Self-compacting concrete: characteristics, technology

• 1 Definition
• 2 Benefits
• 3 Disadvantages
• 4 Properties
• 5 Composition
• 6 Application and installation features
• 7 Diagnostics
• 8 Conclusion

Concrete is a mixture of cement, sand, water and various additives that improve the properties of the solution. Concrete has the following quality characteristics: strength, reliability and longevity. At first glance, it seems that the cement-sand mixture has all the advantages necessary for the construction of various buildings and structures, but construction projects do not stand still and are trying in every possible way to improve the requirements for concrete. So, one of the requirements was to achieve maximum strength gain in a short time, so that less labor was used. Several decades later, they developed a material that fills the formwork without external influences, and called it self-compacting concrete.

Definition

Self-compacting concrete is a material that compacts under the weight of its own, completely filling the formwork even in places where reinforcement is densely laid. SUB concrete is popular for the installation of precast reinforced concrete, for the construction of durable floors without seams and for shotcrete concreting. A type of self-compacting cement-sand mortar:

• highly mobile;
• viscous;
• easy to shape;
• Resistant to delamination.

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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.

Advantages

Laying a protective layer of self-compacting concrete.
Self-compacting concrete has the following advantages:

• speed of installation;
• construction of structures with increased strength;
• absence of defects that arise during the compaction of concrete mortar;
• due to the smooth and dense surface of self-compacting concrete, it is possible to identify the surface and shape of the formwork;
• buildings and structures are obtained with different geometries;
• durability of the material;
• reduction of labor costs, which are due to a decrease in time for pouring concrete and lack of compaction;
• the adhesion of cement to reinforcement has acquired increased strength;
• the possibility of penetration of cement-sand mixture into hard-to-reach areas of the structure;
• supply of a mixture of sand and cement is possible through the formwork;
• reduction of labor costs for the work team;
• safe material manufacturing process;
• no compaction of the cement-sand mixture is required;
• there is no possibility of solution stratification;
• self-compacting concrete is characterized by noise and vibration insulation;
• has an attractive appearance.

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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.

Properties

Self-compacting concrete has the following properties:

• Tensile strength. Self-compacting concrete is higher than that of simple concrete composition.
• Compressive strength. Provided that the mixtures contain equal proportions of cement and water, this type of concrete will provide a denser consistency of components than that of a vibrated mortar.
• Creep. A mortar based on sand, Portland cement, a plasticizer and a chemical modulator has increased creep, but, nevertheless, this coefficient is within the accepted acceptable limits.
• High adhesion. The material has increased adhesion of the solution to the reinforcement.
• Shrinkage. The shrinkage of the solution will depend on the volume of cement adhesive, and the amount of adhesive in self-compacting concrete is not very different from a conventional cement-sand composition.
• Elasticity. In self-compacting concrete, the elasticity is fifteen percent lower than conventional cement-sand mortar. The decrease in elasticity is due to the content of dry components of the mixture of fine fractions and a small amount of ingredients of large fractions.

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Production and transportation

Concrete production plant

The preparation of self-compacting mixtures requires highly qualified personnel and specialized equipment, which does not allow its production at the construction site with your own hands. You can verify this by reading the recommendations and requirements presented below for manufacturers of this material.

Transporting concrete

Instructions and technological requirements used in the production and transportation of commercial self-compacting mixtures:

1. Extreme accuracy of dosing materials before feeding into the mixing plant.
2. Drying and moisture control of fine and coarse aggregates.
3. Use additional water only in accordance with the specifications provided for this grade of concrete.
4. Complete unloading of the mixing plant drum or concrete mixer tank, followed by thorough washing of the tanks.
5. Strictly follow transportation rules. Take into account weather conditions, air temperature - all this can disrupt the condition and design composition of the transported concrete.
6. If the technological instructions provide, or the need arises, for the additional addition of thinning additives at the construction site, it is necessary to strictly adhere to the recommendations of the concrete manufacturer so as not to violate the design parameters of viscosity and spreadability.
7. Transportation of concrete to the construction site should be carried out only by certified vehicles that have passed acceptance tests.

Certified transport

Compound

Self-compacting concrete is made from the following ingredients:

• water;
• Portland cement;
• plasticizers;
• sand;
• chemical modifiers;
• crushed stone of small fractions;
• various fillers, which increase resistance to corrosion and cracking, increase the strength of the material.

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HOW to test the SUB?

There are several procedures for testing the ductile properties of SCC. The flow test, using a conventional cone, is the most popular "field" test recorded by ASTM (American Society for Testing and Materials). The cone is filled with concrete without compaction, raised, and the spread is measured. The spread can range from 45 to 81 cm. Delamination resistance can be tested using the Visual Stability Index (VSI). This measurement is taken at the base whether there is water on the top layer or crowding in the center. Index values ​​range from 0 for highly persistent to 3 for unacceptable persistence.

The viscosity of a mixture can be measured by flow testing with a cone. To do this, you need to measure the time it takes for the mixture to spread 50 cm from the moment the cone is lifted. This is called the T50 measurement and usually has a value of 2 to 10 seconds (although it is not clear what the value is for a self-compacting mixture with a completely acceptable spread value of 45 to 50 cm - approx. transl.). A higher T50 value means a more viscous mixture, more suitable for heavily reinforced objects and deep excavations. Smaller T50 values ​​are suitable for cases where the mixture needs to spread over a long distance horizontally without obstacles.

Preliminary mixture evaluation uses the U-box and L-box tests, where concrete is poured into one part of the box, then the baffle is opened and the ability of the concrete to fill the second part of the box containing the reinforcement is measured. A variation of the cone test is the J-ring, where a reinforced structure is placed around the cone and the ability of the SCC to spread without delamination is measured as the cone is lifted. All of these tests measure the ability of the SCC to overcome dense reinforcement. Another standard test is the column test, which measures the coarse aggregate content at different heights of a cast column as an indicator of persistence (resistance to delamination).

Application and installation features

Self-compacting concrete is used in the following areas of construction:

• during the construction of hydraulic structures;
• for the production of precast reinforced concrete;
• during the construction of monolithic floors without seams;
• to strengthen buildings and structures;
• for structures with a high-quality surface that does not require additional processing;
• during the construction of structures that consist of a large amount of reinforcement;
• the material is used in the construction of fences or thin walls, when a minimum weight of floors is required.

When laying self-compacting concrete, some features should be taken into account:

• a large number of superplasticizers in the solution slows down the setting of the laid mixture;
• in the process of transporting cement liquid with the ability to self-compact for an hour or more, the effectiveness of the superplasticizer decreases, which means the mobility of the solution decreases;
• when supplying a solution with the special property of self-compacting through a pipeline to the working area, a distance that exceeds two hundred meters, delamination and heterogeneous state of the finished product are formed;
• due to possible delamination and inhomogeneous states of products, work time increases, quality deteriorates, and the strength characteristics of the finished mixture decrease;
• when laying the mortar, you should check the formwork for the presence of liquid in it and, if necessary, remove it, since even the slightest amount of excess water can lead to delamination and reduce the properties of concrete;
• During the laying process, it is important to maintain continuous concreting;
• concrete solution with the ability to compact itself does not require additional compaction;
• when the required quality characteristics of the cement-sand mixture decrease, it is restored with special thinners, which are added to the mortar.

To improve the quality of a cement-sand mixture with the ability to self-compact, the following implementations are used, which include:

• the use of sand and crushed stone of small fractions makes it possible to produce high-strength concrete;
• the use of ultra- and microfine fillers increases strength, corrosion resistance and reduces the formation of cracks in the material.

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Characteristics

This type of material has five indicators according to which it is classified into groups: delamination ability, layability, normal viscosity and when the solution flows through a funnel, and also resistance.

1. Classes PA1 to PA2 - in terms of resistance, especially when passing through reinforcement;
2. Classes from SF1 to SF3 - according to the stackability or spreading of the cone;
3. Classes from VS1 to VS2 - based on the spreading of a concrete cone over a circular area 50 cm wide, the viscosity is calculated based on time;
4. Classes from VF1 to VF2 - according to the degree of viscosity calculated when passing through a funnel;
5. From SR1 to SR2 - using sieves, the delamination thickness is measured, according to which this classification is carried out.

Concrete marked S is called highly flexible and, according to GOST 26633-2012, is used in complex or vertical construction, as well as in reinforced structures. F-marked mixtures are called viscous and are poured where high strength or finish is not required.

In terms of their properties, self-compacting materials are in no way inferior to conventional concrete mixtures, for example, their elasticity ranges from 30 to 36 GPA, shrinkage is up to 800, the percentage of air in hardened concrete reaches 4-6, and the volume of water accounts for one quarter. Over time, the resistance of concrete to compression increases: if after four weeks of operation it ranges from 40 to 80 MPa, then after three months it will reach 55-110 MPa.

All these are general characteristics of SCC, however, each mixture, depending on its scope of application, has its own properties. When ordering concrete, it is worth stipulating and clarifying spreadability, viscosity, delamination index - the proportions specified by manufacturers are usually known to suppliers. ACI 237 is engaged in the preparation of a factsheet on flow indicators depending on the type of construction. Those who are working with self-compacting mixtures for the first time can base their performance on regular concrete, since they are similar.

Diagnostics

Standards indicating the classification and description of diagnostic methods for self-compacting concrete mixtures are available only in Europe. To diagnose the workability and fluidity of a concrete liquid, the rheological method is used, which is the science of deformation and fluidity of a substance. Scientific research includes diagnosing a cement-sand mixture with the ability to self-compact according to the following parameters:

• solution mobility;
• viscosity;
• quality characteristics;
• delamination;
• drainage;
• strength characteristics;
• workability.

To analyze the studied parameters, mathematical planning of the experiment was used based on the following factors:

• dosage of plasticizers in the concrete mixture – 0.8%, 1%, 1.3% by weight of cement;
• dosage of stabilizers in the concrete mixture – 0.05%, 0.1%, 0.15%, 0.3%.

Cement samples with dimensions of 10x10x10 cm were studied. After preparing the mixture, the workability of each type was determined and kept in a room with optimal temperature and humidity conditions, and the strength of the concrete was checked on the first day, the third, the seventh, and after two weeks and a month. The data is entered into a table and an analysis is carried out to determine water separation and delamination of the cement consistency.

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Paving stones and paving slabs

Based on the results of research conducted in Japan, since 1988, the main provisions in concreting technology using self-compacting concrete (SCC) have been determined. The development of self-compacting concrete technology has been completed, the use of which has been regulated by the Reinforced Concrete Directive since 2003. This directive defines SCC as “concrete capable of being compacted by its own weight without the influence of additional compaction energy, freed from its air content, conveyed and completely filling the space of the formwork, including between reinforcing bars.” With the introduction of this directive, the use of self-compacting concrete is possible without the consent or general approval of the building authorities, which further promotes its use. Due to its properties, SCC is particularly suitable for the production of prefabricated structures or concrete surfaces that require special architectural solutions.

Self-compacting concrete has the following advantages: - uniform quality of concrete over the entire cross-section - minor restrictions in the design of building elements - improved durability - improved properties of decorative concrete - easier concreting work - reduced construction time - sound insulation and health protection on construction sites and factories ready-made structures

Basic provisions

In accordance with the chosen design principle, SCC is divided into three types, see Table 1. As a rule, compared to vibrated concrete, SCC has a significantly higher content of fine suspended matter. The traditional composition of the concrete mixture, which is dominated by the highest possible content of granular aggregate and low content of fine particles, is not used in the preparation of self-compacting concrete. Instead, a fine suspension (cement + granular aggregate with a grain size < 0.125 mm + concrete additives), mixing water and SUB thinner form an adhesive in which the coarse granular aggregate simply “floats”.

Table 1: Content of fine suspended matter in self-compacting concrete

Two features are decisive for the functional ability of the SUB. On the one hand, the mobility of the concrete mixture must be high enough to facilitate the release of concrete from the air contained in it, create optimal adhesion between steel and concrete even with a high degree of reinforcement, and reduce the risk of defects (for example, accumulation of gravel). On the other hand, the SCC must have a good ability to adhere to individual components and prevent separation of the mixture. When segregation occurs, two effects can occur: - Sedimentation: settling of coarse granular aggregate, sweating - Separation: flowable concrete is not able to transport coarse granular aggregate during pumping.

To simultaneously achieve sufficient mobility and adhesiveness of the components, the optimal ratio of the composition and amount of fine suspension, water and thinners should be observed. The amount of water must be determined in such a way that it exactly matches the water consumption of the fine suspended matter and moisturizes the surface. At the same time, it becomes clear that replacing some fine-grained components with others (for example, replacing cement with fly ash) leads to a change in water consumption, and the changed composition, as a rule, entails a change in the properties of the freshly prepared concrete mixture.

Additional addition of water simultaneously affects the mobility of the concrete mixture and the ability of the concrete components to adhere. This can lead to the fact that while simultaneously increasing the mobility of the concrete mixture, the adhesion between its components will decrease, which will make the concrete structure unstable. A change in the amount of water added of ± 3 l/m may be sufficient to cause slumping, separation, air entrainment or low workability of the concrete mixture. While water affects the mobility and cohesion of the components of a concrete mixture and can cause it to separate, the use of a thinner regulates its flow.

Currently, in the production of self-compacting concrete, almost exclusively new generation liquefiers are used, so-called polycarboxylate-based liquefiers. On the one hand, the interaction between cement and polycarboxylate-based thinner must be known, on the other hand, it is necessary to take into account the mobility characteristics depending on temperature. In addition, some thinners, when mixed in a concrete mixer, can cause an additional thinning effect, which is referred to as the “sludge effect” and can lead to subsequent separation of the concrete mixture.

The addition of pulverized materials (limestone powder or fly ash) improves the workability of the mixture. However, too high a content of very fine particles under the same conditions leads to a decrease in its mobility. Along with the effect of additives on the rheological properties of self-compacting concrete, they also affect the appearance of the concrete surface. By adding limestone or quartz powder, the surface of the concrete becomes lighter than by adding fly ash. The described connections between the individual components of the concrete mixture influence the fact that relatively minor changes in its composition cause significant changes in the properties of self-compacting concrete.

Requirements for freshly prepared concrete mixture

2.1 Fluidity

The flow test without blocking ring (sm) evaluates the flowability of concrete. The spreadability of conventional self-compacting concrete is 700 to 800 mm. The yield test using a blocking ring (smb) evaluates the flowability of self-compacting concrete between reinforcing bars. This primarily determines whether the cement adhesive is capable of moving the coarse granular aggregate between obstacles (for example, between reinforcement bars) or whether the obstruction of the coarse granular aggregate causes it to settle. This effect can also occur if the distance between the reinforcement bars exceeds the maximum diameter of the aggregate grains. Therefore, to carry out the test, the number of rods and the distance between them should be determined depending on the size of the largest grains (Table 2). The diameter of the rods is respectively 18 mm, the diameter of the blocking ring is 30 cm. The use of crushed granular aggregate additionally creates obstacles to the fluidity of the concrete mixture. A mixture of self-compacting concrete is considered suitable for use if the height of the concrete cake is the same outside and inside the blocking ring, there is good movement of the largest aggregate grains through the reinforcing bars and the flow value sm exceeds the smb index by a maximum of 50 mm, see fig. 1.

Table 2: Number of Locking Ring Rods

2.2 Time of flow through the funnel

The viscosity of self-compacting concrete is determined by the time the mixture flows through the funnel (tTr). In this case, the time required for the concrete mixture to flow out of the v-shaped funnel in a uniform stream is measured (Fig. 2). For ordinary self-compacting concrete, this time ranges from 5 to 20 s.

2.3 Tendency to sedimentation

In order to test the sedimentation tendency of self-compacting concrete, the concrete mixture is filled into a cylindrical mold 500 mm high and 150 mm in diameter. The mold is equipped with valves that separate the mixture into three parts. After the cement adhesive has been washed out, the difference in weight of the coarse aggregate determines whether the concrete is prone to sedimentation. If the content of large grains differs by less than ± 20% from the average content of large grains, then the SCC is considered stable to sedimentation.

Research has shown that the interdependent values ​​of spreadability and funnel transit time can serve as criteria for determining the suitability of a concrete mixture. Based on these indicators, determined during standard tests, an individual workability range is established for each type of SCC (Fig. 3), guaranteeing the absence of sedimentation or separation effects, sufficient fluidity, the ability of the concrete mixture to be freed from the air contained in it, and stability with respect to sedimentation.

For the same mix composition, exposure to different temperature conditions will result in different workability ranges. To provide a quick and easy test of fluidity and viscosity suitable for construction conditions, the Federal Union of Cement Manufacturers has developed a combined method that allows both indicators to be determined in one test. The equipment required to perform this test is a discharge cone mounted on a plate to determine the flowability of the concrete mixture (Figure 4).

Concrete production and transportation

Making self-compacting concrete requires high-quality equipment and trained personnel. Due to the sensitivity of the SMS in relation to water consumption, the following requirements are imposed on producers:

— Maximum dosing accuracy in the mixing plant. — Removal before production of moisture accumulating in the hopper under the granular aggregate. — Continuous control of sand moisture content; availability of data on the moisture content of coarse granular aggregate. — The use of residual water is in accordance with only under certain conditions. — Complete emptying of the mixer drum and concrete mixer truck, as well as pumping water for flushing.

Different transport times can affect the consistency of the concrete mixture in such a way that the above-mentioned “sludge effect” (depending on the choice of thinner) will act differently under certain circumstances. In addition, exposure to weather conditions, such as sunlight, can lead to an increase in the temperature of the freshly prepared concrete mixture and thereby change its consistency. In order to respond to possible changes in consistency due to transport, weather conditions, etc. By timely changing the composition of the concrete mixture, the construction site and the ready-mixed concrete plant must constantly exchange information about the properties of the freshly prepared concrete mixture. Additional addition of a thinner at the construction site is necessary to ensure that the freshly prepared concrete mixture reaches the required properties immediately before laying. This assumes that the concrete manufacturer has appropriate dosing data that gives a clear indication of how much thinner must be added to achieve the required flow and viscosity at the existing temperature and consistency of the prepared concrete mixture. These indicators are determined within the framework of type tests.

Due to the sensitivity of self-compacting concrete to production, transport and placement, each vehicle is subject to acceptance tests according to the directive. These tests involve checking the flowability of the concrete mixture with or without the use of an interlocking ring, which is a simple way of assessing the serviceability of self-compacting concrete. Evaluating the interrelated values ​​of spreadability and funnel transit time allows the user to determine whether the SCC is within the range of placement identified by testing. In practice, based on simple use, a combined method with a discharge cone is used. Accurate determination of the delivery time of the concrete mixture is of great importance. Continuous pumping of the concrete mixture must be ensured in such a way that after pumping the mixture from one vehicle, you can immediately begin emptying the other. In this case, attention should be paid to the timely arrival of vehicles with ready-mixed concrete at the construction site, which will allow timely testing of the freshly prepared concrete mixture. These conditions place high demands on the ready-mixed concrete plant in terms of logistics services and are met satisfactorily if type tests are carried out under extreme conditions (eg different temperature ranges) appropriate to the construction site.

Laying concrete mixture

Before laying the concrete mixture, it is necessary to check whether there is any water (for example, rainwater) in the formwork. Even low moisture content can cause self-compacting concrete to delaminate as its water content can rise to unacceptable levels. Before placement, self-compacting concrete must travel a certain distance sufficient to remove the air it contains (Fig. 5). In exceptional cases, when there is no area where air is removed from the concrete, it must be created artificially. In this case, the concrete should not flow too quickly. Air removal occurs when the concrete mixture can flow over the edge until it reaches its final position in the formwork. If the concrete mixture is laid intermittently, then a layer of thick, viscous cement glue, the so-called “elephant skin”, forms on the surface of the concrete. It prevents mixing of different doses of concrete mixture; subsequently, this separating layer can be imprinted on the surface of the concrete. In addition, even due to minor downtime in the formwork, concrete can partially or completely lose its self-compacting properties.

Concreting of building elements in which self-venting is difficult (for example, long and thin columns, formwork with recesses) must be carried out especially carefully. The presence of short sections for air removal and high speed of lifting of the concrete mixture (mainly in tall and thin columns) leads to the fact that the lifting force of the air pores is not high enough for them to penetrate through the layer of freshly laid concrete mixture located on top. Based on this, if possible, the entry of air into the concrete should be blocked, and before laying the concrete mixture, air should be removed from it. Unnecessary air ingress can be avoided when the concrete mixture is not dropped onto the reinforcement from a great height, but is supplied to it by constantly immersing the hose in fresh concrete during pumping. This also prevents the creation of different layers in the concrete structure due to the formation of “elephant skin”. If this is not possible due to the high degree of reinforcement, a good alternative is to fill the concrete mixture from below through the formwork spigot. To do this, it is proposed to use special frame formworks, equipped at the factory with a filling pipe to which the pump hose is connected (Fig. 6). In this case, the mixing device in the loading hopper of the concrete pump must be covered with freshly prepared concrete mixture throughout the entire pumping process.

Based on the properties of self-compacting concrete, continuous placement and continuous supply of concrete mixture is mandatory. If SCC loses the properties necessary for its laying, under certain circumstances, even after a long period of transportation or storage, it is possible to restore the workability of the concrete mixture by adding liquefiers. Concrete cannot be compacted. Self-compacting concrete flows on its own under low shear stress and hardens under high shear stress.

Pressure on formwork

The provisions for determining the pressure on formwork given in DIN 18218 for ordinary concrete do not fully apply to self-compacting concrete. Due to its easy placement, SCC allows high speed concreting of very long and thin building elements, such as columns. The advantage of quick installation is counteracted by increased pressure on the formwork, which requires careful determination of its dimensions. Studies carried out to date and determining the relationship between concreting speed and the properties of freshly prepared concrete mixture on the one hand and the resulting pressure on the formwork on the other hand, give conflicting results. In the absence of accurate results for the corresponding SMS, it is recommended to determine the presence of hydrostatic pressure on the formwork. Both the composition of the concrete mixture and the method of installation have a significant influence. When concrete is fed into the formwork from below, not only hydrostatic pressure is applied to it, but also additional pump pressure necessary to overcome the adhesion friction between the concrete and the formwork (pay special attention when using a piston pump). Therefore, when supplying concrete mixture from below, the calculated pressure on the formwork must be further increased.

Based on the expected maximum pressure on the formwork, its corners must be resistant to torsion. In addition, it is necessary to choose the correct distance between the formwork anchors, since, on the one hand, too many anchor cones should not appear on the surface, and on the other hand, despite the high pressure on the formwork, large curvatures should not occur in the deck, which may adversely affect the appearance of the concrete surface. Sometimes the joints between formwork and decks are not completely closed or the deck has defects of several millimeters (for example, gaps between formwork boards). Such places will subsequently be reflected on the concrete surface. If in vibrated concrete at the site of these defects there is a danger of the formation of accumulations of gravel due to protruding cement glue, then for self-compacting concrete due to the significantly better adhesion between its components there is no such danger.

Properties of self-compacting concrete

In general, the properties of self-compacting concrete correspond to those of conventional concrete. Self-compacting concrete can be designed as normal strength concrete or high strength concrete.

Compressive strength

With the same cement content and water-cement ratio, SCC has a higher compressive strength compared to vibrated concrete due to a more dense connection between its components.

Tensile strength

For the same compressive strength, SCC has a slightly higher tensile strength compared to conventional concrete.

Bond between concrete mixture and reinforcement

Due to the high mobility and strong adhesion between the individual components of the SCC, it has good adhesion to the reinforcement. In this case, the adhesion does not depend on whether the reinforcement is located at the top or bottom.

Elastic modulus

The elastic modulus of SCC is approximately 15% lower than that of traditional concrete. This is explained by the increased content of fine suspended matter and the associated low content of coarse granular aggregate.

Shrinkage

The amount of cement adhesive influences the shrinkage. Since self-compacting concrete differs slightly from ordinary concrete in terms of the content of cement glue, the shrinkage characteristics of both types of concrete are the same.

Creep

According to studies carried out to date, the creep coefficient of SCC is slightly higher than that of ordinary concrete, but is within the generally accepted tolerance limits for ordinary concrete.

Concrete surface quality

The surface of building elements made of self-compacting concrete reflects the structure and structure of the formwork (including pencil inscriptions) down to the smallest detail. SUB can reproduce the smallest relief of the formwork (Fig. 7), so it is very popular when designing the surface (for example, using a matrix). However, this means that defects such as patches or nail holes are also imprinted on the surface. If there are high requirements for the surface of decorative concrete, it is necessary to pay attention to the quality of the deck. In particular, measures must be taken to ensure the necessary coverage of the concrete mixture without imprinting, so that they are comparable to the properties of traditional lightweight concrete.

Self-compacting lightweight concrete

In December 2003, the first approvals for self-compacting lightweight concrete were granted by the building authorities. Self-compacting lightweight concrete is also capable of flowing and releasing the air it contains solely under the influence of its own weight. Despite the fact that the dry density of the lightest self-compacting concrete with approval issued by the building supervision authorities is 1.38 kg/dm2, it also has a sufficient self-compacting coefficient and rheological properties. Regarding mechanical properties, the same requirements apply according to DIN EN 206-1 and DIN 1045.

Based on the special properties of the freshly prepared concrete mixture, self-compacting concrete offers numerous advantages, including the design of special building elements that cannot be made from traditional types of concrete.

conclusions

Self-compacting concrete is concrete with special properties of a freshly prepared mixture. To ensure these properties in practice, the German Committee for Reinforced Concrete has defined in a directive the corresponding standards introduced by the building supervision authorities. As for the properties of rigid concrete, in this case SCC does not differ from ordinary concrete. To these properties of the surface of traces from spacers between reinforcing bars.