Damp Proofing in Buildings

DAMP PROOFING

The damp proof course (DPC) is generally applied at basement levels, which restricts the movement of moisture through walls and floors. Dampness is the presence of hydroscopic or gravitational moisture in the building. One of the basic requirements of the building is that it should remain dry or free from moisture traveling through walls, roofs and doors. The causes of dampness may be

❖ Moisture rising up from the ground to the walls

❖ Rain water travel from wall tops

❖ Rain showers against external walls

❖ Condensation

❖An effective damp proofing material should have the following properties:

❖ It should be impervious.

❖ It should be strong and durable and should be capable of withstanding both dead as well as live loads without damage.

❖ It should be dimensionally stable.

❖ It should be free from deliquescent salts like sulpates, chlorides, and nitrates.

The materials commonly used to check dampness can be divided into the following three categories:

❖ Flexible Materials: Materials like bitumen felts (which may be hessian based or fiber/glass fiber-based), plastic sheeting (polythene sheets), etc.

❖ Semi-rigid Materials: Materials like mastic, asphalt, or a combination of materials or layers.

Rigid Materials: Materials like first-class bricks, stones, slate, cement concrete, etc.

SELECTION OF MATERIALS FOR DAMP PROOF COURSE IN BUILDINGS

The choice of material to function as an effective damp proof course requires a judicious selection. It depends upon the climate and atmospheric conditions, nature of the structure, and the situation where DPC is to be provided. The points to be kept in view while making selection of DPC materials are briefly discussed below:

❖ DPC above ground level: For DPC above ground level with wall thickness generally not exceeding 40 cm, any one of the types of DPC materials above may be used. Cement concrete is adopted material for DPC at plinth level, 38 to 50mm thick layer of cement concrete M15 serves the purpose under normal conditions. In the case of a damp and humid atmosphere, a richer mix of concrete should be used. The concrete is further made dense by adding waterproofing materials in its ingredients during the process of mixing. It is used to apply two coats of hot bitumen over the third surface of the concrete DPC.

❖ DPC Material for floors, roofs: For greater wall thickness or where DPC is to be laid over large areas such as floors, roofs, etc., the choice is limited to flexible materials that provide a lesser number of joints like mastic, asphalt, bitumen felts, plastic sheets, etc. Bitumen felts should be adequately bonded to the surface with bitumen and laid with joints properly lapped and sealed.

❖ DPC Material for differential thermal movements: In parapet walls and other such situations, materials like mastic, asphalt, bitumen felts, and metal (copper or lead) are recommended. It is vital to ensure that the DPC material is flexible to avoid any damage or puncture of the material due to differential thermal movement between the material of the roof and the parapet.

❖ DPC material for Cavity Walls: In cavity wall construction, like cavity over the door or window, it should be bridged by flexible material like bitumen felt, strips or lead, etc.

METHODS OF DAMP PROOFING

The general methods of damp proofing are:

❖ Membrane damp proofing: In this method of damp proofing a water repellent membrane or damp proof course (D.P.C.) is introduced in between the source of dampness and the part of building adjacent to it. Damp proofing course may consist of flexible materials such as bitumen, mastic asphalt, bituminous felts, plastic or polythene sheets, metal sheets, cement concrete. Damp proofing course may be provided either horizontally or vertically in floors, walls etc. Provision of Damp Proofing

Course in basement is normally termed as 'Tanking'.

❖ Integral damp proofing: This consist of adding certain water proofing compounds of materials to the concrete mix so that it becomes impermeable. The quantity of water proofing compound to be added to cement depends upon the manufacturer's recommendations. In general one kilogram of water proofing compound is added with one bag of cement to render the mortar or concrete water proof.

❖ Surface treatment: In the surface treatment method a layer of water repellent substances or compounds are applied on these surfaces through which moisture enters. Pointing and plastering of the exposed surfaces must be done carefully, using water proofing agents like sodium or potassium silicates, aluminum or zinc sulphates, barium hydroxide and magnesium sulphates etc. Surface treatment is effective only when the moisture is superficial and is not under pressure. Sometimes, exposed stone or brick wall face may be sprayed with water repellent solutions.

❖ Cavity wall construction: Cavity wall construction is an effective method of damp prevention. In this method the main wall of a building is shielded by an outer skin wall, leaving a cavity between the two. The cavity prevents the moisture from travelling from the outer to the inner wall.

❖ Guniting: In this method of damp proofing, an impervious layer of rich cement mortar is deposited under pressure over the exposed surfaces for water proofing or over pipes, cisterns etc. for resisting the water pressure. The operation is carried out by use of a machine known as cement gun. It consists of a machine having arrangements for mixing materials and a compressor for forcing the mixture under pressure through a 50 mm dia flexible hose pipe. The nozzle is kept at a distance about 75 to 90 cm from the surface to be gunited. The mortar mix of desired consistency and thickness can be deposited to get an impervious layer. Since the material is applied under pressure, it ensures dense compaction and better adhesion of the rich cement mortar and hence the treated surface becomes water proof.

❖ Pressure grouting: This consists of forcing cement grout under pressure, into cracks, voids, fissures and so on present in the structural components of the building, or in the ground. Thus the structural components and the foundations which are liable to moisture penetration are consolidated and are thus made water-penetration-resistant. This method is quite effective in checking the seepage of raised ground water through foundations and sub- structure of a building.