Bitumen Sarking Felt

Property

Constituent

Combination of asphalt or coal tar pitch with felts, mats or fabrics of organic or inorganic fibres

To prevent rupture or flow of the bitumen a sheet material (felt) is normally incorporated. Felts add strength and spanning ability to the membrane but may, in turn, be responsible for membrane failures

Reinforcement is employed in a bituminous membrane to stabilize the bitumen film, to provide the strength required to span irregularities in its supporting surface, and to distribute local strains over a greater dimension. Several layers of reinforcement are normally used to give greater strength and provide several layers of bitumen in the hope that minor failures in one film will not cause immediate total failure.

Water Absorption

The normal solubility of water in bitumen is in the order of 0.001 to 0.01 % by weight and is so small as to be negligible.

Results of recent tests (source: Canadian Building Digest), on asphalt and coal-tar pitch have reported water absorptions of 0.5 to 2.4 grams/sq ft for commercial coal-tar pitches and 2.0 to 3.9 grams/sq ft for commercial asphalts after one year. This rate of absorption is very low and there is very little difference between the two. It is more significant that the rate of water penetration into the bitumen is also very low.

The waterproofing capability of bitumen is partly due to the non-wetting nature of its surface, but it is primarily a result of its nonporous characteristic. Although absorption of water by bitumens does occur, it is generally very limited and of little significance. For bitumen to perform as a water barrier there must be a continuous film free of any voids, pin holes or cracks. Provided this continuity is achieved the film can be as little as 0.005 inch thick, but deterioration of bitumen and other practical considerations usually dictate the use of a much thicker film.

The water resistance properties of bitumens depend upon the degree of impermeability and water absorption inherent in them. Even a very thin layer (1/64 inch) in a continuous film provides an excellent water barrier. In practical applications the degree of impermeability will be affected by the nature of the filling materials and the continuity of the bituminous coating. For waterproofing and roofing applications, fabrics or felts are used to build up a membrane to provide and maintain continuous waterproofing films.

Adhesion

Adhesion of bitumen to a surface depends upon both the nature of the surface and the state of the bitumen. Because it must make intimate contact with a solid surface, its viscosity must be lowered by heating or dissolving in an acceptable solvent (cut back). In this fluid state good adhesion to most surfaces can be obtained, provided the surface is clean and dry, but surfaces such as metal and concrete require priming with a cut back. Even after bitumen has been successfully applied, the bond can be decreased or destroyed by the migration of water to the interface between bitumen and solid.

Cohesion

The cohesion, or internal strength, and the flow properties of bitumens vary considerably with temperature and stress conditions. Bitumen behaves in a viscoelastic manner, and under the influence of an applied strain its deformation will depend upon the rate of strain and the temperature at which it occurs. At a warm temperature bitumen behaves as a liquid; at lower temperatures it behaves as an elastic solid. As the behaviour of both solid and liquid materials is involved, any mechanical properties such as load-strain must be related to the rate of deformation and temperature. Its thermal expansion also varies with temperature, being greatest when cold.

Masses Per Unit Area of Components and Lengths of Rolls of Class 1 Bitumen Sheets

Source from BS 747: 2000

Physical Properties

Source from ASTM D 6621-00