Aggregates play a vital role in the construction industry, and their quality significantly influences the strength and durability of structures. The specific gravity and water absorption of aggregates are key properties that engineers and builders must assess to ensure the reliability of construction materials. In this comprehensive guide, we will delve into the specifics of these tests, their significance, and the step-by-step procedure for conducting them.

Understanding the Significance

  1. Specific Gravity: A Measure of StrengthThe specific gravity of an aggregate is a measure of its density and strength. It provides valuable insights into the quality of the material. Specific gravity is particularly crucial when identifying different types of stone used in construction.
  2. Water Absorption: Evaluating PorosityWater absorption indicates the porosity of an aggregate. Materials with higher water absorption are more porous and may not be suitable for construction unless they demonstrate adequate strength, impact resistance, and hardness.

Objectives of the Test

The primary objective of this test is to determine the specific gravity and water absorption of aggregates using a perforated basket. These values help engineers and builders make informed decisions about the suitability of aggregates for specific construction applications.

Apparatus Required for Specific Gravity and Water Absorption Test

To conduct the specific gravity and water absorption test on aggregates effectively, a set of specialized equipment is essential.

Here are the required apparatus for this testing procedure:

  1. Wire Basket or Perforated Container: You will need a wire basket with a mesh size not exceeding 6.3mm or a perforated container equipped with thin wire hangers. This container is used for suspending the aggregate sample during the test.
  2. Thermostatically Controlled Oven: To maintain a consistent testing environment, a thermostatically controlled oven is necessary. It should be capable of regulating temperatures within the range of 1 to 110°C.
  3. Container for Water: A container for filling water and suspending the basket is used to ensure that the aggregate sample remains immersed during testing.
  4. Airtight Container: An airtight container with a capacity similar to that of the wire basket is needed for specific stages of the test, particularly during cooling.
  5. Balancing Equipment: A precision balance with a capacity of at least 4 kg and an accuracy of 0.5 g is crucial for accurately measuring the weight of the aggregate sample and the basket during various stages of the test.
  6. Shallow Tray and Absorbent Cloths: You will require a shallow tray and two dry absorbent cloths, each measuring no less than 750 x 450 mm. These cloths are essential for surface drying the aggregate samples during the testing process.

Procedure:

1. Sample Preparation: Begin by obtaining approximately 2 kg of the aggregate sample. Thoroughly wash the sample to remove fine particles and debris, and then drain it. Place the prepared sample in the wire basket or perforated container.

2. Immersion: Immerse the wire basket or container, along with the aggregate sample, into distilled water. Maintain the water temperature between 22 to 32°C and ensure that there is at least a 50mm water layer covering the top of the basket.

3. Air Removal: Remove any trapped air from the sample by lifting the basket containing it approximately 2-fmm above the base of the tank. Allow it to drop 25 times at a rate of about one drop per second. After this, keep the basket and the aggregate completely immersed in water for precisely 24 ± 0.5 hours.

4. Weighing in Water: Weigh the basket and the aggregate sample while they are suspended in water at the same temperature. If you need to transfer the basket and sample to a different tank for weighing, repeat the air removal process by jolting it 25 times. Record the weight in water as W1.

5. Drying: After weighing, remove the basket and sample from the water and allow them to drain for a brief period. Then, transfer the aggregates onto one of the dry absorbent cloths.

6. Secondary Weighing in Water: Place the empty basket back into the water tank, jolt it 25 times, and record the weight in water as W2.

7. Surface Drying: The aggregates on the absorbent cloth should be surface-dried until no further moisture can be removed. Transfer them to the second dry cloth, ensuring they are spread in a single layer. Cover them and allow them to dry completely for at least 10 minutes. Depending on conditions, drying may take between 10 to 60 minutes. Avoid exposing the aggregates to direct sunlight, atmospheric moisture, or other heat sources during this process.

8. Final Weighing: Weigh the surface-dried aggregates and record the weight as W3.

9. Oven Drying: Place the aggregates in a shallow tray and keep them in an oven maintained at a temperature of 110°C for 24 hours. After this period, remove the aggregates from the oven, cool them in an airtight container, and weigh them. Record this weight as W4.

Calculation:

After conducting the specific gravity and water absorption test on aggregates, the next step involves performing calculations to determine these crucial properties. Here’s a breakdown of the calculations involved:

1. Specific Gravity (SG):

Specific gravity measures the density of the aggregate sample compared to an equal volume of water. It is calculated using the following formula:

SG = (Weight of Saturated Aggregate in Water / Weight of Equal Volume of Water)

To compute the specific gravity, divide the weight of the saturated aggregate in water (W1) by the weight of an equal volume of water. This calculation provides a numerical value representing the specific gravity of the tested aggregate.

2. Apparent Specific Gravity (ASG):

Apparent specific gravity is a variation that accounts for the presence of air voids within the aggregate. It is calculated as follows:

ASG = (Weight of Saturated Surface-Dry Aggregate in Air / Weight of Equal Volume of Water Excluding Air Voids in Aggregates)

To determine the apparent specific gravity, divide the weight of the saturated surface-dry aggregate in air (W3) by the weight of an equal volume of water, excluding the air voids present within the aggregates.

3. Water Absorption:

Water absorption quantifies the percentage by weight of water absorbed by the aggregates concerning their oven-dried weight. The formula for water absorption is as follows:

Water Absorption = [(W3 – W4) / (W4 – W2)] * 100

Calculate water absorption by subtracting the weight of the oven-dried aggregate (W4) from the weight of the surface-dried aggregate (W3), dividing the result by the weight of water equal to the volume of the aggregate excluding air voids (W4 – W2), and then multiplying by 100 to express it as a percentage.

Limits for Specific Gravity and Water Absorption

In the realm of materials testing, specific gravity and water absorption serve as critical parameters, helping engineers and builders make informed decisions about the use of aggregates in construction projects. To interpret the results effectively, it’s essential to understand the established limits for specific gravity and water absorption.

1. Specific Gravity (SG) Limits:

Specific gravity values for aggregates typically fall within a range of 2.5 to 3.0. This range signifies the density of the aggregate compared to an equal volume of water.

  • Lower Limit (2.5): Aggregates with specific gravity values nearing the lower limit are less dense and might contain a higher proportion of lightweight materials. While these aggregates may be suitable for certain applications, careful consideration is necessary to ensure they meet the project’s structural requirements.
  • Upper Limit (3.0): Aggregates with specific gravity values approaching the upper limit tend to be denser and could consist of heavier materials. While these aggregates may offer enhanced strength and durability, they may also pose challenges related to workability and transportation.

2. Water Absorption Limits:

Water absorption values for aggregates are generally within the range of 0.1% to 2.0%. This range indicates the percentage by weight of water absorbed by the aggregates concerning their oven-dried weight.

  • Lower Limit (0.1%): Aggregates with water absorption values close to the lower limit are less porous and tend to be highly resistant to moisture. They are typically favored for applications where exposure to water or humidity is a concern.
  • Upper Limit (2.0%): Aggregates with water absorption values near the upper limit are more porous and may require additional measures, such as waterproofing, to mitigate potential moisture-related issues.

(Know about Flakiness and elongation test of aggregate .)

FAQs:

What is the significance of specific gravity and absorption of aggregates?

Specific gravity and water absorption of aggregates are vital properties used to assess the quality and suitability of construction materials. Specific gravity measures density, which can indicate strength, while water absorption reflects porosity and potential durability issues.

How to calculate specific gravity and water absorption of coarse aggregate?

To calculate specific gravity, you divide the weight of saturated aggregate in water by the weight of an equal volume of water. For water absorption, you use the formula [(W3 – W4) / (W4 – W2)] * 100, where W3 is the weight of surface-dried aggregate, W4 is the weight of oven-dried aggregate, and W2 is the weight of the empty basket suspended in water.

What is specific gravity of aggregates?

Specific gravity of aggregates is a measure of the density of an aggregate compared to an equal volume of water. It provides insights into the material’s quality and strength.

What is water absorption of aggregate?

Water absorption of aggregate is the percentage of water absorbed by the aggregate concerning its oven-dried weight. It indicates the porosity and potential for moisture-related issues in the material.