Foundation, Concrete and Earthquake Engineering


2.3 Source 

Almost all natural aggregate materials originate from bed rocks. Based on the mode of formation of rocks, there are three types of rocks namely igneous, sedimentary and metamorphic. 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 subsequently lifted up. Sandstone, limestone, slate and shale are some common sedimentary rocks. Metamorphic rocks are originally either igneous or sedimentary rocks which are subsequently metamorphosed due to extreme heat and pressure. Marble, quartzite and slate are common examples of metamorphic rocks. 

Bricks are obtained by moulding plastic mass of suitably proportioned earth in timber or steel moulds. Moulded bricks are first allowed to dry and then burnt in kilns designed for the purpose. 

2.4 Size of aggregate 

The largest maximum size of coarse aggregate practicable to handle under a given set of conditions should be used [1]. Perhaps, 80 mm size is the maximum size that could be conveniently used for concrete making. The maximum size of the aggregate is governed by the following factors: 

1. It should not be larger than one-fifth of the narrowest dimension of the forms. 

2. It should be one-third of the depth of plane concrete slab. 

3. It should be three-fourth of clear spacing between reinforcing bars or between reinforcing bars and forms. 
Crushed slate aggregate
Crushed granite

4. 40 mm, 20 mm and 10 mm sized aggregates are most commonly adopted in concrete work.

2.5 Shape of aggregate 

Crushed slate aggregate
Crushed slate aggregate
The shape of aggregate is an important characteristic since it affects the workability of concrete. It is difficult to really measure the shape of the irregular body like concrete aggregate. However, as per IS. 383- 1963, shapes of the particles of the aggregate may be round, regular, angular and flaky. Rounded, irregular & angular particles of aggregate show percentage voids of 35%, 37% and 41% respectively. Rounded particles do not have good interlocking where as irregular and angular particles exhibit very good interlocking effect. Flat, elongated or flaky particles are considered weak aggregate.

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