Properties of Aggregates - Mechanical & Physical Properties of Aggregates

What is Aggregate?

When creating concrete or mortar, aggregate is the material that is mixed with cement, bitumen, lime, gypsum, or another adhesive. The aggregate gives the finished product volume, stability, wear and erosion resistance, and other desired physical attributes.

Fine aggregate consists of sand, crushed stone, or crushed slag screenings, whereas gravel (pebbles), broken stone pieces, slag, and other coarse materials make up a coarse aggregate. When a smooth surface is desired, fine aggregate is utilized to create thin concrete slabs or other structural elements; coarse aggregate is used to create more substantial parts.

Properties of Aggregate

Aggregates are examined based on several properties. If these properties of aggregate are aligned, then their use in the suitable work process; otherwise, it's not recommended for any work and not considered that one is as an aggregate. Some important properties of aggregate are as below-

1. Mechanical properties
2. Physical properties
3. Thermal properties
4. Desirable properties
5. Chemical properties

Mechanical Properties of Aggregate

Mechanical properties of aggregate consider its durability in laboratory and field. Mechanical properties majorly we consider its crushing value as the hardness of aggregate, impact value as toughness and abrasion value, etc.

Aggregate crushing value (A.C.V or Hardness): A.C.V is determined by a crushing test apparatus in which a sample that passes from a 12.5mm sieve but is retained from a 10mm sieve is taken and placed into the sample mold. That sample is loaded with a 40-tonne load by machine, and this is after a loaded sample is taken in a 2.36mm sieve.

This A.C.V value for aggregate should not be more than 30% for a surface course of bituminous pavement for road, and for the base course, it should be not more than 45% for a similar one.

Aggregate impact value (A.I.V or Toughness): A.I.V of aggregate samples is taken under the Impact testing machine. In this, a mold of 10.2 mm diameter is placed below the 14 kg hammer, and that hammer is free fall from 380 mm height at 15 times. Due to this impact, aggregate broke sample pass from 2.36mm sieve.

This A.I.V value for aggregate should not be more than 30% for a surface course of bituminous pavement for a road, and for a base course, it should be not more than 45% for a similar one. These values are the same as A.C.V.

Aggregate abrasion value (A.A.V): A.A.V of aggregate samples is taken under the Los Angeles machine. In this machine, 10kg of sample is taken, and this sample is filled in the machine's drum, this drum is rotated 28 to 33 Rpm for 1000 rotations. After this rotation, the complete sample passes through a 1.7mm sieve.

This A.A.V value for aggregate should not be more than 30% for a surface course of bituminous pavement for a road and for the base course it should be not more than 50% for a similar one.

Table: Mechanical properties for aggregate

Physical Properties of Aggregate

We consider the size, shape, texture, elongation, and flakiness value according to the physical properties of aggregate. This property of aggregate is the primary judgment for any selective material. These properties are discussed below-

Surface Texture of Aggregate

By examining the aggregate by hand, its texture is identified. Surface texture increases or decreases the strength of concrete if it is glossy, it will be weak as compared to rough texture aggregate. In this surface, the texture is classified according to a different type of parental rock as-

• Smooth surface aggregate: Slate, Marble, Chalk, rhyolite, etc.
• Rough surface aggregate: Sandstone, oilties, etc.
• Glossy surface aggregate: Black flint
• Honeycombed aggregate: Scoria, pumice, etc.
• Crystal aggregate: Basalt, dolerite, granite, glassic, etc.

Shape of Aggregate

The aggregate shape affects the workability of concrete because different shapes have different bonding natures between particles. Generally, we use angular-shaped for better strength and rounded aggregate for good workability. Some of these are as below-

• Rounded-shaped aggregate: It gives better workability but less strength.
• Angular-shaped aggregate: It gives lower workability but higher strength.

Size of Aggregate

Various types of size of aggregate are mixed with concrete; these are classified according to their mean size name as elongated and flaky aggregate. This criterion affects the strength of concrete because elongated or flaky aggregate does not bond good enough to the cement paste -

• When aggregate length exceeds 1.8 times the mean diameter, called as elongated aggregate. The maximum limit of use in concrete is 15%.
• When aggregate length lowers 0.6 times the mean diameter called flakiness aggregate. The maximum limit of use in concrete is 15%.

Thermal Properties of Aggregate

Thermal properties signify aggregates specific heat, conductivity and thermal expansion. In these different rock material types, aggregate shows different properties. These are as below-

• Specific heat depends upon exterior material coated on aggregate; like in mass concrete work, specific heat is minimum.
• The conductivity of aggregate depends on specific heat and the type of material used for inert aggregate.
• Thermal expansion of aggregate is relative to cement paste at 5.5 x 10^-6/^oC. It varies for different types of cement uses.

Desirable Properties of Aggregate

Desirable properties of aggregate are based on strength parameters. In this tensile and compressive strength is considered by direct or indirect test and cube or core test respectively-

• The compressive strength of aggregate is based on concrete tests. This shows its inner strength as per test and as per loading. When angular aggregate is used with a higher grade of cement, it gives maximum compressive strength.
• The tensile strength of aggregate is weaker than compressive strength but still depends on its properties as rounded aggregate performs lower tensile strength than the angular one.

Chemical Properties of Aggregate

Aggregate is an inert material combined with cement and sand with water. Due to this mixture of alkalies, the aggregate reaction is developed because it ultimately contains silica with basic properties.

In the same context, when silica reacts with cement acidic material, there is a possibility of damaging aggregate, and due to this, the concrete strength decreases. Aggregate that is used in concrete is limited as alkalies do not present above 0.6%.