Following are the five different types or Classification of cement concrete: 1. Water-Proofing Cement Concrete 2. Coloured Concrete 3. Light-Weight Concrete 4. No-Fines Concrete 5. Pre-Cast Concrete 6. Ready-Mix Concrete 7. Transit-Mix Concrete.
Type # 1. Water-Proofing Cement Concrete:
For certain types of works like water storage tanks, reservoirs, basement walls, roofs, swimming pools, sewage units, etc., the impermeability of concrete is absolutely essential. In general, it can be stated that if concrete is made dense and free from cracks, it is watertight.
The dense concrete can be obtained by closely adhering the following essentials:
(i) Using a high class Portland cement of guaranteed quality.
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(ii) Taking extreme care to adopt correct grading and proportioning of the sand, aggregate and cement.
(iii) Using clean and non-porous aggregates.
(iv) Mixing thoroughly to the right consistency using the right amount of water.
(v) Placing, tamping and curing carefully.
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(vi) Making use of suitable water-proofing compound.
The cement concrete to a certain extent may be made impermeable to the water by using hydrophobic cement. All the flat roofs in the modern age are generally constructed of R.C.C. It becomes necessary to give some treatment of water-proofing to such roofs.
Following are the four methods adopted for water-proofing of R.C.C. flat roofs:
1. Finishing
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2. Bedding concrete and flooring
3. Mastic asphalt and jute cloth
4. Use of water-proofing compounds.
1. Finishing:
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For ordinary buildings of cheap construction, the finishing of roof surface is done at the time of laying cement concrete. The finishing of flat roof is carried out in cement mortar of proportion 1:4 i.e., one part of cement to four parts of sand by volume.
2. Bedding Concrete and Flooring:
In this method, the surface of R.C.C. slab is kept rough and on this surface, a layer of concrete is laid. The concrete may be brickbats lime concrete (1:2:4) or brickbats cement concrete (1:6:12). The thickness of the concrete layer is about 100 mm. The surface of the bedding concrete is provided by a suitable flooring such as tiles, terrazzo, Indian patent stone, etc. A convex joint is provided at the junction of parapet wall and roof.
3. Mastic Asphalt and Jute Cloth:
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In this method, a layer of hot mastic asphalt is laid on the roof surface. The jute cloth is spread over this layer. Then one more layer of mastic asphalt is applied so that the jute cloth is sandwiched between the two layers of mastic asphalt. The sand is then sprinkled over the entire surface of roof. For better grip, the lead sheets are inserted at the junction of parapet wall and roof.
4. Use of Water-Proofing Compounds:
Some of the water-proofing compounds like Pudlo, Impermo, etc., are available in the market and when such a compound is added to the cement during construction, it prevents seepage, leakage and damp caused by the capillary absorption of the moisture in cement, mortar and concrete. The quantity of water-proofing compound to be added is also very small, say 2% and thus a bag of cement will require only about 10 N of such compound.
The water-proof compounds are available in the powder form and they are to be mixed thoroughly with cement by hand before the cement is mixed with the aggregate.
The advantages claimed by using a water-proofing compound of good quality are as follows:
(i) It corrects a badly proportioned concrete mixture.
(ii) It cures immature green concrete.
(iii) It makes good concrete from the poor materials.
(iv) It permits less rigid supervision of the workmanship.
Type # 2. Coloured Concrete:
The concrete can be made coloured in the following ways:
(i) By addition of suitable colouring pigments to the extent of about 8 to 10% of the weight of cement.
(ii) By using coloured cement for the preparation of cement concrete.
(iii) By selecting aggregates possessing the required colour.
The coloured concrete is used for the following purposes:
(i) Manufacture of items for public welfare;
(ii) Ornamental finishes in buildings;
(iii) Preparing park lanes;
(iv) Separating lines of traffic of road surfaces;
(v) Underground pedestrian crossings; etc.
Type # 3. Light-Weight Concrete:
The bulk density of ordinary concrete is about 23 kN/m3.
The concrete having bulk density between 5 to 18 kN/m3 is known as the light-weight concrete and it is prepared from the following materials:
1. Binding Material:
The ordinary Portland cement and its varieties can be used as binding material. If local binding material such as lime-slag, lime-cinder, etc. is available, the same can also be adopted as the binding material.
2. Aggregates:
For light-weight concrete, the loose porous materials are used as the aggregates. The natural porous aggregates can be obtained by crushing light-weight rocks. The artificial porous aggregates can be obtained from industrial wastes.
3. Steel:
The light-weight concrete is highly porous and hence it leads to the corrosion of reinforcement, if not properly protected. Hence the light-weight concrete should be made adequately dense when used for R.C.C. work. Sometimes the reinforcement is coated with anti-corrosive compounds, when light-weight concrete is adopted.
4. Water:
It is necessary to use pure drinking water to prepare light-weight concrete. The strength of light-weight concrete mainly depends on the amount of water in the mix. The water-cement ratio for achieving optimum strength of light-weight concrete should be carefully worked out. As water content reaches to its optimum value, there is corresponding increase in the strength of light-weight concrete.
Following are the advantages of light-weight concrete:
(i) The local industrial waste, if found suitable for light-weight concrete, can be economically utilized.
(ii) The reduction in weight of concrete helps easy removal, transport and erection of pre-cast products.
(iii) The use of light-weight concrete results in the reduction of cost to the extent of about 30 to 40% or so.
(iv) The light-weight concrete does not present special problems with respect to freezing and thawing. It is due to the fact that the larger pores in aggregate are unlikely to become saturated, provided the cement paste is protected by air entrainment.
(v) The light-weight concrete has comparatively less tendency to spall. Hence its fire resistance is greater as compared to the ordinary concrete.
(vi) The light-weight concrete has generally a lower thermal expansion than ordinary concrete.
(vii) The sound absorption of light-weight concrete is good because of the fact that the air-borne sound energy is converted into heat in the minute channels of the concrete. The sound absorption coefficient of the lightweight concrete is nearly twice than that of the ordinary concrete.
The only drawback of light-weight concrete is that the depth of carbonation i.e. the depth within which corrosion can occur under suitable conditions is nearly twice than that of normal concrete. Hence special care will have to be taken to provide sufficient cover to the reinforcement of the light-weight structures to grant protection against corrosion.
Type # 4. No-Fines Concrete:
The no-fines concrete consists of cement, coarse aggregate and water. Thus the fine aggregate or sand is eliminated and such concrete has been adopted for cast-in-situ external load bearing walls of single and multi-storey houses, small retaining walls, damp-proofing sub-base material, etc.
Following are the advantages of no-fines concrete:
(i) As compared to the conventional concrete, the drying shrinkage of no- fines concrete is relatively low.
(ii) As there is absence of capillary passages, there is no transmission of water by capillary action.
(iii) It is a type of light-weight concrete and hence it grants the advantages associated with the light-weight concrete construction.
(iv) It possesses better insulating characteristics than conventional concrete because of the presence of large voids.
(v) There is direct saving in material requirements as this concrete does not require sand which results in considerable saving of cement per m3 of concrete.
(vi) The unit weight of no-fines concrete is about two-thirds of the unit weight of conventional concrete. Hence, the pressure on formwork is greatly reduced. Also the formwork need not be watertight and hence it is possible to use cheap formwork.
(vii) As no-fines concrete does not segregate, it can be dropped from a considerable height and placed in very high lifts.
Following are however the limitations of the no-fines concrete:
(i) As no-fines concrete has little or no cohesion in the fresh state, it requires long time for the removal of forms.
(ii) It is highly permeable as compared to the conventional concrete and hence, the rendering of walls becomes essential. However in certain cases, such as drainage layers in soils, the advantage of quality of high permeability of no-fines concrete can be taken.
(iii) The compressive, bond and flexural strengths of no-fines concrete are considerably lower than those of conventional concrete. The use of reinforcement in no-fines concrete is generally not recommended. However, if reinforcement is to be used, it is to be coated with a thin layer of about 3 mm thickness of cement paste so as to improve the bond characteristics and also to improve the resistance to corrosion.
Type # 5. Pre-Cast Concrete:
The main difference between pre-cast concrete and cast-in-situ concrete is that the former is a factory made product while the latter is prepared at site of work. It is possible to prepare well-made pre-cast products by keeping a high standard of finishing. The pre-cast products vary from simple structures such as fencing posts, pipes, paving slabs, etc. to elaborate and complicated artificial concrete blocks.
The procedure for preparing pre-cast products is as follows:
(i) The moulds, which may be of timber, steel or sand, are prepared to the shape of the product.
(ii) The reinforcement, if any, is put up in the moulds as per design.
(iii) The concrete is mixed in the desired proportion and placed in the moulds.
(iv) The finishing of the products is then carried out. The ordinary products such as fence-posts, sleepers, etc. are left as they are, while products such as spun pipes are finished during the process of manufacture.
(v) The products are then sufficiently cured in specially constructed tanks.
(vi) The products are then dispatched for use at site of work. They may be lifted and placed in position by means of light overhead cranes and small mobile cranes.
Advantages:
Following are the advantages of pre-cast concrete:
(i) The concrete of superior quality is produced as it is possible to have better technical control on the production of concrete in factory.
(ii) It is not necessary to provide joints in the pre-cast construction.
(iii) The labour required in the manufacturing process of the pre-cast units can easily be trained.
(iv) The moulds employed for preparing the pre-cast units are of steel with exact dimensions in all directions. These moulds are more durable and they can be used several times.
(v) The pre-cast articles may be given the desired shape and finish with accuracy.
(vi) The pre-cast structures can be dismantled, when required and they can then be suitably used elsewhere.
(vii) The transport and storage of various components of concrete for cast- in-situ work are eliminated when pre-cast members are adopted.
(viii) The work can be completed in a short time, when pre-cast units are adopted.
(ix) When pre-cast structures are to be installed, it is evident that the amount of scaffolding and formwork is considerably reduced.
Disadvantages:
Following are the disadvantages of pre-cast concrete:
(i) If not properly handled, the pre-cast units may be damaged during transport.
(ii) It becomes difficult to produce satisfactory connections between the precast members.
(iii) It is necessary to arrange for special equipment for lifting and moving of the pre-cast units.
(iv) The economy achieved in pre-cast construction is partially balanced by the amount to be spent in transport and handling of pre-cast members. It becomes therefore necessary to locate the pre-cast factory at such a place that transport and handling charges are brought down to the minimum possible extent.
Type # 6. Ready-Mix Concrete:
Ready-mix concrete is a type of concrete that is manufactured in a factory or batching plant, according to a set recipe, and then delivered to a worksite, by truck mounted transit mixers. The inside of a transit mixer uses a simple Archimedes screw to mix and to lift the concrete to the delivery chute.
This results in a precise mixture, allowing specialty concrete mixtures to be developed and implemented on construction sites. The first ready-mix factory was built in the 1930, but the industry did not begin to expand significantly until 1960 and it has continued to grow since then.
Ready-mix concrete is sometimes preferred over on-site concrete mixing because of the precision of the mixture and reduced worksite confusion. However, using a pre-determined concrete mixture reduces flexibility, both in the supply chain and in the actual components of the concrete.
Ready-mix concrete is popularly called RMC, refers to concrete that is specifically manufactured for delivery to the customer’s construction site in a freshly mixed and plastic or unhardened state. Concrete itself is a mixture of Portland cement, water and aggregates comprising sand and gravel. In traditional work sites, each of these materials is procured separately and mixed in specified proportions at site to make concrete.
Ready-mix concrete is bought and sold by volume, usually expressed in cubic meters. RMC can be custom-made to suit different applications. It is manufactured under computer-controlled operations and transported and placed at site using sophisticated equipment and methods.
On some jobs, such as large danger of falling highway jobs, it is possible to use a batch plant that contains its own mixer. A plant of this type discharges ready-mixed concrete into transit mixers, which haul it to the construction site. The truck carries the mix in a revolving chamber much like the concrete mixer.
Keeping the mix agitated in route prevents segregation of aggregate particles. A ready-mix plant is usually portable so that it can follow the job along. It must be certain, of course, that a truck will be able to deliver the mix at the site before it starts to set. Discharge of the concrete from the drum should be completed within one and half to two hours.
Advantages:
Following are the advantages of ready-mix concrete over site-mix concrete:
(i) A centralized concrete batching plant can serve a wide area.
(ii) The plants are located in areas zoned for industrial use, and yet the delivery trucks can service residential districts or inner cities.
(iii) Better quality concrete is produced.
(iv) Storage space for basic materials at site is not required.
(v) It eliminates procurement/hiring of plant and machinery.
(vi) Wastage of basic materials is avoided.
(vii) It saves labour associated with production of concrete.
(viii) It reduces time required to prepare concrete.
(ix) It also reduces noise and dust pollution at site.
Disadvantages:
Following are the disadvantages of ready-mix concrete over site-mix concrete:
(i) The materials are batched at a central plant, and the mixing begins at that plant, so the travelling time from the plant to the site is critical over longer distances.
(ii) Some sites are just too far away, though this is usually a commercial rather than technical issue.
(iii) Access roads and site access have to be able to carry the weight of the truck and load.
(iv) Ready-mix concrete is to be placed within a limited time span after batching at the plant. Concrete is still useable after this time but may not conform to relevant specifications. Modern additives modify precisely that time span however, the amount of additive added to the mix is very important.
Type # 7. Transit-Mix Concrete:
The concrete that is mixed, either wet or dry, en route to a job site is called transit-mix concrete. A transit-mix truck carries a mixer and a water tank from which the driver can, at the proper time, introduce the required amount of water into the mix.
The truck picks up the dry ingredients at the batch plant together with a slip which tells how much water is to be introduced to the mix upon arrival at the site. The mixer drum is kept revolving in route and at the job site so that the dry ingredients do no segregate.