Repair And Strengthening With FRP of Damaged Bridge R/C Slabs

The object of the present study is a viaduct, known as "Fornello", of the Orte-Ravenna highway (E45), Italy, managed by the National Organization for Highways (ANAS). The highway winds along a mountain route and the structure, built in the early seventies, presents damage problems due to freeze-thaw cycles and to abundant use of salt during the winter season, together with lack of maintenance [1] [2]. In the Fornello viaduct particular case, the R/C slab exhibited serious structural deficiencies and, therefore, was destined to demolition and subsequent reconstruction (fig. 1). Cut parts of the removed slab (about 500 x 200 x 1700 mm3) were carried to the LAPS Laboratory of the University of Bologna, Italy, in order to assess a possible strengthening technique for repair (fig. 2). The slab upper sides showed very poor conditions of preservation (i.e. deep cracks, spalling, heavy bars corrosion; fig. 2) and needed to be repaired. A number of 10 pieces of slab were considered in this study; 8 of them were subjected to concrete restoration (fig. 3), 6 of which were also strengthened with FRP bonded on either the original concrete (lower side of the slab) or the restored concrete (upper side of the slab) as shown in fig. 4. Both restoration and strengthening were executed by expert workmanship. The slabs were finally all subjected to four-point bending tests up to failure. In order to evaluate the efficiency of the executed concrete restoration and to assess the validity of the considered strengthening technique, the experimental values of the failure load were compared with the theoretical values estimated by means of the Italian CNR-DT 200/2004 [3]. 4 experimental results were consistent with the theoretical expectations. The detachment between the original and the restored concrete (fig. 5) was observed during 2 tests and the shear failure (Fig. 6) due to lower steel bars bond slip occurred in 2 slabs. Therefore, in such cases the CFRP laminates did not work properly and the slabs didn’t reach the target design performance. The following conclusions can be drawn from the experimental results: • The restoration of the damaged upper side concrete of the bridge slabs is usually carried out in viaduct repair, but the cooperation between the restored and the existing concrete must be ensured. Therefore, appropriate applications have to be executed in order to make the slab work properly, such as steel connectors, which in this work turn out to be adequate to this aim. Alternatively, in order to decrease the workmanship costs, steel trusses can be suitable. Also chemical products can be used, but it’s important to ensure in advance their effectiveness. • The application of CFRP connectors (tow) can decrease the labour costs because their high mechanical properties permit to apply a minor number of elements. However, these connectors can only work as vertical shear reinforcement, because of their uniaxial behaviour and they can not prevent the bond slip between the original and the restored concrete layers. The application of steel connectors gives a better result because they can ensure both the requisites. • The application of CFRP laminates can be done only if the R/C element to be strengthened shows an acceptably low damage level. In the present work, for example, the heavy corrosion of the steel bars does not permit to reach the target design performances. In the case of viaducts or bridges, the lower side surface has to be monitored carefully, in order to ensure the absence of steel corrosion, heavy environmental aggressions, concrete loss of integrity or carbonation.