Q.: Recently, I saw a concrete patio that had a rock-salt finish, which is characterized by many small surface indentations. Can you explain how to produce such a finish?

A.: We recently watched Bob Harris from L. M. Scofield Co. produce such a finish at the World of Concrete '95 Mega Demos. Here's how it's done:

  1. Place, strike off, and bull float the concrete.
  2. Let the concrete bleed and the bleedwater evaporate.
  3. Seed the concrete with coarse rock salt. This product can normally be found in hardware stores. It's used as a deicer during the winter months and is also used in water softeners. The more salt you put on the surface, the more voids you will get. It may take some practice to get the look you desire.
  4. Cover the slab with 1-mil plastic sheeting.
  5. Float and trowel on top of the sheeting. If the concrete is very stiff, you may need to tap the salt into the concrete.
  6. After troweling, remove the sheeting.
  7. After the concrete sets, saturate the surface with water to dissolve the rock salt. Often it helps to sweep the surface with a broom while it's saturated.

You may see dark rings around the pieces of salt. This is a discoloration due to the salt and should fade during washing and with exposure to sunlight.