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.
An isolated footing is strengthened by increasing the size of the footing and the reinforcement steel bars as follows:
The previous steps are illustrated in Fig below.
Fig: Steps for strengthening foundation
Jacketing of Foundations for Strengthening
The following photos (1 - 4) illustrate the practical way of jacketing a footing by reinforced concrete.