Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

١٤٨

EFFECT OF COMPRESSIVE STRENGTH AND
REINFORCEMENT RATIO ON STRENGTHENED BEAM WITH

EXTERNAL STEEL PLATE

Asst. Lec. Hesham Abd AL –Latef Numan
Civil Engineering Department, College of Engineering

Al-Mustansiriya University, Baghdad, Iraq

Abstract:

The present study is an experimental comparison between the effect of increasing the compressive
strength of the section and increasing the reinforcement ratio on the results of strengthening
reinforced concrete beams with external steel plates of constant dimensions.
The experimental program consists of testing ten reinforced concrete beams. Five of them are
without external steel plates to be the original specimens while the other five ones are provided with
steel plates of same dimensions glued at the bottom face of the beams.
Three values of compressive strength (f'c) were used in this study which were (22, 45 and 71MPa)
and also three ratios of internal reinforcement (ρ) which were (0.01411, 0.02116 and 0.03445) to
investigate their effects on the strengthened beams behavior.
The results showed that the cracking load and the ultimate load can be increased up to (150% and
137%) respectively. Also, by increasing the section compressive strength all the properties of the
strengthened beam can be improved while by increasing the reinforcement ratio the deflection and
cracking can be reduced to improve the elastic behavior of the beam..

Keywords: Strengthened beam, external plate, deflection ductility, restraining.

الحديد صفيحة من ب المدعمةالناتئة العتبةعلى ونسبة التسليحالقصوى تاثير قوى الشد

الخارجية

هشام عبد الطيف نعمان

الجامعة المستنصرية كلية الهندسة

:الخالصة

ئج تقوية الدراسة الحالية عبارة عن مقارنة عملية بين تأثير زيادة مقاومة االنضغاط للمقطع وزيادة نسبة التسليح على نتا

خمس . يتكون البرنامج العملي من فحص عشر عتبات خرسانية .العتبات الخرسانية المسلحة باستخدام صفائح فوالذية ثابتة األبعاد

Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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من هذه العتبات غير مزودة بصفائح فوالذية لغرض اتخاذها كنماذج أولية بينما تزود الخمس عتبات األخرى بصفائح فوالذية

.إلى األوجه السفلى لهذه العتباتباألبعاد ذاتها تلصق

وكذلك ثالث نسب من ) ٢ملم/نت 71و 45، 22(ثالث قيم من مقاومة االنضغاط تم استخدامها في هذه الدراسة وهي

.لغرض تقصي تأثيراتها على سلوك العتبات المقواة) 0.03445و 0.02116، 0.01411(التسليح الداخلي وهي

وكذلك فإن بزيادة . على التوالي) %137و %150(لحمل األقصى يمكن زيادتهما حتى كشفت النتائج أن حمل التشقق وا

مقاومة االنضغاط للمقطع فإن كافة خصائص العتبة المقواة يمكن تحسينها بينما بزيادة نسبة التسليح فإن خاصتي االنحراف

. واالتشقق يمكن تقليلهما مما يحسن السلوك المرن للعتبة

Introduction:

An important part of the responsibility of the structural engineer is to select, from many

alternatives, the best structural system for the given conditions. The wise choice of a structural

system is far more important, in its effect on overall economy and serviceability than refinement in

proportioning the individual members (Nilson et al. 2003). In structural engineering, the
maintenance, repair and upgrading of structures are just as important and technical as the design and

construction of new structures. In the case of upgrading this usually involves strengthening of an

existing structure to satisfy a higher ultimate load and /or more stringent serviceability requirements

(Jones et al 1982). One of the more successful methods for strengthening the reinforced concrete
structures is "Plate Bonding Technique". Investigations into the performance of members

strengthened by this technique started in the 1960s. More recently, many researches on plain and

reinforced concrete have been carried out.

The works of Jones et al. (1980), Swamy et al. (1987), Hamoush and Ahmed (1990), Oehlers
et al. (1998) and Kheder et al. (2008) have highlighted a number of features of this technique, some

of which can be summarized as:

� Full composite action can be achieved between a concrete member and a steel plate by the use of
suitable epoxy glue.

� Plating has a considerable reducing effect on both flexural crack width and deflection. The
reduction is greater than would be achieved by using additional internal reinforcement

equivalent to that of external plate.

� Where failure of a strengthened reinforced concrete member is by yielding of bonded plate, the
ultimate strength can be predicted by using conventional reinforced concrete theory accurately.

� This technique can increase the flexural stiffness of the beam at all load stages and consequently
reduce deflections at corresponding loads with a significant increase in serviceability.

� Due to controlling of deflections, cracking and concrete strains, this technique increases the
range of the elastic behavior of the strengthened beams.

However, despite of the plate bonding technique advantages in field of the reinforced

concrete structures, the premature failure trouble is still dominant, as shown in Figure 1, and

Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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must be vanished to attain the technique advantages. Reinforced concrete beams strengthened

externally by plates bonded to the tension face have been noted to fail in a variety of modes,

influenced greatly by the plate thickness. Failure modes include the following (Nguygen et al
2001):

� The flexural dominant mode; characterized by extensive yielding of internal reinforcement and
external plate, deep intrusion of flexural cracks and crushing of concrete in the compressive

zone.

� Premature separations of the plate at the concrete–glue–steel interface; initiated from the zone of
plate curtailment.

� Horizontal tearing of concrete cover; initiated at the location of plate curtailment, the interface
remains intact, with the crack passing through the concrete below the level of main internal

reinforcement and proceeds upwards to the point of loading in a steep vertical ascent (shear

mode of failure).

A hybrid mode of failure in which there is yielding of internal reinforcement and external plate

prior to failure; with actual failure being precipitated by the horizontal tearing of concrete cover

below the level of internal reinforcement (flexure, shear mode of failure).

Aims Of Study:

The aims of this study is to select from two options which one is the best for strengthening

reinforced concrete beams using the plate bonding technique. The two investigated options are the

effects of increasing the compressive strength of the section and increasing the reinforcement ratio

on the strengthened beam behavior, whilst strengthening is done by external steel plate having same

dimensions.

Experimental Work:

The experimental work consists of testing two groups of beams, the first group contains the

original specimens (B1, B2, B3, B4 and B5) while the second group contains their strengthened

specimens (SB1, SB2, SB3, SB4 and SB5) respectively. The beams (B1, B2 and B3) and their

strengthened specimens have the same reinforcement ratio (ρ = 0.01411) but their compressive

strengths are incremented (f'c = 22, 45 and 71MPa) respectively to investigate the compressive

strength of section effect on the strengthened beams behavior. The beams (B3, B4 and B5) and their

strengthened specimens have the same compressive strength (f'c = 71MPa) but their internal

reinforcement ratios are incremented (ρ = 0.01411, 0.02116 and 0.03445) respectively to investigate

the reinforcement ratio of section effect on the strengthened beams behavior.

Ten beams are tested under two point loading up to failure to study their strength and

deformation characteristics in addition to the mode of failure and ductility. Five of the tested beams

Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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are without external steel plates to be the original specimens while the other five ones are provided

with external steel plates of same dimensions glued at the bottom face of the beams.

Materials

Concrete

Three concrete mixes were used to provide three compressive strengths of (22, 45 and 71

MPa), as shown in Table 1. Ordinary portland cement Type (I) complying with the Iraqi standard

specification No. 5/1984. The used fine aggregate was natural river sand with fineness modulus

(F.M.) of (2.73), bulk specific gravity (S.G.) of (2.64) and sulfate content, (SO3%) of (0.31%) by

sand weight, which is less than the limit of Iraqi standard specification No. 45/1984. The used

coarse aggregate is crushed gravel with maximum size of (12mm); the bulk specific gravity (S.G.)

of this aggregate is (2.61) and complying with the Iraqi standard specification No. 45/1984. For

increasing the compressive strength, a superplasticizer (SP) was used to reduce the water content

and compensate the associated reduction in workability, is commercially known as (Glenium 51)

which complies with ASTM C 469–86.

Cylinders and prisms for control tests were cast and stored with each beam and then tested

when the beam was tested. The mix proportions and the average results of cylinder strength f'c,

modulus of rupture fr and Elastic modulus Ec for all beams are given in Table 1:

Reinforcement

Two types of reinforcing steel are used in present work, as shown in Table 2; steel bars used

as internal reinforcement for flexure and shear in all beams and steel plates used as external

reinforcement as well as other internal steel bars in the strengthened beams. Deformed steel bars of

diameter (16, 25mm) are used for the main reinforcement and plain steel bars of diameter (6mm)

are used for stirrups. A steel plate with (1.0mm) thickness is used as external reinforcement in the

strengthened beams by bonding to the concrete surfaces by epoxy resin of mechanical properties

and especially bond strength greater than the concrete tensile strength.

Details of Beams

All the beams were with dimensions of (150x250x2500mm), and their spacing of stirrups and

the limitations of reinforcement were adopted according to ACI Code 318–05, as shown in Table 3
and Figure 2. The shear span (a/d) for all the beams was constant at (4.21) and provided with steel
bar stirrups of (2 legs Ø6mm at 100mm). For the strengthened specimens (SB1, SB2, SB3, SB4 and

Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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SB5), external steel plates with dimensions of (100x1x2500mm) were glued at the bottom faces of

the beams.

Casting and Curing of Beams

Two steel molds were prepared for casting the specimens; so that two beams were cast at the

same time at one day. Six cylinders and three prisms were cast with each two beams for observing

the concrete mechanical properties. The concrete was poured at (3 layers) and compacted about (2

min) by a vibrating table. After (2 days), the two specimens and their control units were removed

from their molds and cured in water containers at a temperature of about (25°C) until the testing age

of (28 days).

For the strengthened specimens, the external steel plates were glued at the specimens surfaces

by the epoxy resin (glue), as shown in Figure 2.

Preparation and Testing of Beams

Before testing, the specimens were painted with a white emulsion to aid the detection of

cracks. Dial gauge with 0.01mm divisions was positioned at the bottom of beam center.

All beams were tested under two–point loading, each equal to (1/2) the total applied load from

the loading machine. Loading was applied in increments of (4kN) to record the deflection. After

each (20kN), the load is kept constant until the required readings of crack widths. Testing was

continued until the beam showed a drop in load carrying capacity with increasing deflection.

Testing was conducted by using MFL SYSTEM of hydraulic universal testing machine type

EPP300, as shown in Figure with a maximum capacity of (3000kN).

Experimental Results:

The experimental test insisted on cracking load, failure load, deflection, cracks and their

characteristics as well as mode of failure. Table 4 contains the exhibited values of the above

properties.

Cracking Load

It is obvious from Table 4 that for the original beams (B1, B2 and B3) which represent the

compressive strength increment the appearance of first crack was at load having ratio (16, 19 and

31%) of their failure loads, while for the beams (B4 and B5) which represent the reinforcement

ratio increment the appearance of first crack was at load having ratio (20 and 15%) of their failure

loads respectively. On the other hand, when strengthen all these beams by constant dimensions

Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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plates the cracking load raised about (8kN) but the ratio of the cracking load at the strengthened

beam to original one was decreased in the both cases; with increasing the compressive strength or

increasing the reinforcement ratio but with by increasing the reinforcement ratio the decrease was

largest as shown in Table 5. From the previous demonstration of results it is clear that increasing the

compressive strength is better than increase the reinforcement ratio for increasing the ratio of

cracking load to failure load and to delay the appearance of the first crack in comparison the beam

ultimate strength.

Failure Load

The failure load can be raised by increasing the compressive strength or increasing the

reinforcement ratio, but from Table 4 the results showed that for the beams (B1, B2 and B3) the
failure loads were (101, 104 and 106kN) respectively and this mains slight increase in the ultimate

strength in comparison with the increase in the compressive strength. While, by comparison

between the beams (B3, B4 and B5) which represent the increase in the reinforcement ratio, their

failure loads were (106, 176 and 253kN) and reflected the considerable in the ultimate strength.

Thus, it is concluded that the reinforcement ratio has the greatest effect on improving the ultimate

strength of the beam more than the compressive strength. For the strengthened specimens (SB1,

SB2 and SB3) the ratio of the strengthened failure load was (1.18, 1.26 and 1.37) respectively and

referred to the activity of increasing the compressive strength in improving the failure load through

the strengthening by plate bonding technique. In contrast with the compressive strength action; the

reinforcement ratio when increased led to reduce the ratio of strengthened failure load as (1.37, 1.28

and 1.17) respectively observed for (SB3, SB4 and SB5) respectively. So that, it is concluded that

the compressive strength has the greatest effect on improving the ultimate strength of the

strengthened beam more than the reinforcement ratio.

Deflection

From Table 4 where the values of deflection were listed; it is noticed that the deflection
proportions to the failure load of the beam, therefore, the deflections of (B1, B2 and B3) were

smaller than the deflections of (B4 and B5). The strengthening process exhibited a considerable

reduction in deflection especially for (SB4 and SB5) in according with the ratio of strengthened

failure load to original failure load when this ratio decreases with the increase in the reinforcement

ratio as shown in Table 5. On the other side, the load-deflection curves in Figure 4 clarify the
similarity in the strengthened beams behavior to that of the original ones.

Ductility

Typically, ductility is calculated by division the value of deflection at failure per the value of

deflection at yield condition and this defined as deflection ductility. That is known by decreasing

the compressive strength, increasing the reinforcement ratio or increasing the yield strength of the

Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

١٥٤

reinforcement the ductility will decrease and the reported effect of flexural strengthening with

external reinforcement is a reduction in the ductility relative to the original condition (ACI

Committee 440 2002). The results of the present study confirms the previous report because all the

strengthened beams exhibited ductility less than that of original specimens and this came from the

increase in reinforcement due to the external steel plate, and for the same reason the ductility of (B4

and B5) was (0.65 and 0.67) which is less than (0.78, 0.76 and 0.77) belong to (B1, B2 and B3)

respectively

Cracking

Cracking was observed by three variables in the present study; crack width (Wu), crack height

(h) and crack spacing (s) as shown in Table 4. The crack spacing is a function of the number of
cracks along the beam. It is noticed that number of cracks was the same in all the strengthened

beams and almost less by one crack than that of their original beams and that conforms too many

previous researches that stated that plate bond technique has a marginal effect on crack spacing. For

the crack width; the original beams (B1, B2 and B3) exhibited reduction in the crack width from

(1.45mm) to (1.35 and 1.25mm) respectively to reflect the effect of increasing the compressive

strength on reduction the crack width, and after this the beams (B4 and B5) exhibited a constant

crack width of (1.25mm) with no effect of increasing their reinforcement ratio. All the strengthened

beams exhibited a reduction in crack width and reflected the activity of the plate bonding technique

in reduction the crack width. For the crack height; the original beams (B1, B2 and B3) exhibited a

crack height of (177, 193 and 199mm) respectively to reflect the effect of increasing the

compressive strength on raising the crack height, but for the beams (B3, B4 and B5) the crack

heights were (199, 195 and 178mm) respectively to reflect the effect of increasing the

reinforcement ratio on diminishing the crack height. The strengthened beams (SB1, SB2 and SB3)

which have the same exhibited a same crack height about (173mm) and when reinforcement ratio in

the beams (SB4 and SB5) the crack height was more diminishing to be less than (173mm) and to

reflect a new prove on the activity of the plate bonding technique in restraining the cracking.

Mode of Failure

The urgent problem of the plate bonding technique is the concentration of stresses at the plate

ends which lead to premature failure and limit the advantages of this technique. Extension the

external steel plate reduces the concentration stresses at the plate ends so that the steel plates were

bonded along the strengthened beams to ensure a desirable failure. Thereby, all the beams failed by

the same manner as mentioned in Table 4 which was flexure mode characterized by developing of
cracks coinciding with rapid increase in deflection continued until the drop of the applied load and

indicated yielding of reinforcement left residual deformations after releasing of the load as shown in

Figure 5.

Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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Conclusions:

1. Increasing the compressive strength is more preferable than increasing the reinforcement ratio for
increasing the cracking load of the strengthened beam with external steel plate and with normal

concrete strength best controlling on the cracking load can be achieved.

2. The reinforcement ratio has the efficiency for increasing the ultimate strength of the beam more
than the compressive strength, but after strengthening; increasing the compressive strength is

more preferable than increasing the reinforcement ratio for enhancing the ultimate strength.

3. The plate bonding technique has a control rule in reduction the deflection of the beam, and this
reduction can be raised by increasing the reinforcement ratio.

4. There is loss in ductility of the strengthened beam due to the further use of reinforcement and this
loss increases with increase the reinforcement ratio. However, the ductility can be restored by

increasing the compressive strength of the beams.

5. The plate bonding technique has a restraint effect for reduction the crack width, the crack width
and cracks number.

6. In spite of the action of external steel plate on increasing the reinforcement ratio and then
decreasing the ductility, but this action enhances the activity of the internal reinforcement for

restraining the cracking and reduction the deflection and leads to increase the elastic behavior

range of the beam.

References

1. Nilson, A. H., Darwin, D. and Dolan, C. W., "Design of Concrete Structures",
International Edition, 3rd Edition, Singapore 2003, 771 PP.

2. Jones, R., Swamy, R. N. and Ang, T.H., "Under–and Over–Reinforced Beams with
Glued Steel Plates", The International Journal of Cement Composites Lightweight
Concrete, V. 4, No. 1, Feb. 1982, PP 19–32.

3. Jones, R., Swamy, R. N., Bloxham, J. and Bouderbalah, A., "Composite Behavior of
Concrete Beams with Epoxy Bonded External Reinforcement", The International
Journal of Cement Composites, V. 2, No. 2, May. 1980, PP. 91–107.

4. Swamy, R. N., Jones, R. and Bloxham, J., "Structural Behavior Reinforced Concrete
Beams Strengthened By Epoxy–Bonded External Plates Reinforcement", The Structural
Engineer, V. 65 A, No. 2, Feb. 1987, PP. 59–68.

5. Hamoush, S. A. and Ahmed, S. H., "Static Strength Tests of Steel Plate Strengthened
Concrete Beams", Materials and Structures, V. 23, No. 134, May. 1990, PP. 116–125.

6. Oehlers, D. J., Mohamed Ali, M. S. and Luo, W., "Upgrading Continuous Reinforced
Concrete Beams by Gluing Steel Plates to Their Tension Faces", Journal of Structural
Engineering, V. 124, No. 3, Mar. 1988, PP. 224–232.

Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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7. Kheder, G. F., Al-Khafaji, J. M. and Ajeel, A. E., "Experimental Design of External Steel

Plates and Study of Concrete Strength and Reinforcement Ratio Effects on
Strengthening Damaged R.C. Beams", Comprehensive Scientific Engineering Conference,
Engineering and Development Journal, V. 12, No. 1, Oct. 2008, PP. 22–44.

8. Nguygen, D. M., Chan, T. K. and Cheong. H. K., "Brittle Failure and Bond Development
Length of CFRP–Concrete Beams", Journal of Composites for Construction, V. 5, No. 1,
Feb. 2001, PP. 12–17.

9. Sharif, A., Al–Sulaimani, G. J. and Ghaleb, B. N., "Strengthening of Initially Loaded
Reinforced Concrete Beams Using FRP Plates", ACI Structural Journal, V. 91, No. 2,
Mar. 1994, PP. 160–168.

10. ACI Committee 440, "Design and Construction of Externally Bonded FRP Systems for
Strengthening Concrete Structures (ACI 440.2R-02)", American Concrete Institute,
Farmington Hills, Mich, 2002, 45 PP.

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Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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TTaabbllee 33:: DDeettaaiillss ooff bbeeaammss

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Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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TTaabbllee 55:: RRaattiiooss ooff ssttrreennggtthheenneedd ttoo oorriiggiinnaall bbeeaammss pprrooppeerrttiieess

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Figure 1: Premature failure hazards (Sharif 1994)

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Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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250mm

190mm

Ø6mm

800mm 800mm 800mm
2500mm

Main
Reinforcement
Ø16 or Ø25mm

150mm

250mm

190mm

800mm 800mm 800mm
2500mm

Ø6mm

External Steel Plate 100x1x2500mm

Main
Reinforcement
Ø16 or Ø25mm

Glued Steel Plate
100x1mm

100mm

150mm

aa)) OOrriiggiinnaall bbeeaammss

bb)) SSttrreennggtthheenneedd bbeeaammss
Figure 2: Details of beams

Figure 3: Testing machine

Al-Qadisiya Journal For Engineering Sciences Vol. 3 No. 2 Year 2010

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aa)) SSppeecciimmeennss wwiitthh iinnccrreemmeenntteedd ccoommpprreessssiivvee ssttrreennggtthh

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bb)) SSppeecciimmeennss wwiitthh iinnccrreemmeenntteedd rreeiinnffoorrcceemmeenntt rraattiioo

FFiigguurree 44:: LLooaadd--DDeefflleeccttiioonn ccuurrvvee ooff tthhee tteesstteess bbeeaammss

Figure 5: Deformation of beam after testing