Properties of AGGREGATE for concrete

Mechanical Properties

STRENGTH AND ELASTICITY

• Strength is a measure of the ability of an aggregate particle to stand up to pulling or crushing forces.
• Elasticity measures the "stretch" in a particle.

High strength and elasticity are desirable in aggregate base and surface courses. These qualities minimize the rate of disintegration and maximize the stability of the compacted material.

Bond

• Interlocking of aggregates and the hydrated cement paste
• Clean and rougher surface improve the bonding

Toughness

• Defined as resistance to failure by impact
• Determined by carrying out aggregate impact test
• Important for concrete used in road pavement

Hardness

The hardness of the minerals that make up the aggregate particles and the firmness with which the individual grains are cemented or interlocked control the resistance of the aggregate to abrasion and degradation.

• Soft aggregate particles are composed of minerals with a low degree of hardness.

Physical Properties

DENSITY AND SPECIFIC GRAVITY

• Density is the weight per unit of volume of a substance.
• Specific gravity is the ratio of the density of the substance to the density of water.

The density and the specific gravity of an aggregate particle is dependent upon the density and specific gravity of the minerals making up the particle and upon the porosity of the particle.

These can be defined as

1) All of the pore space (bulk density or specific gravity)

2) Some of the pore space (effective density or specific gravity)

3) None of the pore space (apparent density or specific gravity)

ABSORPTION, POROSITY, AND PERMEABILITY

The size, the number, and the continuity of the pores through an aggregate particle may affect the strength of the aggregate, abrasion resistance, surface texture, specific gravity, bonding capabilities, and resistance to freezing and thawing action.

· Absorption → the particle's ability to take in a liquid.

· Porosity → a ratio of the volume of the pores to the total volume of the particle.

· Permeability → the particle's ability to allow liquids to pass through.

SURFACE TEXTURE

• the pattern and the relative roughness or smoothness of the aggregate particle.
• affects the workability of hot mix asphalt, the asphalt requirements of hot mix asphalt, and the water requirements of portland cement concrete
• develop the bond between an aggregate particle and a cementing material.

rough surface texture→ produce a stronger bond, and thus creating a stronger hot mix asphalt or portland cement concrete.

PARTICLE SHAPE

Effect :

• The asphalt demands of hot mix asphalt
• The workability and the strength of both portland cement concrete and asphalt pavements

Best aggregates→ crushed stone / crushed gravel

Crushed aggregate→ have irregular, angular particles that tend to interlock when compacted or consolidated.

COATING

• a layer of substance covering a part or all of the surface of an aggregate particle.
• could even increase the quantity of bonding agent required in the mixture.
• prevent a good bond from forming between the aggregate surfaces and the cementing agent.(Coating should be removed )

Natural coating→ mineral deposits formed in sand and gravel by ground water

Artificial coating→ dust formed by crushing and handling