Generally, it is not a good idea to add extra water to ready-mixed concrete once it has arrived on site.

However, under certain circumstances, it may be permissible for water to be added to a load of stiff ready-mixed concrete in order to achieve the specified consistence class.

This is in accordance with BS 8500 and applies when:

• the slump is less than the lower limit of the consistence class
• the quantity of added water is controlled by being measured accurately and is recorded on the delivery docket
• the stiffness is not due to an excessive delay since batching
• the amount of water added does not exceed any specified limit for water:cement ratio.