Exposed Aggregate Concrete - Continued
Placing the Concrete
For maximum strength, the concrete should be as stiff as
possible when it's poured. We prefer about a 4-inch slump; any
stiffer and it's hard to spread. There's a limit to how much
concrete you can place and finish before it starts to set. If
the slab requires more than 8 or 10 yards of material, we'll
pour on successive days. The cedar expansion joints provide
natural stopping points when there's a need to break up the
pour. The dropped areas also function as control joints and are
another convenient stopping point.
We either pour directly from the truck or transport the
concrete in wheelbarrows (Figure 7). Pumping is not usually an
option, because most of our work is in existing neighborhoods
and there is nowhere to dump the excess material that is left
in the hoses.
Figure 7.
When the concrete can't be poured from
the truck, it's transported in wheelbarrows and placed a
section at a time
The size of the pour depends on the weather: We do bigger pours
in cooler weather, smaller ones when it's hot. We also take
into account the weather from previous days, because it can
affect the temperature of the ground and of the stored material
that goes into the concrete. We can offset the impact of the
temperature by putting accelerant or retarder in the mix.
Neither of these additives will affect the way the aggregate
finish looks.
Once the concrete is placed, we spread it with a rake and
screed it level to the top of the forms (Figure 8). The surface
will be generally flat, but we flatten it even more by bull
floating from multiple directions (Figure 9). If we time
everything properly, the concrete will begin to set shortly
after it's bull floated.
Figure 8.
The expansion joints are flush to the
forms, so the author's crew can level the concrete by screeding
across them with a strike-off board.
Figure
9.
A bull float removes any
unevenness produced by striking off the
concrete.
The concrete is ready to finish when the bleed water has
evaporated and the surface is hard enough that you can barely
push your finger into it. We use a hand-held magnesium float to
flatten the slab further and fill any holes in the surface
(Figure 10). Some guys will go over the surface yet again with
a steel trowel. This makes for a smoother surface on many types
of concrete but it will not produce a noticeably better finish
when the aggregate is exposed. We usually float the surface and
leave it at that.
Figure 10.Here, the author uses magnesium hand
floats to do the final surfacing of the concrete. This is the
last thing done before the retarder is sprayed onto the
slab.
Exposing the Aggregate
The aggregate is exposed by removing between 1/16 and 1/8 inch
of cement from the surface. The tops of the uppermost stones
will be visible, but will remain firmly encased within the
slab.
During my early years in the trade, we washed the surface with
water and used brooms to scrub away the top layer of cement.
This method requires careful timing. If you start too soon, it
dislodges the aggregate; if you wait too long, it takes a wire
brush to get the cement off.
Nowadays, finishers use a chemical retarder to slow down the
rate of setting at the surface of the slab. The product we use
is mixed with water and sprayed onto the freshly floated
surface with a pump-up garden sprayer. Oil-based products are
also available. The retarder has some color to it so you can
tell if you've missed any spots, but it will not stain the
concrete.
Most retarders hold the set for up to 24 hours. Many
manufacturers recommend covering the surface with plastic and
waiting several hours or even overnight before washing the
slab. However, there is some risk to covering the slab and
washing the surface the following day. If the weather is hot,
the retarder might not hold the set until you get back. At that
point, it may still be possible to expose the aggregate, but it
will take a lot more brushing. If the plastic should blow off,
the surface may dry so much that it's hard to expose the
aggregate.
We prefer to wash the slab as soon as it's firm enough to stand
on. In warm weather, it will be ready an hour or two after
floating. All it takes to remove the cement is a good stream of
water and some light sweeping with a broom (Figure 11). Once
the excess cement has been removed, the concrete should be
allowed to cure in the normal way.
Figure 11.
The aggregate is exposed by washing away
the top layer of cement. While most of this slab is already
hard, the top layer is fresh because it was sprayed with a
retarder.
A day or two after the slab is poured, the crew comes back to
install the decorative bands (Figure 12). Many contractors set
the masonry with Type N mortar; we use Type S mortar because it
forms a stronger bond.
Figure 12.At left, the author's crew cuts stone to
form an inset medallion in an aggregate finish driveway. Below,
a brick head course set into a patio waits to be
grouted.
Masons who are accustomed to building walls may be tempted to
fill joints with the same mortar that's in the setting bed and
strike them off with a tool. We prefer to use cement mortar and
treat the joints as if they were the grout lines in a tile
floor. Standard mortar contains portland cement, sand, water,
and hydrated lime. Lime makes mortar easier to work with, but
it also reduces its durability. Cement mortar contains no lime,
so it's more akin to concrete.
Figure 13. Here, crew members
have grouted the masonry joints. They will finish them by
screeding flush to the brick and sponge-cleaning the area with
water.
The cement mortar is mixed wet enough that it can almost be
poured into the joints. After it starts to set, we scrape and
sponge away the excess material (Figure 13). This type of joint
will last much longer than a tooled joint because it's flush to
the surface and won't collect water. I've seen horizontal
surfaces grouted this way that are still in good shape more
than 30 years later.
Ron Sansoneis a second-generation concrete
contractor with more than 40 years in the trade. He owns Ron
Sansone Construction in Pacific, Mo.