Moisture Content

Moisture States:

Since aggregates contain some porosity, water can be absorbed into the body of the particles or retained on the surface of the particle as a film of moisture. The following four moisture states are defined:

• Oven-dry (OD): All moisture is removed from the aggregate by heating in an oven at 105  C to constant weight (overnight heating usually is sufficient). All pores are empty.
• Air-dry (AD): All moisture removed from surface, but internal pores partially full.
• Saturated-surface-dry (SSD): All pores filled with water, but no film of water on the surface.
• Wet: All pores completely filled with water with a film on the surface.

Of these four states, only OD and SSD states correspond to specific moisture contents, and either of these states can be used as reference states for calculating moisture contents. The AD and wet states represent the variable moisture contents that will exist in stockpiled aggregates. The SSD state is the best choice as a reference state for the following reasons:

• It represents the "equilibrium moisture" state of the aggregate in concrete; that is, the aggregate will neither absorb water nor give up water to the paste.
• The moisture content of aggregates in the field is much closer to the SSD state than the OD state.
• The bulk specific gravity (BSG) of aggregates is more accurately determined by the displacement method in the SSD condition.
• The moisture content can be calculated directly from measurements of (BSG) using the displacement method.
Absorption Capacity:

Absorption capacity (AC or absorption) represents the maximum amount of water the aggregate can absorb. It is calculated from the difference in weight between the SSD and OD states, expressed as a percentage of the OD weight:

AC = (WSSD - WOD) / (WOD) x 100%

WSSD and WOD represent the weight of the aggregate sample in the SSD and OD states. The absorption capacity is used in mix proportioning calculations and can be used to convert from SSD to OD system or vice versa. Most normal weight aggregates have absorption capacities in the range of 1 to 2%. Abnormally high absorption capacities indicate high-porosity aggregates, which may have potential durability problems.

Effective Absorption:

The effective absorption (EA) represents the amount of water required to bring an aggregate from the AD state to the SSD, expressed as a fraction of the SSD weight:

EA = (WSSD  WAD) / WSSD x 100%

The effective absorption is used to calculate the weight of water absorbed (Wabs) by the weight of aggregate (Wagg) in the mix:

Wabs = (EA) Wagg

If the aggregate is close to the OD condition when batched, it takes the aggregate some time to absorb all the water necessary to reach the SSD condition.

Surface Moisture:

The surface moisture (SM) represents water in excess of the SSD state, also expressed as a fraction of the SSD weight:

SM = (Wwet  WSSD) / WSSD x 100%