What is the water demand of fresh concrete?
The water demand of fresh concrete is the amount of water, in litres per cubic metre, needed to give the mix the required workability (consistence). It refers to the effective water content: the water available for workability and for reacting with the cement, excluding the water absorbed by the aggregates themselves.
What does the water demand depend on?
- Required consistence: an earth-moist mix needs far less water than a plastic or flowing mix.
- Maximum aggregate size: coarser aggregate has less surface area to wet, so the water demand is lower.
- Grading and particle shape: lots of fine sand or angular crushed material raises the water demand; rounded gravel lowers it.
- Fines: fillers and additions such as silica fume need extra water.
- Temperature: in hot weather the mix loses workability faster, and the temptation to add water grows.
- Admixtures: a (super)plasticiser or air-entraining agent reduces the water demand for the same workability.
How is it used?
Water demand is the starting point of every mix design. Once it is known, the target water-cement ratio tells you how much cement is needed. More water therefore means more cement for the same strength, or lower strength at the same cement content.
Practical tips
- Never add water to the truck mixer or barrow to make the mix “easier”. It raises the water-cement ratio, lowers compressive strength and durability, and increases shrinkage.
- Need more workability? Order a higher consistence class or a mix with plasticiser instead.
- Mixing yourself: keep a fixed amount of water per batch and remember that wet sand already brings water with it.
- In hot weather: work quickly, keep materials cool and protect fresh concrete from drying out.
Related terms
- Water-cement ratio
- Mixing water
- Consistence class
- Plasticiser
- Concrete shrinkage
- Compressive strength
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