Strengthening of columns foundations is required in the case of applying additional loads. Widening and strengthening of existing foundations may be carried out by constructing a concrete jacket to the existing footings.
The new jacket should be properly anchored to the existing footing and column neck in order to guarantee proper transfer of loads. This can be accomplished by drilling holes into existing concrete of footing and epoxy grouting the longitudinal reinforcement of jacket.
Another possibility is to provide full anchorage length for longitudinal reinforcement by extending the column jacket at the top of footing.
When the bearing area of footing is not sufficient, the size of the footing should be increased. If the column is also being jacketed, the transfer of load from column to footing becomes easy.
The size of the “jacket” shall be selected such that the average maximum foundation pressure does not exceed the recommended allowable value. Attention shall be given during construction in order that the excavations for the new “jackets” do not affect the existing adjacent foundations.
There can be a split of new concrete from the old concrete surface under the action of loads. To avoid this splitting of concrete, sufficient number of closed rings with sufficient overlap or welded connection should be provided around the footing.
Steps of Isolated Footing Strengthening
An isolated footing is strengthened by increasing the size of the footing and the reinforcement steel bars as follows:
- Excavating around the footing
- Cleaning and roughening the concrete surface.
- Installing dowels at 25-30cm spacing in both directions using an appropriate epoxy material.
- Fastening the new steel bars with the dowels using steel wires. The diameter and number of steel bars should be according to the design.
- Coating the footing surface with a bonding agent in order to achieve the required bond between old and new concrete.
- Pouring the new concrete before the bonding agent dries. The new concrete should contain a non-shrinkage material.
The previous steps are illustrated in Fig below.
Fig: Steps for strengthening foundations
Jacketing of Foundations for Strengthening
The following photos (1 – 4) illustrate the practical way of jacketing a footing by reinforced concrete.
Fig. 1 – 4: Strengthening of footing by Jacketing
If the bearing area of footing has to be increased without strengthening of the column, soil pressure on the extended foundation area has to be transferred to the existing footing.
Transfer of soil pressure to existing footing is difficult as excavation is required below the existing footing. The building has to be properly supported and settlement of foundation has to be avoided.
In this case also, there is a tendency of the new concrete to split from the old concrete. To avoid this, sufficient numbers of well anchored/welded hoops are required.