How do you waterproof a concrete water tank

Getting your concrete underground water tank waterproofed is wise rather than doing a layman job and suffering.

The concrete underground tank may develop a structural crack and lead to leakages both from outside and inside of the tank.

The development of structural Cracks may be due to different reasons

• Insufficient design of the RCC (Reinforced Cement Concrete) tank. Cracks develop due to the external pressure developed by the earth around the underground tank or groundwater pressure, especially during the rainy season.
• Once a crack is developed, underground water percolates into the tank along with soluble salts and cause further damage to the RCC structure – both the reinforcement steel and the concrete.

As the design of the RCC underground water tank is important for the success and durability, the waterproofing of the tank is also very necessary.

The waterproofing helps to negate the effect of negative water pressure and positive water pressure.

Negative water pressure is created by the groundwater on the external surface of the tank.

Positive water pressure is created by the stored water on the internal surface of the tank.

If we consider the financial implications and the ease of maintenance waterproofing becomes all the more important. Because the cost of repairing an underground water tank is far more than the cost of waterproofing during the construction stage itself.

Waterproofing both inside and outside the concrete underground tank surfaces

Here, I will give you the step by step waterproofing solution for your concrete underground water tank.

Note: The solution holds good provided the concrete structure is designed properly.

1. The concrete tank should be cured with water for 28 days after de-shuttering.

1. The surface of the tank should be cleaned of dust and free particulate matter and any loose concrete material with a water jet.
2. If any crack or honeycomb are observed on the concrete surface those should be filled with repair mortar. (The mortar may be Cement: Sand = 1:1 with the addition of non-shrink expansive chemical).
3. Check the volume of the tank should be greater than 4 times the surface of waterproofing.
4.  The inner surface should be damp with water but surface dry.
5.  Apply the waterproof paste in 2 coats. The 1st coat should be applied in a single direction – top to bottom or left to right. The 2nd coat should be applied perpendicular to the 1st coat.
6. Similarly, The outer surface should be damp with water but surface dry.
7. Apply the waterproof paste in 2 coats. The 1st coat should be applied in a single direction – top to bottom or left to right. The 2nd coat should be applied perpendicular to the 1st coat.

The waterproof chemical to be used may be in the powder form or in the liquid form depending on the manufacturer.

Different manufacturers have different products with respective brand names.

For this type of surface application for both positive and negative waterproofing, I will suggest

weberdry tex from Saint Gobain

This is a pre-mixed cementitious waterproof for underground concrete water tanks

The product is available in pack sizes of 5Kg and 20 Kg

When you buy to make sure the pack is not more than 1 year from the date of manufacture.

The paste is to be prepared by mixing weberdry tex: water = 3:1 with a slow speed mixer till consistency and lump-free.

2 coats are to be applied with each coat of 1 mm.

The product provides coverage of 5 Sqm/ 20 Kg bag.

Pros 

• Provide resistance to hard water and chlorinated water.
• Ease of application with a brush or trowel.
• Most suitable for water reservoirs.
• It is effective till 50m of depth.
• Resist water pressure both from inside and outside.
• Lower in VOC.
• Non-Toxic.

Cons

• Not suitable for application on porous surface, brickwork, light-weight concrete.
• Cannot be applied to structures subjected to movements or cracked structures.
• Not suitable for structures exposed to the acidic environment.
• The product is not flexible and tends to crack if there is structural movement.

Hope this gives you a fair idea of How to waterproof a concrete water tank with surface treatment.

Please put your comments if you have already used this technique and having a leakage-free tank.

If you have any success story using some other method please share in comments.

If you are facing any similar problem and do not know what is to be done, post the problem in the comments.