​​​Extreme weather? No problem. How our QC teams keep your job moving.

Weather predictions for this winter are, well, unpredictable.

With El Niño’s return, Toronto and Chicago saw freakishly warm days in December, with the mercury climbing to 60° F (16° C). When warm currents collided with the polar vortex, Chicago temperatures plummeted from 48° to -1° F (9° to -18° C) on the same day in January. The watchword is whiplash, says the Chicago Tribune. Meanwhile, Florida has recorded one of its warmest winter months ever.​

Does this volatility and different climate affect concrete placement? Thanks to extensive experience and carefully defined best practices, the answer is no, say Technical Services experts at our North American affiliates.

“We’ve had a lot of experience with volatile weather, so we’ve developed effective protocols for getting temperatures right while maintaining the placement and finishing characteristics required for a specific project,” says Gary Hall, Senior Technical Support Rep at Prairie Material in Chicago.

Cold-weather protocols at the site, plant

With most outdoor flatwork shut down for the season, the focus in northern markets shifts to structures: high rises, low-rise footings and foundation walls, enclosed floor slabs, caissons and drilled shafts, according to Nat Morlando, Technical Solutions Manager at Canada Building Materials (CBM) in Toronto.

 

High-rise construction has seen a surge in Toronto over the past decade, with more than 90% going toward residential towers to house the 100,000+ new residents arriving every year.

Around 90% of projects are concrete construction, Morlando reports.

 

 

 

n cold temps, high-rise contractors in Toronto and Chicago wrap the floors and use supplemental heat below the elevated decks to warm the deck forms before placement. Poured columns, walls and forms can be wrapped in curing blankets to help with set time. Placement booms and pipes can be insulated. Some projects make use of embedded sensors to determine stripping time based on a strength maturity curve that is matched to the temperature probes and set ahead of time in the lab.

“Our job is to make sure the concrete is delivered to the job at the temperature specs set by the ACI for the application,” says Gary Hall. That means somewhere between 50° to 70° F (10° to 21° C) for cold- weather concrete. “Our target is 65° ± 5° F (16 to 21° C) at time of placement.”

 

Warming materials and drums

When the mercury drops, warming the materials is the only way to meet temperature specs. Hot water is the primary engine, and many yards heat the sand pads as well. The main mixer has temperature sensors that measure sand and water temperatures to properly blend the hot and cold water, achieving ACI target temperatures.

Drivers do their part, too. At the beginning of the day, all truck drums are heated, and after washout the drums are reversed to get rid of any standing water. Since the mass of concrete in the drum helps to maintain temperatures, smaller loads lose temperature faster, so minimum load restrictions are strictly enforced, says Hall.

Accelerators for consistent set time in low temps

Consistency of set time is key to good concrete in the field. Winter mixes add various levels of accelerators, such as calcium chloride for non-reinforced applications and non-chloride accelerators (NCAs) for structural projects.

CBM’s proprietary GetSet mix​ incorporates NCAs with additional cementitious materials to aid in finishing. Prairie Material’s mix experts also use NCA-based mixes, often tailoring the design to control set times for specific customer requirements, such as a 24-36 hour post-tensioning cycle or to maintain a 2-days/floor construction schedule.

Adding more accelerating admixture as the pour progresses will provide a more consistent set by offsetting the effects of cooler mixing water later in the day. It will also help all the concrete in the pour reach set at about the same time to facilitate finishing. Adding more cement to the mix also helps boost strength gain.

Quality control is critical in variable conditions, Hall says. Technicians monitor the temperatures of the first loads and then at least once per hour. “Every time we check a load for slump or take cylinders, the temperature is taken as well. QC in the field is in constant contact with the yard to make sure the concrete properties are right for conditions,” he explains.

No winter pause for below-grade projects

Caissons, drilled shafts and deep tunnel projects continue to pour all winter because concrete is placed below the frost line. Right now, Prairie is providing concrete for caissons in the new ConRAC project at O’Hare Airport. Prairie Yard 32 is also producing mix for caissons below the surface of the Chicago River for Phase Three of the city’s Riverwalk expansion project.

CBM recently completed construction on three subway stations for the Toronto Transit Commission (TTC), all year-round projects. “There was quite a bit of mass concrete at these sites,” Morlando says. “Stations and open-cut stations incorporating walls, inverts and massive roofs required careful preparation and monitoring.”

Enclosed-floor slabs like warehouses and big-box stores can be poured throughout the winter, since they use torpedo heaters to reach the required ambient temperature for placement. Proper venting is critical, since the exhaust from the heaters can be a safety hazard – and can also cause carbonation of newly placed concrete surfaces, resulting in dusting at the surface.

Heat’s on, but no worries for Florida builders

On the other end of the spectrum is hot-weather construction. You’d think in places like Florida, high heat and humidity would be more challenging, but you’d be wrong, according to Kelly Stanton, Commercial Sales and Marketing Manager at Votorantim affiliate Prestige Concrete Products.

“Florida is a great area to place concrete any time of the year,” says Stanton. “Cold is rarely an issue and we know how to deal with the heat.”

All Prestige concrete is formulated for 100° F (38° C) temperatures, per the Florida DOT standard, Stanton says.  The mixes use a Type F fly ash and make use of water-reducing admixtures to keep the heat of hydration low and maintain the plastic properties. Ice and other cooling methods, often employed in the Midwest and Canada, are rarely required in Florida.

Most commercial work is placed at night, but traffic and afternoon rains, not heat, drive the decision. “Generally, we start a pour at 1 or 2 a.m. to be done by 7, and then complete the remaining pours by 3 or 4 in the afternoon, when the rains set in,” Stanton says.

In spring and fall, Florida contractors face 20 to 30 days of dry, hot, windy conditions that require extra care with curing. On those days the contractor may add an evaporation retarder to help reduce shrinkage cracking. Prestige plant personnel also empty aggregate bins more frequently to keep the material cooler “As long as the mix is less than 100 degrees at placement, it can be poured,” Stanton notes.

It’s a different story up north

In northern climates, hot-weather standards are tougher. ACI regulations in Chicago are 80 to 85° F (27 to 29° C) maximum temperature at placement for most concrete. Some high-performance concrete can go as high as 90° F (32° C.) In Canada, the DOT has an upper limit of 28° C (82° F) while the CSA Group limit is 30° C (86° F).

Typically, northern plants will cool the aggregate with soaker hoses and use ice as well as liquid nitrogen in summer months. In Canada, CBM increases the slag in the mix to as much as 50%. Contractors can wet forms, subgrade and reinforcement prior to placement.

Mass pours (anything deeper than 36″, or 92 cm) have their own requirements, which vary by application. For example, the ongoing deep tunnel project in McCook, Illinois has a very tight specification of 70° F (21° C) for mass concrete at placement, set by the US Army Corps of Engineers. To meet that standard, Prairie crews use liquid nitrogen in hot weather to cool the concrete without adding water. CBM also has liquid nitrogen setups at four of its 40 plants, relying on them for mass pours at the Toronto subway stations.

What QC challenges can we help you overcome?

Close collaboration between your engineering and construction teams and our concrete experts is the best way to plan for continuous progress in volatile weather. Let us know how we can support your needs.

Visit our websites for details on all-weather construction in Canada, Florida and the Midwest. ​