How to Carryout Waterproofing with Bitumen Mastic?

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The bitumen mastic has proved to be a successful water-proofing material intended for water-proofing treatment of surfaces such as flat and sloped roofs and tanks.

As the mastic asphalt is reasonably ?uid at the processing temperature, it can be applied to form a dense and continuous layer covering ?at, sloped, or curved surfaces without compacting. Due to great deformation capacity, the mastic asphalt mixture shows strong cracking resistance at low temperatures.

This article discusses the required thickness, remealting, and methods of laying of bitumen mastic for waterproofing treatment.

Thickness of Bitumen Mastic

The bitumen mastic shall be applied in one or three coats, as stated below, to all surfaces, whether sloping, horizontal, or vertical. The thickness shall be as follows-

1. A 10mm bitumen mastic shall be laid in one coat for walls and floors above ground level.
2. For vertical surfaces and for the surfaces steeper than 30 degrees, the bitumen mastic shall be applied in three coats which shall be less than 20mm totally. 
3. For horizontal surfaces and for the sloping surfaces not steeper than 30 degrees, the bitumen mastic shall be applied in three coats which shall be less than 30mm totally. 

Remelting of Bitumen Mastic

1. The remelting shall be done at the site in a mechanical mixer; the mastic temperature shall not exceed 200 degrees C during remelting.
2. The bitumen mastic blocks shall be broken into pieces and then stocked in layers first round the sides and then inwards towards the center of the mixer.
3. The mastic shall be gradually heated to about 200degrees C.
4. When the mastic attain a molten condition, it must be agitated continuously to ensure a uniform consistency.
5. The duration of heating shall be decided such that the properties of bitumen are not impaired.

Method of Laying Bitumen Mastic

1. In the three-coat treatment method, the first layer of bitumen mastic shall be applied thinly on any surface so that it acts as an adhesive layer and prevents blowing.
2. While laying bitumen mastic on a horizontal surface, each coat shall be spread with a float evenly and uniformly over the previously prepared surface.
3. The first coat can be plastered with a metal trowel as evenly and uniformly as possible for laying on a vertical surface.
4. The second and third coats can be applied with a wooden float to achieve a uniform thickness.
5. These bitumen mastic coats should be applied as quickly as possible to prevent dust or dirt accumulation between layers.
6. The blows entrapped in each coat formed by entrapped air or moisture during the laying shall be punctured and repaired while the asphalt is warm and before the next coat is applied.
7. In case of chases, the top of the vertical mastic shall be turned into a chase in the wall not less than 25 mm x 25 mm unless it is being continued horizontally.
8. In a fillet case, an angle fillet not less than 50 mm wide shall be applied in two coats at two planes forming an internal angle.
9. In the case of construction joints, the edges of the mastic already laid should be warmed with hot asphalt and then cut out with a metal trowel to remove any dust or dirt that may have collected. The fresh mastic should be poured before the warmed up surface of the joint cools off.

Water-proofing above Ground Level

1. The water-proofing treatment above ground level shall be laid across the full thickness of walls, excluding plaster, or the base of cavity walls.
2. The damp proofing in the wall shall be continuous with the layer of bitumen mastic on adjacent floors. 
3. Where ever necessary, a vertical damp-proofing course shall be provided on the inner surface of the wall, as in figure 2.

Water-proofing in External Tanking

The following points must be checked for externally applied tanking-

1. The working space outside the walls may be not less than 0.6 m.
2. The base concrete of 100 mm in thickness shall be structurally sound; it shall be extended at least 150 mm beyond the wall's edges to permit angle fillet to form at the junction of horizontal and vertical damp-proofing shown in figure 3.
3. As soon as the layer of mastic asphalt has been applied, it must be covered with a 50mm thick screed of cement and sand to prevent damage. 
4. The horizontal coat of the structural slab shall be laid as quickly as possible.
5. After the vertical damp-proofing bitumen mastic is applied, the outside of the wall shall be protected against damage by the erection of a brick wall.

Water-proofing in Internal Tanking

The following precautions shall be taken before applying bitumen mastic-

1. A space of 300mm outside the wall shall be provided during excavation to keep the wall dry while laying bitumen mastic.
2. The base slab shall be given an even surface, and the walls shall be built to the required height before mastic asphalt laying is commenced.
3. The backfilling of the earth shall not be allowed until three coats of vertical mastic are applied and the loading coat is hardened, as shown in figure 4.
4. As in external tanking, protective layers of screed should be laid, and then the structural floors and walls laid.

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