Bulking of Sand: Understanding the Phenomenon and Practical Implications

What Is Bulking of Sand?

The free moisture content in the fine aggregate or sand results in bulking of its volume. The phenomenon of increase in sand volume due to the addition of water or increase of moisture content is termed as Bulking of Sand.

Bulking of Sand occurs when a film of water is created around the sand particles due to the presence of atmospheric moisture. This film or layer generates the force which then forces the particles to move apart from each other and thus increases the volume of the sand.

Bulking of Sand depends on the size of the particles in the sand and also on the quantity of moisture content in sand. The volume of sand can increase from 20 % to 40 % due to an increase of 5 to 8 % of moisture content.

A finer particle of sand will Increase more in volume due to bulking of sand. Volume increase will be relatively less for a coarse particle of sand because of the large surface area is contributed by fine particles for the same volume contribution.

Bulking of Sand almost disappears when the moisture content is increased beyond 8 to 10 % for coarse-grain sand.

Bulking of Sand Is Caused Due to

Free moisture present in the atmosphere forms a thin film around each sand particle. This thin layer of moisture exerts surface tension which pushes the neighbouring particles away from each other.
Therefore, due to this force no point contact is possible between the sand particles. This caused an increase in volume or bulking of sand.
The percentage of moisture content in the sand and the particle size of the fine aggregate or sand will result to the extent of surface tension and consequently how far the adjacent sand particles are away from each other.
The most interesting point to note is that the bulking of sand increases with the increase in moisture content up to a certain limit and after that the further increase in the moisture content results in the decrease in the volume of bulking of sand and at a moisture content representing saturation point, the sand shows no bulking.

Bulking of Sand Graph

It can be seen from the above graph that fine sand bulks more and coarse sand bulks less. From the above graph we find that the coarse aggregate also bulks but the bulking of the volume is so little that it is always neglected.
Extremely fine sand and particularly the manufactured fine aggregate bulk as much as about forty percent.
Surface moisture that is gained from the surrounding atmosphere on fine aggregate can cause considerable bulking; the amount of bulking varies with the amount of moisture and the aggregate grading.
The increase in volume or bulking of fine aggregate shows a completely unrealistic volume.
Therefore, it is necessary that absolute care must be given to the effect of bulking of sand in proportioning the concrete by volume.
If proper care is not given to the effect of bulking of sand, in case of construction of volume batching, the resulting concrete is likely to be under sanded and harsh.
Bulking of sand will also affect the yield of concrete for given cement content.

Bulking of Sand Formula

The extent of bulking of sand can be estimated by a simple field test. A sample of moist fine aggregate or sand is filled into a measuring cylinder in the normal manner.
Mark that level in the measuring cylinder, and denote the point with “ h₁“. Now pour water into the measuring cylinder and completely inundate the sand and shake it.
As we know that the volume of the saturated sand is the same as that of the dry sand, the inundated sand completely offsets the bulking effect of the sand.
Now mark down the level of the sand, and denote the point with ” h₂“. Then the point h₁ – h₂ shows the bulking of the sample of sand under test.
Percentage of Bulking of Sand = (h₁ – h₂)/ h₂ X 100.
In a similar way the bulking factor of a fine aggregate can be found out by filling the wet sand in a water tight measuring box up to the top and then pour water to inundate the fine aggregate.
After that measure the subsidence of fine aggregate and express it as a percentage. This procedure gives a more realistic reading of the bulking factor of sand.
A bulking factor of sand can also be done using field test to find out the percentage of bulking that could be conducted in a very short time span and the percentage of bulking of sand is found out easily, which could be employed for correcting the volume of fine aggregate to be used any construction.
This method is considered as one of the most important methods of field control to produce quality concrete for construction purpose.
Since volume batching is not adopted for controlled concrete, the determination of the percentage of moisture content is not adopted for controlled concrete; the determination of the percentage of moisture content is not normally required.
The amount of water could be controlled by visual examination of the mixture and by the experience of the worker.
The percentage of free moisture content is required to be determined and correction made only when weigh batching is adopted for the production of quality concrete.

Test Procedure to Determine the Bulking of Sand

In order to determine the Bulking of Sand in a given sample, the following steps can be followed.
First take measuring cylinder; fill the cylinder up to 200 ml mark with a sand sample.
Steel scale can be used in order to get an accurate measurement, but compaction of the given sand sample is not allowed.
Then the given sand sample can be transferred to a container.
Then 100 ml water is filled into the empty measuring cylinder.
Then the transferred sand sample is refilled back to the measuring cylinder with 100 ml water and mixed thoroughly with a steel rod.
Let the measuring cylinder rest for some time so that the mixture of sand and water can settle down.
This time the mixture of sand and water sample will be below the 200 ml mark. Let the present level of mixture sample be “a“.
The bulking of sand of the given sample will be determined by the following equation – Bulking of Sand = (200 – a)/ X 100.
All the above procedure should be repeated at least twice and the average value of all the observations will be the percentage of Bulking of Sand for the given sample of sand.

Frequently asked questions (FAQs) about bulking of sand:

What is bulking of sand?

Bulking of sand refers to the increase in its volume due to the presence of moisture on the surface of sand particles, which causes them to push apart from each other.

Why does bulking of sand occur?

Bulking occurs because moisture forms a thin film around sand particles due to atmospheric humidity. This film creates surface tension, causing particles to separate and increase in volume.

How does particle size affect bulking of sand?

Finer particles of sand experience more significant bulking compared to coarser particles, as the surface area relative to volume is larger in finer grains.

What is the range of volume increase due to bulking of sand?

The volume of sand can increase by 20% to 40% when the moisture content increases by 5% to 8%.

How can bulking of sand affect concrete production?

If not accounted for, bulking can lead to underestimation of the volume of sand in concrete mixes, potentially resulting in under-sanded and harsh concrete.

How can bulking of sand be measured in practice?

Bulking can be measured using a simple test involving a measuring cylinder and water. The difference in volume before and after adding water indicates the extent of bulking.

Does bulking of sand affect the yield of concrete?

Yes, bulking affects the yield by altering the volume of sand needed per unit volume of concrete, which impacts the overall mix proportions.

At what point does bulking of sand diminish?

Bulking diminishes as moisture content in sand reaches saturation, typically around 8% to 10% beyond which further moisture addition reduces bulking.

How can bulking of sand be controlled in construction?

Proper moisture content control and measurement ensure accurate batching of sand in concrete mixes. Techniques such as field tests help in real-time adjustment of sand volume.

Why is understanding bulking of sand important in construction?

Understanding bulking is crucial for ensuring the quality and consistency of concrete mixes. It allows construction professionals to adjust proportions accurately, avoiding potential issues with concrete strength and durability.