Importance of Aggregates

Aggregate play an important part in providing strength to concrete, the transition zone of concrete is the interfacial region between an aggregate and cement paste and is responsible for providing compressive strength to cement. Care should be taken not to use Maximum size aggregate(MSA) more than 20 mm in your houses while there may be rubble of size as big as 80 to 150 mm. However, in the construction of nuclear power plants, use of rubble is advised since they require thick wall section to act as a shield against radiation.

Aggregate should be round and equiangular in size( they should not be irregular and flaky. However, use of angular aggregate is advised since these aggregates have a good interlocking effect, possess minimum surface area and hence require less mortar to cover their surface. During the extraction of aggregate, care should be taken to make the aggregate undergo washing or sieve analysis before its use. Since any presence of harmful materials involving silt, clay, pyrites, acidic salts can affect the strength and durability of the concrete leading to inhomogeneity in composition and failure of the structure when subjected to eccentric loading. The Bureau of the Indian standard specifies four grading zones for fine aggregate Group I to IV in order of decreasing particle size starting from coarse sand and ending up with fine sand. It is advisable not to use fine sand in making RCC structure since they are porous and can cause corrosion in reinforcement.

Fine aggregates include natural sand, crushed natural gravel and crushed natural stone these particles should pass through a sieve of opening 4.75 mm whereas the ones which are completely retained in it are coarse aggregates which include uncrushed gravel and stone and big boulders.

Aggregate should be chemically inert in nature and should be sufficiently tough, hard and should have the good bearing capacity.
Well-Graded Aggregates used in a cement mixture can reduce the presence of air voids and help in providing sufficient strength and durability to concrete.

There are some tests carried out on aggregates such as the Los Angeles test, impact test, abrasion test and test of the soundness of aggregates done by dissolving them in a solution of sodium and magnesium sulphate.

For providing support over concrete dams which has a huge mass, aggregates formed by igneous rocks( specially mafic and ultramafic) should be used.

In the construction of buttress and arch dams moderate to weak aggregates can be employed since the highest level of load acts on the abutments. Care should be taken to restrict the MSA of aggregate according to the nature of the building else’s situation of segregation (separation of coarser particles from cement mix) and bleeding( the rise of water over the surface of concrete and formation of a layer of cement called laitance) take place.

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