Column strengthening is a process used to add or restore ultimate load capacity of reinforced concrete columns. It is used for seismic retrofitting, supporting additional live load or dead load that not included in the original design, to relieve stresses generated by design or construction errors, or to restore original load capacity to damaged structural elements.
There are several techniques which are used to strengthen reinforced concrete columns like reinforced concrete jacketing, steel jacketing, and FRP confining or jacketing.
When strengthening of R.C. Column is needed?
- The load carried by the column is increased due to either increasing the number of floors or due to mistakes in the design.
- The compressive strength of the concrete or the percent and type of reinforcement are not according to the codes’ requirements.
- The inclination of the column is more than the allowable.
- The settlement in the foundation is more than the allowable.
Strengthening Techniques for R.C. Columns
There are three major techniques for strengthening reinforced concrete columns which are discussed below:
1. Reinforced Concrete Jacketing
It is one of the techniques used to improve or restore capacity of reinforced concrete column. The size of the jacket and the number and diameter of the steel bars used in the jacketing process depend on the structural analysis that was made to the column.
Reinforced Concrete Jacketing Process
- Initially, reduce or eliminate loads on columns temporarily if it is required. This is done by putting mechanical jacks and additional props between floors.
- After that, if it is found out that reinforcements are corroded, the remove the concrete cover and clean the steel bars using a wire brush or sand compressor.
- Then, coat the steel bars with an epoxy material that would prevent corrosion.
- If reducing loads and cleaning reinforcement is not needed, the jacketing process begin by adding steel connectors into the existing column.
- The steel connectors are added into the column by making holes 3-4mm larger than the diameter of the used steel connectors and 10-15cm depth.
- The spacing of new stirrups of the jacket in both the vertical and horizontal directions should not be more than 50cm.
- Filling the holes with an appropriate epoxy material then inserting the connectors into the holes.
- Adding vertical steel connectors to fasten the vertical steel bars of the jacket following the same procedure in step 5 and 6.
- Installing the new vertical steel bars and stirrups of the jacket according to the designed dimensions and diameters.
- Coating the existing column with an appropriate epoxy material that would guarantee the bond between the old and new concrete.
- Pouring the concrete of the jacket before the epoxy material dries. The concrete used should be of low shrinkage and consists of small aggregates, sand, cement and additional materials to prevent shrinkage. Steps of reinforced concrete jacketing are illustrated in Fig. 1.
2. Steel Jacketing
This technique is chosen when the loads applied to the column will be increased, and at the same time, increasing the cross sectional area of the column is not permitted.
Steel Jacketing Process
- Removing the concrete cover.
- Cleaning the reinforcement steel bars using a wire brush or a sand compressor.
- Coating the steel bars with an epoxy material that would prevent corrosion.
- Installing the steel jacket with the required size and thickness, according to the design, and making openings to pour through them the epoxy material that would guarantee the needed bond between the concrete column and the steel jacket.
- Filling the space between the concrete column and the steel jacket with an appropriate epoxy material.
In some cases, where the column is needed to carry bending moment and transfer it successfully through the floors, one should install a steel collar at the neck of the column by means of bolts or a suitable bonding material.
3. FRP Confining or Jacketing
For detailed description and design of strengthening reinforced concrete columns using FRP confining, please click here.