In simple terms, self-consolidating concrete (SCC, also known as self-compacting concrete) is a concrete mixture that can be placed simply by means of using its own weight with little or no vibration. The benefit of this is that it creates a highly flowable, non-segregating concrete that spreads into place, fills formwork and encapsulates even the most congested reinforcement.
Additionally, SCC has a better pumpability, creates less friction on pipe lines and the pump pressure can be reduced and pumped for long distances with a cohesive SCC mix. These benefits reduce the labour requirement for construction and thus reduces the overall cost for the entire project. As a concrete, SCC delivers these attractive benefits while maintaining all of concrete’s customary mechanical and durability characteristics.
Traditionally, concrete that had the fluidity of SCC had a high water-to-cement ratio that compromised its compressive strength and durability. However, adjustments to traditional mix design and the introduction of high performance, polycarboxylate-based, high-range water reducers (HRWR) alongside increasing the amount of fine material (decreasing the coarse aggregate) took SCC to the next level. Now it is possible to create a highly flowable and stable type of concrete that doesn’t sacrifice on critical building properties.
For a qualified mix design, there are three major factors to consider. The first is measuring the sensitivity of the mix by looking at the variation in the water content, cement and aggregates variation. Practical experience shows that there is a permissible range of ± 5 litres/m3 variation in water content. The next consideration is the sustainability of rheological conditions (i.e. the effect of temperature) and finally the use of VMA for more robust mix designs.
If needed, low dosages of viscosity modifying admixtures can be used to eliminate unwanted bleeding and segregation. Also, SCC typically has higher amounts of cementitious materials and fines when compared to conventional concrete and so must always have well-graded aggregate distribution.
Client are therefore able to source a concrete solution that exhibits a number of practical benefits, particularly with regard to tall buildings, bridges and precast sections. The fluidity of SCC enables it to turn corners and fill forms much more effectively and at a faster rate than would otherwise be possible. This not only has an obvious immediate benefit but it also reduces both labour costs and wear on equipment.
The workability of an SCC system can be ascertained by conducting a “slump flow” test to check its flowing characteristics. A slump test (as set out in the ASTM C-143 standard test method) is usually carried out to judge plasticity and workability of fresh concrete and the subsequent flow of concrete is measured by the diameter of the spread (as determined in ASTM C1611 – Standard Test Method for Slump Flow of Self-Consolidating Concrete).
Some of the other standard test methods followed globally are Inverted Cone, VSI (Visual Stability Index), Static Segregation Column Technique (ASTM C1610), Static Segregation Test (Field Methods) as well as the most familiar tests conducted in India, such as V Funnel, L Box, U Box, J Ring (ASTM C1621) and Filling Capacity Box .
Even after the concrete has been poured and fully cured, SCC will continue to deliver advantages over traditional concrete. For example, conventional concrete that has been placed into a form and vibrated commonly exhibits defects such as bugholes in the surface that take a lot of time and money to repair. In comparison, after SCC has finished curing it has a much smoother, unblemished and consistent appearance.
Due to these properties, SCC is very advantageous in a number of market sectors and building scenarios, particularly with regard to fast-track construction where monolithic/aluminium form work is utilised for mass construction. In general, SCC has found to be ideal for beam column joints, columns and walls, foundations and basements that require water tightness as well as for areas where it is difficult to place the concrete and for reducing construction joints in retaining walls. Alongside tall buildings, other market sectors that are embracing SCC include tunnel lining projects and mass housing developments that require monolithic formwork.
To ensure that it provides all of the above characteristics, SCC must be carefully produced using best practices for quality control and production. These types of concrete mixtures can be particularly sensitive to a number of factors, including mixture ingredients, aggregate moisture and admixture dosage rates.
SCC typically has higher amounts of cementitious materials and fines when compared to conventional concrete and must always have well-graded aggregate distribution. In addition to these considerations, hauling distances, weather conditions and mixing equipment can also have an impact on the stability of the SCC. To ensure that the flow and stability properties are just right for the project in question, selecting the appropriate high range water reducers along with other products to modify the viscosity of the mixture is essential.
The Euclid Chemical Plastol series of admixtures and Vicstrol (VMA), available from Flowcrete India, allows customers to source polycarboxylate-based, high-range water reducer (HRWR) solutions that have been put to the test at some of the world’s most demanding construction projects.
SCC that utilises Plastol admixtures are known to minimise voids on highly reinforced areas which allows for innovative architectural shapes and features to become a reality. This was illustrated at Michigan University’s Broad Art Museum, where walls were created at incredible angles, tilting over at 70-75 degrees! This impressive feat of architecture was possible thanks to the use of Plastol 6400 as the key admixture in the building’s concrete.
Another important feature of the museum was that the walls portray a blank canvas aesthetic, which meant that the concrete surface had to be smooth and free from imperfections. The use of SCC meant that the bare walls were created without large bugholes or voids to spoil the appearance of the building.
To find out more about self-consolidating concrete admixtures, talk to your local Flowcrete India representative today.
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