13 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 2004 Paper No. 3242 DETERMINATION OF ULTIMATE FLEXTURAL CAPACITY OF STRENGTHENED BEAMS WITH STEEL PLATES AND GFRP Javad VASEGHI AMIRI 1, Morteza HOSSEINALIBEGIE 2 SUMMARY In order to investigate the effect of strengthening of reinforced beams using steel and GFRP plates on the flexural strength and ductility,14 specimens have been designed and tested.the specimen regarding the amount of compressive steel and the condition of strengthening are divided in to two Group I and II.All the beams were simply supported and were tested under two-point loading. The amount of imposed load, strain in concrete, strain in the level of tension reinforcement bars and deflection of mid span in different stages of loading have been measured and recorded. According to test results the amount of change in the flexural strength and ductility of beams have been calculated. The effect of shape and type of the performance,the strengthening of beams on the flexural parameters and ductility have been investigated. Keyword: Strengthening, GFRP, Flexural behavior INTRODUCTION There are different criteria, which increased the demand of strengthening of the structures. Some of them are, the increase of service load, deterioration or aging in the concrete structure, the change of criteria of codes, sudden collisions to the structure by trucks or severe earthquake.for example in Bridges of North of America which were built after the second world war, more often service loads were greater than the design load. This increase has been reported equal to 40 percent. So, the strengthening of structures is necessary. Also the weight of reinforcement must be as light as possible in order to prevent increase of the dead load. Steel and polymer plates (that are known with the abbreviation of FRP) are the most conventional material in strengthening concrete beams. Polymer plates consist of two main components, fiber and polymer unites. Especial type of plates which have fiber glass are called GFRP. Applying glue to the steel and GFRP plates is the simplest way to strengthen the concrete beams. But the most important factor to consider when using the steel plate, is corrosion. Corrosion is a serious danger which will not only decrease the strength of the plate, but will also destroy the bonding between plates and concrete in case of using epoxy resin. That s why the usage of GFRP plates as strengthening material, captured the attention of researchers. The ratio of strength to the weight, easiness to carry it, durability against fire, accessibility in every shape and length are some of advantage of GFRP plates. These materials in comparison with steel show a greater resistance against the electrochemical corrosion. Recently for the 1 Assistant Professor in Mazandaran University, Iran, Email : Vaseghi@ tech.umz.ac.ir 2 Assistant Professor in Mazandaran University, Iran, Email : Baygie@ tech.umz.ac.ir
usage of GFRP for the strengthening of concrete beam a great number of researches had been carried out. In 1991, Saadatmanesh and Ehsuny in Arizona University, U.S.A began a research for strengthening with fiber reinforced polymer (1). They chose 5 beams with rectangle cross section and one beam with T shape cross section, tested under the concentrated load (4 point load).by referring to reference(2) we will see some research which have been carried out in Oxford University on the effect of GFRP on the strength and ductility of beams. Reference (3 and 4 ) are in the same field. EXPERIMENTAL PROGRAM In order to carry out the test, 14 specimens with same dimensions and same flexure and shear reinforcement were used. Beams had square cross section with dimension equal to 200 200 mm and the span equal to 1800 mm. Beams are divided in 2 Groups II, and I which each Group were consisting of 7 beam. One beam was considered as reference and tested without any strengthening, and the other beam were strengthened with steel and GFRP plates. (Three beams were strengthened with steel plates and three beams were strengthened with GFRP plates.) In beams of group I two bars with 8 mm diameter and in beams of group II two bars with 14 mm diameter were used as compressive bars and 3 bars with 14 diameters were used as tension bars. Bars with 8mm diameter were used as stirrup and placed with a distance of 130 mm from each other for each two group. The beams of Group I are strengthened with steel and GFRP plates prior to loading but the beams of Group II were strengthened after cracking load. In all of the test the bearing system is simply supported and the specimen were loaded with two concentrated load which are placed in 1/3 length of span.in Group I steel and GFRP plates are glued with normal epoxy Sicodor-31 to the beams but in beams of group II which were strengthened with steel plates in addition to using glue they used steel strap belts to brace the end of beams. Table 1 shows the characteristic of strengthening material. Figure 1 and 2 shows specimen and cross section and table 2 shows geometric component of strengthening material. TABLE 1 MECHANICAL PROPERTIES OF STRENGTHENING MATERIAL Material Elastic Modules (Gpa) Tension Strength (Mpa) Steel plate 200 220 GFRP 11 180 Epoxy resin 3/4 15-20 The compressive strength of the concrete used in the beams was 330 kg/cm 2 with slump about 60 mm The tensile strength of reinforce used in beams are 3200 kg/cm 2 and the shear strength is 2700 kg/cm 2. The strengthening took place 4 weeks after the casting and grinding took place, then the gluing started. In order to prevent the movement of plates especial clamps were used as end anchorage. After 10 days from strengthening, beams were tested.
FIGURE 1 CROSS SECTION OF BEAMS OF GROUP I AND II TABLE 2 DETAIL OF TEST SPECIMENT AND STRENGTHENING MATERIAL speci men F / c kg/ cm 2 Width ( mm) characteristic of Strengthening Plate (t) Length Effective Fy (mm) ( mm) depth Kg/cm (mm) 2 Type of plate Percent of tension reinforc e Percent of compressiv e reinforce IA 330 - - - - - - 1.32 0.287 IA 1 353. 6 200 2 1500 202 2200 Steel 1.32 0.287 IA 2 320 150 2 1500 202 2200 Steel 1.32 0.287 IA 3 316.8 100 2 1500 202 2200 Steel 1.32 0.287 IA 4 306.9 200 2.5 1500 202 1800 GFRP 1.32 0.287 IA 5 351. 8 200 4 1500 202 - GFRP 1.32 0.287 IA 6 326. 7 140 2.5 1080 202 1800 GFRP 1.32 0.287 IIB 328 - - - - - - 1.32 0.88 IIB 1 323 200 2 1500 202 2200 Steel 1.32 0.88 IIB 2 341.3 150 2 1500 202 2200 Steel 1.32 0.88 IIB 3 337.5 100 2 1500 202 2200 Steel 1.32 0.88 IIB 4 345 200 2.5 1500 202 1800 GFRP 1.32 0.88 IIB 5 315 200 4 1500 202 - GFRP 1.32 0.88 IIB 6 340.4 140 2.5 1080 202 1800 GFRP 1.32 0.88
MEASURED PARAMETERS ULTIMATE LOAD GROUP I The value of ultimate load of reference beam of this Group is equal to 10.83 ton. The largest value of ultimate load in strengthened beam belongs to IA5, which is equal to 17.4 ton, and in comparison to the reference beam it shows 71 percent increase. The lowest amount of increase of ultimate strength in strengthened beams belongs to IA3 beam, which is equal to 12. 5 ton and shows 11 percent increase relative to the reference beam. In strengthened beams by the increase of cross section in the s, ultimate bearing load in the beams increased. Of course in beams, which were strengthened with steel plates, this increase was not considerable. TABLE 3 COMPARISON BETWEEN ULTIMATE LOAD, DEFLECTION, MODE OF FRACTURE IN BEAMS OF GROUP I especie ment Compressive Strength Kg/ cm 2 Ultimate Load (ton) Increase percent in ultimate load Maximum deflection (mm) D S /D m Fracture Mode IA 330 10.83-15.7 1 Yielding of tension bar IA 1 353. 6 12.54 15.8 3.73 0. 238 Separation of IA 2 320 12.35 14 4.85 0. 309 Separation of IA 3 316.8 12.05 11.26 5.39 0.345 Separation of strength plate IA 4 306.9 15.78 45.71 14.52 0.925 Yielding of tension bars and strengthening plate IA 5 351. 8 17.4 60.66 13.96 0.89 Separation of IA 6 326. 7 13.81 27.5 11.07 0.705 Yielding of tension bars and strengthening plate
Beams, which were strengthened with GFRP plates, show more increase in the ultimate strength relative to beams consisting of steel plates. Table 3 shows a comparison between ultimate load, maximum displacement and type of failure of Group I beams. GROUP II The amount of ultimate load of reference beam (IIB) is 12.3. The largest amount of ultimate load in strengthened beam of this Group belongs to IB1 beam, which is equal to 18.2 ton, and in comparison to the reference beam it shows 48 percent increase in strength. The lowest value of the increase of the strength belongs to IIB6 beam that shows 20 percent increase in comparison to the reference beam. In the strengthened beams of this Group by increasing the cross section of plates the ultimate strength increased. The strengthened beams with steel plates of this Group show more ultimate strength relative to similar beams in Group I and it is due to end anchorage. Table 4 shows a comparison between ultimate load of maximum displacement and type of failure of beams of Group II. The strengthened beams with steel plates relative to beams strengthened with GFRP will show more increase in strength. TABLE 4 COMPARISON BETWEEN ULTIMATE LOAD, DEFLECTION, MODE OF FRACTURE IN BEAMS OF GROUP II especiement Compres sive Strength Kg/ cm 2 Ultimat e Load (ton) Increase percent in ultimate load Maximum deflection (mm) D S /D m Fracture Mode IIB 328 12.3-17.8 1 Yielding of tension bar IIB 1 323 18.2 47.97 6.6 0.371 Yielding of tension bar IIB 2 341.3 16.8 36.5 6.69 0.544 Yielding of tension bar IIB 3 337.5 16.6 34.96 10.39 0.584 Yielding of tension bar IIB 4 345 15.2 23.58 14.6 0.82 Separation of IIB 5 315 16.85 36.99 14.38 0.808 Separation of IIB 6 340.4 14.8 20.32 15.9 0.893 Separation of ** D t is measured displacement of mid span of beam and D m is displacement of mid span of reference beam
MID SPAN DISPLACEMENT GROUP I Deflection of mid span in the reference beam (IA) is equal to 15.7 mm.the highest deflection in the strengthening beam is belong to IA4 beam which is equal to 14.52 mm. The lowest amount of deflection belongs to IA1 beam which is equal to 3.73 mm. In this Group with the increase of cross section the deflection of beams reduced which is due to the increase of stiffness in strengthening beams (Table 3). In this Group the strengthened beams with GFRP has a greater deflection relative to strengthening beam with steel plates. GROUP II Deflection of reference beam (IIB) is equal to 17.8 mm. The greatest deflection of these Beams is also similar to Group I belongs to IIB4 beam and the lowest deflection belongs to IIB1 beam which is equal to 6.6 mm. In this Group of beams with the increase of cross section of plates the deflection of beams reduced. Deflection of strengthened beams with steel plates relative to similar beam in Group I is greater and this is because of the presence of anchorage in the end of s. In this Group also the deflection of the strengthened beams with GFRP plates is greater than the deflection of strengthened beams with steel plates. (Table 4) DUCTILITY BEAMS OF GROUP I Figure 3 shows load- deflection curve of Group I beams. Regarding the curve it is clear that the ductility of strengthened beam relative to reference beam reduced with the increase of cross section of plates. The strengthened beam with GFRP has a greater ductility compared to steel plates. With regard to the curve the greatest ductility belongs to IA4 beam and the lowest ductility belongs to IA1 beam. Load(ton) 20 18 16 14 12 10 8 6 4 2 0 IA IA1 IA2 IA3 IA4 IA5 IA6 0 5 10 15 20 Displacement (mm) FIGURE 3 LOAD DEFLECTION CURVE OF MID SPAN BEAMS OF GROUP I BEAMS OF GROUP II Figure 4 shows the curve of load- deflection curve of beams in Group II. In this Group the ductility of the beams reduced with the increase of cross section of. The ductility of all the strengthened beams is less than the reference beam. The beams strengthened with GFRP plates have a greater ductility relative to the beam strengthened with steel plates. The strengthened beam with steel plate in group I, has less ductility relative to strengthened beam with steel plate of this group.
Load (ton) 20 18 16 14 12 10 8 6 4 2 0 0 5 10 15 20 Displacement (mm) IIB IIB1 IIB2 IIB 3 IIB4 IIB5 IIB6 FIGURE 4 LOAD DEFLECTION CURVES OF MID SPAN BEAMS OF GROUP II STRAIN Figure 5 and 6 shows the load- strain curve in the level of tensile bars of beams of group I and II. The result shows that the presence of will change the amount of strain in the concrete. 16 14 Load (ton) 12 10 8 6 4 2 0 IA IA1 IA2 IA3 IA4 IA5 1A6 0 500 1000 1500 2000 2500 3000 Strain 10 6 FIGURE 5 LOAD STRAIN CURVE IN THE LEVEL OF TENSILE BAR OF BEAMS OF GROUP I Load (ton) 20 18 16 14 12 10 8 6 4 2 0 IIB IIB1 IIB2 IIB3 IIB4 IIB5 IIB6 0 1000 2000 3000 Strain 10 6 FIGURE 6 LOAD STRAIN CURVE IN THE LEVEL OF TENSILE BAR OF BEAMS OF GROUP II
In the beams which are not strengthened, stress in the steel bar will increase until the steel reached the yielding limit. The additional stress can be tolerated by large deformations (increasing strain as well as stress known as strain hardening) of steel reinforcement as a result will reduce the increase of compressive strain of concrete. In strengthened beam, tensile stress between bars and strengthened plate will be distributed. Consequently the present stress in the bars are less than stress in bars of reference beam and will not reach the yielding limit. So, the strain of concrete in the strengthened beam is more than the control beam with similar loads. MODE OF FAILURE AND THE CRACK FORMATION Figure 7 (a, b, c, d) shows the state of failure and cracks formation in the beams which are under the test. GROUP I First cracking of the reference beam of group (IA) under the load of 2.25 ton have been observed. With the increase of load the widths and size of cracks increased and in vertical direction were developed towards the upper side of section. By increasing the applied load, more cracks in the pure bending zone (between two applied load) were developed and beam after yielding of tensile bars and considerable deformation accompanied with crushing of the beam failed. In IA1 beam, under the load of 5.5 ton first flexural crack was observed. With the increase of load another flexural crack appeared around the first crack. New cracks developed in the same direction. The growth will only reach the middle of the height of the concrete beam. Shear flexural cracks around the loading point in this beam have been observed which did not appear in the reference beam. The failure of this beam was due to separation of followed by crushing of. First crack of beams IA2, IA3 when the applied load reached to the 4.1 ton and 3.8 ton have been observed respectively. Figure (7.a) : Beam IA in failure state Figure(7.b) : Beam IA4 in failure state Figure (7.c) : Beam IB1 in failure state Figure( 7.d) : Beam IIB6 in failure state
Shape of cracks and type of failure of both beams is similar to IA1 beam. In none of the beams mentioned above did the tensile steel yield. First cracking of beam IA4, was under the load of 2.8 ton. In this beam flexural crack shows a lower growth relative to the reference beam. Shear cracks and also shear- flexural cracks have been observed in this beam. Ultimate failure of this beam was followed by separation of plate with the concrete of the lower part. In this beam tensile bar yields. In IA6, beam under the load of 2.5 ton, first flexural crack have been observed. The formation of crack and type of failure is similar to IA4 beam in this beam tensile bar yields. GROUP II Beams of group II were strengthened after being loaded under the cracking load. First cracking of reference beam (IIB) was under the load of 2.1 ton. The crack of this beam induced in the area of pure flexure. Only few shears flexural cracks near the loaded point have been observed. The failure of this beam was due to yielding of tensile steel and the crushing of concrete at compression side of beam. Beam IIB1 cracked under the 5 ton load. This crack have been induced and developed exactly from the cracking area prior to strengthening. With the increase of load other flexural cracks induced, but this crack had little growth of length and did not reach the middle of the height of beam. Shear- flexural cracks in this beam have been observed near the loading point. In this beam due to the presence of the steel belt and end anchorage the separation of plate did not happened. The ultimate failure of this beam is due to yielding of tensile bar concrete at compression side. The load of first cracking in the IIB2, IIB3 beam were 3.75 ton and 3.7 ton respectively. Formation of cracks and type of failure of this beam is similar to the IIB1 beam. IIB4 beam cracked under the load of 3 ton, and the flexural cracking got wider with applying the load. Shear- flexural cracks induced around the loading point. The ultimate failure of this beam is due to separation of plate with the concrete of lower part. In this beam tensile bar yield. Loads of first cracking in the IIB5 and IIB6 are 3 ton and 2/7 respectively. Shape of cracks and type of failure of this beam is similar to the IIB4. CONCLUSIONS Regarding the tests which have been carried out and experimental results which were presented in this article we can say: _With the strengthening of concrete beam ultimate strength increased and ductility decreased. _In all the strengthened beams load of first cracking increased and in the beams where the width of plate and the width of beam are equal, first cracking induced under a greater load. _Beams which were strengthened with GFRP show more ductility relative to the beams strengthened with steel plate. _Strengthening of the beams after cracking will reduce the ultimate strength relative to the strengthening prior to the cracking. _All the strengthened beams experienced brittle failure under the ultimate loading. REFRENCES 1. Saadatmanesh, H. and Ehsani, M. R. (1991). RC beams strengthened with GFRP plates I: experimental study. struct. Eng. 117(11) 3417-3433. 2. Quantrill, R. J. & Hollaway L. e. and Thorne A. m. (1996a) Part 1.Experimental and analytical investigation of FRP strengthened beam response. Mag. concrete Res. 48(177) 331 342.
3. Hutchinson, A. R. and Rahimi, H. (1993). Behavior of reinforced Concrete beams with externally bonded fiber reinforced plastics. proc 5 The International Conference on structural Faults and Repair, university of Edinburgh, vol 3, pp 221 228. 4.Hollaway, L. C. and Leeming, M.B. Strengthening of reinforced Concrete structures using externallybonded FRP composites in structural and civil engineering, 1999.