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Effect of Aggregates on the Properties of Hardened Concrete

The properties of aggregates and their influence on the properties of the hardened concrete are explained below:

Grading

It is found that the aggregate grading has no kind of influence on the properties of hardened concrete unlike fresh concrete. 


Bulk Density

This has no direct effect but has an indirect effect on the drying shrinkage . Bulk density is an indicator of packing capacity. This is a factor that influences water requirements, which in turn will affect the drying shrinkage of the hardened concrete. For a given particle specific gravity, the higher the bulk densities of sand and stone, the lower the water requirement and hence lower the drying shrinkage.





Particle Specific Gravity

Because aggregates make up about 80% of the mass concrete, Particles specific gravity of aggregates significantly affect the density of the concrete.


Particle Shape

Angular, flat and elongated stone particles tend to increase the flexural strength of the concrete. The effect of particle shape on the compressive strength is negligible.

Surface Texture

Rough surface texture, especially of stone particles , tends to improve the bond between aggregate and the hardened cement paste and hence increase the strength of concrete. Smooth pebble aggregates reduce early age compressive strength and significantly reduce the flexural strength.

Strength

Within the range of conventional concrete strength say 20 to 40 Mpa, aggregate strength has little effect on the strength of the concrete, provided aggregates meet the requirements for crushing strength. Aggregates of high crushing strength are however essential for high strength concrete.


Shrinkage 

Aggregates that are inherently dimensionally unstable in conditions of changing moisture content have a significantly adverse effect on the dimensional stability of concrete.Shrinkage of concrete made with so called "shrinkage" aggregates can be as much as seven times that of concrete made with the normal aggregates. High concrete shrinkage can lead to abnormally high overall shrinkage of structures and excessive deflection of the flexural members

See Effect of Aggregate on the Fresh Properties of Concrete

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