FRP axial strengthening systems are used to improve or enhance the capacity of reinforced concrete columns. It can be used for both circular and rectangular shaped columns but it is more effective in the former shape.
In this article the design of FRP axial strengthening system for columns is discussed.
Fig.1: Applying Continuous FRP Systems for Axial Strengthening of RCC Columns
Advantages of FRP Axial Strengthening Systems for Columns
- Increases the ultimate load carrying capacity of reinforced concrete member
- Improves shear capacity of reinforced concrete element
- Reinforcement bar lap splice capacity of the member is improved due to FRP axial strengthening system application
- The ductility of reinforced concrete column is improved considerably.
Design of Axial FRP Strengthening Systems for ColumnsFRP axial strengthening is usually conducted by providing fiber reinforced polymer (FRP) around reinforced concrete columns. This strengthening technique is specifically influential when the column is circular. However, if the reinforced concrete column is rectangular and the ratio of depth to width of column is larger than 2 or the smallest side of the column is greater than 900mm, then ACI 440.2R-08 is not applied for this strengthening method. Figure-2 illustrates the confined area in different shapes of concrete columns.
Fig.2: Confined Area in Circular, Square and Rectangular Concrete ColumnsThe ineffectiveness of rectangular or square column confinement might be attributed to the non-uniform stress distribution and stress concentration at the corner of the section. This may lead to premature failure of strengthened element. It is essential to wrap reinforced concrete column completely with FRPs in order confine and improve the element effectively. Unlike the flexural and shear strengthening of reinforced concrete beams, the FRPs which surround the column activated only if the member is enlarged laterally and exert stresses on the FRPs. This means that, beam strengthening is an active system whereas column strengthening is a passive system. The FRP system which wrapped around the column creates circumferentially uniform confine pressure that acts against the radial compression enlargement. Figure-3 illustrates how FRP systems create a pressure against the compression expansion of the concrete column.
Fig.3: Forces Acted in a Confined Concrete Column
Fig.4: Reinforced Concrete Column Confined with Continuous FRPs
Strengthening of RCC Column Under Pure CompressionThe compression strength of reinforced concrete column can be increased through confinement of the column. The confinement is achieved by directing FRP systems around the column transverse to longitudinal direction of the column. It should be said that, any FRP system that applied to in other directions should be ignored. The nominal capacity of short, non-prestressed, normal weight reinforced concrete column with tie and spiral reinforcement can be calculated by the following two equations which is provided by ACI 318-11: For tie:
Fig.5: Equivalent Circular Cross Section
Column Subjected to Combined Axial Compression and BendingReinforced concrete column that is subjected to both axial compression and bending can be strengthened by axial FRP strengthening systems. If the eccentricity is smaller than 0.1h then equation-1 and equation-2 can be employed to anticipate the confinement effect on the strength improvement. However, when the eccentricity surpasses 0.1h then the two previous equations is employed to estimate the concrete material properties of the cross-section element under compression. This will be used to construct interaction (P-M) diagram, Figure-6, for the concrete element that has been confined by FRP systems. Moreover, there are several restrictions which should be considered when the member is subjected to axial compression and bending. The first condition which should be considered is that, FRP effective strain must be greater than 0.004. Moreover, if maximum applied bending moment and axial force located below the line that connect the balanced point in the interaction diagram for unconfined member and the origin, then strength improvement should not be considered.
Fig.6: Interaction DiagramRead More: Strengthening of RCC Beams in Shear using Externally Bonded FRP Plates or Strips Strengthening of Foundations