Hydronic Heating in the Curing of Poured Concrete Reduces Construction Project Costs and Time in Subzero Weather

When pouring concrete for large projects under a tight construction schedule during winter months, very precise control of the curing process is necessary. While traditional methods such as direct-fired and indirect-fired forced air heating for winter concrete pouring and curing already exist, hydronic heating is a much more manageable process accelerator.

During the winter, the challenge to managing poured concrete throughout a typical seven- to 10-day curing process benefits from fuel-efficient heater technology capable of continuous operation. In concrete curing, fuel consumption (rather than choice of equipment) is the primary cost factor favoring hydronic heating. Depending on project scope ranging from roads to bridge decks, contractors want to be assured of very precise temperature control of the curing process.

In a typical case, the onus is on maintaining a typical 60 degree surface temperature, regardless of subzero ambient air temperatures. Not only are portable hydronic heating services called on to satisfy the contractor’s insistence on precise control, but hydronic heaters use significantly less fuel than LPG-based direct-fired heaters. Another important factor to consider is that hydronic systems can be configured to cure concrete on individual floor plates in multi-story buildings. The unit can maintain th heater glycol and water fluid temperature at about 60 degrees throughout the curing process over a grid of tubing spaced 30 cm to 45 cm apart. This ensures uniform heating of the poured concrete in a confined area.

Hydronic heating leaves no footprint compared to other methods such as direct-fired heating or forced air. Concrete carbonation concerns accompanying the use of a direct-fired open flame, which emit CO2, have been well documented in the industry. In a few other cases, the contractor may feel that the inefficient fuel consumption and heat loss using indirect air heating outweigh the need to set up and later collect tubing for the glycol-based heating circuit employed in hydronic heating.

These cases are site specific, such as with commercial building construction, where the contractor may want to insist on using indirect heating to serve dual roles of keeping the building warm in cold weather, as requested by other subcontractors (e.g., during the drywall or roofing phase). In other cases, concrete curing in an open flame environment (e.g., direct-fired heating) consumes the oxygen in the facility that’s under construction while creating moisture and other unwanted byproducts. This forces the contractor to call in additional equipment and services to supply make-up air and extract all the pollutants from the resulting combustion process. Considering winter time conditions seen in regions such as Minnesota, the ambient make-up air that has to be blown in needs to be heated in order to maintain concrete curing temperatures in the 60 degree range. This inefficient practice brings additional complexity and extra fuel expense to a tight construction schedule.

The role of hydronic heating continues to expand as the industry demands more from service providers. For instance, improvements to portable diesel-fired generator and heating units, and their glycol-based heating media, continue to emerge in parallel with the use of remote monitoring, lightweight insulating mats and heating circuits placed on top of the slab or permanently embedded in the slab. These rapid deployment features ensure that concrete pouring challenges can be addressed during severe weather.

As an example, tubing can be embedded inside a slab by fixing the tubing to the steel rebar matrix. A glycol/water mixture is heated to specifications and pumped through the tubing—allowing heat to be evenly distributed throughout the slab volume, and optimizing curing. An added benefit to embedding tubing is in a building slab; not only is concrete curing enabled, but after curing is completed the slab itself can be heated by the embedded hose to provide space heating during the construction period, or in some cases provide permanent space heat.

It is not unusual for the first 12 to 24 hours after initial pouring to not allow anything to be placed on the surface of the poured concrete. If the ambient air temperature is low, it may be necessary to protect the freshly poured concrete from cold temperature shock during this time frame. The contractor may choose to build a temporary hoarding over the concrete and inject hot air with an indirect-fired heater. Once the time frame has passed and the concrete can be walked on, then the hoarding will be removed; hydronic tubing and insulating tarps will be installed and curing may proceed for a period of one to two weeks.

In ground thawing applications, the contractor has direct control of the heating circuit. Unlike the typical 60 degree concrete curing temperature, the glycol-water heating circuit can be increased to 185 degrees to conduct heat through frozen subgrades and achieve ground thawing in some conditions by as much as 1 vertical foot per day.

Benefits delivered by hydronic heating range from ensuring durability and crack-free surfaces to avoiding shrinkage and spalling in severe weather conditions. Contractors have a very efficient way to resolve their job-specific curing requirements when partnered with engineers that can provide hydronic heating expertise.

 
Russ Porowski is regional sales leader for Aggreko. For more information, email russell.porowski@aggreko.com.