increase as Mhog reduced. The maximum Mhog occurred at debonding failure

(C in Figure 5.124), after which due to the excessive slip at the plate/concrete interface Mhog reduced

(D in Figure 5.124) and the moment was redistributed to the sagging region, as shown by the

increase in Msag at E, until the plate completely debonded at F.

0

5

10

15

20

25

30

0 20 40 60load (kN)

Mho

g (k

Nm

)

0

5

10

15

20

25

30

0 20 40 60load (kN)

Msa

g (

kNm

)

D

(Mhog)el

(Msag)el

1st flexuralcrack

AB

C

DE

E



- 389 -



increased up to a maximum of 34.8% at A at debonding failure. After failure, as more moment was

redistributed from the hogging region to the sagging region, an increase in %MR was observed at B.

0

10

20

30

40

50

60


%M

omen

t red

istr

ibut

ion

A

1st flexuralcrack

1st debonding

plate completelydebonded

debonding failure

B

max MR;εp.max


5.4.4.5 BEAM NS_S1 (4 X STEEL 0.9MM)

The beam had four 19mm x 0.9mm high yield steel strips near surface mounted to the tension face of

the beam over the hogging region. Flexural cracking was first observed at an applied load P of 8.7kN

(R=3.2kN, Mhog=2.76kNm, Msag=3.81kNm) as shown in Figure 5.126. Upon further loading, a lot of

flexural cracks formed over the hogging region at close uneven spacings. At P=18kN (R=6.5kN,

Mhog=5.64kNm, Msag=7.74kNm), these intermediate cracks caused branching cracks to form at various

locations such as those encircled in Figure 5.126, which indicates that there was slip at the

plate/concrete interfaces.

As loading continued, more IC interface cracks formed, propagating away from the root of the

intermediate crack as shown in Figure 5.127 where the arrows denote the direction of debonding

propagation. Comparing the crack distributions at P=18kN (Figure 5.126) with that at P=45kN (Figure

5.127), it can be seen that most of the flexural cracks formed along the beam at small applied loads,

and as the load was further increased, mostly branching or herringbone cracks formed which signified

slip propagation.


- 390 -

Figure 5.126 Beam NS_S1: Flexural cracking and IC interface cracking (P=18kN)

Figure 5.127 Beam NS_S1: Further development of herringbone cracks (P=45kN)

Minor concrete crushing was observed in the hogging region at P=54kN, however this did not affect

the strength of the region. The beam eventually failed by concrete crushing in the sagging region near

the west applied load at P=60kN (R=23kN, Mhog=16.4kNm, Msag=28kNm). Figure 5.128 shows the

distribution of the herringbone cracks at beam failure, where it can be seen that the branching cracks

concentrated in the region over the interior support and very little IC interface cracks occurred at the

end of the cracked region i.e. near cracks B and C. This indicates that debonding did not propagate

very far along the beam prior to failure. After the sagging region failed by concrete crushing, the

testing was continued under displacement control, where it was found that as the beam deflection

Interior support

A

A westeast

Interior support

A

A

west east

B

B C

C


- 391 -

increased, the plate strain continued to increase while P, Mhog and Msag reduced until one of the strips

fractured near strain gauge SG2 at an average deflection of 55.3mm at Msag.

From the test, a maximum moment redistribution of 39.4% and a maximum plate strain of 0.0418 were

achieved immediately prior to concrete crushing failure. The NSM strips yielded at the interior support

at P=36kN (R=13kN, Mhog=11.8kNm, Msag=15.4kNm). Based on the measured plate strain and from

full interaction analysis, it is estimated that the tensile bars yielded at the maximum hogging moment

position at a load of 29kN (R=11kN). The distribution of flexural cracks in the hogging region upon

failure is shown in Figure 5.128. At failure, an average crack spacing of 69mm was measured with the

last flexural cracks in the region, B and C in Figure 5.128, at approximately 380mm from the interior

support.

Figure 5.128 Beam NS_S1: Hogging region at beam failure (P=60kN)


mean deflection under the applied loads at mid-spans, are plotted in Figure 5.129. After flexural

cracking and IC interface cracking occurred at points A and B in Figure 5.129 respectively, due to the

lower stiffness, the moment in the hogging region Mhog is less than that in the sagging region Msag. As

more load was applied, the difference between Mhog and Msag increased, especially after the plates

and bars yielded, indicating that significant amounts of moment redistribution occurred. The beam

failed due to concrete crushing in the sagging region at C, at the same time maximum moment

redistribution and plate strain were achieved. After failure in the sagging region, the applied load and

Msag reduced while Mhog remained roughly constant until plate rupture occurred at D causing a sudden

reduction in Mhog.

Interior support

A

A east

west

B

B C

C


- 392 -

0

5

10

15

20

25

30

0 10 20 30 40 50 60displacement (mm)

Mom

ent

(kN

m)

A

max %MR

Bbar yield

Msag

Mhog

concrete crushingfailure in sag

plateyield plate

fracture

C

C

D

Figure 5.129 Beam NS_S1: Moment vs displacement



dotted line A in Figure 5.130 represents Mhog at which concrete crushing failure occurred in the

sagging region, and (C) is the SG at the interior support. The strain along the plate increased linearly

with Mhog until the plate yielded at the interior support at B in Figure 5.130. After the plate yielded, the

strains at the interior support i.e. SG2, SG8 and SG14, increased rapidly until plate rupture occurred

near SG2, which caused a sudden reduction in the strains at strip 1 (SG2) and strip 2 (SG8), as

marked by point C in Figure 5.130. This caused the plate strain of strip 3 to suddenly increase over

the interior support at SG14 (D in Figure 5.130) as the plate had to provide the additional tensile

forces carried by the other two strips. The maximum plate strain εp.max achieved before concrete

crushing failure was 0.0418 (E in Figure 5.130).

0

2

4

6

8

10

12

14

16

18

-0.005 0.005 0.015 0.025 0.035 0.045 0.055strain

Mho

g (k

Nm

)

SG1 SG2(C) SG3 SG4SG5 SG6SG7 SG8(C) SG9 SG10SG11 SG12SG13 SG14(C) SG16 SG18SGC19 SGC20εp.y

AA

SG14

SG2

SG8

B

C C D

E

Figure 5.130 Beam NS_S1: Moment vs plate strain


- 393 -





for unplated sections. Much increase in moment redistribution occurred after the plate yielded at D as

evident from the increase in divergence of Mhog/Msag from 1.2. At point E, the maximum moment

redistribution and plate strain were obtained, followed immediately by concrete crushing failure in the

sagging region. After failure occurred in the sagging region Msag reduced, causing the applied load,

and hence, Mstatic to reduce while Mhog remained constant (Figure 5.130). As a result, Mhog/Msag

increased at region F in Figure 5.131 until plate ruptured at G.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Mstatic/(Mstatic)u

Mho

g/M

sag

A

test resultsB

C

1st flexural crack

1st debonding

plate rupture


conc crushingfailure (sag);bar yield

D

EF

max MR; εp.max

plate yieldG

Figure 5.131 Beam NS_S1: hogging-moment/sagging-moment



on elastic analysis at constant EI. The greater the divergence from the elastic moments means that

more moment is being redistributed. It can be seen from Figure 5.132 that the beam behaved

elastically until the first flexural crack occurred at A, after which small amounts of hogging moment

were redistributed to the sagging regions. More moment was redistributed from the hogging to the

sagging region after the plate yielded.


- 394 -

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70load (kN)

Mho

g (k

Nm

)

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70load (kN)

Msa

g (k

Nm

)

1st debonding

platerupture

(Mhog)el

(Msag)el

1st flexuralcrack

concretecrushing

A

B

bar yieldplate yield

B

Figure 5.132 Beam NS_S1: Maximum hogging and sagging moments





the %MR increased up to a maximum of 39.4% when concrete crushing occurred in the sagging

region at B. After failure occurred in the sagging region, the %MR reduced as Msag reduced while Mhog

remained constant (Figure 5.132) until plate ruptured, which caused a sudden reduction in Mhog, and

hence, a sudden increase in %MR at C in Figure 5.133.

0

5

10

15

20

25

30

35

40

45

50


% M

omen

t red

istr

ibut

ion

A

1st flexuralcrack

1st debonding max MR

conc crushingfailure (sag);

plate rupture

bar yieldplate yield

B

C

Figure 5.133 Beam NS_S1: percentage of moment redistribution

5.4.4.6 BEAM NS_S2 (2 X 2 STEEL 0.9MM)

The beam had four high yield steel strips with two 0.9mm thick strips glued together before near

surface mounted to the tension face of the beam over the hogging region. Flexural cracking first


- 395 -

occurred an applied load P of 8.95kN (R=2.98kN, Mhog=3.6kNm, Msag=3.57kNm) as shown in Figure

5.134. As the load was further increased, more flexural cracks formed over the hogging region at

close uneven spacings, causing branching cracks to form at P=14.9kN (R=4.93kN, Mhog=6.0kNm,

Msag=kNm), such as B in Figure 5.134, due to slip at the plate/concrete interfaces. As loading

continued, more IC interface cracks formed, propagating away from the roots of the intermediate

cracks.

Figure 5.134 Beam NS_S2: Flexural cracking and IC interface cracking (P=18kN)

The beam eventually failed by shear failure in the sagging region near the west applied load at

P=60kN (R=22.3kN, Mhog=18.4kNm, Msag=26.7kNm) as shown in Figure 5.135, where concrete

crushing was also evident next to the applied load. Minor concrete crushing occurred in the hogging

region at P=54kN, but this did not affect the strength of the region. Figure 5.136 shows the distribution

of the herringbone cracks over the hogging region at beam failure, where it can be seen that the

branching cracks concentrated in the region over the interior support and very little IC interface cracks

occurred at the end of the cracked region i.e. near cracks C and D. This indicates that debonding did

not propagate very far along the beam prior to failure, also shown in Figure 5.135.

From the test, a maximum moment redistribution of 31.8% and a maximum plate strain of 0.0352 were

achieved immediately prior to failure. The NSM strips yielded at the interior support at P=38.4kN

(R=13.4kN, Mhog=13.9kNm, Msag=16.1kNm). Based on the measured plate strain and from full

interaction analysis, it is estimated that the tensile bars yielded at the maximum hogging moment

position at a load of 35.7kN (R=12.4kN). The distribution of flexural cracks in the hogging region upon

Interior support

east west

A

A

B


- 396 -

failure is shown in Figure 5.136. At failure, an average crack spacing of 104mm was measured with

the last flexural cracks in the region, B and C in Figure 5.128, at approximately 385mm from the

interior support.

Figure 5.135 Beam NS_S2: Shear failure in sagging region (P=60kN)

Figure 5.136 Beam NS_S2: Hogging region at beam failure (P=60kN)


mean deflection under the applied loads at mid-spans, are plotted in Figure 5.137. As loads were

gradually applied and flexural and IC interface cracking occurred, the difference between Mhog and

Msag increased, especially after the plates and bars yielded, indicating that significant amounts of

moment redistribution was present. The beam failed due to shear failure in the sagging region at C, at

the same time maximum moment redistribution and maximum plate strain were achieved.

Interior support

CDC

Concrete crushing

west east P

C D

Interior support

east west A

A C D


- 397 -

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

displacement (mm)

Mom

ent (

kNm

)

flexural cracking

max εp.max;1st debonding

bar yield

Msag

Mhog

shearfailure in sag

plateyield

max %MR

Figure 5.137 Beam NS_S2: Moment vs displacement



dotted line A in Figure 5.138 represents Mhog at which concrete crushing failure occurred in the

sagging region, and (C) is the SG at the interior support. The strain along the plate increased linearly

with Mhog until the plate yielded at the interior support i.e. SG2 and SG8 in Figure 5.138, after which

the plate strains at SG2 and SG8 increased rapidly until shear failure in the sagging region. The

maximum plate strain εp.max was achieved upon shear failure at SG8 (B in Figure 5.138).

0

4

8

12

16

20

-0.005 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04strain

Mho

g (k

Nm

)

SG1 SG2(C) SG3 SG4SG5 SG6SG7 SG8(C) SG9 SG10SG11 SG12SGC13 SGC14εp.y

AASG2 SG8

B

B

Figure 5.138 Beam NS_S2: Moment vs plate strain


- 398 -





for unplated sections. After the formation of flexural and IC interface cracks (D and E in Figure 5.139),

divergence of Mhog/Msag from 1.2 was observed, indicating that moment redistribution has occurred. At

point H, the maximum moment redistribution and maximum plate strain were obtained, followed

immediately by shear failure in the sagging region.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1

Mstatic/(Mstatic)u

Mho

g/M

sag

Atest results

B

C

1st flexural crack

1st debonding


shearfailure (sag);

bar yield

FH

max MR; εp.max

plate yield

G

D E

Figure 5.139 Beam NS_S2: hogging-moment/sagging-moment




more moment is being redistributed. It can be seen from Figure 5.140 that the beam behaved

elastically until the flexural and IC interface cracks occurred, which caused moments to be

redistributed from the hogging to the sagging regions, especially after the bars and the plates yielded,

where large divergence of Mhog from (Mhog)el can be seen.


- 399 -

0

5

10

15

20

25

30

0 20 40 60load (kN)

Mho

g (k

Nm

)

0

5

10

15

20

25

30

0 20 40 60load (kN)

Msa

g (k

Nm

)

1st debonding

(Mhog)el

(Msag)el 1st flexural

crack

shear failure

B

bar yield

plate yield

B

Figure 5.140 Beam NS_S2: Maximum hogging and sagging moments





the %MR increased up to a maximum of 31.8% when shear failure occurred in the sagging region at

B. Compared to specimen NS_S1 (Figure 5.133), which has the same amount of strips but embedded

individually along the hogging region, less moment redistribution and Mstatic were obtained in specimen

NS_S2 (Figure 5.141).

-5

0

5

10

15

20

25

30

35


% M

omen

t red

istr

ibut

ion

A

1st flexuralcrack

1st debonding

max MR;εp.max

shearfailure (sag);

bar yield

plate yield

Figure 5.141 Beam NS_S2: percentage of moment redistribution


- 400 -

5.4.4.7 BEAM NB_F1 (2 X CFRP1.2MM)

This 220mm by 240mm beam had two strips of 15mm x 1.2mm CFRP near surface mounted to the

side face of the beam over the hogging region (Figure 5.74c). In Figure 5.142 to Figure 5.146 the

dotted line denotes the position of the NSM strip on each side face, while the numbers denote the

applied concentrated loads P at each span as shown in Figure 5.4. Flexural cracking first occurred at

an applied load P of 25kN (R=9.83kN, Mhog=6.36kNm, Msag=11.8kNm) where cracks A and B in Figure

5.142 formed. At P=65kN (R=24.8kN, Mhog=18.6kNm, Msag=29.7kNm) the first diagonal ‘branching’ or

herringbone cracks formed at C and D in Figure 5.142. These branching cracks were caused by the

sliding between the plates and the adjacent concrete, hence indicating that debonding or IC interface

cracking had occurred.

Figure 5.142 Beam NB_F1: Flexural cracking and IC interface cracking (P=65kN)

As the load continued to increase, more IC interface cracks formed, propagating further towards the

plate end as shown by E and F in Figure 5.143, with the arrows representing the direction of

propagation. Comparing Figure 5.142 with Figure 5.143, it can be seen that unlike the specimens with

slab shaped cross-sections (NS test series), very little flexural and shear cracks have formed even

when the load was doubled, and that debonding propagated much more dominantly towards the plate

end even at early stages of debonding i.e. the reverse in bondstress and slip was less significant

between adjacent cracks. However, the propagation of the debonding cracks along the plate/concrete

interface was very gradual, as can be seen by comparing the extent of debonding at P=130kN (Figure

5.143) with that at P=170kN (Figure 5.144). Instead, a lot of diagonal cracks were found in both the

sagging and hogging regions as the load was further increased (Figure 5.144), but shear failure of the

RC beam was prevented by the stirrups, and CDC debonding failure was postponed due to the large

amount of internal reinforcing bars used as well as the position and orientation of the NSM strips.

Interior support

NSM strip

east west

A B C D


- 401 -

Figure 5.143 Beam NB_F1: Further development of herringbone cracks (P=130kN)

Figure 5.144 Beam NB_F1: Debonding propagation and diagonal cracks formation (P=170kN)

Further loading from P=170kN (Figure 5.144) to P=220kN (Figure 5.145) resulted in more formations

of diagonal cracks and severe debonding around the interior support, but not further along the beam.

Due to the orientation of the strips and the strong bond that occurred between the NSM plates and the

adjacent concrete, premature debonding was delayed allowing large loads to be applied to the

specimen. At P=218kN (R=84.8kN), minor concrete crushing were observed in the hogging and

sagging regions. This eventually led to concrete crushing failure in the sagging regions near the

applied loads in Figure 5.146 at P=242kN (R=93.9kN, Mhog=65.5kNm, Msag=113kNm). The distribution

of the herringbone cracks at beam failure is shown in Figure 5.146, where it can be seen that although

many branching cracks have developed, which indicated that there was slip at the plate/concrete

interfaces, the debonding cracks did not propagate to the plate end to cause premature debonding

failure (only propagated up to G and H).

Interior support

NSM strip

east

west

F E

Interior support

NSM strip east

west

F

Interior support

E NSM strip


- 402 -

Figure 5.145 Beam NB_F1: Debonding propagation and diagonal cracks formation (P=220kN)

Figure 5.146 Beam NB_F1: At failure (P=242kN)

Interior support

NSM strip east

Interior support

NSM strip west

Interior support

NSM strip

west

east NSM strip

Interior support

Interior support

west east

G

G H

H


- 403 -

From the test, a maximum moment redistribution of 45.5% was achieved at P=158kN (R=62.7kN,

Mhog=38.6kNm, Msag=75.2kNm). Based on the measured plate strain and from full interaction analysis,

it is estimated that the tensile bars yielded at the maximum hogging moment position at a load of

119kN (R=45.8kN).



NB_F1. After herringbone cracks occurred at point A, due to the lower stiffness, the moment in the

hogging region Mhog is less than that in the sagging region Msag. The maximum moment redistribution

was achieved at point C. Minor concrete crushing was first observed in both the hogging and sagging

regions at D, followed by concrete crushing failure in the sagging region at E, which caused a

reduction in both Msag and the applied load P. However, as the hogging region has not yet failed, more

moment is redistributed to the hogging region. Hence, a small increase in Mhog was observed as the

displacement continues to increase until concrete crushing became excessive at the hogging region F,

such that Mhog began to reduce.

0

20

40

60

80

100

120

0 10 20 30 40 50displacement (mm)

Mom

ent (

kNm

)

A

BC

D

E

E

F

minor conc crushingin hog & sag

max εp.max

1st debonding

bar yield

Msag

Mhog

conc crushingfailure in sag

max %MR

conc crushingfailure in hog

Figure 5.147 Beam NB_F1: Moment vs displacement



dotted lines A and B in Figure 5.148 represent Mhog when concrete crushing failure occurred in the

sagging region and at maximum moment redistribution %MR respectively; and (C) is the SG at the

interior support. Steady increases in plate strains were observed until the bars yielded at

Mhog=32.9kNm, after which the increase in plate strains became more rapid. The average plate strain

at the position of maximum hogging moment, SG5 and SG14, was 0.0032 when the maximum


- 404 -

percentage moment redistribution was achieved (B in Figure 5.148). It is worth noting that at

maximum moment redistribution, the plate strains were actually highest adjacent to the position of

maximum moment in both strips, at SG4 and SG15, giving a maximum plate strain of 0.0039. In the

test, a maximum plate strain εp.max of 0.0095 was recorded upon failure (average of the two strips).

After concrete crushing failure occurred in the sagging region at A in Figure 5.148 , as the hogging

region had not yet failed, a small increase in Mhog (Figure 5.147) as well as in the plate strains (Figure

5.148), was observed until failure occurred in the hogging region (at F in Figure 5.147).

0

10

20

30

40

50

60

70

80

-4000 -2000 0 2000 4000 6000 8000 10000 12000strain (x10-6)

Mho

g (k

Nm

)

SG1 SG2SG3 SG4SG5(C) SG6SG7 SG8SG9 SGC19SGC20

0

10

20

30

40

50

60

70

80

-4000 -2000 0 2000 4000 6000 8000 10000 12000strain (x10-6)

Mho

g (k

Nm

)

SG10 SG11SG12 SG13SG14(C) SG15SG16 SG17SG18

A

SGC18

SGC10

SG7

SG3SG2

SG5 εp.max

SG16

SG6

SG4

εp.max

A

SG12

SG13 SG14

SG15

max%MRB

max%MRB

Figure 5.148 Beam NB_F1: Moment vs plate strain



assumed to be constant Mhog/Msag = 1.2, which is shown as line A. The line marked B is the maximum


- 405 -


for unplated sections. When the load was first applied Mhog/Msag should approach 1.2 in region D in

Figure 5.149, however the bedding or settling down of the beam caused divergence from this value.

The maximum moment redistribution occurs when the divergence of Mhog/Msag from 1.2 is the greatest,

which is at point E in Figure 5.149. Minor concrete crushing in the sagging regions near the applied

loads was observed at F, soon after, concrete crushing failure in the sagging region occurred at G,

which coincided with the maximum plate strain. As the hogging region has not yet failed, more

moment was redistributed to the hogging region which resulted in an increase in Mhog/Msag at H. It is

interesting to note how at failure (point G), Mstatic/(Mstatic)u exceeded the limit of 1, and also the

maximum moment redistribution at E was greater than the maximum allowable moment redistribution

for the plated beam (line B). This is because (Mstatic)u was determined based on full interaction

analysis, assuming that concrete crushing failure occurs at a concrete strain of 0.003, but in reality,

concrete crushing does not lead to immediate failure. That is after the concrete crushing strain of

0.003 is reached, the flexural strength of the region can still increase as indicated by SG20 in Figure

5.148, hence the (Mstatic)u is an underestimate of the actual ultimate static moment.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1Mstatic/(Mstatic)u

Mho

g/M

sag

A

test results

B

C

1st flexural crack 1st debonding


conc crushingfailure in sag;

bar yield

F

H

εp.max

max MR

G

D

E

minor conc crushingin hog&sag;

Figure 5.149 Beam NB_F1: hogging-moment/sagging-moment




more moment is being redistributed. The beam behaved elastically until the first flexural crack

occurred, after which small amounts of hogging moment were redistributed to the sagging regions.

More moment was redistributed when debonding occurred as observed by the formation of

herringbone cracks. Minor concrete crushing in both the hogging and sagging region occurred at point


- 406 -

A in Figure 5.150 before the beam failed due to concrete crushing near the applied load. After sagging

region failure, Mhog continued to increase with increase in beam deflection (point B) until the hogging

region also failed due to concrete crushing at C.

0

20

40

60

80

100

120

0 50 100 150 200 250load (kN)

Mho

g (k

Nm

)

0

20

40

60

80

100

120

0 50 100 150 200 250load (kN)

Msa

g (k

Nm

)

minor conccrushing1st debonding

conc crushingfailure (sag)(Mhog)el

A

B

C

(Msag)el

B

A

Figure 5.150 Beam NB_F1: Maximum hogging and sagging moments



such that there should be zero moment redistribution. However, the discrepancy of results at A is

because the beam was still bedding or settling down. A maximum %MR of 45.5% was achieved at B,

before beam failure. Upon further loading, small reductions in %MR were observed, and at point C

concrete crushing failure occurred in the sagging region at 39.9% of moment redistribution. Rapid

reduction in %MR then followed, as marked by region D, due to more moment being redistributed to

the hogging region after the sagging region failed (Figure 5.150).

0

10

20

30

40

50


% M

omen

t red

istr

ibut

ion

A

1st flexuralcrack

1st debonding B conc crushingfailure (sag)

bar yield

max MR

CD

Figure 5.151 Beam NB_F1: percentage of moment redistribution


- 407 -

5.4.4.8 BEAM NB_F2 (2 X CFRP1.2MM)

This 220mm by 240mm beam had two strips of 15mm x 1.2mm CFRP near surface mounted to the

tension face of the beam over the hogging region (Figure 5.74b). Flexural cracks (A, B and C in Figure

5.152) were first observed in the hogging region at an applied load P of 19.6kN (R=6.78kN,

Mhog=7.2kNm, Msag=8.13kNm). As more load was applied, further flexural cracking occurred at

irregular crack spacings. The first diagonal ‘branching’ or herringbone cracks formed at P=78.4kN

(R=29kN, Mhog=24.5kNm, Msag=34.8kNm), which are encircled in Figure 5.152. These branching

cracks were caused by the sliding between the plates and the adjacent concrete, hence indicate that

debonding or IC interface cracking has occurred.


Further loading caused more IC interface cracks to form, mostly in between adjacent flexural cracks

i.e. D in Figure 5.153a, propagating away from the roots of the intermediate cracks such that a

reversal in bondstress and slip is observed, as indicated by the arrows in Figure 5.153a. Minor

debonding, (i.e. E and F in Figure 5.153a) was also observed propagating from the last flexural cracks

G and H towards the plate end. Comparing this beam (Figure 5.153b) with the previous beam NB_F1

(Figure 5.143), which has the same strips but NSM to the sides, more flexural cracks were observed

in NB_F2 and with more obvious slip reversal between cracks.

Interior support

west east A B C

NSM strips

Interior support

west

east A G H

E

D

F

(a) top view


- 408 -


As the beam was further loaded to 220kN a lot of diagonal cracks were found in both the sagging and

hogging region as shown in Figure 5.154a. However, shear failure was prevented by the stirrups, and

CDC debonding failure was postponed due to the large amount of internal reinforcement used as well

as the position and orientation of the NSM strips. Major debonding was also observed in Figure

5.154b over the interior support, propagating gradually towards the plate end. It can be seen by

comparing Figure 5.154b to Figure 5.153a that, as the applied load increased, the debonding between

cracks began to propagate more dominantly towards the plate end as indicated by the arrows in

Figure 5.154b. This is also evident from the crossing of the inclined cracks such as marked by I and J

in Figure 5.154b. Due to the orientation of the strips, debonding was more severe for NB_F2 than the

side plated NB_F1 specimen (Figure 5.145) for the same applied load (P=220kN), and debonding was

found to have propagated further along for NB_F2. It can also be seen from Figure 5.154a that minor

concrete crushing has occurred at interior support as denoted by K and L. In fact, minor concrete

crushing was first observed in both hogging and sagging regions at P=183.7kN (R=70.9kN), but failure

did not occur and the loads continued to increase.

A G H

Interior support

east west

(b) side view

Interior support

west

side view

east

(a) side view

K L

(b) top view


- 409 -

Figure 5.154 Beam NB_F2: IC debonding propagation and diagonal cracks formation (P=220kN)

Prior to failure of the beam, at an average applied load of P=244kN, IC debonding was found to have

propagated to more than 600mm away from the interior support in both east and west spans, as

marked by M and N in Figure 5.155. This indicates that IC debonding has extended past the point of

contra flexure into the compression zone of the sagging region. Also evident from Figure 5.155 is the

severe concrete crushing that occurred next to the applied loads. However this did not cause failure of

the sagging region. Failure was caused by critical diagonal crack debonding, where at P=244kN

(R=94kN, Mhog=67.3kNm, Msag=112.7kNm) the sliding action along the diagonal crack O in Figure

5.156 caused debonding to occur at P, and a catastrophic debonding failure in the west span followed

immediately as shown in Figure 5.157.

Figure 5.155 Beam NB_F2: IC debonding propagation prior to debonding failure (average P=244kN)

Interior support

west east I J

A G H

E

F

west

M

Concrete crushing

direction of propagation G B

N

H C

Concrete crushing

direction of propagation

east


- 410 -

Figure 5.156 Beam NB_F2: CDC debonding prior to failure (average P=244kN)

Figure 5.157 Beam NB_F2: At debonding failure

Comparing Figure 5.153, Figure 5.154 and Figure 5.155, it was found that most of the flexural cracks

formed in the hogging region at early stages of loading, after which further loading resulted in mostly

the formation of herringbone cracks due to debonding propagation, and also the formation of shear

cracks along the span. From the test, a maximum moment redistribution of 40.7% was achieved at

P=219kN (R=85kN, Mhog=58.3kNm, Msag=102kNm) with a plate strain of 0.0085 measured at the

interior support. A maximum plate strain of 0.0102 was recorded at the interior support just prior to

debonding failure at P=243.5kN (R=93.6kN, Mhog=67.4kNm, Msag=112.4kNm). Based on the

measured plate strain and from full interaction analysis, it is estimated that the tensile bars yielded at

the maximum hogging moment position at a load of 109kN (R=40.6kN).

Interior support

A O

P

west

A O

NSM strips

Interior support


- 411 -



cracking occurred at point A, due to the lower stiffness, the moment in the hogging region Mhog is less

than that in the sagging region Msag. As debonding occurs at B, more moment was redistributed to the

sagging region, resulting in greater differences between Mhog and Msag. The maximum moment

redistribution was achieved at point D. Very small increases in Mhog and Msag occurred beyond point D,

indicating that both regions were close to reaching their ultimate capacities. The maximum plate strain

was reached at point E, followed immediately by debonding failure of the NSM strips in the west span

at F.

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35displacement (mm)

Mom

ent

(kN

m)

A

BC

DF

minor conc crushingin hog & sag

max εp.max

1st debonding

bar yield

Msag

Mhog

max %MR debondingfailure

1st flexural cracking

E




dotted lines A and B in Figure 5.159 represent Mhog when the maximum moment redistribution %MR

and the maximum plate strain were achieved respectively; and (C) is the SG at the interior support.

The plate strain at the position of maximum hogging moment was 0.0085 (average of SG5 and SG11)

when the maximum percentage moment redistribution was obtained. As the debonding cracks

propagated further along the beam i.e. in Figure 5.154b and Figure 5.155, rapid increases in plate

strains away from the interior support i.e. at SG2, SG3, SG7 and SG8 were observed (C in Figure

5.159). A maximum plate strain εp.max of 0.0102 (average of SG5 and SG11) was recorded at the

interior support, which was followed immediately by debonding failure of the beam at the west span,

causing a sudden reduction in plate strains near the interior support i.e. at SG4, SG5, SG6, SG10 and

SG11 (D in Figure 5.159). It is interesting to note how the plate strains at SG1 and SG9 decreased

very rapidly at point E in Figure 5.159. This is because the concrete crushing that was occurring next


- 412 -

to the applied loads caused debonding at the plate ends, which were positioned on the compression

surface close to the applied loads.

0

10

20

30

40

50

60

70

-5000 -3500 -2000 -500 1000 2500 4000 5500 7000 8500 10000 11500

strain (x10-6)

Mho

g (k

Nm

)

SG10 SG11(C) SG12 SGC13SGC14

0

10

20

30

40

50

60

70

-5000 -3500 -2000 -500 1000 2500 4000 5500 7000 8500 10000 11500

strain (x10-6)

Mho

g (k

Nm

)


A

SGC13

SG7

SG1SG2

SG5

εp.max

SG3

SG6

εp.max

A

SG10

SG12SGC14

SG11max%MR

B

max%MR

B

SG9SG8

SG4

C

D

D

CE






for unplated sections. When the load was first applied Mhog/Msag approached 1.2 in region D with any

divergence due to bedding or settling down of the beam. Further loading after debonding cracks

formed and bars yielded caused greater moment redistribution. The maximum moment redistribution

occurs at point E in Figure 5.160, soon after, the maximum plate strain was achieved at F, which was

followed immediately by debonding failure at G. It is interesting to note how at failure (point G),


- 413 -

Mstatic/(Mstatic)u exceeded the limit of 1, and also the maximum moment redistribution at E was greater

than the maximum allowable moment redistribution for the plated beam (line B). This is because

(Mstatic)u was determined based on full interaction analysis in which it was assumed that concrete

crushing failure occurs at a concrete strain of 0.003, but in reality, concrete crushing does not lead to

immediate failure. Therefore, the (Mstatic)u is an underestimate of the actual ultimate static moment.

0

0.2

0.4

0.6

0.8

1

1.2

1.4


Mho

g/M

sag

A

test resultsB

C

1st flexural crack

1st debonding


bar yield

F

εp.max

max MR

G

D

E

minor conc crushingin hog&sag;

D

debondingfailure




on elastic analysis at constant EI. The beam behaved elastically until the first flexural crack occurred

at A, after which Mhog diverged from (Mhog)el indicating that moment was redistributing from the

hogging to the sagging regions. In the test, the maximum Mhog and Msag were achieved just prior to

debonding failure at C.

0

20

40

60

80

100

120

0 50 100 150 200 250load (kN)

Mho

g (k

Nm

)

0

20

40

60

80

100

120

0 50 100 150 200 250load (kN)

Msa

g (k

Nm

)

debondingfailure

1st debonding

(Mhog)el

C

(Msag)el

B

A

max MR

B

C



- 414 -



such that there should be zero moment redistribution. The discrepancy of results at A is because the

beam was still bedding or settling down. The amount of moment redistribution increased as more load

was applied, until a maximum %MR of 40.7% was achieved at B, after which further loading caused

%MR to reduce. At point C where the maximum plate strain was recorded, 38.4% of moment

redistribution occurred. Brittle debonding failure then immediately followed at D at 38.7% moment

redistribution.

0

5

10

15

20

25

30

35

40

45

50

0 0.2 0.4 0.6 0.8 1 1.2Mstatic/(Mstatic)u

% M

omen

t red

istr

ibut

ion

A

1st flexuralcrack

1st debonding

B

εp.max

bar yield

max MR

C

D

debondingfailure


5.4.4.9 BEAM NB_F3 (2 X 2CFRP1.2MM)

This 220mm by 240mm beam had four strips of 15mm x 1.2mm CFRP, where two strips were glued

together before near surface mounted to the tension face of the beam over the hogging region (Figure

5.74b). Flexural cracks (A and B in Figure 5.163) were first observed in the hogging region at an

applied load P of 19.4kN (R=6.7kN, Mhog=7.2kNm, Msag=8.04kNm). The first diagonal ‘branching’ or

herringbone cracks formed at P=59.3kN (R=21.8kN, Mhog=19.0kNm, Msag=26.1kNm), which are

encircled in Figure 5.163. Further loading caused more IC interface cracks to form mostly away from

the interior support, i.e. C and D in Figure 5.164, which propagated towards the plate ends. This is

different from beam NB_F2, where in the early stages of debonding, most of the IC interface cracks

formed in between adjacent flexural cracks, propagating in opposite directions away from the roots of

the intermediate crack (Figure 5.152). As the beam was further loaded to 210kN a lot of diagonal

cracks were found in both the sagging and hogging region as shown in Figure 5.165. Major debonding

was also observed in Figure 5.165 propagating further towards the plate end.


- 415 -



Figure 5.165 Beam NB_F3: IC debonding propagation and diagonal cracks formation (P=210kN)

Figure 5.166 shows the debonding propagation of the hogging region of the beam prior to failure at an

average applied load of P=246.5kN. IC debonding was found to have propagated to more than

600mm away from the interior support in both the east and west spans, as marked by E and F in

Interior support

Interior support

East

A B

West

C D

Interior support East

NSM strips

A B West

Interior support

East

A B

West

C

D


- 416 -

Figure 5.166. This indicates that IC debonding has extended past the points of contraflexure into the

compression zones of the sagging regions. It is interesting to note that for this beam, debonding was

more severe away from the interior support (Figure 5.166), as opposed to beam NB_F2 where major

debonding first occurred at the interior support then gradually propagated towards the plate end

(Figure 5.155). Concrete crushing in the sagging region next to the applied loads is clearly evident

from Figure 5.166, however it did not cause a reduction in the applied load and the internal moment,

that is failure did not occur. At the same applied load of P=246.5kN (R=92.8kN, Mhog=73.2kNm,

Msag=111.3kNm) sudden debonding failure occurred in the east span as shown in Figure 5.167.

From the test, a maximum moment redistribution of 36.6% was achieved at P=198kN (R=75.5kN,

Mhog=56.5kNm, Msag=90.5kNm) with a plate strain of 0.006 measured at the interior support. A

maximum plate strain of 0.00833 was recorded at the interior support just prior to debonding failure at

P=246.5kN (R=92.8kN, Mhog=73.2kNm, Msag=111.3kNm). Based on the measured plate strain and

from a full interaction analysis, it is estimated that the tensile bars yielded at the maximum hogging

moment position at a load of 119kN (R=44.8kN).

Figure 5.166 Beam NB_F3: IC debonding propagation prior to debonding failure (average P=247kN)

East

West Interior support

West

A B C D

C B Concrete crushing

Concrete crushing

East

D

F

E


- 417 -

Figure 5.167 Beam NB_F3: At debonding failure (average P=247kN)



cracking occurred at point A, due to the lower stiffness, the moment in the hogging region Mhog is less

than that in the sagging region Msag. As debonding occurs at B, more moment was redistributed to the

sagging region, resulting in greater differences between Mhog and Msag. The maximum moment

redistribution was achieved at point C. The maximum plate strain debonding was obtained

immediately prior to debonding failure of the east span at point D.

0

20

40

60

80

100

120

0 5 10 15 20 25 30displacement (mm)

Mom

ent (

kNm

)

A

C

minor conc crushingin sag

max εp.max1st debonding

bar yield

Msag

Mhog

max %MRdebonding

failure;

1st flexural cracking

minor conccrushing in hog

B

D


NSM strips

East

Hydraulic

jack

NSM strips

Interior support

East Side view

Top view


- 418 -



dotted lines A and B in Figure 5.159 represent Mhog when the maximum moment redistribution %MR

and the maximum plate strain were achieved respectively; and (C) is the SG at the interior support.

The plate strain at the position of maximum hogging moment was 0.006 (average of SG5 and SG11)

when the maximum percentage moment redistribution was obtained. A maximum plate strain εp.max of

0.00833 (average of SG5 and SG11) was recorded at the interior support upon debonding failure of

the beam at the east span.

0

10

20

30

40

50

60

70

80

-3000 -1500 0 1500 3000 4500 6000 7500 9000strain (x10-6)

Mho

g (k

Nm

)


A

SG7SG9SG2

SG5

εp.max

SG3

SG6

max%MR

B

SG9SG8

SG4

SG1

0

10

20

30

40

50

60

70

80

-3000 -1500 0 1500 3000 4500 6000 7500 9000strain (x10-6)

Mho

g (k

Nm

)

SG10SG11(C) SG12SGC13SGC14

SGC13

εp.max

A

SG10

SG12SGC14

SG11

max%MR

B



- 419 -





for unplated sections. When the load was first applied Mhog/Msag approached 1.2 in region D with any

divergence due to bedding or settling down of the beam. The maximum moment redistribution occurs

at point E in Figure 5.170, and the maximum plate strain was achieved at F upon debonding failure.

Like the previous NB_F1 and NB_F2 beams, Mstatic/(Mstatic)u exceeded the limit of 1, and also the

maximum moment redistribution at E was greater than the maximum allowable moment redistribution

for the plated beam (line B). This is because (Mstatic)u is an underestimate of the actual ultimate static

moment.

0

0.2

0.4

0.6

0.8

1

1.2

1.4


Mho

g/M

sag

A

test resultsB

C

1st flexural crack

1st debonding


bar yield

εp.max

max MR

F

D

debondingfailure;

E




on elastic analysis of constant EI. The beam behaved elastically until the first flexural crack occurred

at A, after which Mhog diverged from (Mhog)el indicating that moment was redistributing from the

hogging to the sagging regions. In the test, the maximum Mhog and Msag were achieved just prior to

debonding failure at B.


- 420 -

0

20

40

60

80

100

120

0 50 100 150 200 250load (kN)

Mho

g (k

Nm

)

0

20

40

60

80

100

120

0 50 100 150 200 250load (kN)

Msa

g (k

Nm

)

flex cracks

(Mhog)el

A

B

(Msag)el

B





was still bedding or settling down. A maximum %MR of 36.6% was achieved at B, after which further

loading caused the %MR to reduce. Debonding failure occurred at C with 34% of moment

redistribution.

0

5

10

15

20

25

30

35

40


% M

omen

t red

istr

ibut

ion

A

1st flexuralcrack

1st debonding

B

debondingfailure;

bar yieldmax MR

C

εp.max



- 421 -

5.4.5 SUMMARY AND DISCUSSIONS

A summary of the test results is presented in the following journal paper along with a comparison of

results for the different specimens.

5.4.5.1 JOURNAL PAPER: TESTS ON THE DUCTILITY OF REINFORCED CONCRETE BEAMS RETROFITTED WITH FRP AND STEEL NEAR SURFACE MOUNTED PLATES

This paper focuses on the moment redistribution behaviour of beams with near surface mounted

strips, where the series of tests on NSM beams carried out in this research are presented.


- 422 -

Intermediate Crack Debonding of Plated RC Beams Experimental Investigation on Moment Redistribution Tests on the ductility of reinforced concrete beams retrofitted with FRP and steel near surface mounted plates

- 423 -

Tests on the ductility of reinforced concrete beams retrofitted with FRP

and steel near surface mounted plates

*Liu, I.S.T., **Oehlers, D.J., and ***Seracino, R.

*Ms. I.S.T. Liu Postgraduate student School of Civil and Environmental Engineering The University of Adelaide Corresponding author **Dr. D.J. Oehlers Associate Professor School of Civil and Environmental Engineering The University of Adelaide Adelaide SA5005 AUSTRALIA Tel. 61 8 8303 5451 Fax 61 8 8303 4359 Email: [email protected] ***Dr. R. Seracino Senior Lecturer School of Civil and Environmental Engineering The University of Adelaide Accepted for publications by ASCE Journal of Composites for Construction September 2005

Intermediate Crack Debonding of Plated RC Beams Experimental Investigation on Moment Redistribution Tests on the ductility of reinforced concrete beams retrofitted with FRP and steel near surface mounted plates

- 424 -

Statement of Authorship

TESTS ON THE DUCTILITY OF REINFORCED CONCRETE BEAMS RETROFITTED WITH FRP

AND STEEL NEAR SURFACE MOUNTED PLATES

Accepted for publications by ASCE Journal of Composites for Construction September 2005

LIU, I.S.T. (Candidate)

Performed all analyses, interpreted data and wrote manuscript.

Signed Date

OEHLERS, D.J.

Supervised development of work, edited manuscript and acted as corresponding author.

Signed Date

SERACINO, R.

Supervised development of work, and manuscript review.

Signed Date

Liu, I.S.T., Oehlers, D.J., and Seracino, R. (2005) Tests on the ductility of reinforced concrete beams retrofitted with FRP and steel near surface mounted plates ASCE Journal of Composites for Construction, September 2005

NOTE: This publication is included on pages 423 - 440 in the print copy of the thesis held in the University of Adelaide Library.


- 441 -

5.5 SUMMARY

Through the extensive experimental program performed in this research on moment redistribution of

externally bonded and near surface mounted beams, the following conclusions are drawn:

1. From the tests of seven externally bonded plated beams, it was found that substantial

amounts of moment redistribution can occur. For carbon FRP plated beams this ranged from

28% to 35% and for steel plated beams from 22% to 48%. Hence plated beams have a scope

for moment redistribution.

2. The NSM steel plates achieved strains of up to 0.042 which allowed 39% moment

redistribution and the NSM CFRP plates achieved strains of up to 0.014 which allowed 32%

moment redistribution. Unlike externally bonded plated beams, these tests showed that

beams retrofitted with NSM plates have substantial ductility so that NSM plated beams can be

used to retrofit RC structures that require ductility, which should significantly expand the use

of retrofitting by plating.

3. Although steel is a very ductile material, beams with externally bonded steel plates are still

susceptible to premature debonding depending on the positions and dimensions of the plates.

4. The moment distribution behaviour of continuous plated beams is largely dependent on the

extent of debonding and flexural cracking. Therefore, moment redistribution is dependent on

the variation of flexural stiffness along the beam.

5. The IC debonding behaviour of NSM beams was visually shown through the formation of

herringbone cracks which also shows the directions of the shear flow; this has been studied

through the numerical simulation presented in Chapters 2 and 3. The tests on the NSM

beams also showed the opposing shear flows between flexural cracks when the beam is first

loaded and then the reversal in direction of parts of the shear flow to accommodate the

longitudinal pulling out of the plate.

6. From the series of tests carried out on NSM beams, it was found that the beam shaped

specimens in the NB test series were more susceptible to premature plate debonding due to

higher shear of the beams.


- 442 -

7. Through the tests of both EB and NSM beams, it was found that extending the plates beyond

the points of contraflexure, into the compression region, only delays but does not prevent

premature debonding failure.

8. Comparing the test results for the NSM test specimens with results of the EB test specimens,

it was found that: the percentage of moment redistribution that occurred in the NSM beams

was slightly greater than that reached in EB beams; and much higher plate strains were

achieved in the NSM beams, with the moments obtained in the hogging and the sagging

regions approaching their ultimate capacities. Therefore, in general, beams with near surface

mounted strips are much more efficient than beams with externally bonded plates which are

prone to premature debonding before ultimate capacities can be reached.

5.6 REFERENCES

Ashour, A.F., El-Refaie, S.A., and Garrity, S.W. (2004). “Flexural strengthening of RC continuous beams using CFRP laminates”. Cement & Concrete Composites, 26, 765-775.

Ashrafuddin, M., Baluch, M.H., Sharif, A., Al-Sulaimani, G.J., Azad, A.K., Khan, A.R. (1999). “Peeling and diagonal tension failures in steel plated R/C beams.” Construction and Building Materials, 13, 459-467.

Barros, J.A.O., and Fortes, A.S. (2005). “Flexural strengthening of concrete beams with CFRP laminates bonded into slits.” Cement & concrete composites, 27, 471-480.

Bencardino, F., Spadea, G., and Swamy, R.N. (1996). “Use of non-metallic reinforcements for strengthening/rehabilitating new and deteriorating structures.” Proceedings of international conference on material engineering, 25th AIAS Nat. Conf., Editrice Salentina, Gallipoli, Lecce, Italy, 1185-1192.

Bencardino, F., Spadea, G., and Swamy, R.N. (2002). “Strength and Ductility of Reinforced Concrete Beams Externally Reinforced with Carbon Fiber Fabric.” ACI Structural Journal, 99(2), 163-171.

Blaschko, M. (2003). “Bond Behaviour of CFRP Strips Glued into Slits.” FRPRCS6, Singapore, World Scientific.

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5.7 NOTATIONS

The following symbols are used in this chapter: %MR percentage of moment redistribution %MRh percentage of moment redistributed from hogging to sagging region %MRs percentage of moment redistributed from sagging to hogging region (Ea)l Young’s modulus of adhesive parallel to the directions of the fibres (Ea)p Young’s modulus of adhesive perpendicular to the directions of the fibres (Mhog)EI.const theoretical hogging moment (at maximum moment position) from linear elastic analysis

assuming constant flexural rigidity (Mhog)el maximum hogging moment from elastic analysis of constant EI (Mhog)fail hogging moment experimentally measured at failure (Mhog)test experimental hogging moment at maximum moment position (Mhog)u ultimate hogging moment at maximum moment position (Msag)el maximum sagging moment from elastic analysis of constant EI (Msag)u ultimate sagging moment at maximum moment position (Mstatic)u ultimate static moment b beam width be distance from the edge of the plate to the side of the beam bg width of groove bp plate width dg depth of groove dn neutral axis depth dp depth of plate from concrete surface EA axial rigidity Eb Young’s modulus of reinforcing bars Ec Young’s modulus of concrete EI flexural rigidity Ep Young’s modulus of plate fc concrete cylinder compressive strength ffrac fracture strength fp.u plate fracture strength fp.y plate yield strength ft concrete tensile strength fu ultimate strength fy yield strength h beam depth L span L1 length of plate over hogging region L2 length of plate over sagging region Lp length of plate


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M moment Mel elastic moment Mel elastic moment Mf factor moment Mhog moment in hogging region at maximum moment position Mpl plastic moment Msag moment in sagging region at maximum moment position Mstatic static moment Mult ultimate moment Np number of CFRP sheets P load Pult ultimate load R reaction force at support sp spacing between NSM strips tp plate thickness Vc vertical shear capacity wel load based on linear elastic analysis Wf factor load wnl load based on non-linear analysis wpl plastic failure load wult ultimate load βmax maximum moment redistribution required for wpl

β degree of moment redistribution εp plate strain εp.y plate yielding strain εp.max maximum plate strain ∆ deflection The following acronyms are used in this chapter: c/c centre to centre spacing CDC critical diagonal crack CFRP carbon fibre reinforced polymer EB externally bonded FRP fibre reinforced polymer IC Intermediate crack MR moment redistribution MRh moment redistribution in hogging region MRs moment redistribution in sagging region NSM near surface mounted PE plate end RC reinforced concrete SG strain gauge SGC strain gauge on concrete compression face The following subscripts are used in this chapter: el elastic analysis hog hogging region pl plastic analysis sag sagging region


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Documents

5.4 EXPERIMENTAL S RC B M S /FRP S