Working With Shotcrete, continued
Shooting the Shotcrete
In our area, all shotcrete subs are unionized, but they don't
have a problem working on nonunion residential jobs. Our
regular sub is a company called Dees-Hennessey, which also
specializes in earth-retention shoring. The crew shows up on
the site with a 16- to 20-foot flatbed truck that has a
high-capacity concrete pump in tow. The truck bed contains a
big 350-cfm compressor and a couple of huge reels of 2-inch
pump hose (Figure 4).
Figure 4.In addition to the ready-mix truck
visible at rear (top), a shotcrete job requires a high-powered
compressor mounted on a flatbed truck and a tow-behind concrete
pump. The author's shotcrete sub brings a second concrete pump
to the site as a backup. The batched concrete is discharged
directly into the hopper of the pump and is then forced through
a 2-inch hose to the nozzle, where compressed air is added
The equipment is positioned so that ready-mix trucks can back
up and discharge concrete directly into the hopper of the
concrete pump. The concrete itself is mixed at the batch plant
to the engineer's specifications. These typically call for a 7-
or 8-sack mix — substantially richer than the 5- or
6-sack mix used in poured applications — and
pea-gravel aggregate. We try to park the equipment as close to
the work as possible, but the concrete pump has the power to
move the material a long way if necessary. Our sub has
successfully pumped concrete up to 600 feet horizontally and
300 feet vertically, although not both at the same time.
The nozzleman. The quality
of a shotcrete job is only as good as the nozzleman, as the
person at the business end of the hose is called (Figure 5).
The nozzle has to be oscillated continuously to prevent a
defect called "shadowing," in which the concrete builds up on
the front of the strands of rebar without filling the area
behind. A good nozzleman can keep an eye on the slump of the
material and build its thickness in a series of 3- to 4-foot
lifts without sagging or other defects (an accelerator is often
added at the batch plant to allow the material to stiffen soon
after it's blown on).
Figure 5.The shotcrete nozzleman guns a steady
stream of concrete against a below-grade foundation wall while
the blowpipe operator prepares to blow away unconsolidated
debris, known as rebound. The area to the right has already
been screeded to the level of the guide wires.
Doing all this well takes a lot of practice. To become
certified as a nozzleman by the American Concrete Institute, a
worker must accumulate at least 500 hours of work experience or
training and pass both a written exam and a field performance
Dealing with rebound. The
nozzleman is assisted by a blowpipe operator, who is
responsible for blowing away the rebound, or material that
bounces off the fresh surface rather than sticking. This is
essential because rebound material is mostly aggregate with
little cement paste. If it's allowed to build up and is covered
over, the result will be a permanent weak spot. The blowpipe is
simply a length of 3/4-inch pipe connected to the compressor
with a jackhammer-type hose and controlled with a gate valve.
The operator — who is usually also a nozzleman or a
nozzleman trainee — moves in step with the nozzleman
to maintain a clean leading edge.
Something like 10% or 15% of the pumped-in material typically
ends up as rebound. On a big pour, that's enough to keep
several laborers busy with shovels, sometimes with the
assistance of a material conveyor (Figure 6). The laborers also
help the nozzleman drag the heavy concrete hose from place to
Figure 6.Up to 15% of the concrete used on a given
job ends up as rebound, and several laborers may be kept busy
removing this waste material. A material conveyor is an
efficient way of moving rebound from a basement to ground
Test panels and inspection.
The nozzleman's skill is particularly tested in areas where the
reinforcing steel is tightly spaced (Figure 7), which can make
it difficult to fill around the rebar without leaving voids and
other defects. When this "congested steel" condition is
unavoidable, the engineer will usually require the shotcrete
sub to make a preconstruction test panel. This 4x4-foot mockup
is shot by the same nozzleman who will later do the actual
work, using the specified concrete mix. This test panel is then
cured for five to ten days, cored and inspected for shadowing
around the steel, and crushed for a strength reading.
Figure 7.Heavily reinforced structural elements
such as this pilaster are susceptible to shadowing and other
defects if the shotcrete is applied improperly. To ensure
quality, a mocked-up test panel is often shot, allowed to cure,
and tested before construction begins.
In our area, we're also required to have an inspector on the
site whenever we're applying shotcrete with engineering specs
of 3,000 psi or more. (Practically speaking, this includes
every shotcrete job we do.) This "special inspector," who works
for an independent engineering firm, watches over the entire
process to ensure that nothing is done that could leave hidden
defects in the work.
Screeding and Finishing
After each 3- to 4-foot lift, the nozzleman moves his
operation to allow the finishers to screed the slightly set-up
material to the level of the guide wires (Figure 8).
Figure 8.As each successive 3- to 4-foot lift of
shotcrete is built to the required depth, it's screeded to the
guide wires. The lift below has already been screeded and
troweled; when the current lift has been finished the same way,
the nozzleman will spray another lift over the exposed rebar
visible at the top of the photo.
Once screeding is complete, the guide wires are clipped and
removed. Screeding leaves the surface flat but very rough, so
it's troweled to provide a more acceptable finish. For utility
installations, we usually want a wood or rubber-float finish
(Figure 9). Quite often we apply a steel trowel finish to our
walls if they will be in an area that's exposed to view.
Figure 9.Shotcrete can be finished and ornamented
in various ways, but a simple rubber-float finish is a common
choice for utility applications.
Other more ornamental finishes are also possible. People have
attempted some light stamping, although this has to be done
carefully because it can cause vibration that may lead to
slumping. The concrete can also be finished with stucco. One of
my favorite non-stucco finishes was on a pool structure in
Atherton, where we shot from the inside of the structure,
stripped the forms after curing, and bush-hammered the formed
exterior face, resulting in a woven cloth-like effect. The
inside of the vault was rubber floated, which was fine for the
pool equipment room.
Curing. Once the shotcrete
has been screeded and troweled to the required finish, it's
cured about like ordinary poured concrete. To limit cracking
and prevent shrinkage and other defects, we spray the finished
shotcrete with a chemical curing compound that dries to form a
plastic moisture barrier. The manufacturer of the product we
use specifies that it shouldn't be applied until excess bleed
water has disappeared, but because there's little bleed from
the low-slump shotcrete mix, we don't have to wait long to
apply it to the exposed face. We coat the remaining face after
we strip the forms, usually about five days later.
The only time we don't use the curing compound is when the
concrete will be covered with tile or faced with stone, because
the sealant film can compromise adhesion. In that case, we'll
cure the concrete the old-fashioned way, by covering it with
blankets and keeping them misted with water.
Bill Brownis a specialty concrete contractor in