CEMENT CONCRETE CONSTRUCTION

The cement concrete is amixture of cement, sand, pebbles or crushed rock and water, which when placed in the skeleton of forms and allowed to cure, becomes hard like a stone.

The cement concrete had attained the status of a major building material in all branches of the modern construction because of the following reasons. 

I. It can be readily moulded into durable structural items of various size and shapes at practically no considerable labour expenditure.

 II.It is possible to control the properties of cement concrete within a wide range by  using appropriate ingredients and by applying special processing techniques  mechanical, chemical and physical.

III.It is possible to mechanize completely its preparation and placing processes.

 IV.It possesses adequate plasticity for the mechanical working. 

 In this chapter the building construction carried out with cement concrete will be briefly discussed.

PROPERTIES OF CEMENT CONCRETE

 The cement concrete possesses the following important properties :

I. It has  high compressive strength .

II. It is free  from corrosion and there is no appreciable effect of atmospheric agents on it.

III. It hardens with age and the process of hardening continues  for a long time after the concrete had attained sufficient strength. It is this property of cement concrete which gives it a distinct place among the building materials.

IV. It  is proved to be more economical than steel. This is due to the fact that sand and pebbles or crushed rock, forming the bulk of cement concrete, to the extent of about 80% to 90%, are usually available at moderate cost. The form work  which is of steel or timber, can be used over and over again or for other purposes after it is removed.

V. It binds rapidly with steel and as it is week in tension, the steel reinforcement  is placed in cement concrete at suitable places to take  up the tensile stresses. This  is termed as the Reinforced Cement Concrete or simply R.C.C. 

VI .under the following two conditions , it had a tendency to shrink :

 a. There is initial shrinkage of cement concrete which is mainly due ot loss of water  through forms, absorption by surfaces of forms, etc. 

b. The shrinkage of cement concrete occurs as it hardens. This tendency of cement concrete can be  minimized by  proper curing of concrete .

   VII. It has a tendency to be porous. This is due to the presence of voids which are formed during and after its placing. Following two precautions are necessary  to avoid this tendency :

 VIII. It forms a hard surface, c arable of resisting abrasion.

 IX.  It should be remembered that apart from other materials, the concrete comes to the site in the form of raw materials only. Its final strength  it. However the depend entirely   on local conditions and persons handling it. However the  materials of which concrete is composed may be subjected to rigid specifications.   

 MATERIALS USED IN R.C.C. WORK

Following materials are required for making the R.C.C work :

I. Cement

II. Steel

III. Aggregates

IV. Water.

1.  Cement :   Before the introduction  of ordinary Portland cement, the lime was used as a cementing material. Most of the cement concrete work in building construction is doen with ordinary Portland cement at present. But other special varieties of cement such as rapid hardening cement and high alumina cement are used under certain  circumstances.  The cement should comply with all the standard requirements.

2.  Aggregates :  These are the inert or chemically inactive materials which form  the bulk of cement concrete. These aggregates  are bound together by means of cement.  The aggregates are classified into two categories :  Fine and coarse :

 The material which s passing through BIS test sieve no. 480 is termed as a fine aggregate.  Usually, the natural rive sand is used as a fine aggregate.  But at places, where natural sand is not available economically,  the finely crushed stone may be used as a fine aggregate.

The  material which  is retained on BIS test sieve no. 480 is termed as a coarse aggregate. The broken stone is generally used as a coarse aggregate. The  nature of work decides the maximum  size of the coarse aggregate. For the slabs and walls, the maximum size of coarse aggregate should be limited to one-third the thickness of the  concrete section.

 The aggregates to be used for the cement concrete work should be hard, durable and clean. The aggregates should be completely free from  lumps of clay, organic and vegetable matter, fine dust, etc. the presence of all such debris prevents adhesion of  aggregates and hence reduces the strength of concrete.

 3  Steel :  The steel reinforcement is generally in the form of round bars of mild steel. The diameters of bars vary from 5 mm to 40 mm. sometimes the square bars or twisted bars or ribbed –to steel are used as steel reinforcement.  For road slabs and such other constructions, the reinforcement may also consist  of sheets of rolled steel of suitable thickness. The hyrax which is a steel lath may also be used as the steel.  Reinforcement.

  4  Water : This is the least expensive but most important ingredient of concrete.  The water, which is  used for making concrete, should be clean and free from harmful impurities such as oil, alkali, acid  etc. In general, the  water which is fit for drinking  should be used for making concrete.

 It may be noted that sometimes the ingredients other  than above are added in concrete to give it certain improved qualities  or for changing different physical properties  in its fresh and  hardened stages. These ingredients or substances are known as the admixtures. The addition of an admixture may improve the concrete with  respect to its  strength, hardness, workability, water-resisting power, etc. Following are the commonly used admixtures :

 Alum, aluminium  sulphate, barium oxide, bitumen, calcium chloride, coal ash,  common salt, iron oxide, lime mineral oils,  organic oils,  potassium chloride, silicate of soda, tar products  volcanic ashes, zinc chromate, etc. For instance, when calcium chloride (CaCl2 ) is added as admixture, it absorbs water from the concrete and water cement ratio falls down and can even be brought down up to the limit of 0.25. Thus it gives quick setting concrete. However the use of calcium chloride is not suitable  for concrete with reinforcing bars.

It is necessary to know the complete details of any admixture before its recommendation together with the following factors :

I.Grading curves of aggregates and their respective  properties,

II.Method of construction,

III.Quantity of cement per m3 of concrete,

IV.Requirement of slump and retention.

V.Temperature variation,

VI.Type and make of cement, and

VII.Water-cement ratio.

Depending upon their respective activities in the  concrete mix, the admixtures can be classified in the  following five categories :

I.Accelerators,

II.Air entraining admixtures,

III.High range of water reducers or super plasticizers,

IV.Normal range of water reducers or plasticizers, and

V.Retarders.

It may be noted that some admixtures may have the combined effect of the above  individual activities,  The popularity of various types of admixtures in concrete is  increasing rapidly because of the following advantages available from their use : 

I.Adjusting the final setting times of concrete,

II.Higher early and ultimate strengths,

III.Higher slump and self-levelling concrete,

IV.Increasing durability of  concrete,

V.Lesser water-cement ratios,

VI.Reducing quantity of cement,

VII.Reduction in the permeability of concrete,

VIII.Time savings in terms of repair and maintenance, etc,