Admixtures

I wish your question was in more detail. But, I guess you are probably talking about concrete surfaces which are washed by a water jet the day after it has been placed to expose aggregates.

Concrete surfaces are primarily exposed either for architectural purposes or during mass concreting, when concrete is placed in layers (eg. dams) and one needs a better interlocking between concrete layer is being placed and the previous layer.

Washed concrete surfaces are rough to touch. While washing, typically the cement-sand paste around the coarse aggregate gets washed and the coarse aggregates are exposed.

The depth up to which the concrete surface can be exposed would depend on the amount of surface retarder applied on the formwork or the concrete surface and the pressure at which the concrete is washed. The depth of exposure is generally around 4 to 10 mm and water pressure while washing is around 5 to 10 N/mm2.