Nelson Bustamante: Specific Gravity and Water Absorption Tests on Aggregates

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When it comes to evaluating the properties of construction materials, specific gravity and water absorption tests on aggregates play a pivotal role. These tests offer valuable insights into the strength, quality, and water-holding capacity of the materials, which are crucial considerations in various construction projects.

Specific gravity is a metric that quantifies the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. This parameter serves as a reliable indicator of the strength and quality of a particular material. In essence, it helps determine how robust and resilient the material is.
In general, aggregates with lower specific gravity values tend to be weaker compared to those with higher specific gravity values. This makes specific gravity an essential factor to consider when selecting aggregates for construction purposes. It’s worth noting that the specific gravity test can be conducted using different methods, depending on the size of the aggregates being tested. For aggregates larger than 10 mm or 40 mm, a specific method is employed, while smaller aggregates, less than 10 mm in size, undergo a Pycnometer test.

Aggregate Sample

Essential Apparatus for Conducting Tests

To conduct these tests accurately, specific apparatus is required:

  1. A precision balance with a capacity of approximately 3 kg, capable of weighing with a precision of 0.5 g. This balance should be designed to allow the weighing of sample containers when suspended in water.
  2. A thermostatically controlled oven that can maintain a temperature within the range of 100-110°C.
  3. A wire basket with a mesh size of no more than 6.3 mm or a perforated container of convenient size with thin wire hangers, suitable for suspending it from the balance.
  4. Containers for water filling and suspending the basket.
  5. An airtight container with a capacity similar to that of the basket.
  6. A shallow tray and two absorbent cloths, each measuring no less than 75×45 cm.

The Testing Procedure

To perform the specific gravity and water absorption tests on aggregates, follow these steps:

  1. Begin by washing approximately 2 kg of the aggregate sample thoroughly to eliminate fines, drain it, and place it in the wire basket. Submerge the basket and its contents in distilled water, maintaining the water temperature between 22-32°C, with at least 5 cm of water covering the top of the basket.
  2. Immediately after immersion, remove any trapped air from the sample by lifting the basket 25 mm above the tank’s base and allowing it to drop at a rate of about one drop per second. Keep the basket and aggregate fully submerged in water for a duration of 24 hours.
  3. After 24 hours, weigh the basket and the sample while suspended in water at a temperature of 22°–32°C, recording this weight as W1.
  4. Remove the basket and aggregates from the water, allowing them to drain for a few minutes. Then, transfer the aggregates to dry absorbent cloths. Return the empty basket to the water tank, jolt it 25 times, and weigh it while submerged in water (W2).
  5. The aggregates on the absorbent cloths should be surface-dried until no further moisture can be removed. Then, transfer the surface-dried aggregate to a second dry cloth, spreading it out in a single layer. Allow it to dry for at least 10 minutes until it reaches a state of complete surface dryness and weigh it (W3).
  6. Place the aggregate in a shallow tray and keep it in an oven set at a temperature of 110°C for 24 hours. Afterward, remove it from the oven, cool it in an airtight container, and weigh it again (W4).

Apparatus for testing Specific Gravity and Water Absorption of Aggregates

Interpreting the Test Results

Based on the observations made during the test, you can calculate several essential parameters:

  1. Specific gravity: Specific gravity = W3 / (W3–(W1–W2))
  2. Apparent specific gravity: Apparent specific gravity = W4 / (W4–(W1–W2))
  3. Water absorption: Water absorption = ((W3–W4) / W4) x 100

It’s essential to remember that while a high specific gravity is often associated with high strength, it’s not sufficient on its own to determine the suitability of an aggregate sample for a specific construction application. Mechanical properties such as aggregate crushing, impact, and abrasion values must also be evaluated to make a comprehensive assessment of the material’s suitability for a given project. Moreover, factors like aggregate size and whether the material has undergone artificial heating should be considered as well.
In conclusion, the specific gravity and water absorption tests on aggregates are fundamental procedures that provide valuable data for engineers and construction professionals. These tests are integral in the selection and assessment of construction materials, ultimately contributing to the success and durability of construction projects.

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