The Indian Concrete Institute (ICI), Pune Center, organised a tech-talk on Self-Compacting Concrete (SCC), on 12 January 2019. I was one of the panelists. The brief on the proceedings are as follows:
(a) SCC versus Flowing Concrete: The basic difference between the two is in terms of bleeding and stability. When SCC is selected on the basis of technical requirements like the flow of concrete through closely spaced reinforcement or the uniformity, it cannot be substituted by flowing concrete.
(b) Ensuring Stability of SCC: Stability is the property of fresh concrete to prevent segregation. In many projects, SCC is preferred for stability, despite higher production cost. The conventional method of improving stability is to increase the fines content, using high volumes of filler, reactive (like fly ash) or inert. However, higher fines (paste) content has potential to increase shrinkage and creep. Hence, in some cases, SCC is designed with less fines content (close to normal concrete) and stability is improved, adding viscosity modifying agent (VMA).
(c) SCC Mix Design: Follow European Guidelines for Self-Compacting Concrete as given by EFNARC (www.efnarc.org).
The fluidity and viscosity of the paste is adjusted and balanced by careful selection and proportioning of the cement and additions, by limiting the water/powder ratio and then by adding a super plasticiser (SP) and (optionally) a viscosity modifying admixture (VMA). It reported that modern poly-carboxylate (PC) based SP does not require addition of VMA. Correctly controlling these components of SCC, their compatibility and interaction is the key to achieving good filling ability, passing ability and resistance to segregation. The paste is the vehicle for the transport of the aggregate; therefore the volume of the paste must be greater than the void volume in the aggregate so that all individual aggregate particles are fully coated and lubricated by a layer of paste. This increases fluidity and reduces aggregate friction. The coarse to fine aggregate ratio in the mix is reduced so that individual coarse aggregate particles are fully surrounded by a layer of mortar. This reduces aggregate interlock and bridging when the concrete passes through narrow openings or gaps between reinforcement and increases the passing ability of the SCC.
(d) SP and VMA Mixtures: The mixture of SP and VMA is available in the market. It is advised not to use such mixture. Whereas SP is added to SCC to improve flowability, VMA is added to improve stability and both need to be controlled independently.
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(a) SCC versus Flowing Concrete: The basic difference between the two is in terms of bleeding and stability. When SCC is selected on the basis of technical requirements like the flow of concrete through closely spaced reinforcement or the uniformity, it cannot be substituted by flowing concrete.
(b) Ensuring Stability of SCC: Stability is the property of fresh concrete to prevent segregation. In many projects, SCC is preferred for stability, despite higher production cost. The conventional method of improving stability is to increase the fines content, using high volumes of filler, reactive (like fly ash) or inert. However, higher fines (paste) content has potential to increase shrinkage and creep. Hence, in some cases, SCC is designed with less fines content (close to normal concrete) and stability is improved, adding viscosity modifying agent (VMA).
(c) SCC Mix Design: Follow European Guidelines for Self-Compacting Concrete as given by EFNARC (www.efnarc.org).
The fluidity and viscosity of the paste is adjusted and balanced by careful selection and proportioning of the cement and additions, by limiting the water/powder ratio and then by adding a super plasticiser (SP) and (optionally) a viscosity modifying admixture (VMA). It reported that modern poly-carboxylate (PC) based SP does not require addition of VMA. Correctly controlling these components of SCC, their compatibility and interaction is the key to achieving good filling ability, passing ability and resistance to segregation. The paste is the vehicle for the transport of the aggregate; therefore the volume of the paste must be greater than the void volume in the aggregate so that all individual aggregate particles are fully coated and lubricated by a layer of paste. This increases fluidity and reduces aggregate friction. The coarse to fine aggregate ratio in the mix is reduced so that individual coarse aggregate particles are fully surrounded by a layer of mortar. This reduces aggregate interlock and bridging when the concrete passes through narrow openings or gaps between reinforcement and increases the passing ability of the SCC.
(d) SP and VMA Mixtures: The mixture of SP and VMA is available in the market. It is advised not to use such mixture. Whereas SP is added to SCC to improve flowability, VMA is added to improve stability and both need to be controlled independently.
View Dr J D Bapat's website
Join Dr J D Bapat on Twitter: @cemcretecon , @drjdbapat
Join Dr J D Bapat on Linkedin
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