Influence of aggressive exposure conditions on the behaviour of adhesive bonded concrete–GFRP joints

Construction and Building Materials 12 Ž1998. 427]446

Influence of aggressive exposure conditions on the behaviour of adhesive bonded concrete]GFRP joints P. MukhopadhyayaU , R.N. Swamy, C.J. Lynsdale Department of Mechanical Engineering, Centre for Cement and Concrete, Structural Integrity Research Institute, Uni¨ ersity of Sheffield, Mappin Street, Sheffield S1 3JD, UK Received 29 January 1998; received in revised form 24 July 1998; accepted 19 September 1998

Abstract This paper presents the interim results of an on-going study on the influence of aggressive exposure conditions on the behaviour of epoxy adhesive bonded concrete]glass fibre reinforced polymers ŽGFRP. joints. The type of specimen used in this study is a push-off double lap shear specimen. Twenty-four of these push-off specimens consisting of concrete prisms, 100 = 100 = 300-mm, bonded with 470-mm long, 90-mm wide, and 3.5-mm thick GFRP plates on two opposite faces were tested. The bond length of the plate over the concrete surface was 200 mm. Two different concrete strengths were used, and they were suitably air entrained. The specimens were subjected to three accelerated ageing regimes in the laboratory for approximately 9 months. The accelerated tests consisted of exposing the specimens to alternate wet]dry cycling in 5% sodium chloride solution, cyclic freeze]thaw in air with a temperature of 208C and y17.88C, and a combination of chloride immersion and freeze]thaw cycles. The specimens were comprehensively instrumented, and tested to failure after the exposure regime. The structural performance of the exposed specimens is then compared with that of similar control specimens kept in laboratory environment in terms of load carrying capacity, longitudinal force distribution, shear stress development in the plate, plate end slip, and differential movements between the plate and the concrete substrate. There was clear indication that all the exposure regimes increased the bond transfer length, the magnitude of the shear stress distribution and the plate slip. The combined chloride immersionrfreeze]thaw cycles produced the largest differential movements between the plate and the concrete substrate. The duration of exposure, however, was not long enough to affect the strength of the joints. Overall, the results were very consistent, and showed that accelerated tests could inflict deterioration in the adhesive bonded concrete]GFRP joints. Q 1998 Elsevier Science Ltd. All rights reserved. Keywords: Durability; Concrete]GFRP joints; Plate-bonding

1. Introduction The use of non-metallic advanced composite materials, in the form of fibre reinforced polymers ŽFRP., is now receiving widespread attention of the construction industry, particularly for applications in plate bonding technology to upgrade andror rehabilitate reinforced

U

Corresponding author. Tel.: q44 114 2227712; fax: q44 114 2227890; e-mail: [email protected]

concrete ŽRC. structural elements w1]7x. Most of the published data-to-date, however, relate primarily to the short-term structural behaviour of FRP plate-bonded beams, and there is little information on their long-term performance and durability characteristics. The overall aim of this paper is to report the initial set of data of an ongoing investigation at the University of Sheffield on the durability behaviour of concrete]FRP composites. The rate of natural deterioration of any material can be relatively slow or rapid, but it is generally a steady

0950-0618r98r$ - see front matter Q 1998 Elsevier Science Ltd. All rights reserved. PII S0950-0618Ž98.00030-0

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P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446

process. Experience and intimate knowledge of materials provide us with a good and reliable account of this deterioration process for conventional materials used in normal situations. However, new materials like FRPs do not have that time-tested durability history. Nevertheless, FRPs are generally considered to be highly durable when exposed to natural environment. They are also considered to have relatively high durability when compared to that of metals used in construction. However, the processes of degradation in metals are very different from those in FRPs. In metals, the deterioration processes are mainly electrochemical in nature, whereas in FRPs these are largely physiochemical, involving both physical as well as chemical changes. It is well known that many polymeric materials, like FRPs, are prone to moisture absorption, and the resulting swelling and dissolution can have serious implications on their mechanical properties w8x. However, when FRPs are used in conjunction with concrete to form a plate bonded composite beam, then the long-term serviceability or integrity of the plate]adhesive]concrete member does not solely depend on the plate material but also equally, if not more importantly, on the properties of the interfaces involved in the joint, namely, the plate]adhesive and adhesive]concrete interfaces. Knowledge about this interface durability in a FRP plate-bonded concrete member is currently very limited. Whilst there is considerable appreciation of the technical and economic advantages of using FRP materials for strengthening and upgrading concrete structures, there are still serious concerns about the long-term durability of such materials as shown by the decision not to use carbon fibre reinforced polymers ŽCFRP. to strengthen the A34 trunk road w9x. There is thus an urgent need to establish the durability properties of concrete]FRP adhesive joints. This paper presents the results of accelerated tests on the durability of glass fibre reinforced polymers ŽGFRP. plate-bonded concrete elements. Accelerated ageing tests are fast and repeatable, and although the exact relationship between the real time-tested performance and that obtained from accelerated testing cannot be easily established, they, nevertheless, provide a reliable guide to the performance characteristics of the materials involved. In the study reported here, two basic test regimes, namely, ‘wet]dry in salt water’ and ‘freeze]thaw in air’ were adopted. A third regime, based on a combination of the two, has also been investigated. The plate-bonded specimens were designed to produce shear failure of the bonded joint when loaded to destruction. The results of tests on CFRP]concrete bonded joints will be reported in due course.

2. Background research At the time of initiation of this research programme, the results of tests on the durability of the steel platebonded concrete elements were available from a number of studies. These studies included both full scale specimens tested in real time frame as well as small scale specimens under accelerated ageing conditions Žshort-term.. The researches on the full scale steel plated specimens, exposed to harsh natural environment, were conducted for 10 and 11 years, respectively at the Transport and Road Research Laboratory, UK w10]14x and the University of Sheffield, UK w15,16x. In both researches, random corrosion on the steel plate in various shapes and forms was found at the plate] adhesive interface. Investigations on the corrosion risk using small scale steel plated specimens under accelerated ageing conditions were also done at the University of Sheffield w15,17,18x. The specimens were of two types: Ž1. reinforced concrete plated beams; and Ž2. reinforced concrete push-off specimens. The accelerated ageing conditions were simulated by rapid freeze]thaw and intermittent salt spray cycles. The test variables included different numbers of exposure cycles, type of adhesive and preparation of plate using different primers or grit blasting. This laboratory simulated ageing on the small scale specimens also revealed the real risk of initiation of interface corrosion as observed visually on the plate]adhesive interfaces in real beams. This similarity of the plate corrosion pattern developed from accelerated and real ageing tests clearly established the reliability and validity of the laboratory simulated ageing tests to assess the corrosion risks involved in the plate-bonded joint. There is only limited information available to date on the durability of FRP composites bonded to concrete elements, and subjected to accelerated ageing tests. At the University of Delaware ŽUSA., flexural beam specimens Ž38.1= 28.6= 330-mm., with one internal steel rebar and epoxy bonded E-glass, graphite, and aramid fabrics, were tested after exposing them to 50 and 100 wet]dry cycles, and 50 and 100 freeze]thaw cycles while immersed in 4% calcium chloride ŽCaCl 2 . solution w19x. In general, wet]dry cycling was found to inflict more degradation on the FRP bonded specimens. Debonding of the graphite and aramid fabrics, was also observed during load testing of specimens exposed to wet]dry or freeze]thaw cycles. The performance of reinforced and plain concrete cylinders Ž150 = 300-mm. wrapped with CFRP sheet and RC beam specimens Ž100 = 150 = 1200-mm. bonded with GFRP or CFRP sheet and subjected to 200 and 50 freeze]thaw cycles, respectively has also been reported w20x. The results showed that in simulated cold region weather CFRP wrapping helped to retain the structural performance of concrete cylinders, unaltered in terms of

P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446

strength and ductility, and there were no significant signs of damage in structural behaviour of the RC beams bonded with CFRP or GFRP sheet. These studies on the durability of FRP bonded concrete elements are very useful, and although they highlight the overall structural response of such elements, there is still considerable lack of information on the performance of interface joints of these composites. The data presented here deal with the behaviour of pultruded GFRP plates bonded to concrete prism specimens where the structural performance of the bonded joint is critically evaluated. The selection of the size and type of the plate-bonded concrete specimen for simulated ageing tests is a matter of compromise, and individual judgement. In order to assess the performance of interfaces involved in plate bonding, two types of test specimens have generally been used w15,17,19,21,22x, namely: Ž1. scaled beams to be tested in flexure; and Ž2. double or single lap shear specimens to be tested in pure shear. The main emphasis of the research reported here is on concrete]FRP interface behaviour, and double lap shear specimens were therefore used for this study.

3. Experimental programme 3.1. Details of test specimens The geometry of the test specimens used in this research is shown in Fig. 1 w23x. The concrete prisms used were 100 = 100 = 300 mm, and the GFRP plates bonded to the prisms were 90 = 3.5= 470 mm in size.

429

The concrete prisms were cast with the major axis horizontal to the ground. The plates were bonded to the long opposite side faces of the prism which were vertical at the time of casting in order to ensure that the two bonded concrete substrates were similar in all respects. An adhesive thickness of 1.5 mm was used in the tests. To carry out the push-off test, it was necessary to drill a 40-mm diameter hole at the end of each plate for inserting a pin through at the time of the test. To ensure that during the push-off test the GFRP plate did not tear off prematurely from the weak and reduced section of the drilled hole, a rectangular Ž150 = 90 = 2-mm. piece of mild steel plate was bonded on each side of the GFRP plate, as shown in Fig. 1. 3.2. Details of materials To ensure that the damage during the test regimes occurred at the interfaces, the concrete in the prisms was air-entrained with approximately 7% air entrainment. Two mix proportions were used } mix A with 1.0 Žcement.: 2.05 Žsand.: 3.10 Žgravel. and a water]cement ratio of 0.57, and mix B with 1.0 Žcement.: 0.48 Žsand.: 2.16 Žgravel. with a water]cement ratio of 0.32. Mix A was proportioned to give a 28-day cube strength of approximately 35 MPa, whilst mix B was designed for a 28-day cube strength of approximately 50 MPa. Normal portland cement ASTM Type 1 was used throughout. The sand used was washed river sand, and the coarse aggregate was 10-mm nominal size natural uncrushed gravel. All the aggregates satisfied the grading requirements specified in BS 882: 1992. Concrete cubes cast from each mix were also exposed to each

Fig. 1. Specimen details.

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test environment along with the plate-bonded test specimens in order to assess the exposure effect on the concrete. The strength development of the two concretes in the different environments used in the tests is shown in Table 1. The GFRP plate used in the tests was produced by the pultrusion process, and contained both random continuous filament mat and unidirectional rovings. The stress]strain behaviour of the plate in tension was linear. The fibre build up in the plate material was such that this resulted in different strength and elastic modulus properties in the longitudinal and transverse directions as shown in Table 2. The plates for bonding were therefore always cut in the direction of higher strength, namely, the longitudinal direction. A two-part Žresin and hardener. thixotropic epoxy adhesive was selected after carrying out various initial tests. The resin adhesive had a tensile strength of 24 MPa, flexural strength of 55 MPa and a bulk shear strength of 21 MPa. 3.3. Bonding of plates The substrates for bonding were carefully prepared to ensure high quality interface joints. The concrete surface was roughened using a mechanical grinder to remove all surface laitance and expose the coarse aggregates. Dust from the concrete surface was then removed using a high power vacuum cleaner. The GFRP plate was dried in air after cleaning them under running water. Plate surface was then roughened with fine emery paper taking care not to damage the fibre or their orientation at the plate surface level. The surface

was then made free from dust or free particles using a vacuum cleaner. To keep 1.5-mm constant adhesive thickness, 1.5-mm thick aluminium spacers were stuck on the concrete surface prior to bonding. The resin and hardener were mixed using a slow speed electric drill until the mix showed a uniform grey colour. Once mixed, the adhesive was applied onto the concrete and plate surfaces at the same time, and they were held together in position by dead weights until the adhesive cured. 3.4. Exposure en¨ ironment

The accelerated testing environments chosen for this investigation were based on two conditions, namely: 1. that they represent the basic characteristics of the actual environment to which the plate bonded structure will be exposed; and 2. that the exposure environment would only speed up the deterioration process without altering its pattern. Achievement of these two conditions was intended to help to avoid any spurious deterioration mechanism to occur. One of the worst exposure conditions generally met with is the extreme temperature variations in the form of freezing]thawing cycles, and splashing of salt water continuously or intermittently either from marine environment, or de-icing salts. The three test regimes adopted for this study, therefore consisted of: 1. alternate wet]dry cycles in 5% NaCl solution; 2. cyclic freeze]thaw; and 3. combination of wet]dry cycles in salt water and freeze]thaw cycles in air Ždual exposure..

Table 1 Development of concrete strength Žcubes. Concrete strength ŽMPa.

28 days Žair cured. Control Žair cured.a Freeze]thaw in aira Wet and dry in 5% NaCl solutiona Dual exposurea

Mix A

Mix B

37.1 41.0 39.5 55.0 54.5

48.6 50.3 46.5 69.7 64.6

a

Tested at the end of exposure Ži.e. at an age of approximately 1 year..

In addition, one group of specimens was kept in the laboratory as control specimens. Details of specimens tested under various exposure regimes are shown in Table 3. In the alternate wet]dry exposure, each cycle consisted of complete immersion of the specimens in 5% NaCl solution for 1 week followed by drying in air for another week. A total of 18 such cycles were repeated

Table 2 Properties of GFRP plate Plate thickness Žmm.

Direction

Ult. strain Ž m s.

Ult. stress ŽMPa.

E-modulus ŽGPa.

Fibre content

3.5 3.5

Longitudinal Transverse

14 325 13 500

328 70

22.9 5.2

41.4% by volume

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431

Table 3 Specimens and test programme Exposure regime

Duration

Wet]dry Freeze]thaw Dual

18 cycles Žone cycle s 1 week wet and 1 week dry. 450 cycles Žtwo cyclesrday. 18 cycles Žone cycle s 1 week immersion and 14 freeze]thaw cycles. 9 months

Control

in the ambient temperature Ž18 " 38C. of the laboratory. The amount of salt in the solution was decided from the stipulated guidelines provided by ASTM B 117-85 for salt spray Žfog. testing. The pH of the fresh solution was found typically between 7.5 and 8.0. The quantity of the salt water was enough to submerge the plate-bonded joint completely in it. The freezing and thawing cycles were conducted in a temperature controlled environment, inside an insulated cabinet of size 675 = 630 = 660 mm. The cabinet had provision to program the time with a 24-h time switch. The temperature and its duration are the two most critical criteria of a freeze]thaw cycle, and the freeze]thaw cycle adopted for the tests was based on current specifications w24]26x. A typical cycle of temperature variation with time used in the tests is shown in Fig. 2, i.e. two cycles a day with maximum and minimum temperatures of 208C and y17.88C, respectively. The specimens were exposed to a total 450 of such cycles. The volume of concrete specimens in the cabinet was kept constant throughout in order to have a constant heating and cooling rate. In dual exposure, the specimens were immersed in the salt solution for 1 week, and then placed immediately inside the freeze]thaw cabinet for 1 week where the previous freezing and thawing cycles were repeated. The experiments continued for a total 36 weeks, i.e. total 18-weeks immersion in salt water and 252 cycles of freezing and thawing.

Fig. 2. Temperature variation in a freeze]thaw cycle.

No. of specimens Mix A

Mix B

3 3 3

3 3 3

3

3

Fig. 3. Purpose built test rig.

3.5. Measurements and instrumentation The performance of the test specimens during the different stages of exposure was continuously monitored through measurements of strains in the concrete and the GFRP plates. These strains were measured by means of a Demec mechanical extensometer over a gauge length of 100 mm as shown in Fig. 1. The strains on the plate and concrete were measured in all specimens just before the start of the exposure test Ži.e. day 0., and then at the beginning of the 6th, 12th and 19th day of exposure and at the end. The Demec readings on the specimens, subjected to freeze]thaw cycles, were always taken at the end of the thawing cycle, when temperature stabilized at 208C. In all other cases, the readings were taken at ambient temperature. The specimens were also extensively instrumented prior to load testing ŽFig. 1.. Ten 120-V electrical

432

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resistance strain gauges of 6-mm gauge length were bonded at specified intervals along the bonded plate length to record the strain on the plate. Four linear variable displacement transducers ŽLVDT. with 0.001mm sensitivity were fixed at the top diagonal corners and at the bottom ends of the bonded plate joints to record the displacement slip of the plate relative to the concrete prism. To protect the LVDTs fixed at the top from being damaged by the impact of failure, they had, however, to be removed just after the load level reached 40 kN. All the other measurements were continued up to failure. All instrumentation was simultaneously scanned using a computerised data logging program compatible with IBM AT microcomputer at an interval of 2.1 s. 3.6. Testing apparatus and procedure

A purpose-built testing rig as shown in Fig. 3, was designed, manufactured and assembled in the laboratory, specifically for this test, which was then fitted into a 1000-kN Avery Denison Universal Testing Machine ŽUTM.. The pushing off static force on the concrete prism was applied by means of a roller bearing, 75 mm wide. The test was carried out in the load control mode, and the static load was applied monotonically

until failure. Initially, the load increment was kept at a rate of 2.5 kNrmin, but it was halved after the initiation of cracking at the bonded joint. The initiation of crack was identified from the change in gradient of the load]plate strain curve which was monitored continuously during the progress of the test using the data logging computer screen.

4. Test results and discussion The results directly available from the load tests and the recorded readings were: Ž1. load carried by the bonded plates; Ž2. the plate strains at various locations along the direction of the bonded length; and Ž3. the displacement slips of the plate at the start Žtop. and end Žbottom. of the bonded joint. These test data have been analysed and are discussed below. 4.1. Force transfer between plate and concrete The variation of the longitudinal plate force along the bonded plate length was evaluated from the recorded plate force and the measured strains along the plate. These forces were calculated at every 5-kN load interval, and near ultimate failure for each test

Fig. 4. Typical longitudinal plate force distributions for mix A: Ža. control; Žb. wet]dry; Žc. freeze]thaw; and Žd. dual.

P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446

specimen. Typical distributions of this plate force for the control specimens, and those subjected to the various exposure cycles are shown in Fig. 4 for mix A and in Fig. 5 for mix B. The results obtained in Figs. 4 and 5 show that irrespective of the exposure condition and the strength of the concrete mix, the load transfer from the plate to the concrete at low loads is fairly linear, and occurs at a uniform rate, and that it is confined to between 50 and 100 mm of the bonded length from the free edge A shown in Fig. 1. Thus at low loads, the bond transfer length is fairly short, and it is not adversely influenced by the exposure regime. However, with further increase in load, and particularly as failure approaches, the data shown in Figs. 4 and 5 emphasize two important changes. Firstly, the force distribution becomes much more non-uniform and non-linear, and near the free edge A, it becomes increasingly uniform and constant, implying local debonding at this location. A careful examination of all the plate force distributions of all test specimens indicated that local debonding at the edge A commenced at approximately 60% of the ultimate load. Secondly, both Figs. 4 and 5 also clearly show higher force transfer lengths at higher loads } in other words, both the total force transfer length and the local separation at the edge A increase progressively with increase in the applied loads. Here the exposure regime

433

has a distinct and strong influence on the nature of the bond transfer length. The exposure regime not only increases the length over which the force is transferred from the plate to the concrete, but it also progressively increases the process of debonding at the free edge. 4.2. Shear stress distributions The transfer of the longitudinal force from the plate to the concrete creates shear stresses both in the epoxy adhesive, and at the concrete]adhesive and plate]adhesive interfaces. Assuming a linear variation of the longitudinal force between consecutive strain gauge locations, and with the known values of the width of the plate and the spacing between adjacent gauge positions, the ‘local’ shear stress along the bonded plate length can be computed as, t s D FrbUD L, where, D F s difference in longitudinal force between two consecutive strain gauge locations, D L s spacing between two consecutive gauge locations, and bs width of the plate. These ‘local’ shear stress distributions were evaluated for the control and exposed test specimens of both mixes at three stages, i.e. at three plate force levels of 10 kN, 20 kN and ultimate. These are best shown in the form of histograms, and typical such ‘local’ shear stress distributions are shown in Fig. 6 for the specimens of mix A, and in Fig. 7 for the specimens of mix B, respectively.

Fig. 5. Typical longitudinal plate force distributions for mix B: Ža. control; Žb. wet]dry; Žc. freeze]thaw; and Žd. dual.

434

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Fig. 6. Typical shear stress distributions for mix A: Ža. control; Žb. wet]dry; Žc. freeze]thaw; and Žd. dual.

Although the ‘local’ shear stress data shown in Figs. 6 and 7 are derived from the plate force data in Figs. 4 and 5, the former is, in many respects, more important

than the latter, since they represent the interface shear stresses, and reflect the degree, location and progression of debonding with increase in the applied loads,

Fig. 7. Typical shear stress distributions for mix B: Ža. control; Žb. wet]dry; Žc. freeze]thaw; and Žd. dual.

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and the changes in exposure conditions. The overall trend shown by the data in Figs. 6 and 7 is that at low plate load levels, the bond shear stress is maximum near the free edge, as indeed would be expected, as at these load levels, the force transfer length between the plate and concrete is relatively short, and the maximum plate force occurs at the free edge A. However, with increase in the applied loads, the transfer length increases, and local cracking and debonding at the free edge A push the location of the maximum force and shear stress away from the free edge further into the interior down the bonded length. This change in the location of the maximum plate force shown in Fig. 4a and Fig. 5a, and in the ‘local’ shear stress shown in Fig. 6a and Fig. 7a for the control specimen is further accentuated when the test specimen is exposed to aggressive environmental conditions as shown in Fig. 4b]d, Fig. 5b]d, Fig. 6b]d and Fig. 7b]d. Thus unfavourable exposure conditions do create further damage to the plate]concrete]adhesive interfaces. This progressive movement of the location of the maximum interface shear stress away from the free edge of the plate is consistent with the data obtained at Sheffield from full scale steel plate bonded beams under short-term loading w27x. Thus this confirms that the data in Figs. 4]7 are reliable, and reflect the physical changes occurring at the interfaces of platebonded specimens subjected to various exposure conditions and progressively increasing applied loads. 4.3. Interface shear stresses It is emphasized that Figs. 4]7 represent the results of tests on individual specimens. In the study reported here, three samples were tested for each exposure regime and for each concrete mix ŽTable 3., and every care was taken and effort made to ensure that all the test specimens in a particular group were as identical as practicable in their manufacture and subsequent treatment prior to exposure. In spite of this, local flaws and deficiencies in each test specimen would inevitably cause some variations in the measured strains from test specimen to test specimen, as implied in the Figs. 4]7. In order to bring some meaningful understanding of the process of interface deterioration observed in the tests reported here, and to the numerical values of shear stress data obtained from these tests, the shear stress values for each of the test specimens of each group were computed for three load levels of plate force, namely 10 kN, 20 kN and failure. The results are presented in Table 4 for specimens of concrete mix A, and in Table 5 for specimens of concrete mix B. Tables 4 and 5 present the following data: 1. the ‘average’ shear stress of the bonded joint de-

435

rived by considering the total load carried by the specimen and the total plate bonded area; 2. the ‘local’ peak shear stress as obtained from data similar to that shown in Figs. 6 and 7; and 3. the ‘stress concentration’ index defined here as the ratio of the ‘local’ peak shear stress to the ‘average’ shear stress. The results show that the mean of the ‘average’ shear stress at failure ranged from 1.19 to 1.36 MPa for mix A ŽTable 4. and from 1.40 to 1.57 for mix B ŽTable 5.. Two points emerge from this data. First, there is only some 10% increase in the ‘average’ shear stress at failure between the control specimens, and those subjected to the various exposure regimes. The higher shear stress values at failure of the exposed specimens can be explained by the higher strength of the concrete in these specimens as shown in Table 1. Secondly, the specimens of mix B registered slightly higher ‘average’ shear stresses compared to those of mix A, again because of their higher strength as shown in Table 1. The overall conclusion for these ‘average’ shear stress values is that the ‘average’ shear stress is not a good indicator of the effect of damage induced by exposure condition. On the other hand, the ‘local’ peak shear stress measured in the tests was much higher than the calculated ‘average’ shear stress. Inevitably there were variations in these peak stresses between specimens subjected to the same exposure regime for reasons explained earlier, but in general, these ‘local’ peak stresses at failure ranged from 2.50 to 6.11 MPa for mix A, and from 2.77 to 5.63 MPa for mix B. The exposure regimes showed no particular effect or influence, probably because the duration of the accelerated tests was not adequate to produce visible evidence of interface damage. However, some comparisons of the shear stress data obtained from this study with GFRP plates can be made with previous research on steel plate-bonded push-off specimens at Sheffield w23x. These comparisons show that the ‘average’ shear stress values at failure obtained in this study with GFRP plates are roughly around 50% of those obtained with mild steel plates. The ‘local’ peak shear stress values at failure were also, in general, lower than those obtained with steel plates. These differences can be attributed to the lower stiffness of the GFRP plate, being approximately one-ninth of that of steel. 4.4. Plate slips and plater concrete strains In the tests reported here the local slips were measured by LVDTs ŽFig. 1.. Typical plots of load } recorded slips of the plate at the free end A and the

436

Specimen no.

Exposure regimes Control specimen

At plate force 10 kN

Average ŽMPa. Local peak ŽMPa.a Stress concentration index At plate force 20 kN Average ŽMPa. Local peak ŽMPa.a Stress concentration index At failure Average ŽMPa. Local peak ŽMPa.a Stress concentration index Mean of average shear stresses at failure ŽMPa. a

Wet]dry specimen

Freeze]thaw specimen

Dual specimen

i

ii

iii

i

ii

iii

i

ii

iii

i

ii

iii

0.56 2.33 4.16 1.11 3.94 3.55 1.25 5.16 4.13

0.56 2.57 4.59 1.11 3.59 3.23 1.21 3.13 2.59 1.23

0.56 2.96 5.29 1.11 4.25 3.83 1.22 5.01 4.11

0.56 2.48 4.43 1.11 3.73 3.36 1.29 2.50 1.94

0.56 4.18 7.46 1.11 4.62 4.16 1.51 6.11 4.05 1.36

0.56 2.36 4.21 1.11 3.32 2.99 1.29 3.03 2.35

0.56 2.98 5.29 1.11 2.08 1.87 1.21 3.88 3.21

0.56 2.61 4.66 1.11 3.08 2.77 1.23 3.38 2.75 1.19

0.56 2.89 5.16 1.11 ] ] 1.12 4.36 3.89

0.56 1.91 3.41 1.11 4.36 3.93 1.53 4.16 2.72

0.56 2.55 4.55 1.11 ] ] 1.12 4.29 3.83 1.34

0.56 2.68 4.79 1.11 4.20 3.78 1.37 3.40 2.48

Location of local peak stress can be seen typically in Fig. 6.

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Table 4 Shear stress values for specimens with concrete mix A

Specimen no.

Exposure regimes Control specimen

At plate force 10 kN

Average ŽMPa. Local peak ŽMPa.a Stress concentration index At plate force 20 kN Average ŽMPa. Local peak ŽMPa.a Stress concentration index At failure Average ŽMPa. Local peak ŽMPa.a Stress concentration index Mean of average shear stresses at failure ŽMPa. a

Wet]dry specimen

Freeze]thaw specimen

Dual specimen

i

ii

iii

i

ii

iii

i

ii

iii

i

ii

iii

0.56 3.09 5.52 1.11 4.41 3.97 1.16 4.71 4.06 1.40

0.56 2.92 5.21 1.11 3.43 3.09 1.65 4.43 2.68

0.56 1.34 2.39 1.11 3.62 3.26 1.39 2.77 1.99

0.56 2.46 4.39 1.11 4.32 3.89 1.21 3.79 3.13 1.41

0.56 2.17 3.88 1.11 4.07 3.67 1.59 4.75 2.99

0.56 4.01 7.16 1.11 3.43 3.09 1.43 4.64 3.24

0.56 2.66 4.75 1.11 2.95 2.66 1.59 4.45 2.80 1.55

0.56 3.12 5.57 1.11 2.73 2.46 1.43 4.78 3.34

0.56 3.16 5.64 1.11 3.53 3.18 1.64 5.63 3.43

0.56 2.13 3.80 1.11 3.44 3.10 1.52 3.19 2.10 1.57

0.56 2.31 4.13 1.11 2.14 1.93 1.53 4.17 2.73

0.56 3.01 5.38 1.11 3.04 2.74 1.67 4.53 2.71

Location of local peak stress can be seen typically in Fig. 7.

P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446

Table 5 Shear stress values for specimens with concrete mix B

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438

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Fig. 8. Typical plots of plate slips for mix A: Ža. control; Žb. wet]dry; Žc. freeze]thaw; and Žd. dial.

bottom end of the joint, B ŽFig. 1. are shown in Fig. 8 for mix A and Fig. 9 for mix B, respectively. It is clear from these figures that almost all the debonding and

local slip between the plate and the concrete occurred at the free edge, and that practically no slip took place at the end of the joint even near the ultimate failure.

Fig. 9. Typical plots of plate slips for mix B: Ža. control; Žb. wet]dry; Žc. freeze]thaw; and Žd. dual.

Specimen no.

Control specimen

Wet]dry specimen

Freeze]thaw specimen

Dual specimen

i

ii

iii

i

ii

iii

i

ii

iii

i

ii

iii

Concrete Mix A

Total load 20 kN Total load 40 kN

0.073 0.187

0.087 0.204

0.124 0.189

0.120 0.223

0.061 0.177

0.100 0.196

0.087 0.175

0.085 0.168

0.069 0.116

0.104 0.228

0.086 0.110

0.109 0.179

Concrete Mix B

Total load 20 kN Total load 40 kN

0.098 0.172

0.081 0.141

0.086 0.158

0.132 0.338

0.091 0.250

0.106 0.231

0.068 0.109

0.098 0.146

0.075 0.164

0.078 0.383

0.098 0.190

0.127 0.203

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Table 6 Average plate slip at the top Žmm.

439

440

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Fig. 10. Typical plots of recorded plate and concrete strains at various stages of exposure for mix A: Ža. control; Žb. wet]dry; Žc. freeze]thaw; and Žd. dual.

Fig. 11. Typical plots of recorded plate and concrete stains at various stages of exposure for mix B: Ža. control; Žb. wet]dry; Žc. freeze]thaw; and Žd. dual.

P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446

These results give strong support and credibility to the argument that plate cut-off point is the most critical region in a plate-bonded beam, whether the plates are of steel or FRP, and that this section needs careful design considerations from both stress analysis and durability points of view w7,16,27,28x. To give a more complete picture of the amount of slip occurring in all the test specimens, and the variations between individual test specimens, the slip at the free end has been quantified for two total load levels of 20 and 40 kN, respectively, and these are summarized in Table 6. These results show that higher slips can and do occur in the specimens subjected to the exposure regimes compared to the control specimens, although for reasons explained earlier, these were not observed in all the exposed specimens. However, the generalised trend shown in Figs. 4]9 and Tables 4]6 is that the most immediate and direct adverse effect of exposure to aggressive environmental conditions is most likely to begin at the section of discontinuity between the plate and concrete substrate, and that longer accelerated ageing tests than those reported here are required to bring out the full adverse effect of the exposure regimes on adhesive bonded concrete]FRP joints. To supplement the shear stress ŽFigs. 6 and 7. and the slip ŽFigs. 8 and 9. data, typical plots of strains recorded on the plate and concrete at different ages of exposure are given in Fig. 10 for concrete mix A and in Fig. 11 for concrete mix B. These data emphasize several aspects of the influence of the exposure regime on the integrity of the test specimens. The plate and concrete strains in the control specimens at the end of the exposure period were practically the same, and very small ŽFig. 10a and Fig. 11a. implying that these specimens had undergone little dimensional changes and differential movements between the plate and concrete, and thus remained intact under ambient conditions. All the specimens exposed to aggressive regimes, on the other hand, had showed much higher plate and concrete strains implying that these specimens had undergone much higher dimensional changes than the control specimens. The specimens subjected to wet]dry cycles of both mixes A and B ŽFig. 10b and Fig. 11b, respectively. registered much higher strains of 200]400 m s Žmicrostrains., but the plate and concrete strains were almost identical. These imply that a wet]dry cyclic exposure regime naturally induces dimensional changes of the test specimens, but does not cause differential movements between the plate and concrete. The specimens exposed to freeze]thaw cycles ŽFig. 10c and Fig. 11c., and those subjected to the dual exposure regime ŽFig. 10d and Fig. 11d., on the other hand, clearly show not only dimensional changes but also differential movements between the plate and concrete. These data thus clearly emphasize that these

441

two exposure regimes were much more harmful to adhesive bonded joints than wet]dry cycles alone. The freeze]thaw cycles appeared to create greater movements in the concrete Žas indeed can be envisaged to some extent., whereas the dual immersionrfreeze]thaw cycle regime had caused greater movements in the plate than in the concrete. From a structural integrity point of view, differential movements between the plate and concrete are more critical and harmful to the interfaces, and these tests give clear warnings that these two exposure regimes are likely in the long run to damage the integrity of the adhesive bonded specimens. Obviously, 250 days’ of exposure have not been long enough to create more damage, and longer term accelerated tests are necessary to establish the damaging effects of the exposure regimes. 4.5. Failure loads The failure loads of the control and exposed specimens of both concrete mixes are summarized in Table 7. It is clear that the duration of the exposure reported here was not adequate to have any negative influence on the strength of the joints in spite of the beginnings of the adverse effects of the exposure regimes observed and reported in Figs. 4]11. Indeed, even then, some of the exposed specimens showed higher strength, partly because of the increased strength of the concrete due to ageing ŽTable 1., and partly due to the continued hydration of the concretes, and the consequent improvements in the interface bond strength. The overall conclusion of the data given in Table 7 has to be that the duration of the exposure regimes has not been long enough to affect the overall strength of these adhesive bonded joints, in spite of the adverse indications on the total bond length ŽFigs. 4 and 5., interface shear stresses ŽFigs. 6 and 7., slip ŽFigs. 8 and 9., and differential movements ŽFigs. 10 and 11.. 4.6. Failure patterns As discussed in the previous section, the failure load responses of the tested specimens did not show any adverse effect of accelerated ageing on the overall load carrying capacity of the specimens. However, without visual inspection of the failure surface, and a critical evaluation of the failure patterns, any conclusion would only be superficial and premature. The failure of an adhesive bonded plate]concrete joint subjected to an uniaxial tensile force can occur in three ways: Ža. ‘cohesive failure’ in the adhesive layer ŽFig. 12a.; Žb. ‘adhesion failure’ ŽFig. 12b.; and Žc. ‘concrete shearing failure’ ŽFig. 12c.. After load testing, the failure surfaces on the plate and concrete were carefully inspected and the results are discussed below. The failure in control specimens was largely by ‘con-

442

Specimen no.

Concrete Mix A Concrete Mix B

Control specimen

Wet]dry specimen

Freeze]thaw specimen

Dual specimen

i

ii

iii

i

ii

iii

i

ii

iii

i

ii

iii

45.06 41.77

43.59 59.35

43.84 50.20

46.39 43.62

54.50 57.22

46.33 51.36

43.51 57.19

44.29 51.40

40.31 58.89

54.98 54.69

40.42 55.19

49.14 60.22

P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446

Table 7 Ultimate load at failulre ŽkN.

P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446

crete shearing’ ŽFig. 13.. This mode of failure clearly shows that the adhesive used was capable of providing stronger adhesion bond than the shear strength of the concrete. In the case of specimens subjected to wet]dry cycles, failure was again mainly by ‘concrete shearing’, although there were few spots where clearly the failure surface passed through adhesive]concrete interface ŽFig. 14.. The failure mode was thus predominantly through ‘concrete shearing’ with some loss of adhesion between the concrete and the adhesive. The specimens subjected to freeze]thaw cycles also showed a similar failure mode to those of the wet]dry specimens ŽFig. 15.. The most visible change in failure mode was observed in the specimens exposed to the dual exposure regime ŽFig. 16.. There were large areas of ‘adhesion’ failure between the concrete and adhesive, and these spots were uniformly distributed all over the surface. It was observed that in specimens with concrete mix A Žstrength lower than mix B. there were certainly more signs of ‘adhesion’ failure than in specimens with concrete mix B. This relatively more severe

443

deterioration of the interface with concrete mix A is primarily due to the strength of the concrete. The overall conclusion from an analysis of all the failure surfaces similar to the typical ones shown in Figs. 13]16 is that of all the exposure regimes, the combined chloride immersion and freeze]thaw cycling proved to be the most aggressive, and potentially the most harmful, to the integrity of the interfaces in the bonded joint. Clearly, the duration of the exposure regime was not long enough to affect the ultimate loads of the joints, but there were clear indications that continuation of the exposure regimes will lead to loss of load capacity and failure.

5. Conclusions The major conclusions of this study are as follows: 1. The load transfer from the plate to the concrete at low loads is fairly linear and occurs at a uni-

Fig. 12. Different potential failure modes in a plate bonded joint Ža. cohesive failure through adhesive; Žb. adhesive failure; and Žc. concrete shearing failure.

444

P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446

Fig. 13. Typical failure surfaces of control specimens: Ža. Mix A; and Žb. Mix B.

2.

3.

4.

5.

6.

form rate irrespective of the exposure condition and the concrete strength. At higher load levels, the force distribution becomes much more non-uniform and non-linear, with local debonding at the free edge. The exposure regime increased the length over which the force is transferred from the plate to the concrete, and it also progressively increased the process of debonding at the free edge. As a result, with increase in the applied loads, the location of the ‘local’ peak shear stress moves away from the free edge. Exposure to aggressive environment accentuated the location of the maximum shear stress. The computed ‘average’ shear stress appears to be not a good indicator of the effect of damage induced by the exposure conditions. The plate slip was higher in the specimens subjected to the exposure regimes compared to the control specimens. There was clear evidence that the most immediate and direct adverse effect of exposure to aggressive environmental conditions occurs at the section of discontinuity between the plate and the concrete substrate. All the specimens exposed to aggressive regimes

Fig. 14. Typical failure surfaces under wet]dry cycles: Ža. Mix A; and Žb. Mix B.

7.

8.

9.

10.

showed higher dimensional changes and differential movements between the plate and concrete than the control specimens. There was clear indication that the freeze]thaw cycles and the combined chloride immersionr freeze]thaw cycle exposure regime created the largest differential movements between the plate and concrete, more than those due to wet]dry cycles alone. They are thus likely to be more critical and harmful in the long run to the integrity of the adhesive bonded joints. It was clear from the results that the duration of the exposure regime was not long enough to affect the overall strength of these adhesive-bonded joints in spite of the adverse indications on the total bond length, interface shear stresses, slip and differential movements. Visual observations of the failure surfaces were very informative. Of all the exposure regimes, the combined chloride immersionrfreeze]thaw cycling proved to be the most aggressive, and potentially the most harmful to the integrity of the interfaces in the bonded joint. This regime produced large areas of adhesion failure between the concrete and adhesive. Overall, the accelerated ageing tests produced

P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446

445

Fig. 15. Typical failure surfaces under freeze]thaw cycles: Ža. Mix A; and Žb. Mix B.

Fig. 16. Typical failure surfaces under dual exposure: Ža. Mix A; and Žb. Mix B.

consistent results on the behaviour of the joints emphasizing the reliability of this type of test to assess the integrity of plate-bonded joints.

w6x Swamy RN, Mukhopadhyaya P. Role and effectiveness of nonmetallic plates in strengthening and upgrading concrete structures. In: Taerwe L, editor. Non-metallic ŽFRP. reinforcement for concrete structures. London: E & FN Spon, 1995:473]482. w7x Swamy RN, Mukhopadhyaya P, Lynsdale CJ. Ductility considerations in using GFRP sheets to strengthen and upgrade structures. Third International Symposium on Non-Metallic ŽFRP. Reinforcement for Concrete Structures. Japan: Sapporo, 1997:637]644. w8x Bunsell AR. Long-term degradation of polymer-matrix composites. In: Kelly A, editor. Concise encyclopedia of composite materials. Pergamon Press, 1989:165]173. w9x New Civil Engineer. Magazine of the Institution of Civil Engineers. UK, 1996:4. w10x Calder AJJ. Exposure tests on externally reinforced concrete beams } first two years. Transport and Road Research Laboratory. Supplementary Report 529, 1979. Berkshire, UK: Crowthrone. w11x Calder AJJ. The microstructure of epoxy bonded steel-to-concrete joints. Transport and Road Research Laboratory, Supplementary Report 705, 1982. Berkshire, UK: Crowthrone. w12x Calder AJJ. Exposure tests on externally reinforced concrete beams } performance after 10 years. Transport and Road Research Laboratory. Research Report 129, 1988. Berkshire, UK: Crowthrone. w13x Calder AJJ. Exposure tests on 3.5 m externally reinforced concrete beams } the first 8 years. Transport and Road Research Laboratory, Research Report 191, 1989. Berkshire, UK: Crowthrone. w14x Calder AJJ. The durability of steel plates bonded to concrete

Acknowledgements The authors would like to acknowledge the financial support provided for this research by Centre for Cement and Concrete ŽCCC. at the University of Sheffield. References w1x Chajes MJ, Januszka TF, Mertz DR, Jr, Thomson TA, Jr, Finch WW. Shear strengthening of reinforced concrete beams using externally applied composite fabrics. ACI Struct J 1995;92:295]303. w2x Chajes MJ, Thomson TA, Januszka TF, Jr, Finch WW. Flexural strengthening of concrete beams using externally bonded composite materials. Constr Build Mater 1994;8:191]201. w3x Ehsani RM, Saadatmanesh H. Fibre composite plates for strengthening bridge beams. Comp Struct 1990;15:343]355. w4x Meier U. Strengthening of structures using carbon fibrerepoxy composites. Constr Build Mater 1995;9:341]351. w5x Leeming MB, Peshkam V. A robust solution to strengthening bridges. Proc. 6th International Conference on Structural Faults and Repair } 1995. Engineering Technics Press, 1995:161]164.

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P. Mukhopadhyaya et al. r Construction and Building Materials 12 (1998) 427]446 with structural epoxy adhesive. International Seminar on Structural RepairsrStrengthening by the Plate Bonding Technique. University of Sheffield, SIRIUS, 1990. Hobbs B, Swamy RN, Roberts M. Behaviour of deteriorated plated beams. Final Report on SERC Grant GRrFr29370. University of Sheffield, 1991. Swamy RN, Hobbs B, Roberts M. Structural behaviour of externally bonded steel plated RC beams after long-term exposure. Struct Eng 1995;73:255]261. Hobbs B, Jones R, Swamy N, Roberts M. Accelerated tests to assess the durability of bonded plate systems. International Seminar on Structural RepairsrStrengthening by The Plate Bonding Technique. University of Sheffield, SIRIUS, 1990. Roberts MB. Long-term performance of bonded plate systems for strengthening reinforced concrete beams. PhD thesis, University of Sheffield, 1996. Chajes MJ, Thomson TA, Farschman CA. Durability of concrete beams externally reinforced with composite fabrics. Constr Build Mater 1995;9:141]148. Green MF, Soudki KA. FRP strengthened concrete structures in cold regions. In: Meier U, Betti R, editors. Proceeding of International Conference on Recent Advances in Bridge Engineering } Advanced Rehabilitation. Switzerland: EMPA Dubendorf, 1997:219]226.

w21x Van Gemert DA. Force transfer in epoxy bonded steel]concrete joints. Int J Adhesion Adhesives 1980;1:67]72. w22x Van Gemert DA. Repairing of concrete structures by externally bonded steel plates. Proc. ICPrRILEMrIBK International Symposium on Plastics in Material and Structural Engineering. Prague: Elsevier Scientific Publishing Co, 1981: 519]526. w23x Swamy RN, Jones R, Charif A. Shear adhesion properties of epoxy resin adhesives. Proceedings of International Symposium on Adhesion Between Polymers and Concrete, 1986:741]755. w24x BS 5075. Part 2. Specification for air-entraining admixtures, 1982. w25x ASTM: C 666-84. Standard test method for resistance of concrete to rapid freezing and thawing. w26x RILEM Committee 4-CDC. Methods of carrying out and reporting freeze]thaw tests on concrete without de-icing chemicals } recommendation. Mater Struct, 110Ž58.:209]211. w27x Jones R, Swamy RN, Charif A. Plate separation and anchorage of reinforced concrete beams strengthened by epoxy-bonded steel plates. Struct Eng 1988;66:85]94. w28x Spadea G, Bencardino F, Swamy RN. Strengthening and upgrading structures with bonded CFRP sheets } design aspects for structural integrity, non-metallic ŽFRP. reinforcement for concrete structures. Jpn Concr Instit 1997;1:629]636.