The selection of suitable specifications play an important role in controlling the cost of construction of the building. However, the choice of specifications should be such that the building remains structurally sound, functionally efficient and durable. Techniques for low cost housing construction is discussed in this article.
Low Cost building Construction Techniques and Speedy Construction:
Foundations for Low Cost Building Construction
The factors which influence the choice of specifications for this item of work are
1. Nature of soil and its bearing capacity.
2. Depth of sub-soil water table.
3. The intensity of load on the main walls.
4. 15 to 20 cm thick lime concrete with 40 mm normal size stone over burnt brick aggregates and 40% mortar comprising of 1 lime : 2 sand/surkhi or 1 lime : 1 surkhi : 1 sand.
5. In case the region is infested with termites, it would be necessary to provide termite proofing treatment in accordance with the IS:6313.
6. In case the sub-soil at the site is moist with high sub-soil water table (1.5 m or less below foundation level). It is necessary to provided bed of cement concrete of richer proportion (to function as a lean concrete bed).
7. In case the building is to be cited on soils of filled up nature or on black cotton soil, it would be necessary to give special treatment to the foundation to ensure safe transfer of load from building to the supporting ground. However, in case of black cotton soil, under reamed pile foundation normally proves to be economical.
8. For single storeyed building the depth of foundation under normal conditions of soil should not be less than 60 cm below ground level.
Masonry in Foundation and Plinth
Load bearing wall type of construction can have walls made out of bricks or stones. Hence the specifications for this item of work comprise of brick masonry or random rubble masonry in cement mortar 1:8 (1 cement : 8 sand) or in lime mortar 1:2 (1 lime : 2 sand) or in cement lime mortar 1:1:8 (1 cement : 1 lime : 8 sand).
Following aspects pertaining to this item need special attention:-
1. In case of multi-storeyed load bearing wall type of construction richer mix of mortar should be used to suit the loading conditions on the masonry.
2. In case the sub-soil at the site is moist with high sub-soil water table (1.5 m or less below foundation level), the masonry should be in cement mortar 1:6 or cement lime mortar 1:2:9.
3. In regions where building stone is available at reasonable cost it is desirable to adopt stone masonry is less effected by sub-soil water.
4. For single storeyed construction the width of the foundation under average soil conditions should not be less than 2 bricks.
Damp Proof Coarse (D P C)
Damp proof coarse is provided at plinth level of the building to check the rise of moisture from ground to superstructure. The specifications for this item of work depend upon the intensity of rainfall in the area, the depth of sub-soil water table, and the quality and nature of masonry used in the superstructure.
Economical specifications for this item of work consist of providing 35 mm thick cement concrete (1:2:4) using 10 mm nominal size stone aggregate and painted on top with one coat of bitumen of penetration 80/100 using bitumen @ 1.7 kg/sqm.
Masonry in Superstructure (Load Bearing Wall)
Load bearing walls perform the function of carrying the load from roof and upper floors and transmit the same to the soil through foundations. The walls are also required to withstand the wind and seismic forces besides providing thermal comfort against outside temperature, resistance against rain penetration and insulation against noise. The thickness of the wall to be adopted in a particular situation depends upon the strength of the masonry units (bricks, stone, concrete block etc.) to be adopted, type and mix of mortar used, unsupported length and height of the wall (slenderness ratio), location and dimensions of the openings, location of longitudinal and cross wall, intensity of loading on the wall etc. The method of designing the thickness of the walls based on these parameters is given in the National Building Code.
The strength of masonry depends on the crushing strength of the building blocks and the type of mortar used in the work. It is observed that under normal conditions and moderate spans (as in case of residences etc.) it is possible to provide one brick thick load bearing wall up-to four storeyed construction. Up-to three storeyed residential buildings one brick thick load bearing wall made out of good quality bricks having crushing strength of 75 kg/sq cm can normally be provided without special treatment. However, in case of 4 to 5 storeyed construction with single brick wall there is possibility that in some walls in ground and first floor the stress developed in masonry may be of the order 9 to 10 kg/sq cm. Such walls should be constructed with first class bricks having crushing strength of 105 kg/sq cm and in cement mortar 1:3 (1 cement : 3 sand).
In case, good quality bricks are not available in the area, the thickness of the wall should be designed as per provisions in National Building Code. Under normal conditions brick masonry in superstructure can be constructed in lime mortar 1:2 (1 lime : 2 sand) or ( 1 lime : 1 surkhi : 1 sand) in cement lime mortar 1:1:8 (1 cement : 1 lime : 8 sand) or in cement mortar 1 : 8 (1 cement : 8 sand).
In case the building is to be located in an earthquake zone special provision are required are to be made in the brick masonry in the form of lintel band, vertical steel bars at the jambs of openings, corners and junctions of walls.
In case of stone masonry wall it is observed that 30 cm thick random rubble masonry wall can safely be provided up-to two storeyed residential quarters.
Partition walls are non-load bearing walls which are provided to perform the function of space dividers and to cut off noise. In areas where bricks are easily available , half brick partition walls should be provided in cement mortar 1 :4 (1 cement : 4 sand) with reinforcement comprising of 2 Nos. of 6 mm dia bars at every 4th course and the reinforcement should be bounded and secured at each end adjoining walls.
Depending up-on the climatic conditions, building can be have a sloping roof or a flat roof. The roof should be structurally safe, durable and should provide adequate protection against adverse effect of elements like rain, snow, sun and wind etc. It should provide desired insulation against heat and sound and should have efficient water proofing and drainage arrangements. In areas having rainfall or snowfall sloping roof is preferred to flat roof. In other areas flat roof is provided and it has the added advantage that the roof can be used as terrace for sleeping purposes. Sloping roof can be made by use of timber, structural steel or tubular trusses provided with covering of AC sheets or brick tiles etc. Flat roof can be in the form of cast in situ R.C.C slabs or precast units ( cored units, channel units, cellular units, Waffle units, funicular etc.).
In case of sloped roof it is necessary to provided false ceiling made out of insulating materials like asbestos sheets, timber, fiber-boards etc., to meet the requirements of thermal comfort. In case of flat roof it is necessary to provide terracing required thermal insulation and for drainage of rain water.
Doors and Windows
Doors are provided to perform the function of access to a building and for inter-communication between different rooms. The normal height of door is taken as 2 m and its width varies from 80 cm (for bath room and WC,) to 90 cm for other rooms in residences. In case of public buildings the normal width of door can be up-to 12 m. Windows serve the purpose of providing light, ventilation and vision. The area of windows varies from 10 to 15% of the floor area for buildings in hot and arid regions. In case of humid climate it is necessary to provide excessive ventilation to meet requirements of comfort and as such the area of windows in such regions should vary from 15 to 20% of the floor area.
Filling in Plinth
The function of filling in plinth is to raise the height of the ground floor above the level of the outside ground with a view to prevent ingress of dampness in the ground floor. Under normal conditions the plinth in filled with excavated earth and finished with 15 cm thick layer or sand on top. This layer serves the base on which the layer of base concrete under ground floor is laid. A plinth height of 45 cm is considered adequate for normal construction.
Flooring is provided to have hard, durable smooth, easy to clean, impervious and dust free surface and the materials used in flooring should be strong enough to resist its wear and tear that take place during use. The treatment to flooring for the ground floor is some what different to that for intermediate floor or structural floor. In case of ground floor the water from adjacent ground is liable to rise up by capillary action and make the floor damp. The specification for the floor should therefore be such that the flooring remains free from dampness. It is normal to provide a layer of base concrete under ground floor over 15 cm layer of sand over earth filling in plinth.
Terracing treatment is given over flat roofs to perform the combined function of providing thermal insulation and water proofing simultaneously. It is necessary to provide the layer of terracing materials in slope varying from 1 : 48 to 1 : 60 so as to ensure quick disposal of the rain water. Prior to laying terracing the top of the roof slab is painted with layer of bitumen of penetration 80/100 using 17 kg bitumen for every sq.m. area and impregnated with a coat of coarse sand @ 60 dm³/10 sqm.