Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > Exposed-Aggregate Finishes
Q: How are exposed-aggregate concrete finishes
produced?
A: There are three ways of obtaining
exposed-aggregate finishes on concrete slabs: seeding a select aggregate
into the concrete surface, the monolithic technique where a select
aggregate, usually gap-graded, is mixed throughout the batch of
concrete, and exposing gap-graded aggregates in a special topping
course.
The process for producing monolithic exposed-aggregate
finishes is as follows:
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1) Place the concrete containing the chosen aggregate in a
normal manner in which you fill the forms with the material
and rod the surface with a straight edge (typically a straight
2" X 4" board) with a sawing motion, back and forth
across the form from side to side. Then close the surface as
usual. |
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2) Spray the surface with retarder.
This can be obtained at any contractors supply house. Retarder
does typically contain sugars but the formulations that are
designed for use with concrete are strongly recommended for
a more consistent performance. The retarder will slow the set characteristics of the discrete
surface layer allowing the interior to harden while the exterior
remains soft. |
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3) The third phase is the difficult part. When the concrete
has become hard enough to carry your weight without displacing
the aggregate the surface is washed with a hose and scrub brush
to remove the top layer of cement paste. Care must be taken
not to displace the aggregate and not to expose the aggregate
too deeply (this can cause the aggregate
to lose bond and/or be displaced). Caution: A too long delay
in this part of the process can create extreme difficulty in
the removal of the top paste layer. The retarder
slows the set of the top surface but does not stop it completely.
After the desired surface has been achieved the slab should
be sealed with a clear sealer and curing compound (also found
at your local contractor supply house). |
We recommend "Finishing
Concrete with Color and Texture"
(Publication no. PA124, price $30.00 plus shipping and handling);
it contains a good section which fully describes all three processes
of producing exposed-aggregate finishes.
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