To accommodate the bunker's distinctive design, every subsequent pour had different pumping requirements. The next time a boom pump returned to the job, it was a 36Z-Meter, which placed 300 cubic yards of concrete for the exceptional 16-inch-thick and 10-foot-tall perimeter walls, obviously lacking any window formwork. At a later date, the same boom pump placed a similar amount of concrete for the 8-inch-thick slabs, which involved intricate placement to avoid puncturing heating tubes that were laid for a floor heating system.
Next, interior walls, poured at a 12-inch thickness with 162 cubic yards of 4,000-psi concrete, formed 12 octagonal rooms. The eight-sided shape of the rooms was incorporated into the design for greater strength than square ones offered. This is especially important as 6 feet of dirt along with grass will be placed on top of the structure's roof to conceal the bunker.
Although the bunker will be hidden, three doorways will be exposed. As a result, the entrances are being given extra attention and extra concrete with the help of the boom pumps. At each entry point, a 3-foot-thick slab of concrete sized approximately 15 feet by 20 feet will be placed above heavy-duty blast doors for added protection.
After the interior walls were already placed with concrete, construction crews were faced with a change in the bunker's design. A 4-foot-wide and 10-foot-tall tunnel, resembling a hallway attached to the home, was added around the bunker's irregularly-shaped half circle perimeter.
The addition of the tunnel, situated at the greatest distance from the pump's setup, posed the greatest reach challenge for placing concrete. However, the 40Z, specifically purchased for its reach and maneuverability arrived on site with its 115-foot 3-inch horizontal reach and four-section Multi-Z boom to place concrete at the farthest point without additional delivery line. Consequently, it pumped 335 cubic yards of a 5,000-psi concrete mix for the newly added footings and expanded perimeter wall.
"Without the longer reach of the boom pump, pouring the new perimeter wall would have posed a problem. Dragging line over the top of the interior walls that were already in place would have been extremely difficult and time-consuming," says Kahn Jr. "Fortunately, the 40Z had all the ideal features to easily overcome this job site obstacle."
Waterproofing the Bunker
Once crews installed 400 U.S. tons of steel beams atop the poured walls to hold up the deck, the 40Z pumped a 10-inch thick roof cap with 479 cubic yards of concrete. Again, it was crucial to pump non-stop to avoid cold joints; a backup pump was on site but never needed.
Just as unique as the project was the concrete mix for the roof. As the bunker will eventually be covered with dirt, it is paramount to keep water from entering inside. Xypex, a non-toxic, chemical additive for waterproofing was incorporated into the mix at the batch plant. Becoming an integral part of the concrete, this additive generates a non-soluble crystalline formation deep within the pores and capillary tracts of concrete to seal against the penetration of water.
"Like all mixes on this job, we pumped this highly specialized mix with a smooth flow because of the pump's Free Flow Hydraulics," says Kahn Jr. "This also proved beneficial for the longer 18-hour pours because its closed loop system keeps the oil cool for reliable operation."
Technology on the Move
"I've watched concrete pumping technology rapidly advance," says HFK owner Frank Kahn Sr., who started his Hollywood, Florida-based company with a trailer-mounted line pump 27 years ago. "With today's highly-developed equipment, the concrete pumps had the ideal combination of technological features to accomplish this job with greater efficiency than possible just a few years ago."
In fact, it was just a few years ago that Kahn Sr. moved most of his fleet north when Florida showed signs of a slowdown. Kahn Sr. remained in the sunshine state, but sent his young son Frank Jr. with three operators, four boom pumps and two City Pumps to a niche area in the Carolinas. Frank Jr. aggressively began promoting the company's new location by following ready-mix trucks to job sites and knocking on doors. Hard work paid off; and today, the North Carolina branch of the pumping company is known for tackling unique projects. Consequently, once HFK pumps the three-story lookout tower, which is expected to consume 1,500 cubic yards of concrete, their involvement in the extraordinary project will be yet another challenging feat under their belt.