Methods of test for aggregates for concrete, Part 3: Specific gravity, density, voids, absorption and bulking
1963 Edition

IS 2386 Part 3 outlines four distinct procedures to ascertain the specific gravity of aggregates based on their size classification: Method I applies to aggregates exceeding 10 mm, Method II is suitable for aggregates between 10 mm and 40 mm (either Method I or II may be used), Method III targets aggregates smaller than 10 mm, and Method IV is an alternative method permissible for all sizes. The process typically involves using a pycnometer filled with distilled water, removing entrapped air by rotation, weighing the assembly with the aggregate, emptying and refilling the pycnometer for a control weight, oven-drying the aggregate at 100 to 110°C for 24 hours, and weighing again. This ensures precise determination of specific gravity, apparent specific gravity, and water absorption for various aggregate sizes.

The standard specifies procedures primarily in Clauses 4.1 and 5.4.3 to quantify bulking caused by moisture in fine aggregates. The method involves filling a container with a known volume of water, adding a measured fine aggregate sample, eliminating trapped air, and then measuring the combined volume of water plus aggregate. The volume of water displaced by the sample is calculated by subtracting the combined volume from the initial water volume, allowing computation of bulking percentage. This measurement is crucial to adjust concrete mix proportions to accommodate volume changes due to moisture-induced bulking. Additionally, container sizes for bulk density measurement are prescribed based on particle size to ensure consistency.

IS 2386 Part 3 recommends a minimum sample size of 2000 grams for aggregates larger than 10 mm and about 1000 grams for fine aggregates. For aggregates ranging from 40 mm to 10 mm, a wire basket is used in place of a bucket, and stirring with a rod replaces jolting to remove entrapped air. For aggregates smaller than 10 mm, the sample is transferred to a bucket, covered with water at least 25 mm above the sample, and stirred to eliminate air before carefully adjusting the water level in the tank to prevent loss of fines. Artificial heating of aggregates should be avoided or clearly reported if applied. Two separate tests are recommended for reliability.

Surface moisture in fine aggregates is evaluated through a water displacement technique. A representative sample of at least 200 grams is weighed in its wet state, and the volume of water displaced by immersing the sample is measured. Using the specific gravity of the saturated surface-dry (SSD) fine aggregate, the SSD weight of the sample is calculated. From these values, two percentages are computed: surface moisture relative to the SSD weight and surface moisture relative to the wet sample weight. This method offers a dependable field estimation of surface moisture, which is essential for adjusting concrete mix designs accurately.

The standard provides formulas to calculate water absorption as the percentage increase in weight after saturating oven-dried aggregates, typically using weights obtained from various test clauses. Water absorption is computed by dividing the difference between saturated and oven-dried weights by the oven-dried weight, multiplied by 100. Apparent specific gravity is calculated by ratios involving weights of saturated, dry, and pycnometer-filled samples, distinguishing between bulk and apparent specific gravity based on pore inclusion. Void content percentage is derived by subtracting the ratio of bulk density to specific gravity from one, then multiplying by 100. These calculations are critical for accurately characterizing aggregate properties relevant to concrete mix design and quality assurance.