5.16 : Moisture Content and Bulking of Aggregate

The moisture content of aggregates is a crucial factor in construction, particularly in concrete mixing, as it influences the total water required in the mix. Moisture content represents the water coated on the exterior surface of the aggregate existing in a saturated and surface-dry condition. The total water content of a moist aggregate is the sum of its moisture content and water absorption.

When aggregates are exposed to rain or sit in stockpiles, they absorb moisture, which must be accounted for in concrete batching to ensure the proper water-to-cement ratio. The total water content can be precisely measured using the oven-drying method as the percentage of moisture relative to the oven-dry mass of the aggregate. Innovations such as electrical moisture meters, which measure changes in electrical resistance or capacitance due to moisture variations, provide instant or continuous readings and are useful in automated concrete batching processes.

Moisture also affects the physical volume of aggregates, notably sand, through a process called bulking. This occurs when moisture causes sand particles to repel each other, increasing the overall volume. While preparing the concrete mix, bulking does not directly impact mass proportioning; however, it does affect volume batching, where the increased volume results in a decreased mass per unit volume in the mix.