Properties of aggregate and their behaviour in concrete

Aggregate is one of the building materials used to produce concrete and mortar. When cement is mixed only with fine aggregates it is called cement mortar. Masonry mortar is different from a simple cement mortar because it contains other ingredients as well.

Concrete contains both fine and coarse aggregate. Aggregates add strength to concrete and reduce its potential for shrinkage. In other word, aggregates act as bone of concrete.

Aggregates actually make up 60% to 80% of the volume of hardened concrete. So their properties and characteristics are very important.

Most commonly used aggregates in concrete are sand, gravel, crushed stone, crushed slag and pumice. The discussion about both fine and coarse aggregates are given below-


Fine Aggregates: 

The fine aggregate in concrete is sand. By definition, sand particles are ­­3/16 inch diameter and smaller. Sand, for using in concrete should include a well-graded mix of large and small. It should be free of contaminants that can be harmful to concrete such as silt, clay and organic materials such as leaves and roots.

Sand contamination is little more difficult to detect. There are sophisticated laboratory tests which can determine the exact type and amount of contaminants in concrete aggregates. But there is also a simple field test.

Field test procedure of fine aggregates: put 2 inches of sand in a quart jar. Add water until the jar is about three-quarter full, shake it for one minute. Then let it stand for an hour. If more than 1/8 inch of sediment settles on the top of the sand, it should be washed by drenching with a garden hose the day before it will be used.

Behaviour of fine aggregate in concrete: Fine aggregates fill in the voids between coarse aggregate particles in concrete.

Sharp and angular sand produces harsher concrete mixes with poor workability.

Rounded particles increase workability and make slab surfaces easier to finish.

Masonry sand is generally not appropriate for concrete because it contains only the smaller particles and can cause the mix to be sticky rather than plastic.

3/4" down stone chips

Coarse Aggregate:


Most commonly used coarse aggregates in residential concrete are gravel and crushed stone. Coarse aggregates must be sound, volume stable, non reactive, abrasion resistant, suitably shaped, rough textured, well graded and clean.

Behaviour of coarse aggregates in concrete: Smooth and rounded shape coarse aggregates produce concrete with better workability.

Dirty and contaminated aggregate bonds poorly with the cement paste that can increase mixing water requirements, delay setting and hardening of the concrete, cause stains or popouts, lower strength and durability and increase shrinkage.

Aggregate that is too absorptive produce concrete that has low durability and may suffer from scaling, popouts and excessive shrinkage.

Unsound aggregates produce unsound concrete which is weak, has poor appearance, low durability and may experience cracking, popouts and spalling.

Chemical reactivity, especially with the alkalis in cement, causes internal expansion, cracking, and disintegration of the concrete.

Low abrasion resistance results in low strength and excessive wear in floors and pavements.

Particle shape affects workability and surface texture affects bond of cement paste to the aggregate.

Regardless of the type of aggregate or its particle shape, coarse aggregate should include a well graded range of sizes from small to large.

Coarse aggregate with the largest allowable maximum size produces the most economical concrete mix. However it is not necessary on any single project to use concrete with different maximum aggregate sizes for slabs, beams, piers, and so on. For convenience, the smallest recommended allowable aggregate size may be used throughout the project.


YOUR TURN:
What size of aggregate do you use mostly in your projects?
 

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