Prof. Dr. Hasan Yıldırım
lstanbul Technical University / Civil Engineering Faculty, Materials Science Division
Definition
• Self-compacting concrete (SCC) is a unique type of concrete known for its highly flowable consistency, eliminating the need for vibration and preventing segregation under its own weight.
The following issues are taken into consideration in the design of selfcompacting concrete.
• It should have a very flowable consistency
• It should be homogenous
• Placement without the need for vibration (compression of these concretes occurs solely through gravity) should be feasible
• It should completely fill the mold without any gaps and possess the capability to flow through the reinforcement.
These concrete mixes exhibit structural qualities at least on par with conventional vibrated concrete.
History
It was first implemented in Japan in the late 1980s. It has been used in various projects including aerial tramways, pre-stressed bridges (with pre-stressing on the 3rd day), sandwich-structured subsea tunnels, prefabricated elements for dam galleries, and buildings featuring intricate architecture. The first experiments in Europe were carried out in almost the same application areas between 1992-1995. Greater benefits were achieved in horizontal applications (flooring). Out of the 170,000 m3 of special concrete produced worldwide in 1997, 80,000 m3 constituted SCC. Today, the utilization of both self-compacting concrete and high-performance concrete incorporating its properties has become increasingly high.
One of the paramount features of these concretes is their vibration-free nature. Self-compacting concrete offers significant advantages over vibration compaction, particularly in the following applications.
• Very tall and intricate formwork structures
• Artistic structures with extensive reinforcement Eliminating vibration through SSC usage results in considerable time savings.
• Personnel / material savings
• Increased concrete pouring speed / construction site organization.
• No vibration also eliminates noise
• Improvement in working conditions
Ultimately, the concrete will achieve uniformity and settle with consistent quality, irrespective of vibrator quality, vibrating method, or operator motivation on the day of vibration.
These concretes are highly flowable concretes. By utilizing flowable concrete, meticulous surfaces comparable to those with excellent architectural appearance are achieved. This type of self-compacting concrete must exhibit exceptionally high and uniform fluidity. Interrupted fluidity brings about numerous drawbacks.
• Inhomogeneous concrete leads to issues with filling and leaves traces of sand on the walls
• Uneven surface quality
Properties of SCC
The fresh concrete properties of SCCs are equivalent to those of ordinary concretes. These properties are measured by different experiments.
Mobility of concrete in open environments (Spreading)
Measurement of diffusion in an Abrams cone
Targeted values are between 60cm and 75cm. Simultaneously, the time taken by the concrete to reach a spreading diameter of 50 cm can also be measured.
Mobility of concrete in closed environments (L Box Test)
In the L-Box experiment, the fill rate measurement ratio is determined as H2/H1. Filling ratio must be greater than 0.8. A reinforcement simulation is conducted for lightly reinforced structures.
Simultaneously, to determine viscosity, the time taken for the flow of concrete to reach 20cm and 40cm is measured.
In this experiment, the flow time from the cone is measured.
This experiment enables the measurement of the “stickiness” of concrete. The V-shaped funnel is filled with SSC. Then, the timer starts when the cover at the bottom is opened, and it is stopped when the hole at the bottom becomes visible. This is conducted to assess the concrete’s ability to pass through narrow spaces. Measured values vary depending on the smallest diameter or width of the cone.
Resistance to Segregation (Sieve Test)
This experiment is a stability test using a sieve. The weight of the material passing through a sieve with a diameter of 315 mm and a sieve opening of 5 mm is measured by proportioning it to the weight of the entire sample.
0% < Percentage of material < 15%: satisfactory
15% < Percentage of material < 30%: critical
% < Percentage of material < 30%: bad
Bleeding Test
Measurement of water floating on Tetrachlorethylene at 3, 5, 10, 15, 30, and 60 minutes. This test, recommended by the AFGC working group, is utilized to measure sweating in concrete. Unfortunately, as concrete does not sweat, the result is often zero in many experiments.
J-Ring Test
In this experiment, the J Ring container is used. There are iron bars positioned along the edges of the container. The container is filled with SCC and then released under its own weight, similar to the diffusion test. The internal and external height of the spread concrete is then measured. This test aims to measure the concrete’s capability to flow through reinforcement.
SCC Formulation
The formulation of self-compacting concrete differs slightly from conventional concrete. Optimizations using local materials to meet desired conditions can often be challenging, highly sensitive, and occasionally even unattainable. Formulation, like with all concrete, necessitates precise determination of the expected fresh, and hardened concrete properties.
As there will be no requirement for additional external energy during placement, the concrete’s fluidity must remain constant until the end of casting. Time-dependent rheological protection must be established and maintained.
Components
• The components of self-compacting concrete are the same as those of conventional concrete. (Sand, Aggregate, Cement, Additives, Water).
• The proportions of the components of “liquid” concrete are adjusted based on segregation and sweating criteria.
• The dosage of fine materials (< 80 microns) typically falls between 400 and 500 kilograms. This quantity is determined based on the type of fine material, desired fluidity, surface application type, and additive utilized.
Aggregates
A small Dmax is utilized (the primary target being 10 mm for crushed stone and 12 mm for stream material.) Aggregates should possess good shape and uniformity coefficient.
Sands
A very good sand should be used (around IM 2.5) The consistency of fineness modulus and water absorption needs to be closely monitored. The cleanliness of the sand is the most critical factor in ensuring the desired fluidity and maintaining it over time.
Fine Materials
Fine elements below 80 microns: approximately 500 kg/m3 depending on the intended application type. Cement + Fillers or ash: about 450 kg/m3
Additives
Generally fly ash or limestone fillers Very fine components, such as silica fume, can also be included. Depending on the properties of the raw materials and the desired concrete properties: New generation superplasticizers are used in these concretes.
SSC Production, Mixing
All mixers on the market can produce this type of concrete. However, the mixture requires a longer time compared to traditional concrete. The reason:
• Reducing the size of the aggregate prevents intersection.
• Low energy transfer around the arms of the mixer (smoothness of the mixture).
The efficiency of the mixture varies depending on the type of mixer. (use of arms) Mixing time should be increased in most applications. By timing the addition of cement and water to the mixture, a dry blend of priority components can be achieved. The impermeability of the mixer lids must be regularly inspected to prevent loss of the mixture. The capacity of cement/ mineral additive scales can occasionally pose challenges for full production (500 kg fine/m3). The subsequent addition of superplasticizer may be advantageous to maintain timedependent fluidity. Control of total water is the main factor. We need to regularly check the moisture measuring probes in aggregates and concrete and prevent significant differences. Monitoring the consistency of the concrete using a Wattmeter ensures uniformity throughout the mixture.
SSC Pouring
The high fluidity characteristic of this concrete may pose challenges during transportation. Long distances covered with stationary concrete may lead to segregation within the mixture. It is crucial to ensure that the connection points of the molds are impermeable to prevent any loss of cement slurry. Three suggestions to improve the quality of the poured concrete surface;
• The concrete’s flow rate should be finely tuned to effectively release gases within the mixture. Trapping air bubbles must be avoided by refraining from pouring too rapidly.
• Concrete should be prevented from falling from too high a height.
• To avoid segregation at the end of the mold, the distances that the concrete will flow should be limited.
As a result, thanks to these concretes, there is development towards a more technical profession.
• In terms of regularity of required raw material,
• Careful use of water amounts,
• And in terms of meticulousness in placing concrete
There is progress towards a less difficult profession. At this point, the following question comes to mind, why do some companies regularly pour self-compacting concrete?”
The answer is of course: “Production efficiency”
• A higher efficiency with half the number of people, with a new construction site or factory organization suitable for this type of concrete.
• As working conditions improve, returning to vibrated concrete becomes unthinkable.
• The increasing quality of the surfaces of the produced elements has a very positive impact on the company image.
These are just the first things that come to mind. The following issues come to mind here and should not be forgotten. In these concretes, mold workmanship, precision in concrete consistency, and cost are also important, of course.
As a result, these concretes require great care and precision, and the quality of the company that can pour and apply this concrete is important.
