Temperatures sometimes fell as low as -20 degrees F with winds gusting to 60 miles per hour in the winter of 1973-74 during slipforming of the Canadian National Communications Tower, high above the Toronto waterfront. Special measures were devised to allow the concrete work to proceed and to ensure its safety. There were three kinds of problems: for the slipform to rise steadily there must be a normal rate of setting and hardening of the concrete during the few hours it is in the form; at the time the new concrete emerges below the climbing form and becomes exposed to the elements it has a high moisture content creating a risk for frost damage; and if the concrete has not achieved adequate strength when it emerges into the cold, the continuation of slipforming would be building on weak concrete. The first problem was solved by controlling the initial setting of the concrete through the use of heated concrete and by radiant heaters directed at the slipform a the finishing level. Solving the other two problems- avoiding rapid cooling and ensuring continued strength development- was more complex. It was calculated that if there had been no protection below the slipform the surface of the concrete would have cooled to ambient temperature in about 24 hours and the center wold have been about 10 degrees F warmer. Accordingly, a protection system was adopted which provided graduated insulation and ensured sufficient initial strength development while avoiding sudden cooling. For the first 20 feet, polystyrene 1 inch thick was used; for the next 9 feet this was reduced to one-half inch; for the final 8 feet one-fourth inch plywood was used. To determine if the planned curing temperature regimes were being achieved, thermocouples were used. These were installed in the concrete during each placing shift. The thermocouple wires were coiled on free-turning reels attached to the slipform framework. Then as the form rose the wires automatically unwound into the rising concrete and readings could be taken several times a day at various levels within the protected concrete.