Economic Consideration in Using Slipform
There are many factors that should be considered when decide to use slipform. The building has to be designed with slipform, or a similar forming system, namely, jump forming, in mind. The core has to be repetitive on all floors and fairly simple in design. Slipforming is not economical for buildings under 250 to 300 ft (76.2 to 91.4 m) in core height, because the high initial fixed cost of setup and take-down are not overcome by the low cost of forming such a relatively short height.
The cost of slipforming concrete is very sensitive to building
cross section, height, steel per yard or meter of concrete, embedments, quantity of concrete per foot or meter of height, labor policies, weather, and so on. Obviously, the cost of formwork will remain the same regardless of whether the slipform structure is 50 or 500 ft (15.2 to 152.4 m) high or whether the walls are 6 or 26 in. (152.4 or 660.4 mm) thick.
Openings in the wall not only cost money to form, but they slow down the slide, thereby increasing the cost per volume of concrete proportionally. Similarly, a basic crew of approximately 10 is required whether 10 or 20 yd3 (7.6 or 15.3 m3) of concrete are being placed per hour. Heavy steel, in addition to the increased cost of placing the steel, results in a reduced rate of slide and, therefore, greater concrete cost.
If the weather is cold, the concrete will set more slowly, the rate of slide will decrease, and the cost per cubic yard or cubic meter of concrete will increase. A decision as to the economy of slipforming a structure should be based not only on the cost per cubic yard or cubic meter of concrete but also on the cost savings that may accrue as a result of a decrease in construction time.
There are many costly mistakes that can slow down the slipforming process, thereby making it uneconomical to slipform. Honeycombing is one that occurs frequently. This is usually due to poor vibration of the concrete. The misplacement of inserts and plates, or if the walls become too out of plumb, can lead to going back and ripping out sections of wall, or the redesign of the steel. One of the most costly mistakes that could happen is when concrete sets to the forms, making it impossible for the forms to move. This occurs when the concrete mix is not proper, or the rate of climb of the forms is slowed down for some reason. If this happens, the whole job is stopped and the set concrete is ripped out of the forms. Many times all the sheathing must be replaced. When this occurs on a job, it usually will set the schedule back 4 to 6 weeks, making slipforming an unprofitable venture.
Also, the mere nature of slipforming requires a more experienced crew and superintendent because the process is more difficult to control. Because of complex design of the core of the slip – form, design changes are not easily forgiven. Once the slip is
started, if a change in design is needed, even if the change is near the top of the building, the slip must be stopped in order to allow time for the redesign.