Bulking of Sand – Cause, Tests and Importance

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Bulking of fine aggregate or sand is the phenomenon of increase in sand volume due to the increase of moisture content. Bulking test on fine aggregates has to be performed before using it in construction. This article describes the procedure and importance of determining bulking of fine aggregates.

Causes of Bulking of Sand

The moisture content in the sand makes thin films around sand particles. Hence, each particle exerts pressure. Thus they move away from each other causing increasing in volume. The bulking of the aggregates is dependent on two factors:

1. The fineness of the aggregates
2. Percentage moisture content

As shown in figure-1 below, the bulking of the sand increases with the increase in moisture content. This happens up to a limit beyond which any moisture addition will decrease the volume. A fully saturated fine aggregate does not show any bulking. The rate of bulking is inversely proportional to the size of the aggregates. Hence, fine aggregates bulk more compared to coarse aggregates.

Determination of Bulking of Sand

The apparatus and procedure to determine percentage bulking of fine aggregates is described below.

1. Laboratory Test for Bulking of Sand

Apparatus

1. 250ml measuring cylinder
2. Weighing balance
3. Fine aggregate

Procedure

1. Take 500 grams of fine aggregate over dried at a temperature of 100 to 110 degree Celsius for 24 ± 0.5 hours. This weight is measured as W1
2. The cooled sand is taken in an airtight container. This weight is measured as W2.
3. The water content of the sample is calculated as

Wc =  (W1-W2)x 100/W1   Eq.1

4. In a pan, 250 grams of sand is taken
5. To this 2% by weight of water is added. This is properly mixed
6. The mixture is poured into a 250ml cylinder. This is consolidated by shaking
7. The surface is leveled. The reading is measured as Y1.
8. The test is repeated for the remaining quantity of sand for 2% water by weight each time. The readings are taken as Y2, Y3.....etc until a decreasing reading of the volume is observed.
9. After this level, 4% water is added and the test is continued until the sample become fully saturated.
10. To the standard sample in the measuring cylinder, add about 50 ml water ore and stir the sample well.
11. (xi) Note down the surface level of inundated sand (Y ml).

Calculations  Percentage Bulking of Sand = (Y1-Y)x100/Y Results A graph is plotted with percentage water content along X -axis and percentage bulking along y-axis. From the graph the following results are obtained:

1. % of bulking occurred =
2. % of water content at maximum bulking=
3. % of water content when bulking is zero=
4. % of bulking for the initial water content (W) of the sample =

2. Field Test for Bulking of Sand

Field test determines the necessary adjustment required for buLking of fine aggregates Apparatus

1. 250ml measuring cylinder
2. Fine aggregate

Procedure

1. Pour the sand into 250 ml measuring cylinder up to the 200 ml mark.
2. Fill the cylinder with water and stir well (sufficient water should be poured to submerge the sand completely and it can be seen that the sand surface is now below its original level)
3. Take the reading at the sand surface (Y ml)

Calculations Percentage Bulking of Sand = [(200/Y)- 1] x100 Result Percentage bulking of field sample=

Importance of Determining Percentage Bulking

Unrealistic volume is shown by fine aggregate due to bulking. When concrete proportioning is to be performed, the sand bulking issues is a concern. If the effect of bulking is not studied properly, the concrete designed will have an insufficient amount of sand resulting in a harsh mix. Bulking of sand will affect the yield of concrete for a given cement content.