Placing booms have become an essential tool in the efficient construction of high-rise buildings. A placing boom is simply the articulating boom, more commonly seen on a conventional concrete pump truck, positioned on the uppermost floor of a building construction project. Concrete is supplied through a pipeline from the pump that remains at ground level.
Why use a placing boom?
A conventional truck-mounted pump cannot place concrete directly from the pump line's hose tip above the fourth or fifth floor, which means using extended hoses. A placing boom overcomes this limitation. On post-tensioned decks, the structural engineer or building inspector might not allow dragging hose around the deck, for fear of damage to post-tensioning cable profiles. Placing booms overcome that problem, as well.
One common alternative is a crane and concrete bucket, but that is slow and ties up the crane. A placing boom can supply 70 cubic yards per hour while a tower crane would struggle to supply 30 cubic yards per hour above 15 floors, and performance would drop with every additional floor as the up and down distance grows. That increases the cycle time between bucket loads, while the place and finish crew is still being paid a fixed rate but now is working inefficiently. And while the crane is bucketing concrete, it cannot provide any other services, so the other trades and the general contractor suffer.
Placing booms are manufactured in two types. One type can only be used as a placing boom while the other can be used either attached to a pump truck in the conventional mode, or removed from the truck and flown by crane to the top of the building. The latter is known as a “detach” boom. Concrete pumping companies like the detach feature because it gives their fleet more flexibility.
There are also two pricing options. The first is by the hour, since pumps usually are rented and only used to place elevated decks. A detachable boom is a better fit for this application. The boom section is lifted off the pump for each deck pour, and the pump is connected to the hard piping. At the conclusion of the pour, the boom is returned to the pump truck and the whole assembly leaves the jobsite until the next deck pour.
The second option is to use a dedicated placing boom in conjunction with a ground pump, in which both remain at the jobsite for the duration of the project. The placing boom and pump are rented by the month. It becomes a fixed monthly cost and is used to place all elevated deck and vertical concrete. This option is far more expensive per cubic yard than simply renting a conventional boom truck, but by using it for every bit of concrete, the cost per cubic yard is reduced.
The placing boom can be attached to many different bases. Options include a tower crane mast, a circular mast that clamps to floors below, the existing structure of the building, self-climbing forms, steel beams in an elevator or stair shaft, or even on a barge.
Setting a placing boom on a tower crane mast means it can be located either inside or outside the footprint of the building. It is strong and versatile and can be used on sites where there is not enough room to extend a pump truck's outriggers, or where perimeter power lines make it too dangerous to maneuver a boom with its base outside the building footprint. Unfortunately, it is probably the most expensive option because more base sections must be rented as the building goes higher; tiebacks to the building may be necessary; and it requires either its own foundation or large ballasts.
Perhaps the most common way is to anchor the placing boom atop a tall mast braced to the floors below. This can be located inside the footprint and gives good coverage. The mast supports the weight of the boom and keeps it from falling over while it is stretched out full of concrete, without the need for counterweights.
A placing boom mast essentially is a 24- to 39-inch diameter steel tube extending three or four floors below the boom base. It has holes for attaching it to the existing floors, which also make it lighter. A recent option for masts is hydraulic self-climbing where the mast lifts itself from one floor to the next.