Cold weather curing: a concrete revolution

by Bill and Mary Weaver

A revolution has occurred within the large highway and bridge contractor community — using concrete in cold-winter states since 2010. That was when the American Concrete Institute (ACI) first published the specifications for using hydronic heat for winter concrete work. Now for many large contractors, laying concrete all winter has become routine. Smaller paving contractors might want to consider joining this revolution.

Although cold weather concreting requires considerable attention to detail, being able to complete more jobs per year by working through the coldest weather and being able to keep workers employed year-round both make excellent economic sense. Setting up a hydronic heater, such as Wacker Neuson’s E-3000 — manufactured in Wisconsin — to keep your freshly poured concrete at an optimum temperature for curing in cold weather is simple to do. A contractor renting a Wacker Neuson hydronic heating unit will have thorough and complete professional instruction and set-up through the trained and supported dealer network, including 24/7 customer service.

Hydronic heaters consist of a diesel-run burner (conversion kits are available for natural gas and propane) and two hose reels totaling 3000 linear feet of hose. The hydronic fluid is an excellent heat conductor, being an environmentally friendly glycol/water mixture.

Once heated to the preset temperature, the hoses are laid out in a switchback pattern at specified distances on top of a sheet of plastic on the concrete surface. The plastic protects the newly poured concrete from excessive moisture loss. The hoses are then covered with weighted insulating blankets. Two layers of Wacker Neuson IB 750 blankets (R-value = 5,) are recommended for cold weather. One of these lightweight 6×125-foot blankets can be rolled up and easily carried by one worker, giving excellent cold protection with minimal labor and trucking. The blanket can be cut to size to fit the application without any loss of performance.

The hydronic fluid under the insulation transfers its heat to the concrete as it flows through the hose. The heated fluid completes the loop by returning to the E-3000 unit where the temperature of the returning fluid is checked to make sure the contractor achieves maximum performance. The fluid then recirculates through the burner to be reheated before the next circulating trip.

The action threshold for using cold weather concreting methods is defined by the ACI as when the temperature has fallen, or is expected to fall below 40 degrees F. (4 degrees C) during the protection period. The protection period can vary according to the ambient temperature, and the type of concrete, among other factors. In some cases, protection is only needed until the concrete reaches a compressive strength of 500 psi. Until that time, the water in the capillaries can freeze if its temperature drops below 25 degrees F. This is a serious matter. Once this happens, its potential strength will be reduced by more than 50 percent, and its durability will be adversely affected.

For concrete that will experience freeze thaw cycles, or that will carry an anticipated heavy traffic load, or for roads and bridges exposed to salt, the protection period should be extended.

“For cold weather concrete curing, the contractor can pre-set the temperature of the outgoing fluid from the E3000 to a maximum of 180 degrees, and then adjust downward until the contractor’s embedded concrete temperature sensors indicate the desired, job-specified concrete curing temperature,” explained Luke Sevcik, product application and training specialist at Wacker Neuson Sales Americas, LLC.

“The amount of temperature adjustment required depends on the ambient and concrete temperatures and the distance the E3000 is located from the poured concrete,” (such as high hose-runs up a bridge pier or to a high bridge deck.) Wrapping the heater hose between the E3000 and the concrete pour with insulating blankets reduces heat loss, saving time and money.

A concrete curing temperature of 75° F. is ideal,” explained Sevcik. “Set the temperature of the E3000 carefully according to the ambient temperature.

The next morning, check the temperature of the fluid that is returning to the unit to be reheated, or the temperature indicated by the “cherry sensors” embedded in the concrete, if you have included them. If it is about 75°, you’re fine. If the fluid has cooled to 60 degrees by the time it has circulated over the concrete and returns for reheating, raise the preset temperature. Frequently, the highly efficient burner can raise the temperature of the circulating fluid by ten degrees in only ten minutes after the preset temperature is raised.”

For large curing applications, the Wacker Neuson E3000’s coverage area can be expanded to as much as 18,000 square feet with the addition of external hose reel systems and hydronic pump packs. The unit comes with two onboard positive displacement pumps, which provides consistent fluid flow at heights. The positive displacement pumps can dependably and reliably pump hydronic fluid to bridge decks or bridge piers as high as 200 feet.

Because the fluid filled hydronic hoses are in direct contact with the concrete and are insulated, heating concrete in this way in cold weather uses less than half the fuel that would have been required to heat the air in a temporary construction tent. In a tent, a large portion of air heat will be lost to the surrounding air.

The hydronic hoses filled with the heat-transfer fluid have another very significant advantage for heating concrete in cold weather, compared to tenting and using air heat. Air heat rises and escapes so quickly that it tends to warm only the surface of the concrete. However, ACI’s cold weather concrete specifications require some degree of heating of the full depth of the concrete in cold weather.

It’s important to keep the entire thickness of the concrete slab in a highway or bridge deck as uniform in temperature as possible, for best eventual compression strength and to prevent cracking. “The ACI specifications for winter concreting can be interpreted that the concrete structure shall have differentials no greater than 25 degree F. (14 degrees C) anywhere in the structure.”

This is a difficult goal to meet when using forced air heat. However, because the hydronic heating hoses are in direct contact with the concrete and covered by insulating blankets, the heat will naturally conduct down through the concrete, warming the full depth.

For cold weather concreting of a bridge pier, the hydronic hoses are placed on the outside of the vertical concrete form, but looped a bit closer — about one foot apart. Before the concrete is poured, insulated forms along with the air inside them are heated using the versatile Wacker Neuson hydronic heater, as specified by ACI’s cold weather concreting specs.

In very cold weather ready-mix concrete can also be prewarmed using hot water in mixing. The aggregate can also be prewarmed using a hydronic heater. After the concrete is poured, insulating blankets are placed on top of the form. Sometimes the forms and insulation are completely wrapped in plastic sheets in order to trap the chemical heat of hydration inside the forms.

Since bridge piers are often high in the air and subject to high winds as the concrete cures, trapping the heat of hydration cuts the amount of heat that needs to be added to properly cure the concrete in very cold weather.

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