damp proof course

Damp proof course

What is damp proof course?

At the plinth level above the foundation/plinth beam there is a non-absorbent concrete layer, which is called damp proof course (DPC). It has 2.5-4 cm (1-1.75 inches) thick layer. Damp proof course (dpc)is mainly laid on plinth beam at plinth level or floor level.

Why DPC is provided in building? Or what is the purpose of DPC? It is provided to prevent the super structure from the effects of moisture content present in soil.

If the question arise in you mind, why & how ground water or moisture content flow upward to the building (against gravity) if its down to building?

It is because of capillary action. Capillary action is the phenomenon which let the moisture content move spontaneously in any direction through a porous channel.

That’s why through the homogeneity of structure and pores of concrete & bricks moisture content from ground moves upward to the building. To tackle this, we provide D.P.C, further more a plastic transparent sheet over D.P.C.

The sub-structure/foundation is below the ground and can’t be seen, that’s why there aren’t any arrangements provided to prevent the sub-structure from the bad effect of moisture content.

As it has much importance in structure, it is made up with good quality of concrete. Which concrete mix grade is use for DPC? M15 (1:2:4) ratio of concrete mix is used for DPC.

There’s ready made DPC available in the market and there’s many construction material suppliers, who supplies DPC materials.

Bad effects of dampness:

  1. Dampness can rot the wood work inside the building.
  2. It can cause erosion to metals (steel, aluminum bras) used in building.
  3. Softening and crumbling of plaster.
  4. Bubbles appear on wall paints because of dampness, which crumble the the paint and effect the surface.
  5. Floor and mats also effect by by dampness.
  6. It can effect electric wiring, which can cause shot circuit.
  7. Dampness can cause efflorescence on surface of brick, which effect the brick strength.
  8. It cause smell in building and rooms.
  9. Dampness has really bad effect on human health.

Causes of dampness:

Following are the climatic and structural causes by which dampness enters into the structure.

Rain:

Rain is the biggest reason behind the cause of dampness in structure. Some buildings are capable to bear the heavy rain with interval but it wasn’t able to bear the continuous rain for days.

A good constructed walls prevent the dampness to enter, but the joint of walls and brick pores let dampness enter. It can also enter through the slab joints, slab cracks, holes and the joints b/w wall & slab.

Plinth level of building:

Plinth level of building really have interference into dampness acceptance. It can decrease it and increase it as well. A building constructed above the surrounding ground level surface has less dampness effects comparatively.

The rain water will not accumulate near the building walls and will flow the opposite side if the plinth level is above. The below plinth level will have the opposite case and will be affected by dampness.

Drainability of soil:

The ground soil on which structure is built on, the drainability of that soil also have important role in dampness effects. Sandy soil can absorb the water and shed it, in other hand clay withhold the moisture content.

Seasonal effects:

In cold places atmosphere moisture settle down on ground surface. This type of dampness can be seen as dew drops normally, this dampness will have bad affect on structure. This dampness has less affect on hot places.

Moisture entrapped during construction:

Building walls are wet while construction and this moisture stays for really long. If the hard water or salty water were used in construction, the dampness can stays more longer.

Salt in water absorb the moisture content and withhold it. Construction in rainy seasons or cold weather have more possibilities of moisture content, because of no direct sun light will not let the moisture content dried.

Defective material use:

Defective or low quality material also fascinate to accumulate dampness, e.g. porous bricks, soft stones, especially when that used in outer surface of wall. Salty sand also contribute in dampness accumulation.

Defective construction:

A little carelessness between the joints of walls and slab will left the space between, by which rain water can shed inside of building. The space between the frames of windows & vantilations of outer walls can affect.

Moisture originated in building:

Water accumulation on some spaces in building (washroom, kitchen etc) will result into dampness ultimately. The blockage or breakage of sanitary & sewerage pipes can also cause dampness.

damp proof course is mainly laid on
Damp proof course

Material for damp proof course:

DPC is made up with concrete. The aggregates used in concrete should be less than 12mm (0.5 inch). The aggregates should be pieces of hard stone. It should be washed before using it.

The sand use in it should be coarser and cleaned from organic matters. The average size of sand particles should be 5 mm (3/16 inch). It should be seived before use.

Portland cement is use for DPC making. The water use in it should be clean, having Ph value 6. It shouldn’t have acidity & alkalinity.

Making of damp proof course material:

The proportion of concrete ingredients can be done by volume or by weight. Cement and sand are mixed on water tighten plat form. Put coarse aggregates below, over it put the CS mixture and mix the whole material in dry form.

Then pour the required water quantity, which is almost 45% to weight of cement. Mix it well in wet form. As water proofing agent put 3-5% of padlo w.r.t weight of cement.

Preparation of base for Damp proof course:

Before placing DPC, it is compulsory to prepare the surface of wall. At plinth level the most upper layer should be laid on edge wise, it can be flat but the frog should be downside.

If there is a plinth beam at plinth level then there is no need of laying DPC, but to lay, it is consider better. Upper surface should be clean with steel brush and then wet it well.

Laying of damp proof course:

DPC should be laid according to drawing and laid over equal width of plinth/wall. Straight edge/mould is used to make the edges of DPC plain/straight. Damp proof course is mainly laid on top layer of brick work of foundation or above plinth beam.

Make the surface of DPC plain and compact it. There shouldn’t be any joint in between. After the DPC is laid, make its upper surface rough with the help of steel brush. Rough surface will help to make better joint of masonry.

Curing of damp proof course:

Curing of DPC is done for atleast 7 days. The curing should be start after 12 hours. Masonry work above DPC can be done right after 48 hours but curing will be last for 7 days.

Damp proof course materials:

Following are the materials use to prevent dampness.

Bitumen mastic:

It is also called asphalt mastic. It is a hot non-absorbent mixture of sand and bitumen/asphalt. To prevent the dampness, Use 2.5cm to 5cm thick layer of bitumen mastic. It should be laid in hot form, because a cold form become solid.

Bitumen mats:

Its a 6 mm thick layer, which has width same as wall thickness. It is laid above the plinth/floor level of wall. It is also used for vertically prevention of dampness on walls outer surface.

Hot layer of bitumen:

2 layers of hot bitumen are laid upon the concrete D.P.C. Its average weight should be 1.75 kg/m2. It fills the concrete pores and make it more resistant against dampness.

Metalic sheets:

Steel, copper and aluminum sheets are also used to prevent dampness, but it not economical method. These sheets are wide same as the wall thickness. These sheet are 3mm thick.

Finally use rich cement concrete layer b/w 1.2cm – 4 cm thick and rich cement mortar layer of 2cm thick as damp proof course.

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