Q.: In exposing aggregate on both the treads and risers of concrete steps, can I use a retarder on the forms?
A.: Yes, you can apply a surface retarder to the form surface to retard the concrete adjacent to the form. That should permit you to expose the aggregate on the risers after the forms are stripped. It may take some experimentation to learn how much retarder to put on the forms and how long the retarder will delay the surface set. You can also use a retarder on the treads but it may be difficult to get the same degree of retardation on the treads as on the risers. If so, you may wish to plan on a difference and try to get some contrast between the degree of exposure on the two kinds of surfaces.