Important Questions and Answers on Building Repair Part-3

What Steps or Procedures are to be Followed in Concrete/Masonry Repairs?

The defective (cracked) portion of the concrete or plaster has to be removed taking care that good concrete or plaster is not unnecessarily damaged or chipped off along with the defective portion.

During the repairs, structural load carrying members, e.g., columns, beams, slabs, chhajjas, etc., must be supported/propped/isolated so that the load on these members is reduced and transferred to other good structural members of the RCC framework. The supporting, propping and isolating details must be obtained from the Consultant.

One must be extremely careful during repairs as the weakened member may further lose its strength due to chipping and other removal procedures. Supporting/propping of the structural members must be insisted upon before actual repair work starts on it.

Chipping of defective or deteriorated concrete should be done till all defective concrete work (porous honeycombed concrete) and rust scales are removed.

Remove all rust from steel bars (reinforcement) by tapping lightly on the rusted bars, wire brushing, and cleaning. Rusting on steel in concrete is like cancer in a human body. It must be removed completely before repairs are taken up. If the bars have corroded too much, they will have to be replaced with new reinforcement steel and placed as per the details given by the Consultant. Apply, if specified, a rust converter on rusted steel bars after they are reasonably cleaned.

Wash & clean the whole surface before taking up repairs. Inspect concrete once again, carefully, for weak spots, rusted steel, and any type of porosity. If defects are observed repeat the above steps. If no further defects are observed, then start the repairs.

In the case of brick/block masonry, all joints, especially joints between masonry and reinforced concrete (RCC) should be properly inspected and sealed with mortar and cured for seven days. The surface should be washed and cleaned thoroughly before starting the repairs. If masonry is in a bad condition, it should be replaced.

Repairs, as specified by the consultant, should be carried out. Generally, concrete columns are either gunited or jacketed with concrete or treated with cement or non-cement-based polymer concrete. The slabs and beams are either gunited or treated with polymer concrete. Generally, the guniting method is now not being adopted as it is a messy job and the quality of work greatly depends on the skills of the nozzle men or operators who are scarcely available. These days polymer concrete is mainly resorted to but it is costly. Jacketing columns with concrete is the most economical. However, due to this the size of the column often increases and results in the reduction of the carpet area of the floor around the column locations.

The replaced surface must be scratched and rough-ended to give a good mechanical bond with subsequent finishing coats of mortar or plaster.

After the application of guniting /concrete jacket/polymer concrete, the repaired member should be cured for 7 days. Curing means maintaining over 98% humidity around the repaired surface. This is generally done by ponding water or spraying water at regular intervals. Proper curing is a must as it helps repaired material to gain strength and improves the surface hardness thereby making it impermeable and free from porosity and cracks.

The substrate is then allowed to surface dry and finishing coats of mortar (sand and cement mix) are applied thereafter in two coats. This is also called plaster. Curing is once again done for 7 days after application of each coat of plaster.

The safety of occupants and other persons moving in and around the building must be considered at the planning stage itself. The safety of workers is equally important. Proper insurance cover needs to be taken by the Contractor to avoid unforeseen liabilities on the building owners.

How do we Check at Which Locations Repairs are Necessary and at Which Locations Repairs Need not be Done?

It is very important to note that if there are no visual defects like cracks, spalling (pealing) of concrete, high level of deflection or sagging of beams or slabs, water seepage or dampness, etc. It is advisable not to repair such areas. Disturbing good concrete or masonry can do more harm than good.

Many times, consultants suggest that if a patch is bad, the entire concrete member/masonry has to be chipped and replastered otherwise cracks will develop between old and new concrete/ plaster due to shrinkage of the newly repaired surface. With adequate care and proper selection of materials and methods, harmful cracks can be avoided. While even in cases where concrete or masonry is fully chipped and repaired, if adequate care is not taken, harmful cracks may develop.

Similarly, the entire masonry plaster is often removed on the building face because of a small wet patch or some defect observed in a small area. This is uneconomical and therefore it is suggested that wasteful expenditures can be avoided by proper planning and identification of areas. This helps in avoiding cracks and at the same time cost are controlled.

At times, complaints are received that the repaired masonry wall in the very next monsoon starts showing signs of dampness, which were not existing before repairs.

Many times, concrete carbonation takes place and concrete loses its protective properties which inhibits corrosion of steel within it. However, even if the concrete has carbonated and there are no visual defects it is advisable not to repair the structure.

Often costly methods of guniting are specified on brick/block masonry walls, instead of equally effective manually applied plaster. Guniting on brick or block masonry is technically not justifiable and must therefore be avoided.

Why is Surface Preparation important in Repairs?

Yes, Surface preparation is extremely important. The reasons are as follows:

Dirt, dust, oil, etc. on the over faces must be removed. If these materials are not removed the adhesion of the concrete or repair material over the old surface will not be proper.

Rust on steel reinforcement (bars) must be thoroughly removed. Rust scales are like cancer cells in the human body. They have to be totally removed or else rust will develop again and result in repeated cracking of concrete. When steel, i.e., ferrous, gets rusted it is converted to ferrous oxide and later to ferrous hydroxide. The volume of rusted steel increases within the concrete structure causing tension and resulting in cracking or spalling of concrete.

The old concrete or masonry surfaces should be rough or roughened, if required, to give a proper mechanical bond with the new application over it.

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