Classification Of Aggregate
Aggregate
Aggregate is the major ingredients of concrete. Aggregate is derived from rocks. They are broadly divided into two categories i.e. fine and coarse aggregates. Their properties depend on the mineralogical compound of racks. Crushed rocks are used as coarse aggregates and river sand is used as fine aggregate.Classification Of Aggregate
The classification of aggregate is generally based on their geological origin, size, shape, unit, weight, etc. The aggregate is classified as:-
- Shape Classification
- Geological Origin classification
- Unit weight classification
Geological Origin classification
Natural Aggregate
Almost all natural aggregates originate from bedrocks. Aggregates are reduced to its present size by weathering action of natural agents like temperature variation, rain, wind, etc. or crushing aggregate are obtained by deliberate fragmentation of rock. The geological formation of the parent's rock, weathering processes, petrological structure and texture influence the physical properties of the aggregates.Igneous rocks are formed by the cooling of molten lava at the surface of the crust of earth as basalts or deep beneath crust as granites. The rocks have a massive structure. It is crystalline or glassy depending upon the rate of cooling during formation, whether it was slow or rapid. Percentage of silica content decides acidic or basic nature of the rocks. Igneous rocks are chemically active. In general, aggregates obtained from igneous rock are hard, tough, dense and durable.
Rocks are subjected to weathering action o sun, rain, and wind. This weathering continues to transport and deposit the product of weathering in the ocean bad, whether they are cemented together by carbonaceous, siliceous or argillaceous material. The cemented material gets subjected to the static pressure of overburden and becomes sedimentary rocks available at the surface of the earth. The sedimentary rocks have a stratified structure. The quality of aggregates obtained from sedimentary rocks depends upon the cementing materials, the degree of consolidation, the thickness of layers and contamination. If the thickness of an individual layer is less, it yields flaky aggregates which possess less strength. Some siliceous compact sandstone and limestones yield good aggregates for concrete.
Igneous and sedimentary rocks are subjected to high temperature and pressure which causes metamorphism. It changes the texture and structure of rocks. During volcanic activities, such metamorphosed rocks are brought to the surface of the earth. These metamorphic rocks show a foliated structure. It is not desirable to have a thickness of foliated less than individual aggregates. Normally, the metamorphic rocks like quartzite and gneiss are suitable for the production of good aggregates.
Pumice is a light colored, froth-like volcanic glass obtained by rapid cooling of the froth of molten lava during its formation. Some of the varieties of pumice which are not structurally weak are used for making aggregates for concrete. These concretes have less density of 700 to 1400 kg/m3 and good insulating properties but exhibit shrinkage and high absorption. Volcanic cinders scoria are having similar properties.
Artificial Aggregates
The first type of artificial aggregates is obtained by application of heat in order to expand clay, shale, slate, perlite vermiculite. The suitable raw material is heated in a rotary kiln to a temperature of about 1000 -1200 C. Expansion of material takes place due to the generation of gases which are entrapped in a viscous mass. This porous structure is retained on cooling. The aggregate of suitable size is obtained either by crushing after expansion or by reducing the raw material to the size before calcination. The use of pelletized material produces aggregates which are nearly spherical having a semi-impervious coating over the cellular interior. These aggregates are having lower water absorption and produce concrete of higher workability. Expansion of clay, shale, and slate can aslo be achieved by passing moistened material on the traveling grate under burners. Burning penetrates the full depth and evolved gases are entrapped in viscous mass.Perlite is a glassy volcanic rock when heated to 900 to 1100C expands owing to the evaluation of steam. It forms cellular material with a bulk density of 30 to 240 kg/m3. vermiculite is a platy structure material when heated to 650-1000C expands to several times to its original volume. Concretes made with perlite or vermiculite have very low strength, high shrinkage but are excellent heat insulators.
The second type is obtained from expansion of blast furnace slag by special cooling processes. In the water jet process, a limited amount of water is sprayed in contact with the molten slag during its discharge from furnace steam is generated and gets entrapped into plastic slag which hardness in a porous form. In the machine process, the molten slag is rapidly agitated with a controlled amount o water. Entrapped steam and gases formed due to chemical reactions makes expanded foamed slag after cooling. Bulk density varies between 300-1100 kg/m3. This is a by-product of the iron industry.
Clinker aggregate or industrial cinder from the third type of aggregates. It is made from the well-burnt residue of industrial furnaces, sintered or fused into lumps. It should be free from unburnt coal and hard burnt lime which result in unsoundness due to expansion. Iron or pyrites in the clinker result in staining of surfaces and therefore should be removed. Fly ash is a finely divided residue from the combustion of powdered coal from thermal power stations. It contains a small amount of unburnt fuel. The ash is moistened to form the pellets. are sintered in a suitable furnace. The unburnt fuel from fly ash maintains the process without adding fuel to from sintered nodules. The bulk density of this type of aggregate varies from 1000-1200 kg/m3.
Shape Classification-
In this type of classification depending upon the shaps of particles, the aggregates are rounded, irregular or partly rounded, angular or flaky as follows- Round Aggregate
- Irrengular aggregate
- Angular aggregates
- Flaky and Elongated aggregates
Flaky and Elongated aggregates
An aggregate is termed flaky when its least thickness is less than three-fifths of its mean dimension. The mean dimension of the aggregate is the average of the sieve size, through which the particles pass and are retained, respectively. The particle is said to be elongated when its greatest length is greater than nine-fifth of its mean dimension.Round Aggregates
The aggregate with rounded particles has minimum void ranging from 32 to 33%. It gives the minimum ratio of surface area to the volume, thus requiring minimum cement paste to make good concrete. The only disadvantage is that the interlocking between its particles is less and hence the development of the bond is poor, making it unsuitable for high strength concrete and pavement.Irregular aggregate
The aggregate having partly rounded particles has a higher percentage of voids ranging from 35 to 38%. It requires more cement paste for given workability. The interlocking between particles, thought better than obtained with the rounded aggregates, is inadequate for high strength concrete.Angular aggregates
The aggregate with sharp, angular and round particle has a maximum percentage of voids ranging from 38 to 40. The interlocking between the particles is good, thereby providing a good bond. The aggregate requires more cement paste to make workable concrete to high strength than that required by rounded particles. The angular aggregates are suitable for high strength concrete and pavements subjected to tension.Unit weight classification
Normal weight aggregates
They have a specific gravity between 2.5 to 2.7 produce concrete with unit weight ranging from 23 to 26 KN/m3 and crushing strength at 28 days between 15 to 40 MPa are termed normal weight cocnrete.The commonly used aggregates. i.e. Sands and gravels: crushing rocks such as granites, basalt, quartz, sandstone and limestone: and brick ballast, etc.