Aggregates contain pores of varying sizes; while some are completely enclosed within the particles, others open onto the surface, allowing water to penetrate. The porosity of aggregates is a major factor contributing to the overall porosity of concrete, given that aggregates constitute about three-quarters of concrete's volume.

When all pores in an aggregate are filled with water, the aggregate is considered saturated and surface-dry. If left in dry air, water will evaporate until the aggregate reaches an air-dry state. Prolonged drying in an oven removes all moisture, leading to a bone-dry or oven-dry state.

Water absorption, a critical aggregate property, is measured by the weight loss of a saturated and surface-dry sample after being oven-dried for 24 hours and is expressed as a percentage decrease in mass relative to the dry mass of the aggregate. However, a more realistic duration for measuring water absorption in practical scenarios is 10 to 30 minutes rather than 24 hours. This adjustment acknowledges that in real construction settings, the actual absorption by aggregates in the mixer is less and is also influenced by the sequence of adding ingredients. Moreover, the absorbed water in the aggregate pores may not contribute to the hydration of cement and the workability of the concrete.