The Construction Civil

Self compacting concrete is a concrete which compacts itself, there is no further compaction required for self compacting concrete. Making concrete structures without vibration, have been done in the past. For examples, placement of concrete under water is done by the use of tremie without vibration. Mass concrete, and shaft concrete can be successfully placed without vibration. But the above examples of concrete are generally of lower strength and difficult to obtain consistent quality. Modern application of self-compacting concrete (SCC) is focussed on high performance, better and more reliable and uniform quality.

Recognising the lack of uniformity and complete compaction of concrete by vibration, researchers at the University of Tokyo, Japan, started in late 1980’s to develop Self compacting concrete. By the early 1990’s, Japan has developed and used SCC that does not require vibration to achieve full compaction. By the year 2000, the SCC has become popular in Japan for prefabricated products and ready mixed concrete. The utilisation of self compacting concrete started growing rapidly.

Self compacting concrete has been described as “the most revolutionary development in concrete construction for several decades”. Originally developed in Japan to offset a growing shortage of skilled labour, it has proved to be beneficial from the following points,

1. Faster construction,
2. Improved durability,
3. Reduction in site manpower,
4. Better surface finish,
5. Easier placing,
6. Safer working environment.

Carbonation of concrete is a process by which carbon dioxide from the air penetrates into concrete and reacts with calcium hydroxide to form calcium carbonates. The conversion of Ca(OH)2 into CaCO3 by the action of CO2 results in a small shrinkage.

CO2 by itself is not reactive. In the presence of moisture, CO2 changes into dilute carbonic acid which attacks the concrete and also reduces alkalinity of concrete.

Air contains CO2. The concentration of CO2 in rural air may be about 0.03 per cent by volume. In large cities the content may go up to 0.3 per cent or exceptionally it may go up to even 1.0 per cent. In the tunnel, if not well ventilated the intensity may be much higher.

The pH value of pore water in the hardened concrete is generally between 12.5 to 13.5 depending upon the alkali content of cement. The high alkalinity forms a thin passivating layer around steel reinforcement and protect it from action of oxygen and water. As long as steel is placed in a highly alkaline condition, it is not going to corrode. Such condition is known as passivation.

In actual practice CO2 present in atmosphere in smaller or greater concentration, permeates into concrete and carbonates the concrete and reduces the alkalinity of concrete. The pH value of pore water in the hardened cement paste which was around 13 will be reduced to around 9.0. When all the Ca(OH)2 has become carbonated, the pH value will reduce upto about 8.3 In such a low pH value, the protective layer gets destroyed and the steel is exposed to corrosion.

The carbonation of concrete is one of the main reasons for corrosion of reinforcement. Of course, oxygen and moisture are the other components required for corrosion of embedded steel.

The carbonation of concrete is one of the main reasons for corrosion of reinforcement. Of course, oxygen and moisture are the other components required for corrosion of embedded steel.

Rate of Carbonation:

The rate of carbonation depends on the following factors.

1. The level of pore water i.e., relative humidity.
2. Grade of concrete
3. Permeability of concrete
4. Whether the concrete is protected or not
5. depth of cover
6. Time

It is interesting to know that if pore is filled with water the diffusion of CO2 is very slow. But whatever CO2 is diffused into the concrete, is readily formed into dilute carbonic acid reduces the alkalinity. On the other hand if the pores are rather dry, that is at low relative humidity the CO2 remains in gaseous form and does not react with hydrated cement. The moisture penetration from external source is necessary to carbonate the concrete.

The highest rate of carbonation occurs at a relative humidity of between 50 and 70 percent.The rate of carbonation depth will be slower in case of stronger concrete for the obvious reason that stronger concrete is much denser with lower W/C ratio. It again indicates that the permeability of the concrete, particularly that of skin concrete is much less at lower W/C and as such the diffusion of CO2 does not take place faster, as in the case of more permeable concrete with higher W/C ratio. Depth of cover plays an important role in protecting the steel from carbonation.

Measurement of depth of carbonation:

A common and simple method for establishing the extent of carbonation is to treat the freshly broken surface of concrete with a solution of phenophthalein in diluted alcohol. If the Ca(OH) is unaffected by CO2 the colour turns out to be pink. If the concrete is carbonated it will remain uncloured. It should be noted that the pink colour indicates that enough Ca(OH)2 is present but it may have been carbonated to a lesser extent. The colour pink will show even up to a pH value of about 9.5.

Stone Masonry is the art of building the structures in stones. In some parts of the country, building stones are abundantly available in nature. These stones when cut and dressed to the proper shapes, provide an economical material for the construction of various parts of a building which are located in hilly areas.

Uses of Stone Masonry:

Stone masonry construction is used in,
1. Building foundations, dams, monumental structures.
2. Building walls, piers, columns, pillars, light houses, and architectural works.
3. Arches, domes, lintels and beams
4. Roofs, floors, pavings.
5. Railway ballast, blackboards and electrical switchboards.
6. In the construction of retaining walls.
7. It is used in the construction of integrated living homes and integrated design construction.

Selection of stone for stone masonry:

Selection of stone depends on,
1. Availability,
2. Ease of working,
3. Appearance,
4. Strength and stability,
5. Polishing characteristics,
6. Economy,
7. Durability.

The mosaic flooring consists of tiles available in variety of patter and colours, is commonly used in operation theatres, temples, bath-rooms and superior type of building floors.

For construction of mosaic flooring, first of all, a hard concrete base is laid. Over this concrete base, while it is still wet, a 2 cm layer of cement mortar (1:2) is evenly laid. sometimes, instead of cement mortar using monolithically, a lime surkhi mortar is first spread to a depth of about 6 cm and levelled; over which a layer of paste or cementing material is laid in thickness not exceeding 3 mm. Upon the bed of the cement mortar (cementing material) small pieces of broken tiles are arranged in definite patterns. After this, cement or coloured cement is sprinkled at the top and surface is rolled by light stone roller till the even surface is attained. this surface is left for 24 hours to dry and then it is rubbed with pumice stone to get a smooth and polished surface. The polished surface is finally allowed to dry for about two weeks before use.


Related Posts:

1. Cork Flooring
2. Rubber Flooring
3. Glass Flooring
4. Timber Flooring
5. Brick Flooring
6. Terrazzo Flooring
7. Hardwood flooring
8. Laminate flooring

Terrazzo flooring is a special type of concrete flooring in which marble chips are used as aggregates, and this concrete on polishing with carborundum stone presents a smooth surface. Any desired colour can be obtained by using marble chips of different shades and sizes and also by using different coloured cement. The aggregate shades are exposed by grinding the surface. Normally, terrazzo mix having proportions 1:2 to 3 (1 cement : 2 to 3 marble chips) depending upon the size of marble chips, is used.

The terrazzo flooring is becoming very popular these days for providing floor finishes in banks, hostels, office buildings and other public or social buildings, on account of its excellent wear-resisting properties and decorative effects.

Terrazzo finish is atleast 10 mm thick and comprises a mixture of desired cement (i.e, coloured cement), marble powder and coarse aggregate, such as chippings of marble, quartzite, pearl, glass, etc., of selected colours and of sizes graded from 2 mm to 8 mm. Sometimes, larger particles up to 10 mm are used. This terrazzo finish is installed over the concrete base course in following two ways or methods,

In one method, the cement concrete base is covered uniformly by a 6 mm sand cushion, over which a tar paper is placed. On this paper, a layer of rich mortar (1:3) about 30 mm thick is deposited.

In another method, terrazzo finish is applied monolithically. First of all, a thin coat of cement is spread over the wet concrete base. This layer is then swept with a broom and a layer of cement mortar (1 cement : 2 sand) 12 mm thick is evenly spread immediately over it.

The metal dividing stirrups, about 30 mm in width and 1.3 mm in thickness, are inserted on edge in the mortar bed in desired patterns before it hardens.

When the mortar bed has sufficiently hardened, a terrazzo mixture (1 cement : 3 marble chips), 6 to 12 mm thick, depending upon the size of chips, with water just sufficient to make a workable mix for the mosaic finish, is applied. This terrazzo layer is then rolled lengthwise as well as cross wise. About 85% of the marble or aggregate should be exposed over the finished surface and to achieve this, it may be necessary to add additional chips during the rolling process.

After curing for several days, the surface is carefully polished by means of a grinding machine fitted with carborundum grinding stone disc. During the process of grinding, the surface is kept wet. Holes or pores, if any, are filled with a thin grout of cement paste, having same tint as the terrazzo finish. After this, the surface is again cured for few days and finally ground by grinding machine fitted with a finer carborundum stone disc. Finally, the whole surface is washed with a weak solution of soft soap in warm water. The surface, so produced, is pleasing in apperance and sanitary in nature.



Related Posts:

1. Cork Flooring
2. Rubber Flooring
3. Glass Flooring
4. Timber Flooring
5. Brick Flooring

Brick flooring is suitable for cheap construction, and for places where heavy articles are to be stored as in case of ware houses, stores and godowns. Brick flooring is commonly used in alluvial places, where stone is scarce and well burnt bricks of good quality are readily available. The brick flooring may be laid with bricks laid flat, or on edge arranged in hearing-bone pattern, or set at right angles to the walls.

The bricks, whether laid flat or on edge, are set in ordinary mortar and pointed with cement, or set in hydraulic mortar. Brick-on edge is preferred to bricks laid flat, because the former being less liable to crack under pressure than the latter and also having the higher depth gives a greater thickness in the former case to resist the moisture penetration.

The method of brick flooring is as follows,

First of all, an excavation is made about 40 cm or so below the intended surface or level of the floor depending upon the nature of soil and the type of structure. The earth is then levelled, watered and well rammed, until it becomesdry and hard. On the bed so prepared, the sub-grade should be made with a 25 cm layer of rubble or brick bats, and covered with 10 to 15 cm thick layer of lime concrete or lean cement concrete (1 cement : 3 sand : 6 C.A.). Upon the prepared subgrade, the bricks are laid in desired shape (may be in parallel rows or herring-bone pattern) and set in cement or lime mortar. The joints should be preferably be 1.5 mm in thickness. The joints, sometimes may be required to be pointed to have better appearance. For Pointing purposes, the mortar is first raked out from the joints to a depth of about 2 cm and then pointed with cement mortar. For brick flooring outside the building, the bricks joints are grouted with dry sand and the process is termed as sand grouting.
Merits of brick flooring:

• It offers a durable and sufficiently hard floor surface.
• It provides a non-slippery and fire-resistant surface.
• It is cheaper in initial cost as compared to cement concrete, mosaic, terrazzo flooring, etc.,
• It is easy in maintenance.

Demerits:

• The only drawback of this flooring is that it is water absorbent.
  

The timber flooring is best suited for buildings on hill stations or in localities where the climate is damp. However timber flooring is used carpentry halls, dancing halls, auditoriums, etc., The timber flooring is not much used for ground floors in India. In this type of flooring, the prevention of dampness is most important and hence all possible measures should be taken to check the dampness from rising above.

The entire area of the building below the ground floor of timber is covered with an impervious material in order to prevent dampness. This material may be either cement concrete or asphalt. Generally, a 15 cm layer of concrete known as oversite concrete, is placed all over the bed, and DPC (Damp proofing courses) are inserted throughout the width of the wall immediately below the wall plate.

Timber flooring essentially consists of boarding supported on timber joists called bridging joists or floor joists, which are nailed to the wall plates at their ends. In case of large rooms, where the distance between the wall is considerable, intermediate walls, called sleeper or dwarf walls, are constructed to support the joiss along their length. Longitudinal timber members called 'sleeper plates', are fixed on the top of sleeper walls and the timber joists are secured to the sleeper walls by nailing to the sleeper wall plates.

The hollow space between the flooring and the oversite concrete is kept dry and fully ventilatedby keeping openings in the main walls above the ground level.

The glass flooring can be used for special purposes where it is desired to transmit light from an upper floor to a lower floor such as from a ground floor to a basement. this floor is not commonly used for floors, in general.

In the construction of glass flooring, the structural glass in the form of tiles, blocks, etc., is fitted within frames of various types. The structural glass is available in different forms of varying thicknesses, usually from 10 to 30 mm. The frame-work containing structural glass blocks should be closely spaced that the glass flooring can safely sustain the anticipated loads.

Rubber flooring are being used to a large extent in public and industrial buildings because of their good wearing qualities, resiliency (i.e., elasticity) and noise insulation. The flooring material is made up of pure rubber mixed with fillers, such as cotton fibre, granulated cork or asbestos fibre and the desired colouring pigments. Rubber flooring is manufactured in the form of sheets or tiles, in a variety of patterns and colours. The thickness of tiles or sheets ranges between 3 to 10 mm.

Though rubber flooring is expensive in its initial cost, yet it provides a durable wearing surface. However, oil , grease and gasoline make the floor slippery and difficult to restore it in good condition.

• Rubber sheets are supplied usually in sizes 500 x 90 cm, 350 x 90 cm, and 250 x 90 cm.
• Rubber tiles are supplied usually, in sizes, 20 x 20 cm, 30 x 30 cm, and 45 x 45 cm.
• For the above mentioned three sizes, the corresponding thicknesses are specified as 3.2 mm, 4.8 mm, and 6.4 mm respectively.

Merits of Rubber flooring:

• It provides an attractive, resilient, durable and cheap surface.
• It offers surface which can be easily washed and cleaned.
• Being moderately warm with cushioning effect, it gives comfortable living and working conditions.
• It offers adequate insulation against noise and heat.

Demerits:

• It is subjected to rotting when kept wet for sufficient time and its use is not recommended for basements.
• Rubber Flooring does not offer resistance against fire, being combustible in nature.
• This covering when applied over wooden base may get torn, under excessive sub-floor movements.