One way to reduce the cost of materials for colored flatwork finishes is to place the color only at or near the surface. There are two common approaches for doing this: Dry-shake finishes or the two-course method. Both are compatible with stamped finishing techniques.
Colored flatwork has brighter and truer colors when it contains white cement as the binder, but does not require colored concrete throughout its full depth.
The dry-shake method is an excellent finish treatment for concrete flatwork. It consists of applying a prepared, packaged, dry color material that contains pigment, white portland cement, and specially graded silica sand or fine aggregate. Surfaces that receive dry-shake toppings are sealed to keep out excess water and prevent damage from freezing and thawing. Dry-shake finishes on surface treatments, however, may wear away over time or become damaged by impact, and expose the underlying concrete.
Another alternative to achieve colored finishes on flatwork is to use the two-course method. A thin colored wearing surface is placed over a freshly laid base course of untinted concrete. The topping and base courses bond intimately to one another, creating a monolithic slab.
The topping, which normally ranges in thickness from 1/4 to 2 inches, consists primarily of sand and cement, although coarse aggregate can be used as long as its size is limited to about one-third of the topping thickness. When concrete is to receive a stamped finish, a maximum nominal size of coarse aggregate of 3/8 inches should be specified, provided the topping thickness can accommodate it. Increased thickness may be necessary to accommodate a stamped finish with a 3/8 inch aggregate. In cold climates, topping mixes should be air-entrained to provide protection for freezing and thawing durability.
Pigments in topping mixes are optional. Maximum dosage for pigment should not exceed 10 percent by mass of cement. Much lower dosages are frequently effective in reaching the desired color when working with white cement because it provides a neutral tinting base. Installers should also choose sand with care because it, too, can impart color to the resulting concrete.
The ideal time for application of the topping is when the base slab is firm enough to support a concrete finisher. Two-course finishes are economical because the material savings usually more than offsets the higher labor costs.
In cases where the base slab has already hardened, it may be desirable to place a separate bonded or unbonded topping. See Resurfacing Concrete Floors or Concrete Floors on Ground EB075 for more information on these techniques.
For more information on colored and stamped flatwork, see: Finishing Concrete with Color and Texture PA124.