Placing and Finishing Concrete - Design and Control of Concrete Mixtures - page 209

Fig. 11-35. Patterned, textured, and colored concretes are very attractive. (59031, 53598, 59003, 47835, 44898)

Since timing is important, test panels should be made
to determine the correct time for exposing the aggregate
without dislodging the particles. On large jobs, a water-
insoluble retarder can be sprayed or brushed on the sur-
face immediately after floating, but on small jobs this may
not be necessary. When the concrete becomes too hard to
produce the required finish with normal washing and
brushing, a dilute hydrochloric acid can be used. Surface
preparation should be minimized and applicable local
environmental laws should be followed.
Two other methods for obtaining an exposed aggre-
gate surface are: (1) the monolithic technique where a
select aggregate, usually gap-graded, is mixed throughout
the batch of concrete, and (2) the topping technique in
which the select exposed-aggregate is mixed into a topping
that is placed over a base slab of conventional concrete.
The aggregate in exposed-aggregate concrete can also
be exposed by methods other than those already dis-
cussed. The following techniques expose the aggregate
after the concrete has hardened to a compressive strength
of around 28 MPa (4000 psi):
Abrasive blasting is best applied to a gap-graded
aggregate concrete. The nozzle should be held per-
pendicular to the surface and the concrete removed to a
maximum depth of about one-third the diameter of the
coarse aggregate.

Waterblasting can also be used to texture the surface
of hardened concrete, especially where local ordinances
prohibit the use of sandblasting for environmental rea-
sons. High-pressure water jets are used on surfaces that
have or have not been treated with retarders.
In tooling or bushhammering, a layer of hardened
concrete is removed and the aggregate is fractured at the
surface. The surfaces attained can vary from a light scaling
to a deep, bold texture obtained by jackhammering with a
single-pointed chisel. Combs and multiple points can be
used to produce finishes similar to some finishes used on
cut stone.
Grinding and polishing will produce an exposed-
aggregate concrete such as terrazzo, which is primarily
used indoors. This technique is done in several successive
steps using either a stone grinder or diamond-disk
grinder. Each successive step uses finer grit than the pre-
ceding one. A polishing compound and buffer can then be
used for a honed finish.
Regardless of the method employed, it is wise for the
contractor to make a preconstruction mock-up (field
sample) for each finish to determine the timing and steps
involved; in addition, the mock-up is used to obtain
aesthetic approval from the architect and owner. For
more information see Kosmatka (1991) , PCA (1972) , and
PCA (1995) .