Cyclic Tests of Existing R.C. Columns Repaired or Retrofitted by Mean of Jacketing Technique

Seismic assessment of reinforced concrete structures as well as their rehabilitation and strengthening is dealt with in many experimental and theoretical researches. New trends in seismic assessment essentially refer to “performance based design”. Experimental research both on seismic assessment of existing elements as well as on the efficiency of different retrofitting techniques is essential. Much research is currently being directed towards retrofitting solutions for existing reinforced concrete structures under seismic load. Among these, fibre-reinforced polymer (frp) confinement of elements stressed under axial and/or bending action is an increasingly popular solution, with a similar trend in research effort. Confining concrete can in fact significantly enhance both its resistance and stiffness properties; the exterior frp jacket, besides, protects the inner core from temperature and humidity effects, adding to its durability. This paper focuses on current experimental tests at the Laboratory of experiments on materials and structures of the University of Roma Tre on eight 1:6 scaled column specimens representative of tall and squat 2,500 m diameter circular r.c. piers of regular (14-21-14 m tall) and irregular bridges (14-7-21 m tall) designed according to EC8 and Italian Code before 1986. In previous research some columns were tested until collapse by pseudodynamic tests but others are still entire. An accurate study to detect the level of degradation in materials, as the case of real structures after an earthquake, is now performed. Piers designed according to Italian Code show dearth in transverse reinforcement resulting in concrete core crushing in compression and longitudinal bar buckling and rupture. Concrete cover spalling and yielding of hoops are clearly visible at pier base over a height of about 200-300 mm. Squat piers are also affected by shear cracking which spread with an about 52°-56° inclination angle. EC8 pier damage is restricted to concrete cover spalling and longitudinal bar buckling due to localized hoop arrangement. Based on the evaluation done, EC8 columns are repaired and Italian ones retrofitted by mean of FRP jacket with the aim of enhancing ductility and shear capacity. Repairing operations are described in detail. Mechanical removal of damaged concrete cover followed by cleaning of substrate from residue particles is performed to provide a good bond. Concrete core is repaired with resin injections. Two different restoring operations are forseen for damaged longitudinal reinforcing bars: cutting of the damaged portion of the bars and restoration with welded stainless bars on one hand, setting of 1 m length stainless bars anchored in foundation to support existing bars on the other hand. New bars are applied in pairs to avoid asymmetries. Casting self compacting concrete is used to restore damaged pier section. Its low shrinkage, elastic modulus not higher than the one of the substrate and high tensile strength allow to reduce tensile stresses in the material. In addition, good workability and resistence to segregation and remarcable filling and passing ability make self compacting concrete suitable to intervene on existing structures, since it’s neccesary to assure that the new material could really restore element continuity and homogeneity (crossing new and existing reinforcing bars without causing hollow spaces inside the element and discontinuity in the contact surface). C-FRP (Mapewrap C UNI-AX 300/10) jacket is applied to upgrade ductility in tall piers and to increase shear strength in squat piers to match EC8 prescriptions. Confinement increases concrete compressive strength and ultimate strain and also provide longitudinal bars constraint against buckling preventing cover spalling. According to Monti et al. approach, using 0.167 mm thick C-FRP, 1 layer is needed to obtain the required ductility upgrade. FRP jacket to get EC8 shear strength is designed following guide lines proposals (EC8, OPCM 2003 n. 3274, CNR 2004 DT 200, FIB 2001 n. 14) either adopting suggested values or considering experimentally measured parameters neglecting safety coefficients: using 0.167 mm thick C-FRP, 2 layers and 1 layer are needed respectively. Wrapped strips are unidirectional 100 mm wide with interval of 60 mm with the exception of the first one which is only 30 mm. FRP strips are set over the plastic hinge zone in case of tall piers and over the whole height in case of squat piers. The test equipment is composed by a system for the application of vertical loads: the specimen is placed within a testing frame realized using a 1000 kN hydraulic jack fixed to a transverse beam which is linked to the ground by means of two  60 mm steel tendons with hinged connections. A 250 KN MTS hydraulic actuator, connected on one side to the top of the pier and on other side to a reaction wall, is used to impose displacements or loads to pier. The specimen footing is restrained to the laboratory strong floor using two tranverse beams fixed to ground with steel tendonds in order to avoid any base horizontal displacements and rotations. Horizontal displacements (or loads) are applied by using the built-in MTS control system composed by an LVDT with stroke +/-125 mm (or a 250 kN load cell), the digital controller Testar II and the acquisition software TestWare. The same system is used to acquire horizontal displacements and loads. A parallel acquisition is performed by using an external data acquisition system (DEWETRON DEWE-RACK) with 16 channels togheter with a 500 kS/s National Instruments E-Series DAQ (AT-MIO-16E-1) with 16 analog inputs. In particular, the horizontal displacements of three point of the pier, placed respectively at the hight 230 mm, 450 mm and 1290 mm, are recorded using 3 linear potentiometers (stroke +/-50 mm), whereas other 12 linear potentiometers are used for the acquisition of vertical displacements of other three different pier sections in order to estimate their curvature along the height. Finally the vertical load is acquired by means of a 1000 kN load cell. Actual cyclic quasi-static tests aim to evaluate existing piers and the effectiveness of the efficiency of repairing and retrofitting techniques. Pseudodynamic tests are foreseen in the next future. Six cyclic tests (5 displacement controlled and 1 force controlled) have been performed on a squat EC8 pier both to evaluate its elastic stiffness and to check and calibrate test instrumentation equipment functioning. Moment-curvature diagrams and top pier total displacement, distinguishing contributions due to flexure and shear, are presented.