Self-consolidating concrete

Self-compacting concrete (SV- concrete or SCC, international: SCC: "self compacting concrete" ) is a concrete, the vented solely by gravity and encloses the reinforcement. This is possible by a very fluid consistency which is achieved by modern Hochleistungsbetonverflüssiger (HBV) on polycarboxylate.

History of the SV - concrete

The patent for the invention of the self-compacting concrete holds Gerald Schlung. Then the development went out in the 1980s from Japan, where it was frequently due to unskilled workers at the construction sites to deficiencies in the durability of concrete as a result of false compaction. There SVB is used economically particularly for large projects. In Europe, especially in Sweden and the Netherlands some experience with SCC. At the same time the new building material is now also used in most other countries of the EU and the USA. In Germany SCC was used in the major project of the Science Center in Wolfsburg phæno and the BMW Central Building in Leipzig.

Types of SV - concrete

There are three types of SCC:

• The type of grain flour
• The Stabilisierertyp
• The type of combination

When grain flour type, the self-compacting fresh concrete properties are achieved by the fines content is increased. In contrast Stabilisierertyp stabilizing admixtures are used to obtain an SCC, does not suffer from separation and sedimentation. The combination type stabilizer is additionally added with increased fines content. All grades, however, have in common that they based on polycarboxylate their properties can not develop without HBV.

Properties

The self-compacting properties are achieved by a fundamental change in the composition of the mixture. Thus, the rheological basis is of particular importance, as these are completely redefined by the use of novel HBV.

The new HBV have an excellent dispersing the cement and Feinststoffpartikel the concrete. The internal friction values ​​between the particles are greatly reduced as well as the water demand is reduced by reducing the electrical bonding forces.

This is due to molecular design of HBV, which is characterized by very long Polycarboxylatseitenketten. Since the molecules attach to the cement and Feinststoffpartikel, is formed by the side chains of a three-dimensional repulsion to each other which prevents agglomeration of the particles. HBV molecules act thereby as a support structure between the particles of the suspension.

The same properties as a normal concrete are made generally to a self-compacting concrete, however, additional requirements will be added to the self-compacting properties. Of primary interest are:

• Viscosity
• Tendency to lock (important with high degree of reinforcement and concrete pumping )
• Self-venting

The properties are highly dependent on the choice of additives, as SVB extremely sensitive to variations in the quality and quantity of mixture components, and then possibly not achieved the desired properties. This requires meticulous documentation and extensive production line testing, since the properties in retrospect can no longer be influenced.

Mix design

The foundations for the design of mixtures for SCC were placed in Japan. Generally this has become necessary to normal concrete a completely different and unusual at first sight approach in comparison.

At the beginning of the design of the type of SCC and thus the gross composition and the additive is determined. It is important to:

• Preselection of a notional air void content of about 4-6 vol - %
• Limitation of the coarse hammer (d> 2 mm ) due to the marked rise in internal stress to approximately 50-60 % of the total surcharge volume without air pores
• Analogously, a boundary of the fine hammer ( 125 microns

Under these conditions, initial experiments are conducted at the finest components of the mixture future. These include all of the components up to a diameter of 125 microns. Five Feinststoffsuspensionen with different ratio of water to Feinststoffvolumen (w / FP ) are used for each mixture approach, in each case, the water content is varied. There the respective slump of the mixtures is then determined using the slump cone - to - Haegermann table, which is entered through a formula in the "relative slump flow " and converted into a graph. Are all five values ​​determined followed by a linear regression. The y -axis portion of the resulting straight line in this case represents the water demand, the gradient is a measure of the sensitivity of the mixture.

In the next stage sand and HBV are added to the mixture, the above limitations apply to the sand. Varies will be the content of HBV. A review of the properties is done using a special V - funnel and a Judge Sets - to - Haegermann table. Be determined both viscosity and the slump. Both values ​​certain limits must be observed.

The final stage of the design also includes the coarse aggregate as a mixture component. Again viscosity experiments are driven with a V-funnel and Ausbreitversuche with the slump. The mixture has reached the desired characteristics, followed by the final tests for processability.

To test the processability of the concrete tests with the L-box, the " Kajima " box and the ring J are performed. The L-box is used to test the blocking tendency and Nivellierverhaltens and consists of a steel box having two spaces which are separated from each other by a slide valve. In the passage between the rooms a reinforcement obstacle is installed. The Kajimabox consists of a plexiglass box in the barriers are installed. This unit is the degree of filling of the concrete - that is, the property, fill the voids - determined. In both experiments defined limits must be observed. The J- ring finally determined additional consistency properties of SCC. It is measured the same to reach next to the slump the time. Both values ​​must move within certain limits.

Manufacture and installation

The manufacture and installation of SCC requires experienced and trained professionals, especially because variations in the properties of fresh concrete can not be compensated during compaction.

In manufacturing it is especially important to meter the amount of water closely, taking as other concrete, the humidity must be considered in the award. In particular, at very low water - cement ratios for high-strength SCC can have a strong impact on the final product small variations in moisture content. Furthermore, the water contained in truck mixers included in the calculation or an impact by field trial should be determined in such cases low water cement ratios or a known sensitivity of the mixture. Today's mixing technology is usually well enough equipped to meet these increased demands. After filling of the mixer may be added under any circumstances water or flow agents, as this would lead to a significant disruption of the mixture.

On the job site, the concrete should be tested prior to installation with the slump flow test and the funnel flow test to ensure that the concrete is self-compacting and the deviations from the target values ​​in the context of the tested specifications will remain.

Before installation, continue to pay attention, that the increased requirements are met to the formwork. This includes both the tightness of the formwork as well as the stronger version of the formwork due to the higher concrete pressure.

Areas of application

A problem with the use of self-compacting concrete, inclined surfaces, such as bridge decks, because the planned manufacture of a gap without lock casing is problematic. Well suited for the use of self-compacting concrete, however, are very delicate components or components with a very high degree of reinforcement.

This type of concrete is excellent for the following requirements and resulting applications suitable:

• View concrete components due to high surface quality ( columns, beams, ...)
• Hydraulic structures due to the very dense structure
• High wall components (with appropriate formwork)
• Large ceiling sections
• Precast plants

In addition, SCC has high cement and fly ash content due to the strong flow additive, a high reducing power with respect to the water - cement ratio. This results in a higher strength than conventional concrete and leaves other component dimensions to.

Construction management aspects

A consideration of possession planning Motzko / Huth Darmstadt University of Technology shows that SCC also economically is an extremely interesting building material. Be mentioned here in particular time and personnel savings that fail disproportionately high in relation to the additional costs for material and manufacture. The full report is given as a reference in the web links.

As usual components in a complete operation either being eliminated or at least significantly reduced, the cost of working equipment, maintenance, and site safety are reduced. Fewer personnel can perform the same work, so also eliminates a portion of the passenger transport and vehicles required, together with social spaces on the site. Total Economically like that for a smaller building site not a significant impact, as the amount is missing. On large construction sites but this should very well be included in the decision making.

In any case, it can be said that the employee hazard is also reduced in the range of concreting by the reduced manpower. Also vibration and noise exposure of employees to be reduced.

Regulations for the use of the SV - concrete

The use of self-compacting concrete is regulated in Germany by DIN EN 206-9:2010. Earlier, the German Committee had issued a directive on the application of self-compacting concrete for reinforced concrete 2003.