Absorption of Aggregates

Absorption Capacity (AC):

Absorption capacity refers to the amount of water an aggregate material, such as sand, gravel, or crushed stone, can absorb. It includes both the water that becomes part of the aggregate's pore structure (capillary absorption) and the water that adheres to the surface of the aggregate particles (surface absorption).

AC is typically expressed as a percentage of the aggregate's dry weight. It's an important parameter to consider in concrete mix design because it affects the water-cement ratio and, consequently, the workability and strength of the concrete mixture.

Aggregates with higher absorption capacity will require more mixing water to achieve the desired workability, potentially affecting the concrete's overall performance.

Effective Absorption (EA):

Effective absorption refers to the portion of the absorbed water in the aggregate that is available to contribute to the hydration process and chemical reactions within the concrete mixture.

Not all the water absorbed by aggregates participates in the hydration of cementitious materials. Some of it may remain trapped in the aggregate's pores or evaporate during mixing.

Effective absorption is a crucial parameter in concrete mix design because it helps determine the amount of water that needs to be added to the mixture to achieve the desired water-cement ratio.

It's determined by subtracting the absorbed water that doesn't contribute to hydration from the total absorbed water by the aggregate.

The steps below can be used to calculate the absorption capacity of aggregates.

A basic outline of how the test is performed:

Sample Preparation:

Obtain a representative sample of the fine aggregate to be tested. The sample should be clean, contaminant-free, and sufficiently large for accurate testing.

Drying:

Dry the fine aggregate sample in an oven at a temperature of approximately 110°C to 115°C (230°F to 239°F) until it reaches a constant weight. This ensures that all moisture is removed from the sample.

Cooling:

Allow the dried fine aggregate sample to cool to room temperature in a desiccator or other suitable container to prevent moisture absorption from the atmosphere.

Weighing:

Weigh the dried fine aggregate sample accurately to determine its dry weight. Record this weight for use in calculations.

Immersion:

Immerse the dried fine aggregate sample in water and allow it to soak for a specified period, typically 15 to 24 hours.

Removal of Excess Water:

After soaking, remove the fine aggregate sample from the water and allow excess water to drain from the surface.

Weighing After Immersion:

Weigh the fine aggregate sample again after immersion and draining. Record this weight.

Calculate the absorption capacity (AC) of fine aggregate using the following formula:

Absorption Capacity (AC %) = (Wwet - Wod) ÷ Wod * 100

Where:

Wwet = weight of the fine aggregate sample after immersion and draining (in grams).
Wod = the oven-dry weight of the fine aggregate sample (in grams).

Note: as long as you use the same units for the calculations, instead of grams, you can use other weights like pounds, ounces, kilograms, etc..

Example Calculation:

The oven-dried weight of the sample = 934 grams
After soaking, the weight of the sample = 1145 grams
(1145 - 934) ÷ 934 x 100 = 22.6%
The absorption capacity of the aggregate is 22.6%.

Calculate the effective absorption (EA) of fine aggregate using the following formula: Effective Absorption (EA %) = (W_ssd-W_ad)÷ W_ssd*100 Where :

Wssd = the weight of the sample with all of the internal pores full and no surface moisture.
Wad the air-dry weight of the sample with no surface moisture and the internal pores partially filled.

Example Calculation:

The saturated-surface-dry weight of the sample = 1055 grams
the air-dry weight of the sample = 989 grams

(1055 - 989) ÷ 1055 x 100 = 6.2559 %

The effective absorption of the aggregate is 6.3%