The volume increase of fine aggregate due to presence of moisture content is known as bulking. Fine sand bulks more as compared to coarse sand. Extremely fine sand particularly the manufactured fine aggregate bulks as much as about 40%.
Note: Fine aggregate do not show any bulking when it is absolutely dry or completely saturated.
What Causes Bulking of Aggregate?
The moisture present in aggregate forms a film around each particle. These films of moisture exert a force, known as surface tension, on each particle. Due to this surface tension each particles gets away from each other. Because of this no direct contact is possible among individual particles and this causes bulking of the volume.
Bulking of aggregate is dependent upon two factors,
Percentage of moisture content
Particle size of fine aggregate
Bulking increases with increase in moisture content upto a certain limit and beyond that the further increase in moisture content results in decrease in volume. When the fine aggregate is completely saturated it does not show any bulking. Fine sand bulks more as compared to coarse sand, i.e. percentage of bulking in indirectly proportional to the size of particle.
Why to Determine Percentage of Bulking?
Due to bulking, fine aggregate shows completely unrealistic volume. Therefore, it is absolutely necessary that consideration must be given to the effect of bulking in proportioning the concrete by volume. If care is not given to the effect of bulking, in the case of volume batching, the resulting concrete is likely to be undersanded and harsh. It will also affect the yield of concrete for a given cement content.
How to Determine Percentage of Bulking?
The extent of bulking can be estimated by a simple field test.
Fill a sample of moist fine aggregate (sand) into a measuring cylinder in the normal manner. Note down the level, say h1.
Pour water into a measuring cylinder and completely cover the sand with water and shake it. Since the volume of the saturated sand is the same as that of the dry sand, the saturated sand completely offsets the bulking effect. Note down the level of sand, say h2.
Subtract the final level h2 from initial level h1 (i.e. h1-h2), which shows the bulking of sand under test.
Calculate percentage of bulking using formula given below.