What is absorption water?
Absorption water is the water that is taken up by the aggregates (sand, gravel) when they come into contact with the mixing water during concrete production. This water penetrates the pores of the aggregate particles and is therefore no longer available for the hydration of the cement.
Why is absorption water important?
When designing a concrete mix, the water-cement ratio (w/c ratio) is precisely specified. The w/c ratio is the weight of the effective water divided by the weight of the cement. The effective water is the water that actually participates in the cement reaction — which is the total mixing water minus the absorption water.
If the aggregates are dry, they absorb more water than when they are already moist:
- Dry aggregates — absorb more water, leaving less effective water available. The concrete becomes stiffer and harder to work
- Wet aggregates — contribute additional water, raising the w/c ratio and potentially reducing strength
Application
In concrete practice, absorption water is accounted for as follows:
- Determination of absorption capacity — the absorption capacity of the aggregate is measured in the laboratory (to BS EN 1097-6)
- Correction of mixing water — the required amount of mixing water is adjusted for the moisture condition of the aggregates
- Moisture measurement at the plant — the ready-mix plant continuously measures the moisture content of sand and gravel and adjusts the recipe automatically
Typical absorption values
| Aggregate | Absorption (% by weight) |
|---|---|
| River gravel | 0.5–1.5% |
| Crushed rock | 1.0–3.0% |
| Recycled concrete aggregate | 3.0–8.0% |
| Lightweight aggregate | 5.0–25.0% |
Accurate correction for absorption water is essential for consistent concrete quality.
Related terms
- Concrete
- Concrete mix
- Mixing water
- Water-cement ratio
- Aggregate
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