Strengthening of Shear-Damaged Reinforced Concrete T-Beam Bridges with CFRP Strips

Transcription

1 Strengthening of Shear-Damaged Reinforced Concrete T-Beam Bridges with CFRP Strips Riadh Al-Mahaidi, Geoff Taplin, John Susa Department of Civil Engineering, Monash University, Clayton, Vic., Australia 3168 Abstract This paper provides a broad overview of the strengthening techniques used for reinforced concrete beam components in bridge decks, outlining material properties and listing important design aspects related to their application. A detailed discussion of the shear-strengthening repair with FRP is undertaken. In evaluating the effectiveness of this shear strengthening technique, the conclusions reached within the literature review are then applied to a strengthening proposal for nineteen shear-damaged RC T-beams. 1. Introduction As society has developed, the pressures placed upon our current bridge network have also grown. Continual upgrading of service loads and the large increase in traffic volumes has meant that hundreds of thousands of bridges worldwide are in desperate need of repair. The impact of these damaging factors has been compounded in the earlier built bridges due to errors within the standards they were designed to. Requiring only half the shear reinforcement of current design standards, some of the bridges constructed in the first five decades of this century are perceived to be deficient in shear requiring shear strengthening. In addition, bridge beams that have already been damaged in shear require strengthening as well. The situation is no different here in Australia with many bridges in service requiring similar shear repair. With limited financial resources, the responsible transportation departments (such as Vicroads and the RTA) are looking towards advanced modern materials in the hope of rectifying these bridge defic iencies in a costeffective manner. One such innovative strengthening technique that has received significant attention lately is the use of Fibre Reinforced Plastic (FRP). Where a structure is found to be structurally deficient there are a range of strengthening methods that may be employed in restoring the structure. Traditionally repair and retrofitting have been achieved through section enlargement, posttensioning or bonded steel plates (Farmer and Gee 1997). Even though these conventional alternatives are preferred to demolition and reconstruction, these repair methods are expensive and can cause the bridge to be out of service for considerable periods. Advancements in material technology have created a fourth strengthening category; the use of externally bonded fibre reinforced plastic (FRP) composites. An evaluation of the newly developed technique has shown that this retrofitting mode is largely superior to traditional forms. Meier (1997a) and Farmer and Gee (1997) reported that the use of this advanced technique overcomes many of the drawbacks associated with traditional repair methods, yet possesses many of its desirable strengthening attributes. The attractiveness of the newly developed strengthening technique is understandable in consideration of the outstanding material properties and practical ease of application. The cost associated with utilising such an advanced material and its desirable properties are quite substantial. However in considering all of the project costs (F. Seible 1997), the ease of installation ensures that the method is cost competitive with conventional repair methods. 2. Shear Strengthening with CFRP The attractiveness of the newly developed retrofitting technique occurs as the advanced material's attributes have been tailored to the strengthening function. The properties of the CFRP are so unique, that they cannot be achieved with the use of conventional materials. The advanced material consists of a plastic matrix embedded with a reinforcing material. Apart from holding the reinforcing material together, the matrix also protects the fibres against mechanical damage. The reinforcing fiber (Emmons et al 1998) provides the unidirectional tensile strength, in addition to the chemical and environmental durability. Fibre reinforced plastics are manufactured by the pultrusion process and commonly utilise either 1

2 one of carbon, glass or aramid fibres. Meier (1997) assessed each of the available reinforced fibers and concluded that Carbon Fiber Reinforced Plastic (CFRP) is the best suited to the post-strengthening function within bridges. As a consequence of this evaluation, research and commercial product development in the field have been generally limited to the Carbon reinforced variety. Considerable research has been conducted worldwide to study the implication of retrofitting reinforced concrete beams with FRP. The Swiss Federal laboratories (EMPA) along with study at King Fahd University were part of the first research groups to demonstrate that advanced composite materials can be used to strengthen existing reinforced concrete infrastructure (Buyukozturk and Hearing 1998). A large number of successful strengthening applications have also confirmed the reliability of the technology. With Meier (1997) stating that up-until 1996 no failures have been reported within any of the 700 field applications of the retrofitting technique. 3. Shear Failure Mechanisms One of the most important aspects in the application of the shear strengthening technology is that the failure criteria are established (Ziraba et al 1991). By accounting for each failure mode, the possibility of an unexpected premature failure occurring is reduced. For RC beams strengthened in flexure a failure mode classification was developed by Buyukozturk et al. (1998). To the authors knowledge, there does not seem to be a similar failure classification for shear strengthening. Consequently this section of the paper will describe each of the failure modes observed in the research in establishing the failure criteria for FRP shear repaired members. 3.1 Shear Delamination This variety of failure is similarly related to the formation of diagonal shear cracking. The twostage failure mode where delamination follows the formation of shear cracking was observed within the research completed by Triantafillou (1998), Al-Sulaimani et al. (1994) and Chaallal et al. (1998). Each of the beams that had failed in this manner had been repaired by the application of FRP strips and had failed in an explosive manner. Rather than surface cracking offsets promoting failure, delamination occurred as insufficient shear resistance had been provided. The actual failure mechanism within this category typically involves a combination of failure modes incorporating debonding at certain areas and facture at others. 3.2 Shear Fracture This fracture mode is largely different to the other failure modes, in that failure is governed by a maximum interface bond strain. The failure mechanism occurs where adequate anchorage and FRP is available yet the strengthening arrangement is unable to develop the necessary increase in shear capacity. Chajes et al. (1995) experiments showed evidence of this failure type, with the wrap strengthened beam tearing along the inclined shear crack once the maximum bond strain had been reached adjacent to the shear crack. 3.3 Failure Categories The following is a summary of the failure modes evident where shear deficient RC beams has been strengthened in shear by applying FRP strips. The failure list incorporates all of the observed experimental failure modes with some of the categories integrated being governed by identical factors. Shear crack debonding: Local FRP peeling due to surface offsets and movement developed at the shear crack. Delamination: Occurs where there is insufficient bond interface stress transfer limiting the strengthening method's shear contribution. Stress concentration: FRP debonding occurring due to insufficient anchorage and the formation of stress concentrations at the ends. Maximum bond elongation: The maximum allowable strain of the composite bond reaches limiting capacity. The shear categories of failure listed above are not significantly different to those published by Buyukozturk for the flexural application. This similarity is understandable in that the strengthening principles are identical and the factors critical to its application success are common to both shear and flexural strengthening. 4. Strengthening Proposal The overall objective of this paper is to devise a FRP strengthening proposal for 19 RC T- 2

3 beams. All of these test beams have been previously loaded to failure and are consequently extensively damaged in shear (Taplin and Al- Mahaidi 1998). These test beams are variations on 50% scaled versions of beams found within the Kiewa Valley Highway Bridge in Victoria which is typical of many of the T-beam bridges built prior to the 1950 s. Detail of the standard T beam, that was only marginally deficient in shear before pre-loading, is shown in Figure 1. A number of other variations have been made to this standard beam. Table 1 provides a summary of the variables investigated. Further information on dimensions and reinforcement of each of these experimental beams is provided in Taplin and Al- Mahaidi (1998). Loaded to failure, the beams have lost a considerable amount of their original shear strength. With significant slippage occurring between the stirrups and the concrete during loading, further shear resistance by the stirrups is unlikely. As a result, the stirrups are assumed to provide no shear contribution in the shear strengthening design. The total shear capacity of a concrete member (V uc ) is the sum of the shear resistance of the uncracked concrete, the dowel shear resistance and the vertical component of aggregate interlock. The pre-loading would have largely reduced the uncracked concrete shear resistance in being extensively damaged. The shear contribution from the other two components is difficult to assess. Unable to assess the shear contribution s of these components, an upper and lower shear contribution bound is assumed. Upper bound: assuming full shear capacity V uc Lower bound: no shear contribution V uc = mm 305 mm f c = 25 MPa 75 mm Reinforcement tensile reo. : 6 16 bars f sy = 718 MPa 140 mm 3500 mm 250 mm stirrups: 6 legs mm Figure 1 Typical T Beam details (Beam Number 1) Table 1: Summary of Experimental Beams Variable Beam Number Standard beam 1 Width of flange 2 to 5 Compressive strength of concrete 6 and 7 Spacing of stirrups 8 and 9 Level of shear reinforcement 10 and 11 Flexural reinforcement 12 and 13 Repeated loading 14 to 16 Negative bending 17 to 19 3

4 5. Repair Strategy 5.1 Arrangement of CFRP Strips In repairing the T beam s, there are a number of shear strengthening arrangements available. From the literature surveyed, it seems that the L-plate strengthening is the most appropriate. Bonded into the flange and wrapped around the soffit of the beam, the major advantage of this strengthening system is the level of anchorage that it provides at both ends of the beam. With extensive cracking located between the flange and the web of the beam a simple U plate arrangement would be unable to provide the necessary shear force resistance in similar circumstances. Cored and epoxy bonded into the flange, the L plate anchorage arrangement is able to provide the necessary shear transfer across these cracks as depicted in Figure 2. A further anchorage concern specific to the use of L-plates is the bursting failure at the soffit of the beam. Further detail of this failure mode observed within Bleibler s et al. (1998) research is illustrated within Figure 3 below. In trying to limit the occurrence of this failure mode, a mechanical anchorage system positioned across the overlapping plates is to be tried within the experimental research. Figure 4 provides further information on this anchorage system where the same adhesive is used to bond the steel plate. Extensive Cracking Traditional shear strengthening application does not contribute to shear at this point. L plate anchorage system - provides necessary shear force transfer. Figure 3: L-plate bursting failure Figure 2: T beam anchorage concerns Epoxy Bonded Steel Plate Beam Soffit Figure 4 : Details of L-plate beam soffit anchorage 4

5 5.2 Surface Preparation As the beams that are to be repaired are extensively damaged, additional surface preparation measures may need to be completed. In epoxy bonding FRP to the outer surface of a member, an extremely flat concrete surface must exist. Even though major surface offsets are unlikely at cracking on the web, the slightest of lateral movements may have disastrous consequences promoting local debonding failure. Therefore it is important that a level concrete surface is provided and rectification work in the form of concrete surface sanding is to be carried out where it is required. The extensive surface damage evident at the shear crack at many of the beams may require the application of an epoxy putty or resin crack injection in developing the necessary bonding surface. Further guidance on this aspect may be obtained from the supplier. 6. Analytical Model The earlier analytical models that were first developed were quite simple, based on traditional reinforced concrete design principles. Numerous CFRP strength contribution methods have been published, with the vast majority utilising compatibility and equilibrium criteria. These initial methods such as those proposed by Sharif, Al- Sulaimani, Sharif, Basunbul, Baluch and Ghaleb (1994) were quite comparable to experimental results when the predicted failure mechanism did occur. However in instances where an alternative variety of failure occurred, the simple theoretical models continually overestimated the load capacity (Ziraba, Baluch, Basunbul, Sharif, Azad and Al- Sulaimani 1994). By not considering critical design items such as composite slippage, curtailment stresses and plastic material behaviour, these oversimplified methods are inadequate on their own in detailed CFRP strengthening design. New and more complex analytical modes have been and are continuing to be developed, replacing the earlier unsophisticated models. These advanced design methods are accounting for non-linear behaviour and a greater range of failure mechanisms. Experimental research application and model testing has shown encouraging results with theoretical and actual failures in close agreement (Malek and Saadatmanesh 1998). A number of analytical models have been published in the area, however not one of them deals specifically with the shear strengthening contribution of L-plates. Consequently in design at this proposal stage, a new model and design process has been formulated. Derived from first principles the design procedure is governed by the maximum strain and interface stress criteria. 6.1 Limiting Strain Criterion The following formula has been derived in analogy to internal steel stirrups. Where instead of limiting yield stress, the shear capacity contribution of the FRP is governed by a maximum interface strain. AfrpE frpε frpdo Vfrp = S Triantafillou (1998) in his research into such shear strengthening has developed an effective FRP strain model representing this limiting interface strain value. ε frp = ( ρ frpe frp) ( ρ frpe frp) 6.2 Maximum bond interface stress criterion The other failure mechanism that needs to be considered within the design is the limiting maximum interface stress. Sika, the supplier of the strengthening system, states that the maximum interface bonds strength is greater than 2 MPa. In the proposal, the shear contribution of the FRP is calculated on the basis of this value. Research undertaken by Al-Sulaimani et al. (1998) suggests that this figure is quite conservative with a maximum interface strength experimentally derived at 3.5 MPa. The shear capacity prediction under this criterion utilises the model published by Al-Sulaimani s et al. (1998) in predicting the shear contribution of the strip shear strengthening arrangement. As the L-plate is assumed to be perfectly bonded the average shear stress is replaced by the maximum stress within this original model. V frp = τ 2 max tsh 2 S s d 7. Shear Strengthening Summary The standard shear-strengthening repair scheme will incorporate 4 L- plate approximately 250 interval. Other modifications that will be investigated during the experimental testing include a smaller L-plate 150 mm, no flange anchorage and the addition of the steel 2 5

6 anchorage plates at the bottom of the beam soffit. Further shear-strengthening details and the predicted capacities are summarised in Table 2. Table 2 : Shear Strengthening Proposal Beam No. Varying parameter Repair category L plate Spacing (mm) V uc (kn) V frp ε (kn) σ Lower Bound V frp 1 Standard Standard flange width Standard flange width No flange anchorage Rectangular Strips fc' Standard fc' Standard stirrup spacing L-plate spacing stirrup spacing L-plate spacing stirrup L-plate spacing flexural strength steel plate anchorage flexural strength steel plate anchorage span Standard Repair Negative bending Standard Repair Negative bending Standard Repair Upper Bound V uc + V frp 8. Conclusions This paper presented the case for using CFRP L- strips to strengthen RC T-beams damaged in shear. Such beams are typically found in bridge decks requiring upgrading to meet current and future traffic loading. Literature search has shown that L- shaped CFRP strips are the most effective elements for this kind of repair. A proposal for strengthening 50% scale RC T-beams was outlined with lower and upper bound theoretical predictions of strength of the repaired beams. Repair and experimental testing based on this proposal will be implemented in February References 1. Al-Sulaimani, Sharif, A., G. J., Basunbul, I. A., Baluch, M. H. and Ghaleb, B, N. (1994) 'Shear Repair for Reinforced Concrete by Fibreglass Plate Bonding' ACI Structural Journal, 91:3 pp (July-Aug.). 2. Arockiasamy, M., Sowrirajan, R., Shahawy, M. and Beitelman, T., E. (1995) 'Repair of Damaged Pre-tensioned Solid Slab using CFRP laminates' in Proceedings of Second Non-metallic (FRP) reinforcement for Concrete Structures (FRPRCS-2), (ed.) L. Taerwe: E & FN Spoon, pp Bassett, S., (1997) 'Carbon laminates Solve Structural Problems in Bridges across Continent', High-Performance Composites, pp (March-April). 4. Bleibler, A., Meier, H. and Steiner, W. (1998) 'External Strengthening of Reinforced Concrete Structures Using Bonded CFRP Plates' in Proceedings Australasian Structural Engineering Conference, pp Buyukozturk, O., Hearing, B. (1998) 'Failure Behaviour of Pre-cracked Concrete Beams Retrofitted with FRP', Journal of Composites for Construction, pp , (Aug.). 6

7 6. Chaallal, O., Nollet, M., J. and Perraton, D. (1998) 'Shear Strengthening of RC beams by Externally Bonded Side CFRP Strips', Journal of Composites for Construction, pp , (May). 7. Chajes, J., M., Januszka, T., F., Mertz, D., R., Theodore, A., Thomson, Jr. and William, W., Jr. (1995) 'Shear Strengthening of Reinforced Concrete Beams Using Externally Applied Composite Fabrics', ACI Structural Journal, 92:3, p , (May-June). 8. Emmons, P., Alexander, V., M. and Thomas, J. (1998) 'Strengthening Concrete Structures, Part I & II', Concrete International, pp (March-April). 9. Farmer, N., Gee, T. (1997) 'Strengthening with CFRP laminates', Construction Repair, pp2-4, (Jan.- Feb.). 10. Lane, J. S., Leeming, M. B. and Fashole-Luke, P. S. (1997) 'Testing of Strengthened Reinforced and Prestressed Concrete Beams', Construction Repair, pp (Jan.-Feb.). 11. Malek, A., Saadatmanesh, H., (1998) 'Ultimate Shear Capacity of Reinforced Concrete Beams Strengthened with Web-Bonded Fiber-Reinforced Plastic Plates', ACI Structural Journal, 95:4 pp , (July-Aug.). 12. Malek, A., M., Saadatmanesh, H. and Mohammad, E. R., (1998) 'Prediction of Failure Load of R/C Beams Strengthened with FRP Plate Due to Stress Concentration at the Plate End', ACI Structural Journal, 95:1, pp (Jan.-Feb.). 13. Malek, A. M., Saadatmanesh, H. (1998) 'Analytical Study of Reinforced Concrete Beams Strengthened with Web-Bonded Fiber Reinforced Plastic Plates or Fabrics', ACI Structural Journal, 95:3, pp (May-June). 14. Meier, U. (1997) 'Repair Using Advanced Composites', in Proceedings of International Conference Composite Construction; Conventional and Innovative, Austria, pp Meier, U., (1992) 'Carbon Fiber-Reinforced Polymers: Modern materials in Bridge Engineering', Structural Engineering International, 92:1, pp Meier, U. and Winistorfer, A. (1995) 'Retrofitting of Structures Through External Bonding of CFRP Sheets', in Proceedings of Second Non-metallic (FRP) reinforcement for Concrete Structures (FRPRCS-2), (ed.) L. Taerwe: E & FN Spoon, pp Midwinter, K., Hewitt, L. (1997) 'Plate Bonding Carbon fibre and steel plates', Construction Repair, pp.5-9, (Jan.-Feb.). 18. Nanni, A. 'CFRP strengthening', Concrete International, pp Norris, T., Saadatmanesh, H., and Ehsani, M., R. (1997) 'Shear and Flexural Strengthening of R/C Beams with Carbon Fiber Sheets', Journal of Structural Engineering, pp , (July). 20. Seible, F. (1997) 'Advanced Composites for Bridge Infrastructure Rehabilitation and Renewal', in Proceedings of International Conference Composite Construction; Conventional and Innovative, Austria, pp Sharif, A., Al-Sulaimani, G. J., Basunbul, I. A., Baluch, M. H. and Ghaleb, M. (1994) 'Strengthening of Initially Loaded Reinforced Concrete beams using FRP Plates', ACI Structural Journal, 91:2, pp , (March-April). 22. Sharif, A., Al-Sulaimani, G. J., Basunbul, I. A., Baluch, M. H. and Husain, M. (1994) 'Strengthening of Shear-damaged RC beams by external bonding of steel plates'. Magazine of Concrete Research, 47:173 pp (Dec.). 23. Sheikh, S., A., Vecchio, F., J., De Rose, D. and Bucci, F. (1998) 'Behaviour and Analysis of FRP-Repaired Elements' in Proceedings of International Conference on HPHSC, Perth, pp Spadea, G., Bencardino, F. and Swamy, R. N. (1998) 'Structural Behaviour of Composite RC Beams with Externally Bonded CFRP' Journal of Composites for Construction, pp , (Aug.). 25. Steiner, W. (1997) 'Strengthening of Structures with Carbon Fiber Laminates', in Proceedings of International Conference Composite Construction; Conventional and Innovative, Austria, pp Swamy, R.N. and Mukhopadhyaya, P. (1995) 'Role and Effectiveness of Nonmetallic Plates in Strengthening and Upgrading Concrete Structures' in Proceedings of Second Non-metallic (FRP) reinforcement for Concrete Structures (FRPRCS-2), (ed.) L. Taerwe: E & FN Spoon, pp Taplin, G., Al-Mahaidi, R. (1998) An experimental and Theoretical Investigation of the Shear Strength of Model Reinforced Concrete T-beams, Report prepared for VicRoads, Department of Civil Engineering, Monash University, Clayton, Australia. 28. Triantafillou, T. C., (1998) 'Shear Strengthening of Reinforced Concrete beams using Epoxy -Bonded FRP Composites', CA Structural Journal, 95:2 pp (March- April). 29. Ziraba, Y. N., Baluch, M. H., Basunbul, I. A., Sharif, A. M., Azad, A. K., Al- Sulaimani, G. J. (1994) 'Guidelines toward the Design of Reinforced Concrete Beams with External Plates', ACI Structural Journal, pp (Nov.-Dec.). 7