Aggregate free moisture and absorption adjustment

What free moisture is, and what it is not

Aggregate sits somewhere on a scale of four moisture states. Oven-dry has no water at all. Air-dry has partly filled internal pores. Saturated-surface-dry — SSD — is the state where the internal pores are full and the surface is exactly dry. Wet is SSD plus a film of water on the outside.

Mix designs are written against SSD, because SSD is the only state in which the aggregate neither takes water from the paste nor gives water to it. It is the neutral point, and every correction is measured from there.

Free moisture is the water above SSD: the film on the surface, which joins the mix water. Absorption capacity is the water the particles hold internally at SSD, which does not. Total measured moisture content contains both, which is why it cannot be used directly.

Subtracting absorption from measured moisture leaves the free moisture — as a percentage of the aggregate mass, and as the actual mass of water that batch of aggregate contributes.

The measured moisture itself comes from a field test, and the tests differ in what they burn off. Gravimetric drying — an oven, a hotplate, or the frying-pan method — drives water out by heat and weighs the loss. A calcium carbide meter reacts the moisture chemically and reads a gauge in minutes. Whichever instrument produced the number, it reports total moisture, so the absorption subtraction here still applies to it.

When this calculation is used

• Correcting batch water at the plant against this morning’s stockpile moisture reading.
• Reconciling a delivered mix that came out wetter or stiffer than the design predicted.
• Converting an SSD-basis mix design into as-batched masses for the material actually on site.
• Checking whether a dry stockpile is about to steal water from the paste.

Worked example

A stockpile of sand is weighed into a batch as delivered. This morning’s moisture test shows a total moisture content several points above the aggregate’s published absorption capacity.

The free-moisture percentage is what the surface water contributes, and the free-moisture mass is that percentage applied to the batch. That mass is subtracted from the mix water — the aggregate has already delivered it. If the free moisture comes back negative, the aggregate is drier than SSD and the sign reverses: the mix needs that much MORE water, because the stone will otherwise take it from the paste.

Both figures are produced by the certified engine when the calculator loads, from this pack’s own declared example. Nothing on this page is a stored answer.

What each input represents

Assumptions and limits

• Absorption is quoted for this aggregate, not for aggregate in general. A published range is a starting point for a trial mix and a poor substitute for the supplier’s own figure.
• The moisture reading is representative of what will actually be batched. Stockpiles drain, so the bottom of a pile and the top of it are frequently different materials for this purpose.
• Fine and coarse aggregate are corrected separately. Averaging them across a batch hides the fact that sand holds far more surface water than stone.
• A negative result is a real answer, not an error. Aggregate below SSD is common in hot, dry conditions and the water it will absorb has to be added to the mix.

What the guards protect against

• A moisture content above the range of ordinary correction practice is refused rather than computed. Material that wet is free-draining, its moisture is changing while it is being weighed, and a confident number for it would be worse than no number.
• An absorption capacity beyond the normal-weight band is refused too, because it almost always means a lightweight aggregate has been entered — and lightweight aggregate needs a different method entirely, not a correction on this one.
• The guards protect the method’s domain, not the arithmetic. The subtraction is always valid; what stops being valid is the assumption that surface water and absorbed water separate cleanly.

Provenance

Neville, A. M., Properties of Concrete — aggregate moisture states and free-moisture correction

Free moisture = total moisture content less SSD absorption capacity

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