Foundation pouring is the crucial stage of construction. An important factor that ensures the strength of the foundation of the house is not only the quality of concrete, but also the weather conditions. At what temperature can the foundation be poured in order not to disturb the setting and hardening of concrete? It depends on the brand of concrete, as well as on the presence of additives that lower the temperature of crystallization of water.
At what temperature can the foundation
be poured? Concrete hardening process and its characteristics
After pouring the foundation, the concrete passes through two stages: setting and hardening. The setting time does not exceed a day, in this phase the concrete solution passes from the liquid phase to the solid phase. But its strength is not high yet. After setting, the hardening process begins, it lasts much longer and is determined by many factors: the brand of concrete, its composition, the presence of additives, the temperature and humidity regime.
The optimum conditions for setting and hardening ordinary concrete are considered to be a temperature range of 3 ° to 25 ° C.At the same time, the process speed has a direct temperature dependence - the warmer, the faster the setting of concrete and the strength set, and it also increases with the increase in the grade of concrete.
The concrete grade is a characteristic of its compressive strength after aging under normal conditions for 28 days. That is, after four weeks, concrete of any brand acquires its design strength, after which its hardening continues for several years with extremely low speed.
There is another characteristic of concrete - critical strength. It is expressed as a percentage of the design and indicates that the degree of hardening, in which the effect of low temperatures does not entail irreversible changes in the structure of the concrete and curing processes. The share of different brands is different, its values are given in the table.
Table between the critical strength depending on the grade of concrete
The rate of hardening of concrete varies in time nonlinearly and depends on temperature. It can be determined from the graphs shown in the figure. The graphs are constructed for different ambient temperatures, with daily fluctuations taken into account and the lowest temperature value. The temperature of concrete above 35 ° C is achieved by heating the filled solution.
The strength of the concrete, depending on the aging time in the
clock. When the temperature drops below 3 ° C, the phase transformation of water into ice begins, with the structure of the non-cured concrete broken, and microcracks appear that will subsequently lead to its destruction.
Lowering the temperature below the zero mark leads to a complete stoppage of the hydration process of cement, the hardening process completely stops. After raising the temperature and thawing, the process can resume, but the reverse restoration of the destroyed bonds will not happen. However, if the concrete has reached critical strength, lowering the temperature will not cause irreversible changes in it, and after thawing the hardening process will resume.
Example of determining the possibility of foundation pouring
Condition: the foundation is poured at a temperature of 5 ° C, with the weather forecast promising cooling to minus temperatures after 72 hours. Will this affect the strength and characteristics of the M300 concrete?
For M300 concrete, the critical strength is 40%.As can be seen from the graph, at a temperature of 5 ° C the critical strength of concrete in 40% will be collected only after 130 hours. Conclusion - the foundation can not be filled.
How to fill the foundation at minus temperature?
To accelerate the setting and hardening time to critical strength, the following techniques can be used:
- Provide an increase in the temperature of the mortar or the embedded foundation.
- Choose a higher brand of concrete.
- Use additives and plasticizers.
The temperature rise of the solution is achieved by heating its components to 30 ° C and using hot water at a temperature of 70 ° C.When mixing the solution, hot water is added to the granular filler, and only then cement is injected into the solution.
The poured foundation is either insulated, covered with foil and heat-insulating materials, or erected a temporary heated structure. As heat insulation materials, you can use straw, sawdust, peat, snow.
You can increase the brand of concrete by increasing the cement content in it or using a higher grade dry cement. Determine the content of components can be done by calculation.
Additives and plasticizers are used to give concrete a variety of properties: accelerate the hardening process or lower the freezing point of water. Possible additives are listed in the table. When using the additive, mix with water.
Additives and plasticizers applied to concrete
Large construction companies use these technologies for the erection of concrete structures at any time of the year. But with private development it is necessary to evaluate the feasibility of foundation pouring in cold weather. Perhaps, it is easier to use the rule: fill the foundation at a temperature above 5 ° C with the condition that it is not expected to decrease in the next 28 days. In addition, it should be remembered that you can not leave an unloaded shallow foundation for the winter - this will lead to its destruction.
