Methods for Repairing Concrete Columns in Old Buildings
Before repairing concrete columns in old buildings, loads such as the axial dead, live load, horizontal load and their related bending moments must be known.
Concrete column repairs can broadly be divided into two categories — Surface repair and structural repair. Surface repair (or cosmetic repair) only takes into consideration the local deterioration while structural repairs, on the other hand, restore and strengthen those areas of the columns which have been seriously affected.
Some of the methods of repairing columns in old concrete buildings are as follows –
· By jacketing, or enlarging the cross section of the column.
· Re-alkalization of steel to prevent corrosion.
· Chloride extraction is also done to prevent corrosion.
· Increasing the shear capacity of intermediate floors by adding shear collars.
Parameters to Follow When Repairing Columns
The column needs to be unloaded so that the cross-section is able to carry the load of the new design in case concrete deterioration is significant. If unloading is not done, the new repair will be unable to carry any load.
Shrinkage of new material can further reduce the load. The problem, however, is that unloading columns is quite expensive. The costs escalate further for high-rise buildings.
The enlarged cross-section can provide confinement to the existing column if the present load is not removed before beginning repairs. This is because the percentage of direct load which can be taken by an enlarged cross-section is less than twenty-five percent of the jacket strength.
If removal of load from the column is not an option, then an additional column system can provide a method of support in conjunction with the repair of the existing column.
Redistributing the Load
In case of corrosion along with significant concrete deterioration, the load has to be redistributed in the structure before repairing. This is especially important if a new pattern is to be designed as adjoining members can get affected by the redistribution.
Supplemental Reinforcing Steel
At the time of the repairs, column ties cannot be disturbed as it might lead the longitudinal bars to get buckled. As a result, the supplemental vertical bars may need to be placed outside the tie bars. The dimensions of the column also need to be increased to provide requisite cover. Apart from these, hairpin ties, usually of stainless steel are used to provide lateral support to the supplemental bars.
Concrete removal within a column cage can only be done for unloaded columns. If not unloaded, the longitudinal bars will buckle, which can lead to the column’s compression failure.