Fourth International Conference on FRP Composites in Civil Engineering (CICE2008) 22-24July 2008, Zurich, Switzerland

- 1 -

1 INTRODUCTION

While the world post-strengthening technique of reinforced concrete (RC) structures using ex-ternally bonded (EB) fiber reinforced polymer (FRP) materials developeded in the 1980s in Western Europe, Polish experience started in 1997 with the first FRP post-strengthening appli-cation of the bridge crossing the river Wiar in Przemyśl (Siwowski & Radomski 1998). This technique was straight ford continuation of the European practice of strengthening RC bridge girders using of externally bonded steel.

Simultaneously with the first FRP applications in Polish civil engineering infrastructure, started experimental research on RC beams externally strengthened with FRP. The laboratory of the Department of Concrete Structures (DCS) at the Technical University of Lodz (TUL), as a pioneer, has been conducting an extensive study on FRP strengthened RC members starting from 1997. Experimental work started with RC rectangular beams strengthened in flexure with CFRP laminates (Research Grant nr 7 T07E 030), carried out by the author in 1998 (Kotynia 1999). It was the first step in understanding debonding mechanisms and their causes. The test results limited expected concept of possibilities of FRP utilization and determined strengthening effectiveness. The following research project carried out by the author on externally FRP strengthened beams (Research Grant nr 8 T07E 006 21, supported by the Polish Ministry of Scientific Research and Information Technology) aimed to delay the intermediate crack debond-ing of the bottom composite laminate, to increase the load carrying capacity and the CFRP strength utilization ratio. Many experiments have been carried out by the author on RC mem-bers strengthened for flexure with EB FRP materials indicating a low efficiency of this tech-nique, caused by premature FRP debonding. The failure mode significantly limited the effi-ciency of the CFRP used for retrofit, due to plate-end debonding and concrete cover separation or due to the intermediate crack-induced debonding (Kotynia & Kamińska 2003).

In order to improve serviceability of strengthened structures and to increase the stiffness and the load capacity of RC members, the FRP prestressing technique has been proposed. Fatigue tests of RC slabs strengthened with prestressed and gradually anchored CFRP strips, carried out at EMPA - Dübendorf in cooperation with the TUL were the continuation of the previous static test on the similar RC girders (Kotynia et al. 2008) The aim of the tests was to investigate the

Advances and challenges in FRP strengthening of concrete structures - research at Technical University of Lodz

R. Kotynia Department of Concrete Structures, Technical University of Lodz, Lodz, Poland

ABSTRACT: The paper presents state-of-the-art in the field of fiber reinforced polymer (FRP)strengthening concrete structures based on the research carried out at the Technical University of Lodz (TUL) over a last decade. The paper points out the main achievements found in the FRP strengthening for flexure, shear and confinement using both externally bonded (EB) and near surface mounted (NSM) techniques. Challenge of the FRP strengthening of concrete structures is supposed the combined NSM technique with FRP prestressing that sufficiently may improve serviceability and capacity of strengthened structures.

- 2 -

fatigue durability of large scale RC slabs strengthened with prestressed and gradually anchored CFRP strips, under cyclic loading and at elevated temperature.

Generally, RC elements which need to be strengthened in flexure, should be strengthened in shear as well. Hence, simultaneously with the tests on FRP strengthened beams in flexure car-ried out at TUL, the research project on externally strengthened beams in shear started. Several configurations of the shear FRP reinforcement such as straight strips bonded to the sides, L-shaped strips bonded both to the sides and the tension face of the beam, U-jackets and full wrapping of the whole cross-section were investigated. The test results indicated that EB shear strengthening can not mobilize the full strength of the FRP materials due to their brittle and premature debonding. Anchorage of the FRP in concrete may sufficiently increase the strength-ening effect (Kamińska et al. 2003 )

One of the efficient FRP applications analyzed at TUL was the external confinement of con-crete columns. Many experimental tests in the world have been carried out to investigate this phenomenon. The main object of the research project performed at TUL was to investigate the external longitudinal FRP composite reinforcement ratio, concrete cross-section shape and load eccentricity influence on the strengthening effect The test results indicated that the confinement of the concrete core using FRP wraps effectively limits the transverse strains of the concrete, hence the concrete in some cases is able to carry several times higher load than the unconfined specimen.

Near surface mounted (NSM) technique has become promising and attractive for flexural and shear strengthening of RC structures. Due to a better anchorage of NSM FRP reinforcement embedded into pre-cut slits opened in the concrete cover of the RC element, this technique has been significantly more efficient than EBR system. The experimental tests carried out by the au-thor on RC beams NSM strengthened in flexure confirmed substantial increase in the stiffness, and strength of the concrete beams, and increase in the FRP strain efficiency from 54% to 80% of the ultimate strain. Shear tests indicated that use of NSM CFRP strips effectively enhanced the shear capacity of RC beams, depending on the FRP spacing.

An advancement in NSM strengthening refers to the bond behavior investigated in the pull-out beam test in order to understand the complex bond phenomenon that characterizes FRP debonding failure. The bond behavior between NSM FRP and concrete is under ongoing test (Research Grant nr 0322 T02 2006 31) carried out by the author at TUL and supported the Pol-ish Ministry of Scientific Research and Information Technology.

The present work summarises the advances in the experimental tests carried out on RC members strengthened using EB and NSM FRP techniques at TUL during last decade. Based on the FRP experience, the author made an attempt to specify the challenge of the FRP in civil en-gineering infrastructure. The strengthening RC structures with the combined NSM and prestressed FRP materials is supposed to be the most efficient technique in terms of an im-provement of serviceability and the load capacity of strengthened structures and an increase in the strength utilization of FRP materials.

2 EXTERNALLY BONDED FRP 2.1 Passive FRP strengthening in flexure

Numerous tests carried out on RC beams and slabs externally FRP strengthened in flexure in-dicated that the full FRP strength can not be utilized due to premature brittle FRP debonding from the concrete surface. Two main types of FRP debonding were observed in the preliminary author’s test (Research Grant nr 7 T07E 030), classified on: THE end plate debonding and the intermediate crack debonding - referred to as mid-span debonding (Kamińska & Kotynia 2000). First type of failure appeared at the end of the anchorage FRP length and it refers to the plate end debonding together with a concrete cover (Figure 1a). Extension of the FRP strip across the entire shear span to the support, providing transverse reinforcement and mechanical anchorage can mitigate the FRP end debonding. The intermediate crack debonding is characterized as flex-ural or flexural-shear debonding (Figure 1b). It initiates in the high bending moment region and then develops towards one of the strip ends. The maximum strain of the bottom FRP recorded at the FRP midspan debonding failure was

- 3 -

about 6-7 ‰ (Kotynia & Kamińska, 2003), that corresponds with the FRP strain utilization ratio about 35%, depending on the cause of delamination. In order to delay midspan debonding and increase efficiency of strengthening, following the application of the bottom strips, additional longitudinal U-jacket wet lay-up CFRP sheets were applied over the middle of the beams (Fig-ure 1c). The additional longitudinal U-jacket sheets increased the load carrying capacity of 20% and the FRP strength utilization fe fuε ε to 0.56, where feε and fuε are the maximum FRP strain registered in the test and the ultimate FRP strain. Moreover the CFRP flexible sheets made the flexural behavior more ductile than the stiff strips made.

a) b)

c) d) Figure 1. Failure modes FRP strengthened beams a) end plate debonding; b) location of the critical sec-tion (CS); c) midspan debonding; d) midspan debonding in the beam with the bottom strip and longitudi-nal U-jacket wet lay-up sheets.

2.2 Strengthening in flexure with prestressed FRP To take the best advantage of the EB strengthening, to improve serviceability of strengthened

structures, to effectively reduce crack widths, to relieve stress in the internal reinforcement, that may control the crack distribution, to limit deflection, to increase the stiffness and the load ca-pacity of RC members, the FRP prestressing has been proposed (Meier 1995). Experimental tests on RC specimens strengthened with active composites indicated that pre-stressing level should be at least 25% of the FRP ultimate strength. For the prestressing level above 70% of the FRP ultimate strength, failure due to fracture of the composite was observed. However, for the prestressing level below 60% of the FRP ultimate strength the strip’s debonding caused the fail-ure. To overcome anchorage problems with shear stress at the ends of the strips, the prestressing force can be reduced gradually towards both ends of the strip eliminating the risk of shearing-off in the concrete. This new method was developed and successfully tested in the Swiss Federal Laboratories for Materials Research (EMPA) in Dübendorf-Zurich (Meier 2007). To adapt this technique for strengthening real bridge structures, the fatigue and temperature influence on the bond behaviour of the active system had to be explored. Fatigue tests of RC slabs strengthened with prestressed and gradually anchored CFRP strips, carried out at EMPA - Dübendorf in co-operation with the TUL, were the continuation of the previous static test on the similar RC gird-ers (Kotynia et al. 2008a). The aim of the tests was to investigate the fatigue durability of the large scale RC slabs strengthened with prestressed and gradually anchored CFRP strips, under cyclic loading and the elevated temperature. Two strengthened slabs were first preloaded mono-tonically to the maximum cyclic load of σmax = 0.85 fys and σmax = 0.26 fys, respectively.

- 4 -

After the static preloading the fatigue loading was applied at constant amplitude for each slab. An elevated temperature up of 116°C, was additional investigated parameter in one slab. Fa-tigue failure of several longitudinal tensile steel rebars followed by the local strip debonding ini-tiated the collapse of the slab (Figure 2a). After the rupture of steel rebars, the width of the flex-ural cracks suddenly increased, that led the local strip debonding and finally fracture of the strips (Figure 2b). This failure mechanism occurred in a very brittle manner in the slab with the maximum steel stress, corresponding to the upper cyclic load level of 85 % of the yield stress, after 331 000 cycles. The failure of the slab with the upper cyclic load level of 26% of the yield stress occurred after 12 000 000 cycles. The fatigue test of RC slabs strengthened with prestressed and gradually anchored strips indicated that the CFRP temperature up of 75ºC did not influence on bond CFRP behaviour between CFRP strips and the concrete. The temperature above 100ºC (close to the glass transition temperature), accelerated the gradual bond loss be-tween CFRP strips and concrete, that led the local CFRP strips debond, moving from the middle of the slab towards one of the supports.

a) b) Figure 2. Failure of the slab strengthened with active strips a) steel rebars rupture; b) FRP fracture.

2.3 Strengthening in shear Insufficient internal shear internal steel reinforcement, increased service load or construction

errors are the main reasons of the shear FRP strengthening necessity. Several tests carried out on RC beams externally bonded with FRP indicated significant increase in the shear resistant of RC beams. Brittle shear failure of RC beams is one of the most disastrous and unpredictable failure modes. The cited papers emphasize the shear failure modes and the FRP efficiency in RC beams strengthened with externally bonded CFRP composites observed in the tests directed by the au-thor at TUL (Research Grant nr 8 T07E 006 21) (Kamińska et al. 2003, Waśniewski & Kamiń-ska 2004). The main aim of the reported shear tests was to evaluate the effect of several parame-ters on the shear performance of the strengthened RC beams, such as: the EB CFRP ratio, the internal steel reinforcement ratio, the shear span to depth ratio, the cross section of the beams (rectangular and T-cross section), the anchorage of both ends of the CFRP strips in negative and positive moment regions (in double and single span beams, respectively). The preliminary test contained T-section RC, double span beams, CFRP strengthened in the middle support region with transverse U-jacked sheets and L-shaped strips, due to insufficient internal steel shear rein-forcement. The CFRP materials were not anchored in the flange. Extreme supports had suffi-cient shear steel reinforcement ratio. Sudden debonding of the external CFRP reinforcement was the reason of the beams’ failure occurred after diagonal cracking. The strip separation was the result of the low anchorage length of the strip intersected by the major shear crack (Figure 3a, 3b).

In order to delay the shear failure and improve the shear strengthening effect, the additional anchorage of the lateral CFRP strips was applied in the following beams’ series. The aim of the rectangular, double span RC beams test was to investigate the influence of the CFRP strips an-chorage on the shear capacity and the mode of failure. Tested beams were strengthened in shear with transverse L-shaped strips and straight strips applied at an angle 90 and 60 degrees. As an investigated parameter was the anchorage of the lateral strips, hence several strips were an-

- 5 -

chored with additional straps of the CFRP sheets, bonded on the tension and compression strip’s ends in different configurations. The test results confirmed considerable influence of the strips’ anchorage on a delay of the shear failure and on the increase in the shear capacity. The shear failure was due to the debonding of the lateral strips or anchorage from the concrete surface, af-ter internal steel stirrups yielding (Figure 3c, 3d).

a) b) c) d) Figure 3. Failure modes of failure a) T-section beams strengthened with U-jacked sheets; b) T-section beams strengthened with U-jacked L-shaped strips; c) rectangular beams strengthened with diagonal lat-eral strips; d) rectangular beams strengthened with vertical lateral anchored strips. The test results indicated low efficiency of the FRP shear strengthening without anchorage. An-choring of the lateral stirrups at both ends increases the shear capacity of the beam, up to 81% over the reference beam. The diagonal application of the CFRP reinforcement was more effec-tive than the vertical. The following test of T-section beams strengthened with sheets and L-shaped strips anchored in the beam’s flange has been going on by the author at TUL. 2.4 FRP confinement

Strengthening effects of FRP confined rectangular, square and circular specimens were ana-lyzed in the test carried out at TUL in terms of several aspects, such as the influence of: the ex-ternal longitudinal composite reinforcement ratio, the cross section shape, the initial preloading prior FRP application, the load eccentricity on strengthening effect (Kamińska & Ignatowski 2003). The tested concrete specimens contained: cylinders with 150 mm diameter and 300 mm height, specimens with square cross section of 100 mm and 200 mm height and specimens with rectangular cross section of 105 x 200 mm and 200 mm height. All strengthened specimens failed due to a sudden rupture of the CFRP transverse sheets (Figure 4). The test results con-firmed, that the use of CFRP confinement is an effective method of concrete strengthening. The reason of the increase in the concrete strength is the limit of the transverse concrete strain, that causes advantageous triaxial compression state in the concrete core confined with CFRP. The strengthening effect depends on the transverse composite reinforcement ratio and on the shape of the concrete cross section. The highest increase in the strength occurred for cylinders, less for the specimens with square cross section and the lowest for the specimens with rectangular cross section.

- 6 -

Figure 4. Failure modes of CFRP confined specimens. In case of the eccentrically loaded specimens with the rectangular cross section, the strength-

ening effect of the transverse composites is clearer than for the axially loaded specimens. The CFRP application under loading was not significant for the strength and strain increase. The longitudinal composite reinforcement was effective only with simultaneous application of the transverse CFRP reinforcement.

3 NEAR SURFACE MOUNTED FRP STRENGTHENING 3.1 NSM strengthening in flexure

Installation of the NSM CFRP laminates began by making one, two or three parallel slits cut into the concrete cover in the longitudinal direction at the tension side of the element with a diamond blade and then bonding FRP reinforcement into the slits. Recent studies examined flexural behavior of RC beams strengthened with near surface FRP reinforcement have intro-duced several parameters influenced on load carrying capacity of strengthened elements, such as: geometry of the concrete structure, steel and FRP ratio, concrete strength, static scheme, spacing between adjacent FRP laminates and the minimum distance from the CFRP laminates to the concrete edge. Several experimental tests indicated benefits of NSM technique such as: in-crease in the load carrying capacity of deficient reinforced concrete members, easy to apply and cost effective. Advantages of using the near-surface mounted FRP reinforcement with respect to externally bonded FRP laminates are the possibility of anchoring the reinforcement into adja-cent RC members and minimal installation time. Both round and rectangular unidirectional FRP bars made in pultrusion processes are common used for NSM strengthening. Laboratory tests performed by the author in the TUL contained three beam series NSM strengthened with rec-tangular CFRP strips. In general, the beams failed due to the intermediate crack debonding of the NSM CFRP laminates with the detached concrete cover below the tensile steel (Kotynia 2006). Full composite action between CFRP laminates and concrete was observed. (Figure 5). The NSM laminates detached from the beam with the concrete cover attached to the laminates on the whole depth of the slits. Strengthening of RC beams with the NSM CFRP laminates sub-stantially increased both the stiffness and strength of the concrete beams.

Figure 5. Failure modes of NSM CFRP strengthened beams in flexure.

- 7 -

The author’s test results indicated significant influence of the steel and CFRP reinforcement ratio as well as their elasticity modulus on the ultimate loads and the CFRP strain limitation that is shown on Figure 6. The increase in the CFRP stiffness makes the increase in the ultimate loads but it causes the decrease in the CFRP debonding strain. A strain utilization of the NSM CFRP strips ranged from 54% to 80% of the ultimate strain (Kotynia 2008b).

Figure 6. Effect of steel and CFRP stiffness on the limit strain and the strengthening ratio.

The tests confirmed that the higher is NSM CFRP ratio the lower are limit strain during its

debonding and the CFRP strain utilization. The increase in the CFRP stiffness makes the in-crease in the ultimate loads but unfortunately it makes the decrease in the CFRP limit strain. 3.2 NSM strengthening in shear

Advantages of the NSM FRP technique strengthening confirmed in flexure induced the au-thor to apply it in shear strengthening. The main aim of the test was to analyze the shear behav-ior, failure mode of the strengthened beams and the strain distribution in the NSM FRP strips. The influence of the type of NSM strips and their spacing were considered (Kotynia 2007b). The research program consisted of five RC T-section beams, simply supported and strengthened in support regions with NSM CFRP 45 degrees laminates in different spacing. The beams strengthened with strips in spacing 360 mm and 210mm failed in shear due to debonding of the NSM strips and splitting of the concrete cover of the longitudinal reinforcement. The strip sepa-ration occurred as a result of the low anchorage length of the strip intersected by the major shear crack (Figure 7). This mechanism was prevented by decreasing of the strip’s spacing, which provided a larger bond length. Test results indicated effective enhanced the shear capacity of RC beams of NSM strengthened beams. The beams with low FRP percentage below 0.32% failed in shear due to debonding of the NSM strips and splitting of the side concrete cover of the internal steel stirrups. The FRP percentage above 0.33%, affected the failure due to flexure and an increase in the capacity of 73% in comparison with the unstrengthened beam. Cutting of all steel stirrups during FRP strips installation did not influence the failure mode and the ultimate load due to improved anchorage condition of the NSM strips.

Figure 7. Failure of the shear NSM CFRP strengthened beams.

5

7

9

11

13

15

0,0 0,1 0,2 0,3 0,4 0,5ρfEf/ρsEs (-)

Lim

it st

rain

εt (

‰)

A1

C3

C2

C1B2

B1

A2

1

2

3

4

5

0,0 0,1 0,2 0,3 0,4 0,5ρfEf/ρsEs (-)

Stre

ngth

enin

g ra

tio Fu

/Fuo

(-)

A1C3

C2

C

B2

B1

A2

BI-2/3A BI-2/3B

- 8 -

4 CHALLENGES IN FRP STRENGTHENING

The available in the literature on the experimental research on RC elements NSM strength-ened in flexure shows that the enhancement in the ultimate state is much higher than that in the serviceability limit state and due to FRP debonding this technique can not be effectively ex-ploited. Based on the previous EB FRP prestressing experience, the author intends to introduce FRP prestressing into NSM technique in flexure and shear. Certainly the NSM FRP prestressing is the most effective technique of strengthening that improves serviceability of strengthened structures, effectively reduces crack widths, relieves stress in the internal reinforcement, in-creases the stiffness and the load capacity of RC members. The future author’s research at TUL will focus on NSM prestressing systems for flexure and shear in order to invent the most effec-tive strengthening technique in aspect of the capacity, serviceability of the RC elements and a cost effect. The experimental work on NSM prestressing should develop strengthening practice and elaborate design guidelines.

5 ACKNOWLEDGEMENTS

The following persons made within the last decade direct or vicariously highly appreciated con-tributions at author’s research at TUL: M. E. Kamińska, A. Czkwianianc, A. M. Brandt, W. Ra-domski, U. Meier, M. Motavalli, K. W. Neale, S. H. Rizkalla, R. El-Hacha, M. Ehsani, K. Har-ries, J. A. O. Barros, P. Ignatowski, Ł. Sowa, T. Waśniewski, R. Walendziak, technical staff of the laboratory of the Department of Concrete Structures in the technical University of Lodz. The author wises to acknowledge the support provided by the Sika-Poland, Research Grant nr 7 T07E 030), supported by the Polish Research Scientific Committee, Research Grant nr 8 T07E 006 21 and Research Grant nr 0322 T02 2006 31, supported by the Polish Ministry of Scientific Research and Information Technology, University of Lodz.

6 REFERENCES

Ignatowski, P., Kamińska, M. E. 2003. About concrete confinement of the slender RC columns with CFRP composites, 73-97, Journal for Restoration of Buildings and Monuments.

Kamińska M.E., Kotynia R. 2000. Experimental Research on RC beams strengthened with CFRP strips, Report No. 9, ISSN 1230-6010. Dept. of Concrete Structures, Tech. Univ. of Lodz, Poland.

Kotynia, R. 1999. Ductility and Load Capacity of Reinforced Concrete Members Strengthened with CFRP Laminates, Ph.D. Dissertation, Technical University of Lodz, Poland.

Kotynia R., Kamińska M.E. 2003. Ductility and failure mode of RC beams strengthened for flexure with CFRP, Report No. 13, ISSN 1230-6010. Dept. of Concrete Structures, Tech. Univ. of Lodz, Poland.

Kotynia, R. 2006. Analysis of reinforced concrete beams strengthened with near surface mounted FRP re-inforcement. Archives of Civil Engineering, LII 2, 2006, 305-317.

Kotynia R. 2007. Shear strengthening of RC beams with NSM CFRP laminates, Proc. of 8th FRPRCS-8, ID 8-14, Patras, Greece.

Kotynia R., Baky H. A., Neale K. W., Ebead U. A. 2008. Flexural Strengthening of RC Beams with Ex-ternally Bonded CFRP Systems: Test Results And 3-D Nonlinear FE Analysis, Journal of Composites for Construction, CC/2006/022793 (accepted for publication).

Meier U. 1995. Strengthening of structures using carbon fibre/epoxy composites, Construction and Build-ing Materials, V. 9, No. 6.

Meier, U. 2007. Is there a future for automated application of FRP strips for post-strengthening. Proc. of FRPRCS-8 conference, ID K-1, Patras, Greece.

Siwowski, T., Radomski W. 1998. Pierwsze krajowe zastosowanie taśm kompozytowych do wzmacnia-nia mostu, 382-388, Inżynieria i Budownictwo, Nr7, (in Plish).

Waśniewski T., Kamińska M.E. 2004. Shear capacity of RC beams strengthened with CFRP composites, Proc. of 5th PhD Symposium in Civil Engineering, 1153-1161, Delft, The Netherlands.