Let us go through some of the FAQs related to bulking of sand in construction.
1. What is the bulking of sand?
The increase in the volume of the sand, when it becomes moist by the absorption or addition of water up to a certain degree, is called the bulking of sand.
As you can see in the graph above, the bulkage of sand is maximum when the moisture content is between 4 to 8% and it reduces gradually as the water is added further. The bulkage becomes negligible at the 20-25% moisture level, and the volume of the sand becomes equal to that of the dry sand when water is added further up to the saturation level by compacting the sand grains.
From the graph, you can observe that the bulkage of sand is maximum in the fine sand and it reduces as the grain size of the sand increases and are at the minimal level in the coarse-grained sand.
It is tested that, fine sand reaches the bulkage of up to 40% of its original volume when it gains 4 to 5% of the moisture content to its volume.
The reason is, water creates a thin film surrounding the sand particles, creating a void and distancing them from each other. In fine-grained sand, the count of sand particles is more when compared to coarse sand, hence creating more voids, which further makes more bulkage of sand. When the percentage of water is increased beyond 20%, this thin water coating diminishes, reducing the voids between the sand particles, and the wet sand attains the regular dry sand volume.
2. What is the importance of the bulking of sand?
For eg., if we take moist sand having 20% bulkage in a dry mix, and the final wet mix concrete needs 1.52 times the dry mix ratio to achieve the required volume, the sand content will be 30% lesser in the final wet concrete when they are added volumetrically. This shortage of fine aggregate (sand) results in segregation, bleeding, and poor compaction, which in turn creates a honeycomb structure of the concrete.
So, it becomes necessary to take dry sand when preparing the concrete mix volumetrically, or we have to take extra sand to compensate for the sand bulkage, after calculating the percentage of bulking of sand on the construction site.
We can reduce the sand bulkage effect considerably by preparing the mix proportion by weight instead of volume, using a weigh batching machine on the site.
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| Concrete weigh batcher. |
3. How to calculate the bulking of sand on the field?
1. Take a cylindrical jar of any size that is available in the field.
2. Take the sample sand in the container, a little below its top surface level.
3. Measure the height of the moist sand from the bottom of the jar and let us record it as Hm as shown in the above fig.
4. Now, add water to the jar up to the sand level, and shake the jar well by closing the jar mouth with your palm.
5. After shaking, keep the jar for the settlement of sand and measure the saturated level of sand as Hs as shown in fig. above.
Now,
bulking of sand is given by the formula,
= [{(Hm - Hs ) / Hs } × 100]
For eg.,
if the height of the sample( moist) sand taken in the jar is 225mm. and the height of the saturated sand is 190mm.,
Then bulking of sand
= [{( 225 - 190) / 190} × 100]
= 18.42 %
4. What is the bulkage factor of sand?
The bulkage factor is simply the ratio of the height or volume of the sample sand to the height or volume of the final saturated sand i.e. taken in the same container (observe above image). The bulkage factor will be always greater than one.
Here, bulkage factor = [Hm /Hs]
= [225 / 190]
= 1.18
So, if you want to take this tested sand for the concrete mix, then you have to take 18 % extra sand or in other words, 1.18 times the needed volume proportion to compensate for the bulking of sand.
I hope you understood the concept, if not, you can ask your query in the comment section below.
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Have a GOOD DAY😃.
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