Cold Weather Concreting with Hydronic Heaters

A rapidly growing number of contractors have turned to hydronic heating equipment to help solve their cold-weather concreting problems. This paper takes a look at how hydronic heaters work and the benefits they provide to contractors.

Background

ACI 306R-88 delineates construction procedures which make it possible to successfully place concrete in cold weather. Quoting directly from ACI 306R: 

"Cold weather is defined as a period when, for more than 3 consecutive days, the following conditions exist: (1) the average daily temperature is less than 40ºF, and (2) the air temperature is not greater than 50ºF for more than one-half of any 24 hour period."

Under this definition, "cold-weather" conditions exist three to six months per year in many areas of the northern U.S. and Canada. The National Climate Data Center publishes local daily temperature data which covers the last 30 to 60 years for most cities. Following is a sample of North American cities, showing the average number of days per year that the low temperature falls below freezing:

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Not every freeze day is necessarily an ACI 306R "cold-weather" day, but easily 80% of them are cold-weather days. Until they see the data, most contractors do not appreciate how many ACI 306R cold-weather days they face each year.

Cold-Weather Problems
Concrete is sensitive to temperature. Three problems confront contractors when placing and finishing concrete in cold weather:

• Time to initial set is lengthened, necessitating the finishing crew staying on the job longer. Extended regular hours plus overtime hours cause cost overruns, which eat into margins;
• Early freezing permanently damages concrete.
  Freshly mixed concrete freezes at 29ºF 
  (-2ºC). If concrete freezes before attaining 500 psi (3.5 MPa) compressive strength, voids caused by ice formation make the concrete less durable, weaker, and possibly unusable. Replacing freeze-damaged concrete is expensive; and 
• Slow strength development.
  Reduced temperatures reduce the rate of strength gain. This increases the time before protections can be removed and concrete can be safely stripped or loaded. Project delays increase project costs.

Solution
All three problems can be remedied by one, simple, cost effective solution . . . HYDRONIC HEATING.
Concrete that is placed at 60 to 70ºF (16 to 21ºC) and maintained at 60 to 70ºF for 2 to 7 days will set quickly, not freeze, and will rapidly develop strength. Contractors can readily get 60 to 70ºF concrete delivered to the construction site during cold weather. The problem is what happens to the concrete after it arrives at the site. It is often placed on cold soil or into cold forms and left exposed to cold air. Hydronic heaters can be used to help keep the concrete warm.

What is hydronic heating?
"Hydronic heating" is a heating system which involves the transfer of heat by circulating a fluid in a closed system of pipes or hoses. Baseboard hot-water heat and in-floor radiant heat are common examples of fixed installation hydronic heating systems.

Portable hydronic heaters
Portable hydronic heaters are used to thaw frozen ground as well as to cure concrete. 
A properly designed portable hydronic heater for construction site heating is self-contained and easily transported. A glycol/water solution is heated and pumped through flexible tubing, usually rubber hoses, forming closed circulation loops. Temperature control is effected through an adjustable thermostat. A convenient mechanism is provided for transporting, unwinding, placing, and rewinding the hoses.
Insulation blankets are used to contain the hydronically delivered heat against the formwork or concrete. Hydronically heated hoses can be positioned in any configuration to heat concrete placements. The hydronically delivered heat enables the contractor to continue concrete work throughout the winter -- both indoors and outdoors.

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Hydronic Advantages
Hydronic heaters provide a unique advantage over hot-air heaters. Water/glycol heat transfer fluids are 970 times more dense than air. The specific heat of water/glycol solutions is more than 6 times greater than air. As a result, hydronic heaters can deliver very large quantities of BTU's at very low temperature differentials (DT) between the heat transfer hose and the concrete. Low temperature differentials eliminate hot spots and provide even heat distribution to the concrete. Cracking and curling induced by temperature gradients within the concrete are essentially eliminated. Because the DT are typically 10ºF (-12ºC) or less, there is no danger of accidentally overheating the concrete and impairing long-term strength development.

Applications

Ground Thawing/Tilt-up Concrete
The versatility of hydronic heaters is best illustrated by examining a few typical applications. First, consider a tilt-up wall application in New Hampshire for a 550,000 ft2 (51,000 m2) distribution warehouse. The project involved placing and curing tilt-up wall panels on site, November through January. This job illustrates several of the capabilities which can only be provided by hydronic heaters.

The contractor first thawed the frozen ground before placing a slab on grade by placing the heat transfer hoses on the ground. The hoses were then covered with insulation blankets. Thawing progressed at a rate of 1 ft. of depth per 24 hours of operating time. After the ground was thawed, sand was hauled in and graded. The ambient air temperature was in the low 20º F (˜-6º C). The heat transfer hoses were placed on top of the sand, blankets were laid, and the sand was heated for 24 hours from a beginning temperature of approximately 35º F (2º C) to roughly 70º F (21º C). A properly insulated and properly spaced hose array will heat soil 1.5 to 2º F (˜0.5º C) per hour.

The contractor then did something unique. Instead of removing the hoses and placing a slab on grade, he left the heat transfer hoses in place and poured concrete on top of the hoses, creating a casting slab that could be heated from below.

The casting slab was continued around the entire perimeter of the building footprint. From November to January, the contractor formed, placed, and cured 167 wall panels, each approximately 44 ft. (13m) high x 18 ft. (5.5m) wide x 17 in. (430mm) thick.

Wall panels were sophisticated "structural sandwiches", consisting of a release agent, decorative rustications, reinforcing steel, concrete, 2 in. Styrofoam insulation, more concrete, more reinforcing steel, and "skin" concrete.

Ambient air temperatures during the daytime were in the mid 20ºs F (˜-4º C). Overnight lows would sometimes dip below 0º F (-18 C). The combination of heat transfer hoses and insulation blankets kept the wall panels between 55 and 60º F (13 and 16º C). Each wall panel was cured for seven days.

The foregoing illustrates some important capabilities of hydronic heaters:

• The ability to thaw frozen ground;
• The ability to preheat cold soil prior to pouring a slab on grade; and
• The ability to heat a casting slab to cure tilt-up wall panels.

Another capability, which was not illustrated by the tilt-up wall job, is the ability to cure a slab on grade without pouring the concrete on top of the hoses. The procedure would have been the same as used to pour the casting slab, except that the hoses would have been removed before the placement. After the slab had taken a final set, it would be covered with a polyethylene vapor barrier, followed by the heat transfer hoses and concrete curing blankets. The slab would be maintained at 60 to 70ºF (16 to 21ºC) until sufficient strength had developed.

Poured walls
Hydronic heat was also used for cold-weather concrete curing on a poured wall condominium/hotel project in Breckenridge, Colorado. First the hydronic heater was used to thaw the ground and frost-protect the footings. Formwork was then erected for the wall.

Heat transfer hoses were attached to the outside of the plywood forms, spaced approximately 16 in. (410mm) apart. Concrete was placed using a concrete pump.

The formwork was then blanketed, and the hydronic heater was operated for seven days to ensure proper strength development.

The role of the hydronic heater was to provide a curtain of warm air outside the formwork, thus preventing the concrete from cooling. The hydronic heater consumed only $35 of fuel per day. The contractor estimates that his labor/heating/curing costs were cut by 70 to 80% versus building and maintaining a heat containment enclosure and using hot-air heaters. Additionally, the fire hazard associated with hot-air heaters was eliminated.

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Elevated slabs
Hydronic heaters have been used to cure parking decks, highway bridge decks, and the poured floors of multi-story buildings. Hoses and blankets are placed on top of the reinforcing steel to preheat reinforcing steel and formwork. After preheating, the procedure is identical to the procedure used to heat and cure slabs on grade, that is, place the heat transfer hoses on top of the slab following final set, place blankets, and maintain concrete at 60 to 70ºF (16 to 21ºC) until proper strength has developed.

When ambient air temperature is 15ºF (-9ºC) or below at the time of the placement, it is usually advisable to also tent the underside of the formwork and warm it with hot air. The additional cooling effects of wind should also be taken into consideration when calculating total heat requirements.

Heating work enclosures
Hydronic heaters can be coupled with commercially available liquid-to-air heat exchangers to produce extremely clean hot air. Hydronically heated work space air is free of CO, CO₂, H₂O, smoke, and all other combustion by-products.

Benefits
Hydronic heaters offer contractors many benefits:

• Thawing and curing results are reliable and predictable;
• Project schedules can be met;
• Costs are predictable and can be accurately calculated and quoted; and
• Compared to alternative methods, costs are economical.

• Because hydronic heaters deliver clean, pollution-free heat, there is no risk of carbonation, chalking, or dusting.
• Because hydronic heaters deliver high BTU’s at low temperatures and the hoses evenly distribute the heat, curling and cracking are minimized.
  

Summary

Cold-weather concreting often requires the use of supplementary heat.  Hydronic heaters offer a unique combination of high performance at a lower cost.  Low temperature differentials between hoses and the concrete ensure safe, even heating.