Design matters

Design matters
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First Published: Wed, Dec 02 2009. 08 45 PM IST
Updated: Wed, Dec 02 2009. 08 45 PM IST
When the mason we appointed created pockmarks on the existing wall plaster to create a gripping surface for the layer of plaster of Paris (POP) to be applied, the existing plaster started coming off. At points it came off in clumps, at others it held up just enough to start the POP work. Has this happened due to lack of cement in the plaster? Should we redo it entirely, as suggested by the mason? We are already behind schedule. Will fixing murga jaali (chicken-wire mesh) on the existing surface help? How much extra would I have to pay for this?
— Deepa Mohindra
The wall plaster will fall off in clumps only if it has not bound well with the wall. Redo the entire plaster. The approximate cost in a high-rise building is Rs8-10 per sq. ft for dismantling, and Rs12-18 per sq. ft for application of a 15mm thickness of plaster (1 part cement: 6 parts sand). Expect an 8-15% discount for plaster area more than 1,000 sq. ft. The additional time required would be one-two weeks (approximately), depending on the surface area to be covered, ambient room temperature and humidity.
The wall plaster could also dust off (rather than fall off in clumps) due to two conditions: inadequate cement quantity or inadequate curing (keeping the plaster moist until it achieves full strength). In both conditions, the plaster dusts off like dry sand when its surface is disturbed. You could redo the plaster or strengthen the existing surface with a brush application of cement-water slurry (also called cement-water “solution”) on the surface, curing it before applying the second coat, followed by a layer of POP.
Application of cement solution on the existing surface is recommended only if the plaster is slightly weak, and not if it is falling off in clumps. Moisten the existing plaster an hour before applying the cement slurry. The cost of cement slurry application is approximately Rs3-5 per sq. ft per coat. The additional time required will be approximately one or two days per coat.
Murga jaali or chicken-wire mesh is nailed to the existing (brick or plaster) surface to avoid shrinkage cracks that appear on the final plaster or POP surface when (a) excess water is used to make the plaster or POP paste; (b) the POP layer is very thick; or (c) the plaster and POP cover the junction between two different materials, such as a brick wall and RCC (reinforced cement concrete) column/beam. The wire mesh provides nominal reinforcement to the plaster, similar to mixing in metal or polymer fibre, easily available in the market today. The cost is approximately Rs8-12 per sq. ft. However, this will not help if the underside of the plaster is weak, and is coming off in clumps. The additional time required would be approximately one or two days.
We want our basement wall to be 6 inches of RCC, plus a brick wall inside, but our consultant is insisting on a 13-inch brick wall on three sides, with RCC plus brick on only one side, which is all that is required, according to him. What should we do?
— Manish and Varun Paul
An RCC wall is recommended with a (column-beam) frame structure as it helps to tie the building frame together better, even if a raft foundation is not being employed. A 13.5-inch brick wall, on the other hand, is all right if it touches the basement wall of the adjoining house and no thrust is expected on its surface.
Though not waterproof by itself, RCC will help if you want to use a pressure grouting technique for waterproofing. Chase-cutting for electrical and plumbing pipes is not recommended in an RCC wall, therefore a 4.5-inch brick wall can be erected along its inner face to accommodate those. This inner brick wall could also be erected before casting the RCC wall. This way, the thin brick “skin” binds well to the RCC. The only problem with this is that you will never be able to see any honeycombing (hollow spaces) in the embedded RCC wall, so repairing it becomes a challenge.
For walls erected along the free edge of your plot, with no adjoining house to support the RCC on the other side, the RCC wall can be embedded between two 4.5-inch brick walls, so that these ultimately sit on either side of the RCC core.
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First Published: Wed, Dec 02 2009. 08 45 PM IST