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Contribution of Admixtures for Making Sustainable Concrete

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Sustainable development is“Development that meets the needs of the present without compromising the ability of future generations to meet their own needs.”Brundtland Report – Oct , 1987

Admixtures have a huge role in making concrete better and sustainable. This article brings about the big contributions of admixtures.
As we are all aware there are two major categories of admixtures

–    Chemical admixtures
–    Mineral admixtures

Chemical admixtures are materials which are added up to about 5% by weight of cement, which can improve, modify the properties of concrete.

The other categories of admixtures are additives basically which contain primarily minerals, which react chemically in the hydration process of cement forming durable concrete. They are also known as pozzolonas.

They are also defined as follows

“Siliceous or siliceous and aluminous materials, which in themselves possess little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with Ca(OH)2 at ordinary temperatures to form compounds possessing cementitious properties”.

Contribution of chemical admixtures;

Admixtures are chemicals added in very small amounts to the concrete to modify the properties while the concrete is still fluid and also after it has hardened and is in service. The quantity added is less than 0.4% and usually less than 0.2% of the concrete weight but even at this low level, admixtures have a very significant effect on the concrete properties.

Increased fluidity: reduces noise and energy requirements during placing.

Plasticisers /superplasticisers

–    Will increase slump (against control mix) when added at same w/c ratio giving a similar strength concrete.
–    Can give same Slump (as control mix) at reduced w/c ratio giving increased strength.
–    Can give similar slump (as control mix) ,similar strength at  similar w/c ratio but reduced cement content.

Table 1The table 1 also indirectly shows another interesting point. Assume we want to have a concrete without admixture but which can give similar slump to that mix containing the admixture. So can we do this?

One way is to increase the water content of control mix approximately equal to to the % of water reducing ability of admixture. In  other words the control mix having a water content of 180 liters  will have to be increased to about 212.4 litres.In this case to maintain same strength the cement content will move up to 212.4/0.45=472 kg/m³.

So this means we are having cement savings every time we are using a superplasticiser!

Optimized mix design: reducing embodied carbon dioxide and energy by enhancing the effectiveness of the cement component.

Normal Plasticising Admixture use can typically reduce the CO2 by more than 20kg per cubic meter of concrete. Superplasticisers use could make this more than double .Looking at this the co2 reductions globally by using superplasticisers is huge.

Table 2The ultra modern PCE based admixtures have pushed the boundaries of concrete technology to unprecedented /unheard of levels. With water reducing abilities of the order of 30 to 40% the strength increase or cement reduction or CO2 reductions are phenomenal.

Reduced permeability: increases the durable life of the concrete.

The water reduction ability of the SNF and PC based admixtures indirectly means, today we are able to easily achieve M60 and above with these advanced materials. Apart from strength the durability parameters are pushed to amazing levels.  With lot of research and use of pozzolonic materials modern concretes have moved to next Table 3level of durability. Judicious use of PFA, GGBS, Microsilica, Metakaolin etc with carefully selected chemical admixtures have made the design of durable structures simpler.  Here we need to note, while it is possible to easily achieve High strength concrete that may not necessarily ensure durable service life of the structures made with such concretes. Chemical admixtures usage along with SCMs have made possible lowest levels of RCPT values which can not be achieved with OPC alone!!. Further highly durable concretes can be produced with very low water absorption (permeability) using good admixtures and pozzolonic materials. It is established beyond any doubt that SCMs when used in conjunction with high quality admixtures provide a low porosity, fine pore structure and hence enhanced durability.

Reduced damage from harsh environments: including marine, freeze-thaw and sub zero situations.

Superplasticised, water reduced concretes with high levels of cement replacement materials such as Flyash and GGBS have shown remarkable improvements in sulphate ion resistance. Using this beneficial effects around the world lot of concretes are poured with OPC+GGBS.

Air entraining admixtures provide a network of micro air bubbles which significantly enhances the freeze thaw resistance in cold climates.
There are several new varieties of highly hydrophobic admixtures which impart very high impermeability to concrete making the concrete highly durable.

ECMAS 4Improved quality: better finish and reduced service life repair.

With the advent of PCE based hyperplasticisers Self compacting concrete is quickly becoming a single solution to durable structures especially in case of thin structures or wherever congested reinforcement is there.SCC is a natural choice of architects when specifying thin structures, curved structures, form finish structures etc. Generally SCC contains good amount of SCMs and are mostly done at low water cement ratios using PC based admixtures and hence SCC exhibits more durability compared to conventional concretes made with Portland cements alone.

There are other classes of admixtures such as Corrosion inhibitors, Shrinkage reducing admixtures which adds to the service life of concrete.

ECMAS 6There are also new class of admixtures which can create self curing concretes. When used in a effective manner they can totally eliminate the need for curing. There are lot of efforts going on in various countries to produce self compacting and self curing concretes. Such concretes when successfully commercialized will make perfect examples of sustainable concrete as they almost eliminate human intervention in placing, curing of concretes ling to leading to highly durable concretes.

We can list the contribution of admixtures to sustai-nability as follows:

Superplasticisers and Hyperplasticisers viscosity modifiers. Have changed the construction technologies. Because of them we are now able to make highly pumpable high slump concretes, self compacting concretes resulting in higher production, reduced labour, reduction in construction time.

ECMAS 5Using host of SCMs like Metakaolin, GGBS, Flyash in conjunction with Corrosion inhibitors, shrinkage reducing admixtures, hydrophobic admixtures we can push concrete to highest durability limits

High performance admixtures  &SCMs  can make
–    Concrete with higher strength ::::> Use less concrete
–    Concrete with greater durability ::::> Longer service life
–    Strength with less water::::>Use less water
–    Fluid concrete ___>>> Reduced batching energy
–    Pumpable concrete___>>> Reduced transportation energy
–    Self-consolidation___>>> Lower placing energy (vibration)
–    High workability ___>>> Reduced construction times

Conclusion:

The contribution of chemical admixtures to achieve sustainable concretes is very important. Proper usage of chemical admixtures in conjunction with SCMs have great impact on the construction methods. In short, the sustainable constructions have got invaluable fillip with the use of both chemical and mineral admixtures.

References:

–    Evaluation of Chloride Ion Permeability of Triple Blend SCC, By V.R.Kowshika, Bharathi Ganesh, H.Sharada Bai.(ICI-IWC).
–    ”Sustainability of concrete construction” by Tarun.R.Naik
–    ”Sustainable concretes with Metakaolin and High performance admixtures” by V.R.Kowshika and Raj Pillai

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