Here in Colorado, we regularly experience “fool’s spring.” That’s when the weather pattern and our high altitude bring us lovely spring weather, then slam us with a final snowy blast or two before winter finally cedes to spring.
For example, the first week of April brought us highs in the 70s and a couple of days that neared 80° F. On mid-April day brought mixed snow and rain in the afternoon and overnight lows in the 20s, leaving up to 9 inches of snow overnight near Denver. There was a 70% chance of another inch the next day, and more snow predicted for the next week.
Likewise, in the middle of a typically balmy Rocky Mountain autumn, an early snowstorm regularly visits in October or even as early as September. If you have a standard pour scheduled at that time, you will likely encounter challenges that a volumetric mixer truck helps you easily solve.
Weather here is more dynamic than in many parts of the US and worldwide, but it’s a reminder that being prepared to pour concrete in non-ideal and even extreme temperatures, particularly when conditions can change rapidly, can keep jobs on time and customers happy. There’s no better way to adapt to cold weather or scorching temperatures than with a volumetric mixer truck.
Read on for details, but the biggest advantages a volumetric mixer brings to a pour in non-ideal temperatures include eliminating issues with timing, radically shortening the time where controlling or influencing the temperature is necessary, and the flexibility to modify the concrete mix on the fly, minute by minute, based on conditions and if they change.
The Impact of Various Temperatures
The “ideal” temperature to pour concrete is generally accepted to be between 40- and 60-degrees Fahrenheit (approximately 4° to 15° C). When fall arrives and temperatures start to drop, cold creates a couple of key problems with your pour.
First, there’s the threat of the concrete freezing before it gains strength. Second, the cold slows curing. It slows significantly below 50 F, and below 40 F the hydration reaction effectively stops, and the material doesn’t gain strength. Extremes of either hot or cold can lead to reduced strength, more cracking, and compromised durability.
For this reason, in many places with temperatures that approach freezing and drop even lower than that, concrete work has a clear seasonal shift that slows significantly in the winter. So much so that we have a video on how to winterize our trucks.
When hot weather comes, basically the opposite scenario occurs. Water evaporates too quickly. Curing is accelerated. The material can become harder to handle quickly. The temptation to add water can compromise quality and strength and shorten the time available to place and finish the material.
Depending on how cold it gets, filling your volumetric mixer with hot water can provide enough heat to keep concrete in the temperature range where it cures properly. In some cases, particularly extreme cold, you can even heat aggregates and add chemicals to the mix to work around mother nature. Warming the ground is also an option, as its temperature changes much more slowly than air temperature, and it can be much colder than the air temperature, causing problems if you pour concrete directly onto it. The additional time and energy to take these steps does, of course, mean added expense.
Adding accelerators is also an option and using calcium chloride (CaCl₂) is common given its cost and effectiveness. Be sure to carefully calculate how much to use, or it can cause problems like rebar corrosion or discoloration. Pricier accelerators are also available, along with plasticizers to help combat the cold and get the finished concrete you need.
Mass is also a significant factor. A larger pour will retain heat longer and lead to slower drying and more time to cure. If possible, especially if the temperature varies significantly from day to night, schedule your pour during the warmest part of the day. Finally, you can cover the pour with insulating blankets to slow heat loss and lengthen cure time.
Pouring freshly mixed concrete on site eliminates any issues with timing, how far/long it is to the site, and more. If the weather changes, no problem. When a storm rolls in and temperatures drop, or the weather breaks and the sun comes out, just change the mix to adapt to changing temperatures.
We also have techniques to ensure a better pour when the weather is hot enough to cause problems with traditional pours.
Temperature of the air and ground, along with humidity, can make adjusting the concrete mix challenging. Failure to adapt impacts the strength and durability of the finished slab. Rapid evaporation of water accelerates setting time. Water rapidly leaching out of the surface layer can render your mix too dry and lead to surface cracking. Loss of slump means loss of entrained air. The volume of your wet slab is larger than your dry slab, and more evaporation means more shrinkage. At the same time, adding water can also reduce strength and durability.
Higher temperatures mean a faster set time that makes it difficult to level and finish in time and place joints correctly (place at smaller intervals than when pouring in the cold), leading to surface deformities and unevenness. If the hydration process happens too quickly, compressive strength suffers, while cooler concrete has more time to hydrate and allow crystals to form and strengthen.
What can you do to manage these variables? Pour during the coolest part of the day. Dampen the subgrade to keep moisture in the mix. Shade/cover and wet down all your equipment until using it. along with the forms and reinforcement. Store component materials out of the sun to minimize heat they absorb, and pre-wet aggregate to cool it off. Use chilled water, or even add ice to the water in your volumetric mixer. In some cases, liquid nitrogen to cool water, aggregates, or the mixed concrete itself is an option. Consider adding fiber, or additives that slow setting time, improve strength, and provide corrosion protection.
Adequate hands-on deck – more workers to help finish the slab – is key so they can begin finishing as soon as possible. Be sure your crew is protected and hydrated, too. Use sunshades and wind breaks to slow evaporation.
Control For A Better Pour
Like with any concrete pour, controlling or at least buffering all the variables you can control will lead to a better pour. While you can’t control ambient temperature, and you can only influence surface temperature a bit, you can control pour temperature and how you protect the concrete, while doing whatever you can to control the rate of either cooling or heating of the slab, as needed.
The only option that gives you the most control over these variables is to mix and pour volumetric concrete on site, in real time.
Do you regularly pour concrete in hot or cold temperatures outside the ideal range for curing concrete? Contact us today, or schedule a meeting today to learn how volumetric mixers give you several ways to control the variables that cause problems with traditional pours in hot or cold weather and allow you to provide a quality pour, every time.