The bulking test should be made at the start of work, and after that on each new consignment of sand and after any material in the weather – rain or hot to ensure that the bulked volume of the sand is not changed.
When sand is delivered and is used on the job, quite frequently it contains moisture which causes films of water to form on the surface of the particles, fluffing them apart. This is called bulking and for moisture content of about 5 or 6 percent it may be as much as 20, 30 or even 40 percent depending upon the grading of the sand. Bulking of sand is more in fine sand than coarse sand.
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The standard test for bulking is based on the fact that while damp sand bulks the volume of saturated sand increases.
When we use fine aggregate (sand) to make concrete, it is necessary to know how much sand that you are using has bulked. This is determined by bulking test. This test is based on the fact that while damp sand bulks, the volume of saturated sand completely inundated with water is the same as if the sand were dry. Thus, you can measure volume of saturated sand and the result will be the same as that of dry sand.
To make the test you will require:
01. a straight sided container such as a 1 kg jam jar, or a clean, empty cane
02. a steel measuring rule
03. a steel rod to rod the sand
04. a second container to dip it into, and some water.
First fill the container about two-thirds full with the sand you are testing. Drop it loosely, do not pack it.
Level off the top of the sand and pushing the steel rule down through the bottom, measure its height. Suppose it is 15 cm.
So now you know the height of the damp, bulked sand. Next step is to find the height of the same sand when saturated with water. You can then compare the two.
Empty the sand in the other container, taking care to see that none of it is lost in the process, and half fill the first container with water. Now put the sand back into the water, bit by bit, so that it is entirely saturated.
First put back about half the sand and rod it thoroughly to remove any air. Then add the rest and rod again in the same way and level off top. Now push your rule through the sand as before and measure the new height then it is find that it has sunk noticeably. Say it now measures 12.5 cm.
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Result shows that when sand saturated (or dry) measured has bulked to 15 cm when damp. Therefore bulking is 2.5 cm on 12.5 cm of dry sand and that is to say 20 percent.
In gauging sand for the mix, therefore you should add 20 percent more sand than that quoted in the specification or as suggested by mix design.
Suppose the mix specified is 1:2:4, then the actual quantity of sand to be used will be 1.20 X 2 = 2.4 litres giving a field mix of 1:2.4:4.
If you neglect to make this correction for bulking the actual dry sand measured will be X 2 = 1.67 litres. The mix will then be 1:1.67:4 in terms of dry sand.
This reduction in the ratio of sand causes a reduction in the quantity of concrete produced with each bag of cement, and in most cases there will be insufficient fine material to give a workable mix which can make concrete with voids their by reducing its strength and durability both . It may be noted that coarse aggregate is little affected in volume by moisture.