|
Precast building components like precast RC roofing and flooring elements, precast thin RC lintels, precast RC door and window frames, were successfully tried out under the NBO experimental projects.
Precast Roofing systems:
Roofing accounts for up to 25% cost of construction of a house by using precast roofing components, 20 to 30% economy could be achieved in the cost of roof construction. Some of the popular precast roofing / flooring techniques promoted by the NBO, which have manifold advantages of saving in cost, consumption of cement and steel and increased pace of construction, are described below. All these systems make use of components which are fabricated on the ground, ready in all respect for erection in building. In – situ concreting, work is minimized in these systems so that the speed of construction is not affected. The necessity for putting up centering and shuttering as in the case of cast – in – situ reinforced concrete (RC slab completely) is eliminated under these roofing systems. The provision of centering and shuttering is a time- consuming process and elimination of the same leads to considerable saving in erection time.
Precast RC lintel:
In building construction, 15 cm thick RC lintels are provided over door, window openings. Lintels can be as well designed as a composite member consisting of 75 mm thick RC lintel with the masonry on top. The precast thin lintel 75 mm thick, and 230 mm wide with 3 numbers of mm dia M.S. bars could be used for opening up to 1.8 m provided bricks used have minimum crushing strength of 10N/mm2 and mortar is not leaner than 1:6 cement – sand and the height of masonry above the lintel is at least 45 cm. use of precast lintels speeds up the construction of masonry walls besides eliminating shuttering and centering. About 50% saving in material and overall cost is achieved apart from saving in construction time by adoption of precast lintel.
Precast RC frames for Doors and Windows:
With fast depletion of the forest resources timber has now become a scarce material. This has necessitated search for substitute materials for making frames for doors and windows. Precast RC frames are alternate to traditional timber frames for doors and windows and economical compared to frames made out of traditional timbers like teak and deodar. They are more suitable than timber frames in wet areas like lavatories, bathrooms and kitchens and are resistant to termite attach and growth of fungus. The RC frames are better resistant to fire as compared to wooden frames.
Economical walling systems
Wall is one of the important structural in building and housing construction which, on an average, accounts for 25 to 30% of the cost of construction in civil work. Traditionally, brick masonry and stone masonry are the two most popular wailing systems used in building construction in India. Burnt clay brick is one of the predominantly used basic construction materials in the Indo-Gangetic plains while in states like Rajasthan and Gujarat, locally available stone is used as an important building material for wall construction.
As a result of research work undertaken at the CBRI, Roorkee, several innovations in design construction of load bearing brick masonry have been evolved. These include single brick chick load bearing masonry walls for four and five-storey construction, nineteen centimeter thick walls for upto two-storey construction, half brick thick walls, fly-ash gypsum brick masonry and precast stone block masonry. Adoption of these systems has not only resulted in economy but also in less consumption of bricks and mortar, utilization of industrial waste and saving in time.
Single Brick Thick Load Bearing Wall
Design of load bearing walls was earlier made on empirical methods based on the established experience of architects. For example, wall thickness for a four-storeyed construction as per the empirical method was kept as 46 cm, 34 cm, 23 cm, for ground, first, second and third storeys respectively. Through a rationalized design method , it has now become possible to construct single brick thick load bearing wall in multi-storeyed construction through use of better quality bricks. In present construction practices , single brick thick load bearing walls could be adopted for construction of four-and-five storeyed residential buildings provided bricks of good quality having compressive strength of 7-10.5 N / mm2 are available. Apart from saving in consumption of bricks, cement and steel , such types of load bearing brick structures provide more covered area, resulting in upto 15% economy in cost of construction.
Nineteen Centimeter Thick Load Bearing Brick Masonry Wall
It has been a common practice these days to adopt 23 cm thick load bearing walls using conventional bricks for normal house construction. However, by a rational design of the structure it is possible to construct 19 cm thick load bearing walls for upto two storeys using burnt clay bricks.
In ordinary brick masonry construction, English Bond and Flemish Bond are generally adopted for getting the required wall thickness. In case of 19 cm thick walls a different type of bond is adopted and construction also differs from the traditional type. Under this technique of all construction, bricks are laid on edge as well as flat. In order to obtain a height of 23 cm, three bricks are placed on bed whereas adjacent to it two bricks are placed on edge. The next 23 cm height is obtained by placing two bricks on edge and three bricks on bed on the same side of the wall. The pattern is then repeated to obtain full height of the wall. Corners and junctions of such walls are also constructed in a different manner to break the continuity of joints.
Such walls can be designed according to the provisions of IS: 1905-1980. A saving to the extent of 16 % has been achieved in cost of bricks and mortars employing this technique.
Half Brick Thick Load Bearing Wall
Generally, half brick thick walls are constructed as non-loading bearing walls. But with proper planning, it is possible to put up half-brick thick (115mm) walls behaving as load bearing walls. In construction of such walls, over-lapping of half brick thick walls at regular intervals act as pilasters of 23 cm x 23 cm size. These pilasters transfer the load of the roofing systems to the foundation.
Masonry work in brick of 10 N/mm2 strength in cement sand mortar 1:4 is generally adequate for constructing such walls. A ring beam of 23 cm width and 7.5 cm depth with 10 mm. dia. MS bars is required to provide bearing for the roof. Saving to the extent of 15% is achieved when compared with the traditional 23 cm thick load bearing walls.
Flyash Gypsum Bricks for Masonry
Flyash gypsum bricks are a good substitute for burnt clay bricks for the areas in which flyash is readily available. These bricks can be manufactured with a composition of 92 % flyash, 4.5% powdered kankar lime and 3.5% gypsum in volumetric proportions. A compressive strength of 5-6 N/mm2 (50-60 Kg/cm2) is expected to be achieved from these bricks. They also have good insulation properties and consequently provide better thermal comfort to the occupants in comparison with the common bricks
The method of manufacture of these bricks is very simple and does not require elaborate arrangements. Flyash kankar lime and gypsum are powdered and mixed in the desired proportion volumetrically in dry state. A measured quantity of water is then added and mixing is done in a pan till the mixture forms a stiff paste. The paste is poured into steel moulds. The mould is released when the mix has hardened sufficiently. The bricks are then cured for at least 10 days by stacking them properly and occasionally sprinkling water in order to keep them wet. The bricks are subsequently subjected to curing by air for about a fortnight. These bricks do not require to be burnt in a kiln.
Stone Block Masonry
The precast stone block masonry is an improvement over the traditional stone masonry which requires excessive materials and more skilled labour and is time-consuming. The precast blocks can be easily produced at factory or at site with semi-skilled workers without any special equipment.
These blocks are used both for load bearing as well as non-load bearing walls. The thickness of the wall depends upon the load coming over it. This construction system provides an economy of the order of 15 to 20% in the wailing cost against the conventional method of construction, viz, random rubble masonry. Stone block masonry provides finishing of brick masonry on both sides of the masonry wall.
Single Stack System of Plumbing
In the conventional system of drainage for building two separate pipes are provided , one for the soil which takes discharge from the water closet, urinals, etc. and the other for the discharge from baths, sinks and wash basins etc. Besides, a vent pipe is also fitted with both the pipes to prevent breakage of seal of the traps of different appliances. The single stack system of plumbing which replaces the above system, is essentially a one pipe system in which the wastes from kitchen, both, etc. and soil from toilets is carried out of the building in a single stack. The ventilation is also completely omitted and the service stack itself serves the purpose of a ventilation pipe. It embodies the merits of both the conventional two pipe system and the modern one pipe system. The use of this system effects saving in the quantity of pipes and fittings and also gives a cost saving of about 30% in plumbing work.
Single stack system of plumbing has been used in large number of the NBO experimental projects. Beside this, has been also used in mass housing programmes undertaken in different parts of the country.
|
Participants