What is Self Compacting Concrete SCC

Self-Compacting Concrete (SCC) has properties that differ considerably from conventional slump concrete. Self Compacting Concrete is highly workable concrete that can flow through densely reinforced and complex structural elements under its own weight and adequately fill all voids without segregation, excessive bleeding, excessive air migration (air-popping), or other separation of materials, and without the need for vibration or other mechanical consolidation.

What about highly flowing concrete? Is that Self Compacting Concrete ?

ASTM C1017 defines flowing concrete as “concrete that is characterized as having a slump greater than 190 mm (71⁄2 in.) while maintaining a cohesive nature”. Flowing concrete can be proportioned with an even higher slump to be self-leveling, which means that it is capable of attaining a level surface with little additional effort from the placer.

According to ACI 212, these self-leveling characteristics “have given rise to a false belief that such concrete does not require vibration”. ACI 212 stresses that, unlike SCC, to obtain a properly consolidated self-leveling concrete, “some compaction will always be required”. Thus Self Compacting Concrete is always “highly flowing”, but “highly flowing” may not qualify as Self Compacting Concrete . The same is true of “self-leveling” concrete.

How is Self Compacting Concrete different from conventional slump concrete?

Self Compacting Concrete  must be highly workable so that it can move under the force of gravity without vibration, during mixing, transportation, handling, and placement. It is so highly flowable

that the conventional slump test cannot distinguish between different levels of Self Compacting Concrete flowability – all would be 280 mm+ (11 in.+) in slump. However, SCC must also be viscous enough so that the mortar suspends and carries coarse aggregate, maintaining a homogenous, stable mixture, resistant to segregation, bleeding, excessive air migration, or paste separation.It must have dynamic stability during mixing, transportation, handling and placement, and static stability during protection and curing. SCC’s workability is a function of its rheology. Conventional concrete brought to 280 mm+ (11 in.+) slump does not have this stability.

What are the limitations of Self Compacting Concrete ?

Application:

Caution should be taken when using SCC in flatwork as it has limited bleeding

characteristics and may be subject to plastic shrinkage cracking if not properly protected and cured. Higher powder contents bleed less than conventional concrete and can also lead to plastic shrinkage cracking if not properly cured.

Production and Quality Control:

Self Compacting Concrete  requires a higher level of quality control than conventional slump concrete. Combined aggregate grading, tightly controlled mix water, controlled cement source, and the use of advanced admixtures require a greater awareness on the part of all production personnel. Processes must be put in place to compensate for normal variation of materials. Key items to monitor are:

• Coarse and fine aggregate grading

• Coarse aggregate void volume

• Aggregate moisture