AGGREGATES

AGGREGATES

 Aggregates are materials mixed with binding materials like cement, lime or mud in the preparation of mortar or concrete. These shall consist of naturally occurring stones, gravels, and sand and shall be hard, strong and durable. Natural aggregates are formed by the process of weathering and abrasion, or by artificial crushing of larger parent mass. The aggregates occupy 7080% of the volume of concrete, their impact on various characteristics and properties of concrete is considerable.to know more about the concrete one should know more about the aggregates which constitute more volume of the concrete

PROPERTIES OF AGGREGATES

 Physical Properties

The physical properties of aggregates are those that refer to the physical structure of the particles that make up the aggregate. 

a) Absorption, Porosity, And Permeability

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

Porosity is a ratio of the volume of the pores to the total volume of the particle. Permeability refers to the particle's ability to allow liquids to pass through it.

 The porosity, permeability and absorption of 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. 

b) Surface Texture

Surface texture is the pattern and the relative roughness or smoothness of the aggregate particle.

 A rough surface texture gives the cementing material something to grip, producing a stronger bond, and thus creating a stronger hot mix asphalt or Portland cement concrete.

c) Aggregate Voids

 Aggregate particle voids influence the specific gravity and absorption of the Aggregate materials.

d) Strength And Elasticity

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.

e) Density And Specific Gravity

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 

f) Hardness

Hard aggregates have good cementation and are more resistant to abrasion and degradation.

 Whereas soft aggregates have poor cementation and are not resistant to abrasion and degradation.

 g) Particle Shape

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 irregular and angular aggregates give more strength as compared to round aggregates but are less workable than round aggregates

TESTS FOR AGGREGATES LOS ANGELES ABRASION TEST

Objective

 To determine the resistance to degradation of small size coarse aggregate in Los Angeles machine.  Several mechanical properties of aggregates are of interest, especially when the aggregates are to be used in road construction or are to be subjected to high wear. Strength, toughness, hardness or resistance to wear is an important property of concrete used in roads and floor surfaces subjected to heavy traffic. 

Los Angeles abrasion test studies all possible reasons causing wear.in the Los Angeles abrasion machine attrition, abrasion, and crushing are all present as follows

 Attrition: by the friction between the aggregate particles.

Abrasion: by the friction between the steel balls and the aggregates

 Crushing: by hitting the walls of the testing machine Los Angeles test is suitable for coarse aggregates of different sizes and is not used for fine aggregates

Apparatuses

 Los Angeles machine,

 Sieves,

Balance,

Steel balls,

 Scoop,

 Sample trays,

 Oven,

 Wire brush

Procedure

Prepare the sample according to gradation

Ø  Place the test specimen and abrasive charge in the Los Angeles Abrasive Testing Machine and close the opening with the dust-tight cover. 

Ø  Start the testing machine and allow operating for the required number of revolutions at a speed of 30 to 33 rpm. 

Ø  When the testing machine has completed rotating the required number of revolutions, remove the cover and carefully empty the entire contents into a pan. Remove the abrasive charge from the pan.

Ø  Separate the test specimen on the 4.75-mm sieve, and then sieve the passing 4.75-mm material on the 1.70-mm sieve. Combine the material retained on the 4.75 and 1.70-mm sieves. Weigh and record these values to the nearest 1 g.  

Ø  Wash the aggregates retained on sieve no 12.n the Place these aggregates in oven for drying and then weighed.

Now find the abrasion value in percentage by the formula as given below

Abrasion value= (W1˗ W2\ W1) ×100                                  

Where  W1=weight of material before test

W2=weight of material after test