Tilt-up Curing in Tilton, N.H

The E3000’s Hydronic Heat Cures Tilt-Up Walls
With more than a decade of experience in tilt-up concrete construction, Clayco Construction, based in St. Louis, Mo., is considered the best in its field. As with any concrete construction job, cold weather always presents a problem. In addition to dramatically reducing the concrete curing rate, cold temperatures decrease both labor and equipment productivity, which slows overall project progress. Because of these concerns, many companies choose not to place concrete in the winter. Tilt-up concrete construction is no exception. In fact, the problems caused by cold weather are the reason that tilt-up construction is mostly used in warmer climate states, and has only recently gained attention in the North.

The Tilton, N.H. job involves the construction of a 550,000-square foot office building and distribution warehouse for DM Management, a mail-order catalogue company. Valued at more than $17 million, the job presented a problem that even Clayco had not yet faced. According to Pete Woolley, project superintendent for Clayco Construction, the weather conditions were difficult to overcome. "We’ve never done a job in conditions like they have here in Tilton – I mean this is extreme," Woolley said. "We had 24 inches of snow in November alone and winds up to 50 miles per hour."

Though Clayco Construction had overcome extreme winter conditions in the past, it was the high winds that made this tilt-up job extraordinarily difficult. "Propane heaters and tents were totally out of the question. We didn’t even think twice about it," Woolley said. "The cold weather is bad enough, but the high winds make it ten times worse. The wind chill can freeze the face of the concrete, causing integrity problems and requiring patchwork. We had to find a way to get this job done, and because of scheduling demands, postponing was not an option."

While Woolley was searching for a way to keep the job on schedule, a Wacker Neuson dealer who came up with the solution. “They suggested we use the Ground Heater to help facilitate the cure," Woolley said. "Though the Ground Heater was designed for thawing frozen ground and curing concrete, it had never been used for a tilt-up curing application, but we saw the potential." Clayco decided to rent five E3000 Ground Heaters from the dealer for this large curing job.

"Normally, in a non-winter job, we would cast our panels on the concrete floor of the building, but winter conditions require a different approach," Woolley said. "In winter, we construct a below-grade, temporary casting slab for pouring the wall panels. We poured the casting slab over the Ground Heater’s hoses and basically made a large radiant heating pad to keep the wall panels warm during the curing process."

The preparations began by deciding where to create the casting slab and then laying out the E3000’s hoses accordingly. "Using all five Ground Heaters, we laid out roughly 40,000 feet of hose for this slab – over seven miles of hose," Woolley said. With the E3000s running, the Clayco 30-person crew poured concrete over the hoses to make the large radiant heating pad.

Once the casting slab was ready, the crew began positioning the wood wall forms on the slab. A release agent was sprayed into the forms and on the casting slab itself. Next, foam-strip rustications were placed into the forms to provide a decorative appearance on the face of the finished concrete wall. Galvanized mesh was then placed into each form, followed by a 2.5- to 5.5-inch concrete pour (depending on wall size) known as a "skin." A 2-inch wall insulation was then laid on top of this "skin," rebar was set, and the remainder of the concrete, known as "backs," was poured and trowel finished.

We ran the Ground Heaters full bore, pumping out 180° F into the casting slab and heating more than 12,000 square feet of slab with each unit," Woolley said. "The idea was that the heat going through the slab would conduct up through the wall panels, facilitating the cure. For a proper cure, we wanted to keep the panels at 50° F or more." With average temperatures below freezing and sometimes as low as 0° F, Clayco placed insulation blankets on top of the panels to retain the heat and prevent the face of the concrete from freezing. Checking the gauges on the E3000s, Woolley found that the Heat Transfer Fluid coming back into the boilers was between 70° to 80° F, meaning the concrete walls were sufficiently warm to cure properly. "We did a seven-day cure on the walls, running the Ground Heaters 24 hours a day. To ensure the concrete achieved proper strength gain, we did five-day breaks on our test cylinders. Everything worked great."