Hardening of concrete - additives to speed up the process,

Hardening (or setting) of concrete is a very complex physical and chemical process. When mixing water and cement, the two components interact, resulting in a new compound. This article will examine in detail the hardening of concrete in various conditions and provide useful recommendations.

General information

Concrete mass in normal conditions hardens very quickly, and over time it becomes even more durable, as the water penetrates deeper into the cement particles. Even after several months, the process of hydration of the latter continues.

Tip: if you need to process concrete structures after curing, you will be helped by a service - cutting reinforced concrete with diamond circles with the help of professional equipment.

In this regard, in wet conditions, the material seizes more firmly than in air. In dry conditions, it turns out fragile, since the hardening of concrete occurs much faster due to the small amount of moisture needed to penetrate the grain of the cement.

Under optimal conditions, the concrete sets quickly, for example, after preparation, the mixture gains about 70% of the possible strength within 7-14 days. After this period, the increase in strength decreases. If the temperature is below normal, the process of hardening of concrete slows down, and at low temperatures (below 0⁰ C) - almost stops. At elevated temperatures and good humidity, hardening accelerates.

In the process of solidification, the volume of the concrete mass decreases. At the same time, setting takes place faster on the surface, it shrinks and cracks may appear if there is insufficient moisture. Also, their formation is a consequence of the uneven and insufficient heating of the material, another reason for its insufficient strength and fragility.

Hardening Additives

When performing construction work, the rate of hardening of concrete plays an important role. Often in the winter and in the manufacture of products from reinforced concrete it is necessary to accelerate the process of solidification of the mixture.

For this purpose, apply various additives with strict adherence to proportions, their price depends on the method of use.

Among the most commonly used are the following:

• 4% calcium or sodium nitrate, calcium nitrite, calcium nitrite-nitrate-calcium chloride, sodium nitrite-nitrate-sulfate;
• 2% sodium sulfate;
• 2% calcium chloride (for reinforced structures);
• 3% calcium chloride (for non-reinforced structures).

Tip: if the design is pre-stressed, additives such as calcium chloride and nitrite nitrate calcium chloride are not used.

Grasping

If concrete work is performed at low temperatures, the mixture is prepared using heated materials.

For heating applied:

• steam;
• electric current;
• lime-pot;
• exothermic cements;
• hardening accelerators are added to the mixture.

The considered methods can be used both separately and simultaneously.

Instructions for the preparation of heated concrete is as follows:

1. The concrete mass is heated to a temperature of 80-90⁰ C, and supplements - up to 40-50⁰ WITH.
2. After mixing, the mixture is covered with insulation so that the heat is kept for 2-5 days.

Concrete structures are heated with steam or electric current. To save heat, the place of work is covered with a foil. Works on heating concrete with electric current look like this - passing a current of 220 V, they gradually increase the temperature of the mixture to 70⁰ C. The same method can also be used to heat horizontal surfaces of structures.

From the above, it is clear that the strength of concrete directly depends on the temperature regime at which it is solidified.

For example, when:

• low temperature process is terminated;
• increased - accelerates (provided sufficient moisture).

Tip: when working in the winter, use additives to speed up the hardening process.

Works after laying

Immediately after the installation, it is necessary to keep constant control over the material; this will help avoid its destruction and the appearance of inevitable consequences.

To achieve high-quality setting and good structural strength of fresh concrete, proper care is required:

• maintaining sufficient moisture;
• protection from direct sunlight, shock, any damage and shaking;
• Avoid sudden temperature changes.

To do this, laying poured a small layer of sand or sawdust, as well as cover their hands with a tarpaulin, burlap or other coatings that retain moisture. In addition, the concrete surface is periodically watered with water, splashing the stream to keep it moist.

Tip: in the summer, when the air is dry and hot, you need to water the formwork.

Remember, if there is a probability of contact between the laid concrete mass and groundwater, it is necessary to isolate or divert water for 3-14 days (time depends on the quality and composition of the cement).

Advice: diamond drilling of holes in concrete with professional crowns will help to make a technological hole in the hardened concrete mass.

Recommendations

1. It is unacceptable to move on freshly-concrete surfaces and structures.. Movement, installation of formwork and scaffolding is possible only when concrete reaches a strength of 1.5 MPa.
2. Vehicles and concrete-laying machines on concrete surfaces are allowed when the concrete reaches the strength specified in the construction project..
3. Before performing construction and production work, the laboratory must necessarily approve the necessary measures related to the protection and care of concrete, the procedure for carrying out the necessary work, the timing of their implementation and the schedule of concrete hardening.
4. The design of the work to be carried out and the design of the building itself should govern the conditions for the hardening of the concrete., for example, humidity and temperature from the beginning of the laying of the material in the structure to the monolithing of the seams between the blocks.

Tip: for foam concrete, aerated concrete and similar materials there is a special GOST 31360 2007 for cellular autoclaved concrete, in which all processes are regulated.

Conclusion

Proper hardening of the concrete mix is ​​a prerequisite for obtaining the desired result from the material. It is required to make the maximum amount of effort, especially in the winter period, for the material to acquire the required strength. The video in this article will help you find additional information on this topic.