SEISMIC RETROFITTING OF RECTANGULAR BRIDGE PIERS WITH FRC JACKETS

Transcription

1 SEISMIC RETROFITTING OF RECTANGULAR BRIDGE PIERS WITH FRC JACKETS Dominic Vachon and Bruno Massicotte École Polytechnique de Montréal, Canada Abstract The research project described in this paper was aimed at exploring the used of a thin jacket made out of fibre reinforced concrete (FRC) for enhancing the performance of lap regions of rectangular columns. It is divided in three main parts. The first one presents the deficiencies of existing bridge piers and the rationale of the proposed strengthening technique. The second part describes the development of self compacting FRC mixes using various types of fibres at high volumes and the selection of the most effective one in term of workability and flexural strength. Finally, the paper presents the results of an experimental program in which large scale column specimens were fabricated with reinforcement details reproducing the conditions that prevailed before modern recommendations. Three conditions were tested: reference, with a FRC cover, and with a FRC cover reinforced with a welded wire mesh anchored to the column core. The specimens were tested in reverse cyclic loading with increasing displacement. The tests results showed the significant improvement of the column performance, the failure mode changing from a brittle one, for the reference specimen, to a very ductile one with plastic hinges forming outside the lap region, for the two strengthened specimens. 1. Introduction Observations made following major earthquakes that occurred in the last three decades worldwide showed that often lap splice details were not adequately designed and contributed to the collapse of structures. Until recently, a common design practice was to use dowel bars in the footing that were lapped with continuing bars at the bottom of the column. Modern design recommendations prohibit lap splices between the footing and the column where plastic hinges may form during a severe earthquake. More appropriate reinforcement detail requirements have been included in recent codes to insure sufficient ductility to the structure. However the safety of existing bridge piers 1247

2 must be addressed. Several solutions have been proposed and applied for strengthening bridge piers. However, most of them were developed for circular and square columns, while no solutions have been proposed for rectangular columns with a large aspect ratio. A research project was initiated at École Polytechnique de Montréal with the objective of developing a method to retrofit highly rectangular bridge piers with lap splice at the base. As illustrated in Fig. 1, strengthening solutions for circular, square or slightly rectangular (b/h 2) columns consist in placing a jacket made out of reinforced concrete, welded steel plates or FRP wrapping around the existing column. These jackets have the capability of increasing the confinement of the concrete core and thus increase the lap splice resistance. However these techniques cannot be easily adapted for rectangular columns with an aspect ratio higher than 2 because: the large dimensions of the jacket reduces the efficiency of the confining material, and bridge piers cannot be widened excessively due to traffic lanes or water flow. This paper addresses the strengthening of highly rectangular cross sections by introducing a new technique that uses a thin jacket made of fibre reinforced concrete (FRC) in the lap splice regions at the base of column. h b Fig. 1 Conventional retrofitting of circular, square and rectangular columns with steel or FRP jackets 2. Failure mode of rectangular bridge piers and research objectives Several failure modes can be observed in bridge piers subjected to severe earthquakes. Priestly and Chai (1992) concluded from their research and field observations on rectangular columns that many pier failures are due to insufficient flexural capacity in the longitudinal direction or inadequate shear strength in the transverse direction. They pointed out that many piers exhibited splitting crack type failures in the lap splice regions. Fig. 2 illustrates a typical splitting failure mechanism associated with inadequate lap splice detailing. Splitting cracks may form at the tensile face, as shown in Fig. 2, but can also develop in the plane of the reinforcing bars, parallel to the tensile face. 1248

3 The aim of the research project was to develop a strengthening technique that would mitigate splitting crack propagation, enabling yielding of the reinforcement in the lap splice region. Only flexural failure modes in the longitudinal direction were considered in this study. NGravity VLongitudinal VTransverse Flexural crack Continuing reinforcement Splitting crack Plastic hinge regions Lap splice length Dowel bars Fig. 2 Typical failure mode of rectangular bridge piers in the longitudinal direction due to inadequate lap splice detailing 3. Description of the proposed retrofitting technique The proposed concept relies on the ability of the fibres to mitigate the propagation of splitting cracks in the concrete cover. Tests on joint region entirely made of SFRC presented in a recent paper by Harajli and Rteil (2004) indicated the improved resistance of lap splice regions due to the contribution of fibres. The concept proposed in the present paper addresses the use of FRC jackets for retrofitting existing columns that are used only as cover in the lap splice region. The jacketing method considered in the research project carried out by Vachon (2004) at École Polytechnique de Montréal consists of replacing the existing concrete cover by a FRC jacket as presented in Fig. 3. Two options have been considered for the FRC jacket: fibre concrete only, and fibre concrete with a welded wire mesh anchored to the concrete core (FWWM). In the options considered, the existing concrete cover is removed and the dowel bars and the continuing reinforcement are exposed in order to pour the FRC cover. The aim of this technique is to enhance the anchorage performance of bars by eliminating the splitting failure mode and, thereby, enabling the dowel bars to yield outside the lap splice region and provide the required ductility. 1249

4 To achieve this objective, a structural fibre concrete mix with high workability was developed. This an essential characteristic that is required to enable the pouring of the FRC cover from the top of repaired zones with congested reinforcement. Existing FRC cover FRC cover with welded wire mesh and anchors Fig. 3 Fibre reinforced concrete jacket 4. Development of the self-compacting FRC Based on past experience in developing fibre concrete mix for structural applications (Massicotte et al 2000), a mix containing 80 kg per cubic meter of steel fibres ( 1% in volume) was selected as a reasonable compromise between strength and workability. Fibres of small length ( 30 mm) were chosen to facilitate the pouring of the FRC. During the preliminary phase of the project three types of fibres were investigated: steel fibres with flat ends (type F), steel fibres with hooked end (type H) and synthetic fibres (type S). A self compacting concrete with a compressive strength of 50 MPa was developed to meet the required strength and workability. The development of a workable fibre concrete mix followed the three essential steps that are recommended for structural mixes that use moderate to high fibre dosage. First, the concrete mix must be designed without fibres and the volume of constituents selected. The second step consists of replacing an equal amount of aggregates by fibres according to Baron-Lesage method (Rossi 1998) which allows the determination of the maximal compactness of the granular structure for each type of fibre mix. An efficient approach to obtain this consist of relating the flowing time of mixes in an apparatus called the workability meter to the sand to coarse aggregate ratio (S/A). The maximum compactness corresponds to the S/A ratio with the minimum flowing time. Fig. 4 shows the two optimization curves for the two steel fibre mixes. This method was not used for the synthetic fibre (type S) since it has been impossible to obtain a flowing concrete at a fibre dosage of 6.8 kg/m 3. For both steel fibre mixes, the optimum was 1250

5 obtained for an S/A ratio of 1.0. The final step of the technique consists of adjusting the superplasticizer and other admixtures to obtain the required fluidity. Time (s) Type H 30 Type F ,8 0,9 1 1,1 1,2 1,3 S/A Fig. 4 Optimization of the mix compactness Following the mix optimization phase, the mechanical characteristics of hardened concrete were measured on notched prisms tested according to the RILEM (2000) procedure. The specified prism dimensions are mm. To minimize wall effects and to consider the segregation effect, the three prisms were cast vertically in one piece and sawn later as shown in Fig. 5. The specimen casting orientation was dictated by the importance of simulating actual field conditions and their effects on fibre orientation and associated mechanical properties. Specimens cast vertically in one block Cutting lines P Direction of fibre action Anticipated splitting crack Fig. 5 RILEM prisms and pouring method for the specimens Bending test results consisting of the average of three specimens of each mix are shown in Fig. 6. Fibre mixes providing high post-cracking strength at small crack opening are required to stop the propagation of splitting cracks in the concrete cover. The two steel fibre options showed comparable initial performance. The post-cracking strength of synthetic fibres at small crack opening was less than half that of the two steel fibre mixes and was therefore rejected. Flat-end fibres exhibited the most 1251

6 appropriate mechanical characteristics at small and large crack opening and were therefore selected. 16 Load (kn) Type H Type F Type S Deflection (mm) Fig. 6 RILEM Test results 5. Evaluation of the retrofitting method on column specimens Bridges pier specimens were fabricated to evaluate the effectiveness of the FRC retrofitting method. At this exploratory stage, mm specimens simulating the middle portion of a rectangular bridge pier were designed. As shown in Fig. 7, the specimen contained three 25 mm bars on each face that were splice at the base of the column over a length of 600 mm which corresponds to 24 bar diameter, a common value used in the 1960's. The shear reinforcement was spaced at 300 mm in the lap splice region and were overlapped in the concrete core, a common detailing practice for rectangular columns in the 1960's. The specimen nominal concrete strength was 30 MPa while the steel strength was 400 MPa to be representative of existing columns. One reference specimen was fabricated. For the retrofitted specimens, the SFRC jacket was poured vertically in order to reproduce actual field conditions for fibre orientations and compaction. 5.1 Specimen behaviour The specimens were submitted to a quasi-static cycle loading. Two different structural behaviours were observed as shown in Fig. 8 and Fig. 9. The reference specimen reached the design resistance in one direction only and presented a hysteretic behaviour that showed a rapid loss of strength at the second cycle. In the opposite, the response of 1252

7 the two retrofitted specimens exhibited a significantly enhanced hysteretic behaviour as the specimens kept their stiffness and strength throughout the test. Actual Specimen Reinforcement detail Fig. 7 Design of the specimens Load (kn) Deflection (mm) Fig. 8 Hysteretic behaviour of reference specimen 1253

8 90 Charge (kn) Déformation (mm) Fig. 9 Hysteretic behaviour of retrofitted specimen FWWM 5.2 Observed failure mode Observations made on the crack pattern obtained in the laboratory illustrate the governing failure mode. The reference specimen exhibited the formation of splitting cracks along the bars in lap splice region that led to the sliding of the bars and consequently to the brittle failure of the specimen without any sign of ductility. The retrofitted specimens showed a completely different failure mode. The FRC jacketing prevented the initiation of the splitting cracks and, as expected, allowed the concrete in the lap splice region to maintain the load transfer mechanism between the bars and therefore to develop sufficient strength to resist the applied loads. Furthermore, a plastic hinge formed at the base of the retrofitted specimens in the dowel bars, below the lap splice region and above the footing. A ductile failure was observed for both retrofitted specimens with better performance for the FWWM specimen. 5.3 Ductility enhancement and energy dissipation Fig. 8 and 9 show that the proposed strengthening method changed the failure mechanism from a brittle one to a ductile one but also significantly improved the aspect of the hysteretic responses, without any pinching, indicating the efficiency of the proposed technique. The energy dissipated by each specimen can be compared with the energy that would be dissipated by an idealized elasto-plastic system (shown as a dotted line in Fig. 8 and 9). The comparison of the dissipated energy is shown in Fig. 10 and allows comparison of the hysteretic behaviour of the specimens. 1254

9 Energy Ratio 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 FWWM Fibre Reference Ideal elasto-plastic system Cycles Fig. 10 Comparison of the dissipated energy by each specimens Fig. 10 shows that the retrofitted specimens dissipated a significantly larger amount of energy than the reference specimen. This specimen (R) dissipated only 11% of the energy of the perfect elasto-plastic system while the retrofitted specimens (F and FWM) dissipated 38 % and 58% of that of a perfect elasto-plastic system respectively. Table 1 gives the amount of cycle of the loading protocol each specimen has accomplished during the test as well as the amount of energy dissipated by each specimen. For the test, 9 cycles were required according to the New-Zealand load protocol (Vachon 2004). Table 1 Energy dissipated by the specimens Specimen Reference Fibre only FWWM Number of cycles accomplished Total energy dissipated in the tests (kj) 19,75 38,36 57,78 From Fig. 10 and table 1 it can be concluded that the used of a FRC jacket is a very efficient technique for retrofitting bridge piers. It allows the specimens to dissipate a larger amount of energy, to exhibit a larger ductility and to show a good and stable hysteretic behaviour. The FRC jacket with a wire mesh anchored in the concrete core is the most efficient. 1255

10 6. Conclusions and recommendations for future research The experimental results obtained on the reference and retrofitted specimens showed that FRC jackets can increase significantly the ductility of rectangular bridge piers by modifying the nature of the failure mechanism. In order to fully evaluate the efficiency and improvement provided by FRC jackets in the context of seismic retrofitting of rectangular bridge piers and to develop a design method based on actual material properties, further researches are required. In future studies, parameters such as the thickness of the FRC cover, the depth of demolition, the presence of anchors, the use of a welded wire mesh, the type and volume of fibres, the size of the bars, the geometry and length of lap splices, and the column aspect ratio and reinforcement ratio will have to be considered. References 1. Priestley, M.J.N., Chai, Y.H., Design guidelines for assessment retrofit and repair of bridges for seismic performance, Report No. SSRP-92/01, University of California, San Diego (California), (1992) 266 pages. 2. Harajli, H.M., Rteil, A.A., Effect of Confinement Using Fiber-Reinforced Polymer or Fiber-Reinforced Concrete on Seismic Performance of Gravity Load-Designed Columns, ACI Structural Journal, January-February 2004, pp Vachon, D., Renforcement sismique de piles de ponts rectangulaire avec béton renforcé de fibres, Mémoire de maîtrise, Département des génies civil, géologique et des mines, École Polytechnique de Montréal, (2004)192 pages. 4. Massicotte, B., Degrange, G., Dzeletovic, N., Mix design for SFRC bridge deck construction. Proceeding of the Fifth RILEM Symposium of Fibre-Reinforced Concrete, Lyon, France, September , pp Rossi, P., Les bétons de fibres métalliques, Presses de l'école nationale des ponts et chaussées, Paris, (1998) 309 pages. 6. Réunion Internationale Des Laboratoires Et Experts Des Matériaux, Systèmes De Construction Et Ouvrages (RILEM)., Test and design methods for steel fibre reinforced concrete (RILEM TC 162-TDF), Materials and Structures, January-February, (2000) p