...

Monolithic works: pouring concrete in winter

This post looks at using monolithic pours to set concrete in winter. Monolithic pouring offers cost-savings, speed, and strength, as well as reduced labor costs. It also reduces stress cracking, offers greater crack control, and cuts overall construction time. Additionally, the superior surface finish and heightened durability that comes with monolithic pours make them the preferred choice for winter concrete works.

Recommendation points



There is an unspoken rule in construction: lay the foundation and erect the box in the summer, and in the cold season, engage in glazing, interior decoration and communications. But to get winter discounts from construction companies and material suppliers, you have to carry out concrete work in winter.

Pouring the foundation in the winter

Features of the behavior of concrete at low temperatures

Portland cement is the main binder in vintage concrete. This substance is of the hydration principle of hardening, that is, it only needs increased moisture to form a monolithic structure. Acquisition of design strength by concrete occurs within 28 days, while in the first 1.5โ€“2 weeks, concrete hardens most intensively and gains about 2/3 of the design indicators.

The first thing you need to know is that at low temperatures the curing of concrete does not stop, it slows down greatly, but at the same time it is restored when favorable conditions appear. Thus, freezing of the concrete mixture does not lead to its irreversible deterioration, it is only important to avoid intense mechanical stress during this period..

Foundation in winter

Another feature of the chemical reaction of cement hardening is that it is exothermic, that is, accompanied by the release of heat. Often this fact alone is enough to maintain an acceptable level of heat in the concrete mass for normal curing..

The third nuance is the ability to increase the resistance of the concrete mix to freezing. Using special additives, it is possible to increase the flowability of the concrete mixture and ensure the initial setting even at temperatures below zero. But the use of frost-resistant additives is not a panacea at all; there are restrictions on the maximum concentrations of these substances and the minimum permissible temperatures. Our site offers to consider the main methods of pouring concrete in winter.

Method 1: formwork insulation

Self-heating of concrete mix is โ€‹โ€‹a double-edged sword. On the one hand, with increasing temperature, strength gain accelerates, on the other hand, due to uneven heating, heterogeneous stresses arise in concrete, leading to the formation of cracks. Therefore, in construction, it is customary to use cement binders with low heat release (about 200 J / g).

Since most foundations and monolithic plinths are subsequently insulated, it is reasonable to immediately use thermal insulation as a formwork element. On the one hand, this will reduce heat leakage from the concrete structure and make it more evenly heated. On the other hand, this way you can avoid the work of installing insulation: liquid cement laitance provides excellent adhesion to most foam polymers. Obviously, when using formwork insulation, it is better to order concrete with normal (250 J / g) or increased (280 J / g) heat release.

Thermal insulation of formwork for concreting in winter

The wider the width of the concrete structure, the less the thickness of the insulation, sufficient to effectively contain the heat during curing. For example, for a concrete tape 350 mm thick, which is an established MZLF standard, a PSB-S shell with a thickness of only 40 mm on the sides and 50 mm on top is sufficient. In this case, a foundation with a thickness of 500 mm can be insulated with a shell of 25 mm from all sides.

Fixed formwork made of expanded polystyrene under the basement walls

The installation of thermal insulation is extremely simple, but for this you need to set the appropriate allowances during the installation of the formwork. Plates are fixed to the walls of the mold using wire, or temporary fixation with glue or strong cement mortar. It is important to prevent the slabs from shifting during tying of the reinforcing cage, and before pouring the concrete mixture, provide a stop from above to prevent the foam from squeezing out under the action of Archimedean force.

Method 2: anti-freeze additives

The essence of the action of additives that allow concrete to harden at low temperatures is to prevent the entry of water into the crystalline phase. The same effect is observed, for example, in salty sea water, which does not freeze at zero, but at lower temperatures. Being in a liquid state, water is capable of reacting with silicates and aluminates even in a highly cooled state, however, the rate of hydration is significantly reduced.

Antifreeze additives for concrete

Most antifreeze additives negatively affect the strength properties of concrete, thus, to obtain the design characteristics, it is required to use concrete one or two strength classes higher or use a higher-quality binder for preparation. It is also important that most additives contain chlorides and sulphates, which have a corrosive effect on the reinforcement and the concrete itself, which requires careful monitoring of compliance with the recommended concentrations.

According to the principle of action, antifreeze additives are of two types. The so-called cold ones simply do not allow the water to crystallize, but there are also โ€œwarmโ€ ones that increase the initial heat release of concrete for the period before it is laid in the mold. The latter type of additives should be used only in cases where it is planned to heat the concrete structure for at least 2 weeks.

Method 3: heating the mixture

The most effective method of winter concrete construction is considered to be artificial maintenance of a positive temperature mixture during the period of concrete gaining strength and its controlled cooling. Internal and external heating methods can be distinguished.

Heating up concrete with heat guns

In the simplest case, a hothouse is built around a reinforced concrete structure โ€“ a sealed polyethylene film sheath with a small internal volume. Hot air is blown under the film, while in the daytime the heating power can be reduced due to the action of the greenhouse effect. This method is the most costly from an economic point of view, but such a solution can be considered the only acceptable one in the construction of critical structures from heavy high-quality concrete..

Heating concrete with cable

Internal heating of the mixture is carried out either by a timely installed heating cable, or by heating the steel reinforcement with an electric current. The latter is beneficial in two aspects: you do not need to buy a heating element that is used only once, plus, in addition, heating is important precisely in the zone where the concrete mass touches the reinforcement, where the most extreme concentration of loads is observed. To heat the concrete through the reinforcement, it is enough to bring out two contact elements when the frame is knitted. In this case, a contact circuit should be formed between the connection points, which does not break and does not have shunts in the form of jumpers with low resistance. To warm up the fittings, special DC-to-current transformers are used, which automatically adjust to the circuit resistance.

Conclusion

Conducting monolithic work at subzero temperatures only seems to be a difficult task. As already mentioned, concrete does not deteriorate during freezing, but at the same time laying the foundation in winter contributes to a more uniform stabilization of the supporting soil layers before erecting the box.

It is extremely important to remember that the concrete mass before the final strength gain is very sensitive to the action of frost heaving forces, especially lateral ones. Therefore, if monolithic works are planned for the winter, an incompressible hygroscopic cushion should be arranged under the foundation, for example, from 15โ€“20 cm of sand and gravel mixture. At the same time, the sinuses of the pit do not fall asleep until the onset of stable heat or until the full set of strength in the event that the concrete products are heated.

Rate the article
( No ratings yet )
Recommender Great
Tips on any topic from experts
Comments: 2
  1. Waverly

    Is it possible to pour concrete in winter for monolithic works?

    Reply
  2. Lincoln Bennett

    Can pouring concrete in winter affect the quality and durability of monolithic works?

    Reply
Add comments