Characteristics of agrregates( concerete technology)

 Characteristics of aggregate 

Aggregate must be clean, hard ,strong , properly shaped and well graded. The aggregate must possess chemical stability, resistance to abrasion and to freezing and thawing.they should not contain deleterious material which may causes physical ir chemical changes .

The properties of aggregate for concerete are discussed under the following heads 

1) Particles size and shape 

2) Surface texture 

3) specific gravity 

4) Bulk density 

5) Voids 

6) Water absorption 

7) surface moisture 

8) Bulking of sand  

9) Deleterious material in the aggregate  

1) Particles size and shape

The size and shape of aggregate significantly influence the mobility i.e.. the workability fresh concerete and the bond between the aggregate and the mortar phase .The rounded aggregates are available in the form of river or seashore gravel which are fully waterworm or compeletly shaped by attrition, where irregular or partially rounded aggregates are partly shaped by attrition and have rounded edges.

The round aggregate require lesser amount of water and cement paste for a given workability .the water content could be reduced by 5 to 10%. 

Surface texture 

The surface texture is a measure of the smoothness or roughness of the aggregate. Based on the visual examination of the specimen, the surface texture may be classified as glassy , smooth , granular, rough , crystalline porous and honeycombed. The strength of the bond between aggregate and cement paste depend upon the surface texture.

Specific gravity 

The specific gravity of an aggregate is defined as the ratio of the mass of solid in a given volume of sample to the mass of an equal volume of water at the same temperature.

Bulk density 

The bulk density of an aggregate is defined as the mass of material in a given volume and is expressed in kilograms per litre.The bulk density of an aggregate depends on how densely the aggregate is packed in the measure.

The other factors affecting the bulk density are the particles shape ,size the grading of the aggregate and the moisture content. 

Voids 

The empty space between the agrregate particles are termed as voids .it is the difference between the gross volume of aggregate mass the volume occupied by the particles alone.

Water absorption 

The pores may become reservoirs of free moisture inside the aggregate.the percentage of water absorbed by aggregate when immersed in water is termed as absorption of aggregate .

Surface moisture 

The surface moisture expressed as a percentage of the weight of the saturated surface dry aggregate is termed as moisture content .since the water absorption represents the water contained in the aggregate in the saturated surface dry conditions and the moisture in the water in excess of that the total water content of moist aggregate is equal to the sum of absorption and moisture content.

Bulking of sand 

The increase in the volume of a given mass of sand caused by the presence of water is known as bulking. The bulking of sand is caused by the film of water which push the particles apart. The extent of bulking depends upon the percentage of moisture present in the sand and its fineness. It is seen that bulking increases gradually with moisture content upto a certain point and then begins to decrease with further addition of water due to mergin of films. Until when the sand is inundated .At this stage the bulking is practically nill. 

Deleterious Material in the aggregate 

The material whose presence may adversely affect the strength, workability and long term performance of concerete are termed as deleterious material.

These are considered undesirable as constituent because of their intrinsic weakness , softness ,fineness or other physical or chemical characteristics harmful to the concerete behaviour.