Concrete shrinkage during mortar dehydration. losses during

In the construction of one of the serious problems is the shrinkage of concrete during curing. Since this beautiful material appeared, builders around the world have been trying to reduce the rate of precipitation of the cement composition. It depends on a number of factors.

Initially, the shrinkage of reinforced concrete is due to the loss of water, over time, the composition shrinks due to the physical and chemical processes occurring in it.

Existing types of shrinkage

If you go from the very beginning, then, first of all, it is worth saying about the contraction compression of concrete.

• This type of compression occurs in the process of mixing the solution. Hydrates, which are formed as a result of the interaction of water with cement, have a smaller volume than the original material.
• This type of shrinkage has a greater effect on the porosity of the composition than on its volume. Larger volume losses occur during dehydration.

Dehydration solution

Dehydration or wet shrinkage of reinforced concrete is 7–12 times higher than contraction performance. Experts divide it into 2 stages.

The first stage is conventionally called plastic, the second hydraulic.

• The processes of plastic settling of the solution intensively take place within 3 to 6 hours after pouring. It is caused by the active loss of moisture during evaporation, a large percentage of the formwork and the base are taken over by themselves. The smaller the composition of the water, the lower the rate of sitting. Also, a product with good reinforcement loses less in volume.
• After the solution is grabbed, plastic upsetting smoothly into the hydraulic. It does not happen so quickly and intensively, but, given the volume of the foundation, it can be much more dangerous. Concrete dries unevenly, with the result that cement stone also shrinks unevenly. This is where internal stresses arise, which can lead to cracks.

It is important: if the pouring is carried out at high temperatures in a hot climate, then due to the lack of water, the formation of cracks is possible at the first plastic stage of settling the solution.

• At 1 m fill, the total precipitation of the solution can be up to 3 - 5 mm. At first glance it may seem that the figures are not significant, but when converted to the volume of large construction, the losses can be quite serious.

Losses during operation

After complete solidification of the concrete solution, the processes of settling in it do not stop. No wonder that experts recommend giving the foundation stand before construction begins.

Depending on the quality of the cement, the time of settling the foundation can vary from six months to 2 years.

• As a result of carbonization under the influence of carbon dioxide in the atmosphere, as well as due to seasonal temperature differences, concrete products can walk even up to 5 mm.
• It is for this reason that the foundations are poured in the spring or in the first half of summer, so that before the frost the concrete dries as best as possible and does not tear it in winter.

The nuances of shrinkage in different conditions

Vibration work

This method is the most common and has been used for over 100 years. Its essence lies in the fact that, as best as possible, squeeze out excess air from the solution. In the literature, such actions may be called compaction of the concrete mass.

You can act manually or by technical means.

• Compaction with your own hands is done by improvised means, by repeatedly punching, sticking and tamping the total mass of the solution.. It can be quite effective, but requires skill and considerable labor costs.
• When using vibrators, the qualification is not so important, the quality of work also increases many times, the only significant disadvantage here is the high price of the vibratory machine itself.

The video in this article shows the pouring of the solution and the work of the deep vibrator.

Important: the coefficient of shrinkage of concrete during vibration ranges from 0.8 to 0.6%. That allows to use much more dry mixes and mixes with the increased rigidity. Concrete shrinkage in percent is regulated according to GOST 24544-81.

• After carrying out vibration laying of the screed on large areas, so that it does not crack due to internal stresses and deformations, cutting of reinforced concrete with diamond circles is used. To compensate for stresses when pouring volumetric foundations, diamond drilling of holes in concrete is used.

The video in this article shows the process of cutting a shrink seam with a diamond wheel.

Use of new formulations

Currently, new types of concretes are widely used, these are foam and aerated concrete.

Manufacturers claim that they do not shrink at all, but experts say the opposite:

• Any composition in which there is cement, gives a certain shrinkage. So the shrinkage of the house of aerated concrete according to GOST 25485 should be within 0.3 mm. (See also the article Aerated concrete facing: how to do.)
• Shrinkage of foam concrete can also vary from 0.55 to 3 mm. Depending on the method of production of blocks. For private construction, the figures are of course miserable, and they have little effect on the final result, but they are still better taken into account when building.

Composition fluidity

Of course, solutions with high fluidity, greatly facilitate the life of most builders. But the addition of a large amount of water entails a change in the characteristics of concrete is not for the better.

Therefore, it is very important to maintain the correct proportions.

• For cement-containing compositions, parameters have been introduced that determine the ability of the composition to spread under the action of vibration. In different sources, they are called differently, the mobility of the solution, workability or shrinkage of the cone of concrete.
• In the documents, as a rule, this value is indicated by the letter “P”, followed by the value of the coefficient, determined on a five-point scale. More rarely, in old overheads, the name “sediment cone” occurs and the coefficient ranges from 10 to 15.

• For most traditional monolithic works, a solution with a P-1 or P-3 flow coefficient is used. For products with thick reinforcement, pouring narrow aisles or high columns, it is supposed to take the composition of high yield which is marked as P-4. Here the draft of the cone can reach up to 20 cm.
• A mixture with a draft cone of about 12 cm in modern conditions is labeled as P-2. For work with large, densely charged areas and especially where the use of a concrete pump is meant, a solution of at least P-4 is used. This mixture is still good because it does not require vibratory compaction.
• Diluting the solution with water directly in the mixer is not recommended. But, despite the fact that the instruction forbids, in the vastness of our vast country during the construction process, this is often done. The mixture in this case, of course, hardens, but it will significantly lose in the fortress. So when diluted with cement cement M300, in fact we get the quality of M200 or even M100.
• In modern conditions, the shrinkage of the cone of concrete can be increased by the addition of synthetic plasticizers. With such additives, it is possible to easily bring the fluidity of the composition to the characteristics of P-4 and even P-5. They allow construction all year round, working both in heat and in sub-zero temperatures without loss of quality characteristics.

The video in this article shows a method for determining the fluidity of the composition.

Conclusion

As mentioned earlier, the shrinkage of concrete during the pouring process is inevitable. The whole question is how much the mixture can settle. Modern science has already invented plasticizers, which reduce these processes to almost zero, such solutions are called non-shrinking.

In the west, they are used everywhere. We, as the price is high, are used only during repair work and in road construction.