242 Strengthening of reinforced concrete structures

242

9Design and specifications for FRP plate

bonding of beams

M B L E E M I N G A N D J J D A R B Y

9.1 Introduction

The culmination of the research, the experimental testing in the laboratory,the modelling by numerical methods and the full scale beam testing on site,is the practical application of the technique to real situations. The first stageof this is calculating the extent of the strengthening required and determin-ing the quantity of the plates to be bonded. The next aspect that concernsthe designer is the specification of how the work has to be carried out. Thepractical aspects of undertaking the work on site have been covered inChapter 10.

9.2 Practical design rules and guidelines

Throughout the design process the client has to be satisfied that the tech-nique is sound and will give him the service life required. For techniquesthat have stood the test of time, a competent designer applies codes anddesign rules that have been formulated by experts and have had the ap-proval of appropriate authorities. In the case of new techniques, however,authenticated design rules do not exist and an experienced designer mustinterpret the research carried out. It is necessary to understand the limitsof the research and if there are areas where detailed knowledge is lacking,a conservative approach is necessary.

One of the objectives of the ROBUST programme of research was thedevelopment of practical design rules and guidelines for flexural strength-ening of reinforced (RC) and prestressed (PC) concrete beams. These rulesand guidelines were to be based on the experimental research and numeri-cal verification carried out. The preliminary rules will need to be modifiedlater by the results of further research and by taking account of the longterm performance of actual structures. The accuracy of numerical verifica-tion has been demonstrated in Chapter 8 by comparing those results withthe experimental ones reported in Chapters 4 and 5. The numerical

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methods used were developed for parameter studies of plate geometry andmaterial properties and although the technique can be used for design it isgenerally considered by engineers to be expensive, time consuming andcomplex for practical design. Consequently, simpler methods based onempirical formulae which will be derived from the numerical and experi-mental results must be developed.

9.2.1 Failure modes

The first step in the formulation of design rules is to determine how thestrengthened beam will fail under certain loading conditions and then toapply empirical formulae to predict that ultimate limit state of failure. Themember may have to satisfy more than one ultimate failure mode. Theapplication of suitable factors of safety will ensure that the failure mode isnot reached in practice.

Three main failure modes have been identified in the literature andwithin the programme as follows (see Fig. 9.1):

• Flexural failure, modes 1, 2 and 3 in Fig. 9.1, occurring either in thecarbon fibre reinforced polymer (CFRP) plate as tension failure, yieldof the steel reinforcement in tension or in the concrete as a compressionfailure, the first being likely to occur when the beam is under-reinforcedand the latter when over-reinforced. Yield of the steel reinforcement islikely to occur before either the concrete or the CFRP plate fails butwhile this may contribute to the ultimate failure of the beam it is not theprime cause of failure.

Figure 9.1 Typical failure modes for strengthened beams. Failuremodes 1–8 on figure. Failure mode 8, adhesive failure at concrete/adhesive interface; failure mode 9, adhesive failure at adhesive/FRPplate interface; failure mode 10, interlaminar shear within FRP plate.

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244 Strengthening of reinforced concrete structures

• End anchorage peel, modes 6 and 7 in Fig. 9.1. The abrupt termina-tion of the plate can result in a concentration of stresses, some normal tothe plate, which cause the plate to peel off towards the centre of thebeam.

• Peel at a shear crack, modes 4, 5 and 8 in Fig. 9.1. This is a complexmechanism where debonding may occur due to strain redistribution inthe plate at the crack and/or the formation of a step in the soffit of thebeam causing shear peel. The delamination can then propagate towardsthe end of the plate. Whether mode 5 or 8 occurs depends on theamount of the shear reinforcement in the unstrengthened beam.

There are a number of other possible but unlikely modes of failure whichhave been identified in the literature such as delamination of the compositeplate or in the glue line, but these have generally not been experienced inthe research to date as the strength of these materials is higher than that ofconcrete. This type of failure could happen in practice if the installation hasbeen less than perfect or there is a defect in the manufacture of the plate.

9.2.2 Failure in flexure

Meier (1987) states that with the substitution of appropriate properties,classical reinforced concrete beam theory can be applied when using ad-vanced composite plates to strengthen beams in flexure. Strengthenedfull scale reinforced concrete beams tested by Kaiser (1989) at EMPA alsoseem to validate the strain compatibility method in the analysis of cross-sections. These statements were examined carefully in the light of theresults from beam tests in the ROBUST programme. A major study wascarried out using normal reinforced concrete theory to predict the load/deflection history, the deflection of the beams and the modes of failure ofthe beams.

In flexure, classical theory assumes that plane sections remain plane, thatis, strain compatibility is assumed. Deflection of the beams can be calcu-lated using simple formulae. Tracing the load/deflection history using nor-mal reinforced concrete theory gives a reasonable fit to the experimentalvalues, bearing in mind the inherent assumptions in the method. However,in most cases, while classical theory predicts that the plate will fail intension, this phenomenon has not been experienced in any of the tests onbeams with unstressed plates. The concrete compressive strain is usuallylimited to 0.35% for normal design purposes, whereas the value in the testshas been somewhat lower, in the range of 0.19–0.25%. Figure 9.2 comparesthe actual strains measured on one of the 2.3m beams tested at OxfordBrookes University with the calculated strains at a load of 90% of the actualfailure load in the test.

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Design and specifications for FRP plate bonding of beams 245

The strains on the concrete in the experimental work at Oxford BrookesUniversity were measured by demountable mechanical (Demec) gauges onthe side of the beams. It was not therefore possible to obtain readings atfailure load. The correspondence between measured readings and thosecalculated becomes worse as the failure load is approached. This is probablydue to the extensive cracking in the concrete that occurs in the tensionflange and to the yield of the reinforcement. However, the prediction is onthe safe side and will be more accurate at working load.

With the use of the classical reinforced concrete design theory and simpleformulae for deflection, load deflection graphs can be drawn and the resultscompared with the experimental results. The correspondence is good withthe calculated deflection following the changes in the slope of the experi-mental results as the reinforcing steel yields. The use of classical reinforcedconcrete theory is adequate for the purpose and has been found to predictstresses in strengthened beams better than the unstrengthened beams.

To apply reinforced concrete theory to strengthen beams it is necessaryto know the ultimate strength of the plates and their modulus and use anappropriate material factor of safety. The ultimate strength of the platesand their modulus is obtained from the characterisation tests carried out onthe plates. A material factor of safety, γm, of 1.5 is proposed for the RO-BUST CFRP plates, based on properties derived from test specimens fromthe production of plates, assuming a pultruded plate fully cured at the

Figure 9.2 Comparison of measured strains on a 2.3m beam withcalculated strains from RC theory.

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works, and operating temperatures not in excess of 50°C combined witha heat distortion temperature greater than 90 °C. With these parametersEurocomp (1996) would advise that γm should not be taken as less than 1.5.This value can be compared with γm for steel of 1.15 and for concrete 1.5.The calculations can be carried out at the ultimate limit state, balancing thecompressive and tensile forces which have been calculated from straincompatibility using appropriate modular ratios for the various materials.

9.2.3 End anchorage peel

The mechanism of end anchorage peel has been studied by a number ofresearchers: Jones et al. (1988, 1989), Swamy et al. (1989), Roberts (1989),Taljsten (1994), Jansze (1995), Jansze and Walraven (1996), Zhang et al.(1995), Raoof and Zhang (1996), Oehlers and Ahmed (1996). The theory ofJones et al. (1988) relates to relatively stiff steel plates and calculationswithin the programme have shown that stresses predicted by the theory aremuch reduced due to the thinner more flexible CFRP plates. These stressesare said to be due to:

1 The force resulting from making the plate conform to the curvature ofthe beam; this force is 1/370 th of an equivalent steel plate.

2 Plate curl due to the eccentricity of the plate force to the bondline; thisforce is 1/14 th of the equivalent steel plate.

3 Interface bond stress or longitudinal shear; this force is still importantbut is reduced due to the CFRP plate being thinner, since the closer theoutermost fibre is approached the lower the longitudinal shear stress.

Swamy et al. (1989) suggested a design method of doubling the longitudi-nal shear stress and using an ultimate interface shear strength of √2� thetensile strength of the concrete.

The theory developed by Roberts (1989) is based upon partial interactiontheory. Incorporating the results obtained from the 1m beams into thetheory, anchorage shear/peel stresses at failure in the order of 14–18MPaare predicted. These are clearly not sensible, being considerably greaterthan the tensile strength of concrete.

Täljsten (1994) uses fracture mechanics for his predictions. Again thedata obtained from the tests on 1m beams at the University of Surrey andalso from the 2.3m beams at Oxford Brookes University were input into thetheory. Anchorage shear/peel stress at failure in the order of 1.3–1.8MPaare predicted for the 1m beams and from 1.8–3.4MPa for the 2.3m beamswhich did not appear to fail by this mechanism. These results are morereasonable with respect to the expected shear or tensile strength of concretealthough a value of at least double would have been expected for the 1mbeams.

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Design and specifications for FRP plate bonding of beams 247

Zhang et al. (1995) and Raoof and Zhang (1996) present a theory basedon a series of cantilever teeth of plain concrete between cracks in the coverzone anchoring the plate and is limited by the tensile strength of the con-crete. The spacing of cracking is difficult to determine but a minimum andmaximum spacing is said to give an upper and lower bound solution whichbrackets the data.

A simplistic analysis was carried out by calculating an anchorage stress ina similar manner to the bond stress for a reinforcing bar by taking the forcein the plate at the point of maximum moment at failure and dividing it bythe length multiplied by the width of the plate beyond that point. Stressesfrom this exercise varied from 2.6–4.2MPa which are of an order that mightbe expected although there is still a wide variation. Breaking these figuresdown into groups depending on plate width gives the results in Table 9.1.

None of the above theories presents an adequate method for generaldesign. End peel stresses have been shown to be critical for thick andrelatively stiff steel plates but it is thought that they are not significant forthin and flexible composite plates. There is no doubt that peel stresses occurat the end of a plate that is curtailed at a point some distance from thesupport but none of the above theories fitted the experimental results fromthe ROBUST programme.

The calculations for anchorage length suggested by European thinkingare based on tests carried out when a CFRP plate is bonded to a concreteblock and a force applied to pull the plate from the concrete in a directionparallel to the bond line. The tests are often done as a double shear lap test.In these tests the peel is initiated not at the end of the plate but at thefront face of the concrete. The peeling mechanism is not therefore dueto end peeling stresses but to high longitudinal shear stress at the leadingedge of the concrete. This condition would be closer to the situation at theedges of a crack. The failure mode is considered to replicate FRP peelingoff at the outermost crack in the uncracked anchorage zone. Calculationsbased on fracture mechanics give good agreement with experimental re-sults. Usually when a plate peels under these conditions only a thin layer ofconcrete a millimetre or two in thickness adheres to the plate but in somecases its failure line may divert into the plate causing interlaminar platefailure.

Table 9.1 Anchorage stresses for the 1m beams

Plate width (mm) 90 65 45Plate thickness (mm) 0.5 0.7 1.0Maximum stress (MPa) 2.62 3.34 4.22Minimum stress (MPa) 2.53 3.09 3.99

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Rostasy (1993) looked at this situation and cites work by Ranish (1982)who studied bond stresses in the anchorage length of steel plates bonded toconcrete blocks. It was found that a maximum value of bond stress (max τb1)was reached which equalled four times the mean surface tensile strength ofthe concrete established by a pull-off test. The necessary anchorage length(lbl) was found from equation [1]:

nec bl lu bll F E t b522

1 1 12 515 10. . .max . .( ) [1]

where lbl is between 500–1500 mm, Flu is the limiting plate force, El is themodulus of the plate (MPa), t1 is the thickness of the plate for valuesbetween 5–15mm and b1 is the width of the plate (about 150mm). Thisformula was introduced into early design procedures for CFRP plateswhere the thickness of the CFRP laminate was expressed as the thickness ofan equivalent steel laminate in the ratio of their respective strengths. Themax τbl was also given in a table with values according to Tausky (1993)based on the measured tensile strength of the concrete at the surface. Thistheory assumed a minimum bond length of 500mm and the force to beanchored was the tensile force in the CFRP laminate at the design level inthe location of the maximum moment.

Neubauer and Rostasy (1997) carried out 51 bond tests using CFRPplates bonded to concrete blocks where the bond length, plate width, platethickness and concrete cube strength were varied. It was found that anchor-age lengths of between 195–330mm were required to sustain a plate forcethat could not be exceeded and that longer anchorage lengths were noteffective. Fracture mechanics was used to analyse the results which wereexpressed in the formulae [2]–[4]:

l E t ft. ctm max . .� √( ) ( )1 1 2 [2]

T k b E t fck. b 1 ctm 0.5.max . . . .� √( )1 1 [3]

where 1.06. 2 1b 1k b b b� � � �√ ( )( ) (( )( )1 4001 [4]

where lt.max is the maximum bond length in mm, E1 is the modulus of theplate, t1 is the thickness of the plate in mm, fctm is the concrete surface tensilestrength in Nmm�2 which should not be taken as greater than 3Nmm�2,Tck.max is the maximum plate force that can be anchored in Newtons, b1 is theplate width in mm and b is the width of the beam soffit or the plate spacingin slabs in mm.

Chajes et al. (1993) first investigated adhesives with moduli from5–0.2 GPa with various types of surface preparation in similar tests.The adhesive with the lowest modulus failed in the adhesive while theothers failed in the concrete. Fusor together with a Chemglaze primer (anorganofunctional silane) gave the best results and was chosen as the adhe-

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Design and specifications for FRP plate bonding of beams 249

sive for further tests where three different concrete strengths were used.These few results were used to predict a shear stress failure in the concreteof 11.1 √fc psi, where fc is probably the cylinder strength although not stated(0.92 √fc in MPa). Further lap shear tests were then carried out using 50 mm,100 mm, 150 mm and 200mm bond lengths with a 36.4N mm�2 concrete.These tests showed that a maximum force of 12 kN could be anchored by a25 mm wide strip with a 95mm anchor length and that longer anchoragelengths did not increase the force that could be anchored. An average shearstress in the concrete was estimated at 4.945 Nmm�2. Chajes et al. (1993)therefore came to the same conclusion as Neubauer and Rostasy (1997)that there is a certain anchorage length that will sustain a plate force thatcannot be exceeded. Table 9.2 compares the results from these two papers.

Table 9.2 Comparison of anchor lengths

Neubauer and Rostasy (1997) Chajes et al. (1993)

Adhesive Sikadur 31 Fusar

Tensile 24.81 30.6strength(MPa)

Elongation 0.41 3.0(%)

Elastic 5.1721 1.584modulus(GPa)

CFRP CarboDur PrepregPlates

Tensile 2000–3000 1655strength(MPa)

Elongation 3–5 1.5(%)

Elastic 150–230 108.5modulus(GPa)

Results Concrete Plate Anchor Plate Concrete Plate Anchor Platestrength thickness length force strength thickness length force(MPa) (mm) (mm) (kN) (MPa) (mm) (mm) (kN)

25 1.2 22825 2.4 330

36.4 1 95.25 1255 1.2 19455 2.4 275

1 Figures from Chajes et al. (1993) for Sikadur 31.

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Detachment of the plate was a common failure mechanism in the RO-BUST tests. However, the plate became detached so quickly that there wasalways doubt about whether the failure started at the end of the plate andpropagated towards the centre or started peeling at a crack and propagatedtowards the end of the plate. Even recording the point of failure with a highspeed video camera could not resolve the question. It is now thought thatpeel initiating from a shear crack is the more likely mechanism.

9.2.4 Peel at shear or at wide shear/yield crack

Where the shear span to effective depth of the beam is less than about 6, thenormal mode of failure of a reinforced concrete beam is usually due to theformation of a diagonal flexure–shear crack. This effect is illustrated in Fig.9.3 where the results of the tests on 1m long beams at the University ofSurrey are presented. The effect of the width of the plate is evident, thewider the plate the greater the ultimate moment. At low values of shearspan to depth ratio, effective anchorage of the ends of the plates alsoincreased the ultimate moment. The shear span to depth ratio is related to

Figure 9.3 Effect of shear span to depth ratio and plate width onultimate moment of beams strengthened with CFRP plates.

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Design and specifications for FRP plate bonding of beams 251

the longitudinal shear stress between the plate and the concrete but with thescatter of the results a lower bound longitudinal shear stress could not beassessed. Whether this reduction in ultimate strength was due to shear orend peel effects could not be ascertained. It is clear that where the shearspan to depth ratio is low, calculating the required amount in strengtheningby flexure alone is not sufficient and shear and end peel effects must beconsidered or the plate ends must be anchored adequately. At a point whena crack occurs at the end of the plate due to shear and bending, end shearpeel is likely to occur and the whole depth of the cover concrete togetherwith the plate will detach and peeling will occur at the level of thereinforcement.

If a shear crack in a beam strengthened with a CFRP plate approaches anangle of 45°, a step can occur in the soffit of the beam as the end of the beamrotates about the compression zone of the top flange (see mode of failure 4in Fig. 9.1). This failure mode was identified by Meier and Kaiser (1991).This mechanism can cause the plate to peel due to stresses normal to theplate and may be aggravated by stress redistribution across the crack.Triantafillou and Plevris (1992) address the problem and propose the rela-tionship P � λ(Gs.As � Gp.Ap) where P is the debonding load, G is theshear modulus, A the area of the main reinforcement and the plate, respec-tively and λ is a constant determined by experiment. From the three experi-ments they found that λ approximates to 0.011 using a value of 4.4GPa forthe shear modulus of the CFRP plates.

Many failures within the ROBUST programme are now thought to haveinitiated from a near vertical crack at a point about a beam depth away fromthe load point associated with triangular cracking of the concrete followedby peeling towards the end of the plate. The vertical crack occurred be-tween the shear link reinforcement. The beams were heavily reinforced inshear to avoid shear failure of the type described above. A possible mecha-nism for failure mode 8 in Fig. 9.1 is the formation of a wider crack at thepoint where the steel reinforcement yields, leading to debonding of theplate to redistribute the high strains across the crack which then propagatetowards the end of the plate. This mechanism may be aggravated by somesmall vertical movement at the soffit due to shear strain across the crack.The factors influencing the failure will be:

• shear transfer in the compression zone of the concrete,• shear transfer due to aggregate interlock in the crack below the neutral

axis,• dowel action of the steel reinforcement,• dowel action of the CFRP plate, although this must be small and should

be discounted,• a shear step in the soffit of the beam causing plate peel. This latter

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mechanism is dependent on the angle of the crack and the extent of theshear strain across the crack,

• the formation of the wide crack due to yielding of the steelreinforcement.

The main objective of the programme was to study CFRP plate bondingin flexure but it is clear that with low shear span to effective depth ratiosused in some of the experiments, the mode of failure was a shear peelingcausing the removal of the layer of concrete cover to the internal tensilereinforcement. All the beam tests in the programme were carried out underfour point bending. This loading configuration gives constant shear in theshear span with a high bending moment at the loading point that diminishesto zero at the support. This form is commonly used in loading tests as itleads to simple analysis but is not typical of loading in practice whereuniformly distributed loads are more common and point loads, when theyoccur, are frequently moving such as in wheel loads. With uniformly distrib-uted loading there is no clear shear span, and anchorage and shear effectsmay be less pronounced. Shear effects are not well understood in reinforcedconcrete theory and the relative effects of shear transfer in sound concrete,aggregate interlock in cracked concrete and the dowel action of thereinforcement are difficult to quantify separately. Taylor (1974) gives thefollowing proportions of the shear force taken by the above actions: com-pression zone shear 20–40%, aggregate interlock 33–50% and dowel action15–25%. Regan (1993) gives a more recent overview of research into shearover the last 100 years. It is not thought that the dowel action of the CFRPplate is significant. The extent of a shear step in the soffit of the beam isdifficult to quantify and is also probably of little significance. The situationwill be on the safe side if the beam satisfies the normal code requirementsfor shear ignoring the contribution of the CFRP plate.

The stresses in the plates and in the steel reinforcement can be calculatedthroughout the length of a beam according to classical RC bending theoryon the basis that the stresses vary in accordance with the moment at anypoint. This is illustrated in Figs. 9.4 and 9.5 for the half spans of two beams,one with a single point load and the other with uniformly distributed load-ing. The case of the single point load is similar to that of the shear span onlyof a beam loaded in four point bending as is usual in experimental beamtests.

Longitudinal shear stress between the plate and the concrete will berelated to the rate of change of the force in the plates and hence to the stressin the plate for constant section. It can be seen from Fig. 9.4 for the case ofa single point load that the greatest slope on the curve of the stresses in theplate is towards the middle of the beam where the steel reinforcement hasyielded and can no longer contribute any greater force to balance the

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Figure 9.4 Tensile stresses under a single point load.

Figure 9.5 Tensile stresses under uniformly distributed load.

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increasing moment. The longitudinal shear stress, at this point, can beshown to be approximately equal to S/(by) where S is the shear force, b thewidth of the plate and y the distance from the middle of the concrete stressblock which is effectively the distance of the centroid of the plate from thetop of the beam less half the neutral axis distance. For the case of auniformly distributed load the greatest slope is between about 0.6 and 0.7 ofthe half span. The former case is a coincidence of high moment and shear;the latter of high moment but low shear.

Combined with the above, higher stresses will occur in the CFRP plate atthe location of the crack due to the increase in strain over the width of thecrack. This instantaneous high stress can only dissipate by debonding eitherside of the crack, possibly causing a triangle of concrete to detach from oneside of the crack. This phenomenon was found in the testing of the 18mbeams where the strains in the plates were found to be very high at thepoints where the prestressing cables had been cut by coring through the sideof the beam. These strains were found to be 1.25 to 1.5 times the averagestrains. The debond will release the peak stress to a point where the rate ofchange of stresses in the plate no longer exceeds the longitudinal shearstress that the concrete can withstand.

Once the length of the debond is sufficient for the difference in the forcesin the plate on either side of the debond to exceed the maximum longitudi-nal shear stress of the concrete, the debond will propagate further and willultimately reach the end of the plate. The three factors involved in failuremode, namely, high longitudinal shear stress at the interface between theplate and the concrete, stress redistribution in the plate over the crack andshear step effects, do not lend themselves to simple analysis for designpurposes. It is suggested, therefore, that in the design the strain in the plateis calculated at the point where the internal steel reinforcement starts toyield and the ultimate load for this failure mode should then be taken as thepoint of yield strain plus an additional limiting strain. The additional limit-ing strain has yet to be determined from experiment.

9.2.5 Limit states and partial safety factors relevant tocomposite plate bonding

The design of strengthened bridges should be considered for both theserviceability limit state (including the checking of stress limitations) andultimate limit state, in accordance with the relevant clauses of BS 5400: Part4 (1990), except where amended in this chapter. The appropriate loads andload factors should be taken from BS 5400: Part 2 (1990).

Partial safety factors to the characteristic strength of the CFRP plates ofγm � 1.5 for ultimate limit state (ULS) and γm � 1.00 for serviceability limitstate (SLS) should be used. Where the characteristic strengths of the exist-

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ing concrete and reinforcement are not known characteristic strengths maybe derived from test evidence.

The ‘over-reinforcement’ of a concrete section can result in a brittlefailure. Sections to be strengthened should, therefore, be checked to ensurethat this does not occur, in particular by using the requirements of Clause5.3.2 of BS 5400: Part 4 (1990).

9.2.6 Plate dimensions and spacing

When determining the size and thickness of each individual plate forstrengthening a slab, the effective width of the section should not be greaterthan the width of the plate plus twice the distance of the plate to the neutralaxis of the section.

9.2.7 Fatigue and creep

Fatigue is discussed in Chapter 7, Part 2 ‘Fatigue Behaviour’. The stressrange for fatigue purposes in the existing steel reinforcement of reinforcedconcrete beams should not be increased when the beam is strengthenedby CFRP plate bonding. In prestressed concrete it should be ensured thatstress transfer due to creep in the concrete will not result in excessivecompressive forces being induced into the plates.

9.2.8 End and intermediate fixings

Intermediate fixings are not required as is the case with steel plate bondingas the plates are much lighter. The ‘grab’ of the adhesive is such that theplate remains in position after having been rolled into the adhesive layerwithout further support.

Fixings may be required at the ends of the plates to resist concentrationsof peel and shear stresses in this region, where these stresses exceed theproduct of the pull-off strength of the concrete and γm. The fixings should bedesigned as described above. In the ROBUST programme of research,15 mm thick �45° glass fibre reinforced polymer (GFRP) end tabs werespecially bonded to the ends of the plates and stainless steel bolts werepassed through both the plate and the end tab at suitable spacings and wereanchored at a depth into the concrete below the steel reinforcement. Thebolts should be supplied with large washers and tightened up to a predeter-mined torque to prevent crushing of the composite materials. It is notadequate to omit the end tabs and merely to drill through the compositeplates and thence to anchor them at their ends with bolts. Drilling holesthrough unsupported composites will sever the unidirectional fibres and theconcentrated compressive forces under the bolt head will weaken the plate

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further; it is not possible for the forces in the plate to be transmitted into thebolt. The bond between the ROBUST end tabs and the plates was designedto take the whole force to be anchored. This force is then transferred intothe concrete by the fixing bolt. Other forms of fixing were not found to beso effective. Tests by Poulsen (1996) show that the ultimate anchorage forcecan be increased by up to three times by utilising three bolts, but increasingthe number of bolts beyond this value is not effective. The effect of bondingshort lengths of laminate across the end of the plate was also examined.One transverse laminate increased the ultimate force anchored by 75% butfurther transverse laminates gave little increase in anchorage force.

Plates should not extend into areas of compression without further veri-fication about their adequacy, as plate buckling may occur causing tensile/peel stresses in the adhesive normal to the plate. The maximum com-pressive force in the plate is limited by the maximum permitted strain inthe concrete at compressive failure, normally taken for design purposes as0.0035. Where the plates are fully bonded they can be considered fullyrestrained against buckling. The out-of-plane forces can be calculated andeven an out-of-straightness of 5mm in 1m produces out-of-plane forces thatare well within the tensile strength of the concrete which is the weakest link.However, an unbonded length as short as 40mm for a 1.4mm thick platecould be liable to buckling failure. Clearly the length of voids in thebondline must be limited to well below the critical buckling length depend-ent on the thickness of the plate. During construction, very careful monitor-ing of voids in the bondline is required for sections of plate that will go intocompression during any part of the loading cycle. This is an area that wouldbenefit from further research.

9.2.9 Vandalism and fire

Where fire damage and vandalism are expected the plates may be coveredby a suitable polymer-modified cementitious screed or by suitable intumes-cent coatings. Fire tests have been carried out at EMPA where CFRP plateswere found to last considerably longer than steel plates. The reason for thisis that the adhesive is the most vulnerable factor and steel conducts the heatrapidly to the glue layer. Carbon does not conduct heat to the same extentas steel and to some extent protects the glue layer. Composite materials arebeing increasingly used in offshore structures as panelling for blast and fireprotection (Wu and Gibson, 1994).

9.2.10 Durability

At present there are no standard accelerated laboratory testing methods topredict the long term performance and durability of the system. The dura-bility of composite plate bonding is discussed in Chapter 6.

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9.3 Application of the technique

9.3.1 General

The technique of bonding CFRP plates to structures can be used in alllocations where there is a requirement for additional flexural reinforcementin the tensile zone. Their use for strengthening in compression requiresfurther justification. Careful consideration should be given where the head-room of bridges is critical or where there is evidence of frequent damage tothe soffit from vehicles. Special provisions may be necessary to protect theCFRP plates under such circumstances. Where plating to soffits of bridgesabove carriageways is being considered the available headroom should bechecked. Due allowance should be made for any fixings required.

The Highways Agency (1994) requires that, for steel plate bonding, anyelement of a bridge structure to be strengthened should be capable ofsupporting nominal dead load, superimposed dead load factored by thepartial safety factor for loads, γfL � 1.2 and nominal HA (normal) live load(i.e. unfactored) when checked at ULS. There seems no reason to alter thisrequirement for CFRP plate bonding at present.

Where plates are bonded to the top surfaces of slabs and beams andsubsequently buried by the road surfacing it would be impractical to pro-vide inspection facilities for the plates. In such cases, special care should betaken during bridge inspections to identify any areas of plates that may havedebonded as indicated by local breakup or reflective cracking of the surfacein the location of the plates. It is essential that accurate drawings indicatingthe location of all plates are maintained and are readily available for suchinspections.

9.3.2 Investigations and tests

Before plate bonding is considered for a structure, investigations should becarried out to ensure that the risk of corrosion in the existing member is lowand that the structure is sound enough (including any repaired areas) forstrengthening by plating.

Surfaces that are damp or subject to leakage, particularly if contaminatedwith chlorides, should only be plated after satisfactory remedial measureshave been taken. If the reinforcement is corroding, the expansive rustproducts may disrupt the concrete and eventually cause debonding of theplate. Therefore, unless repairs have been carried out, plate bonding shouldonly be considered for members where chloride values are generally lessthan 0.3% by weight of cement and half-cell potential measurements arenumerically generally less than �350mV (e.g. �200 mV) with respect to acopper/copper sulphate electrode. CFRP plates are less vulnerable to dura-bility problems than steel plates and therefore surfaces that are damp and

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258 Strengthening of reinforced concrete structures

subject to leakage are less critical. However, steps should be taken to rectifythe dampness and prevent the leakage. Tests in ROBUST carried out byOxford Brookes University showed that there was no loss in pull offstrength when Sikadur 31 was used to bond the dollies to saturated surfacedry concrete. Nonetheless, it is advisable to avoid these conditions to ensurethat the highest standards are used in installation of the technique. Whiledampness may not affect the composite plate and adhesion to the concretesignificantly, it could cause further corrosion of the steel reinforcementleading to spalling of the cover concrete to which the plate may be bondedand would also be susceptible to freeze/thaw damage causing delaminationof the concrete cover.

9.4 Materials

9.4.1 Composite plates

A variety of composite plates have been used in the ROBUST and otherresearch programmes, using glass, carbon or aramid fibres manufacturedby the pultrusion process or made up from prepreg material. In the UK andin Europe the use of plates containing about 60% by volume fraction ofunidirectional carbon fibre manufactured by the pultrusion process in anepoxy or a vinylester resin matrix is favoured. The plate in ROBUST was90mm wide by 1 mm thick and used T300 carbon fibre in a vinylesterresin (BASF A430) combined with a peel-ply inbuilt surface finish on bothsides of the plate. This method of surface finish provided a suitable surfacefor bonding without an extra manufacturing process and kept the surfaceto be bonded clean until just before the adhesive was applied. The choiceof T300 fibres was made on cost and availability at the time with a penaltyof a lower strength and modulus. T700 fibres are much more widely avail-able now and there is no reason why this superior strength of fibre shouldnot be used in plates of different widths and thickness. Other plates fromEurope have used T700 fibres and are available in various sizes but requireabrading and wiping clean before use to obtain a satisfactory bondingsurface.

The modulus of glass is too low to give satisfactory performance inplate bonding requiring at least three times the thickness of an equivalentCFRP plate to give the same stiffness and is therefore not being usedeconomically. Aramid has no special advantages over carbon and is moreexpensive.

The use of any plating material must have adequate data to support thequoted properties resulting from characterisation tests carried out in ac-cordance to appropriate standards such as BS2782 Part 3 (1996) and Part 8(1994), ASTM D3039/3039M (1995) and Crag (1988). These standards re-

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Design and specifications for FRP plate bonding of beams 259

quire a test sample 25 mm wide. Tests have been carried out on full widthsamples which gave significantly lower results. This size effect must beresolved before reliance can be placed on results from coupon specimens.The material factor of safety used in design should reflect this uncertaintyand will depend on the number of tests carried out and their variability.

9.4.2 Concrete – suitability for bonding

A range of tests is available to determine the suitability of the concretesurface for bonding plates. The most suitable test method involves bondinga metal dolly to the concrete using the same adhesive used to bond theplate, and subsequently pulling the dolly off. Examination of the failuremechanism and failure load gives useful guidance. The preparation is con-sidered suitable when a cohesive failure occurs in the concrete. This testshould be carried out in accordance with BS 1881 (1992) or with the latestedition of prEN 1542 (awaiting translation before ratification). A minimumvalue of 1.5 Nmm�2 should be obtained.

9.4.3 Adhesive

On the basis of long term experience epoxy resin adhesives have beenfound to be suitable for steel plate bonding. Their durability has beenestablished by use over a period of twenty years. The same epoxy resinadhesives have been used in research and in practice for CFRP plate bond-ing with acceptable results. An adhesive must demonstrate an acceptabletrack record of use in steel and CFRP plate bonding or must be subjectedto additional tests to prove its acceptability for strengthening proposalsdetailed below.

Epoxy resin adhesives require care in use. Manufacturers or formu-lators commonly supply two-part resins in containers suitably propor-tioned for mixing. It is important that all the hardener is added to theresin in its container and mixed with a slow speed mechanical mixer. Highspeed mixing entrains air and is less efficient. The resin and hardener shouldbe of different colours and adequately mixed to produce a uniform colour.The speed of the chemical reaction increases with the temperaturegenerated.

Sikadur 31PBA was used in the ROBUST programme of research. Thisis a two-part cold cured, epoxy-based structural adhesive composing a resinwhich is white in colour and a black coloured hardener. They should bemixed together to form a uniform grey colour. The adhesive should beapplied onto the CFRP and/or concrete surfaces within 20min. Sikadur 30has been used more extensively in Europe. The properties of Sikadur31PBA and Sikadur 30 are given in Table 9.3.

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260 Strengthening of reinforced concrete structures

Table 9.3 Properties of typical adhesives used in plate bonding

Sikadur 31PBA Sikadur 30

Heat distortion temperature (°C) 43 62Flexural modulus (kNmm�2) 8.6 12.8Tensile strength (Nmm�2) 21.8Moisture resistance

Design and specifications for FRP plate bonding of beams 261

9.5.3 Application of adhesive

The bonding of CFRP plates to concrete members is carried out by applyingthe structural epoxy resin to the plate and/or concrete, offering the plate tothe soffit of the member and applying pressure to ensure intimate contactwith the concrete. It is important to spread the adhesive immediately aftermixing to dissipate the heat generated and extend its workability time.Common practice is to spread the adhesive slightly more thickly in a domedprofile along the centre line of the plate than at the sides of the plate. Thisreduces the risk of forming voids when pressing the plate loaded withadhesive against the concrete surface. Excess adhesive squeezed out of theplate can be scraped away and a fillet formed at the plate edge. Small glassbeads (ballotini) dropped onto the epoxy adhesive surface prior to fixing canbe used to maintain the minimum required adhesive thickness of 1–2 mm.Procedure trials should always be carried out to prove the method ofapplication and acquaint the operatives with the material. The ambienttemperature and dew point temperature should be determined and if theambient temperature is less than 10 °C or dew point is more than 10 °C,artificial heating and dehumidifiers may be required to maintain the ambienttemperature and humidity at acceptable levels for installation and curing.The manufacturer’s instructions on safe use of resins should be followed.

Based on experience in ROBUST up to three plates may be bondedin layers to give the required total thickness, each plate being bondedseparately.

9.6 Quality control

9.6.1 Adhesive bond/shear tests

In bonding to CFRP, assurance of the quality of adhesive bonded jointsshould take place prior to bonding by ensuring adequate control of all theprincipal steps in the bonding process; these steps include surface prepara-tion, mixing, application and curing of the adhesive. The purpose of the testis to check adhesion and the adhesive material properties. Simple single lapshear tests are recommended, with a 10mm overlap, see below. Threesingle lap shear tests should be performed for each batch delivered to thesite. The CFRP adherends should be prepared in the same manner as theplates to be bonded. The load and locus at failure is to be recorded andreported and the test should be deemed acceptable if the bond strength ofthe adhesive to the CFRP is greater than 12 Nmm�2.

It is recommended that representative samples of the prepared surface beused to make single lap shear joints generally in accordance with ASTM

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262 Strengthening of reinforced concrete structures

D3163 (1992). The dimensions and arrangement of the test are shown inFig. 9.6. The failure load will indicate whether or not the adhesive materialhas been mixed and cured adequately, while the locus of the joint failurecan provide information on the soundness of the bond. Interfacial failureand low joint strengths indicate poor adhesion, whilst failure of the joint inthe composite material itself can at least provide some lower bound confi-dence level in bond performance.

An alternative to the single lap joint test is the wedge cleavage testillustrated in Fig. 9.7 and carried out generally in accordance with ASTMD3762 (1979). The purpose of the test is also to assess adhesion and tough-ness of the adhesive material. The procedure involves first bonding togethertwo strips of composite 150mm long and then, once the adhesive has cured,driving a wedge between them to provide a self-stressed specimen. Thisinduces a cleavage stress on the bondline and if adhesion is poor a crackwill grow between the composite and the adhesive layer. If the crack growsthrough the adhesive only, then satisfactory adhesion is assured; the frac-ture energy of the adhesive material can additionally be calculated from aknowledge of specimen geometry and material properties, enabling a checkon the toughness properties of the cured adhesive. The length of the crackand its position should be recorded.

Figure 9.6 Single lap joint (all dimensions in millimetres).

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Design and specifications for FRP plate bonding of beams 263

Figure 9.7 Wedge cleavage test (all dimensions in millimetres).

Tests should be carried out over a range of temperatures specificallyincluding �25 °C, �20 °C and �45 °C using CFRP adherends. The tempera-tures are measured by means of thermocouples attached to the CFRPsurface of the joint. The minimum average lap shear stress is 12Nmm�2 at20 °C.

9.6.2 Tests for new materials – epoxy resin adhesives forCFRP plate bonding

9.6.2.1 Materials

The material consists of a two-part cold cured, epoxy-based adhesive com-prising resin and hardener, the resin being based on the diglycidyl ether of‘bisphenol A’ or ‘bisphenol F’ or a blend of the two. The hardener or curingagent should be from the polyamine group in order to achieve a betterresistance to moisture penetration through the adhesive layer. Inert fillersmay also be incorporated with the resin component to improve the applica-tion or performance characteristics of the adhesive. The filler should be anelectrically non-conductive material, be highly moisture resistant, be able towithstand temperatures up to 120 °C without degradation and have a maxi-mum particle size of 0.1mm. The materials should be supplied in two packs,in correct weights for site mixing. The two components should have differ-ent colours in order to aid thorough mixing.

Other adhesives offering performance equivalent to that required by thisspecification for long term durability will be acceptable. In the absence ofsatisfactory laboratory accelerated durability tests, however, the assessmentof the durability of an alternative adhesive should be based on the provisionof evidence that it has, in practice, performed satisfactorily in conditionssimilar to the use proposed for a period of not less than 15 years afterinstallation.

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264 Strengthening of reinforced concrete structures

9.6.2.2 Mixing

Mixing should take place in accordance with manufacturer’s instructions.

9.6.2.3 Placing

The adhesive should be capable of being applied readily to both concreteand CFRP surfaces in layers from 2–10mm thick.

9.6.2.4 Cure time and temperature

The adhesive should be capable of curing to the required strength between10 °C and 30°C in relative humidities of up to 95%. For repairs andstrengthening works the adhesive should cure sufficiently to give the speci-fied mechanical properties at 20 °C in not more than three days. On curingthe adhesive should undergo negligible shrinkage.

9.6.2.5 Usable life

Mixed adhesive, before application to the prepared surfaces should have ausable life in excess of 40min at 20 °C. Test method as in BS 5350 :Part B4(1993).

9.6.2.6 Open time

The time limit in which the joint should be made and closed should notexceed 20min at a temperature up to 20°C. Test method as in BS 2782 :Part8 Method 835 (1994).

9.6.2.7 Storage life

Shelf life of both the resin and the hardener should be not less than 6months in original containers at 5°C and at 25 °C.

9.6.2.8 Mechanical properties of hardened adhesive

Cure and storage temperature, as stated in the appropriate standards,should be used for all tests and a minimum of five tests should be under-taken for each test requirement and the average of the results taken.

9.6.2.9 Moisture resistance

Minimum moisture transport through the adhesive should be ensured, withwater content not exceeding 3% by weight after 28 days immersion in

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Design and specifications for FRP plate bonding of beams 265

distilled water at 20 °C. Specimens should be 40mm by 40 mm and between1–2 mm thick. The procedure and calculations are used in accordance withBS 6319 :Part 8, Clause 6.2.7 (1984) to determine water content.

9.6.2.10 Heat distortion temperature

The adhesive should have a heat distortion temperature (HDT) of at least40 °C. The sample under test should be placed in a temperature controlledcabinet at 20 °C and loaded in order to achieve a maximum fibre stress of1.81 MN m2 in accordance with ISO 75 (1987). The HDT should be takenas the temperature attained, measured by a thermocouple attached to thespecimen, after undergoing a further 0.25 mm deflection while subject to asurface heating rate of 0.5 °C min�1.

9.6.2.11 Flexural modulus

Flexural modulus, without creep effects, at 20 °C should be in the range of4–10 kN mm�2. A specimen 200 mm by 25 mm by 12mm deep tested in fourpoint bending should be used. The sample should be loaded at the thirdpoints to achieve a deflection at a rate of 1 mm min�1 and the centraldeflection recorded. From the resultant load–deflection curve, the secantmodulus at 0.2% strain is calculated.

9.6.2.12 Tensile strength

A minimum value of 12N mm�2 at 20 °C should be achieved. A dumb-bellspecimen should be manufactured in accordance with BS 2782 : Part 3(1996) with a cross-section of 10 mm by 3mm. The specimens should be castin polytetrafluoroethylene-lined moulds. Adhesive ductility may be meas-ured with appropriate strain monitoring equipment.

9.6.3 Concrete

The suitability of the preparation of the concrete surface should be demon-strated by a ‘pull-off’ test using the adhesive to be used in the bonding of theplates. The test should be deemed a failure unless the failure plane is withinthe concrete.

The bond of the plate to the concrete may be demonstrated after bondingby suitable tests such as coin tap, ultrasonic pulse velocity or thermography.

9.6.4 Trial panels and procedure trials

A trial panel and procedure trial for approval by the engineer should beprovided for the strengthening with a plate of the same width and thickness

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266 Strengthening of reinforced concrete structures

and incorporating the same fixing details as those designed for the structure.This should demonstrate the following:

• preparation of the plate• preparation of the substrate surface• use of spacers• application of adhesive• installation of fixings• finishing of installation• compliance with quality control items

9.7 In-service inspection and maintenance

9.7.1 Manual of procedures

A manual of procedures for inspection and maintenance for the strength-ened structure should be prepared for the maintaining authority. A typicalsummary of inspection procedures is given below but this should not beregarded as exhaustive. A draft list prepared during design can be extendedduring construction should it become necessary.

It is suggested that inspections should take place every six months for atleast two years after completion of the work. The frequency of further in-spection should be agreed with the maintaining authority after a review ofthe inspection reports.

The inspection procedure manual should include all relevant technicalliterature relating to products used in the work. Photographs of criticaldetails and maps of cracking are also useful. Procedures to be adopted ifdeficiencies occur should also be clearly defined.

The deck soffit should be checked to ensure:

• There is no visible evidence of sealed cracks in the concrete opening.• There is no visible evidence of the plates debonding.• There is no audible evidence of the plates debonding by tapping the

plates with a coin or suitable light hammer.• The plate fixing bolts are not loose.

The top surface of the deck should be checked to ensure:

• There is no visible evidence of the road surfacing cracking or deforming.• There is no visible evidence of any cracks in the edge parapet detail

increasing in width or new cracks forming.

Hammer tapping must not damage the system or any nuts slackened onthe fixing system.

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Design and specifications for FRP plate bonding of beams 267

9.8 ReferencesASTM D3163 (1992) Standard Test Method for Determining Strength of Adhesively

Bonded Rigid Plastic Lap-Shear Joints in Shear by Tension Loading, ASTMPhiladelphia 15.06.

ASTM D3039/3039M (15.03) (1995) Test Method for Tensile Properties of PolymerMatrix Composite Materials, ASTM Philadelphia.

ASTM D3762 (1979) Standard test method for adhesive bonded surface durabilityof aluminum (Wedge test), ASTM, Philadelphia.

BS 1881 : Part 207 (1992) Recommendations for the Assessment of Concrete Strengthby Near-to Surface Tests, British Standards Institution, London.

BS 5400 : Part 4 (1990) Code of Practice for Design of Concrete Bridges – Steel,Concrete and Composite Bridges, British Standards Institution, London.

BS 5400 : Part 2 (1990) Specification for Loads, British Standards Institution,London.

BS 5350 : Part B4 (1993) Determination of Pot Life – Method of Test for Adhesives,British Standards Institution, London.

BS 2782 : Part 3 (1996) Mechanical Properties 1976, Method 320 B/C, TensileStrength, Elongation and Elastic-Modulus Method of Testing Plastics, BritishStandards Institution, London.

BS 2782 : Part 8 (1994) Other Properties 1980, Method 835C, Determination ofGelation Time of Polyester and Epoxide Resins using a Gel Time, BritishStandards Institution, London.

BS 6319 : Part 8 (1984) Method for the Assessment of Resistance to Liquids, BritishStandards Institution, London.

Chajes M J, Finch W W, Januszka T F and Thomson T A (1993), ‘Bond and forcetransfer of composite material plates bonded to concrete’, ACI J 93(2) 208–217.

CRAG (1988) Test Methods for the Measurement of the Engineering Properties ofFibre Reinforced Plastics, ed. P T Curtis.

EUROCOMP (1996) Structural Design of Polymer Composites – EUROCOMP –Design Code and Handbook, ed. J L Clarke, E & FN Spon, London.

Highways Agency (1994) Strengthening of Concrete Highway Structures Using Ex-ternally Bonded Plates, BA 30/94.

ISO 75 (1987) Plastic and Ebonite – Determination of Temperature of Deflectionunder Load.

Jansze W (1995) ‘Anchorage of externally bonded plates under static flexural load-ing’, Progr Concrete Res 4, February.

Jansze W and Walraven J C (1996) ‘Anchorage of externally bonded steel plates’,International Concrete Congress, In Service of Mankind, Dundee, Scotland, 24–28June 1996.

Jones R, Swamy R N and Charif A (1988) ‘Plate separation and anchorage ofreinforced concrete beams strengthened by epoxy-bonded steel plates’, The StructEng 66(5/1) 85–94.

Jones R, Swamy R N and Charif A (1989) ‘Plate separation and anchorage ofreinforced concrete beams strengthened by epoxy-bonded steel plates’, The StructEng 67(13/4) 250.

Kaiser H (1989) Strengthening of Reinforced Concrete with Epoxy-Bonded Carbon-Fiber Plastics, Thesis submitted to ETH, Zurich, Switzerland, in partial fulfilmentof the requirements for the degree of Doctor of Philosophy (in German).

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Meier U (1987) ‘Bridge repair with high performance composite materials’, J MaterTechnik 4 125–128.

Meier U and Kaiser H (1991) ‘Strengthening of structures with CFRP laminates’,Advanced Composites Materials in Civil Engineering Structures Proceedings, MTDiv/ASCE/Las Vegas, Jan. 31 1991.

Neubauer U and Rostasy F S (1997) ‘Design aspects of concrete structures strength-ened with externally bonded CFRP-plates’, Structural Faults & Repair – 97,Edinburgh, July 1997, Vol 2, pp 109–118.

Oehlers D J and Ahmed M (1996) ‘Upgrading reinforced concrete beams by boltingsteel side plates’, Bridge Management 3, Inspection, Maintenance and Repair,Guildford, 1996, E&FN Spon, pp 412–419.

Poulsen E (1996) Anchorage of Sika“ CarboDur Laminates Bonded to Concrete,CEN TC104 SC8 WG3 meeting in Paris 5/6 Dec 1996.

prEN 1542 Products and Systems for the Protection and Repair of Concrete Struc-tures – Test Methods, the Pull-off Test, Awaiting German translation beforeratification, not available publicly.

Ranish (1982) Zur Tragfähigkeit von Verklebungen zwischen Baustahl uns Beton –geklebte Bewehrung, Institute for Construction Materials, Massive type Construc-tion and Fire Protection, Technical University Braunschweig, Issue 54.

Raoof M and Zhang S (1996) ‘Peeling failure of reinforced concrete beams withexternally bonded steel plates’, Bridge Management 3, Inspection, Maintenanceand Repair, Guildford, 1996, E&FN Spon, pp 420–428.

Regan P (1993) ‘Research on shear: a benefit to humanity or a waste of time?’ TheStruct Eng 71(19) 337–346.

Roberts T M (1989) ‘Approximate analysis of shear and normal stress concentra-tions in the adhesive layer of plated RC beams’, The Struct Eng 67(12/20) 229–233.

Rostasy F S (1993) ‘Strengthening of R/C and P/C structures with bonded steel andFRP plates’, Proc. 5th Int. Conf. On Structural Faults and Repair 1993, Vol 2, pp217–224.

Swamy R N, Jones R and Charif A (1989) ‘The effect of external plate reinforce-ment on the strengthening of structurally damaged RC beams’, The Struct Eng67(3/7) 45–56.

Täljsten B (1994) Strengthening of Existing Concrete Structures with Epoxy BondedPlates of Steel or Fibre Reinforced Plastics, Doctoral Thesis, Luleå University ofTechnology, Division of Structural Engineering, Sweden. August 1994.

Tausky (1993) Betontragwerke mit Aussenbewehrung, Birkhäuser Verlag.Taylor H P J (1974) ‘The fundamental behaviour of reinforced concrete beams in

bending and shear’, Proc. ACI-ASCE Shear Symposium, Ottawa, 1973 (ACIPublication SP42-3) American Concrete Institute, Detroit, 1974, pp 43–77.

Triantafillou T C and Plevris N (1992) ‘Strengthening of RC beams with epoxy-bonded fibre-composite materials’, Mater Struct 25 201–211.

Wu Y S and Gibson A G (1994) ‘Composites offshore: applications and opportuni-ties’, Composites 94, Birmingham 22/23 November 1994, British Plastics Federa-tion Publication No 293/10 pp 45–56.

Zhang S, Raoof M & Wood L A (1995) ‘Prediction of peeling failure of reinforcedconcrete beams with externally bonded steel plates’, Proc Inst Civ Eng StructBuilding 110(Aug) 257–268.

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9.9 Bibliography

BD2 (DMRB 1.1) Technical Approval of Highway Structures on Motorways andOther Trunk Roads.

BA 35 (DMRB 3.3) The Inspection and Repair of Concrete Highway Structures.SB 1 (DMRB) (Scotland) The Inspection of Highway Structures.DB 24 (DMRB 3.1) Design of Concrete Highway Bridges and Structures, Use of

BS5400 : Part 4: 1990.DB 37 (DMRB 1.3) Loads for Highway Bridges.BD 21 (DMRB 3.4) The Assessment of Highway Bridges and Structures.

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Table of Contents9. Design and Specifications for FRP Plate Bonding of Beams 9.1 Introduction 9.2 Practical Design Rules and Guidelines 9.3 Application of the Technique 9.4 Materials 9.5 Workmanship 9.6 Quality Control 9.7 In-Service Inspection and Maintenance 9.8 References 9.9 Bibliography

Index