Fine aggregate or sand can experience an increase in volume due to the presence of moisture. This is known as bulking, and it is important to test for it before using sand in construction projects. The purpose of the bulking test is to determine the extent of the volumetric increase that occurs when moisture is present.
To perform the bulking test on fine aggregate, a sample of sand is taken and its dry volume is measured. The sand is then saturated with water and any excess water is removed by shaking. The volume of the sand in its saturated state is then measured. The difference in volume between the dry and saturated states is the bulking volume.
The bulking test is important in determining the correct amount of sand to be used in construction projects. If the bulking volume is not taken into consideration, the amount of sand needed may be underestimated, resulting in a shortage of material. Conversely, if the bulking volume is overestimated, the project may use more sand than necessary, leading to unnecessary expenses. Therefore, conducting the bulking test is crucial to ensuring accurate calculations and successful construction projects.
Causes of Bulking of Sand
When sand contains moisture, it forms thin films around the particles, which results in each particle exerting pressure. As a result, the particles move away from each other, causing an increase in volume. This phenomenon is known as bulking of aggregates, and it depends on two factors: the fineness of the aggregates and the percentage of moisture content.
As illustrated in figure-1, the bulking of sand increases with an increase in moisture content up to a certain limit. However, beyond this limit, any additional moisture will cause a decrease in volume. Therefore, it is crucial to control the moisture content in sand to prevent any negative effects on the volume and quality of the construction material.
Fig.1: Variation of Bulking of sand with moisture content
The absence of bulking is observed in a fully saturated fine aggregate. In contrast, the extent of bulking is inversely related to the size of the aggregate. This implies that smaller aggregates tend to exhibit more bulking than larger aggregates. In particular, fine aggregates have a higher tendency to bulk than 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
To determine the percentage bulking of sand, the following apparatus are required: a 250ml measuring cylinder, a weighing balance, and fine aggregate. The procedure starts with taking 500 grams of fine aggregate and drying it at a temperature range of 100 to 110 degree Celsius for 24 ± 0.5 hours. The weight of the dry sand is measured and recorded as W1. The cooled sand is then placed in an airtight container and its weight is measured and recorded as W2. The water content of the sample is calculated using Equation 1.
To perform the bulking test, 250 grams of sand is taken in a pan, and 2% water by weight is added to it. The mixture is thoroughly mixed and poured into the 250ml measuring cylinder. The sand is consolidated by shaking, and the surface is leveled. The initial reading of the volume is measured as Y1. The test is repeated by adding 2% water by weight each time until a decreasing reading of the volume is observed. Then, 4% water is added, and the test is continued until the sample becomes fully saturated.
To determine the percentage bulking of sand, about 50 ml of water is added to the standard sample in the measuring cylinder and stirred well. The surface level of the inundated sand is noted down as Y ml. The percentage bulking of sand is then calculated using the formula: Percentage Bulking of Sand = (Y1-Y) x 100/Y.
A graph is plotted with the percentage water content along the X-axis and the percentage bulking along the Y-axis. The results obtained from the graph include the percentage of bulking occurred, the percentage of water content at maximum bulking, the percentage of water content when bulking is zero, and the percentage of bulking for the initial water content (W) of the sample. These results provide important information for the design and evaluation of concrete mixtures, as the bulking of sand can affect the volume and stability of the final product.
2. Field Test for Bulking of Sand
The field test is a method used to determine the amount of adjustment needed for the bulking of fine aggregates. The apparatus used in this test includes a 250 ml measuring cylinder and the fine aggregate being tested. The procedure involves pouring the sand into the measuring cylinder up to the 200 ml mark, filling the cylinder with water, and stirring the mixture thoroughly. The amount of water poured should be sufficient to submerge the sand completely, and it should be noticeable that the sand surface is now below its original level.
The reading is then taken at the sand surface, which is labeled as Y ml. To calculate the percentage of bulking of sand, the formula [(200/Y)- 1] x100 is used. The result obtained from this calculation gives the percentage of bulking of the field sample, which is an essential factor to consider in determining the necessary adjustment required for the fine aggregate being tested.
Importance of Determining Percentage Bulking
When proportioning concrete, the issue of sand bulking must be taken into consideration as it can lead to unrealistic volume measurements of fine aggregate. Failing to properly study and account for the effect of bulking can result in insufficient sand being used, resulting in a harsh concrete mix. Additionally, the bulking of sand can have a significant impact on the yield of concrete when a specific amount of cement is used. Therefore, it is crucial to understand and address the issue of sand bulking in order to accurately and effectively design concrete mixes.