Conventional strengthening and innovative-material retrofit

Conventional strengthening methods

1. RCC jacketing of columns, beams and walls to enlarge the section and increase confinement.
2. Steel jacketing/plates/angles with grout; steel bracing (X or V) in frames.
3. Adding RC shear walls or infill walls to give lateral stiffness.
4. Wire-mesh/ferrocement plastering on masonry walls.
5. Strengthening foundations: enlarging footings, underpinning, adding tie beams, grouting the soil.
6. Strengthening connections and anchoring roofs; adding bands and vertical bars to masonry.
7. Epoxy/cement grouting of cracks and honeycombs.
8. Reducing seismic demand: removing upper storeys, heavy parapets and tanks.

Disadvantages: these methods add mass, enlarge sections, are slow, and disturb the occupants.

Retrofit using innovative materials

1. FRP (fibre reinforced polymer) wraps/laminates: carbon (CFRP), glass (GFRP) or aramid sheets bonded with epoxy to columns (confinement), beams (flexure/shear), walls and slabs. They are light, strong, corrosion-free and quick to apply, with no increase in section, but they are costly, weak in fire, and need skilled work.
2. Fibre reinforced concrete / shotcrete and ECC (engineered cementitious composites) for jackets and overlays.
3. Steel fibre or polypropylene mesh, geosynthetics and GFRP grids in plaster overlays for masonry (e.g. polymer mesh used in Nepal after 2015).
4. Shape memory alloys and prestressed straps: give active confinement and close cracks.
5. Base isolators and dampers (rubber bearings, viscous dampers): reduce the force reaching the structure; used in important buildings.
6. Textile-reinforced mortar (TRM) and high-performance mortar overlays.

These materials give high strength with little added weight, so they are suitable for retrofitting without major disturbance.