Adding water on -side to concrete mixtures such as exposed aggregate concrete and pattern paving concrete has been a controversial topic for as long as concrete has been used as a construction material.
The addition of water is clearly covered in Australian standard for Ready Mix Concrete. The standard does allow to add the water on side to make concrete workable.
Although the general rule is that water on side can be used to adjust the slump of the concrete as long as the maximum specified water -cement ratio is not exceeded. You can allow use from 5 to 10 l of water for cubic meter. A small addition of water can be beneficial to the quality of the hardened concrete.