Moisture Content

Since aggregates are porous (to some extent) they can absorb moisture. Generally this is not a concern for HMA because the aggregate is dried before HMA production. However, this is a concern for PCC because aggregate is generally not dried and therefore the aggregate moisture content will affect the water content (and thus the water-cement ratio also) of the produced PCC and the water content also affects aggregate proportioning (because it contributes to aggregate weight). In general, there are four aggregate moisture conditions (Figure 1):

Figure 1. Aggregate moisture states

  1. Oven-dry (OD). All moisture is removed by heating the aggregate in an oven at 105°C (221°F) to constant weight (this usually constitutes heating it overnight). All pores connected to the surface are empty and the aggregate is fully absorbent.
  2. Airdry (AD). All moisture is removed from the surface, but pores connected to the surface are partially filled with water. The aggregate is somewhat absorbent.
  3. Saturated surface dry (SSD). All pores connected to the surface are filled with water, but the surface is dry. The aggregate is neither absorbent nor does it contribute water to the concrete mixture.
  4. Wet. All pores connected to the surface are filled with water and there is excess moisture on the surface. The aggregate contributes water to the concrete mixture.

Note that pores not connected to the surface are not considered.

Equations

These conditions are used to calculate various aggregate properties. The moisture content of an aggregate is expressed as:

If the moisture content is positive, the aggregate has surface moisture and will contribute water to the PCC, while if the moisture content is negative the aggregate is air dry to some degree and will absorb moisture from the PCC.

Related Tests

Typical moisture tests are:

  • ASTM C 70: Surface Moisture in Fine Aggregate
  • AASHTO T 85 and ASTM C 127: Specific Gravity and Absorption of Coarse Aggregate
  • AASHTO T 84 and ASTM C 128: Specific Gravity and Absorption of Fine Aggregate
  • AASHTO T 255: Total Evaporable Moisture Content of Aggregate by Drying
  • ASTM C 566: Total Moisture Content of Aggregate by Drying