What is bulking of sand?
Bulking of sand refers to the phenomenon where the volume of the sand gets increased due to the presence of moisture content.
The moisture content in the sand forms a thin film of water around each particle. This film of water exerts a surface tension force that keeps the neighboring particle away.
This force that is exerted by the surface tension keeps every particle away from each other. This results in the bulking of volume.
How much bulking is happening?
The amount of surface tension and consequently how far the adjacent particles are kept away will depend upon two factors namely,
- Percentage of moisture content and
- The particle size of the aggregate.
1) Percentage of moisture content
It is interesting to note that the bulking increases with the increase in moisture content up to a certain limit and beyond that the further increase in the moisture content results in decreased volume.
When the aggregate is completely saturated, there is no bulking.
For a moisture content of 4 to 6%, there will be a 20 to 40% increase in the volume of fine aggregate.
2) Particle size of the aggregate
The particle size of an aggregate plays an important role in the bulking phenomenon.
It is seen that the fine sand bulks more and the coarse sand bulks less. The coarse aggregate also bulks but the bulking is so little that it is always neglected.
The extremely fine sand bulks as much as 40%.
The graph mentioned below indicates how much the coarseness of sand and the moisture content in sand influence the bulking phenomenon.
Problem due to bulking of sand
Due to the bulking, fine aggregate shows a completely unrealistic volume.
When the concrete proportioning is done by volume batching, due to bulking of sand, the mixed volume of the fine aggregate is much lesser than the correct volume.
While in weight batching, bulking of sand is not taken into account because here we didn’t measure materials by their volume, we only batch the materials according to their weight.
If we do not consider the bulking, the entire concrete quantity is going to be lessened and the quality of the concrete also gets affected severely.
Now that we learned all the basics, let’s see how we can calculate the percentage of bulking that happened.
Determination of bulking of sand (Field method)
The following are the steps that are followed in the field to find out the percentage of bulking. Refer to IS 2386: 1963 part 3 for more information.
Step 1: Take a sufficient quantity of sand and fill 2/3rd of the container in the loose condition.
Step 2: Measure the height of the sand and mark it as h1.
Step 3: Pour water into the container till the sand submerges. Shake the container to make sure the sand settles and consolidates.
We use water here because of the fact that the volume of the saturated sand is the same as that of the dry sand. Instead of waiting hours for the sand to dry, we simply just make the sand saturated so that the bulking effect goes off.
Step 4: Now surely the height of the sand decreases. Note the height of the saturated sand and mark it as h2.
Height of sand sample, h1 = 20cm
Height of saturated sand, h2 = 16cm
Bulking of sand = ((20-16)/16) x 100 = 25%
How to find the correct volume of sand using bulking percentage?
Let’s say we use volume batching and we need 1m3 of sand to prepare a particular volume of concrete. We found out that the bulking of sand is 25%.
That means we have to add 25% extra sand to the mix so that the volume of the concrete needed doesn’t have to change.
If we don’t add an extra 25% of sand, then the final volume of concrete gets reduced by 25% because we add water to the concrete mix and it breaks the bulking effect and results in lesser volume.
So in our case, we need to add 1.25m3 of sand to get the same volume of concrete.
This correction is considered as one of the important processes to produce quality concrete.
Now as you learn all the concepts, why don’t you try some quizzes?
Hope you understand everything you need to know about the bulking of sand. If you have any queries (or) if you find this article helpful, let us know in the comment section.