SCC Concrete - Self compacting concrete

What is SCC concrete?

Definition:

Self-compacting concrete (SCC concrete) is an innovative, high performance concrete that can flow easily and spread readily under its own weight, completely filling formwork and restricted sections, even in the presence of congested reinforcement structures without the need of mechanical vibration and without undergoing any significant separation of material constituents. The hardened self-compacted concrete has the same engineering properties and durability as traditional concrete which is dense and homogeneous. There are many names for this type of concrete such as, self-consolidating concrete (SCC concrete), self-leveling concrete (SLC concrete), or self-placing concrete. 

Congested reinforcement

Self-compacting concrete rely primarily on the modern generation of admixtures that reduce the water content and increase the workability without any effect on the strength of concrete. 

Generally, the higher the required flow ability of the SCC concrete mix, the higher the amount of fine material needed to produce a stable mixture. However, in some cases, a viscosity-modifying admixture (VMA) can be used instead of, or in combination with, an increased fine content to stabilize the concrete mixture.

Difference between SCC concrete and normal concrete

Concrete is one of the most commonly consumed materials in the world, after water. Working with the fresh concrete needs to involve skilled and experienced technicians using heavy and often hazardous mechanical vibration to ensure suitable compaction of the concrete to obtain the full strength and durability of the hardened concrete. 

Concrete pouring in a construction site

It was against this background that self-compacting concrete (SCC), which eliminates the need for compaction unlike regular concrete, has been developed and its advantages utilization. 

SCC concrete is much more needed in its fresh state than of traditional concrete which needs vibration and special treatment to reach the full strength, and it became clear that the remaining important obstacle to the expanded use of Self compacted concrete in Europe was the absence of suitable test methods to identify and specify its three key fresh requirements: filling ability, pass ability, and segregation resistance. 

Although there were a number of test methods, these had often been developed for specific applications with little or no attempt to ensure that they were more generally applicable, and there was no immediate prospect, anywhere, of a standardized tests. This was delaying the increased use of SCC concrete since it was tough to corroborate mix designs except by full-scale trials (feasible in special projects only). Establishment of a standardized tests was therefore an essential prerequisite to understanding the benefits of SCC concrete and to easing its widespread use in general construction.

Constituents of SCC concrete:

The constituent materials of SCC concrete  shall be suitable for the intended use in concrete and not contain harmful ingredients in such quantities that may be detrimental to the durability or quality of the concrete, or cause reinforcement corrosion. 

The SCC concrete contains the following:

1- Aggregates:

Coarse grains (gravels): The shape and particle size distribution of the aggregate is very important as it affects the packing void content, water absorption, grading of all aggregate should be closely and continuously supervised and must be taken in account in order to produce SCC concrete with good quality. The size of aggregate has a limit of 20mm or less. Aggregates of size 10 to12 mm is desirable for structures having congested reinforcement. If the coarse aggregate contact exceeds a certain limit, then blockage will occur between steel reinforcement, so it's recommended that the limit value of coarse aggregate is around 1/2 the solid volume.

Fine grains: The fine gravel that used in traditional concrete is the same gravel used for SCC, manufactured sand, natural, rounded and crushed sand are suitable. There is a limit that the fine aggregate shouldn't be more than 40% of the volume.

2- Typical Portland cement:

The preferred cement to use is of specific gravity of 2.90, and fineness of 2800 

cm²

/gm.

3- Mixing water:

Water quality must be established on the same line as that for using reinforced concrete or pre-stressed concrete. 

4- Chemical admixtures: 

Superplasticizers are an essential component of SCC concrete to provide the necessary workability, also they contribute to decrease the water content of mixture of about 12% that is used to produce the mixture.

Role of Super Plasticizers:
Although, manipulating the water-powder ratio leads to improved workability of the cement paste, it also leads to decreased viscosity. For the achievement of self-compact ability, therefore, a super plasticizer is indispensable. With super plasticizers, the paste can flow with little concomitant decrease in viscosity. Figure (4) below shows the action of super plasticizer.

5- Additions:

1. Fly Ash: Fly ash is a fine inorganic material with pozzolanic properties, which can be added to SCC to improve its properties.
2. Silica Fume: Silica fume gives very good improvement of the rheological as well as the mechanical and chemical properties. Improves also the durability of the concrete.
3. Ground (Granulated) Blast Furnace Slag: GGBS is a fine granular mostly latent hydraulic binding material, which can also be added to SCC concrete to improve the rheological properties.
4. Ground Glass Filler: This filler is usually obtained by finely grinding recycled glass.
5. Pigments: Used for the coloration of building, and filling material to improve the properties of fresh and hardened concrete. This material was brought from local market, then it was used in the concrete mixes after passing sieve number 0.075 mm.

6- Fibers:

Fibers may be used to enhance the flexural strength and toughness of SCC concrete in the same way as for normal concrete, in other words it's used to enhance the mechanical properties. Polymer fibers may be used to reduce segregation and plastic shrinkage, or to increase the fire resistance. Ease of mixing and the placing processes proposed, shall be demonstrated by site trials to the approval of the engineer.

Requirements of self-compacting concrete

The workability of SCC concrete can be characterized by the following requirements:

• Filling ability (high workability).
• Pass ability.
• Segregation resistance.

1- Filling ability (high workability):

The ability of the concrete to flow freely under its own weight, both horizontally and vertically upwards if necessary, and to completely fill formwork of any dimension and shape without leaving voids.

This is inquired by:

1. Increasing the liquidity of mixture by using superplastisizers admixtures.
2. Reduce internal friction between the grains by decreasing the percent of granular soil in the mixture or using of the percentage of powdered soft gradient.

2- Passing ability:

The ability of concrete to flow freely in and around dense reinforcement (congested) without blocking and under its own weight. This property is done by reducing the coarse aggregate content.

3- Segregation resistance:

During placement and while flowing, the concrete should retain its homogeneity. There should be no separation of aggregate from paste or water from solids using admixtures that decreases the viscosity, and no tendency for coarse aggregate to sink downwards through the fresh concrete mass under gravity.