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. 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.
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. The surface will be generally flat, but we flatten it even more by bull floating from multiple directions. If we time everything properly, the concrete will begin to set shortly after it's bull floated.
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.
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.
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. Once the excess cement has been removed, the concrete should be allowed to cure in the normal way.
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. Many contractors set the masonry with Type N mortar; we use Type S mortar because it forms a stronger bond.
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.
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. 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.