Self-compacting concrete - effective solution for a wide

In this article we will talk about what a self-compacting concrete is, what is the history of the appearance of this material, and what are its differences from ordinary mortars prepared on the basis of Portland cement.

Key Features

The material, which will be discussed in this article, is unique because it has the ability to condense under its own weight. Simply put, when conventional solutions are poured into the formwork, a compactor is needed for concrete. The type of solutions considered by us is itself capable of completely filling in any form even in the manufacture of densely reinforced structures.

The technology is new, but despite this, it is becoming more and more used. The most promising area for the use of such concretes is the industrial production of concrete products and the installation of monolithic structures, such as, for example, high-strength seamless concrete floors. The technology uses no less demand for the gunning of concreting, reinforcement and restoration of structures that are already in operation.

A bit of history

In the 70s of the last century, a number of high-strength concrete modified by a series of superplasticizer additives was developed. For example, such compounds in 1970 were used in the construction of monolithic structures on oil production platforms operated in particularly difficult climatic conditions.

During the construction and subsequent operation of these facilities, recommendations and restrictions were formulated with respect to the introduction of superplasticizers:

• The introduction of superplasticizers at a certain dosage can result in a slower setting of the mixture.
• When transporting the mixture for 1 hour or more, the effect of the added additive decreases, as a result, the degree of solution mobility decreases.
• The use of superplasticizers provokes the separation of a dense mixture during transportation by pipeline over a distance of more than 200 meters. As a result, the quality of finished products is significantly reduced. This circumstance must be particularly taken into account in the course of industrial construction using special gas pumps.

The widespread management of ambitious construction projects dictated the need for the manufacture of fundamentally new concrete that meets the highest operational requirements. Among these requirements, the tight deadlines that are necessary both for carrying out concreting and for obtaining a brand strength by the finished product deserve special mention.

The solution was found through a series of theoretical studies and practical implementations implemented in order to:

• efficient use of multi-fraction aggregates;
• the introduction of ultrafine and microdispersed fillersdesigned to increase the corrosion resistance and strength of materials reinforced with metal structures;
• control of rheology of high mobility mixtures;
• production of fundamentally new types of modifiers and other regulators of operational properties of concrete.

In 1986, Japanese scientists developed a highly durable composition of self-compacting concrete with a number of unique qualities. New material called Self-Compacting Concrete.

A unique property of the solution was the possibility of compaction due to the mechanical load of its own weight. As a result, the development of a new type of material made it possible not to use a concrete compactor.

The industrial application of the solution was started in 1996 of the last century, and a little later, full-scale material tests were carried out under natural conditions. And only in 2004 the features of the operation of self-compacting concrete were finally determined, and their classification was carried out:

Classification of mixtures used in the manufacture of self-compacting concrete

• For non-reinforced or low-reinforced concrete structures (pipelines, floor slabs, foundations, lining of tunnels, etc.), high-mobility SF1 mixtures are used. Mixtures of SF2 and SF3 are used to construct conventional structures, including vertical structures and complex architectural forms.
• For the manufacture of structures and products with high quality surface treatment and for structures of low strength class, viscous mixtures like VS1, VF1 or VS2, VF2 are used.

Important: Due to the high separation, transportation of such materials over long distances is not recommended.

• In the construction of vertical structures and structures reinforced with a pitch of not more than 100 mm, easily formable mixtures of PA 1 and PA 2 are used.
• For the construction of high-rise elements (except thin beams) and vertical structures with a reinforcement pitch of not more than 80 mm, SR1 and SR2 mixtures resistant to delamination are used.

Performance Features and Benefits

In accordance with GOST 7473-2010, self-compacting concrete has a lower water-cement ratio (not more than 0.4), at the same time high workability of the material is achieved on average - 70 cm. The strength parameters of the finished structures are at least 100 MPa.

Important: Constructions cast from this material are distinguished by a dense structure with a complete absence of large pores. As a result, the water does not penetrate deep into the concrete products and the percentage of corrosion of the reinforcement is extremely small.

An important component in the composition of self-compacting concrete is polycarboxylate - a polymer that performs the function of a highly efficient complex chemical modifier.

The principle of the component is to adsorb onto the surface of the cement grain and to impart to it a negative charge. As a result, cement grains repel each other, which, in turn, set in motion the solution and contributes to its compaction.

The important point is the length of the chain, which is created by molecules of superplasticizer. The longer the chain, the more intense the repulsion of the grains. As a result, the efficiency of plasticization increases, and its action can be prolonged by means of constant mixing.

Important The duration of the plasticization effect using polycarboxylates is 3 times longer than using traditional superplasticizers. This quality is ensured due to the larger size of polycarboxylate molecules.

Among the advantages of such solutions we note the following:

• A longer time during which the mixture can be transported;
• The ability to transport sweep long distances from the plant to the construction site;
• Optimum quality of ready-made concrete products, namely ideally smooth surface without the need for additional processing;
• Short deadlines for the implementation of construction and, as a result, accelerated commissioning of the object;
• No concrete vibrator required;
• A smaller number of skilled workers employed on the construction site and, as a result, a lower price for the entire construction;
• Lower noise level compared to conventional construction, which is especially important if concreting is carried out in the center of the settlement.

Requirements for manufacturing conditions

GOST for self-compacting concrete involves the production of high-quality equipment and qualified personnel. That is why such materials are not made with their own hands, but mainly in the factory.

Due to the sensitivity of the material in terms of water consumption, production must meet the following requirements:

• Ensuring high precision dosing of components before they are fed into the mixing plant.
• Mandatory warning of the presence of moisture accumulations under the granular aggregate in the bunker before the start of the production process.
• Ensuring continuous moisture control of used sand.
• Providing continuous moisture control in a coarse aggregate.
• Timely cleaning of the mixer drum and mandatory verification of its internal volume before loading the ingredients in the mixture.

Features of transportation

The time and conditions of transportation affect the quality of all concrete, and self-compacting concrete is no exception. The process of segregation (stratification) of concrete results in a decrease in the quality of the building material and products made with its use.

The so-called “sediment effect”, when using various thinners, is characteristic of building mortars, if they are transported over 1 hour without mixing. That is why the transportation of mortar is carried out through special automixers - mixers on a wheeled chassis.

Another important point that should be taken into account when planning transportation is the negative impact of weather conditions. For example, the long-term exposure of the sun to the surface of the shipping container will most likely lead to an increase in temperature, both of the tank itself and of the concrete mix. An increase in temperature, in turn, can lead to a change in the consistency of the material.

In order to prevent such a development, special vehicles should be used for transportation. In addition, the manufacturer must provide the customer with detailed and reliable information about the properties of the material.

In order for the mixture at the construction site to acquire an optimal working texture, a thinner is mixed into its composition. The amount of diluent is selected in accordance with the recommendations of the manufacturer and based on the content of the technological requirements.

Upon delivery of concrete to the facility, it is necessary to ensure continuous pumping of the mixture so that the material is applied as intended without significant delay.

Important: When pumping and subsequent use of the mixture, it is necessary to have accurate information about how much time the material has spent on the road and how much time is devoted to its unloading and application until the moment of supposed loss of performance.

Features of laying concrete mix

By and large, self-compacting concrete fits around just like most other mortars.

But there are some features to keep in mind:

• Before proceeding with the installation, you must make sure that there is no water in the formwork for the concrete product. If there is water, it must be completely removed, since even a small amount of water may cause the mixture to separate.

• Before being used for laying, the solution must cover a distance that will be sufficient to remove the air contained in it. As a rule, this distance is the length of the sleeves from the concrete pump to the formwork. If priming is performed without using a pump, this distance should be created artificially.
• Instructions for laying the finished solution does not provide for downtime. If a technological break is necessary for one reason or another, it should not exceed half an hour. The fact is that due to long-term contact with open air, the surface of the material is compacted. The resulting compaction will prevent the mixing of two layers of alternately stacked solution. According to statistics, even minor downtime during installation can contribute to the loss of the ability of concrete to compaction.

• If the laying of the mixture in the formwork, from which it is difficult natural air discharge, concreting requires a special approach.

For example, filling the form of narrow columns, you should not dump the mixture down. It would be better to lower the sleeve into the formwork and gradually lift it as the form is filled. The smaller the height from which the concrete is fed into the formwork, the less air will be dissolved in its thickness, and the more efficiently it will be removed from the form.

Conclusion

So, we have also considered the features of the material, due to which you do not need a manual vibration compactor for concrete. As a result, the construction work planned by you can be implemented in a short time without compromising the quality of the finished result.

Order and purchase self-compacting concrete is easy. But where it is more important to ensure compliance with the requirements of transportation and use of the material. Do you have any questions? You can find more useful information by watching the video in this article.