ZOO’S NATURAL ROCK WALLS EMERGE FROM WET-SHOTCRETE
In a project to create natural-looking artificial rock formations for a city zoo, contractors compared application methods before determining that a high volume of shotcrete could most efficiently be placed by a concrete pump.
The project for the city-owned John Ball Zoo in Grand Rapids, MI, calls for 360 lin ft of simulated rock in a horseshoe shape to form containment walls for the zoo’s new chimpanzee exhibit. Heated structures for winter housing of the chimps will be built later at the open end of the U-configuration under a separate contract.
General contractor Wolverine Construction, Grand Rapids, subcontracted Jolly Miller Landscape Construction Co., a Seattle-based contractor that specializes in the design and construction of naturalistic habitats. Jolly Miller, in turn, retained the pumping services of Superior/Cross Concrete Pumping, Inc., Grand Rapids. The project was started in June 2000, and was scheduled for completion by November 1, 2000.
How to build a naturalistic, simulated-rock wall with jutting outcrops and rough-textured surfaces that will contain a band of leaping, scrambling apes might seem a challenge to many designers and builders. To Jolly Miller designers it was relatively old hat.
“We know that the best chimpanzees can make a running vertical leap of 15 ft,” says the company’s construction superintendent Erik McCormick. “We’re building the walls 16 ft high. That doesn’t seem to leave much margin for error, but the walls are designed with a 5-degree incline. They won’t get out.”
Prior to shotcrete application an intricate forming system is created. It begins with a steel reinforcing-bar armature that is “bent and contorted to an organic shape that is predetermined to provide a random and natural looking finish,” according to McCormick. A backing material of metal lath is then applied (burlap may be used on smaller projects), held by wire ties to the armature. Metal chairs, similar to those used to support rebar on road grades, are placed to maintain a 2- to 3-in. space between the pliable lath and the armature.
Finally, a layer of chickenwire is applied over the lath to contain the first layer of shotcrete that is blown through the wire. The elaborate formwork is time consuming, McCormick says, and, combined with curing requirements, limits actual pumping time to four to five hours a day, twice a week.
“We selected a Schwing concrete pump for the job,” says McCormick. “The pump and pumping firm were recommended to us by another shotcreter in the Grand Rapids area.”
A muddy site
Cross/Superior Concrete Pumping delivered a 17-meter Schwing truck-mounted boom pump to the zoo site last June. The unit is equipped with a 1418-2.5 concrete pump kit that is used to push all 800 cu yds of the low-slump shotcrete required to complete the job. The boom is not used, as the contractors opted to extend up to 300 ft of 2-in. pipeline directly from the pump to the most distant reaches of the pour.
“We could have used the boom for shorter pours,” McCormick explains, “but with its 5-in-diameter line we would have needed a line reducer to connect a 2-in. extension line beyond the boom. And we needed the smaller line to pump at the low pressures you need for shotcrete application.”
The method has proven effective. With muddy site conditions surrounding the wall pour areas, Cross/Superior positions the truck/pump combo on stable ground as workers move the smaller line with relative ease, McCormick says, normally with two men per 50-ft section. The boom truck’s location enables mix trucks from Grand Rapids Gravel to approach and unload in the Schwing pump hopper, avoiding the mushy, puddled clay areas near the walls.
The Jolly Miller crew initially placed an 8-in.-thick structure coat that is topped later with a texture coat that ranges from 2 to 6 in. thick. The structure coat is placed in two lifts, starting at grade and poured up to 5 ft. high. Crews return the following day to pour the second lift to reach the top of the wall. Because the texture coat is placed from the top of the wall on down, the structure coat must be completed from top to bottom.
The texture application, also called a “carve coat,” is finished by trowel to create the natural-looking rock formations. The Jolly Miller crews are also building separate formations – called “mud banks” – within the enclosure that are shaped with trees, roots and branches.
For the high wall pours the contractor erected scaffolding for a four-man crew to move and direct the line from the Schwing pump. Using the wet shotcrete method, air is compressed through a separate line and introduced at the hose nozzle to the wet mix that is pumped at low pressures. The result is a consistent spray of material distributed evenly over the wide array of random planes and jutting surfaces.
A wise decision
“We considered both methods – wet and dry –application,” McCormick recalls. “We use gunite on smaller jobs, but on a large job like this your production is slower with a dry-mix when you apply the structural coat. Speed is not important for the texture coat, where precision for detailed work comes first. With the pump, we can keep it at the same location all day to get the job done without interruption. The mix trucks have no problems unloading because the pump is always on stable ground. On days we don’t pump, Cross/Superior moves the Schwing offsite to do other jobs.”
The owner of the company that bears his name, Jolly Miller, cites another reason for pumping the zoo project: “We’re working with an excellent material-supply firm and an excellent pumping firm. Often on wet shotcrete jobs you get mix segregation or loss of slump when the mix arrives. Here, the supplier and pumper are maintaining excellent control of the material.”
The use of the boom pump “may seem like overkill,” Miller says. But he points out that the unit’s availability and the flat pumping rate charged by Cross/Superior do not increase costs over a trailer-towed line pump. “In the best of both worlds, we would use a wet mix for the structural application, and come back with a dry mix for the texture coat,” says Miller. “But for this job the concrete pump is the most efficient and productive way to go. We’ve used many different pumps in 30 years of shotcreting, and the Schwing has always performed beautifully for us.”
Shotcreting – either wet or dry – is by nature a slow operation. The largest individual pours, totaling 55 cu yds for a waterfall and pool display within the chimpanzee exhibit, saw 10-yd truck loads emptied in 45 minutes, according to McCormick. Average output, however, is closer to 5 to 6 yds/hr, including the slower texture-coat applications.
The reliable and consistent mix from Grand Rapids Gravel is a rich 7-1/2-bag design with sand, pearock and 4 percent flyash. Delivered at the required 3-in. slump, the mix achieves 3,000 psi in 7 days. It achieves a strength of up to 5,000 psi in 21 days. Earthen color tones are added at the site.
By mid-October about 85 percent of the total 800 yds of shotcrete was in place. And with the hollow wall average about 1 ft thick on each side, the voids within provide the benefit of concealing the exhibit’s utilities. Electrical conduit, sprinkler boxes and pumps, pipes and valves for the waterfall exhibit are all conveniently placed and well hidden from a spectator view.
The project was on schedule for its November wrap-up, McCormick reported in October. And, he added, “the wall, itself, may have to be wrapped up. We’ve got insulation blankets at the ready in case overnight temperatures approach freezing. So far we’ve been lucky.”