4It is a mixture of binding material,fine aggregates, coarse aggregatesand water in suitable proportions.Cement and lime are generally usedas binding materials where as sand,surkhi, etc are used as fineaggregates and crushed stones,gravel, broken pieces of bricks, etcand water free from organicimpurities are used as coarseaggregates in the manufacturing ofdifferent types of concrete.
The required strength ofconcrete can be obtained bycareful selection of itsingredients and their propergrading, accurate watermeasurements and adopting agood work man ship, propermixing, finishing and curing ofconcrete during theconstruction work.
Concrete is a brittlematerial, it is strong incompression but weak intension, so to over comethat weakness steel barsare used in the R.C.Cconstruction.
MANUFACTURING OF CONCRETE:Following operations are mainlyinvolved during manufacturing ofConcrete.
Mixing:For preparing concrete well washed fineaggregates, coarse aggregates andcement or lime are spread on puccaplatform and then it is mixed thoroughly tilluniformity is achieved. Machine mixingcan be done, if the magnitude of work sodemands.Only that much concrete should be mixedwhich can be laid within two hours. Ifcement has been used in the mix then theconcrete should be laid in position within30 minutes after water has been added toit.
Placing:Laying and construction of concrete shallbe paced within three hours of addingwater in case of concrete prepared withlime mortar and within one hour in caselime cement mortar has been used.Concrete should be used in layers notexceeding 15 cm in thickness whenconsolidated. Next layer of concrete is laidonly when the previous one has beenthoroughly consolidated and it should notbe dropped from heights as it would resultin segregation of aggregates. If it is to belaid at a depth then chutes should beused.
Curing:Concrete shall be well cured for aperiod of at least 07 days after itslaying. Curing shall be done byspreading wet sand or gunny bags etc,and watering frequently.
12Different types of concrete areused depending uponrequirements in different typesof engineering works. Somewell-known types of concreteare described as under:
13(1) Cement Concrete:This is the plain concrete prepared byusing Portland cement as the bindingmaterial and the remaining ingredients, asusual, are the fine aggregates-sand, coarseaggregates-broken stone, gravel and water.It is a concrete of high strength. Forcommon constructional works, the ratio ofcement. Fine aggregates, coarseaggregates by volume are 1:2:4 or 1:3:6.For a very high strength this ratio may be1:11/2 : 3 and for a very ordinary work notrequiring much of strength it may be 1:4:8.
14(2) Lime Concrete:This is another plain concretewhich is prepared by using limeas the binding material and theremaining ingredients such asfine aggregates may be sand orsurkhi, coarse aggregates maybe broken bricks, broken stoneor gravel and water. It is acheap variety of concrete.
15(3) Reinforced Cement Concrete(R.C.C):This is the cement concrete furtherreinforced by steel commonly knownas R.C.C. The reinforced cementconcrete has several advantages overplain concrete and structural steel.
16(4) Pre-stressed concrete:It is that type of concrete in whichsteel as well concrete of very highstrength are used, thus it can sustainexcessively high loads.
17Except above mentionedConcrete types, there are someother types of concrete such as:(5) Precast Concrete,(6) Mass Concrete,(7) Light Weight Concrete, (lightweight aggregates are used)(8) Non-fines Concrete (withoutcoarse aggregates)(9) Coloured Concrete.
18Uses and advantages of Concrete:Concrete as a constructional material hasthe following advantages:(1) Concrete is economically used in thelong run as compared to other buildingmaterials.(2) Concrete possesses a highcompressive strength and also resist wellto weathering effects. When it is properlyprepared its strength is equal to that of ahard natural stone.
19(3) Green concrete (fresh concrete)can be easily handed and mouldedin any size and shape according todemand/specifications. The formwork can be reused a number oftimes for similar jobs, resulting ineconomy.(4) It is strong in compression andhas unlimited structuralapplications in combination withreinforcement.
20(5) The concrete is strengthly used inthe construction of foundations,walls, beams, lintels, roofs ofbuildings, air fields, water retainingstructures, dams, bridges, etc.(6) It is durable and fire resistant andrequires very little maintenance.(7) Concrete can be evenly spreadon and filled in fine cracks, fissuresfor repairing the components ofstructures by grouting process.
21PROPERTIES OF CONCRETE INPLASTIC STATE:A freshly mixed concrete mustpossess the under mentionedproperties.(i) Good workability:Workability of concrete can bedefined as the ease with which it canbe mixed, transported and placed inposition in a homogeneous state.
22(ii) No segregation:Segregation may be defined as theseparation of coarser materials fromother mass, if the quantity of waterin the mix is more, the larger sizedaggregates will be separated thuscasing segregation, so concreteshould be free from segregation.
23(iii) No bleeding:Bleeding may be defined asseparation of water or water sandcement from freshly mixedconcrete. This is caused due toexcess water content in concrete.
24PROPERTIES OF CONCRETE IN HARDENEDSTATE:A hardened concrete must possess the undermentioned properties:(i) Adequate Strength:Strength is defined as the resistance of ahardened concrete to rupture under differentloadings and is accordingly designated indifferent ways, i.e. tensile strength, compressivestrength, flexural strength, etc. A good qualityconcrete in hardened state must possess thedesired crushing strength.
25(ii) Durability:Durability is defined as the period oftime up to which concrete inhardened state withstands theweathering effects satisfactorily.This property is mainly affected bywater cement ratio . A good qualityconcrete in hardened state must bedurable.
26(iii) Impermeability:The impermeability of hardenedconcrete may be defined as theproperty to resist entry of water. Thisproperty is achieved by using extraquantity of cement in concrete mix.A concrete in hardened state mustbe impermeable.
27(iv) Elasticity:As hardened concrete is brittlematerial, it is desired that it shouldpossess adequate elasticity.
28(v) Low shrinkage:A hardened concrete shouldexperience least shrinkage. Thisproperty is guided by water cementratio. Shrinkage is less if the watercement ratio is less.(vi) Least thermal expansion:A hardened concrete shouldpossess least coefficient ofexpansion.