Self-compacting concrete: properties, pros and cons

Calculation of concrete composition

SUBS is characterized by the following indicators:

• low water-cement ratio (0.38–0.4);
• high rate of workability (up to 70%);
• absence of large pores and capillaries in the structure;
• compressive strength B60–B80, but can reach up to 100 MPa;
• elastic modulus - within the range of 30–36 GPa;
• air bubble content no more than 6%;
• mobility P5;
• frost resistance F400;
• water resistance W62 and higher;

As a result, we have the following advantages of SCC in comparison with classical concrete:

1. High mobility, non-delamination even during long-term transportation of the material.
2. Avoid using vibrators.
3. High structural strength and ideal surface quality of finished products.
4. Possibility of installation in thin-walled, densely reinforced structures of complex geometric shapes.
5. Significant reduction in construction time.
6. The production price is comparable to the cost of producing standard heavy mixtures, and if you analyze the economic effect of using this type of material, it is even lower.

Classification

Depending on the method of ensuring resistance to water separation and delamination, there are two basic types of self-compacting mortars:

• finely dispersed type - a significant increase in the volume of the finely dispersed fraction in comparison with conventional concrete;
• stabilizer type - production of solutions using stabilizing additives.

Fine aggregate
Fine aggregates increase the resistance of self-compacting mixtures to delamination and reduce the blocking of the movement of concrete mortar when it flows in densely reinforced structures.

This:

• granulated blast furnace slag;
• limestone powders;
• fly ash;
• siliceous drifts.

Volume of fine suspended matter in SUB

Stabilizing additives help achieve optimal viscosity while providing the perfect balance between resistance to delamination and movement - the opposite properties that occur when water is added.

Stabilizing additives

When a stabilizing modifier is introduced, a persistent microgel is formed on the surface of cement grains, which ensures the formation of a “bearing frame” in the cement paste and prevents delamination. At the same time, the resulting structural skeleton allows the filler (crushed stone, sand) to move freely, thereby without disturbing the workability of the solution.

In international practice, according to quality indicators, the following classification of self-compacting compounds is accepted:

• SF1–SF3 - classes according to the value of workability (cone spreading);
• VS1–VS2 - viscosity classes, method T500 (time required for a standard concrete cone to spread around a circle with a diameter of 500 mm);
• VF1–VF2 - viscosity classes, taking into account the time of flow of the material through the V-shaped funnel;
• PA1–PA2 - classes that study the ability of a self-compacting suspension to pass obstacles (resistance of reinforcement) in an L-shaped tank;
• SR1–SR2 - classes of resistance to delamination, studied by the amount of delamination using the sieve method.

Based on the indicators of the above test methods, the purpose and scope of application of self-compacting mixtures are determined (see table below).

Self-compacting concrete - application

Characteristics

Polycarboxylate is an essential component that is used in the manufacture of self-sealing material.
The quality and characteristics of different types of concrete depend on the additives used during the preparation of the solution. The composition of self-compacting concrete varies depending on the manufacturer, but polycarboxylate is an indispensable component for its preparation. In many ways, this complex polymer modifier provides the unique properties of the finished product.

Self-compacting concrete has the following characteristics:

• water-cement ratio - 0.38-0.4;
• workability - up to 70 cm;
• compressive strength - up to 100 MPa;
• elasticity parameter - 30-36 GPa;
• air level - no higher than 6%.

SCC properties

Thanks to the specific formulation and obtained characteristics, self-compacting concretes are characterized by high compressive and tensile strength. In addition, the solution has the following qualities:

• Creep. This property is determined by a number of additives used, but does not exceed the permissible values.
• Adhesion. Concrete adheres well to reinforcement cages.
• Shrinkage. How the solution will set depends on the volume used when preparing the glue solution. Self-compacting concrete contains approximately the same amount of cement glue as classical mixtures.
• Elasticity. This indicator depends on the proportion of dry additives of different fractions. Traditional cement-sand mixtures are 15% more elastic than self-compacting concrete.

The process of pouring concrete structures

Monolithic and prefabricated reinforced concrete are made in strict accordance with technology

It is imperative to pay attention to each stage of creating a structure in order to achieve the desired parameters and properties

Construction of formwork

The creation of a monolithic reinforced concrete structure begins with the installation of formwork, which will not allow the liquid solution to spread and will support the concrete at all stages of hardening.

What types of formwork are there:

• Collapsible panel - includes several separate blocks for rigidity, can be manufactured independently on site.
• Block - used for a single fill of not one, but several walls at once without ceilings with a supporting structure.
• Pneumatic - with a durable shell, allows air to pass through, it is made for complex cavities of small volumes.
• Volume-adjustable - for installation of monolithic floors and walls in high-rise buildings, constructed using a crane.
• Sliding - used to create high-rise buildings, installed along the perimeter, and during the process of solidification of the monolith at different levels, it is gradually lifted up with jacks.
• Fixed - for decorative finishing.
• Tunnel – for pouring mortar into two walls with a ceiling.

Creating formwork during the construction of a structure is one of the simplest stages of construction

It is important to properly strengthen the panels, check the evenness with a building level, and choose sufficiently rigid and reliable materials so that the structure does not deform under the influence of the weight of the mortar.

Preparation of the solution

After installing the formwork, a solution is prepared, which for pouring monolithic reinforced concrete must include the following components: part of cement of at least M350 grade, 2 parts of sifted fine sand, 3 parts of filler (crushed stone, gravel), water in sufficient quantity to obtain a solution of the required consistency. First, mix all the dry substances, only after thorough mixing add water a little at a time.

To improve the characteristics of the solution, detergents are added to it (a teaspoon of anti-grease agent per bucket of liquid concrete will increase strength and reduce shrinkage), PVA glue (200 milliliters per bucket to improve fluidity and increase adhesion of materials), liquid glass (to increase heat resistance and acceleration setting at the initial stages of pouring).

You can prepare the cement mortar yourself in a concrete mixer or order the required volume and organize its continuous supply to the site.

Reinforcement and filling

The reinforcement frame of monolithic reinforced concrete is created from steel ribbed rods of different diameters. In the formation of large elements, rods with a cross-section of 15-25 millimeters are used; for ordinary walls, they are taken with a cross-section of up to 10 millimeters. The frame is tied with knitting wire and lowered into the formwork. The reinforcement must stand on special clamps at a height of at least 30-50 millimeters from the concrete surface.

After the reinforcement is completed, concrete is poured: the formwork is gradually filled, feeding liquid concrete from the gutter. Small objects are poured at a time, large ones are divided into sections (horizontally) and tiers (vertically). First of all, the grips of one tier are filled, then the reinforced concrete structure is subsequently poured further.

After pouring is completed, the solution is compacted with a vibrating tool. Dry the concrete, covering it with film so that the water does not evaporate quickly and the concrete is strong. During the first days, it is advisable to periodically spray it with water to prevent cracks.

Application and installation features

SCC is used in the construction of various objects, in particular for the construction of hydraulic structures (piers, port buildings, breakwaters, hydroelectric dams).

This material is also widely used in the following areas:

1. In the production of prefabricated reinforced concrete elements and structures.
2. For the construction of monolithic seamless floors.
3. For strengthening and strengthening buildings and supporting structures.
4. When constructing objects that initially require a high-quality, flat surface that does not require additional processing (for example, a runway).
5. During the construction of various concrete structures with a high content of reinforcing elements.
6. For the construction of fences or thin-walled premises (for example, when the building requires a minimum weight of load-bearing and supporting floors).

When laying self-compacting concrete, it is worth remembering some of the features and nuances of using this material. For example, due to the increased content of superplasticizers, the setting time of the concrete mixture significantly increases. Self-compacting concrete, when transported in a concrete mixer for more than one hour, begins to lose its self-compacting property; plasticizers also negatively perceive long-term transportation and, as a result, the mobility of the solution is lost. Professionals recommend not saving money and manufacturing SCC directly at the construction site.

When transporting concrete through a pipeline more than 200 meters long to the working area, delamination may appear and, as a consequence, heterogeneity of the mass. This point should also be taken into account especially carefully and try, if possible, to avoid the use of long supply pipelines when filling. The presence of heterogeneous composition and delamination can negatively affect the final result and significantly reduce the physical and mechanical properties of the finished structure.

Before pouring self-compacting concrete into the formwork, you must make sure that there is no liquid, since even a small volume of water can disturb the composition. If there is water, then it is worth removing it, and it is desirable that the surface is completely dry, this will significantly improve the adhesion coefficient

When pouring SCC, it is important that all components are supplied continuously, otherwise homogeneity may be impaired and the quality of the final result may decrease. The solution cannot be touched for some time, since it does not need compaction, and any manipulations can disrupt the uniformity and create surface unevenness after hardening.

Important: If during transportation or storage the self-compacting mixture has lost some of its quality characteristics and thickened, they can be restored with special chemical thinners, which are fed directly into the concrete mixer and mixed until the mass becomes homogeneous and the original properties are restored. To improve the quality characteristics of class B15 concrete during the preparation of the working mixture, the following additives can be used:

To improve the quality characteristics of class B15 concrete during the preparation of the working mixture, the following additives can be used:

• sand and crushed stone of smaller fractions - when used, the strength increases significantly, but the viscosity also increases (since the surface area of ​​the particles of the fine fraction in total is much larger than the large one;
• the addition of micro- and ultrafine filler to the mixture also significantly increases strength and reduces the risk of corrosion on the reinforcement and the appearance of cracks during shrinkage and subsequent operation.

Features of laying SUB

During the installation of SCC, the following nuances must be taken into account:

1. Before pouring self-compacting concrete, make sure that there is no liquid in the formwork. If it contains even a small amount of water, the properties of the composition may change, which will lead to delamination of the concrete.
2. During the installation process, you need to constantly stir the SUB (if the composition has thickened, add a thinner).
3. The length of the pipeline for supplying the solution to the workplace should not be more than 200 m. If the mixture goes on for too long, it may become heterogeneous.

1. The concrete solution does not need to be further compacted.
2. If you are pouring a concrete mixture into a form for a narrow column or into another formwork with poor air drainage, it is not recommended to drop the solution from a great height. It is better to lower the construction sleeve as low as possible into the formwork and raise it as the form is filled. The lower the height, the less air will enter the concrete thickness.

Main advantages

When pouring, mixtures of this type fill all parts of the formwork equally quickly and without voids. That is, self-compacting concrete can be used not only in the construction of, for example, residential buildings, but also complex architectural structures. In addition, such mixtures are simply ideal for pouring densely reinforced structures. Near the frame elements, no air bubbles form in their thickness, and the layer itself is as homogeneous as possible.

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Another undoubted advantage of self-compacting concrete is that structures poured from it have absolutely flat surfaces. There is no need to perform any actions to correct them at the final stage. For example, when pouring floors from such concrete, there is no need to use leveling material before installing the finished floor.

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Builders who use such concrete in their work, of course, consider its great advantage that there is no need to purchase expensive vibration equipment. Such concrete is compacted simply under its own weight. Therefore, pouring various types of structures using it turns into a very simple procedure.

Vibration equipment is known to have a high degree of noise. Since there is no need to use such units when laying self-compacting concrete, work with this material can also be done at night.

Self-compacting mixtures are, of course, more expensive than conventional cement ones. However, due to the absence of the need to use vibration equipment, structures made from them are often even cheaper.

In terms of performance characteristics, these two types of solutions are similar. Mixtures of heavy and self-compacting concrete allow the construction of equally strong and durable structures. Sometimes structures built using solutions of the latter type have even better performance characteristics.

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Key Features

Vibrating screed, the use of which can now be abandoned

The material discussed in this article is unique because it has the ability to compact under its own weight. Simply put, when ordinary mortars are poured into formwork, a concrete sealant is needed. The type of solution we are considering is capable of completely filling any shape, even in the manufacture of densely reinforced structures.

The technology is new, but despite this, it is increasingly being used. The most promising area for using such concretes is the industrial production of reinforced concrete products and the construction of monolithic structures, such as, for example, high-strength seamless concrete floors. The technology is no less in demand when carrying out shotcrete concreting, strengthening and restoration of already in use structures.

A little history

The photo shows the pouring of supports using self-climbing formwork

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

The photo shows the consistency of solutions with a self-compacting function

During the construction and subsequent operation of these structures, recommendations and restrictions were formulated in relation to the introduction of superplasticizers:

• The introduction of superplasticizers at a certain dosage can result in a slowdown in the setting of the mixture.
• When transporting the mixture for 1 hour or more, the effectiveness of the introduced additive decreases, as a result, the degree of mobility of the solution decreases.
• The use of superplasticizers provokes separation of the dense mixture when transported through a pipeline over a distance of more than 200 meters. As a result, the quality of finished products is significantly reduced. This circumstance must be especially taken into account during industrial construction using special gasoline pumps.

The widespread implementation of grandiose construction projects dictated the need for the production of fundamentally new concretes that meet the highest operational requirements. Among these requirements, special mention should be made of the short deadlines required both for concreting and for gaining grade strength in the finished product.

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

• effective use of multi-fraction fillers;
• introduction of ultrafine and microfine fillers designed to increase the corrosion resistance and strength of materials reinforced with metal structures;
• control of the rheology of highly mobile mixtures;
• production of fundamentally new types of modifiers and other regulators of the performance properties of concrete.

In 1986, Japanese scientists developed a particularly strong composition of self-compacting concrete with a number of unique qualities. The new material is 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.

Industrial use of the solution began in 1996 of the last century, and a little later full tests of the material were carried out under natural conditions. It was only in 2004 that the operational features of self-compacting concrete were finally determined and their classification was carried out:

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

The photo shows formwork filled with self-compacting concrete with a reinforcing frame.

• For unreinforced or low-reinforced concrete structures (pipelines, floor slabs, foundations, tunnel linings, etc.), highly fluid SF1 mixtures are used. Mixtures of SF2 and SF3 are used for 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 of type VS1, VF1 or VS2, VF2 are used.

• When constructing vertical structures and structures reinforced with a pitch of no more than 100 mm, easily moldable mixtures of PA 1 and PA 2 are used.
• For the construction of high-rise elements (except for thin beams) and vertical structures with a reinforcement pitch of no more than 80 mm, delamination-resistant mixtures SR1 and SR2 are used.

Application environment

Monolithic reinforced concrete is used in individual and large-scale construction to perform a variety of tasks - using this technology, cottages, private one-story houses, buildings with a large number of floors, and many other structures are built. Before constructing a foundation, floors, or walls, be sure to carry out all calculations, determine the expected loads, and choose the right steel.

What is built using monolithic reinforced concrete technology:

• Strip foundation - usually laid under walls and columns.
• A monolithic slab under the base of the entire structure is the simplest option. Modifications using box-shaped and ribbed slabs are possible.
• Pile foundation is the current choice for weak soils.
• Load-bearing internal and external structures.
• Columns of various cross-sectional shapes, used as vertical supports with or instead of walls.
• Production of ribbed, hollow or solid monolith slabs for floors with beams.
• Various types of flights of stairs - can be made of screws, straight, or combined.
• A wide variety of decorative architectural elements - thanks to the good plasticity of concrete, you can design different columns, pediments, arches.
• Tunnels - for the metro, laid under roadways and complex bridges.
• Bridges – monolithic reinforced concrete is ideal for the construction of such objects.
• Sites that will withstand severe loads - test sites, airfields and others.

Where is SMS applied?

The performance characteristics and composition allow the use of self-compacting concrete in various fields of construction. Compositions differing in mobility and viscosity are used for pouring structures of complex shape, including those with dense reinforcement, vertical concrete elements of buildings and structures. According to construction standards, SMS is applied:

• for the production of prefabricated reinforced concrete elements and structures;
• during the construction of structures for hydraulic structures;
• for repair and restoration of objects, including using shotcrete;
• when installing a durable floor without seams, designed for high loads;
• to obtain durable surfaces that do not require additional processing;
• when pouring elements with a dense reinforcing mesh;
• during the construction of buildings and structures, the strength of which is subject to increased requirements;
• for the production of Lego bricks;
• for thin and durable elements with minimal weight - partition walls, fencing elements.

Advantages and disadvantages

The advantages of the material are:

• Quality of finished products. Objects have a smooth surface that does not require additional processing. The structure of the structure is completely free of large pores, so moisture does not penetrate inside, and the wear resistance of the building increases.
• No vibration compaction. Thanks to this, the overall construction time is reduced, the noise level during the work process is reduced, and fewer builders are required.
• Increased hardening time. Achieved through the introduction of plasticizers. This allows material to be transported to the site from long distances.

Disadvantages include the higher cost of self-compacting concrete and some restrictions in use.

How to make it yourself

The technology for creating artificial stone includes the use of already known components. Home craftsmen use the following proportions:

• liquid glass – 250 g;
• potassium hydroxide – 200 g;
• slag and fly ash - 750 g each;
• water – 130 g.

But what sand concrete is best used for floor screed can be found here in the article.

The recipe is designed to prepare 1 liter of concrete mixture. After making calculations, you can obtain the consumption of components to create the required amount of environmentally friendly concrete. To date, there are no scientific publications with the exact composition. When creating a concrete mixture according to this recipe, some changes in its quantitative composition may be necessary.

The video explains how to make geopolymer concrete with your own hands:

All of these components can be easily purchased in the retail chain. However, it should be taken into account that the cost of ready-made concrete based on polymerization technology will be higher than that of a concrete mixture made from Portland cement. The mixture sets very quickly. To slow down this process, tetraborate decahydrate is introduced into the formulation. Additionally, water-based resins, latexes and PVA can be added to the composition.

Cooking technology

First you need to prepare your tools and containers. You will need:

• gloves, goggles, overalls, a respirator and other means of protection against the action of aggressive substances;
• mixing container;
• a mixing tool that does not react with alkalis (wooden spatula);
• scales to monitor compliance with proportions;
• molds for casting (formwork).

From the article you can find out what kind of concrete can be used for the foundation of a house.

When mixing the components of geopolymer concrete, heat is generated. This must be taken into account when choosing a mixing container

If there is a need to speed up the hardening of the mixture, electrolytes can be placed in the formwork, but this is important to take into account when calculating the proportion of concrete for the foundation of a house

KON is quite aggressive. Upon contact with the skin, and especially mucous membranes, it causes serious burns. You need to work with it wearing glasses and gloves

You need to work with potassium metasilicate (liquid glass) no less carefully. It also has a crystallized structure and is very hygroscopic

Composite concrete is one of the few existing methods for recycling slag and ash. Fly ash is waste from the combustion of solid fuels. This component can constitute up to 75% of the geopolymer concrete mixture. The substance has a dubious environmental reputation. But its use makes concrete durable and strong.

All components are mixed in a concrete mixer in the following sequence:

• pour water;
• slag and fly ash are filled in;
• mix the components well;
• polymers are added;
• mix the concrete until ready.

The molds must be poured in a dry place. When hardening, this type of concrete forms a film. It tends to swell upon contact with water. To prevent this from happening, you need to protect the filled forms from high humidity.

But if you do everything correctly, then as a result you will get the following material:

• with high compressive strength;
• with low shrinkage;
• with resistance to high temperatures and acids;
• comparable to granite in impermeability.

Composition and properties of self-compacting concrete

The composition contains the following ingredients:

• water;
• chemical modifiers;
• crushed stone of small fractions;
• Portland cement;
• plasticizers;
• sand;
• various fillers.

The internal structure of self-compacting concrete is more compact. This explains why the durability-related parameters when tested are higher than those obtained with conventional concretes of equivalent compressive strength.

The fire resistance of self-compacting concrete is no different from conventional concrete made using the same type of aggregates.

Depending on the location of the reinforcement, it is recommended to specify the most appropriate maximum aggregate size. The optimal range is considered to be from 12 mm to 18 mm, depending on the specific case.