Alkali-Silica Reaction in Concrete: How to Assess Aggregate
Some siliceous aggregates react with cement alkalis and crack concrete. Learn the signs, the ASTM tests and the usual ways of managing this risk.
Quality and technical information
Aggregate Moisture and Absorption: Correcting Concrete Mix Water
Surface moisture in sand changes the water-cement ratio. Learn the moisture states, saturated surface-dry condition and how to correct batch weights and mix water.
Concrete mix designs are usually written for aggregate in the saturated surface-dry condition, yet site materials are almost never in that state. If the difference is not corrected, the real water in the mix, and so the strength of the concrete, drifts away from the design.
Four moisture states
| State | Internal pores | Particle surface |
|---|---|---|
| Oven dry | Empty | Dry |
| Air dry | Partly full | Dry |
| Saturated surface-dry | Full | Dry |
| Wet | Full | Extra water |
Dry material takes up some of the mix water and wet material adds to it. The saturated surface-dry state is the balance point.
The numbers you need
- Absorption: from ASTM C127 for coarse and ASTM C128 for fine aggregate
- Total moisture: from ASTM C566, by drying a sample
Free moisture equals total moisture minus absorption. If the figure is negative, the material will take water from the mix.
Worked example
This example is hypothetical. The design calls for 800 kg of sand in the saturated surface-dry condition. The sand has 5 percent total moisture and 1.5 percent absorption, so free moisture is about 3.5 percent.
- Water carried by the sand: about 28 kg
- Wet sand to weigh: about 828 kg
- Added water: 28 kg less than the design
Without this correction, the concrete would have 28 kg of extra water and 28 kg less sand.
Site notes
Sand does not dry evenly in a stockpile; the lower part is usually wetter. Measure moisture again across work shifts and after rain. On how moisture affects volume, read Converting tonnes and cubic metres.
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