Self-compacting concrete ... superplasticisers play a key role.
Rheodynamic signals concrete revolution
Dubai-based construction chemicals company MBT highlights the advantages of its Rheodynamic concrete which it says has the potential to revolutionise concrete construction.
01 September 2002
Contrary to popular understanding, steel-reinforced concrete is vulnerable to corrosion and is a material that deteriorates easily through external aggression.
The accessibility of the steel reinforcement to aggressive media is invariably due to both incorrect mix proportions and, in particular, to inadequate placement and compaction. This last aspect is especially relevant today, where forms can be very congested and a serious challenge to labour, the quality of whose work has worsened over the years.
Increasing structural performance has led to greater reinforcement volumes which, together with a growing trend towards the use of smaller diameter bars to limit cracking, has created a higher incidence of heavily congested formwork in mat foundations and moment resisting structures.
There is a growing interest in reducing the potential for deterioration of a structure due to poor compaction, by specifying highly flowable high-performance concrete (HPC).
Originally developed in Japan to offset a growing shortage of skilled labour, the concept of self-compacting concrete (SCC) was proposed by Okamura in 1986.
A highly fluid mix that had excellent deformability, resistance to segregation, that could entirely fill and self-compact under its own weight in heavily reinforced formwork, was first developed at the University of Tokyo in the late 1980s.
SCC can be defined as concrete that completely fills the formwork and encapsulates the reinforcement without vibration, achieving compaction only through the action of gravity.
To achieve this, the concrete must be highly fluid and stable: it must remain homogeneous during the entire process from plant to form, without separation of the aggregate, mortar and water.
The first applications of SCC focused on the challenges of producing homogenous high-performance concrete and achieving optimal compaction in situations of heavily congested reinforcement or difficult access conditions.
The late 1980s and 1990s saw large volumes of SCC used in major Japanese civil engineering structures, such as massive LPG storage tanks and the huge anchorage blocks for the 2-km-span Akashi-Kaikyo suspension bridge, each anchorage of which contains a complex mixture of dense reinforcement and cabling encased in 120 kg/cu m of SCC.
In the intervening years, SCC has received enthusiastic acceptance by a European concrete construction industry eager to profit from the multitude of benefits of its use in both readymixed and precast applications.
Practical application has been accompanied by considerable research of the physical and mechanical properties of SCC.
While SCC was originally developed as part of a strategy to enhance the durability of concrete, particularly in complicated, heavily civil engineering structures with reduced labour, it soon became evident that there were other substantial benefits to be gained from its use in mainstream construction.
SCC can be employed in most applications where traditionally vibrated concrete is used and whilst it exhibits superior installation properties in densely reinforced structures, it also offers benefits in unreinforced applications such as backfilling and tunnel lining.
SCC can be fibre-reinforced, modified with corrosion inhibitors or air-entraining agents and is equally appropriate for precast as well as in-situ concrete.
Choosing the right products
Getting the mix right is crucial to achieving successful SCC. By controlling material grading and moisture contents, segregation is avoided and maximum fluidity is achieved.
MBT's proven admixture products help to achieve the optimum mix, from advanced technology superplasticisers to complementary products, which enhance the performance of SCC both in its wet and fully-hardened stages.
Mix design for SCC
The mix design of SCC is critical to its success and MBT's worldwide experience of SCC formulation has made it the market leader in transferring this pioneering technology acquired through international research projects.
Mix design criteria for SCC
SCC is the result of a skilful combination of chemical and mineral admixtures, along with a higher proportion of fine aggregates than is usual in conventional mixes. Typical mix design criteria include cementitious contents of 450 kg/cu m for horizontal mixes and 500 kg/cu m for vertical mixes and pozzolans such as PFA or slag can be used. The correct choice of release agent achieves a blemish-free surface.
Characteristics of SCC
SCC differs from conventional concrete in that its fresh properties are critical to its ability to be placed satisfactorily. There are three key aspects of workability, which require to be carefully controlled to ensure satisfactory performance during its wet phase and for successful classification as SCC:
The special rheological properties of SCC are achieved through the use of chemical and mineral admixtures. Superplasticisers capable of effecting a water reduction of >20 per cent are essential to provide the necessary workability, but high fluidity increases the tendency of a mix to segregate and maintaining homogeneity is an important aspect of the control regime.
The addition of fine powder with a particle size smaller than 0.125 mm reduces the risk of segregation and blocking if added in sufficient proportion usually at 30-40 per cent of binder content.
Fines derived from high fibre fines content sand (>10 per cent) stone powder, fly ash, GBFS, PFA, silica fume and other fillers are suitable. In particular silica fume can significantly improve long-term durability of the concrete.
Limiting the volume of coarse aggregates reduces the potential for collisions between aggregate particles and increases the passing ability. The consequent increases in paste volume, low water/ powder ratio and the addition of the superplasticiser provides the fluidity necessary to reduce air entrapment and increase cohesiveness to resist segregation.
The inclusion of viscosity modifying admixtures (VMAs) helps to ensure homogeneity and reduce the tendency for a highly fluid mix to segregate. Their use will also suppress some of the affects of poor aggregate grading and insufficient fines.
A SCC mix should have high workability and high viscosity. The fluidity of the mix is assured when there is no friction between the internal particles and the concrete can flow freely. Segregation occurs when the components of the concrete separate out into mortar and course aggregates. Reaching the right balance between fluidity and resistance to segregation - apparently opposing properties - is key to the successful production of SCC.
Rheodynamic concrete, the future
The enhanced form of SCC developed by MBT has the potential to revolutionise concrete construction.
Rheodynamic concrete is the means by which, one day, readymix operators could supply and lay concrete slabs, and how precasters will eliminate noisy external vibration and the energy costs of heat curing needed to achieve high early strength.
Distinguished from SCC by the proven Glenium superplasticiser technology, Rheodynamic concrete can be produced easily from familiar, locally-available materials.
It is suitable for the large-scale industrial production of concrete in conventional applications, where durability, time, labour, energy and direct savings on costs of construction and life cycle are sought.
Rheodynamic concrete self compacts and virtually levels itself from a single pouring location. It requires no vibration or tamping and only a minimum of trowelling to produce a smooth finish. Plastic settlement is eliminated by the high degree of compaction and struck concrete is almost always blemish free.
The incredible properties of Rheodynamic concrete are due to the breakthrough technologies of Glenium SCC and Glenium Stream VMAs used in skilful combination with mineral admixtures and lower water-powder ratio than in conventional mixes.
The Glenium mechanism represents the first "new" superplasticiser technology for almost 40 years. Its unique dual action permits water reductions of up to 40 per cent of an unmodified control or 15 per cent greater reduction than a typical superplasticised HPC mix.
