HOW AGGREGATE PROPERTIES AFFECT THE PROPERTIES OF HARDENED CONCRETE?

PROPERTIES OF AGGREGATE AFFECT THE PROPERTIES OF HARDENED CONCRETE

The following properties of aggregate affect the properties of hardened concrete.

1. GRADING

Aggregate grading has no significant effect on the properties of hardened concrete.

Aggregate

2. BULK DENSITY

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

3. PARTICLE SPECIFIC GRAVITY

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

4. PARTICLE SHAPE

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

5. SURFACE TEXTURE

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

6. STRENGTH

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

Aggregates of high crushing strength are however essential for high strength concrete.

7. 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 “shrinking” aggregates can be as much as seven times that of concrete made with normal aggregates. High concrete shrinkage can lead to abnormally high overall shrinkage of structures and excessive deflection of flexural members.