Physical Properties of Aggregates 1

Prepared and Composed By: Ahmed Bilal Siddiqui

Physical Properties of Aggregates Porosity: Volume of pores to the total volume of water is called porosity. If the rock pores are not connected, a rock may have high porosity and low permeability. Absorption: Absorption : The ability of aggregate to absorb water is called absorption. The water absorption of aggregate is determined by measuring the increase in mass of an oven dried sample when immersed in water for 24 hours (the surface water being removed). The ratio of the increase in mass to the mass of the dry sample, expressed as percentage, is termed as absorption. Permeability: The ability of aggregate to pass water through it is called permeability. If the rock pores are not connected, a rock may have high porosity and low permeability.

Texture: It is relative roughness and smoothness of aggregate. Surface texture of the aggregate affects its bond to the cement paste and also influences the water demand of the mix, especially in the case of fine aggregate. It seems that the shape and surface texture of aggregate influence considerably the strength of aggregate. Strength: It is ability to resist crushing. The strength of aggregate depends on its composition, texture and structure. Thus, a low strength may be due to the weakness of constituent grains or the grains may be strong but not well knit or cemented together. A good average value of the crushing strength of aggregate is about 200 MPa (30000psi) but many excellent aggregates range in strength down to 80 MPa (12000psi). Particle Shape: The shape of particle in aggregate is known as shape of aggregate. The shape of the aggregate particles affects such things as: 1) The asphalt demands of hot mix asphalt 2) The workability and the strength of both portland cement concrete and asphalt pavements

Hardness: The resistance of aggregate to degradation is called hardness. Soft aggregate particles are composed of minerals with a low degree of hardness. Weak particles have poor cementation. Neither type is acceptable.

Prepared and Composed By: Ahmed Bilal Siddiqui

Density: Mass per unit volume of aggregate is called density of aggregate. 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 may be defined as follows: 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)

Specific Gravity: Ratio of density of aggregate to the density of water is called specific gravity of aggregate. The specific gravity of aggregate depends on the specific gravity of minerals of which the aggregate is composed and also on the amount of voids. The majority of natural aggregate have a specific gravity between 2.6 to 2.7. Qualities of Good Sand: There are following qualities of good sand. 1) Good sand should have coarse and angular grains of pure silica. 2) The grain of sand should be hard, strong and durable. 3) It should be free from silt, clay or any such salts that may hamper setting or attacking the reinforcement. 4) It should not contain any organic matter.

5) It should be well graded i.e. Should contain in suitable proportions particles of various sizes. 6) It should not contain any hygroscopic matter.