Retrofitting and jacketing

Retrofitting

Retrofitting is the strengthening or modification of an existing building so that it can resist earthquake forces better than it was originally designed to. It improves the strength, stiffness, ductility and load path of the existing structure without rebuilding it.

Jacketing

Jacketing is the most common retrofit method for RCC members. A new layer of material is placed around an existing column, beam or wall to increase its strength, stiffness, ductility and confinement.

RCC jacketing

A new concrete layer with additional steel is cast around an existing column or beam.

1. Remove loose concrete, roughen the surface and clean the bars.
2. Provide dowels (12-16 mm bars), drilled and epoxy-grouted into the footing and slab.
3. Place new longitudinal bars and closely spaced closed ties (6-8 mm at 75-100 mm c/c).
4. Apply a bonding agent and cast high-slump, small-aggregate (10-12 mm) concrete or micro-concrete; the jacket is 75-100 mm thick.
5. Cure for at least 14 days.

   Before          After (RCC jacket)
  +------+       +--------------+
  |      |       | +----------+ |
  | old  |       | | old col  | |
  | col  |       | +----------+ |
  +------+       +--------------+
                  new bars + ties

Steel jacketing

Steel plates (or angles with battens/straps) are placed around the column and welded or bolted together, and the gap (about 25 mm) is filled with non-shrink cement grout. It adds confinement and shear strength with little increase in size and is quick to execute, but it needs fire and corrosion protection.

   Steel plates      Angles + battens
  +==========+       L----------L
  | grout    |       | batten   |
  |  column  |       |  column  |
  +==========+       L----------L

Advantages of jacketing: increases the axial, flexural and shear capacity and the ductility of columns and beams. Limitations: the section size grows (RCC jacket), and workmanship at the old-new interface is critical.