Contd. from Part-I …..
This entire unit consisting of the slip-forms, cross-beams, platforms etc. are raised in synchronisation along the jack rods by means of hydraulic jacks of suitable capacities. The jack rods, which are usually embedded in the concrete walls of the core itself, can be either sacrificed or removed later on if so planned. For a mid-rise building these can also be suspended from a crane in stead of embedding in concrete walls. Besides fixing re-bars, these platforms are also used for fixing doors and window frames, inserts/embedments, keeping provisions for supports and other openings and so on. Lowermost platform is used mostly for quality related purposes such as monitoring concrete quality, continuous surface finishing of freshly stripped concrete surfaces, curing, rectification of shortcomings, if any, and so on.
The rate of lifting of forms primarily depends upon the rate of setting of the poured concrete. The lowermost portion of the poured concrete inside the slipform must set sufficiently in order to gain enough strength to support itself as well as the layers above it (upto the top of forms) without getting deformed/damaged while the forms are slipped up. At the same time, it should not set too hard and thus resulting in excessive friction between the forms and the concrete surface. That could lead to difficulities in lifting the forms which in turn could damage the still-fresh surface of the concrete as well.
A 300mm lift per hour is common in this method. However, this speed can be somewhat lesser or more as well depending on actual conditions or requirements etc. Proper monitoring of setting characteristics of concrete is hence important in this method. Concrete is continuously poured in layers of suitable thickness and by the time the pour reaches almost top of the slipform the lowermost portion, as mentioned already, must set adequately. Hence, rate of pouring too needs to be pre-planned and monitored accordingly. The forms are normally lifted in a series of small crawls of about 1 inch or so in order to achieve the planned speed of 300 mm per hour or whatever. This process of concrete pouring and lifting progresses continuously without any break unless absolutely necessary or the pour plan demands it. The progress achieved in this method is substantially higher than that of any other conventional method. Typically, one storey height of core can be completed in 2 to 4 days. If round the clock construction with proper planning is carried out, a progress rate of one storey per day too is not impossible.
Contd. in Part-III …..