Cold weather presents one of the most persistent challenges in concrete construction. When temperatures drop, the hydration process that gives concrete its strength slows dramatically or stops entirely. This is why concrete heating during curing is not just a convenience but a necessity for winter placement. In this article, we examine the practical methods used to heat concrete during curing, drawing lessons from a major subway expansion project in North Toronto where Curing of High Performance Concrete Methods and Duration played a central role in project planning. Understanding how to maintain proper curing temperatures directly affects the durability, strength, and long-term performance of any concrete structure.
Understanding the Challenges of Cold Weather Concrete Curing
Why Temperature Matters During Concrete Curing
Concrete curing is a chemical process called hydration, in which cement reacts with water to form crystalline structures that bind aggregates together. This reaction generates heat internally, but in cold weather, the ambient temperature pulls heat away faster than hydration can produce it. When concrete falls below freezing before it reaches sufficient strength, the water inside expands and damages the internal structure, leading to cracking, scaling, and reduced load-bearing capacity.
ACI 306, the standard for cold weather concreting, recommends that concrete be maintained at a minimum temperature of 50°F (10°C) for the first three to seven days after placement. Achieving this requires active heating when ambient conditions fall below this threshold. The consequences of inadequate curing include:
- Reduced compressive strength at 28 days, sometimes by 50 percent or more
- Surface scaling and delamination caused by freeze-thaw cycles
- Increased permeability, leading to corrosion of reinforcing steel
- Delayed formwork stripping and construction schedule disruptions
- Higher long-term maintenance and repair costs
The North Toronto Subway Expansion: A Real-World Case
A concrete heating case that illustrates these challenges well is the North Toronto subway expansion project. Concrete was being placed in an underground shaft for this major transit infrastructure project. The contractor faced a difficult situation: the cold and damp conditions deep in the excavation prevented proper curing during winter months. If concrete placement had to stop during winter, the entire project timeline was at risk, leading to significant cost overruns and delayed public benefits.
The jobsite presented specific complications that made standard heating approaches difficult. Crews were working 150 feet below ground level across nine separate placement areas spread over half a mile. The solution required a custom-engineered heating system that could operate reliably at depth while meeting the stringent requirements of underground infrastructure work.
Heating Equipment and Configuration for Deep Shaft Placement
Selecting the Right Heating Equipment
For the North Toronto project, Aggreko designed a system built around industrial-grade heaters and generators capable of operating in demanding underground conditions. The equipment package included nine 150 kW heaters paired with three 300 kW generators with auxiliary fuel tanks. Each generator powered two of the 150 kW heaters, while the remaining three heaters drew power from site utilities.
The following table summarizes the key equipment and specifications used in this application:
| Equipment Type | Quantity | Specification | Function |
|---|---|---|---|
| Industrial Heaters | 9 | 150 kW each | Provide heated air for curing concrete |
| Generators | 3 | 300 kW each | Power heaters in areas without site electricity |
| Auxiliary Fuel Tanks | 3 | Integrated with generators | Extended runtime without refueling |
| Step-Down Transformers | Multiple | 600 V to 480 V | Match heater voltage requirements to site power |
| Skid Mounts | 9 | Custom-fabricated | Allow forklift mobility of heaters in shaft |
| Ductwork | Custom | Varied lengths | Route heated air to strategic placement points |
Power and Electrical Configuration
One of the technical challenges in this setup was the voltage discrepancy between the site utilities and the heater requirements. The heaters required 480 V, but the house voltage on site was 600 V, standard for Canadian industrial installations. Step-down transformers were used to reduce the voltage, allowing the heaters to operate safely and efficiently.
The electrical installation followed a clear division of responsibility:
- Aggreko provided the heaters, generators, transformers, and primary cable runs
- The customer’s own electricians performed all final connections and grounding
- Aggreko craned the equipment down the shaft and ran hoses and cables between components
- Ductwork was routed to distribute heated air at key locations throughout the shaft
This collaborative approach ensured that the heating system met both operational needs and local electrical code requirements. Proper grounding was especially critical given the damp underground environment, where electrical safety hazards are elevated.
Installation Logistics and Operational Considerations
Moving Equipment Into Deep Shafts
Installing heating equipment 150 feet below ground is a complex logistical operation. For the North Toronto project, the equipment was craned down into the shaft in a carefully sequenced process. Each heater and generator had to be lifted, lowered, and positioned without damaging components or creating safety hazards for workers below.
Once in the shaft, mobility became the next challenge. The heaters were mounted on custom-fabricated skids so that a forklift truck could move them from one placement area to another as work progressed across the half-mile-long site. This flexibility was essential because concrete placement happened in nine different zones, and each area required heated air at different times during the curing cycle.
Cable Management and Ductwork Distribution
Operating at depth introduced specific constraints around cable and ductwork routing. The team had to hang cables without placing excessive weight on any single support point. This required careful load calculations and the use of multiple suspension points distributed along the shaft walls. The key considerations included:
- Weight distribution: cables and hoses were supported at regular intervals to prevent overloading any single anchor
- Clearance: all runs were positioned to avoid interference with ongoing concrete placement and formwork operations
- Accessibility: routing allowed for inspection and maintenance without disrupting other trades
- Safety: all cables were secured away from walkways and work areas to prevent tripping hazards
Addressing Generator Cold-Start Problems
The generators themselves faced challenges starting and stopping in the cold environment. Even though they were providing heat to the concrete, the equipment had to be able to start reliably in near-freezing conditions. This required additional cold-weather starting aids and careful monitoring during the early stages of each heating cycle. Battery heaters, block heaters, and synthetic oil with better low-temperature viscosity were among the measures used to ensure reliable operation.
Results and Best Practices for Cold Weather Concrete Curing
Project Outcomes
The heating system delivered measurable results. Aggreko designed, delivered, and installed a custom heating solution that met all project specifications. The temporary heaters sped up the curing process sufficiently to allow concrete placement to continue through the winter. The North Toronto subway expansion project was completed with no further delays directly attributable to cold weather curing issues.
This outcome reinforced several important principles for cold weather concrete work. Properly designed heating systems can maintain curing temperatures even in extreme conditions deep underground. The investment in temporary heating equipment is far smaller than the cost of schedule delays, winter shutdowns, or remedial work on improperly cured concrete. Contractors working on similar projects should also review How to Protect Freshly Placed Concrete During Curing for additional protective measures that complement active heating.
Key Takeaways for Contractors
Based on this case study and established industry standards, here are the essential steps for planning concrete heating during cold weather curing:
- Assess the temperature requirements early. Review ACI 306 guidelines and determine the minimum temperature needed for your specific concrete mix design. Factor in ambient conditions, formwork type, and section thickness.
- Size the heating equipment properly. Calculate the heating load based on the volume of concrete, surface area exposed to cold, and desired temperature differential. Undersized heaters will not maintain temperature, while oversized units waste fuel and create uneven conditions.
- Plan for power distribution. Verify voltage compatibility between heaters and site power. Include transformers, cable runs, and grounding in the planning phase. Coordinate with the electrical contractor early to avoid delays.
- Design for mobility. In projects where concrete is placed across multiple zones, heating equipment must be movable. Skid mounting, wheeled carts, or trailer mounting should be considered from the start.
- Address equipment reliability. Cold weather affects not just concrete but also the heating equipment itself. Cold-start aids, proper fuel management, and regular maintenance checks are essential.
- Monitor and document temperatures. Use temperature sensors and data loggers to track concrete temperature throughout the curing period. This documentation is valuable for quality assurance and can be critical for warranty and dispute resolution.
Integrating Heating with Broader Curing Practices
Active heating should never be the sole curing strategy. It works best as part of a comprehensive approach that includes insulating blankets, windbreaks, heated enclosures, and proper formwork management. For decorative concrete applications, additional considerations around color consistency and surface finish apply, which is why Colorful Concrete Tiles a Complete Guide to Decorative provides useful guidance on maintaining quality in challenging conditions.
In congested reinforced concrete members where rebar spacing is tight and concrete flow is restricted, proper curing heat distribution becomes even more critical. The heating system must deliver uniform temperatures throughout the section, including around dense reinforcement. For tips on managing concrete placement in these conditions, refer to a Guide On How to Consolidate Concrete in reinforced sections.
Concrete heating during curing is a proven technique that enables year-round construction in cold climates. The North Toronto subway expansion project demonstrates that with proper planning, the right equipment, and a systematic approach, concrete can be placed and cured successfully even 150 feet underground in winter conditions. The key is to start planning early, select equipment that matches the specific demands of the site, and integrate heating into a complete curing program that addresses temperature, moisture, and protection requirements.