Seizure of concrete: the effect of additives on the process.

As you know, all solutions have a certain period of viability, during which they can be aligned, applied and filled in forms. But after some time in the composition are irreversible processes, and the material begins to acquire certain properties. It is very important that the optimum external conditions are respected - the setting of concrete at low temperatures can last for several days, as a result of which strength and durability will significantly decrease.

What is the seizure

Главная причина, по которой бетон приобретает высокие прочностные свойства это то, что цемент при взаимодействии с водой приобретает очень высокую прочность и превращается в так называемый цементный камень. По-научному процесс называется гидратация, и проходит он в несколько этапов, главными из которых является первоначальное схватывание и дальнейшее твердение (читайте также статью «Фибропенобетон: особенности и область применения»).

In this article, we will look at how much concrete is set and how ambient air temperature affects it. It is worth noting that this process, in contrast to hardening, is very transient: it occurs on the first day, or even hours, after a concrete structure is poured. That is why he needs to pay the closest attention, as strength depends on him.

The setting time of concrete depends on the temperature conditions, consider this dependence in the table:

How can you influence the seizure

If earlier at low and negative temperatures, all work was suspended, nowadays there are many ways to speed up or slow down the setting, which allows concreting almost all year round. It is them that we consider in more detail.

Additives

There are many special compounds on the market that can accelerate or slow down the setting, while the negative effect on strength will be minimal.

All of them can be divided into the following groups:

• Additives в бетон для быстрого схватывания, чаще всего используются, когда необходим быстрый набор прочности, при этом естественно сокращается и период первичного твердения. For example, some of them allow the material to gain up to 50% of the strength in just 2 days. The use of such compositions should be limited to sites where high strength is very important, since the performance is still decreasing.
• Frost-resistant accelerator setting concrete - this option is used at low temperatures. The composition reduces the temperature threshold of freezing moisture and accelerates the initial setting without significant loss of strength. In such conditions it is important that the water does not freeze - it has irreversible consequences for the strength characteristics.

Note! It is worth remembering that the lower the temperature, the greater the proportion of addition of the composition. This factor is fundamental in the preparation of concrete.

• Concrete setting retarders - this type of composition is used in the following cases: at high ambient temperature, during transportation of concrete by mixers over long distances, during long-term workflow, when the composition must retain its original properties for a long time. Also retarders are indispensable for the device of monolithic structures: this way you can avoid the occurrence of layers and the weakening of concrete.

Tip! It is worth remembering that stirring the solution does not allow the hardening stage to begin, so if you need to prolong the viability of the solution for half an hour, then with this operation you will avoid premature setting.

If hardening is calculated by day, then the setting is transient, this is its biggest feature. And it is very important at what temperature the concrete sets - it should be as close as possible to the optimum, otherwise it will be problematic to achieve a good result without additives.

The use of various systems and structures for heating concrete

Сразу отметим, что используются они при отрицательных температурах и направлены на ускорение схватывания и твердения (см.также статью «Время схватывания бетона и его механизм»).

The following options are most commonly used:

• Electric heating designs. Its features are as follows: a special heating wire is laid into the structure before it is poured or metal electrodes are inserted while the concrete is still fresh. By flowing a current through the system, it is possible to heat the entire structure, preventing it from freezing and speeding up the setting.

• The use of thermoactive formwork, this option involves the use of electric heating or water jacket in order to maintain a positive temperature in the structure. It is used quite rarely due to the complexity of the device.
• Another option that is suitable for large-scale construction is the installation of a greenhouse, it can be a collapsible design or a canopy bed curtain system. It is important that a positive temperature is maintained inside.

It is worth remembering that diamond drilling of holes in concrete and cutting of reinforced concrete with diamond circles are allowed only after gaining at least 50% strength. You should not hurry with these works, otherwise you can break the structure of the material.

Conclusion

As you can see, adhesion is of paramount importance to concrete, and any disruption to the process reduces the strength of the material. The video in this article will vividly show some features of the process in question.