What is Bulking of Sand? See Its Standard Values, Graph & Much More

Bulking of sand is a phenomenon occurring in fine aggregates. The free moisture content present in sand contributes for the phenomenon bulking of aggregates.

For a moisture content of 5 to 8 %, the bulking of sand may vary from 20 to 40 %.

Because of bulking of sand, dry and moist sand may have similar weight, but the volume of moist sand will be more as compared to dry sand.

What is Bulking of Sand?

The phenomenon of increase in volume of fine sand at particular moisture content is called bulking of sand. The finer the sand is, the more bulking will be observed.

The bulking of sand can be explained as follows:

• The free moisture present in the sand forms a film around an individual particle of sand.
• The film exerts surface tension due to which the neighbouring particles of sand are prevented from coming close to each other.
• Every particle has the water film and this film is exerting surface tension to other particles. Hence, the particles are kept away.
• This results into no point contact between the particles, which would have been otherwise.
• The no-contact between particles lead to increase in volume of sand called bulking.

Causes of Bulking of Sand

The bulking of sand occurs due to moisture content present in fine sand forming a film around particles. The film causes surface tension and the sand particles are kept away.

Factors affecting Bulking of Sand

Following are the two factors that greatly affect bulking of sand:

1. Percentage of moisture content: As the moisture content increases, the bulking of sand increases till a certain limit called saturation point. At the saturation point, the sand shows no bulking at all. After that, increase in moisture content shows decrease in volume.
2. Particle size of fine aggregate: The smaller the size of fine aggregate, the more is the bulking.

Bulking in Coarse Sand

As discussed above, bulking depends on size of aggregate.

Due to large size of the coarse aggregates, very little bulking is observed. Hence, bulking in coarse aggregate is so negligible that it is always neglected.

Maximum Bulking of Sand

Bulking of sand is highest in extremely fine manufactured sand. The bulking observed may be up to 40 %.

Bulking of Sand Test Procedure

The test procedure of bulking of sand is described below:

• Take a measuring container.
• Fill sand in the container till two-third.
• Note the height of the sand from the cylinder. Say h₁.
• Then remove the sand from the container.
• Fill the container with water and add the sand gradually.
• Stir the sand in container with a rod.
• Measure the height of the sand in the cylinder. Say h₂.

The percentage of bulking of sand is then calculated as follows.

Bulking of Sand Calculation

Or,

Standard Values for Bulking of Sand

Percentage of bulking by volume is tabulated against particular moisture content below:

Moisture Content (in %)        Bulking of Sand (in %)

2 %                                                          15 %

3 %                                                         20 %

4 %                                                         25 %

5 %                                                        30 %

Bulking of Sand Graph

Bulking of Sand Field Test

• A sample of sand is filled in measuring cylinder normally and its volume is noted down. Say v₁.
• Water is added in the cylinder and the sand is shaked. The sand becomes saturated. As saturated sand shows no bulking, its volume may be considered as the volume of dry sand, which is also noted down. Say v₂.
• Bulking of the sand sample is the difference between volume of sand and volume of saturated sand.

Bulking of Sand IS Code

The test procedure for bulking of sand is mentioned in the Indian Standard Code of Practice- IS 2386 (Part 3) – 1963.

Adjustment Necessary for Bulking of Sand on Field

Bulking of sand is to be considered while making purchase of sand. The reason is explained below:

• Bulking of sand affects the volumetric proportioning of sand to a large extent.
• The volume of sand is less than the observed volume due to the bulking. Thus, more volume is required.
• If volume adjustment for bulking of sand is not taken into consideration, the volume of sand required will be increased and thus the total cost of mortar or concrete will increase.

During volume batching, the concrete formed will be under-sanded. Hence, it will be harsh. Yield of concrete at particular cement content will also be affected.

Bulking of sand is one of the reasons why Volume Batching of Concrete is not Advised. To know the
recommended method of batching as well as volume batching, Read Batching of Concrete.

Importance of Bulking of Sand

The field test to determine the bulking percentage is employed to find out the moisture present in the sand. However, as weigh batching is adopted instead of volume batching, the determination of percentage of moisture is not necessary.

For production of quality concrete, the moisture content is needed while weigh batching of concrete too. Hence, bulking factor plays an important role in quality control of concrete.

Reduction in quantity of fine aggregates also makes the concrete mix with reduced sand causing difficulty in placing.

For example, if bulking of sand is 30 % and concrete mix is taken as 1:1.5:3, then

Actual volume of sand to be used will be 1.3 * 1.5 = 1.95 units instead of 1.5 units taken otherwise. If bulking factor is not taken into consideration, the actual volume of dry sand will just be 1/1.3 * 1.5 = units.

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