Why curing of concrete is important?

Curing offers the following functionalities :

1. Resist the concrete from getting dried throughout hydration. If this happens, the strength of the concrete is reduced significantly. The damage can’t be recovered. If concrete is dried, the cement grains will produce an impervious layer of hydration product around them, and it will resist recurring of hydration, though several occasion it is dampened again.

2. To maintain heat at the surface.

Curing is carried out for the following purposes :-

a. To get rid of frost damage (under 5°C, 40°F)
b. To raise initial strength
c. To minimize temperature gradients

When low temperatures decrease initial strength, the effect does not remain for a long time in case the concrete has not frozen, and it is consequently retained at higher temperatures. As a result, a sample that is retained at 5°C (40°F) will not completely hydrate (specifically if there exist a pozzolan in it), but even after a number of months it will hydrate again with higher temperature.

Since hydration occurs more slowly, cements containing pozzolans and GGBS normally need longer curing. It is therefore necessary that these concretes should be recognized on site, and cured sufficiently. The pozzolanic reaction will then produce extra hydration products to block some of the pores among the cement grains, and attain good strength.

Given below, some recognized process of curing :

• Cover materials (e.g., columns) in polythene once the shutters are detached.
• Spray with curing membrane as soon as detachment of shutters.
• Wrap slabs with polythene (and pour ground slabs on polythene).
• For heat retention, the polystyrene should be utilized on the back of shutters (particularly, steel ones)
• Just leave shutters in exact position for a few extra days (particularly wooden ones).
• 50 mm of sand is well suited on slabs.
• Ponding (i.e., developing a pool on the concrete surface) will be definitely most suitable.

Note about curing:

• Ensure that curing is provided immediately as possible. A few hours may provide significant effect.
• Spray-on curing membranes are less effective, and in windy conditions they should not be used. On complicated areas (like columns), they should be used as there is no other options.
• Keep in mind that PFA, GGBS and, especially, CSF requires much better curing (frequently 5 days, in spite of 3 days).
• Allowing the bleed water to dry off will lead to more bleeding, and plastic cracking.
• Slabs on ground should contain a polythene sheet that is arranged under them, to get rid of excessive water absorption with dry soils.



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Published By
Rajib Dey
www.constructioncost.co
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Types of plasticizers commonly used and their benefits

Plasticizers & super-plasticizers facilitate concrete to enhance it's workability.

Due to it’s strong fluidity, shotcrete is highly beneficial. When the homogeneous mix is applied pneumatically, it simply conforms with even rugged surfaces whereas maintaining the initial resistance to compression to offer good structural support from the beginning.

A family of chemical polymers like plasticizers and super-plasticizers are liable for the material’s fluidity. These are also called water reducers to minimize the total water-to-cement ratio, providing it a more ‘liquid’ consistency devoid of diluting the mix with water.

These additives are normally added all through the mixing process and allow the concrete mix to become yielding until application keeping its consistency unchanged.

Plasticizers and super-plasticizers provide a temporary dispersing effect, that produces a comprehensive hydration of each cement particle, making the fluidity of the mix better.

This mix is amalgamated with volcanic fly ash to form the type of hydraulic concrete, a fully hardy and weatherproof material along with extremely erosive salt water.

Plasticizers: first steps - First generation of plasticizers is known as Lignosulphonates. It belongs to a byproduct originated from wood processing that is frequently applied in recent times to form a workable mix with only basic raw materials.

These additives, known as Mid-Range Water Reducers [MRWR] affix themselves to the surface of a cement particle, that bears both positive and negative charges. Plasticizer polymers, which are negatively charged, neutralize the positive charges on the cement surface, transforming the entire surface completely negative.

It causes a physical effect that allows the negatively charged cement particles to keep away each other, providing a dispersing effect to develop better water infiltration. This mix can now function well devoid of adding more water, and allows for a cutback in the overall amount of water necessary, reducing the water-cement ratio by around 10%.

But these additives can defer the curing process, which may sequentially produce further complicacies. If curing is not done within a certain timeframe, a greater amount of hydrostatic pressure could amass in a formwork column over a long lasting period, causing the formwork to burst.

The second generation: Plasticizers 2.0: This type of plasticizer can significantly lessen the water-to-cement ratio of around 25%. Polysulfonates like naphthalene and melamine offer same type of working mechanism to the first generation of plasticizers, producing an electrical dispersing albeit of superior intensity.

These polymers conform to the cement particles, charging them negatively to form repulsion among related particles, allowing water to flow and hydrate the mix in an efficient manner.

This similar repulsion activity also activates major air occlusion, raising the workability of the mix but at the same time developing pockets of air that reduces its resistance and settle its structural integrity.

This type of polymer may also put other challenges, since it provides a very narrow window of ‘workability’: as soon as the cement is hydrated, it is likely to develop a crust-like byproduct that makes application complicated.

Super-plasticizers: the third generation - Super-plasticizers stands for the additives which bring huge benefits along with a water-to-cement ratio curtailment of around 40%.

Polycarboxylates alias High Range Water Reducers (HRWR) function on the base of sterical in spite of electrostatic repulsion. A major steric effect is steric hindrance for which a chemical reaction can’t occur. In this case it stops cement particles from agglomerating.

Polycarboxylates stands for complex co-polymers which are applied to satisfy several functions, and comprise of a negatively charged ‘backbone’ molecule with polymeric side chains.

Most of these additives can be amalgamated jointly, and blended with other types like air-entraining, accelerating and retarding additives
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Published By

Rajib Dey

www.constructioncost.co

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