A salt finish is a simple and inexpensive way to dress up what
would otherwise be plain concrete flatwork. It's achieved by
pressing coarse rock salt into the surface of fresh concrete,
allowing the concrete to set, and then washing the salt away
with a stream of water. The random indentations produced by
this process give the surface a weathered and textured look.
Long popular in the warm Western and Southern states, the
finish is rarely seen in colder regions, where water could
freeze in the indentations and cause the surface to
spall.
Earlier this year, I visited a residential project in Alamo,
Calif., and watched a crew form and pour a new driveway and
walk. It was a big job that involved several loads of material,
so crew leader Magdaleno Avarra decided to form and place the
concrete in stages.
Complicating matters slightly, the customer wanted a colored
surface. Although it's possible to order integrally colored
concrete (color is added to the mix itself), there's always the
chance that one load will not perfectly match the others.
Avarra avoided this problem by using color hardener — a
commercially produced mix of dry pigment, portland cement, and
finely ground aggregate — to tint the surface of the
concrete after it was placed. To ensure uniformity, he ordered
enough color hardener to do the entire job and verified that it
all came from the same production run.
The job's first few steps were the same as they would be for
any concrete slab. Crew members formed the edges, installed
reinforcing material, placed the concrete, and then
bull-floated (1) and edged it. Next, they manually broadcast
color hardener over the still-wet concrete (2), and floated (3)
and troweled it into the surface. When the concrete was firm
but not completely set, they broadcast salt over the surface
(4), partially embedding it by tamping down with a magnesium
float (5). (Some contractors use rollers to press in the
salt.)
Avarra's crew allowed the slab to set up overnight and then
cleaned it the following day with a pressure-washer (6). The
water dissolved the salt, leaving a series of irregular dimples
where the salt crystals had been. Once the surface was dry,
crew members sprayed it with a curing compound.
To give the driveway more visual interest, the crew
salt-finished only the main thoroughfare (7); curbs and border
areas got a light broom finish instead. — David
Frane