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:
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:
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.
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.
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%
It is used to calculate the additional water (Wadd) added to the concrete with the aggregate.
Wadd = (SM) Wagg