June 26th 2013 marks the 40th anniversary of the completion of the CN Tower's concrete "slipform" and the first bucket of concrete being poured. June 26th also coincidentally, was the first public opening for the tower in 1976.
The slipform was used to create the concrete shaft from the foundation all the way to the 1464ft level (which was completed on Feb 22 1974), being jacked up one inch every 5mins. The slipform was designed, engineered and put into operation by Doug Sumner, P.Eng.
The slipform is a wooden form of 2 parallel walls using 4ft high plywood. Construction workers cross-tied rebar into the empty portion of the form, poured concrete into the wooden form, jacked the form upward, and repeated until the entire shaft was completed. The entire slipform structure, and its jacking mechanism, sat on steel rods placed within the curing concrete walls. Concrete is lifted from trucks on the ground to the slipform using a fairly simple and effective cable-hoist system, transferred to human transported wheel barrels on the top platform, and then poured into holes located on the top platform which route the concrete down flexible and moveable "funnels" to the second level where workers hand pour the concrete into the wooden forms.
The slipform had 3 main levels: the top level deck was where the workers carted the concrete via wheel barrels to the holes in the upper desk. The mid-level deck was where the concrete dropped down via a shute into the wooden forms (as seen in the lower-left image above). The lower-level outside and inside decks were where the cured concrete was manually finished off with toweling.
Upwards of 50 men would work in each shift, both on the slipform and on the ground as support personnel. There were 3 shifts per day allowing for continuous concrete pouring for 14 months, except for several weeks due to strikes. No work was done on weekends due to the cost of overtime. Work was done in rain, sleet and snow, regardless of the weather. Note: as a common myth and misconception, the entire shaft was not poured in one continuous run until completed – strikes by workers and weekends would bring the work to a halt.