Following four materials are required for making R.C.C.: 1. Cement 2. Aggregates 3. Steel 4. 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 done 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 is passed through BIS test sieve no. 480 is termed as a fine aggregate. Usually, the natural river 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 thin 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 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.

The aggregates may also be classified in the following two categories:

(i) Natural aggregates


(ii) Artificial aggregates.

(i) Natural Aggregates:

The term natural aggregate is used loosely to designate aggregates which need only be removed from their natural deposits as unconsolidated sediments. The aggregates obtained from such deposits are called gravel and sand while those produced from ledge rock, boulders or cobble stones are known as crushed stone.

Thus the natural aggregates can be divided into the following three types:


(a) Crushed rock aggregate;

(b) Gravel;

(c) Sand.

(a) Crushed Rock Aggregate:


The crushed rock aggregate is obtained by crushing rock pieces into suitable sizes. It is evident that the quality of the crushed rock aggregate will be controlled by the nature and type of rock from which it is crushed. The rocks are classified into three major divisions according to their origin, namely, igneous rocks, sedimentary rocks and metamorphic rocks.

The igneous rocks are formed originally by cooling from a molten rock material known as magma. They are further classified as plutonic rocks, hypabyssal rocks and volcanic rocks. The plutonic varieties are brittle due to the presence of large crystals and the main types of rocks under this variety are granite, syenite, diorite, etc.

The hypabyssal igneous rocks are medium-grained and they generally possess inter grown texture and hence, they are among the best road stones. The main types of rocks under this group are porphyry, dolorite, porphyrite and diabase. The volcanic types of igneous rocks are fine-grained with basalt and andesite as main varieties. They are excellent for building construction.

The sedimentary rocks are formed by the deposition of products of weathering on the pre-existing rocks. They are further classified as calcareous; siliceous and argillaceous. In calcareous variety, the calcium carbonate predominates and the main types include lime stones, dolomites and chalk.


The lime stones and dolomites are suitable because they have excellent adhesion to cement and bituminous binder. The chalk is unsuitable for construction purpose.

In siliceous variety, the silica predominates and sandstones and quartzite are the main types of stones of this group. The sandstones are frequently used in construction work and as road stones where they are locally available. The quartzite is quite hard and its adhesion to cement is very good but to bitumen is poor. In argillaceous variety, the clay predominates and the main types are clay, shale and mudstone. They are all poor stones for construction purpose.

The metamorphic rocks have either igneous or sedimentary origins but, as a result of intense heat or pressure or both, they have been altered or metamorphosed into rocks having significantly different properties. The hornfels which are formed due to the thermal metamorphism are considered the best from the point of view of road construction.

The gneiss and granulates have the same qualities of road making as those of the granites of coarse variety. The slate and schist are quite unsuitable for being used as aggregates.

It should be remembered that all the above three classes of rock have been used successfully as aggregate. The suitability of rock from a given deposit cannot be determined merely from its origin or method of formation, but is estimated from a combination of experience, physical tests and mineralogical examination.

It may also happen that the most widely used tests may prove misleading and hence, the best possible index of aggregate suitability is its performance in earlier construction of the type under consideration.

(b) Gravel:

The term gravel is used to mean the coarse material resulting from the disintegration of natural rock due to weathering and carried away by water and subsequently deposited on the river banks.

The properties of gravel will by and large be governed by the properties of the basic rock constituents and usually, the hard varieties of gravel are found dumped along river banks or along strata which had earlier been under water. The larger varieties of gravel, known as boulders, do not require special tests, if they are to be used as soling or base course.

(c) Sand:

The final residue of the resistant mineral grains resulting from the weathering action upon the rocks is known as sand and the final form has often been reached after many cycles of deposition and weathering. The most important mineral in sand is quartz and it is hardly affected by the ordinary weathering agents.

(ii) Artificial Aggregates:

The blast furnace slag is perhaps the only artificially prepared aggregate which is used in the construction. It is obtained as a by-product in the manufacture of steel. If slag is specially manufactured under controlled conditions, it can certainly prove to be an excellent aggregate of uniform quality.

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-torsteel 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 hyrib which is a steel lath may also be used as 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 upto 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 detail 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 plasticisers,

(iv) Normal range of water reducers or plasticisers, 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.

Sea Water for Making Concrete:

It is advisable to use clean water fit for drinking purposes for making cement concrete. However, at places where sea water is available in abundance and potable water is costly, the sea water can be used for making cement concrete.

The problem of using sea water for making cement concrete has to be studied from the following two aspects:

(1) Strength

(2) Corrosion of reinforcement.

(1) Strength:

Table 8-1 shows the analysis of average sea water. It contains about 3.50 Per cent of dissolved salts. The approximate Percentages of various salts are 78 per cent of sodium chloride, 15 per cent of magnesium chloride and magnesium sulphate and the rest 7 per cent of calcium sulphate, potassium sulphate, etc.

Now all chlorides tend to accelerate the setting of cement and to improve the strength of concrete in early stages. On the other hand, the sulphates tend to retard the setting of cement and to discourage the strength of concrete in early stages.

It is found that the net effect of these two contradictory actions is the fall in strength of concrete to the tune of about 8 to 20 per cent. Hence the sea water can be used for making cement concrete for structures where such fall in strength is permissible or where it is possible to correct the same by adjusting water-cement ratio, cement content in concrete, etc.

The sea water tends to develop dampness and efflorescence. Hence it can be adopted for concrete structures where finishing characteristics are not important or where persistent dampness of the surface is permissible.

(2) Corrosion of Reinforcement:

It is found that the sea water does not lead to the corrosion of reinforcement, provided the concrete is dense and there is enough cover to the reinforcement.

The minimum cement content for concrete permanently under sea water should be 3 kN per m3 and the minimum cover over the reinforcement should be 75 mm. However it is not advisable to take the risk of corrosion of reinforcement for pre-stressed concrete and hence the sea water should not be used for making pre-stressed concrete.