(This article is intended to present the technology and materials of construction, as practiced for a G+4 building in Chennai. This is not intended to be of diagnostic or design significance.)
Minor Damages
Jacketing with micro-concrete was adopted for columns in initial stage of corrosion with minor damages such as narrow cracks and discolouration, without appreciable damage to existing steel. The brief specifications and methodology for the same are as follows:
- Existing structure shall be propped to ensure support during rehabilitation work
- Plaster/concrete on distress areas shall be removed and steel exposed. The entire surface is cleaned preferably with sandblasting.
- Zinc based epoxy primer shall be applied on exposed surface of reinforcement in one or two coats to ensure complete protection.
- Packaged cement-based micro-concrete shall be provided to sufficient thickness all round existing columns. It is only needed to add water to packaged micro-concrete.
- If after cleaning, fresh concrete area is larger than 100mm, concrete shall be provided with larger aggregate instead of micro-concrete. In this case, graded aggregates are to be mixed with packaged micro-concrete.
- Before concrete, the column surface shall be soaked in water for few hours. Concrete surface may be provided with slow setting epoxy adhesive primer film to ensure composite action of old and fresh concrete.
- Curing shall be done for 7 days.
- A polyurethane outer coating may also be provided, if necessary
- All chemical application shall be done by skilled contractors and shall be in accordance with manufacturers specifications.
Jacketing with carbon-fibre-composite micro-concrete was adopted on columns with later stage of corrosion and moderate damages such as cracks, spalling and some damage to existing steel.
The brief specifications and methodology for the same are as follows:
- Existing structure shall be propped to ensure support during rehabilitation work
- Plaster on distress areas shall be removed and steel exposed. The entire surface is cleaned preferably with sandblasting.
- Zinc based epoxy primer shall be applied on exposed surface of reinforcement in one or two coats to ensure complete protection.
- Depressions on surface may be filled with epoxy putty and/or cement-based micro-concrete. Large depressions shall be filled using micro-concrete with graded aggregates.
- One or more layer of carbon fibre fabric shall be applied onto the surface sandwiched between two layers of suitable saturant polymers.
- Before concrete, the column surface shall be soaked in water for few hours. Concrete surface may be provided with slow setting epoxy adhesive primer film to ensure composite action of old and fresh concrete 24 hours prior to application of jacketing.
- Curing shall be done for 7 days
- A polyurethane outer coating may also be provided, if necessary
- All chemical application shall be done by skilled contractors and shall be in accordance with manufacturers’ specifications.
Severe Damages
Where columns are badly spalled and column steel is considerably eaten away, structural repair was done with RC Jacketing. The brief specifications and methodology for the same are as follows:
- Existing structure shall be propped to ensure support during rehabilitation work
- Excavation shall be done to expose footing top below.
- Plaster on distress areas shall be removed and steel exposed. The entire surface is cleaned preferably with sandblasting.
- Zinc based epoxy primer shall be applied on exposed surface of reinforcement in one or two coats to ensure complete protection.
- Depressions on surface may be filled with epoxy putty and/or cement-based micro-concrete. Large depressions shall be filled using micro-concrete with graded aggregates.
- The column size is proposed to be increased all round using M20 micro-concrete with graded aggregates. New main steel and lateral steel are to be provided. Steel shall be HYSD TMT bars. Clear cover of 50mm may be provided. Steel for use shall be clean and free from dust, rust or oily material.
- Holes shall be drilled into overhead soffit and footing beneath. The depth of hole should accommodate load transfer. Rotary percussive drills may be used with air or water flushing. Diamond drilling is not considered suitable.
- Pumpable grade polyester resin grout (paste/capsule) shall be used for filling overhead and vertical drill holes.
- The bars are now pushed into the drill holes and positioned. Butt welding of bars may be permitted in case of lap splices. After the respective curing period of the grout, steel ties shall be tied round the steel and formwork for concrete may be made ready.
- Before concrete, the column surface shall be soaked in water for few hours. Concrete surface may be provided with slow setting epoxy adhesive primer film to ensure composite action of old and fresh concrete 24 hours prior to application of jacketing.
- M20 cement based micro-concrete shall be applied in the formwork to jacket the column
- Curing shall be done for 7 days
- A polyurethane outer coating may also be provided, if necessary
- All chemical application shall be done by skilled contractors and shall be in accordance with manufacturers’ specifications.
Summary of Chemicals for Rehabilitation
Chemical | Forsoc | Pidilite | Sika |
Zinc based epoxy primer (for corrosion protection of steel) | Nitozinc Primer | Dr. Fixit Epoxy Zinc Primer | -- |
Cement-based Micro-concrete (with/without graded aggregates) | Renderoc RG | Dr. Fixit Micro-concrete | SikaRep Microcrete |
Epoxy putty | Nitocote VF | -- | -- |
Carbon fibre fabric with saturant | Nitowrap EP | -- | SikaWrap |
Pumpable grade polyester resin grout (paste/capsule) | Conbextra capsule & Lokfix | -- | AnchorFix |
Slow setting epoxy adhesive primer (for bond between new & old concrete) | Nitobond EP | Dr. Fixit Epoxy Bonding Agent | SikaDur - 32 |
Aliphatic polyurethane coating (optional over finished surface) | Nitocote UR512 | -- | Sikalastic |
Wax based Concrete curing compound | Concure | -- | Antisol |
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