DESPITE frosts and heat

Talking about laying concrete in construction, we have always had in mind that the construction work is carried out under normal conditions, ie, before the onset of winter weather, or at temperatures not exceeding 35 Yea C. In this case, no additional care project hardening concrete is required. However, given that the hardening requires constant moisture to prevent early drying even at these temperatures, it hid from direct sunlight. In Russia, the air temperature in different areas are very diverse: from-70є C to +50 C. Yea past winter construction almost completely stopped, and the construction in the southern regions of the country required the development of special conditions of hardening concrete. However, the scale of construction in our country require construction activities all year round and in all weather conditions.

FROST IS AFRAID OF CONCRETE?

Yes, fresh concrete frost danger. And, above all because of the effect of low temperature on the setting and hardening of cement. Concrete is very sensitive to cold. This affects primarily on setting and hardening rate. For example, when the temperature drops from 20 to 5є with concrete setting slows to 2 - 5 times. But most dramatically seen this slowdown with further decrease in temperature - up to 0 º C. However, if returns to normal temperature curing, curing it again takes the usual pace. And if the temperature drops to concrete from 0 º? Hardening stops completely. This is explained by the fact that the freezing of the concrete it contains free water freezes, and the formation of cement slows. Therefore, stop and hardening of concrete. Freezing in concrete, the water expands by 9%. As a result, the pores of the concrete develops a lot of pressure, which causes the destruction of the structure has not yet hardened concrete. Accumulated on the surface of the grains of coarse aggregate by freezing water forms a thin film of ice, which separates the aggregate surface from contact with the cement paste. As a result of deteriorating situ concrete. If you freeze the concrete at an early age, the ice crystals will destroy many cement glue. If the mixing of concrete was conducted prior to freezing, and curing of concrete has not yet started, it will not start and after freezing. But if you started hardening, it is suspended until the free water in the concrete will be in the form of ice. When thawing frozen concrete free water into a liquid, and the hardening of concrete is resumed. There are the same process as before freezing, but with a changed structure. These changes in the structure of concrete, reduces its strength and adhesion of concrete with reinforcement. The ultimate strength of the concrete will be the lower than before the concrete subjected to freezing.

The most dangerous freezing concrete during cement setting. Concrete is also harmful and repeated freezing and thawing it in the initial period of hardening (thawing and freezing).

POSSIBLE LEE WINTER CONCRETING?

Yes, this is the work of Russian scientists argue SA Mironov VP Sizov and IG Sovalova have developed and put into practice the theory and methods of winter concreting.

It is about creating normal conditions for hardening in the winter. This means that, during the period, which is determined by the achievement of the specified strength of concrete, to maintain the required temperature and humidity, using the internal heat of the concrete or additional heat curing concrete.

As always, it all starts with the concrete mix, which preparation in the winter is a very important operation. First of all, you need to carefully check the quality and condition of the raw materials. So, for example, sand, crushed stone and gravel should not be contaminated and mixed with snow and ice. Therefore, they are stored on the dry upland areas under the eaves or in enclosed spaces. Of course, we can not allow the storage of cement in it got cloudy.

Prepare concrete mix should be in heated rooms. Internal heat reserve in concrete create, fueling its components. Heating the aggregates can be a single-stage, when both materials are thawed and heated, and two-stage, when some plants aggregates previously thawed, but on the other - is heated to the design temperature (40є C). At the same time in the tanks with steam to heat water to the desired temperature - 30 to 80є SA Cement and additives finely heat prohibited. As for accessories, it must be cleared of snow and ice and warmed up with hot water or steam. Temperature of the concrete mix at the time of loading the mixer must be such as to ensure the desired temperature of concrete at the exit of the mixer and panning - at least 5є C.

So, ready-mixed concrete. But it must be transported to the place of installation with minimal heat loss. Heat loss of the transportation of concrete is less than the transfer operations. Therefore, in the winter time it is brought to a place without laying overload. It should ensure that shipping containers were insulated and heated. If the concrete mixture is transported in the back of the dump, the body harboring a tarp or heat the exhaust gases. This provides a concrete mixture over the heat curtain. While transporting concrete in buckets and bins of wood covered warmed lids; externally insulated with felt and then sheathe with plywood. When pumping concrete insulated transport as room where concrete pumps and betonootvody.

Дата добавления: 2015-08-27