Strengthening of reinforced concrete columns is needed when:
1. The load carried by the column is increased due to either increasing the number of floors or due to mistakes in the design.
2. The compressive strength of the concrete or the percent and type of reinforcement are not according to the codes’ requirements.
3. The inclination of the column is more than the allowable.
4. The settlement in the foundation is more than the allowable.
There are two major techniques for strengthening reinforced concrete columns:
1 REINFORCED CONCRETE JACKET
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.
In some cases, before this technique is carried out, we need to reduce or even eliminate temporarily the loads applied to the column; this is done by the following steps:
- Putting mechanical jacks between floors.
- Putting additional props between floors.
Moreover, in some cases, where corrosion in the reinforcement steel bars was found, the following steps should be carried out:
- Remove the concrete cover.
- Clean the steel bars using a wire brush or sand compressor.
- Coat the steel bars with an epoxy material that would prevent corrosion.
- If there was no need for the previous steps, the jacketing process could start by the following steps:
- Adding steel connectors into the existing column in order to fasten the new stirrups of the jacket in both the vertical and horizontal directions at spaces not more than 50cm.Those connectors are added into the column by making holes 3-4mm larger than the diameter of the used steel connectors and 10-15cm depth.
- 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 1 and 2.
- 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.
The previous steps are illustrated in Fig 1.
Fig:1. Increasing the cross-sectional area of column by RC jacketing.
2 STEEL JACKET
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.
This technique is implemented by the following steps as shown in Fig 2:
1. Removing the concrete cover.
2. Cleaning the reinforcement steel bars using a wire brush or a sand compressor.
3. Coating the steel bars with an epoxy material that would prevent corrosion.
4. 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.
5. Filling the space between the concrete column and the steel jacket with an appropriate epoxy material.
Fig:2. Increasing the cross-sectional area of column by steel jacketing.
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.
Fig:3. shows a column which was strengthened with steel angles.