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Strengthening concrete to add more floors

Micro concreting and composite fibre wrapping are the most popular techniques.

As cities across the world revise their master plans to permit higher floor-area ratios and join the trend towards vertical growth, architects and engineers are faced with new challenges in the strengthening and repair of concrete structures.

For example, adding more floors to a building beyond the designed height calls for new foundations and columns. But that option is ruled out if the building has been constructed up to the boundary line or if the interiors lack enough space to accommodate new members. The alternative is to strengthen existing foundations and columns to support higher loads.

Likewise, the columns, beams, slabs and retaining walls will have to be strengthened if a structure is to take more than the designed load. Flaws in structural design and construction require expensive repairs.

Until recently, the accepted methods of strengthening were concrete jacketing, guniting or steel-plate bonding, all cumbersome, labour intensive and problematic. These techniques add to the size of members and increase deadweight.

Modern science has equipped the construction sector with technological options to overcome most of the challenges in structural rehabilitation. Micro concreting and composite fibre wrapping are the most popular techniques in use today.

Micro concrete is a ready-to-use blend of dry powders which require only addition of clean water to produce a free-flowing, non- shrinking repair medium. The material is based on Portland cements, graded aggregates, filters and additives. The low water requirement ensures early strength and long-term durability. “The high compressive strength makes it ideal for strengthening works,” says K. Sajikumar, chief executive of Intdecor, a city-based firm marketing the Fosroc range of engineering products.

Micro concrete is applied after roughening the existing concrete surface and adding more steel rods before installing the shuttering. A special epoxy resin agent is applied to the old surface to ensure bonding with the micro concrete.

One of the advantages is that it can be pumped or poured into small areas where access is restricted and where vibration of the material is difficult or impossible. Application of micro concrete increases the thickness of the column by just about 5 cm. The highly fluid composition allows for placement without vibration.

Composite fibre wrapping system is a more advanced method used for strengthening of concrete. Available in rolls, the material resembles a cloth and is just about that thick. Since it is wrapped around the surface, it is ideal for any type of profile or shape. “Composite fibre wrapping is gaining acceptance as a concrete- strengthening medium because it is easy to install, saves time and labour and is resistant to chemicals. Because it protects the concrete from corrosion, it can strengthen bridge piers and jetty piles constantly exposed to water,” Mr. Sajikumar says. Other applications include columns, beams, slabs, retaining walls, pipes, chimneys and tunnels.

Both micro concreting and composite fibre wrapping system are accepted methods of strengthening structures for earthquake resistance.

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