What does Saturated Surface Dry (SSD) mean in terms of aggregate moisture?

The term Saturated Surface Dry (SSD) refers to a specific condition of aggregate where the particles have absorbed moisture to the point that their voids are filled with water, but the external surface is in a dry state. This means that while the internal spaces of the aggregate particles are fully saturated with water, the outer surface does not have visible moisture.

This condition is crucial for accurate batch calculations in concrete mixing because it helps determine the amount of water that must be added to achieve the desired concrete properties. When using SSD aggregates, the calculated water content in a mix design does not need to compensate for moisture on the surface since those surfaces are dry. Understanding this state plays a key role in ensuring the water-to-cement ratio is correct, which directly influences the strength and durability of the concrete.

The other options incorrectly describe different levels of moisture. Full submersion indicates that the aggregates are totally covered in water, which is different from SSD. Air-filled voids suggest that the aggregates are completely dry and not absorbing any moisture, which contradicts the SSD definition. Lastly, while the surface being wet conveys some truth, it does not accurately represent SSD, where the external surface must be dry for it to qualify as such.