A local contractor we supply is doing after some renovation work in an upscale area and has asked us about exposed aggregate. Specifically, he wants to know the best way to install the exposed aggregate finish and if there are durability and maintenance issues to be concerned about. What can we can pass along on this subject?

This is an area where experience counts. Experienced contractors can get some beautiful effects by exposing the aggregate. Although you can use a standard concrete mix, special aggregate sometimes can be used to take advantage of its color or shape. Another option is to broadcast special aggregate on the surface of the concrete once it is in place, being sure to work it into the surface enough so it bonds well with the cement paste.

One of the tricky things about installing exposed aggregate pavement is getting the right depth of exposure, plus getting an even appearance throughout the pavement. If a contractor reveals too much of a stone, it won’t have adequate bond and can break loose from the slab.

There are two ways to reveal the aggregate. One is to wash the surface with water as the concrete is getting hard. Another way is to spray the surface with a retarding agent and waiting until the next day to reveal. Using this technique, the concrete is usually covered with polyethylene sheeting overnight.

The second method is the safer approach because only the surface paste is retarded. The concrete below gets very hard and makes a strong attachment to the stones. The depth of reveal also tends to be controlled in the process too.

Assuming the concrete has good aggregate to begin with, enduring freezing and thawing is the primary durability issue. That applies to all kinds of exterior pavements, not just those with exposed aggregate. You should work with the contractor to be sure the mix design has adequate air entrainment in it for the area in which you are working.