Aggregates and classification of aggregates (Type of aggregates)

Aggregates:

Aggregates is the board category of basis materials used in construction, including sand, gravel, crushed stone, slag saw dust,, broken bricks etc. It constitutes about 85% of the volume of the concrete. They are chemically inert and forms mortar and concrete when mixed with cement. Aggregate do not take part in the various chemical reactions taking place in concrete during setting and hardening of concrete, thus they are called inert materials. The aggregates are generally used to increase the volume of concrete. They are economical and cheaper than cement. Aggregates also contribute toward greater volume stability and durability to concrete.

Requirement of good aggregates : A good aggregates should meet certain requirements which are as follows:

1.     It should be chemically inactive.

2.     It should be sufficiently strong.

3.     It should be hard enough.

4.     It should be durable.

5.     There  should be limited pores (porosity).

6.     It should have rough surface.

7.     Its surface should be clean and free from clay and other materials.

8.     It should be proper shape i.e. either spherical or cubical.

9.     It should be well graded.

10.  It should be resistant to wear and tear.

Classification of aggregates :

            Aggregates can be classified into four general categories as follows:

1. Classification According to source or nature of formation :  The aggregates that are either available naturally or which are reduced to the required size and shape by applying forces or other process like crushing, cutting etc. Therefore, they are further divided into two categories:

(a) Natural aggregates : These aggregates are used as such as they are available in nature. Main sources of these aggregates are natural deposits of sand, and gravel or obtained from quarries by cutting rocks. Natural sand and gravel prove to be the cheapest as the various natural agents like wind, water and snow  etc. Aggregates obtained from the river beds are very good quality and the most commonly used. The rocks are generally of three types i.e. Igneous, Sedimentary and Metamorphic rocks. The aggregates obtained from Igneous rocks match the requirements to a great extent. Also Sedimentary rocks provide good quality aggregates but metamorphic rocks are not used for obtaining aggregates because of formation of thin structure are not desirable.

            (b) Artificial Aggregates : These aggregates are generally obtained from natural aggregates by performing certain chemical reactions, applying force, heating the natural aggregates etc. The example of artificial aggregates are broken bricks, fly ash , fly ash or broken bricks are not suitable for reinforced concrete work and is used in mass concrete.

2. Classification according to size : The size of aggregates varies to a broad range from centimeter to micrometer. So on the basis of size of aggregates are classified into four categories.

(a) Fine Aggregates

(b) Coarse Aggregates

(c) All in aggregates

(d) Single size aggregates

(a) Fine aggregates: Fine aggregates are those aggregates which can pass through 4.75mm IS-sieve, the residue of the sieve are not considered. The minimum limit of size of sand particles is approximated to be 0.07mm.  Silt are the materials that covers a range of 0.06mm and 0.02mm, of clay, silt and sand which is  in equal proportations. Fine aggregates are natural sand, crushed stone and sand and crushed gravel stone. Fine aggregates contribute toward reducing the number of voids, increasing the workability, increases the volume, reduced the cost and proper density is provided fine aggregates may be further divided into coarse, medium and fine sand depending on variation of sizes.

            (b) Coarse aggregates : The aggregates which are the reduce of the 4.75mm IS sieve and pass through 75mm IS sieve. Coarse aggregates consists of  crushed gravel, uncrushed gravel and partially crushed gravel. These aggregates are formed by natural disintegration of rocks or by artificial crushing of rock or gravel. the maximum particle size of coarse aggregates is 75mm and minimum size is 4.75 mm. Cyclopean aggregates are the aggregates with size greater than 75 mm. the aggregates size depends upon the reinforcement and the types of works.

(c) All-in-aggregates: all-in-aggregates, as the name specifies is the mixtures of fine aggregates and course aggregates. It contains various fractions of fine and coarse aggregates High Quality concrete can not be prepared using these aggregates. If some aggregates lack in particular aggregates then these aggregates are added to make required all-in-aggregate.

(D) Single size aggregates: The aggregates which lie in a very short range of size are called single size aggregates. for example, for 20mm single size aggregates maximum quantity passes through 20mm IS sieve put a greater quantity is retained on a 10 10mm IS sieve.

 

3. Classification according to shape: The degree of packing of aggregates very much depends upon the shape of aggregates.

On the basis of shape it can be classified as:

A.     Rounded aggregates

B.     Irregular aggregates

C.     Angular aggregates

D.    Flaky and elongated aggregates.

shape of aggregates

A.) Rounded aggregates : The ratio of surface area to the volume is minimum in case of bounded aggregates. Therefore, less amount of cement is required for preparing concrete of good quality. These are 33 to 35% voids in rounded aggregates. Due to smooth and rounded structure there is poor locking among the rounded aggregates and between aggregates and reinforcement etc. Thus reducing strength of  structure but they provide high workability.

B) Irregular or partly rounded aggregates: These aggregates vary in surface as compared to rounded aggregates. They are not perfectly round. The workability is less for  concrete in which these aggregates are used. They have  35 to 37% of voids. Because of more surface area they required more cement paste. The interlocking provided by these aggregates is  not upto the required level but better than rounded aggregates. The concrete prepared from these aggregates is not of high strength.

C) Angular aggregates: These aggregates have angular structure i.e. having sharp and well defined edges, and are rough particles. The voids percentage lies in the range of 37 to 40%. Due  to rough and angular structure the interlocking is quite good and also required  more cement paste for a workable concrete with high strength. crushed rock is an example of angular aggregates.

 

D) Flaky and elongated aggregates : The shape of aggregates becomes important in case of high strength concrete where very low water-cement ratios are to be used.

            Flaky aggregates: The flaky aggregates are those aggregates in which the least dimension of the aggregates (thickness) is less than 3/5the times of mean dimensions.

4. Classification according to unit Weight: The aggregates can be classified into three type on the basis of unit weights as:

a)     Normal weight aggregates

b)     Heavy weight aggregates

c)      Light weight aggregates

(a) Normal weight aggregates: sand and gravels, crushed rocks etc. are common examples of normal weight aggregates which have a specific gravities between 2.5 and 2.7 and the unit weights ranging between 23 to 26KN/m3. they have the 28 day crushing strenght of about 15 to 40 MPa.

(b) Heavy weightaggregates: Magnetites, bartyes and scrap iron and generally used as heavy weight aggregates which have a specific gravities of about 2.8 to 2.9 and unit weight ranging between 28 to 29KN/m3. they have a high compressive strength of the range of 20 to 21MPa.concrete containing these aggregates do not have good workability and the light and heavy particles separate out due to variation in weights of materials. 

(c) Light weight aggregates: The aggregates of this category are either natural or manufactured. They have unit weight 12KN/m3 for fine aggregates and 10KN/m3 for coarse aggregates. the concrete produced from these aggregates provides fire resistance and thermal insulation but the workability is poor due to rough surface.