Achieving the Saturated Surface Dry (SSD) condition for aggregate involves removing all surface moisture while the internal pores remain full of water.
Methods to Obtain Saturated Surface Dry Aggregate
The key to reaching the SSD condition is to remove moisture from the aggregate surface without allowing the water within its pores to evaporate. Based on common practices in materials testing, a standard method involves careful drying:
- Step 1: Prepare the Material
Ensure the aggregate is thoroughly saturated initially. This is usually done by immersing the aggregate in water for a specified period (e.g., 24 hours). - Step 2: Remove Surface Moisture
Once saturated, the surface moisture must be carefully removed. A widely used technique, as described in reference 6.3.4, is to:- Roll the material in the cloth and rub with dry ends until all surface moisture has been removed.
- This process uses absorbent cloth to wick away surface water.
- Step 3: Alternative Drying Method
As an alternative to using a cloth, the aggregate may also air dry at room temperature to SSD. This requires careful monitoring to ensure the surface moisture is gone but the aggregate doesn't start drying internally. - Step 4: Critical Point - Stop Drying at SSD
It is crucial to Do not dry past the SSD condition. Drying beyond this point means water is evaporating from inside the aggregate pores, leading to a dry or air-dry state, not SSD. Recognizing the SSD state often involves visual inspection (the surface no longer appears damp or shiny but still looks darker than fully dry aggregate) or specific test methods depending on the aggregate size.
Why is SSD Important?
The Saturated Surface Dry condition is a critical reference state for determining several important properties of aggregate, particularly for concrete mix design.
- Accurate Absorption Measurement: The difference in weight between the SSD state and the oven-dry state determines the aggregate's water absorption capacity.
- Correct Mix Water Calculation: When designing concrete mixes, knowing the absorption allows engineers to calculate the correct amount of water needed. Aggregate in the SSD condition will neither contribute water to nor absorb water from the fresh concrete mix. Aggregate drier than SSD will absorb mix water, affecting workability and potentially strength. Aggregate wetter than SSD will contribute excess water, altering the water-cement ratio.
Properly achieving the SSD condition ensures consistent and accurate material testing and concrete mix proportioning.
