PACKING DENSITY OF AGGREGATE

Packing Density of Aggregate

In a unit volume filled with particles, packing density, or packing degree is the volume of solids in this unit volume and is equal to one minus the voids. The packing density of aggregate gives an indication of how efficiently aggregate fill a certain volume and for that reason is such an important concept in concrete technology.

If a high volume of particles can be packed in a certain volume, the necessity for binder, which usually is much more expensive, to fill the voids and glue particles will be decreased.

 

 

 

Where:

Vs = volume of solids

Vt = total volume = volume of solids plus volume of voids

e = Voids = volume of voids over total volume

The packing density is a function of the combined effect of shape, texture, and grading of the components.

Besides grading, the shape factor and the convexity ratio are the major factors affecting packing density.

Determination of Packing Density

The packing density of individual aggregate in a volume fraction of total aggregate or over all aggregate is determined from its maximum bulk density of mixture and specific gravity from the following relation.

Packing density = (Bulk density x Weight fraction) / Specific gravity

The packing density or packing degree not only depends on the aggregate characteristics, but also on the compaction method and on the dimensions of the container.

If the sample is just poured, the packing density will be lower than that corresponding to a sample tapped with a rod or a sample vibrated.

Unfortunately, a correlation among packing densities obtained using different compaction methods has not been established, as it depends on the size, shape, and texture of the particles.

In fact, it is possible that a crushed aggregate has a lower loose density than a rounded one, while the vibrated density could be the opposite.

The packing density could be evaluated in air (dry packing), or in water or any other liquid.

As for particles retained in the No 200 sieve (i.e., 75 micron), dry packing, either in oven dry or in SSD state, has been used successfully to predict concrete behavior.

On the other hand, due to the important effect of inter-particle forces, dry packing could not be a good indicator of the actual behavior of very fine particles (passing No 200 mesh) under the saturated conditions found in fresh concrete.

In fact, water demand tests have been proposed to measure the packing density of powders such as cement, fly ash, and aggregates particles less than 75micron.

In concrete mixtures, theoretically, the higher the packing density of aggregates the less paste or the less the water required for given workability.

As cement is the costliest large component of concrete, and as cement paste is responsible for shrinkage, heat development, and some durability problems, optimization calls for concrete that complies with all the specifications using, within limits, the least cement.

The best concrete mix with required workability typically came from the densest packing of sand and coarse aggregate.

One of the objectives of optimization of concrete mixtures is to combine available aggregates in such a way that maximum or near maximum packing degree is obtained while maintaining an acceptable level of workability.

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