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Cost Effective Use of Colored Concrete in Flatwork
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 white
cement is used as the binder, but does not require colored concrete
throughout its full depth.
Dry-Shake Method
 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. As surface treatments, however, dry-shake finishes
may be worn away over time or become damaged by impact, thereby
exposing the underlying concrete.
Two-Course Method
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 6 to 50 mm
(1/4 to 2 in.), consists primarily of sand and cement, although
coarse aggregate can be used as long as its size is limited to about
1/3 of the topping thickness. When concrete is to receive a stamped
finish, a maximum nominal size of coarse aggregate of 9.5 mm (3/8
in.) should be specified, provided the topping thickness can accommodate
it. Increased thickness may be necessary to accommodate a stamped
finish with a 3/8 in. 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% by mass of cement. Much lower dosages are
frequently effective in reaching the desired color when white cement
is used because it provides a neutral tinting base. Sand should
be chosen 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 for more information on these techniques.
For more information on colored and stamped flatwork, see:
Finishing Concrete
with Color and Texture
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