aspects of behaviour of cfrp reinforced concrete beams in bending

1、Aspects of behaviour of CFRP reinforced concrete beams in bendingMuhammad Masood Rafi *, Ali Nadjai, Faris Ali, Didier TalamonaFire Safety Engineering Research received in revised form 24 July 2006; accepted 30 August 2

2、006 Available online 18 October 2006AbstractThe corrosion of steel poses a serious problem to the durability of reinforced concrete structures and fibre reinforced polymer (FRP) has emerged as a potential alternative mat

3、erial to the traditional steel. The results of a test series consisting of carbon FRP (CFRP) and steel bars reinforced concrete beams are reported in this paper. The results indicated that the behaviour of CFRP and steel

4、 reinforced beams was similar in many aspects. Both type of beams failed in their predicted modes of failure. The strength design method underes- timated nominal moment capacity of CFRP reinforced beams. The deflection o

5、f CFRP reinforced beams was satisfactory at service load level, corresponding to theoretical load capacity. The deformability factor of CFRP reinforced beams was more than 6 indicating their ductile nature of failure. ?

6、2006 Elsevier Ltd. All rights reserved.Keywords: Fibre reinforced polymer; Carbon FRP; Mode of failure; Concrete beams; Reinforcement; Moment; Deflection; Deformability1. IntroductionReinforced concrete (RC) structures h

7、ave profited from the unrivalled dominance of steel over all other reinforc- ing materials for more than 100 years. Superior qualities of steel, in terms of strength and compatibility with con- crete, make steel an effec

8、tive concrete reinforcement. Steel is, however, highly susceptible to oxidation when exposed to chlorides. Although, alkaline environment of concrete protects steel from corrosion and makes it very durable it is not alwa

9、ys possible to provide an efficient protection. Factors such as insufficient concrete cover, poor design or workmanship, poor concrete mix and aggressive environ- ments can break down the protection layer and may lead to

10、 corrosion of the steel rebars. These destructive environ- ments include marine surroundings, use of deicing salts on bridges and parking garages, and the use of salt contam- inated aggregates in the concrete mixture. Th

11、e initial signs of distress are usually cracking and spalling of concrete, which provides access to other environmental agents likemoisture to further intensify the oxidisation of steel. As corrosion goes on it causes a

12、reduction in the cross-sec- tion of a steel bar, which leads to loss of bond between rebar and concrete. To arrest the rusting of steel remedial work has to be carried out in order to achieve the full potential of the st

13、ructure. This structural maintenance incurs exorbitant costs annually. Unreliable durability of these structures as a result of corrosion of steel is thus a serious problem. Recent efforts and research have been focussed

14、 towards the introduction of innovative non- metallic materials in the construction industry. Fibre rein- forcement polymer (FRP) materials have evolved as a result of new developments in the fields of plastics and fibre

15、 composites. A significant amount of research work has been exe- cuted to investigate various aspects of the use of FRP bars with concrete. As a result of these efforts, the applications of FRP bars are becoming increasi

16、ngly common as a rein- forcing material. Carbon FRP (CFRP) bars are mostly used in prestressing applications due to their high tensile strength, which is comparable with steel strands. This paper presents the results of

17、tests carried out on concrete beams reinforced with CFRP bars. Testing of these beams0950-0618/$ - see front matter ? 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.08.014* Corresponding author. T

18、el.: +44 28 903 68734; fax: +44 28 903 68726. E-mail address: rafi-m@ulster.ac.uk (M.M. Rafi).www.elsevier.com/locate/conbuildmatAvailable online at www.sciencedirect.comConstruction and Building Materials 22 (2008) 277–

19、285Construction and BuildingMATERIALSContinuous carbon fibres with a volume fraction of 60% by volume were used. The nominal tensile strength and tensile modulus of these fibres were 4.83 GPa and 234 GPa, respectively. T

20、he resin used to bond fibres was bisphenol epoxy vinyl ester. A textured surface was provided on the bar through surface treatment in order to increase the bond with the concrete. The bar had a widely spaced spiral wind-

21、 ing imprint with textured surface in between helical rings. The surface texture was formed with the same resin without involving external fibres. Properties of CFRP bars are given in Table 2. The manufacturer provided r

22、esults of ten- sile tests on these bars.2.2.3. Steel bars Tension rebars in BRS beams consisted of 10 mm diam- eter high strength deformed bars, as shown in Fig. 2. Mate- rial properties of these bars are given in Table

23、2. These properties were determined by the tensile tests in the labo- ratory. The bars were chosen because their nominal cross- sectional area was approximately equal to the CFRP bars. It was, therefore, possible to posi

24、tion the steel and CFRP bars identically in the control and in the test beams. Steel reinforcing bars of the identical area of the CFRP bars were not available. Top bars were of 8 mm diameter high strength deformed steel

25、 for all beams. Both top and bottom steel bars were hooked at each end. The mechanical properties of top rein- forcing bars are shown in Table 2. The reinforcement cages (Fig. 1) were tied with iron wire. Smooth 6 mm dia

26、meter closed rectangular stirrups spaced at 100 mm centre to centre were chosen to comply with the criteria of the ultimate strength design of FRP reinforced beams given by the ACI code [2,3]. Results of tensile tests on

27、 6 mm bars are also included in Table 2.2.3. InstrumentationThe instrumentation was set up to measure the deflec- tion of the beam and the deformation of the reinforcingbar. Strain gauges were used to measure deformation

28、 and to monitor bond of tension bars. The deflection at midspan was recorded using linear variable differential transducers (LVDTs). These LVDTs were placed on both sides of the beam at the centre. Horizontal LVDTs were

29、used at the end of the BRC beams additionally to measure the slip of the CFRP bars. Computer aided data acquisition systems were used to record continuously, load, deflection, slip and strain. Thus this data could be obt

30、ained easily at any time during each test.2.4. Test procedureThe specimens were placed on half-round supports, which were spaced at a distance equal to the test span of the beam. Loading was applied in small increments,

31、through 38 mm diameter rollers, by means of a 200 kN hydraulic jack. Steel plates 25 mm wide and 5 mm thick were placed under each roller on the top of the beam in order to avoid local crushing. Each load increment was 2

32、.5 kN for BRS and 5 kN for BRC beams and was mea- sured with a 200 kN load cell. All beams were tested to fail- ure. The beams were 2–3 months old at the time of testing. Immediately after the load increment, cracks were

33、 identi- fied using a magnifying glass and marked. The ends of the cracks were labelled with the corresponding load step. Three minutes were allowed for the completion of the pro- cess before the next increment of load w

34、as applied. The monitoring of cracks continued over the entire loading spectrum. The operator manually controlled load, which was dis- played on the monitor screen, and made the necessary adjustments to keep load as cons

35、tant as possible. For all tests the load was removed after the applied load dropped substantially below the ultimate load. A complete test took approximately 1 h.3. Analysis of test results3.1. Cracking behaviourConcrete

36、 is a weak material in tension and cracks when subjected to high local tensile stresses. Its low tensile capac- ity can be attributed to the high stress concentration under load [4]. Flexural cracking in beams is not onl

37、y unavoid- able but is necessary to allow tension reinforcement to play its part. The formation and propagation of cracks depend on the tensile strength of the concrete. When the principal tensile stress in concrete exce

38、eds its tensile strength, cracksFig. 2. CFRP and tension steel bar.Table 2 Mechanical properties of rebarsBar type Nominal ultimate strengtha (MPa) Ultimate strainaElastic modulus (GPa)CFRP 1676 0.0145 135.9 Steel – 10 m