Craning concrete and placing it high up poses numerous challenges.
One of the greatest challenges is dealing with the weight. “Although tower cranes are very efficient at what they do, they don’t have large capacities when it comes to tonnage,” says Stuart Galloway, President, Canadian Concrete Expo.” This weight restriction also means that you are constricted to working in a smaller radius since the crane’s lifting power diminishes the further out you go from the tower of the crane.”
The weight of a bucket of concrete (also called a skip) is often the heaviest load to be craned so, sometimes, companies have to use a larger sized crane on a project to lift the skip. This can mean using a tower crane instead of a city crane.
Smaller cranes, such as city cranes, are self-erecting and you only need an engineer report that certifies that the ground underneath and around the crane’s working area is solid and stable before you can erect it.
To assemble a larger crane, you need mobile cranes. And, with the larger cranes, you have to mount them into a concrete sub-structure. So, first, an engineered footing gets poured, then wait for the concrete to harden—then you can install the crane.
Also, craning up concrete increases the time it takes to place the concrete since the labourers have to wait for the empty skip to descend potentially dozens of storeys and then return again.
“Lowering the bucket to the mixers below and then hoisting it up again can take five minutes on a 20-30-storey building,” says Galloway.
Also, contractors tend not to fill the skip all the way to avoid spillage, so they are not even getting a full load on each lift.
Craning concrete has additional safety challenges.
Lifting a bucket of liquid or loose materials will move differently than lifting a solid block. The center of gravity of a bucket containing concrete will shift. Operators need to slow down and be smoother on the controls when craning concrete versus craning a pallet or a solid object. They also have to be more careful when choosing a lift path; you don’t want to lift over people and risk spilling it on them.
“Concrete is a liquid and its center of gravity changes when it moves, but a solid item has a fixed center of gravity, and this changes how it moves,” says Galloway.
Also, craning concrete poses greater safety risks. Labourers pouring concrete on the top floor are working under a suspended load. There is the potential to be pinned between the bucket and a wall or some other obtrusion or for the bucket to land on your boots.
“From reach to fighting gravity to pumping power to the extra time needed to complete the work, there are many challenges to placing concrete high up,” says Galloway. “Once you know the circumstances of your pour, you can choose the equipment you need for safe and efficient high up concrete placement.”
1. This blog is courtesy of the Canadian Concrete Expo, Toronto, Jan. 22-23, 2020. For more, visit: https://canadianconcreteexpo.com/.