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  • Writer's pictureBenny Kuriakose

Learn to Repair Concrete Slabs in Flood Affected Buildings

Increasing urbanization, global warming, and climate change have led to more frequent floods in India. The frequency of flooding seems to be increasing, with some places like Kerala and Chennai facing them almost every year. Floodwater entering houses can cause permanent damage to concrete slabs, floors, and ceilings. The repair of buildings can be expensive, and if not tended to can even result in the collapse of the building. With this in mind, this next chapter of our series of how to repair flood affected buildings specifically explores the different methods that are used to repair cracks in concrete slabs; be it for floor, ceiling, or roof.


List of Contents



1. Repairing Cracks in Concrete Slabs


Cracks in a concrete slab are a serious hazard to the structure. Rainwater or floodwater seepage can cause corrosion of reinforcement, which can lead to further cracking and severe damage. In such cases, seeking advice from experts is the best option. The steps to repairing cracks in concrete slabs are as follows:


1.0.1 Steps to Repair Minor Cracks in Concrete Slabs

  • Form a ‘V’ groove along the crack.

  • Remove all the loose particles using a wire brush and water.

  • Fill in the voids using a non-shrinkable cement grout.

  • Carry out curing for a minimum of 15 days.


Repair Minor Cracks in Concrete Slabs
Repairing Minor Cracks in Concrete Slabs

1.0.2 Steps to Repair Major Cracks in Concrete Slabs


In addition to the steps to be taken for repairing minor cracks, stitching may be used to make sure that the tensile strength of concrete slabs is retained. The steps to repair major cracks is as follows:

  • On both sides of the crack, drill holes in the concrete slab

  • The chicken wire mesh should be fixed along the wall using nails and it should have proper cover to avoid any corrosion in future.

  • U Shaped metal rods, 4-6 mm diameter is inserted as shown below.

  • Fill the voids using non-shrink cement grout and mortar.

  • Carry out curing for a minimum of 15 days.


Repairing Major Cracks in Concrete Slabs
Repairing Major Cracks in Concrete Slabs

2. What to do About the Dampness on the Underside of Concrete Slabs


The pressure exerted by floodwater will force itself into the pores of a concrete slab if it does not have good water-proofing qualities. Dampness might appear on the sides of a concrete slab that is exposed to the weather. But, it is more likely to be seen on the underside (the ceiling), where corrosion of reinforcement can happen.


The salts in the floodwater are responsible for the corrosion of the reinforcement bars. The presence of chlorides will cause the breakdown of the protective oxide layer on the iron bars. When the oxide layer is eaten away, and iron is exposed to the atmosphere, it expands and cracks, causing more corrosion.


Dampness on the Underside of Concrete Slabs
Dampness on the Underside of Concrete Slabs

3. Repairing Cracks in Concrete Ceiling


The cracking of the concrete may happen in beams, columns and slabs. The ceiling can be damaged due to the air pressure build-up that happens as the water level rises in the slab, and the air gets trapped in between them. Initially, the cracking will be straight; followed by the falling of concrete pieces, exposing the iron bars to the atmosphere. The repairs shall be done as early as possible when these cracks appear. The damaged portion of the reinforcement is to be removed, and rust inhibiting chemicals can be applied over the iron bars. After that, rich cement concrete or mortar mixed with non-shrink chemicals can be applied over the reinforcement to prevent further atmospheric contact.


However, if the decay is in the advanced stage, then expert opinion should be sought before starting the repair of the concrete ceiling. If a substantial cross-section of the reinforcement bar has been eroded by corrosion, then additional iron bars will have to be added to strengthen the concrete slab. The cement grout will have to be applied to hide the reinforcement bars.


Repairing Cracks in Concrete Ceiling
Repairing Cracks in Concrete Ceiling

4. How to Get Rid of Dampness on Floors


Water may leak through the joints of a tiled floor during floods. The water that is trapped in the soil below the floor comes up through the tile joints because of the upward thrust, leading to floor cracks. These floor cracks will have to be filled up depending on their width. The problem will disappear once the water has receded and proper drainage around the building is planned.


Dampness on Floors
Dampness on Floors

5. Lifting of the Floor Tiles


The floor tiles will sound hollow because of lost adhesion between the tile and the base of the floor. In some cases, the tiles might also get detached from the floor base. This will be worse in cases of poor workmanship.


In the event of serious damage, the affected portion of the floor will have to be re-laid or re-tiled. In other situations, cement grout mixed with non-shrink additives can be injected through the joints.


Lifting of Floor Tiles
Lifting of Floor Tiles

6. Salt Crystallization on Floors


The floors will have white patches after the floodwater has receded. The white patches will disappear when the floor is washed or wiped; on drying they reappear. These white deposits are the salts that would have found entry into the floors through the contaminated floodwater. The salts dissolve when the floor is washed, but when the water evaporates, the salts are left behind again.


They can be removed using a simple technique. Apply a layer of paper pulp (newspaper immersed in water for a few hours and crushed by hand) over the surface of the floor, and leave it to dry. As the paper pulp dries, the water evaporates and the salts will be left behind in the paper pulp. Continue this process a minimum of three times so that the salt which is present below the floor can also be completely removed.


Salt Crystallization on Floors
Salt Crystallization on Floors

7. Step to Repair Floor Cracks


If the floor crack is a hairline crack or is minor, it is better not to do any repair work. If the repair is attempted, it might damage the floor and make it unpleasant. If the floor crack is major, then follow the procedures for filling in the cracks as mentioned in section 1.0.1.


Repairing Floor Cracks
Repairing Floor Cracks

8. Flaking of Plaster


There are many cases where flaking of plaster (small loose masses formed on the plaster) takes place, and the plaster starts to flake off after the seepage of floodwater. The affected areas may be randomly located in the wall. Once the water finds a way between the brickwork and the plaster, it will lose its adhesion (grip). Once the adhesion is lost, the affected area will sound hollow when tapped with a stick. Initially, only a small portion of plaster will peel off the wall. If possible, apply cement grout using a syringe in these hollow-sounding portions and the plaster will continue to hold its position. The areas where the plaster has fallen off the wall will have to be replaced with fresh plaster.


Flaking of Plaster
Flaking of Plaster

9. Blistering of Paintwork


If water has found entry inside the painted surface through some crevices, the paint will peel off the walls or timber elements. The remedy for this is to paint the surface again. Proper specifications of the paint manufacturer will have to be used, including the use of a primer. Before painting, make sure that the wall or timber is completely dry, otherwise, it will fail again.


Blistering of Paintwork
Blistering of Paintwork

10. Poor Workmanship


The biggest challenge the construction industry in India is facing today is ensuring good construction quality. The durability of a building depends on the quality of the workmanship and the design details. Many well-constructed buildings have survived the floods, while poorly constructed buildings have been damaged. The quality of workmanship has come down drastically during the last two or three decades.


11. Let’s Review


Damages due to flooding can be a costly affair. The recent Chennai flood has resulted in extensive damage to buildings, and infrastructure. Rainwater falling on a building occupies the pores in the concrete slabs, and get trapped. Prolonged exposure to rain, and the trapped moisture causes corrosion of reinforcement bars present in the concrete slabs. As a result, the strength of the slab reduces, and it may eventually fail. Temporarily fixes, and DIY solutions can be ineffective and could endanger the lives of numerous people living in the building. Therefore, timely professional help should be sought to carry out the repair of buildings affected by flooding. Proper proportioning, mixing, curing and waterproofing of concrete can only help in minimizing the impact of floods, and in extending the lifespan of a building.


Check out the other two parts of this series of blogs on repairing flood affected buildings: