Warming up concrete, why is it needed and how to organize it
When concreting foundations and pouring monolithic structures at low temperatures (minimum below 0°C and daily average below 50°C), heating of concrete according to SNiP "Bearing and enclosing structures" must be carried out without fail. A variety of methods can be used to ensure warming up, and we will describe the most popular in our article.
In order for the solution to solidify well in the cold, it must be additionally heated.
Using cables
Laying the heating wire in the formwork
Another technique involves the use of heat-carrying cables, which are laid in the formwork and, when current passes through them, heat the solution:
For work, we take PNSV conductors in polyethylene or PVC insulation. The second option is preferable for use in a reinforced structure, since PVC does not melt, which means that the risk of a short circuit to the reinforcement will be minimal.
Note! PVC loses elasticity in the cold, so when laying the wire, you need to be careful not to damage the insulating layer at the fold. Usually heating is carried out by pieces of PNSV wire with a diameter of 1.2 or 1.4 mm
The material is cut into standard pieces (17 or 28 m depending on the configuration) and twisted into spirals with a diameter of about 30 mm for a more compact installation.
Usually heating is carried out by pieces of PNSV wire with a diameter of 1.2 or 1.4 mm. The material is cut into standard pieces (17 or 28 m depending on the configuration) and twisted into spirals with a diameter of about 30 mm for a more compact installation.
Typical wiring diagram for concrete heating
- Then the spirals are connected into several "triangles" or "stars" (the diagrams are shown in the figures), and are assembled into several common tires.
- Since the energized PNSV cable quickly burns out in air due to low heat dissipation, the heating circuits inside the formwork are connected to a current source using thick aluminum wires - the so-called "cold ends".
TSZP transformer
"Cold ends" are connected to the terminals of the step-down transformer. For work, it is best to use systems such as SPB-40, KTPTO 80 and their analogues, since they provide regulation of the activity of the entire heating system.
The heating process itself is divided into several phases:
| Phase | Temperature dynamics |
| Primary curing | No current is applied, the temperature of the solution is maintained due to the chemical reactions of the material |
| Preheat | The current is supplied to the terminals of the transformer, the solution gradually warms up to 700C. The rate of temperature rise should not exceed 100C per hour. |
| Isothermal heating | The longest stage. The current is supplied during the entire time of curing, incorporated in the project. Heating control is carried out: it is impossible to raise the temperature above 800C, otherwise the cement granules will begin to sinter, which will disrupt the hydration process. |
| Cooling | The decrease in temperature occurs gradually, at a rate of about 4-50C per hour. |
During all this time, the transformer regulates the strength of the current flowing to the conductors. Upon completion of the heating, the contact conductors are dismantled, and the PNSV wire remains in the thickness of the concrete.
Strength of concrete with antifreeze additives
Antifreeze additives are added to concrete taking into account the ambient air temperature in which concrete will have to be worked. The strength of such concrete by the time of cooling to the design temperature (according to the amount of additives) should be in ...%:
- 30% - when using concrete grades up to M200 inclusive
- 25% - when using concrete grades M300 and M400
Concrete above the specified grades, having gained strength of 30% and 25%, can be frozen, but after thawing, concrete structures must gain the remaining strength to 100% under conditions that will ensure the development of this design strength, before loading these structures with a load.
The set of strength of concrete is ensured by the correct preparation of concrete during its preparation, as well as by protecting structures after concreting, from the effects of contact with wind and sub-zero temperatures.
