What is Aggregates ? Classification of Aggregate ?

 

Aggregates

Aggregates are defined as inert, granular, and inorganic materials that normally consist of stone or stonelike solids. Aggregates can be used alone (in road bases and various types of fill) or can be used with cementing materials (such as Portland cement or asphalt cement) to form composite materials or concrete.

 

The most popular use of aggregates is to form Portland cement concrete. Approximately three-fourths of the volume of Portland cement concrete is occupied by aggregate. It is inevitable that a constituent occupying such a large percentage of the mass should have an important effect on the properties of both the fresh and hardened products.

 

As another important application, aggregates are used in asphalt cement concrete in which they occupy 90% or more of the total volume. Once again, aggregates can largely influence the composite properties due to its large volume fraction. 

Classification of Aggregates

Aggregates can be divided into several categories according to different criteria.

(a)In accordance with size:

 

Coarse aggregate: Aggregates predominately retained on the No. 4 (4.75 mm) sieve. For mass concrete, the maximum size can be as large as 150 mm.

 

 

Fine aggregate (sand): Aggregates passing No.4 (4.75 mm) sieve and predominately retained on the No. 200 (75 μm) sieve.  

 

(b)In accordance with sources:

 

Natural aggregates: This kind of aggregate is taken from natural deposits without changing their nature during the process of production such as crushing and grinding. Some examples in this category are sand, crushed limestone, and gravel. 

 

Manufactured (synthetic) aggregates: This is a kind of man-made materials produced as a main product or an industrial by-product. Some examples are blast furnace slag, lightweight aggregate (e.g. expanded perlite), and heavy weight aggregates (e.g. iron ore or crushed steel).

(c)In accordance with unit weight:

 

Light weight aggregate: The unit weight of aggregate is less than 1120kg/m3 . The corresponding concrete has a bulk density less than 1800kg/m3 . (cinder, blast-furnace slag, volcanic pumice).

 

Normal weight aggregate: The aggregate has unit weight of 1520-1680kg/m3 . The concrete made with this type of aggregate has a bulk density of 2300-2400 kg/m3 .

 

Heavy weight aggregate: The unit weight is greater than 2100 kg/m3 . The bulk density of the corresponding concrete is greater than 3200 kg/m3 . A typical example is magnesite limonite, a heavy iron ore. Heavy weight concrete is used in special structures such as radiation shields.  

(d)In accordance with origin:  

 

Igneous rock Aggregate:

 

• Hard, tough and dense.

 

• Massive structures: crystalline, glassy or both depending on the rate at which they are cooled during formation.

 

• Acidic or basic: percentage of silica content.

 

• Light or dark coloured.

 

• Chemically active: react with alkalis. 

 

 

Sedimentary rock Aggregates:

 

• Igneous or metamorphic rocks subjected to weathering agencies

 

• Decompose, fragmentise, transport and deposit deep beneath ocean bed are cemented together.

 

• Can be flaky.

 

• Range from soft-hard, porous-dense, light-heavy.

 

• Suitability decided by: degree of consolidation, type of cementation, thickness of layer and contamination.

 

 

 Metamorphic rock Aggregate:

 

• Rocks subjected to high temperature and pressure.

 

• Economic factor into consideration.

 

• Least overall expense.

(e)Particle shape:

• Rounded Aggregate: Good workability, low water demand, poor bond.

 

 

 

• Angular Aggregate: Increased water demand, good bond.

 

 

• Flaky Aggregate: Aggregate stacks give workability problems.

 

• Elongated Aggregate: May lack cohesion and require increased fines 

 

• Irregular Aggregate: Fair workability, low water demand. Irregular shape with rounded edges.