Built-in Insulating Properties of ICF Protect Concrete as it Cures
by Joe Harnois
January 10, 2024
Pouring concrete in cold weather might sound daunting, but if you’re working with insulated concrete forms (ICFs), there’s a built-in advantage that helps you keep temperatures in the safe range for curing properly and completing your project.
ICF walls are designed with two layers of insulation, providing a protected environment for freshly poured concrete. The exothermic nature of cement generates heat when mixed with water. In simpler terms, the chemical reaction between water and cement generates heat and the dual insulation walls of ICF construction help to retain that heat.
ICFs Protect Concrete Temperature and Aid Curing
With ICFs boasting substantial insulating foam on both sides of the wall structure, the concrete can harness its own heat. Nudura® ICF forms are available in a variety of thicknesses that allow for in winter conditions. This self-generated warmth effectively maintains temperatures above the critical 40°F threshold required to achieve the intended, fully cured strength of the concrete.
Studies have shown that if concrete reaches a minimum compressive strength of 500 psi before it freezes, its ultimate strength will not be affected. This typically happens within the first 24 hours. The rule of thumb is that if the concrete can be kept above 40°F for 24 hours, it will reach design strength.
While it is not recommended to pour concrete below 40⁰, both anecdotal information in the field and studies show that 4” and 6” ICF walls retained a temperature above 40⁰ for four to five days following a concrete pour.
Experience in the Field
Installers working on a three-story dormitory project during a brutal Northfield, Minnesota winter decided to test the performance of an ICF installation. This unscientific temperature test was conducted with a meat thermometer pushed through the ICF foam into the freshly-poured concrete core. Even though the average temperature on the jobsite* was approximately 19°F, the thermometer within the concrete consistently registered around 100 degrees for 24 hours following the pour. A study commissioned by the Portland Cement Association fundamentally confirmed the findings of the rudimentary field testing effort.[i]
Working with Rebar
Understanding that most ICF walls also contain rebar, it is not recommended to pour concrete when the temperature of the rebar is below 40⁰. Cold rebar will contract when it comes into contact with warm cement, resulting in poor curing and possible loss of design strength. Rebar must be warmed before pouring concrete if the rebar temperature has dropped below 40 degrees.
Tips for Cold-Weather Concrete Pours
Keep forms clean, dry and free of ice, snow and other precipitation.
You can’t pour concrete at any temperature if the interiors of forms are wet or have collected foreign materials. Cover the tops of forms with plastic or other materials to keep the interiors dry and free of snow, ice and foreign materials. Ice in forms must be melted with forms dried completely before pouting concrete.
Do not install forms for ICF construction over frozen footings.
In a study commissioned by the Portland Cement Association, recorded temperatures indicated that concrete in direct contact with the frozen footings prematurely froze, thus damaging the concrete.[ii]
Protect Tops of ICF Forms to Ensure Proper Curing Temperature
While concrete produces heat which is maintained in the ICF form, the top of the form is particularly susceptible to exposure to cold. It must be protected while the concrete cures to ensure that the proper temperature for curing is maintained, allowing the full height of the ICF to reach its intended design strength. In addition to maintaining the proper cure temperature, tops of forms also should be protected from precipitation until concrete is fully cured.
Tops of forms can be covered with or protected in the following ways:
- Insulating blankets
- Tarps or plastic sheeting
- Insulation batts
- Polystyrene (EPS) panels cut to the width of the wall
- Plywood or OSB panels cut to the width of the wall
- Curing blankets
- Temporary roofs, shelters or enclosures
The Bottom Line
The inherent properties of Nudura ICF, featuring dual insulated panels, play a pivotal role in successful cold-weather concrete pours. The heat that is naturally generated during the concrete hydration process is protected inside the forms. This retention of warmth within the ICF wall helps to ensure that concrete cures to its full design strength, even during cold weather. It’s important to note that while the concrete core of ICF walls retain heat, care must be taken to cover the exposed top of each form to protect against heat loss and the damage that can occur if precipitation falls before the concrete is fully cured.
*Follow ACI-306R – Cold Weather Concreting for recommended concrete temperatures for concrete walls >12” (305mm). Testing has shown that concrete may be placed in an ICF at temperatures well below freezing (32°F/0°C).
[i] https://www.icfmag.com/2013/11/cold-weather-concrete-pours/, featured study, Cold-Weather Construction of ICF Walls, commissioned by Portland Cement Association (PCA).