Self-compacting concrete
Self-compacting concrete (SV-Beton or SVB, English: SCC: "self compacting concrete") is a concrete that ventilates solely by gravity and surrounds the reinforcement . This is made possible by a very flowable consistency, which is achieved by modern high-performance concrete plasticizers (HBV) based on polycarboxylate .
History of self-compacting concrete
The patent for the invention of self-compacting concrete holds Gerald Schlung. In the 1980s, developments began in Japan, where unskilled workers on construction sites often led to deficiencies in the durability of concrete as a result of incorrect compaction . There, self-compacting concrete is used economically, especially for large projects. In Europe there is some experience with self-compacting concrete, especially in Sweden and the Netherlands. At the same time, the new building material is now also used in most other EU countries and the USA. In Germany, self-compacting concrete was used in the large-scale project of the Science Center phæno in Wolfsburg and the BMW central building in Leipzig.
Types of self-compacting concrete
There are three types of self-compacting concrete:
- the flour grain type
- the stabilizer type
- the combination type
With the flour grain type, the self-compacting properties of fresh concrete are achieved by increasing the flour grain content. With the stabilizer type, on the other hand, stabilizing concrete admixtures are used in order to obtain a self-compacting concrete that does not suffer from separation and sedimentation . With the combination type, additional stabilizers are added if the flour grain content is increased. What all types have in common, however, is that they cannot develop their properties without high-performance concrete plasticizers based on polycarboxylate ether.
properties
The self-compacting properties are achieved through a fundamental change in the composition of the mixture. The rheological fundamentals are of particular importance, as they are completely redefined through the use of new types of high-performance concrete liquefier.
The new high-performance concrete plasticizers have an excellent dispersing effect on the cement and fine particles of the concrete. The internal friction values between the particles are extremely reduced and the water requirement is reduced by reducing the electrical binding forces.
This is due to the molecular design of the high-performance concrete plasticizer, which is characterized by very long polycarboxylate side chains. Since the molecules attach to the cement and fine particles, the side chains create a spatial repulsion that prevents the particles from agglomerating with one another. The high-performance concrete liquefier molecules act as a support structure between the individual particles of the suspension .
In general, self-compacting concrete has the same properties as normal concrete, but there are additional requirements for the self-compacting properties. Of primary interest are:
- viscosity
- Blocking tendency (important with a high degree of reinforcement and pumped concrete)
- Self-venting
The properties mentioned are very dependent on the choice of additives, since self-compacting concrete is extremely sensitive to fluctuations in the quality and amount of mixture components and then may not achieve the desired properties. This requires meticulous documentation and extensive testing of ongoing production, as the properties can no longer be influenced afterwards.
Mix design
The foundations for the design of mixes for self-compacting concrete were laid in Japan. In general, a completely different and at first glance unusual approach compared to normal concrete has become necessary here.
At the beginning of the design, the type of self-compacting concrete and thus the rough composition as well as the additives are determined. The important thing is:
- Preselection of a calculated air void content of approx. 4 to 6 % by volume
- Limitation of the coarse allowance (d> 2 mm) due to the strongly increasing internal stress to approx. 50 to 60% of the total allowance volume without air pores
- Analogous to this, a limitation of the fine aggregate (125 µm <d <2 mm) to 40% by volume of the mortar volume
Under these conditions, first tests are carried out on the finest components of the future mixture. These contain all components up to a diameter of 125 µm. Five fine material suspensions with different ratios of water to fine material volume (w / FV) are prepared for each mixture, whereby the water content is varied in each case. The respective slump of the mixtures is then determined with the setting funnel-to-Haegermann table, which is converted into the “relative slump flow rate” using a formula and entered in a diagram. Once all five values have been determined, a linear regression follows. The y-axis section of the resulting straight line represents the water requirement, the slope is a measure of the sensitivity of the mixture.
In the next stage, sand and high-performance concrete liquefier are added to the mixture, whereby the above restrictions apply to the sand. The content of high-performance concrete liquefier is now varied. The properties are checked with the help of a special V-funnel and a setting funnel for the Haegermann table. Both the viscosity and the slump are determined. Certain limits must be observed for both values.
The last stage of the draft also includes the rough surcharge as a mixture component. Again, viscosity tests are carried out with a V-funnel as well as spreading tests with the spreading table. Once the mixture has achieved the desired properties, the final tests for processability follow.
In order to test the workability of the concrete, tests are carried out with the L-box , the “Kajima” box and the J-ring . The L-Box is used to test the tendency to block and the leveling behavior and consists of a steel box with two rooms, which are separated from each other by a slide. A reinforcement obstacle is built into the passage between the rooms. The kajimabox consists of a plexiglass box with built-in barriers. This device is used to determine the degree of filling of the concrete - i.e. the property of filling the cavities. Defined limit values must be observed in both experiments. Finally, the J-ring determines additional consistency properties of the self-compacting concrete. In addition to the slump, the time to reach it is also measured. Both values must be within certain limits.
Manufacture and installation
The production and installation of self-compacting concrete requires experienced and trained specialists, especially because deviations in the properties of fresh concrete cannot be compensated for during compaction.
During production, it is particularly important to dose the amount of water very precisely, whereby, as with other concrete, the moisture in the aggregate must be taken into account. Especially with very low water-cement values for high-strength self-compacting concretes, small fluctuations in the moisture content can have a strong effect on the end product. Furthermore, in such cases of low water-cement values or a known sensitivity of the mixture, the water contained in truck mixers should be taken into account or an effect should be determined by means of field tests. Today's mixing technology is usually well enough equipped to meet these increased requirements. After filling the mixer, under no circumstances should water or superplasticizers be added, as this would lead to a considerable disruption of the mixture.
At the construction site, the concrete should be tested with the flow slump test and the funnel discharge test prior to installation to ensure that the concrete is self-compacting and that the deviations from the target values remain within the tested specifications.
Before installation, it must also be ensured that the increased requirements for the formwork are complied with. This includes both the tightness of the formlining and the thicker design of the formwork due to the higher fresh concrete pressure.
application areas
Inclined surfaces, such as bridge panels, are problematic when using self-compacting concrete, since the planned creation of a slope without counter-formwork is problematic. On the other hand, very filigree components or components with a very high degree of reinforcement are well suited for the use of self-compacting concrete.
This type of concrete is ideally suited for the following requirements and resulting applications:
- Fair-faced concrete components due to high surface quality (columns, beams, ...)
- high wall components (with appropriate formwork)
- large ceiling sections
- Precast concrete plants
Furthermore, self-compacting concrete with a high cement and fly ash content has a high reducing power with regard to the water-cement value due to the strong addition of superplasticizers. This results in a higher strength than with conventional concretes and allows other component dimensions.
Construction management aspects
A construction-related consideration by Motzko / Huth of the Technical University of Darmstadt shows that self-compacting concrete is also an extremely interesting building material from an economic point of view. In particular, time and personnel savings are mentioned here, which are disproportionately high in relation to the additional costs for material and production. The full report is included on the web links for your reference.
Since a complete work step is either omitted or at least significantly reduced in conventional components, the expenses for equipment, maintenance and construction site safety are also reduced. Fewer people can do the same work, which means that part of the passenger transport and required vehicles and social rooms on the construction site are no longer required. This may not have a significant impact on the overall economy for a smaller construction site because the amount is missing. In the case of large construction sites, however, this should very well be included in the decision-making process.
In any case, it can be said that the risk to employees in the area of concreting is also reduced due to the reduced use of personnel. Vibration and noise exposure of employees are also reduced.
Regulations for the use of self-compacting concrete
The use of self-compacting concrete is regulated in Germany in DIN EN 206-9: 2010. The German Committee for Reinforced Concrete had previously issued a guideline on the use of self-compacting concrete in 2003.
Web links
- Further reports on self-compacting concrete
- Diploma thesis on self-compacting concrete as a PDF download
Individual evidence
- ↑ Patent No. 106555 from June 20, 1974 of the Berlin Patent Office
- ↑ Detailed studies of the form pressure can be found e.g. B. in ( PDF ( Memento of the original dated December 4, 2016 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this note. ).
