Strengthening of RC Columns

Details of FRP jacketing methods for columns are discussed in Section 5.3.2, so they are not repeated here. 

There are basically three types of failure modes observed in existing RC columns with inadequate transverse reinforcements and/or seismic detailing. The first and the most critical 

It is a well-established fact that lateral confinement of concrete can substantially enhance its compressive strength and ductility [58-61] and it is beneficial to controlling, delaying and, in some cases, even preventing all three modes of failure mentioned above. Therefore, the most common form of FRP strengthening of columns involves external wrapping of FRP sheets or straps around them. 

Many researchers have studied the stress-strain behaviour of concrete confined with FRP composites. Enhancement in the compressive strength of concrete as a result of external wrapping of FRP was first demonstrated by Fardis and Khalili [62,63]. Although the mechanical aspects of FRP confinement of concrete leading to an improvement in its behaviour is very similar to that of steel, there are some major differences between the two One of the problems with FRP confinement of concrete is that the strength of FRP jacket cannot be fully utilised until the lateral strain in the confined concrete is very high. In some cases, the concrete will crush before the potential strength of FRP jacket is fully mobilised [65-67]. 

The idea of confining concrete by winding continuous resin-impregnated fibre strands aroimd RC columns (filament winding) is another technique used for FRP strengthening of RC columns. In this process, an FRP jacket with controlled thickness, fibre direction and volume fraction can be obtained. This procedure is more appropriate for circular columns and usually, special computer-controlled winding machines are used for this purpose. 

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Ye, L.P., Zhang, K., Zhao, S.H., and Feng, P. (2003). Experimental study on seismic strengthening of RC columns with wrapped CFRP sheets . Construction and Building Materials, 17 499-506. 

Perrone, M., Barros, J.A.O., and Aprile, A. (2009) CFRP-based strengthening technique to increase the flexural and energy dissipation capacities of RC columns . Journal of Composites for Construction, 13(5) 372-383. 

Wu, Z.S., Gu, D.S., Wu, G., and Hirahata, H. (2006b). Seismic performance of RC columns strengthened with Dyneema fiber-reinforced polymer sheets . In Proceedings of the Fourth International Conference of Earthquake Engineering, Taipei, Taiwan. 

Seismic retrofit of columns and shear strengthening of RC bridge structures... 

Strengthening of concrete columns can be achieved by reinforced concrete (RC) jackets. This is an intrusive and time-consuming procedure, but in some cases it is necessary. However, the effect of strengthening a column using RC jackets should be examined carefully as this method will increase column flexural strength and stiffness more than a steel jacket, FRP composite jacket, or wire wrap, with potentially undesirable effects on bridge performance. 

In this entry, special emphasis is given on the technique of reinforced concrete (RC) jacketing. RC jacketing is a widespread technique used for the strengthening of existing columns, beams, and walls. The use of epoxy resins for the crack repair of damaged stmctural elements is also presented. 

A wide section of the guideline focuses on the brittle failure mechanisms. In particular, according to experiences gained from examining the performances of RC structures after seismic events, a wide section focuses on the most common brittle collapse mechanisms resulting from shear failure of partially confined beam-column joints (i.e. exterior or comer joints on the perimeter of the stmcture in some cases, where frames are only in one of the plan directions, they could be also interior). The typical joint failures observed in the L Aquila post earthquake are described and a local strengthening procedure by means of FRP, steel jacketing or CAM technique is discussed. 

The experimental tests clearly pointed out the potential of the recommended FRP solution to increase the seismic performances of beam-column joints designed according to non-seismic code provisions. The experimental validation of this local strengthening technique may strongly encourage the use of local interventions based on the use of FRP reinforcement to significantly increase both the local and global seismic capacity of RC structures. 

As an introductory example, consider the load paths followed in the case of an RC column, before and after a critical earthquake, vis-a-vis those followed during a subsequent intensive earthquake acting on the same column strengthened by means of an RC jacket. Let the existing and the added materials be indicatively... 

The monolithicity coefficient values for the design of strengthened RC columns presented herein depend on the type of the jacket (four or three sided), the normalized axial load value, and the thickness of the jacket. The proposed values are more realistic than using one value to cover all situations, as is the case with present code recommendations (Eurocode 8 2005 Code of Structural Interventions 2012). It was also found that in many cases, monolithicity coefficient... 

Antonopoulos CP, Triantafillou TC (2003) Experimental investigation of FRP-strengthened RC beam-column joints. ASCE J Compos Constr 7(l) 39-49... 

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