We're helping out on a high-rise job, where the owners are concerned with speeding up the construction cycle. They want us to find some electrically heated forms to obtain higher early strengths in the concrete. Where can we find such a product?
There are several patents in this field, and successful work has been reported with such forms in both Europe and the United States, some of it dating back to the 1970s. Typically, electrical wires are embedded in the form material and temperatures are controlled by thermostat. The form must be substantial enough to distribute the heat evenly to the concrete. Both steel and fiberglass-reinforced plastic in combination with wood or plywood have been used for electrically heated formwork. However, we haven't found a current source of such forms for cast-in-place concrete. As recently as 1988, electrically heated forms were offered at World of Concrete, but that manufacturer appears to be out of business. We asked Anthony Adonetti of Structural Contours Inc., who has bid on some of the work for which heated forms were once proposed. He believes that such forms would not be cost-effective today, and suggested alternative ways of accelerating the strength development of the concrete. Heated enclosures are one answer; electrically heated pads laid on top of the slabs are another way to speed concrete curing. Effectiveness varies depending on the surrounding temperature. Another way to speed stripping time is to change the concrete mix so that it develops the necessary strength earlier in the work cycle. The time saved is frequently well worth the extra cost of the concrete. This approach helped speed work on the Riverfront III high-rise job reported by Concrete Construction in November 1991 (p. 783). The Riverfront contractors achieved their stripping strengths in three days, using column-mounted deck forms for about 70% of the job. The column-mounted forms delivered their load to the building's columns, greatly reducing loads on the newly stripped slabs below.