An experimental study of the failure modes of reinforced concrete beams strengthened with prestressed carbon composite plates

PII: S1359-8368(97)00043-7

ELSEVIER

Composites Part B 29B (1998) 411-424 © 1998 Published by Elsevier Science Limited Printed in Great Britain. All rights reserved 1359-8368/98/$19.00

An experimental study of the failure modes of reinforced concrete beams strengthened with prestressed carbon composite plates H.N. Gardena't and L.C. Hollaway a'*

aComposite Structures Research Unit, Department of Civil Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK (Received 6 February 1997; accepted 15 September 1997) Concrete structures deteriorate for various reasons and upgrading has been achieved for over 20 years by bonding steel plates using epoxy resins. Disadvantages of this method include transporting, handling and installing heavy plates and corrosion of the plates. The use of composite materials overcomes these problems and provides equally satisfactory solutions. The rehabilitation of concrete structures represents a large demand for efficient strengthening methods and composite materials are well suited to this application. Further advantages are gained by prestressing the plate before bonding to the concrete. The benefits of external prestressing using polymeric composite materials have been investigated only relatively recently and further work in this field is needed in order to understand the behaviour of members prestressed with composite materials, thereby allowing full advantage to be gained from the ease with which composites can be handled and applied, and from their excellent duarability. This artcile is concerned specifically with the failure modes of reinforced concrete beams prestressed in this way. Reinforced concrete beams of 1.0 and 4.5 m lengths were tested in four point bending after strengthening them with externally bonded carbon fibre reinforced polymer plates. The plates were bonded without prestress and with prestress levels ranging from 25% to 50% of the plate strength. The non-prestressed beams failed by separation of the plate from the beam, associated with concrete fracture in the cover to the internal rebars, while most of the prestressed beams failed by plate fracture. The plate prestress prevented cracking of the adhesive layer, a phenomenon associated with shear cracking in the concrete. The bonded plates failed progressively by longitudinal splitting and interlaminar fracture, rather than suddenly without warning. Under a shear span-beam depth ratio of 3.40, plate separation was initiated by a shear displacement in the concrete: a high prestress was required to enable the ultimate plate strain to be reached before the shear displacement reached its critical value. © 1998 Published by Elsevier Science Limited. All rights reserved (Keywords: reinforced concrete; carbon fibre reinforced polymer (CFRP); external prestressing; adhesive)

NOMENCLATURE e Mself

P

eccentricity of external prestress from centroid of section bending moment due to self weight of beam magnitude of external prestress force

Specimen identity notation Beams are numbered in ascending order in each table. Specimens without plate prestress are given the subscript, U, such as beam 2u, 1.0 m Specimens with plate prestress are given the subscript, P, such as beam 4p, 1.0 m The size of beam tested, and the number of rebars in the 4.5 m beams, are indicated as subscripts following the prestress designation, such as beam 2p, 4.5 m, 2 ba~s *Corresponding author. Tel: +44 (0)1483 259280; Fax: +44 (0)1483 450984; eEmaih [email protected] tCurrently at Taywood Engineering, Ltd., Southall, Middlesex.

INTRODUCTION Concrete structures deteriorate for reasons including internal reinforcement corrosion, freeze-thaw action, excessive loading and poor initial design. Many bridges and other structures are no longer considered satisfactory in terms of load carrying capacity for these reasons or changes in the loading specifications of design codes. In order to maintain efficient highway networks and to keep buildings operational, older structures must be upgraded so that they meet the same requirements demanded of structures built today and in the future. It is becoming both environmentally and economically preferable to upgrade structures rather than to rebuild them, particularly if rapid, effective and simple strengthening methods are available. The choice between upgrading and rebuilding is based on factors specific to each individual case, but certain issues are considered in every case. These are the length of time during which the structure will be out of service or providing a reduced service, relative

411

Failure modes of reinforced concrete beams : H. N. Garden and L. C. Hollaway

P - A I ~ ' Centr°ilalaxisJ " ' g J - - P ..........~ i

.................................... J ~ ...............Miiff ...................................../~'Iself

Prestress centteline

a. Axial force

b. Bending moment

c. Self weight

Combinationof forcesand momentsestablishedby prestressing

costs upgrading and rebuilding in terms of labour, materials and plant, and disruption of other facilities. The need to upgrade bridges, for example, is a worldwide one which places considerable importance on strengthening techniques 1. Bonding of steel plates to the tension faces of structural members has shown to be a successful technique; the first reported application in the UK was the strengthening of bridges at the M5 Quinton Interchange (Worcestershire, England) in 19752. In this particular application, four bridges were strengthened after cracking was noticed during a routine inspection3. Disadvantages in the use of steel plates include transporting, handling and installing heavy plates, corrosion of plates, limited delivery lengths of plates which necessitate the work and difficulty of forming joints, the need for massive and expensive falsework to hold plates in position during adhesive cure, and the need to prepare the steel surface for bonding (this being labour intensive and time consuming). Fibre reinforced polymer (FRP) materials, however, possess the qualities of high strength-to-weight ratio and corrosion resistance, resulting in low maintenance costs. FRP materials have mechanical and physical properties superior to those of steel, particularly with respect to tensile and fatigue strengths, and these qualities are observed under a wide range of temperatures. The prime material types which find uses as reinforcing fibres in FRPs are glass, carbon and aramid4. The main disadvantages in using these materials are high material cost and brittle failure modes. Carbon fibre reinforced polymer (CFRP) materials are around ten times more expensive than mild steel but material cost usually constitutes around 20% of the total cost of a strengthening project5, the remaining 80% being labour. The easy handling of FRP plates reduces labour costs considerably. The problem of having to join limited lengths of steel plate is overcome by the fact that FRP plates may be delivered to site in rolls of 300 m or more 6. Therefore, the use of polymeric composite plates to replace steel presents significant benefits. Further advantages are gained by prestressing the plates before bonding and it is these with which this paper is concerned. The benefits of external prestressing using polymeric composite materials were investigated only relatively recently. Further work in this field is needed in order to understand the behaviour of members prestressed with composite materials, thereby allowing full advantage to be gained from the ease

412

with which composites can be handled and applied and from their excellent durability. This article is concerned specifically with the failure modes of reinforced concrete beams prestressed in this way. The subject is introduced before the current state of knowledge in the field is assessed by a review of previous studies; the failure modes of beams with and without plate prestress are then described and compared. The article is the first of two, the second considering the deformation responses and load carrying capacities of the beams 7.

EXTERNAL PRESTRESSING The principles of conventional prestressed concrete apply also to extemally prestressed members, in which the prestressing force acts outside the concrete, as shown in Figure 1. The effect of the prestress is to apply an axial load [Figure l(a)] and a bending moment [Figure l(b)] that opposes the self weight of the beam [Figure l(c)]. The external post-tensioning of structures has been practised for several years 8. This method involves the tensioning of high strength cables by reacting against the structure to be reinforced. An improvement in the range of elastic behaviour, ultimate capacity and fracture behaviour are benefits gained from this technique; a reduction in structural material weight and improved fatigue behaviour also result9. However, various problems are associated with the method, both at the time of construction and during the service life. These include the need to maintain lateral stability of girders during post-tensioning and the need to protect the cables against corrosion 1°. External prestressing can be applied to new and existing structures. The fatigue behaviour of concrete members is improved by this technique because prestressing the tension flange of a beam reduces the tensile component of the stress cycle it, thereby delaying or preventing fatigue crack initiation and growth in steel girders or reinforcing bars in concrete. A significant advantage is gained by prestressing in segmental construction 12. This is caused by the compressive strains generated by the prestress at the joints between segmental units, these locations experiencing high tensile strain in the absence of prestress. This high tensile strain is associated with high compressive strain at the top of a segmentally constructed member, resulting in failure by concrete crushing. Depending on the prestress magnitude,

Failure modes o f reinforced concrete beams : H. N. Garden and L. C. Hollaway the likelihood of crushing at joints is either reduced or eliminated. The present authors confirmed the high strains that occur at segmentally joined units by load testing a plated beam in which a saw cut was introduced at midspan 13.

PREVIOUS WORK WITH PRESTRESSED COMPOSITE PLATES

The rehabilitation of concrete structures represents a large demand for efficient strengthening methods, though the external prestressing of concrete beams using composite materials has been studied only relatively recently. Beam strengthening using non-prestressed bonded composite plates has been studied far more widely than the use of prestressed plates. The use of CFRP plates for the external reinforcement of concrete beams was pioneered at the Swiss Federal Laboratories for Materials Testing and Research (EMPA). External prestressing using composite plates was also introduced at EMPA 14-17 and was noted to be a more economical alternative to conventional prestressing methods used in new construction TM. Meier et al. 19 reviewed work undertaken at EMPA on the use of both nonprestressed and prestressed plates, including a prestressed beam that had been subjected to 30 million cycles of fatigue loading without any evidence of damage to either the concrete or the plate. This particular plate was prestressed to 50% of its strength so the mean stress level in the cyclic loading was high. Meier 6'2° recalled some of the conclusions drawn in the study undertaken by Deuring 15 concerning the use of prestressed CFRP plates. It was noted that CFRP materials exhibit no plastic deformation so the greatest flexural resistance of a strengthened section is reached when the plate fractures in tension, either after or at the same moment as internal steel yield, but before compressive failure of the concrete. The beams tested by Deuring 15 failed largely by peeling of the plate from the concrete, caused by the vertical relative displacement between the two halves of a shear crack. The compression transferred into the concrete by the plate tension either delays or prevents this type of failure. The failure mode is influenced by the cross sectional area of the plate and the prestressing force. The sum of the crack widths is reduced more with prestressed plates than with non-prestressed plates 6'2°. Saadatmanesh and Ehsani 21 conducted an experimental study of the strengthening of reinforced concrete beams using non-prestressed and prestressed GFRP plates. One of the two prestressed beams contained a relatively small amount of internal tensile steel reinforcement, while the other contained larger bars and was pre-cracked prior to bonding of the plate. The plate prestress in the pre-craked case closed some of the cracks, indicating the benefit of prestressing from a servicability point of view. Triantafillou et al. 22 tested reinforced concrete beams in three point bending with various quantities of internal reinforcement and magnitudes of CFRP plate prestress. Improved control of concrete cracking was brought about

not only by a greater internal reinforcement provision, but also by higher plate prestress, indicating the serviceability advantage gained by prestressing the composite. It was noted that prestressed composite plates can potentially act as the sole tensile reinforcement in new concrete construction, and prefabrication is also possible because of the simplicity with which composites may be handled and applied. The confinement imposed by the initial compressive stress at the base of the beam may improve the shear resistance of concrete beams. Also, an advantage from a cost point of view is that the same strengthening to failure may be achieved with a prestressed plate of relatively small cross-section, as that achieved with a larger non-prestressed plate 22. The particular anchorage method used at the ends of the plates was to continue the plates under the beam supports. The present study to be discussed in this article includes this system but shows that bolted systems also remain intact up to and after the collapse of the externally prestressed beam. Char et al. 1° conducted an analytical parameter study to determine the effects of varying the cross sectional area and material type of the composite plate and the prestress in the plate. The parameter study revealed that prestressing a GFRP plate would not necessarily increase the ultimate moment capacity over that of a beam with a non-prestressed plate, for the particular beam configuarion and prestress level considered. This was because both the non-prestressed and prestressed beams failed by plate fracture. The present study reveals that prestressing may, in practice, increase the ultimate capacity of a beam but the magnitude of the increase depends on the similarity or difference between the failure modes of the beams with and without prestress. The failure mode of the prestressed beam depends on the prestress magnitude. Wight et al. 23 reported encouraging data on the strengthening and stiffening achieved with prestressed CFRP plates. The control of concrete crack widths and numbers of cracks was improved by prestressing the plates. The beam with a non-prestressed plate failed by concrete fracture in the cover thickness within one of the shear spans of the four point loaded beam, whereas the prestressed concrete near the beam soffit, caused by the plate prestress, was sufficient to reduce the magnitudes of vertical displacements across shear cracks and to transfer failure into the plate, a finding also recorded in the present study. The avoidance of concrete failure in the shear spans was associated with a much improved ultimate load.

EXPERIMENTAL WORK Material characterization

The component materials of the plate-strengthened beams were characterized before beam systems were examined under load. Characterization provided material properties for analytical and numerical work not reported here, and determined the ultimate strain of the composite material for comparison with beam test data in reference7.

413

Failure modes of reinforced concrete beams : H. IV. Garden and L. C. Hollaway Table 3

Table 1 Concrete material properties

CFRP reinforcement properties

Beam size (m)

28-day strength (MPa)

56-day modulus (GPa)

56-day tensile strength (MPa)

Beam size (m)

Strength (MPa)

Modulus of elasticity (GPa)

Ultimate strain (microstrain)

1.0 4.5

54 47

35 35

2.5 2.6

1.0 4.5

1414 1284

111 115

12343 10770

Table 2 Steel bar reinforcement properties Beam size (m)

First yield stress (MPa)

Maximum stress (MPa)

Modulus of elasticity (GPa)

1.0 4.5

350 556

436 639

215 220

Concrete. Concrete mixes were prepared for 1.0 and 4.5 m beams. The concrete from which the 1.0 m beams were made contained ordinary portland cement with the material proportions free water/cement = 0.4 and cement/coarse aggregate/fine aggregate = 1.0/1.1/1.9. The coarse aggregate was of 10 mm maximum size. The water/cement ratio of the 4.5 m beams was also 0.4 and the cement/coarse aggregate/fine aggregate ratios were 1.0/2.7/1.9. The coarse aggregate in the 4.5 m beams was graded in sizes of 5 - 1 0 mm and 10-20 mm and their material proportions (relative to the cement) were 1.1 and 1.6, respectively. Standard cube crushing tests 24'25 were undertaken to find 28 day compressive strengths, the results of which are listed in Table 1 for the two batches of concrete used. Standard tests of rectangular blocks 26 were undertaken to find the compressive moduli of elasticity of the concrete at an age of 56 days, as shown in Table 1. Necked rectangular specimens were tested at an age of 56 days to find the uniaxial tensile strengths presented in the table. Each result shown is the average of three. After manufacture, curing conditions commonly found on site were simulated by placing the beams under polythene until testing, having demoulded at an age of 24 h. Internal steel reinforcement.

For the 1.0 m beams, three steel bars (similar to those used as internal reinforcement in the beams) were tested in tension in an "Instron" loading machine. The bars were instrumented with electrical resistance strain gauges and the properties listed in Table 2 were obtained. Tensile testing in an "Amsler" loading machine revealed the properties shown for the high tensile bars used in the 4.5 m beams.

Carbon fibre composite plates. The CFRP plates bonded to the 1.0 m beams were made from prepregs manufactured by Cytec ( " C Y C O M " 919HF-42%-HS-135-460 (P/N 02098)) and contained 58% weight fraction of high strength fibres (Toray T300) and 42% epoxy resin. The composite plate material used for the 4.5 m beams was a pultrusion manufactured by Techbuild Composites Ltd. and contained weight fractions of 67% and 33% for the fibres and vinylester resin, respectively. For characterization testing, plates were cut into strips 25 mm wide and 300 mm long and instrumented with electrical resistance strain gauges for

414

tensile testing in an "Instron" tensile loading machine fitted with wedge jaws. The crosshead movement was set at 0.5 mm min -1, though testing at other rates indicated no effect on mechanical properties. Table 3 lists the characteristics of the CFRP materials used. The strengths are quoted as the stresses at first fracture of the composite material. Further fractures occured after the first damage and more load was carried in some cases. The electrical resistance strain gauges bonded longitudinally to the coupon specimens tended to fail before the first onset of specimen damage. Strain gauge data were no longer recorded beyond a stress of, typically, around 80% of the stress at first fracture. For this reason, the ultimate strains shown in Table 3 were calculated from the stress at first fracture and the modulus of elasticity determined in each test. This calculation assumes the behaviour of the composite material remains linear to first fracture. The material behaviour was linear up to the last strain gauge reading, which was taken at a high proportion of the first fracture stress, so this assumption is reasonable. Fracture of the coupons occurred by a combination of longitudinal splitting throughout the specimen width and intedaminar fracture. Longitudinal splitting began along the edges of the specimens.

Adhesive.

The adhesive used throughout this work was Sikadur 31 PBA (plate bonding adhesive), a two-part cold-curing epoxy resin adhesive. A uniform mid-grey colour indicates adequate mixing of the white silica-filled resin and black hardener. Although data sheets with mechanical properties of the adhesive were provided, bulk tests were carded out to determine material properties after a period of de-airing; published values are for the material as applied on site (i.e. with air voids). Uniaxial tensile tests were conducted but the strengths of bonded joints made with the adhesive were not investigated; tests were undertaken at room temperature. Test pieces derived from BS 278227 were used and the tests were conducted to this standard. To ensure measurement of the tensile properties of the pure adhesive and to encourage constistency in the manufacture of the test specimens, it was important to expel as much air from the material as possible during both the resin-hardener mixing and adhesive casting stages. The resin and hardener were mixed in the ratio 3:1 (by mass) of resin to hardener in a vacuum chamber. An "Instron" universal testing machine with wedge jaws was used for specimen loading and a clip extensometer was attached centrally to the necked length of each specimen. The average tensile strength and modulus (of 12 results) were 31 MPa and 12GPa, respectively. Extension rates of 0.1 mm min -1 and 2.0 mm min -1 were applied but no consistant effects on mechanical properties resulting from test speed were observed.

Failure modes o f reinforced concrete beams • H. N. Garden and L. C. Hollaway 100ram

-I I

- 10mm

- 15ram

15ram--

• R3 link.~ @ 51ram oenll~

IId

LI i

2mm adhesive

0.8mm CFRP

!

\

3 no. R6 main bars

~T10mm

I I1[11|

I-

,I 65mm

Reinforcement layout of l.Om beams 145mm

_fl 5mm

,I J_

~2 no. R 8 t o p b~rsC

• R 6 1 i n k s @ 150mmc~atres [

-20mm

20ram

2ram adhesive

2 o r 3nn. T12

n~n bars

~3mm CFRP

{

Tll3mm P

90ram

Reinforcement layout of 4.Sm beams Figure 2

Internal and external reinforcement of beams (not to scale)

Beam testing Introduction and objectives. The main objective of the study was to compare the failure modes of plated beams with and without prestress. Reinforced concrete beams of 1.0 and 4.5 m lengths were tested in four point bending; this loading configuration allowed the effects of both shear and bending loads to be observed simultaneously. The beams were underreinforced before plating to encourage flexural failure and the achievement of full bending capacity; the steel area ratio (based on effective depth) was not greater than 1% in any of the beams. Load spacings (or constant moment regions) of 220 and 1250 mm were used in the 1.0 and 4.5 m beams, respectively, to achieve shear spaneffective depth ratios of 4.05 and 7.44, respectively. The dimensions of the composite plates were 0.8 × 67 mm for the 1.0 m beams and 1.3 × 90 mm for the 4.5 m beams. All beams were strengthened with the bonded CFRP plates described before, the plates having been prestressed to different levels before bonding to the concrete. The internal reinforcements in the two sizes of beam are shown in Figure 2. Each shear link in the 1.0 m beams consisted of two separate R3 links joined together to achieve a sufficient area of steel to eliminate shear failures in both the plated and unplated beams. The 4.5 m beams were reinforced internally with either two or three T I 2 tensile bars or so that different levels of beam ductility could be encouraged.

Surface preparation and bonding procedure.

The preparation of adhered surfaces before adhesive application was necessary throughout the work. For the concrete, this was achieved by gritblasting beam soffits using "Guyson"

angular chilled iron grit (grade 07 "Turbobead ''28) of 0.18 mm nominal particle size, delivered under a pressure of 80 psi. The nozzle of the gritblaster was passed back and forth until uniform exposure of aggregate was achieved. CFRP surfaces were roughened for bonding by carefully removing a pre-attached peel ply, a technique first employed at Oxford Brookes University 29 for composite plate bonding. The adhesive was applied by palette knife to both the concrete and composite to ensure there was sufficient excess to avoid a starved joint and to prevent the formation of air bubbles by the spread of adhesive from one surface to the other. Ballotini (glass spheres) was used to achieve the desired glueline thickness of 2 mm. Bonding pressure was applied by raising the beams on scissor lifts up to the desired level underneath the prestressed plates and reacting against the force provided by the tendency of the plates not to deform out of plane under load; the beams were raised until adhesive was no longer seen to be displaced from between the plate and beam in each case.

Application of plate prestress.

Figure 3 shows the frame used for the application of prestress to the composite plates. In each case the plate was initially positioned with its bonding surface uppermost and a small amount of prestress was applied so that adhesive could be spread onto the plate. Having turned the plate again, so that the bonding surface now faced the beam, the remaining prestress was applied. Prestress load was monitored on the load cell connected in series with the plate. Having turned the plate again, so that the bonding surface now faced the beam, the remaining prestress was applied. Prestress load was monitored on the load cell connected in series with the plate. Electrical

415

Failure modes of reinforced concrete beams : H. N. Garden and L. C. Hollaway

Hydraulic jack

Prestressing frame

,

....................................

/

Composite plate

Adhesive

........................................................

Supporting bed

Plate end

Supporting thread

Load cell

\ I Concrete h e ~

ooupling Figure 3 Frame used to apply prestress to composite plates

CI~ plat/

Bearingpad~.

Adhesive-filled bore

Support roller

a: Plate ends under supports pae

AnchOrage

\

block

Mild steel bolt

b: Plate ends bolted to beam Figure 4 Details of end anchorages for composite plates

resistance strain gauges were bonded to the plate on the face opposite the bonding surface. Prestress load was applied by a hydraulic jack located in series with the plate (Figure 3). The tensile load to be applied to the plate in each case was determined by multiplying the nominal prestress percentage by the ultimate load of the plate. The latter was based on the strengths found in the material characterization tests. Upon completion of the prestressing stage and its associated anchorage installation (described in the next section), the prestress was transferred into the concrete by releasing the pressure in the pump used to operate the jack. The length of composite material extending beyond the bond to the concrete was then cut free and the beam was removed from the prestressing frame. In addition to prestressing the plate, beam 6p, 1.0 m was subjected to an axial precompression load of equal nominal magnitude to the prestress tension in the plate. This was to simulate the prestressing of the composite plate by reacting against the beam, in a similar fashion to conventional post-tensioning with steel tendons. Strain readings taken on the concrete beam during the prestressing operation indicated that the compressive strain was of very low magnitude so the precompression of beams was discontinued after this trial. Plate end anchorage systems used. Triantafillou and Deskovic 3° and Karam 31 analysed the short-term mechanical behaviour of externally prestressed beams, concluding that failure in the beam may occur at the ends of the plate when the prestress is transferred into the concrete if the prestress is too high. This necessitates the installation of a plate end anchorage system to counteract the stresses that

416

tend to cause fracture in the concrete. Meier et al. ]9 reported that this fracture occcurs due to the development of high shear stresses in the concrete near the level of the bonded plate, and shear failures at the plate ends may occur under a plate prestress of over 5% of the strength of the plate material. However, prestressing levels of as much as 50% of the plate strength may be required for a viable strengthening system 19. Therefore, the presence of plate end anchorage is clearly very important in beams externally prestressed with composite plates. For this reason, all prestressed beams in the present study were provided with anchorage to avoid plate end concrete fracture before the beams were load tested. The anchorage also avoided concrete fracture at the ends of the plates during the load tests. The need for plate end anchorage also arises during the load testing of beams strengthened with initially unstressed plates; this was demonstrated by the authors in a previous investigation of anchorage 3a'33. The anchorages adopted in the present study were either the continuation of plates under the supports of beams or the installation of steel bolts through the composite plate and into the concrete. The former method (Figure 4), which was used in the load tests of the 1.0 m beams only, generated a normal force against the plate equal to the shear force acting on the beam at any applied load. This technique provides the greatest anchorage force possibe, without the use of an externally fitted anchorage system. The bolted system did not provide an external normal force but plate end concrete failure was prevented by the presence of an anchorage block bonded to the CFRP and secured in position by the bolts, as shown in Figure 4. Since the 1.0 m beam tests were

Failure modes o f reinforced concrete beams • H. N. Garden and L. C. Hollaway Table 4

Prestress levels, ultimate capacities and failure modes of the beams

Beam 1.0 m beams Unplated 1 u,10 m 2 u,10 m 3 u,10 m 4p,10

m

5 P,l~0m 6p,10 m 4.5 m beams Unplated, 2 bars Unplated, 3 bars 1 u,4.5 m.2 bars 2p,4.5 m,2 bars 3 P,4.5 m,3 bars 4p,4.5 m,3 bars 5 P,4.5 m,3 bars

Anchor method . -None Supports Bolts Supports Bolts Supports . --None Bolts Bolts Bolts Bolts

Nominal prestress(%) .

.

. --0 25 25 40 50

Maximum load (kN)

Failure mode

-0.0 0.0 0.0 23.5 23.7 47.2

14.85 34.00 49.65 45.50 49.60 43.65 50.50

Steel yield followed by concrete crushing Plate separation at base of shear crack (Figure 8) Plate separation at base of shear crack (Figure 8) Plate separation at base of shear crack (Figure 8) Plate separation at base of shear crack (Figure 8) Shear failure outside plated length at end Tensile fracture of composite plate

--0.0 26.5 26.2 33.6 46.6

28.5 42.3 60.0 63.8 70.5 72.8 76.5

Steel yield followed by concrete crushing Steel yield followed by concrete crushing Plate sep. from near load point (Figures 5 and 6) Tensile fracture of composite plate Conc. crush followed by plate tensile fracture Tensile fracture of composite plate Tensile fracture of composite plate

.

-0 0 0 25 25 50 .

Actual prestress(%)

m

.

preliminary trials, steel anchorage blocks were used for these specimens, but GFRP blocks were used for the 4.5 m beams. The bores to accomodate the bolts were drilled after the anchorage blocks were bonded to the composite plate. After the anchor bores were drilled, adhesive was placed in the bores as well as being applied to the bolt threads; the bolts were then inserted into the bores. The bolts extended into the concrete to distances of 40 mm in the 1.0 m beams and 70 mm in the 4.5 m beams. The bonded anchorage blocks were intended to have a greater length of bond in front of the bolts than behind; Figure 4 shows this detail. This choice of anchorage detail was adopted in an attempt to reduce the likelihood of splitting of the composite plate, by providing a sufficient bond distance over which load could be transferred into the bolts. The bolted system comprised two bolts through the plate width, located midway between the internal rebars.

Test procedure. The 1.0 m beams were instrumented with the following: five pairs of demec discs, located at 20 mm vertical centres over a gauge length of 100 m at midspan, to allow measurement of bending strains throughout vertical sections; electrical resistance (ER) strain gauges bonded to the plates at plate ends, in the shear spans, at midspan and under point loads; and one displacement transducer positioned on either side of each beam at midspan to monitor deflections. The demec gauge length in the 4.5 m beams was 407 mm and eight pairs of discs were located at 30 mm vertical centres. Strain gauges were located on the plates of the 4.5 m beams also, in positions corresponding to those on the 1.0 m beams. The compressive faces of the beams were also instrumented with strain gauges, in positions adjacent to the loading points. Midspan deflections and plate strains were recorded by a Schlumberger "Orion" datalogger, while demec strains were noted manually at load intervals of 4 and 8 kN for the 1.0 and 4.5 m beams, respectively, until concrete cracking warned of imminent failure. In the 1.0 m beam tests, load was applied in increments of 1 kN by a "Denison" testing machine; the increments were reduced to 0.5 kN as failure approached. Similar increments of load were applied to the 4.5 m beams using an " A v e r y " testing machine.

Results.

The prestress magnitudes and failure modes of the beams are shown in Table 4; the ultimate capacities are also included for reference. The prestress levels are given as proportions of the strength of the composite materials.

The failure mode of the unplated beams. The unplated beams failed in a conventional flexural manner with wide cracks in the concrete throughout the region of uniform bending moment. The data obtained from demec gauge measurements indicated a large upward shift of the neutral axis after yield of the internal steel reinforcement. Compressive failure of the concrete in a shallow layer became greater with increasing deformation of the beams, but no further load was carried. Shear cracks were present throughout the shear spans of the unplated beams, although these cracks were not wide since the beams were sufficiently reinforced in shear. The failure modes of the plated beams without plate prestress. In all plated beams without prestress or plate end anchorage, failure was complete when the bonded composite plate became separated from the concrete beam with a layer of concrete still bonded to the plate. In each case, beam failure occurred due to plate separation in one shear span only. The shear span in which failure initiated in any particular beam is referred to here as the "failed shear span". The separation of the layer of concrete usually extended into the non-failed shear span, though this was a secondary effect caused by the momentum of the separating plate. The thickness of this concrete layer, and the length of plate over which it remains bonded, have been found generally to be variables dependent on the shear spandepth ratio under which beams are loaded 34. A greater number of shear cracks developed through the depth of the concrete in plated beams than in unplated specimens, due to the greater opportunity for shear cracking provided by the higher load carrying capacities of the plated beams. In the non-prestressed beams with plate end anchorage, the concrete at the end of the plate in the failed shear span could not separate from the bulk of the beam, so a sagging length of plate remained in the failed shear span.

417

Failure modes of reinforced concrete beams : H. N. Garden and L. C. Hollaway

Thin layer of s e p m ' ~ d ¢onerete

Figure 5

Exposed plate end

Plate separation in 4.5 m non-prestressed beam

~

To end o f beam

Stage 1: shear crackformation

Stage 2: tributary crackformation

S

r

Profile of Level of internal .--T CFRP plato

Stage 3: relative vertical displacement

Thin

Stage 4: after collapse of beam Figure 6

Progressive failure near loading position in beam 1 u, 4.5 m, 2 bars

The 4.5 m non-prestressed beam (lu, 4.5m, 2 bars) experienced the separation of the whole thickness of cover concrete from the internal rebars over a short length of plate adjacent to crack X in Figure 5. Beyond this short length of exposed rebars, towards the end of the plate, the thickness of separated concrete diminished to a thin layer composed of cement and weak aggregate-cement interfaces. During loading of this beam, the greatest indication of concrete damage was at crack X. The damage observed was the progressive widening of the crack, leading to the isolation of a "triangular" piece of concrete as shown sequentially in Figure 6. The crack in the concrete at this position was the widest of all those in the failed shear span. The crack appeared to be predominantly flexural in nature, though a gradual change of direction towards the loading point was seen. As Figure 6 shows, a tributary crack formed adjacent to the main crack (stage 2), forming a "triangular" shape bounded by the main crack, the tribuatry crack and the soffit of the beam. Widening of the main crack at beam soffit level (stage 3) was associated with the formation of a relative vertical displacement at position A between the two halves of the main crack. This relative displacement became

418

more pronounced with increasing applied load on the beam; its magnitude was around 2 mm shortly before failure. After plate separation (i.e. beam collapse) the cracks bounding the "triangular" piece of concrete, shown shaded in Figure 6, had widened further, leaving this piece of concrete separated from the rest of the beam, but still firmly attached to the internal rebars. As the profile of separated concrete in Figure 6 shows, the thickness of the separated concrete in the cover layer decreased towards the end of the plate. Figure 7 shows the appearance of this failed region through the width of the beam in section S-S. The failure of the non-prestressed 1.0 m beams also occurred by a relative vertical displacement effect, but this was located at approximately half way along the failed shear span, as shown exaggerated in Figure 8. The plate appeared to separate first between positions B and C in the direction BC, before separating further between positions B and A in direction BA. The internal steel rebars were exposed over the length AB and a short distance beyond B, but not over the remainder of length BC. The profile of separated concrete shown in Figure 6, and the void shown in Figure 7, were seen in these 1.0 m beams also.

Failure modes of reinforced concrete beams : H. N. Garden and L. C. Hollaway

1- Internal tensile reber

Rough surface of sofia left behind by separation of thin layer of cono~e Figure 7 Section S-S (Figure 6) through the width of the 4.5 m beam

l

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crack

C

Vertical displacement

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Figure 8 Shear crack displacement in 1.0 m beams

m

Concrete crack propagating at soffit level

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B

CD

To end of beam

E

/

/A Composite plate

Concrete beam Figure 9 Plan on soffit showing propagation of concrete cracking in 4.5 m beam

The width of separated concrete was similar to the plate width throughout the failed shear span in the nonprestressed 1.0 and 4.5 m beams without plate end anchorage, and in the 1.0 m non-prestressed beams with anchorage. However, in the 4.5 m beam, 1 u, 4.5 m, 2 bars, the separated width at the location of the exposed rebars was equal to the beam width, reducing to the plate width at soffit level, as shown in Figure 9. In the 1.0 m beams, however, the width of separated concrete was similar to the plate width over the distance AC in Figure 8. The interface between the adhesive and composite plate was damaged in beams 2u, 1.0 m , 3 u, t.0 m and 1 u, 4.5 m,2bars;the plate separated from the adhesive near the end of the plate in the failed shear span (giving rise to the exposed plate end

shown in Figure 5), with traces of adhesive remaining on the plate. This failure was caused by the propagation of concrete shear cracks through the adhesive and along the plate-adhesive interface towards the plate end, as shown in Figure 10. The cracks penetrated the adhesive after propagating along a level ABC in the concrete; this level lay at approximately 1 mm from the beam soffit. Similar adhesive-composite interfacial damage was found at various other locations throughout the failed and nonfailed shear spans of these beams, although the plate and adhesive became visibly separated in the plate end region only. Plated beams with plate prestress. Beam 4p, 1.0 m failed in a manner similar to that described above for the 1.0 m

419

Failure modes of reinforced concrete beams : H. N. Garden and L. C. Hollaway

Conventional shear crack in concrete beam

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Figure 11 Separationof a layerof concreteaftercompositefracturein beam 6p, a.0m

beams without plate prestress but with plate end anchorage. Adhesive-plate interface cracking occurred in this beam. Beam 5p, L0m was exceptional in the way failure occurred, the mode of failure having affected the recorded maximum load. Rather than failing because of effects arising from bending within the loaded span, the beam failed in shear through its depth at one end, between the end of the bonded plate and the beam support. This is thought to have occurred because two of the tensile rebars were broken at the location during the drilling operation to provide bores for the bolted plate end anchorage. The rebars presumably moved at some stage during manufacture of this beam; the problem was not experienced in other beams. The bending moment at the section through which failure occurred was very small because the affected position was close to a beam support and shear was the predominant load. Final failure occurred by widening of the crack in the concrete with an associated rise in the rate of change of beam deflection (with respect to applied load), leading to the eventual explosive fracture of the single intact rebar. The recorded failure load was relatively low in comparison with that of the beam 4p, 1.0 m" The failure mechanism observed for beam 5p,1.0 m did not become apparent until loads near ultimate were reached. Concrete cracking and rebar yield occurred first in the constant moment region of the beam in the absence of beam end failure development, so data relating to cracking and yield are considered to be valid. Beam 6p, 1.0 m failed by plate fracture before compressive or soffit level concrete damage could occur. The fracture occurred by strands of material separating from the bulk of the plate, the total number of separated strands having

420

increased with rising applied load on the beam. The first fracture was at midspan, followed by fracture under the point load positions. Once approximately one third of the cross sectional area of the plate had fractured progressively, the remainder failed as a single unit, leaving two lengths of plate bonded to the concrete, one in each shear span as shown in Figure 11. The concrete cover to the internal steel separated from the steel in the constant moment region; the thickness of separated concrete decreased throughout the shear spans towards the support pad of the beam, as shown by the hidden detail in Figure 11. The plate came to rest in a permanent curved shape, presumably caused by some of the prestress remaining in the composite plate; the prestress is unlikely to have been lost completely because the plate remained bonded to a layer of concrete, as Figure 10 shows. Longitudinal splitting occurred along several lines throughout the width of the plate of beam 6p, 1.0 m. Longitudinal fracture of the adhesive and the separated concrete cover was observed along these lines. Inspection of the composite plate, after the beam test, revealed delamination of the CFRP throughout its thickness. Beam 2p, 4.5 m, 2 bars failed by fracture of the composite plate; compressive failure of the concrete did not occur. The plate fractured into many narrow strands in the constant moment region, while fewer and wider strands were found in the shear spans. Plate fracture began shortly before the maximum load was reached and occurred by the progressive failure of strands of material in the constant moment region of the beam. The concrete cover to the internal rebars did not separate from the bulk of the beam in this case, although the adhesive layer peeled away from the soffit of the beam,

Failure modes of reinforced concrete beams : H. IV. Garden and L. C. Hollaway

A

I

Surfaceconcrete

Fracturedcomposite

i

Figure 12

B/

Adhesivelayer

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Crackingof the adhesivelayerin beam 2p.4.5m,2bars

as shown in Figure 12. This Figure shows that the adhesive layer in the constant moment region was fractured, thought to have been caused by the separation of the plate when composite tensile fracture occurred at position P, for example. The adhesive cracks represented in Figure 10 did not form in beam 2p, 4.5 m, 2 bars, though adhesive cracks were seen in line with some of the flexural cracks in the concrete. The cracking pattern depicted in Figure 6 occurred near a loading position in one of the shear spans shortly before beam collapse, though these cracks did not appear to influence the final failure mode of the beam and were narrower than in the non-prestressed beam, 1 u, 4.5 m,2 bars.No damage to the GFRP plate end anchorages was found. The composite plate experienced interlaminar fracture, as shown in Figure 12, and this extended along the plate as far as the plate end anchorages but did not damage the bond between the GFRP anchorage block and the CFRP plate. Beam 3p. 4.5 m, 3 bars also experienced plate fracture, but compressive failure of the concrete occurred first. The composite plate again fractured into relatively narrow widths in the constant moment region and wider strands in the shear spans. Concrete crushing started at 70 kN, just below the maximum load carried by the beam (Table 4), and the beam then experienced relatively high deflection until the onset of plate fracture. This result indicates that plate fracture was caused by crushing. The composite plate again fractured throughout its thickness, this interlaminar failure having propagated to the plate end anchorage positions, but the integrity of the anchorages was not impaired. As in beam 2p, 4.5 m,2 bars,the adhesive was damaged in an arched fashion and the concrete cover layer did not separate from the internal reinforcement. Adhesive cracks of the form depicted in Figure 10 were not present. Beam 4p, 4.5 m, 3 barsexperienced the failure of fibres in the CFRP plate at a load of 60 kN in the constant moment region of the beam. This continued until the maximum load of 72.8 kN, at which plate fracture progressed more rapidly with failure of a larger number of strands between the loading positions of the beam. The accelerated failure of the plate caused a drop in load to 66 kN, at which the plate

broke completely. Compressive failure of the concrete occurred in the time during which the applied load fell from 72.8 to 66 kN. The load tests of all the beams were loadrather than deflection-controlled but opening of the loading valve did not apply any further load to beam 4p, 4.5 m, 3 bars after 72.8 kN had been reached. This was because the beam was deflecting rapidly due to the progressive failure of the plate. The plate end anchorages were again intact after beam collapse and the widths of failed plate strands were again narrowest in the constant moment region. The failure of the plate into two separate lengths was associated with separation of the concrete cover to the internal rebars in the constant moment region, rather than the arched adhesive failure observed in the previous two beams with a 25% nominal prestress level. Again, adhesive cracking of the manner shown in Figure 10 did not occur. Composite plate fracture occurred in beam 5p, 4.5 m, 3 bars also. Neither concrete crushing, nor plate end anchorage damage, were recorded. The fracture of the plate was associated with the exposure of the internal rebars, due to separation of the concrete cover layer over a short length in one of the shear spans. The plate fractured into wider strips in the shear spans than in the constant moment region and interlaminar failure also occurred, but adhesive cracking was not evident.

DISCUSSION It is common in conventional reinforced concrete design to ensure that beams are underreinforced so that, in the event of collapse, the tensile steel yields before the concrete fails in compression. The analogy in externally plated beams is that the steel should yield, then the plate should fracture, both before the concrete fails in compression. However, since underreinforcement is intended to ensure a ductile failure rather than a brittle one, the desirability of plate fracture is questionable. This is a brittle mode of failure and pieces of concrete invariably fall from the plate during the explosive fracture; the consequences of this are potentially dangerous. It has been found in this study, however, that the

421

Failure modes of reinforced concrete beams : H. N. Garden and L. C. Hollaway pultruded composite plates, similar to those used in practice, fail progressively when plate fracture occurs, and considerable further load may be carried after the first fracture. This was observed with the realistically sized 4.5 m beams. Such a failure in practice would provide a visual warning of imminent structural collapse, unless the structure was vastly overloaded in a short period of time. Strands of plate material are seen to hang from the composite, usually at the edges first. The beams without plate prestress failed by concrete fracture in the tension zone of the beam, but it is seen from strain measurements that they were closer to failing by plate fracture than by concrete compression 7. Beam 3 p,4.5 m,3ba~ was the only specimen to have failed in concrete compression before plate tensile fracture, although tensile fracture followed shortly after so the failure was almost balanced. Given that this 25 prestressed 4.5 m beam with three rebars failed first by concrete crushing, it may be assumed that a comparable non-prestressed beam would have failed similarly, although concrete crushing would probably have been followed by concrete fracture in the cover layer rather then plate fracture. With further prestress above 25%, concrete compression failure was eliminated in the 4.5m beams with three rebars. This was clearly due to the higher initial strain in the plate before the beams were externally loaded, resulting in beam failure by plate fracture. It appears, then, that the nominally 25% prestressed beam with three rebars failed in a manner analogous to that in overreinforced conventional beams, i.e. concrete crushing, because the plate did not fracture first. However, the fact that the addition of further plate prestress was able to encourage an underreinforced failure indicates that the sequence of failure events can be tailored by varying the prestress level. The ultimate load of beam 2u, ~.0m was almost identical to that of beam 4p. 1.0 m, reflecting the fact that the 25% prestress level did not improve the strength of the beam or modify the failure mode. Since the failures in both of these beams appeared to be initiated by the vertical relative displacement (Figure 8), it is reasonable to assume that the plate separated when this vertical displacement reached a given magnitude in both cases. A given vertical displacement magnitude would presumably have been associated with a given width of concrete shear cracking at the location of failure initiation. Indeed, Triantafillou and Plevris 18 stated that the plate separates when the ratio of the vertical to the horizontal displacement at the base of the shear crack reaches a given magnitude. These authors related the applied shear force, V, at plate separation to this displacement ratio by the following expression: V

Vo~ w.~.G.A, where v and w are the vertical and horizontal displacements, respectively, and ~G.A is the total shear stiffness of the steel reinforcement and the composite plate. The similarity in the ultimate loads indicates that the total shear stiffness of the member was not changed by applying the plate prestress. This is an important result because it shows that, while prestressing improves the flexural stiffness and serviceability

422

loads of concrete members, it does not improve their shear capacity. Therefore, the theoretically higher flexural capacity of a prestressed beam will not necessarily be achieved since the beam may be in need of shear rehabilitation in order to reach the elevated load. The shear span-depth ratio of this 25% prestressed 1.0 m beam was 3.40; the shear displacement behaviour changes to a flexural response with increasing shear span-depth ratio, as seen in the 4.5 m non-prestressed beam in which plate separation initiated in a region where the bending moment was a relatively high proportion of its maximum value. The flexural, rather than shear, dominated behaviour of the larger beams enabled these to fail by tensile plate fracture so their full theoretical flexural capacity was reached. As mentioned in Section 4.1.3 (characterization of plate material), the coupon specimens failed by longitudinal splitting and interlaminar fracture, as did the bonded plates. In both coupons and externally bonded plates, the first fractures occurred along the edges of the CFRP material, and further load was carried before complete failure was reached. It is likely that the process of cutting the material, to make coupons, disturbed the edges by severing fibres that may not have been completely unidirectionally aligned along the length of the specimens. At any longitudinal section throughout the width of the bonded plates, the material on either side of that section would have assisted in carrying load. This would not have been the case along the edges, however, so the edges are likely to have been more highly stressed, consistent with first fracture along the edges. It has been shown that each crack in the adhesive layer was located along a soffit level crack associated with a shear crack in the concrete (Figure 10). The prestressed beams were found not to exhibit adhesive cracking, except beam 4p, 1.0 m, the prestress of which was presumably insufficient to prevent adhesive crack development. Beam 6p, 1.0 m, with a much higher prestress level than beam 4p, 1.0 m, did not experience adhesive cracking, so it is clear that the presence or absence of adhesive cracks is dependent on the magnitude of initial plate tension. Adhesive cracking was not observed in the nominally 25% prestressed beam 5p, 1.0 m, the failure load of which was 43.65 kN. The maximum shear force in this beam was only 12% lower than that in the similarly prestressed beam 4p, t.0 m, in which adhesive cracks did occur. This indicates that the adhesive cracks in beam 4p, 1.0 m were likely to have occurred first at loads nearing ultimate. It was also difficult to determine when adhesive cracks first formed in the 4.5 m beams, although these were again not present until loads near ultimate were reached. However, the absence of adhesive cracks at loads above the internal rebar yield load confirms that they were not present at the serviceability load of the beam. This finding has durability implications since the presence of adhesive cracks provides additional access for moisture into the concrete and the adhesive layer. A detailed discussion of durability factors is outside the scope of the paper, but the ingress of moisture and salt spray in highway bridge beams, for example, is detrimental so it is

Failure modes o f reinforced concrete beams • H. N. Garden and L. C. Hollaway clearly desirable to minimize the number of localized positions of moisture ingress. From a structural point of view, however, no obvious deterioration in performance was attributed specifically to the presence of adhesive cracks. As shown by Garden and Hollaway7, relatively rapid increases in deflection and plate strain occurred immediately before failure, but these were associated with the propagation of concrete or plate fracture, rather than with adhesive cracking.

REFERENCES 1.

2. 3. 4.

CONCLUSION 5.

Composite materials are readily handled and applied on site so they are well suited to the rehabilitation of concrete structures. A significant advantage of composites over steel is their excellent durability. No prestress loss should be experienced due to long term corrosion of composite plates; consequently, they are ideal materials with which to restore the prestress in structures whose tendons have suffered corrosion. Carbon fibre reinforced polymers are high strength materials so the concrete itself may fail at the ultimate limit state in non-prestressed beams. Prestressing of the composite may shift failure away from the relatively weak tensile zone of the concrete. The integrity of the adhesive material used in this study was maintained up to loads approaching the ultimate limit state, confirming its adequacy for the application. It has been shown that the adhesive may crack shortly before beam failure, though this does not appear to have any direct infuence on structural performance. GFRP blocks bonded to the composite plate are suitable for accommodating anchorage bolts and do not suffer a loss of integrity when the composite plate fractures in tension. If plate separation initiates at the base of a shear crack due to the vertical displacement at this position in a non-prestressed beam, then a high prestress is likely to be required to enable the full flexural capacity of the section to be reached. An externally prestressed member may not be able to reach its full flexural capacity unless the section is additionally reinforced in shear; work is currently in progress on this topic.

ACKNOWLEDGEMENTS This research was undertaken as part of the ROBUST project, one of several ventures in the UK Government's DTI LINK Structural Composites Programme. The industrial members of the project are L.G. Mouchel and Partners (lead partner), the Royal Military College of Science (subcontractor to lead partner), Oxfordshire County Council, Balvac Whitley Moran Ltd., Techbuild Composites Ltd. (formerly GEC Reinforced Plastics Ltd.), Vetrotex (UK) Ltd., James Quinn Associates Ltd., and Sika Ltd. The academic partners are the University of Surrey and Oxford Brookes University. The authors express their appreciation for the financial support provided by the Engineering and Physical Sciences Research Council (EPSRC).

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