Aggregate: Factors Affecting the Grading of Aggregate | Concrete Technology

Factors that affect the grading of aggregate are: 1. Surface Area of the Aggregate 2. Relative Volume Occupied by the Aggregate 3. Workability of the Mix 4. Segregation.

Factor # 1. Surface Area of the Aggregate:

For a given maximum size of aggregate, larger the size of aggregate particles, lesser the surface area of aggregate in a given mix, which will require lesser amount of water to wet its surface and greater portion of the water will be available for imparting workability to the mix for a particular water-cement ratio.

The water/cement ratio of the mix is generally fixed from strength consideration, but at the same time, the amount of cement paste should be sufficient to cover the surface of all the particles so that lower the surface area of the aggregate, the less paste and thus less water is required.

Assuming the aggregate particle as spherical of diameter D, the ratio of the surface area to volume comes out to be 6/D. This ratio of the surface of the particle to their volume is called specific surface, which is inversely proportional to the particle size, but the coefficient of 6/D will be different for different shape of particles. It is also defined as the surface area per unit weight.

In case of graded aggregate, the grading and the overall specific surface are related to one another. If the grading extends to a larger maximum aggregate size, the overall specific surface is reduced and the water requirement decre­ases, but the relation is not linear.

For example if the size of aggregate is increased from 10 mm to 63 mm, under certain conditions, the water/cement ratio can be decreased by 0.15. VAriation of water reduction with the increase in the aggregate size is shown in Fig. 4.12. From the study of the Fig. 4.12 it will be clear that by the increase of particle size about 50 kg. water per cubic metre of concrete can be reduced.

The specific surface is found to give somewhat misleading picture of the workability to be expected. An increase in the specific surface of the aggregate for a constant water/cement ratio has been found to lead to a lower strength of concrete. The reasons for this are not quite clear.

It may be possible that a reduction in density of the concrete consequent upon an increase in fineness of the aggregate is instrumental in lowering the strength. Though it seems that the surface area of the aggregate is an important factor in determining the workability of the mix, but the exact role played by the finer particles has not been ascertained.

Factor # 2. Relative Volume Occupied by the Aggregate:

For a mix to be satisfactorily workable, it must contain a sufficient amount of material smaller than 300 micron size including cement particles. Thus a richer mix requires a lower content of fine aggregate than a lean mix. If the grading of fine aggregate (sand) is such that it is deficient in finer particles, in that case increase in fine/coarse aggregate ratio will not prove useful as it may lead to an excess of middle sizes resulting in harshness of the mix.

(A mix is said to be harsh when one size fraction present is in excess, as shown by steep step in the middle of grading curve so that particle interference results). For this reason the minimum contents of particles passing 300 and 150 micron are laid down. Sometimes particles passing through 150 micron are suggested as shown in Table 4.22.

The volume of entrained air can be taken as equivalent to one half of the volume of fines. This approach has been adopted by the Germans and Dutch for the mix design.

The requirement that the aggregate should occupy as large a relative volume as possible is due to economic considerations, as aggregate is cheaper than the cement paste. Technically also, too rich mixes are undesirable, as they suffer from the defect of excessive shrinkage.

Further it has also been assumed that gre­ater the amount of solid particles that can be packed into a volume of concrete higher the strength. Thus the maximum density theory has suggested the grading curves as parabolic in shape or partly parabolic and then straight as shown in Fig. 4.13. However it has been obser­ved that aggregate graded to give maximum density produce a harsh and unworkable mix.

Factor # 3. Workability of the Mix:

The workability is improved when there is an excess of paste above that required to fill the voids in the fine aggregate, and also an excess of mortar above that required to fill the voids in the coarse aggregate.

Factor # 4. Segregation:

It can be defined as separation of the constituents of a heterogeneous mixture so that their distribution is no longer uniform. In the case of concrete the difference in the size of particles and in the specific gravity of the mix constituents is the main cause of segregation, but its extent can be controlled by the choice of suitable grading and by care in handling.

Types of Segregation:

Segregation can be divided into two categories as follows:

1. The Coarser Particles Tend to Separate:

In this case the coarser particles tend to travel further along a slope or to settle more than finer particles. This type of segregation occurs in lean mixes when with some grading’s too dry mix is prepared. Addition of water will improve workability.

2. Grout Separates Out from the Mix:

This type of segregation occurs in wet mixes. The grout (cement slurry) separates out from the rest of the mix. If in a lean mix, excessive water is used, this type of segregation will occur.

Segregation is affected by grading of aggregate. The grading should be such that the mortar should be prevented from passing freely out of the voids in the coarse aggregate. It is also essential for the voids in the combined aggregate to be sufficiently small to prevent the cement paste from passing through and separating out.

The actual extent of segregation depends on the method of handling and placing of concrete. While handling concrete it should not be allowed to drop from a height of more than 1 m. dropping of concrete from a considerable height, passing along a chute, particularly with changes of direction and discharging against an obstacle lead to segregation. Hence by adopting correct method of handling and placing, segregation of concrete can be checked.

Thus there is no doubt that the grading of aggregate is a major factor in the workability of a concrete mix. Workability in turn affects the water and cements requirements, controls segregation and bleeding and influences the placing and finishing of concrete. These factors influence the properties of green as well as hardened concrete such as strength, shrinkage and durability.

Grading is thus of vital importance in proportioning of concrete mixes, but its exact role in mathe­matical terms is not still fully known. Finally it can be said that ensuring a constant grading is far more important than devising a good grading, otherwise variable workability will produce concrete of variable strength.