Aggregates and Testing of Aggregates

 Aggregates |Testing of Aggregates| Classification of aggregate| Source of aggregate.

General:- 

A aggregates are the important constituents inconcrete.

 They give body to the concrete, reduces shrinkageand effect economy. 

Earlier, aggregatesw wereconsiderd as chemically inert materials but nowi ithas been recognised that some of the aggregates  arechemically active and also that certain aggregates  exhibit chemical bond at the interface of aggregate and paste.

 The mere fact that the aggregates occupy 70–80 per cent of the volume of concrete, their impacton various characteristics and properties of concreteis undoubtedly considerable. 

To know more aboutthe concrete it is very essential that one should knowmore about the aggregates which constitutem majorvolume in concrete. 

Without the study of the aggregate in depth and range, the study of the concrete is incomplete. Cement is the only factory made standard component in concrete.

 Other ingredients namely, water and aggregates are natural material sand can vary to any extent in many of their properties.

Concrete can be considered as two phase materials for convenience; paste phase and aggregate phase. Having studied the paste phase of concrete in the earlier chapters, we shall now study the aggregates and aggregate phase in concrete in this chapter. 

The study of aggregatescan best be done under the following sub-headings:-

(a) Classification.

(b) Source.

(c) Size.

(d) Shape.

(e) Texture.

(f ) Strength.

(g) Specific gravity and bulk density.

 (h) Moisture content.

(i ) Bulking factor.

 ( j) Cleanliness.

(k) Soundness.

 (l ) Chemical properties.

(m) Thermal properties.

 (n) Durability.

(o) Sieve analysis.

 (p) Grading.

A) Classification:- 

Aggregates can be classified as 

(i) Normal weight aggregates, 

(ii ) Light weight aggregates

 (iii) Heary weight aggregates. 

(iv)Light weight aggregate 

(v) Heavy weight aggregate 

will be discussed elsewhere under appropriate topics. In this chapter the properties oformal weight aggregates will only be discussed.

Normal weight aggregates can be further classified as:-

I) Natural aggregates and

II) Artificial aggregates.

I) Natural Aggregate:- 

Sand, Gravel, Crushed ,Rock such as Granite, Quartzite, Basalt, Sandstone

II) Artificial Aggregate:- 

 Broken Brick, Air-cooled Slag, 

Sinteredfly ashl , Bloated clay.

Aggregates can also be classified on the basis of the size of the aggregates as coarse aggregatend fine aggregate.

B) Source:- 

• Almost all natural aggregate materials originate from bed rocks. 

• There are three kinds of rocks, namely, igneous, sedimentary and metamorphic. These classifications are based on the mode of formation of rocks.

•  It may be recalled that igneous rocks are formed by the cooling of molten magma or lava at the surface of the crest (trap and basalt) or deep beneath the crest
• (granite). 

• The sedimentary rocks are formed originally below the sea bed and subsequentle lifted up. 

• Metamorphic rocks are originally either igneous or sedimentary rocks which are
• subsequently metamorphosed due to extreme heat and pressure.

•  The concrete making properties of aggregate are influenced to some extent on the basis of geological formation of the parent rocks together with the subsequent processes of weathering and alteration.

• Within the main rock group, say granite group, the quality of aggregate may vary to a very great extent owing to changes in the structure and texture of the main parent rock from place to place.

a) Aggregates from Igneous Rocks:- 

• Most igneous rocks make highly satisfactory concrete aggregates because they are normally hard, tough and dense. The igneous rocks have massive structure, entirely crystalline orvwholly glassy or in combination in between, depending upon the rate at which they were cooled during formation. 

• They may be acidic or basic depending upon the percentage of silica content.
• They may occur light coloured or dark coloured. 
• The igneous rocks as a class are the most chemically active concrete aggregate and show a tendency to react with the alkalies in cement.
• This aspect will be discussed later. As the igneous rock is one of the widely occurring typeof rocks on the face of the earth, bulk of the concrete aggregates, that are derived, are of igneous origin.

b) Aggregates from Sedimentary Rocks:- 

• Igneous rocks or metamorphic rocks are subjected to weathering agencies such as sun, rain and wind. 
• These weathering agencies decompose, fragmantise, transport and depos the particles of rock, deep beneath the ocean bed where they are cemented together by some of the cementing materials. 

• The cementing materials could be carbonaceous, siliceous or argillaceousin nature. At the same time the deposited and cemented material gets subjected to static pressure of water and becomes compact sedimentary rock layer.

• The deposition, cementation and consolidation takes place layer by layer beneath the ocean bed. These sedimentary rock formations subsequently get lifted up and becomes continent .

• The sedimentary rocks with the stratified structure are quarried and concrete aggregates are derived from it. 

• The quality of aggregates derived from sedimentary rocks will vary in quality depending upon the cementing material and the pressure under which these rocks are originally compacted. Some siliceous sand stones have proved to be good concrete aggregate Similarly, the limestone also can yield good concrete aggregate.

C) Aggregates from Metamorphic Rocks:- 

• Both igneous rocks and sedimentary rocks may be subjected to high temperature and pressure which causes metamorphism which changes the structure and texture of rocks.
• Metamorphic rocks show foliated structure. The thickness of this foliation may vary from a few centimetres to many metres. If the thickness of this foliation is less, then individual aggregate may exhibit foliation which is not a desirable characteristic in aggregate. 

• However, many metamorphic rocks particularly quartizite and gneiss have been used for production of good concrete aggregates.

C) Size:- 

The largest maximum size of aggregate practicable to handle under a given set of

conditions should be used. Perhaps, 80 mm size is the maximum size that could be

conveniently used for concrete making. Using the largest possible maximum size will result in

(i) reduction of the cement content.

 (ii) reduction in water requirement.

 (iii) reduction of drying shrinkage.

 However, the maximum size of aggregate that can be used in any given condition

may be limited by the.

following conditions: 👈 click here

(i ) Thickness of section; 

(ii) Spacing of reinforcement;

(iii) Clear cover; 

(iv) Mixing, handling and placing techniques.

D) Shape:-

Shape of Aggregate

The shape of aggregates is an important characteristic since it affects the workability of concrete.

It is difficult to really measure the shape of irregular body like concrete aggregatew which are derived from various rocks. Not only the characteristic of the parent rock, but also the type of crusher used will influence the shape of aggregates, e.g., the rocks available round about Pune region are found to yield slightly flaky aggregates, whereas, good granite rock

as found in Banglore will yield cubical aggregate. The shape of the aggregate is very much influenced by the type of crusher and the reduction ratio i.e., the ratio of size of material fed into crusher to the size of the finished product. Many rocks contain planes of parting or jointing which is characteristic of its formation. 

It also reflects the internal petrographics structure As a consequence of these tendencies, schists, slates and shales commonly produce flaky forms, whereas, granite, basalt and quartzite usually yield more or less equidimensional particles Similarly, quartizite which does not posses cleavage planes produces cubical shape aggregates.

E) Texture:- 

Surface texture is the property, the measure of which depends upon the relative degree to which particle surfaces are polished or dull, smooth or rough. Surface texture depends on hardness grain size, pore structure, structure of the rock, and the degree to which forces acting on the particle surface have smoothed or roughend it. Hard, dense, fine-grained materials will generally have smooth fracture surfaces. Experience and laboratory experiments have shown that the adhesion between cement paste and aggregate is influenced by severalc complex factors in addition to the physical and mechanical properties.

F) Strength:- 

When we talk of strength we do not imply the strength of the parent rock from which the aggregates are produced, because the strength of the rock does not exactly represent the strength of the aggregate in concrete. Since concrete is an assemblage of individual pieces of aggregate bound together by cementing material, its properties are based primarily on the quality of the cement paste. 

This strength is dependant also on the bond between the cement paste and the aggregate. If either the strength of the paste or the bond between the paste and aggregate is low, a concrete of poor quality will be obtained irrespective of the strength of the rock or aggregate. But when cement paste of good quality is provided and its bond with the aggregate is satisfactory, then the mechanical properties of the rock or aggregate will influence the strength of concrete.