ALL YOU WANT TO KNOW ABOUT COARSE AGGREGATE (GRAVEL)

ALL YOU WANT TO KNOW ABOUT COARSE AGGREGATE (GRAVEL) AS CIVIL ENGINEER 

STONE AGGREGATE

“Coarse aggregate” or stone aggregate  means which is broken from rocks using explosives & crushed into pieces using machines & coarse aggregate sizes 6mm,12mm,20mm,40mm,60mm

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LCETED TRIVIA

• Typically the most common size of aggregate used in construction is 20mm. A larger size, 40mm, is more common in mass concrete.

• Larger aggregate diameters reduce the quantity of cement and water needed

A)   PROPERTIES OF COARSE AGGREGATE

1) Crushing Strength: The crush resistance of the stone, as a rule, must be greater than the degree of concrete in which it is to be used. Therefore, it is of great importance for high strength concretes. When deciding to obtain stone aggregate from a particular quarry, this should be verified and subsequently verified that the material is supplied from the same quarry.

2) Abrasion Resistance:  Stone particles when rubbed together or with an external environment should not degrade and are measured by the abrasion resistance of the aggregate. It should not be more than 30% for wear surfaces and 50% for other structures.

3) Shape Of Particles: The crushed stone aggregate should be angular, cubic in shape, and not flaky or elongated. A cubic shape has all 3 dimensions of length, width, and height equal, and the crushed stone should be as close to this as possible. A particle having the ratio of one of the dimensions of less than 0.6 times or more than 1.8 times the nominal size of the aggregate is called flaky or elongated and such particles give comparatively less strength and less durability, keeping other things the same. The shape of the particles depends not only on the crushing jaws but also on the quality of the rock.

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LCETED TRIVIA

A) Angular aggregates are used for the development of high strength because of -

1. The higher cement demand for a particular W/C ratio (due to increased quantity of water required for lubrication compared to rounded aggregates)

2. Efficient interlocking between the aggregates (which leads to better bond development)

B) Whereas flat-surfaced particles like the flaky & elongated ones, having a larger specific surface area makes very poor concrete. Water demand & cement requirement for a given strength is very high for such particles. Presence of such particles is usually avoided in good concrete.

4) Water Absorption: The water absorption, when the stone aggregate is kept soaked in water for 24 hours, should not exceed 2% by weight.

5) Soundness Of Aggregate: A stone aggregate is said to be not solid when it undergoes excessive changes in volume with increasing or decreasing temperature. This is mainly important in areas where freezing and thawing can produce such changes.

6) Alkali Aggregate Reaction:In some of the aggregates, the alkali present in the cement reacts with the stone constituents. Such reactions will cause a harmful expansion of the aggregate and will result in an expansion of the concrete that will cause cracking. Therefore, this is not considered appropriate for using such an aggregate. Dolomitic limestones and some other known stones can be avoided in concrete. However, in India, such aggregates are found in very few places and should not be used.

All the above properties have to be ascertained at the time of selecting a particular source or quarry. At the time of execution of work, the site Engineer is to merely ensure that the source is not changed and carry out some field tests.

B)   FIELD TESTS FOR COARSE AGGREGATE

i) The particles should be angular cubic shaped. Flaky and elongated particles should be minimum.

ii) Absorption of water on being soaked for 24 hours should not be more than 2% by weight.

iii) Size/grading should be as required.

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