Self-compacting concrete
During the 1980s a number of countries including Japan,
Sweden and France developed concretes that are self-compacting.
These self-compacting concretes (SCCs) are characterized by:
extreme fluidity as measured by flow, typically between 700-750mm,
rather than slump. SCC allows concrete to be placed
fully compacted without segregation and with no additional energy
(vibration). It currently makes up about 5% of the Japanese
concrete market and around 15% of the Danish and Swedish markets.
SCC has economic, social and environmental benefits over
conventionally-vibrated concrete. The uptake of SCC technology
within the UK has grown rapidly in recent years particularly in
precast concrete factories but less quickly in cast-in-situ
construction.
SCC is made from the same basic constituents as
conventional concrete but with the addition of a
viscosity-modifying (anti-segregation) admixture and high levels of
superplasticising admixtures to impart high workability. The cement
(hydraulic binder) content of the concrete is relatively high
(typically around 500 kg/m3). The ratio of fine to
coarse aggregates is increased, with fine aggregate often making up
more than 50% of the total aggregate fraction. Fine fillers, such
as ground limestone, may be used in addition to cement.
Overnight strengths typically reach 30-40N/mm2
and two-day strengths can break the 100N/mm2
barrier enabling easier and more reliable demoulding.
Fly ash is often used in
self-compacting concrete, because it imparts the rheology needed to
make high quality formed finishes with no compactive effort. The
rheology of the mortar fraction must be such to flow around the
formwork and reinforcing and be able to expel entrapped air using
its own self weight. Therefore, the proportions and nature of the
finer materials, cement, additions such as fly ash, fine aggregate,
etc can have a significant bearing on the final concrete. Fly ash
is often included within SCC because of its water-reducing
properties and the effect the spherical particle has on the
thixotropic nature of the concrete. This reduces segregation of the
coarse aggregate particles within the mortar matrix. Segregation
defines the difference between a well designed SCC concrete and a
badly designed one. Depending on the particle size distribution and
particle shape of the fine aggregate, varying proportions of fly
ash can be utilized both as a Type I and II addition
1.
A column produced using self-compacting fly ash concrete.
References and further
information
1. UKQAA, Technical Data Sheet 1.3 Applications for PFA in Concrete, 2006. View at: www.ukqaa.org.uk