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Adding versatility to concrete
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The advent of chemical admixtures has added a whole new dimension to the process of preparing concrete mixture, making it versatile enough for modern construction designs and placing methods.
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New dimension: Engineers employ a wide range of admixtures these days.
Until a few years ago, concrete was just concrete. Preparing the concrete mixture was just a matter of adding cement, sand and water to the required quantity of aggregates. The advent of chemical admixtures has added a whole new dimension to the process, making it versatile enough for modern construction designs and placing methods. Today, engineers employ a wide range of admixtures, including special-purpose varieties f
or various applications.
Admixtures are essentially ingredients other than water, aggregates and cement that are added to the concrete batch immediately before or during mixing. Classified as construction chemicals, they are liquid or powder products which modify and improve the properties of fresh and hardened concrete or mortar. They are used to improve quality, accelerate or retard the setting time, enhance resistance to weather conditions, impart strength and workability and provide better finish.
Various types
There are several types of admixtures like plasticisers, superplasticisers, accelerators, retarders, and corrosion inhibitors. Other types include integral water proofing compounds, sprayed concrete accelerators and hyper plasticisers.
Plasticisers or water reducing admixtures are agents added to cement for improved density and quality of concrete. By decreasing cohesion, they make it easier to pour and compact the concrete. Plasticisers contribute to significant saving in cement without reduction in strength and workability.
At the work site, concrete has to be loose enough to pour and compact by vibration. But adding water beyond the specific ratio reduces the strength of the mixture. By mixing plasticisers, the concrete becomes `looser’ without adding water. Plasticisers usually reduce the water demand by seven to 10 per cent.
Superplasticisers, also known as high-range water reducing admixtures, are generally used in the construction of high rise structures where it becomes necessary to strengthen the columns supporting the building without adding to their size. They also find wide application in the production of pre cast elements, beams, shell forms and concrete railway sleepers. They make compaction easier in the construction of heavily reinforced concrete bridges, foundation columns, beams and narrow formwork units.
Hyperplasticisers are high performance third generation ploycarboxylic-based admixtures and cohesion agents applied mainly to self compacting concrete to achieve high early strength in skyscrapers and for underwater concreting. Because of the special nature of their application, they are yet to find use in Kerala.
Mortar plasticisers are ideal for minimising shrinkage cracks and improving the trowelling properties of mortar mix.
Accelerators constitute another class of admixtures. Accelerating admixtures are added to concrete either to increase the rate of early strength development or to shorten the time of setting, or both. They shorten the set time of concrete, allowing a cold-weather pour, early removal of forms, early surface finishing, and in some cases, early load application.
On the other hand, retarding admixtures employ the reverse process to delay the setting of concrete and avoid ‘cold joints’ where delays in transport and placing of concrete are inevitable. They are mostly used in construction of towers, chimneys, high-rise buildings, slipform shutters, tunnel or shaft lining, offshore construction and piles.
Retarding admixtures slow down the hydration of cement, lengthening set time.
They are beneficially used in hot weather conditions in order to overcome accelerating effects of higher temperatures and large masses of concrete on concrete setting time.
Retarding admixtures form a film around the cement compounds, thereby preventing or slowing the reaction with water. After a while, this film breaks down, and normal hydration proceeds.
Retarders are preferred by contractors for a variety of reasons. For one, they facilitate transportation of concrete, thus eliminating the cost of relocating central mixing plants.
They allow more time for texturing or grooving of concrete surfaces as well as for hand finishing work. They also eliminate cold joints in multi-stage paving and in the event of equipment breakdown.
Readymix concrete plants use superplasticisers-cum-retarders to create the optimum blend that will enable transportation and pumping to the work surface.
T. NANDUKUMAR
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