At what temperature can concrete be poured outside: minus, minimum, in frost

There are particular difficulties when pouring concrete in cold weather. Knowing the ideal circumstances for exterior concrete pouring can mean the difference between a structurally disastrous project and a successful one. Concrete’s curing process slows down in colder weather, which may have an impact on the material’s strength and durability. For any construction project, knowing the minimum temperatures required to pour concrete is essential.

It is possible to pour concrete in cold weather, but there are some safety measures to consider. In general, temperatures below 40°F (4°C) should not be used to pour concrete. Further precautions must be taken to guarantee that the concrete cures properly if the temperature drops below this point. These precautions could include applying chemical accelerants, insulating blankets, or heaters to aid in the proper setting and curing of the concrete.

Pouring concrete becomes even more difficult in very cold weather, such as when the temperature falls below freezing. The water in the concrete mix may freeze at these temperatures, weakening and cracking the concrete. Contractors frequently use heated enclosures or specially formulated mixes that can withstand the cold to pour concrete in subfreezing temperatures. By using these methods, the water in the concrete is kept from freezing and the proper setting of the mixture is ensured.

In the end, even though concrete can be poured in cold weather, it is crucial to be aware of the risks and take the appropriate safety measures. Concrete poured in cold weather can retain its strength and integrity over time with the right planning and techniques. Taking the proper precautions can result in successful concrete projects even during the winter, regardless of whether the weather is freezing or just mildly cold.

The process of gaining strength of concrete structures

It is first necessary to take into consideration, at least briefly, the characteristics of the process of gaining strength in a monolith in order to determine the temperature at which concrete can be poured. When cement and water are mixed, a reaction takes place. The concrete is still workable and fluid in the early hours but starts to solidify after a few hours, thickening and eventually becoming completely hard.

Hydration is the term used to describe the interaction between cement and water. The mixture hydrates in two stages: setting and hardening. When aluminates are used in the setting, needle-shaped crystals that are interconnected form. These crystals form a sort of framework, or skeleton, after 6 to 10 hours. The concrete starts to solidify.

Depending directly on the surrounding temperature, the entire setting process may take anywhere from 20 minutes to 20 hours. The cold season is when the process takes the longest; at around zero degrees Celsius, the concrete starts to set after six to ten hours, and the stage lasts for fifteen to twenty hours.

Clinker minerals react with the water in the solution during the hardening process, progressively forming a silicate structure. The reaction causes tiny crystals to form, which come together to form a distinctive structure with fine pores. This concrete retains its shape and structure but already has increased brand strength and durability in just 28 days.

The ideal temperature and humidity levels for the hardening stage are likewise +20 degrees and 100%, respectively.

Deviations from the parameters have a significant impact on strength: the hardening rate varies over time, and the monolith takes several years to fully mature (although the set of design strength must be completed 28 days after pouring).

The effect of negative temperatures on the hardening of concrete

As was previously mentioned, the outside temperature has a significant impact on the rate of hydration. Thus, hardening happens five times slower on average when the temperature drops from +20 to +5 degrees. The reaction rate decreases with decreasing temperature. Hydration completely ceases when the temperature falls below zero (the water just freezes).

Water has a tendency to expand when it freezes, which raises the pressure inside the concrete solution and breaks down crystal bonds that have already formed. It is no longer possible to restore the concrete’s structure because it has been destroyed. Furthermore, big fillers may become encased in the ice that forms in the mixture, which would prevent them from sticking to the cement. All of this considerably weakens the structure’s monolithicity and diminishes its strength.

Hardening proceeds when the water thaws, but the concrete’s structure has already been distorted. It is possible to see deformation, delamination, cracks, and the separation of reinforcement and large fillers from the monolith. The strength indicator will decrease with the time that freshly poured concrete freezes.

• When the ambient temperature is at +5 C and below, and no measures are planned to warm up or increase the frost resistance of concrete.
• In the off-season – when the temperature is unstable, strong jumps in both the thermometer readings and humidity are noted.
• If the thermometer shows a temperature of +25 degrees and above, and the air humidity is below 50%. At such times, it is better to use special cements or not to carry out work, since the hydration process will occur very quickly: the water will evaporate, and the concrete will not have time to gain strength, as a result of which cracks, deformations, delamination, etc. often appear.d.

• Pouring concrete at sub-zero temperatures without warming up for at least 3 days to a mark of +10-30 degrees.
• When concrete with special additives has already been prepared, and a thaw suddenly sets in outside or the air humidity becomes higher than 60%, it starts to rain, etc.d.
• In case of inability to determine the optimal heating mode, adjust devices, control concrete in frost. After all, both frost and overheating are equally terrible for concrete.

Temperature of concrete: +30, air temperature: +20, humidity: 100%.

Concrete needs particular conditions to cure, so pouring it in the winter creates its own set of difficulties. Although temperatures as low as 40°F (4°C) can be used to pour concrete, extra care must be taken when working in frost or below freezing temperatures. To speed up the curing process, this involves utilizing additives, heated enclosures, and insulated blankets. Concrete’s strength and durability can be guaranteed even in winter conditions by being aware of and responding to these issues.

Concreting in winter

In a number of situations, such as when it would be unfeasible to halt construction for the entire season or for urgent purposes, concrete may need to be used in frost. Concrete needs to be heated due to the negative effects of below-freezing temperatures on the material and its technical properties. Deformations may occur if the temperature inside the solution is higher than the outside temperature.

The critical strength indicator is reached by heating the concrete. In the event that the design documentation lacks this information, the value is interpreted as being 70% of the design strength. The critical strength is 85% of the design when water resistance/frost resistance values are required.

• Heating the components themselves to prepare the mixture.
• Using the thermos effect.
• Implementation of electric heating.
• Using steam heating.

Therefore, the lowest temperature at which concrete can be poured is beyond all doubt. In order to maintain the material’s technical characteristics and the fundamental standards for strength, dependability, and durability, the task is to prepare the mixture and the object as best as possible given the working conditions.

Heating all of the materials needed to prepare the concrete is the easiest and least expensive option. They are heated to a minimum of +35–40 degrees when the concrete is poured.

Crushed stone/sand is heated to +60, water to +90, and cement is simply kept in a warm room until it reaches room temperature. All other materials are heated, with the exception of cement. After that, everything is combined and poured.

The thermos method

This choice is important when pouring large structures. The laid mixture must show a temperature of at least +10 degrees (better more), but no further heating is supplied. By using this technique, the poured mixture will have enough time to reach a critical strength while cooling.

According to this method’s basic idea, the concrete undergoes an exothermic reaction that initiates the hardening process and releases heat. Concrete will therefore self-heat. When heat loss is taken into account, concrete can warm up to +70 degrees Celsius.

Heat-insulating materials dependable shield the formwork, preventing heat loss from the concrete during the hardening process. Water does not freeze, and the internal structure of the concrete monolith is not destroyed as it progressively gets stronger. Since it doesn’t require any equipment, this method is thought to be the easiest and most cost-effective when pouring foundations during the winter.

Electric heating of concrete mix

Many people consider electric heating as a solution when considering the temperatures at which concrete can be poured. Numerous techniques are available for carrying out heating, including the use of electrodes, the induction method, and different electric heating devices.

• Electrodes are introduced into the freshly poured mixture.
• Then current is applied to the electrodes.
• As the current passes through the electrodes, they heat up, transferring heat to the concrete.

Since direct current will trigger the electrolysis process, which results in the release of gas, the current must be alternating. All electrode surfaces are shielded by the gas, which causes a large increase in current resistance and a discernible decrease in heating. Concrete that has reinforcement inserted into it may be utilized as an electrode.

It is essential to make sure the concrete is heated uniformly and to a maximum of +60 degrees in order for this method to function. In these situations, the average amount of electricity used per cubic meter of concrete mortar is between 80 and 100 kW*h.

Since there are several challenges in implementing induction heating, it is not widely used. This kind of concrete mixture heating operates on the basis of high-frequency currents heating electrically conductive materials contactlessly. Thus, current is run through an insulated wire that has been wound around the steel reinforcement. As a result, induction occurs, heating the concrete and the reinforcement simultaneously. A cubic meter of concrete typically uses 120–150 kW*h of electricity.

Several techniques are used when using electric heating devices to lessen the detrimental effects of frost on the mixture’s hydration process. For instance, these could be heating mats that are connected to the network after being arranged on the concrete. Over the poured monolith, you can erect a tent-like structure, insert a heat gun, and apply heat.

Here, it’s crucial to keep the concrete moist to prevent it from drying out during the heating process, which also has a detrimental effect on its strength and quality (when freezing). One cubic meter requires 100–120 kWh of power, assuming an ambient temperature of about -20 degrees.

Steam heating of concrete in winter

Heating the concrete with steam makes sense when the outside temperature is zero or lower. This technique works particularly well for buildings with thin walls. The inside of the formwork has channels created for the passage of steam. Occasionally, two walls are formed with double formwork, and steam is circulated between them. It is possible to put pipes in the concrete and run steam through them.

You can heat the concrete to between +50 and +80 degrees using this method. The hardening process is accelerated multiple times by the high temperature and ideal humidity levels. Thus, concrete that has been steam-heated for two days gains strength equivalent to what it would under normal circumstances harden in a week.

This method’s main disadvantage is how much money, time, and effort it takes to put into practice.

Use of additives in frost

Anti-freeze additives and specific chemical accelerators for concrete hardening are widely used today. These additives are typically calcium carbonate, sodium nitrite, and chloride salts, among others. Additives initiate the hydration of cement, thereby lowering the freezing point of water and raising the temperature at which concrete hardens.

It is possible to prevent the need for heating by adding additives to the mixture. Certain additives can make concrete so much more resistant to frost that it eliminates the need to consider whether concrete can be poured in below-freezing temperatures because hydration can happen even at -20 degrees outside.

However, additives do have some drawbacks in addition to all of their benefits.

• They adversely affect the reinforcement – the process of corrosion may begin, so it is relevant to introduce additives only into uninhabited concrete.
• Additives allow concrete to gain strength equal to a maximum of 30% of the design strength, and then when the mixture thaws (at a positive temperature), the strength gain process continues. In this regard, according to SNiP, additives cannot be introduced into concrete operating under dynamic loads (hammers, vibratory machines, etc.d.).

– make cement’s silicate components more soluble by reacting with hydration products to produce basic and double salts, which lower the liquid’s freezing point in the solution.

Condition	Temperature
Minus	Concrete should not be poured below 5°C (41°F)
Minimum	Ideally, pour concrete at temperatures above 10°C (50°F)
In frost	Special precautions are needed for pouring below 0°C (32°F)

Although pouring concrete in cold weather has its own set of difficulties, it is completely doable with the right safety measures. Concrete can still be poured in below-freezing temperatures as long as precautions are taken to guarantee that the mix cures properly. Concrete can normally be poured in as low as 5 degrees Celsius (41 degrees Fahrenheit), but when the temperature gets closer to freezing, extra caution is required.

Using cold weather concrete mixes and curing-accelerating additives is essential for a successful concrete pour in colder climates. Preheating the aggregates and mixing water beforehand can also aid in preserving the proper temperature. During the crucial first curing stage, protective measures like heated enclosures or insulated blankets can keep the concrete from freezing.

It’s crucial to keep an eye on the weather report and make preparations for temperature drops. Concrete integrity can be protected by taking preventative measures, such as using insulated forms or curing compounds, if frost or freezing temperatures are predicted. Recall that strong, long-lasting concrete requires careful planning and patience, especially in cold weather.

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